Phew... Turned out the HF rolloff was caused by a small capacitor in the wrong place, shunting the input to ground.
The whole setup is basically an unregulated 400V supply with an extra, mosfet buffered 0-30V output for the grids. I built this circuit some weeks ago for testing and matching my 808s and now I hacked it into a primitive class A2 output stage to test the OPT. The capacitor was there to remove some noise when measuring the DC conditions of the tubes and I forgot that it was there.
With a functional driver stage the transformer just makes it past 20kHz before it starts to roll off, indicating that they probably can be used for hifi if we don't expect hi-end performance.
Sorry for the confusion, but I', glad I caught my own mistake here before spreading any more misinformation re. the performance of these transformers.
Still, my test equipment is very old and the tone generator in particular is not very linear som please take my measurements with a grain of salt.
The whole setup is basically an unregulated 400V supply with an extra, mosfet buffered 0-30V output for the grids. I built this circuit some weeks ago for testing and matching my 808s and now I hacked it into a primitive class A2 output stage to test the OPT. The capacitor was there to remove some noise when measuring the DC conditions of the tubes and I forgot that it was there.
With a functional driver stage the transformer just makes it past 20kHz before it starts to roll off, indicating that they probably can be used for hifi if we don't expect hi-end performance.
Sorry for the confusion, but I', glad I caught my own mistake here before spreading any more misinformation re. the performance of these transformers.
Still, my test equipment is very old and the tone generator in particular is not very linear som please take my measurements with a grain of salt.
Yes, @Fuling - Thanks for these first investigations ! 
So with your equipment and 808 tube prototype, the Xfo being wired in 5K/8R, you reach "a 23Hz to a bit over 20kHz" bandwidth @-3dB.
I'll see what I have with the Tiny 10Y/VT25 tube...
About your measurements :
T
So with your equipment and 808 tube prototype, the Xfo being wired in 5K/8R, you reach "a 23Hz to a bit over 20kHz" bandwidth @-3dB.
I'll see what I have with the Tiny 10Y/VT25 tube...
About your measurements :
- What is the plate current with the 808 tube ?
- Did you measured the max. output power before clipping, or "undistorted" ?
T
5k/8R is correct, I used a 7,5R dummy load on the 8R tap and most of the measurements where taken with a 12Vpp output signal (2,4W). I ran 80-90mA through the transformer most of the time. An 808 with a 5k plate load should be able to deliver closer to 10W but the power supply I'm using for the tests is not quite up to the task.
I'll come back later tonight with some more exact numbers, I was very very tired last night
I'll come back later tonight with some more exact numbers, I was very very tired last night
Some new test results:
With the tube running at 331V 91mA I get about 6,6W (10V peak into 7,5R) before visible clipping. I'm aiming for something like 450V 100mA in the final version and 10-12W output power. As far as I know 808 should be operating with a dull red color on the plates and that happens somewhere north of 40W.
With 8V peak (4,2W) as a reference point, the transformer rolls off -1dB at 53Hz and 25kHz and -3dB at 24Hz and 33kHz. Not great, not terrible
My vintage tone generator is a bit shaky in the 4k-50k range but these numbers shouldn't be too far from the truth.
Worth mentioning is that the 808 is a high mu triode with pentode-like plate curves, a "real" triode with lower Rp would probably produce different test results from the transformer.
With the tube running at 331V 91mA I get about 6,6W (10V peak into 7,5R) before visible clipping. I'm aiming for something like 450V 100mA in the final version and 10-12W output power. As far as I know 808 should be operating with a dull red color on the plates and that happens somewhere north of 40W.
With 8V peak (4,2W) as a reference point, the transformer rolls off -1dB at 53Hz and 25kHz and -3dB at 24Hz and 33kHz. Not great, not terrible
My vintage tone generator is a bit shaky in the 4k-50k range but these numbers shouldn't be too far from the truth.
Worth mentioning is that the 808 is a high mu triode with pentode-like plate curves, a "real" triode with lower Rp would probably produce different test results from the transformer.
I see @Fuling.
The 808 tube draws a 90 to 100mA current as projected in your circuit. Needless to say that it impacts the resulting inductance of the output transformer, and hence the bass extension. As you wrote it : not great, but not terrible.
In comparison, on my U-KT120 which uses unkown Chino output transformer, with a 100-110mA plate current and 370-380V cathode-plate, the bandwidth is :
The Guy who sold me those transformer replaced it with basic Hammond 125 series...
T
The 808 tube draws a 90 to 100mA current as projected in your circuit. Needless to say that it impacts the resulting inductance of the output transformer, and hence the bass extension. As you wrote it : not great, but not terrible.
In comparison, on my U-KT120 which uses unkown Chino output transformer, with a 100-110mA plate current and 370-380V cathode-plate, the bandwidth is :
- @10.00VRMS output / 12.5WRMS 8R : 16Hz-22kHz @-1dB ; 10Hz-40kHz @-3dB
- @2.83VRMS output / 1.0WRMS 8R : 7Hz-23kHz @-1dB ; 4Hz-42kHz @-3dB
The Guy who sold me those transformer replaced it with basic Hammond 125 series...
T
Being a 60 Euro guitar amp OPT, they seem quite decent after all. Especially considering how good they look, which is a factor for me as I want to avoid modern-looking components next to my 808s
Due to the high Rp of these tubes, I must resort to some form of negative feedback anyway to get any kind of damping factor. A bit of local feedback around the output tubes should help a good bit with the LF extention.
It's difficult to guess how they will perform "hotwired" to 10k/8R in a zero NFB 10Y SE amp. Personally, I would take a closer look at something from Piemme for a nice looking budget option.
I must finish my hideously expensive and complex 6S4S PP amp before I do anything else but it would be interesting to hear these transformers in a standard, zero NFB SET before I put them in service with those weird 808s. 300B would be nice, and I just happened to order two pairs from TT at the same time as I ordered the transformers. The intention there was to build a 300B SET with a pair of leftover LL1688 transformers sometime in the future, it would be fun to compare the 60E TT transformers to the 400E Lundahls...
Due to the high Rp of these tubes, I must resort to some form of negative feedback anyway to get any kind of damping factor. A bit of local feedback around the output tubes should help a good bit with the LF extention.
It's difficult to guess how they will perform "hotwired" to 10k/8R in a zero NFB 10Y SE amp. Personally, I would take a closer look at something from Piemme for a nice looking budget option.
I must finish my hideously expensive and complex 6S4S PP amp before I do anything else but it would be interesting to hear these transformers in a standard, zero NFB SET before I put them in service with those weird 808s. 300B would be nice, and I just happened to order two pairs from TT at the same time as I ordered the transformers. The intention there was to build a 300B SET with a pair of leftover LL1688 transformers sometime in the future, it would be fun to compare the 60E TT transformers to the 400E Lundahls...
Being a 60 Euro guitar amp OPT, they seem quite decent after all. Especially considering how good they look, which is a factor for me as I want to avoid modern-looking components next to my 808s
Yes, sure they are !
Yes. It would be a good idea to experiment some FB, let's say the usual 6dB value, to see how it behaves. My U-KT120 have a 6dB FB level with trimmer to match gain between the two blocks.
Yes, true. Notably for the 10K reflected primary Z when connecting 8R on the 4R secondary tap. Soemtimes, this trick doesn't work as expected...
For the bass extension, I'm more confident : the low 20mA current and low power output of the 10Y will sure help in that direction. I'm more suspicious for the treble extension, with possible overshoots / wavy square signal tops, due to a possible "average" winding design. Maybe yes, the Piemme transformers would be better on that point ?
I'll test those TT transformers when I'll be delivered, for sure... And keep an eye on the Piemme model !
I made a test with a nice rectagular encapsulated transformer coming from a 50s SW civil aviation receiver, which has only a mere 7.5H inductance without any current at the primary, but offering a convenient 10K/8R ratio.
Used with the 10Y, at 40Hz/1W, the square waves are... Spikes ! Tested alone with no DC, on my signal generator, the 40Hz square waves becomes pretty decent with a 20% slope. With a 6dB FB, the bass extension is 35Hz.
These transformers were designed for speech, that's obvious... And too bad, since I have 2 or 3 of them !
T
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Yes, some kind of NFB will be necessary here. I can possibly get 4-5dB cathode feedback if I connect the cathode to the 16R tap instead of to ground, but this doesn't work well with all transformers and must be evaluated in a working circuit. Plate to grid feedback or output plate to input cathode feedback are also options, regular global feedback would be a last resort.
The claimed 160mA airgap in the TT transformer feels, to say the least, like a bit of an overkill for the 10Y, but practicals tests is probably the best way to know how it will work IRL.
Transformers are funny things after all... I've had my pair of LL1688s on the shelf for fifteen years or so without ever using them, while I gladly use any junk transformer that I possibly can find as an output transformer. The amp I currently use is based on a pair of surplus mains toroids that happened to work extremly well as PP OPTs and they even had extra windings for 15% CFB.
The claimed 160mA airgap in the TT transformer feels, to say the least, like a bit of an overkill for the 10Y, but practicals tests is probably the best way to know how it will work IRL.
Transformers are funny things after all... I've had my pair of LL1688s on the shelf for fifteen years or so without ever using them, while I gladly use any junk transformer that I possibly can find as an output transformer. The amp I currently use is based on a pair of surplus mains toroids that happened to work extremly well as PP OPTs and they even had extra windings for 15% CFB.
Yes, sort of. Seems to be some kind of balanced feedback there, one phase connects to the cathode of the output tube and the other, presumably, to the cathode of the driver through a resistive divider. An elegant circuit for sure!
Personally, I usually employ the full secondary winding for CFB whenever possible. Not every transformer likes this and with high transformer ratios and low mu triodes the amount of feedback becomes vanishingly small, like 1-2dB or so. Often it can still be a small step in the right direction re. DF, bandwidth and distortion at the expense of exactly zero extra components, but it must be evaluated from case to case.
With the high mu 808 and the 16R tap at the TT transformer, I should get something like 4-5dB of CFB which might be just enough if I'm lucky.
It seems like CFB from the 16R tap was good enough for Mr Shishido when he designed his 808 SET:
Personally, I usually employ the full secondary winding for CFB whenever possible. Not every transformer likes this and with high transformer ratios and low mu triodes the amount of feedback becomes vanishingly small, like 1-2dB or so. Often it can still be a small step in the right direction re. DF, bandwidth and distortion at the expense of exactly zero extra components, but it must be evaluated from case to case.
With the high mu 808 and the 16R tap at the TT transformer, I should get something like 4-5dB of CFB which might be just enough if I'm lucky.
It seems like CFB from the 16R tap was good enough for Mr Shishido when he designed his 808 SET:
Yes, the same principle. Sometimes a part of the primary winding is "cut off" and connected between cathode and ground, allowing much higher feedback ratios. An effective way to reduce distortion and output impedance, but the driver stage must work harder to deliver large voltage swings instead.
Hello tubelectron,
Great build, very nice amplifiers you have there!
I am looking forward to see your VT25/10Y amplifier.
I have an VT25A amplifier, using LL2735B-25MA. I get an -3db upper frequency of 36KHz, for 1W into 8ohm.
I strongly suggest you to use VT25A.
Best Regards,
George
Great build, very nice amplifiers you have there!
I am looking forward to see your VT25/10Y amplifier.
I have an VT25A amplifier, using LL2735B-25MA. I get an -3db upper frequency of 36KHz, for 1W into 8ohm.
I strongly suggest you to use VT25A.
Best Regards,
George
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Ah, yes
Ah yes - one little difference though, noticed afterwards : on my circuit I made the choice to have the cathode current which doesn't pass through the loudspeaker output windings, unlike on yours.
But it may be not that important : did you measured any mV DC offset on your speaker in your configuration ?
Thanks @filenet !
In fact, I have a little stock of RCA and Sylvania 10Y/VT25 for years, so it will be the occasion to use them satisfactorily.
Yes, they are different from the VT25A, which is closer the the VT52 tube.
On this thread, it was the occasion to share about output transformers with @Fuling, who gave me valuable informations on the subject.
Of course, when I'll start my compact Stereo Amplifier N°10Y project, I will open a thread, and you will be welcome, indeed !
T
tubelectron: Your circuit is definitely very clever, using one section of the winding for CFB and the rest for speakers and GNFB.
I've never had a real problem with DC offset due to running the cathode current through the secondary, but it could potentionally be a problem if you run a lot of current through a winding with an unusually high DCR and connect a small, sensitive fullrange speaker to it.
In the 808 circuit this could actually become a bit problematic with ~100mA plate current + 20mA or so grid current.
Edit: Had to check: The TT transformer measures ~0,5-06R from 0 to 16R and ~0,3R from 0 to 8R. 35-40mV DC across the speaker terminals should be OK for most speakers.
I've never had a real problem with DC offset due to running the cathode current through the secondary, but it could potentionally be a problem if you run a lot of current through a winding with an unusually high DCR and connect a small, sensitive fullrange speaker to it.
In the 808 circuit this could actually become a bit problematic with ~100mA plate current + 20mA or so grid current.
Edit: Had to check: The TT transformer measures ~0,5-06R from 0 to 16R and ~0,3R from 0 to 8R. 35-40mV DC across the speaker terminals should be OK for most speakers.
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Aha - of course, this is not dangerous in any way, but this may be enough to displace the resting position of the cone of the speaker though...
While writing this, I saw that I did not posted the schematic of the U-KT120... Here it is, with all numbers :
As you can see, on this one I did not used the cathode-speaker winding trick, the transformer proved good enough to avoid it. But there's a global 6dB trimmable FB loop though, plus a screen-to-plate bandwidth/overshoot correction (1n 500V).
T
Nice looking schematic, I bet that amp sounds good!
Yes, 40mV will displace the speaker cones a little bit but I don't think it will be a real problem for me, as my main speakers uses 18Sound 12" pro drivers as woofers. I would be much more bothered if I had Lowthers or similar. By the way, I think Nelson Pass mentioned that he is happy with up to 100mV DC on the output.
Yes, 40mV will displace the speaker cones a little bit but I don't think it will be a real problem for me, as my main speakers uses 18Sound 12" pro drivers as woofers. I would be much more bothered if I had Lowthers or similar. By the way, I think Nelson Pass mentioned that he is happy with up to 100mV DC on the output.
Another advantage of having the secondary in the cathode side is you increase the primary impedance more than you might expect.
I run an 845 amp with a 5K primary and use the 0-16 on the cathode side.
Primary impedance increases to more like 6K in that case. Increasing the impedance was mostly my reason for going that route.
It has been my daily driver for about 6 years.
I run an 845 amp with a 5K primary and use the 0-16 on the cathode side.
Primary impedance increases to more like 6K in that case. Increasing the impedance was mostly my reason for going that route.
It has been my daily driver for about 6 years.