Same Lampizator?
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The DAC I've used for years!
24bit/192KHz TUBE DAC DIY KIT - Lampucera1.0 + Lampizator CS8416 + CS4397 | eBay
@johnss
Many thanks for this helpful update. I have been hoping someone might have walked this same path before me with the BEWITCH amp - and here you are! Wonderful! Many thanks too, for the great pics!
Do you still own the amp and enjoy it?
It looks like you have invested heavily in premium parts on your BEWITCH amp upgrade. There's some real nice bits in those pics.
You didn't mention how much improvement your final upgraded amp delivered, over and above the original circuit with the original parts. I'd be keen to hear a lot more from you about that.
The way that you tell your story, it seems like you may have adjusted the GNFB - and finally settled on half of the original GNFB - BEFORE you tried the LAMPIZATOR upgrades.
I'd like to ask you what halving the GNFB - without any other circuit changes - made on the sound of your original BEWITCH amp?
You also noted that upgrading the coupling caps made big improvements. Are you referring to C1, C2, C3 and C4? Any further thoughts on this?
I'm thinking that I might try these two simple changes on our BEWITCH amp - halving GNFB and fitting superior C1, C2, C3 and C4 caps - without taking the LAMPIZATOR route at all.
Any further thoughts that you can share would be appreciated.
You are welcome. I did the upgrades in stages. First did the coupling caps, installed Jupiter vintage tone. then replaced them with Jensen Copper foil caps.
After that tried experimenting with the GNFB. Lampizator says he removed it all (lifted one leg on the GNFB resistor) I tried that but got too wooly sounnding and lacked definition. I finally soldering in a stereo pot with shielded leads so I could adjust GNFB while listening. I ended up at about half of the original GNFB the amp came with stock.
Next tried the lampizator mods where you change the plate and load resistors to enable 6SN7s in all the four octal sockets across the front. and went one step further by installing MILLS Wire wound resistors in place of the carbon film resistors that came from the factory.
If I were to rate the sonics improvement on which mod made the most significant improvement in SQ (sound quality). The top 4 choices would be the coupling caps, reduction of GNFB, selection of better quality input tubes, installation of Shuguang KT-88-T power tubes.
The other changes made small improvements but those were the biggest improvements mentioned above.
I have gone through a similar process on a number of Chinese made and sourced tube amps. The thing to remember though is the parts costs add up quickly. If you sum the original purchase price of the amp, plus the cost of the parts, you are quickly approaching what you can purchase many US made tube amps for on the used market.
For example, in similar power levels, the Cary CAD-50s Mk1 and Mk2, you can get these for around 1100 to 1300 a pair USD. Or some of the older CJ MV-50 and MV-55 amps, or some of the earlier VTL amps.
I kept an excel sheet for most of the china amps I did. I some cases I exceeded the original purchase price of the stock amp with added after market parts, which is a bit nuts from a logical point of view. you will never get those funds back out if you sell the amp.
I have bought and sold over 18 Chinese made tube amps over the last 5-6 years. The only one I have hung on to, and use for a reference to compare any new purchases to, is the inexpensive Music Angel EL34. Not sure if its even made any more. But that amp is the bench mark any new comers (new amps purchased) must beat. If they can't beat it sonically, the new comer gets listed and sold off.
As for your Q as to what I am using now, the Bewitch was put on a shelf for the time being. I have an assortment of tube power amps that I use and rotate through from VAC, Cary (the V-12, SLAM-100, CAD-50MK2), VTL, Primaluna, CJ and if super efficient speakers are used, the little Inspire KT-88 amp.
I'll add some photos of the music angel. With not too power hungry speakers, and Shuguang 6CA7 fat bottle power tubes, it has one of the best glue bond joints between your rear and the listening chair (ability to keep you listening for extended periods) of any amp I have tried.
the Key on the music angel is to add bypasses to the electrolytic cathode caps (amp is cathode biased not fixed bias). and the coupling cap upgrade made a huge improvement.
After that tried experimenting with the GNFB. Lampizator says he removed it all (lifted one leg on the GNFB resistor) I tried that but got too wooly sounnding and lacked definition. I finally soldering in a stereo pot with shielded leads so I could adjust GNFB while listening. I ended up at about half of the original GNFB the amp came with stock.
Next tried the lampizator mods where you change the plate and load resistors to enable 6SN7s in all the four octal sockets across the front. and went one step further by installing MILLS Wire wound resistors in place of the carbon film resistors that came from the factory.
If I were to rate the sonics improvement on which mod made the most significant improvement in SQ (sound quality). The top 4 choices would be the coupling caps, reduction of GNFB, selection of better quality input tubes, installation of Shuguang KT-88-T power tubes.
The other changes made small improvements but those were the biggest improvements mentioned above.
I have gone through a similar process on a number of Chinese made and sourced tube amps. The thing to remember though is the parts costs add up quickly. If you sum the original purchase price of the amp, plus the cost of the parts, you are quickly approaching what you can purchase many US made tube amps for on the used market.
For example, in similar power levels, the Cary CAD-50s Mk1 and Mk2, you can get these for around 1100 to 1300 a pair USD. Or some of the older CJ MV-50 and MV-55 amps, or some of the earlier VTL amps.
I kept an excel sheet for most of the china amps I did. I some cases I exceeded the original purchase price of the stock amp with added after market parts, which is a bit nuts from a logical point of view. you will never get those funds back out if you sell the amp.
I have bought and sold over 18 Chinese made tube amps over the last 5-6 years. The only one I have hung on to, and use for a reference to compare any new purchases to, is the inexpensive Music Angel EL34. Not sure if its even made any more. But that amp is the bench mark any new comers (new amps purchased) must beat. If they can't beat it sonically, the new comer gets listed and sold off.
As for your Q as to what I am using now, the Bewitch was put on a shelf for the time being. I have an assortment of tube power amps that I use and rotate through from VAC, Cary (the V-12, SLAM-100, CAD-50MK2), VTL, Primaluna, CJ and if super efficient speakers are used, the little Inspire KT-88 amp.
I'll add some photos of the music angel. With not too power hungry speakers, and Shuguang 6CA7 fat bottle power tubes, it has one of the best glue bond joints between your rear and the listening chair (ability to keep you listening for extended periods) of any amp I have tried.
the Key on the music angel is to add bypasses to the electrolytic cathode caps (amp is cathode biased not fixed bias). and the coupling cap upgrade made a huge improvement.
@johnss
This is invaluable information. Many thanks again for this, John.
This 100% falls in-line with my expectations.
Aside from a couple of other 'good-house-keeping" tweaks that know, I will probably limit my upgrades to include these 4 changes.
In other words, I won't be travelling the LAMPIZATOR route.
I will report how these changes progress, at this forum.
Additionally, over the longer term I will try to figure out how to install the individual cathode bias resistors for each 6550 - and probably use MILLS resistors in those positions.
This is precisely what I was going to do... You beat me to it!
As others have commented, this amp relies on a level of GNFB to function, so this GNFB value is the all-important question IMHO.
Would you be willing to share what value resistor that you eventually installed in the GNFB position? (R14 and its counterpart)
Looking at your photos below, it seems you also applied this tweak to your BEWITCH. Compared to the MUSIC ANGEL amp, did this tweak deliver a worthwhile impact on the sound of your BEWITCH amp?
[Please forgive the 50 question inquisition... 😱]
This is invaluable information. Many thanks again for this, John.
This 100% falls in-line with my expectations.
Aside from a couple of other 'good-house-keeping" tweaks that know, I will probably limit my upgrades to include these 4 changes.
In other words, I won't be travelling the LAMPIZATOR route.
I will report how these changes progress, at this forum.
Additionally, over the longer term I will try to figure out how to install the individual cathode bias resistors for each 6550 - and probably use MILLS resistors in those positions.
This is precisely what I was going to do... You beat me to it!
As others have commented, this amp relies on a level of GNFB to function, so this GNFB value is the all-important question IMHO.
Would you be willing to share what value resistor that you eventually installed in the GNFB position? (R14 and its counterpart)
Looking at your photos below, it seems you also applied this tweak to your BEWITCH. Compared to the MUSIC ANGEL amp, did this tweak deliver a worthwhile impact on the sound of your BEWITCH amp?
[Please forgive the 50 question inquisition... 😱]
Your photo below appears to show a 330K / 2W resistor in the GNFB position. Was this your final choice?
CLASSIC WILLIAMSON IDEA: Try removing Common Cathode Bypass Capacitor???
I read with interest johnss' post about how much his EL34 MUSIC ANGEL amp improved, as a result of bypass in EL caps used in the Cathode Bypass Capacitor.
So I was thinking to try bypassing the Common Cathode Resistor Caps on the BEWITCH, with some no EL caps.
[Johnss' photos give me some ideas about what cap values to try...]
After a bit more research on this subject however, I discovered that one of the time-honored amp circuits of the past - 1947 Williamson Amplifier - very successfully used common cathode bias resistors... But with no bypass capacitor at all!
Evidently, the Williamson circuit sounds better this way.
I'm wondering if removing the Common Cathode Bypass Capacitors altogether - from the BEWITCH circuit - might be an interesting experiment?
I read with interest johnss' post about how much his EL34 MUSIC ANGEL amp improved, as a result of bypass in EL caps used in the Cathode Bypass Capacitor.
So I was thinking to try bypassing the Common Cathode Resistor Caps on the BEWITCH, with some no EL caps.
[Johnss' photos give me some ideas about what cap values to try...]
After a bit more research on this subject however, I discovered that one of the time-honored amp circuits of the past - 1947 Williamson Amplifier - very successfully used common cathode bias resistors... But with no bypass capacitor at all!
Evidently, the Williamson circuit sounds better this way.
I'm wondering if removing the Common Cathode Bypass Capacitors altogether - from the BEWITCH circuit - might be an interesting experiment?
Attachments
Success!!!
Meanwhile, having returned our BEWITCH 6550 Integrated tube amp back to its totally original factory design - complete with its complement of run-of-the-mill Russian tubes - I have completed two upgrades.
I will call these upgrades "changes" because I accept that the results are highly subjective.
CHANGE 1:
A trick I learned from an old Ham Radio enthusiast... I installed a ferrite ring type power filter just after the power socket. This has worked wonders on the preamps I have owned and typically really opens up the frequency extremes.
This change delivered a very solid improvement in sound quality - not unlike stepping up to an exotic and expensive power cable; but at a fraction of the cost.
The tops opened-up nicely and the bass sounded tighter and slightly more extended.
This change delivered good results, but was comparatively minor compared to the second change.
CHANGE 2:
I bypassed the coupling caps in C1, C2, C3 and C4 positions with high-quality CORNELL DUBILIER 940C series .01uF polypropylene caps.
I have successfully used these bypass caps before - in speaker crossover projects - but nothing could have prepared us for the improvement this second change delivered.
Unbelievably - after Change 2 - in the BEWITCH, we now have an amp that rivals any amp we own or have ever owned.
Having owned highly regarded amps - in some cases for many years - this is a a delightful upset.
For reference, our own power amp history includes: -
CARY 300SE Signature Monoblocks (300B SET, US)
THETA 6550 Monoblocks (Dynaco derivative, US)
LUXMAN AVANCE Z501 (Solid State, JAPAN)
FIRST WATT J2 (US)
TRANSCENDENT SOUND SE-OTL (Original, US)
HOLFI Power 150 (DENMARK)
LINX PULSAR (UK); and
LEBEN CS300F (JAPAN) - our current resident amp.
[And we have tried countless others along the way...]
In short, we can barely tear ourselves away from this BEWITCH amp. Yes. It's that good, after just a few minor changes.
I will continue to try new ideas and report back - but this current recipe is going to be very hard to beat.
Meanwhile, having returned our BEWITCH 6550 Integrated tube amp back to its totally original factory design - complete with its complement of run-of-the-mill Russian tubes - I have completed two upgrades.
I will call these upgrades "changes" because I accept that the results are highly subjective.
CHANGE 1:
A trick I learned from an old Ham Radio enthusiast... I installed a ferrite ring type power filter just after the power socket. This has worked wonders on the preamps I have owned and typically really opens up the frequency extremes.
This change delivered a very solid improvement in sound quality - not unlike stepping up to an exotic and expensive power cable; but at a fraction of the cost.
The tops opened-up nicely and the bass sounded tighter and slightly more extended.
This change delivered good results, but was comparatively minor compared to the second change.
CHANGE 2:
I bypassed the coupling caps in C1, C2, C3 and C4 positions with high-quality CORNELL DUBILIER 940C series .01uF polypropylene caps.
I have successfully used these bypass caps before - in speaker crossover projects - but nothing could have prepared us for the improvement this second change delivered.
Unbelievably - after Change 2 - in the BEWITCH, we now have an amp that rivals any amp we own or have ever owned.
Having owned highly regarded amps - in some cases for many years - this is a a delightful upset.
For reference, our own power amp history includes: -
CARY 300SE Signature Monoblocks (300B SET, US)
THETA 6550 Monoblocks (Dynaco derivative, US)
LUXMAN AVANCE Z501 (Solid State, JAPAN)
FIRST WATT J2 (US)
TRANSCENDENT SOUND SE-OTL (Original, US)
HOLFI Power 150 (DENMARK)
LINX PULSAR (UK); and
LEBEN CS300F (JAPAN) - our current resident amp.
[And we have tried countless others along the way...]
In short, we can barely tear ourselves away from this BEWITCH amp. Yes. It's that good, after just a few minor changes.
I will continue to try new ideas and report back - but this current recipe is going to be very hard to beat.
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I also favor the 940C. Did you adjust the GNFB yet? Another tweak that increases the difficulty tearing your ears away when done right.
So I hear - although reducing NFB did not improve things much with the preamps I have modified in the past.
I've ordered a bunch of possible resistor values. Let's see...
It appears that after using a pot to vary the GNFB - and actually listening hard for the final best results - johnss settled on 330K GNFB position, for his BEWITCH.
I have ordered:
100K (in case I want to return to stock)
220K
270K
330K
They should arrive tomorrow.
I too believe in listening for final tuning. My wife's younger ears are an awesome tuning tool too!
I will probably jump straight 330K and give it a try.
I will report.
REPLACEMENT RESISTOR POWER HANDLING: Advice please?
As described below, I have had a range of new 2W resistors arrive, for the purposes of further tests on the GNFB position - which currently sits at 100K, ex factory.
Although the schematic calls for 2W resistors, I have noticed that the new 2W resistors that have arrived are considerably smaller than the resistors that came with the original factory built amp, in those positions.
Perhaps the factory had a bin full old 3W or 4W resistors they wanted to use up? Maybe they thought 3W / 4W resistors sounded better in those positions? Who knows?
QUESTION: Will a 2W resistor in the GNFB position on this circuit, provide adequate power handling?
As described below, I have had a range of new 2W resistors arrive, for the purposes of further tests on the GNFB position - which currently sits at 100K, ex factory.
Although the schematic calls for 2W resistors, I have noticed that the new 2W resistors that have arrived are considerably smaller than the resistors that came with the original factory built amp, in those positions.
Perhaps the factory had a bin full old 3W or 4W resistors they wanted to use up? Maybe they thought 3W / 4W resistors sounded better in those positions? Who knows?
QUESTION: Will a 2W resistor in the GNFB position on this circuit, provide adequate power handling?
TEST 1: 330K / 2W Tested in GNFB Position...
Well I threw caution to the wind and tried the 2W / 330K resistors in the GNFB position (R14). I figured... what's the worst that could happen?
After a few hours of warm-up, this new value changed the sound of BEWITCH dramatically.
PROS:
Amp still idles tomb-silent - even at high volumes. (Impressive!)
Spacious and open
Relaxed
Intimate
Palpable
CONS:
Ponderous
Lame and tuneless bass
Serious loss of dynamics
Total loss of boogie factor and swing
Total loss of listening enjoyment
So keeping to the original BEWITCH circuit, the 330K / 2W resistor in the GNFB position is not a keeper. Not terrible by any stretch, but definitely not for us.
Frankly I had to turn it off, it sounded so mediocre.
Tomorrow's test... 220K / 2W in the GNFB position.(I'll pass on the 270K...)
I will report.
Well I threw caution to the wind and tried the 2W / 330K resistors in the GNFB position (R14). I figured... what's the worst that could happen?
After a few hours of warm-up, this new value changed the sound of BEWITCH dramatically.
PROS:
Amp still idles tomb-silent - even at high volumes. (Impressive!)
Spacious and open
Relaxed
Intimate
Palpable
CONS:
Ponderous
Lame and tuneless bass
Serious loss of dynamics
Total loss of boogie factor and swing
Total loss of listening enjoyment
So keeping to the original BEWITCH circuit, the 330K / 2W resistor in the GNFB position is not a keeper. Not terrible by any stretch, but definitely not for us.
Frankly I had to turn it off, it sounded so mediocre.
Tomorrow's test... 220K / 2W in the GNFB position.(I'll pass on the 270K...)
I will report.
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TEST 2: 220K / 2W Tested in GNFB Position...
This is more like it!
With the 220K value, the sound of BEWITCH headed back towards the previous flavor of the original value (100K).
Still the bass was not quite as impressive or enjoyable as with the 100K, but it is possible that the midrange and tops were a tiny bit more open at 220K. So here, I am finding that subjective taste comes into play - and I can certainly see why some amp designs include a variable GNFB dial.
For someone who listened to mainly laid-back vocals, 220K might be the ticket. For us - with wider tastes that span from CLASSICAL, JAZZ and REGGAE to SOUNDTRACKS, ELECTRONICA and ROCK - the original 100K value provided a better balance.
One fly in the ointment: The original 100K GNFB resistor looks to be either METAL FILM or METAL OXIDE in construction. For these tests, I have been using CARBON FILM resistors. Probably an oversight, but this too has brought new learnings.
To our ears, these CARBON FLIM resistors - used in the GNFB position - tend to introduce a "warm smooth blanket" over everything. Nothing sounds quite as crisp and textured. This is an interesting takeway as this would be a fine fix for an overly "dry" sounding amp.
In any event, the BEWITCH was never "dry" sounding, so we will revert to either METAL FILM or METAL OXIDE resistors for the final choice.
Perhaps at that time, I will try non-magnetic WIREWOUND (MILLS / OHMITE GOLD?) versus METAL FILM v METAL OXIDE - in the final value.
I welcome any wider suggestions and experience others have with GNFB Resistors.
This is more like it!
With the 220K value, the sound of BEWITCH headed back towards the previous flavor of the original value (100K).
Still the bass was not quite as impressive or enjoyable as with the 100K, but it is possible that the midrange and tops were a tiny bit more open at 220K. So here, I am finding that subjective taste comes into play - and I can certainly see why some amp designs include a variable GNFB dial.
For someone who listened to mainly laid-back vocals, 220K might be the ticket. For us - with wider tastes that span from CLASSICAL, JAZZ and REGGAE to SOUNDTRACKS, ELECTRONICA and ROCK - the original 100K value provided a better balance.
One fly in the ointment: The original 100K GNFB resistor looks to be either METAL FILM or METAL OXIDE in construction. For these tests, I have been using CARBON FILM resistors. Probably an oversight, but this too has brought new learnings.
To our ears, these CARBON FLIM resistors - used in the GNFB position - tend to introduce a "warm smooth blanket" over everything. Nothing sounds quite as crisp and textured. This is an interesting takeway as this would be a fine fix for an overly "dry" sounding amp.
In any event, the BEWITCH was never "dry" sounding, so we will revert to either METAL FILM or METAL OXIDE resistors for the final choice.
Perhaps at that time, I will try non-magnetic WIREWOUND (MILLS / OHMITE GOLD?) versus METAL FILM v METAL OXIDE - in the final value.
I welcome any wider suggestions and experience others have with GNFB Resistors.
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Are you asking for math help?
Self-biased P-P 6550. 35-45 Watts dead clean, maybe 80 Watts gross distortion.
The NFB is taken from a "4 Ohm" tap.
The 100k (or larger) NFB resistor has a small resistor at the end. But basically it is in parallel with the 4 Ohm 80 Watt load, sees the same voltage, will dissipate 4/100,000 the power.
80W * (4/100,000) = 320/100,000 = 0.003,2 Watts.
A 2W part is 625 times bigger than truly needed. We usually go 2X or 10X more than "needed" to ensure low stress and long life. 600X is quite generous, but not a problem.
It may be a distributor special close-out, as you suggest. It could also be that some magic material is only available in larger sizes.
@PRR
Many thanks for this solid reassurance. The math is currently beyond me. I'll get there.
There is so much info on the web suggesting that "bigger resistors sound better, due to reduced noise". Here is a great example: -
Technical Q&A
So perhaps many of these claims are true? I am not in a position to discern the truth of it, but this logic might explain the widespread fitting of bigger resistors than are actually required.
Perhaps there's some truth to it?
As you say, maybe the factory just got a great bulk deal on a big supply of 3W resistors, so 3W resistors were used wherever the Ohm value fitted.
Many thanks for this solid reassurance. The math is currently beyond me. I'll get there.
There is so much info on the web suggesting that "bigger resistors sound better, due to reduced noise". Here is a great example: -
Technical Q&A
So perhaps many of these claims are true? I am not in a position to discern the truth of it, but this logic might explain the widespread fitting of bigger resistors than are actually required.
Perhaps there's some truth to it?
As you say, maybe the factory just got a great bulk deal on a big supply of 3W resistors, so 3W resistors were used wherever the Ohm value fitted.
TEST 3: 130K / 4W Tested in GNFB Position... (& TEST 4: METAL FILM is back!)
With a 327K/2W Carbon Film Resistor paralleled with a 218K/2W Carbon Film Resistor - delivering a combined 130K of resistance and 4W of power handling, this test went very well.
Everything started to fall back into place and sound very good. Dynamics were back in full force and the bass had regained control.
Nevertheless, for the last couple of test values, my wife has commented that she felt the sound was a bit imbalanced - with the MIDS-AND-ABOVE sounding very open, detailed and revealing, but the MIDS-AND-BELOW sounding a bit murky and muffled. So, still not quite right.
Oddly, I don't recall hearing this imbalance before I started messing with the GNFB values, so this is puzzling. Perhaps the imbalance was there all along? I'm unsure, but we are heading full circle, so we will find out.
Further, the annoying smoothness of the Carbon Film resistor prevailed, so later in the evening, I cobbled together what was (effectively) a 136K Metal Film Resistor; albeit with only 1/2W power handling ability.
After a warm-up period, we sat down for a critical listen, to discover that the annoying smoothness was now completely gone. This was progress. Metal Film Resistor easily pulls ahead of the cheaper Carbon Film Resistor, in this GNFB position.
But we could still hear the imbalance that my wife found so noticeable. Still the sound-balance strongly favored the MIDS-UPWARD in terms of acceptable brilliance and clarity - while the MIDS-AND-BELOW continue to sound unclear - even though the bass in all other respects is sounding first rate.
Tomorrow, we will be trying a 100K / 1W Metal Film Resistor in the GNFB position, so we will be back to where we started from, in the GNFB tests.
In the event that the imbalance in sound between MIDS UP and MIDS DOWN prevails, I will look into changing the original SOLEN MKP coupling caps - which I have already bypassed - for more of the wonderful CORNELL DUBILLIER 940C Series Polypropylene caps.
In any event, I will report.
With a 327K/2W Carbon Film Resistor paralleled with a 218K/2W Carbon Film Resistor - delivering a combined 130K of resistance and 4W of power handling, this test went very well.
Everything started to fall back into place and sound very good. Dynamics were back in full force and the bass had regained control.
Nevertheless, for the last couple of test values, my wife has commented that she felt the sound was a bit imbalanced - with the MIDS-AND-ABOVE sounding very open, detailed and revealing, but the MIDS-AND-BELOW sounding a bit murky and muffled. So, still not quite right.
Oddly, I don't recall hearing this imbalance before I started messing with the GNFB values, so this is puzzling. Perhaps the imbalance was there all along? I'm unsure, but we are heading full circle, so we will find out.
Further, the annoying smoothness of the Carbon Film resistor prevailed, so later in the evening, I cobbled together what was (effectively) a 136K Metal Film Resistor; albeit with only 1/2W power handling ability.
After a warm-up period, we sat down for a critical listen, to discover that the annoying smoothness was now completely gone. This was progress. Metal Film Resistor easily pulls ahead of the cheaper Carbon Film Resistor, in this GNFB position.
But we could still hear the imbalance that my wife found so noticeable. Still the sound-balance strongly favored the MIDS-UPWARD in terms of acceptable brilliance and clarity - while the MIDS-AND-BELOW continue to sound unclear - even though the bass in all other respects is sounding first rate.
Tomorrow, we will be trying a 100K / 1W Metal Film Resistor in the GNFB position, so we will be back to where we started from, in the GNFB tests.
In the event that the imbalance in sound between MIDS UP and MIDS DOWN prevails, I will look into changing the original SOLEN MKP coupling caps - which I have already bypassed - for more of the wonderful CORNELL DUBILLIER 940C Series Polypropylene caps.
In any event, I will report.
TEST 4: 100K / 1W Tested in GNFB Position... (METAL FILM is back!)
So we have now gone full circle and are back to the original design GNFB value of 100K.
Without chasing every dog and kicking every rabbit, we feel that this is the best compromise of all:
* Dynamics are back to full swing.
* All frequencies sound well-balanced.
* Enjoyment factor is back to very high.
I use the word compromise, because there are individual aspects of the sound that benefited from less GNFB. Less GNFB delivered: -
* Slightly more sparkling and open tops
* Slightly more spaciousness and dimensionality
In other words, more of what tubes are famous for.
But in terms of balance, the 100K GNFB position has delivered.
We are now back to a METAL FILM resistor in the GNFB position.
As always, I am keen to remove as much ferrous (magnetic) metal from the signal path as possible, so I will focus further tests on what type of resistor delivers best in this GNFB position. The current short list includes: -
* Wirewound (MILLS, MUNDORF M-RESIST SUPREME, etc.)
* Ceramic (OHMITE AUDIO GOLD)
* Tantalum Non-magnetics (AUDIO NOTE, SHINKOH, etc.)
I welcome any suggestions from those who have been on a similar journey, as it relates to finding resistors that define feedback levels.
I will report.
So we have now gone full circle and are back to the original design GNFB value of 100K.
Without chasing every dog and kicking every rabbit, we feel that this is the best compromise of all:
* Dynamics are back to full swing.
* All frequencies sound well-balanced.
* Enjoyment factor is back to very high.
I use the word compromise, because there are individual aspects of the sound that benefited from less GNFB. Less GNFB delivered: -
* Slightly more sparkling and open tops
* Slightly more spaciousness and dimensionality
In other words, more of what tubes are famous for.
But in terms of balance, the 100K GNFB position has delivered.
We are now back to a METAL FILM resistor in the GNFB position.
As always, I am keen to remove as much ferrous (magnetic) metal from the signal path as possible, so I will focus further tests on what type of resistor delivers best in this GNFB position. The current short list includes: -
* Wirewound (MILLS, MUNDORF M-RESIST SUPREME, etc.)
* Ceramic (OHMITE AUDIO GOLD)
* Tantalum Non-magnetics (AUDIO NOTE, SHINKOH, etc.)
I welcome any suggestions from those who have been on a similar journey, as it relates to finding resistors that define feedback levels.
I will report.
QUESTION: Shared Cathode Resistor (Safety) Change?
Despite some judicious parts replacement - better coupling caps, etc. - our BEWITCH 6550 integrated amp is now fully configured as per the original factory schematic. Sounding better than ever for it too!
We have no plans to roll-in KT88 tubes, as it will likely put the transformers under additional stress. The stock NOS Russian 6550C tubes sound great.
I wanted to address the potential risk around the low-ish resistance value of the Shared Cathode Resistors. It has been made clear earlier in this thread that each individual 6550 should have its own (approx. 500-Ohm) Cathode Resistor.
Opinions seem to vary about exactly how much is a safe minimum, but it is evident that (the current) 375-Ohms resistance shared between two 6550 tubes working in triode, may not be enough to be safe.
In the present Shared Cathode Resistor configuration (R13), each pair of output tubes shares a cathode resistor - two paralleled 750-Ohm / 5W sandcast resistors, effectively presenting a 375-ohm resistance to each pair of 6550 tubes - with a total of 10W power handling. Interestingly, the BEWITCH 6550 factory schematic specifies 250-Ohms and the actual circuit-board labelling asks for 390-Ohms, so it seems that even the factory design team has had concerns around the resistor values in use here.
Until I figure out a way to install an individual Cathode Resistor / Bypass Cap for each individual tube - which could be months or years away - it is in my thinking to immediately reduce the risk of failure by: -
Replacing existing two paralleled 750-Ohm / 5W sandcast resistors, with two paralleled 2.4K-Ohm / 12W wirewound resistors, effectively presenting a combined total of 1200-Ohms resistance to each pair of 6550 tubes - with power handling increased to 20W-24W.
What do others think of this workaround please? Will it reduce my risk of melt-down? I'm really hoping the increase in resistance value won't change the sound of the amp. I'm not too bothered if we lose a few watts in total amp output power - due to increased burn-off - as we use efficient speakers anyway.
One further question I have is: If I make this increased resistor change, will I also need to vary/increase the value of each of the two stock 220uF Shared Cathode Resistor Bypass Caps?
I'd really appreciate some expert thoughts here.
Despite some judicious parts replacement - better coupling caps, etc. - our BEWITCH 6550 integrated amp is now fully configured as per the original factory schematic. Sounding better than ever for it too!
We have no plans to roll-in KT88 tubes, as it will likely put the transformers under additional stress. The stock NOS Russian 6550C tubes sound great.
I wanted to address the potential risk around the low-ish resistance value of the Shared Cathode Resistors. It has been made clear earlier in this thread that each individual 6550 should have its own (approx. 500-Ohm) Cathode Resistor.
Opinions seem to vary about exactly how much is a safe minimum, but it is evident that (the current) 375-Ohms resistance shared between two 6550 tubes working in triode, may not be enough to be safe.
In the present Shared Cathode Resistor configuration (R13), each pair of output tubes shares a cathode resistor - two paralleled 750-Ohm / 5W sandcast resistors, effectively presenting a 375-ohm resistance to each pair of 6550 tubes - with a total of 10W power handling. Interestingly, the BEWITCH 6550 factory schematic specifies 250-Ohms and the actual circuit-board labelling asks for 390-Ohms, so it seems that even the factory design team has had concerns around the resistor values in use here.
Until I figure out a way to install an individual Cathode Resistor / Bypass Cap for each individual tube - which could be months or years away - it is in my thinking to immediately reduce the risk of failure by: -
Replacing existing two paralleled 750-Ohm / 5W sandcast resistors, with two paralleled 2.4K-Ohm / 12W wirewound resistors, effectively presenting a combined total of 1200-Ohms resistance to each pair of 6550 tubes - with power handling increased to 20W-24W.
What do others think of this workaround please? Will it reduce my risk of melt-down? I'm really hoping the increase in resistance value won't change the sound of the amp. I'm not too bothered if we lose a few watts in total amp output power - due to increased burn-off - as we use efficient speakers anyway.
One further question I have is: If I make this increased resistor change, will I also need to vary/increase the value of each of the two stock 220uF Shared Cathode Resistor Bypass Caps?
I'd really appreciate some expert thoughts here.
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