Hi All, I've started playing around with some first gen valvestate amps and am interested in any mods that people know of, or have tried.
The two I'm currently playing with are the 8080 and 8100. I've had some success with repairing and servicing valvestates, but am looking to try the tone and gain mods I have seen mentioned online.
The following post has some info:
http://www.diyaudio.com/forums/instruments-amps/183691-marshall-valvestate-8100-mods-upgrades.html
Does anyone have the info from the indyguitarist.com link for the gain mod, the domain has been taken over, and I haven't managed to find the info over there.
I'd also be interested in any mods to other model valvestates, as I have a couple of 8040's, and am thinking about grabbing an 8240 in a local second hand shop.
Schematic Link (preamp): diyAudio
cheers.
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LIST OF MODS & INFO
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Usual Caveat: This thread is for my own information only, and anyone else trying this stuff needs to ensure they are appropriately skilled. I cannot recommend or take responsibly for these mods on anyone else's equipment.
1/ The "Fenderish" Tone Mod as described above.
2/ The "Gain Mod" while I'm not sure of exact details, I have had some very helpful advice, and some great articles. Armed with this info I'll start out with the following:
...... 2.1/ Try a 220K Plate bias resistor on the first plate (also try a 470 if this gain isn't enough)
...... 2.2/ Try a bypass cap in parallel with the cathode resistor. (try 1uf, and .68uf (suggested as "lead sound" on another page), 25uf (Suggested as "rythym sound" on another page)
3/ Larger Heatsink for output stage durability
4/ PC style fan to move some air through the amp.
5/ Relocating the "hot resistor" near the preamp valve that I read about somewhere (can't recall where, and whether it was in regards to the 8080/8100 or the 8240)
6/ Possibly try a completley different discrete pre amp for the solid state section, maybe something from Elliot Sound Products or similar.
The two I'm currently playing with are the 8080 and 8100. I've had some success with repairing and servicing valvestates, but am looking to try the tone and gain mods I have seen mentioned online.
The following post has some info:
http://www.diyaudio.com/forums/instruments-amps/183691-marshall-valvestate-8100-mods-upgrades.html
Does anyone have the info from the indyguitarist.com link for the gain mod, the domain has been taken over, and I haven't managed to find the info over there.
I'd also be interested in any mods to other model valvestates, as I have a couple of 8040's, and am thinking about grabbing an 8240 in a local second hand shop.
Schematic Link (preamp): diyAudio
cheers.
=================
LIST OF MODS & INFO
=================
Usual Caveat: This thread is for my own information only, and anyone else trying this stuff needs to ensure they are appropriately skilled. I cannot recommend or take responsibly for these mods on anyone else's equipment.
1/ The "Fenderish" Tone Mod as described above.
2/ The "Gain Mod" while I'm not sure of exact details, I have had some very helpful advice, and some great articles. Armed with this info I'll start out with the following:
...... 2.1/ Try a 220K Plate bias resistor on the first plate (also try a 470 if this gain isn't enough)
...... 2.2/ Try a bypass cap in parallel with the cathode resistor. (try 1uf, and .68uf (suggested as "lead sound" on another page), 25uf (Suggested as "rythym sound" on another page)
3/ Larger Heatsink for output stage durability
4/ PC style fan to move some air through the amp.
5/ Relocating the "hot resistor" near the preamp valve that I read about somewhere (can't recall where, and whether it was in regards to the 8080/8100 or the 8240)
6/ Possibly try a completley different discrete pre amp for the solid state section, maybe something from Elliot Sound Products or similar.
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Without digging too much, I can suggest one thing to try. I looked quickly at the schematic (from your link) and see that the 12AX7 is used as a DC coupled cathode follower, this is the critical "tube overdrive" stage giving you the tube tone. I just built an amp and used a 220K plate resistor on the gain triode, and left the 100K cathode resistor on the follower. This increased the gain quite a bit. In my amp it sounds awesome. Just looked again at that schematic and you can also significantly increase the gain by adding a cathode bypass cap (something like 680nF, but you can play with this value) to the first triode. You may want to try the bypass cap first, then the 220K plate resistor if you want even more gain.
Thanks Doozerdave, In additon to your much appreciated post, I have also found some great webpages and articles talking about plate voltages in preamps, and modifying tube preamps in general.
I'm thinking, in regards to point 2 (Gain Mod), that I'll try the 220K resistor first, or possibly a pot in series with R109 to carefully play with the resistance dynamically, maybe 200K linear. Another option is to add 120K in series and have a bypass switch to allow both. Hopefully I'll get a chance to tinker this coming weekend.
I'm thinking, in regards to point 2 (Gain Mod), that I'll try the 220K resistor first, or possibly a pot in series with R109 to carefully play with the resistance dynamically, maybe 200K linear. Another option is to add 120K in series and have a bypass switch to allow both. Hopefully I'll get a chance to tinker this coming weekend.
From what I gather the cap selectively adds gain in the highs depending on value giving, it a little more brightness, I might add this with a switch as well to have a play and see if I like it. Evidently this adds gain and brightens up the sound a little due to the AC bypass of the resistor.
Yes, you can play with the value of the cathode bypass cap to change the frequency of the shelf filter. Check out this page, Adam's Amplifiers: Triode Calculator
It shows you how much of a difference a bypass cap makes (bypassed vs unbypassed in the results). If you fully bypass Rk with something like 10uF you'll boost all frequencies, but this could make the bass muddy. The bypass also keeps the DC bias more stable, since AC is being bypassed by the Ck.
It shows you how much of a difference a bypass cap makes (bypassed vs unbypassed in the results). If you fully bypass Rk with something like 10uF you'll boost all frequencies, but this could make the bass muddy. The bypass also keeps the DC bias more stable, since AC is being bypassed by the Ck.
One other thing, I wouldn't recommend putting a pot in the plate circuit. It's high voltage which is somewhat dangerous with pots, plus you'll get a lot of noise when turning the pot. You shouldn't need to adjust it like that anyway. Just try a few values. I'm guessing you'll find a value you like within 2 tries.
hi again, and thanks for that link to the triode calc, thats very nice!!
In regards to the pot, I was only suggesting it in a bench test type scenario to carefully arrive at a value that I like. I realise its running a significantly higher voltage. How much noise do you think it would generate? I could always turn it down a little I guess before adjusting. Mind you I might just try the 220k first and see how that sounds anyhow.
In regards to the pot, I was only suggesting it in a bench test type scenario to carefully arrive at a value that I like. I realise its running a significantly higher voltage. How much noise do you think it would generate? I could always turn it down a little I guess before adjusting. Mind you I might just try the 220k first and see how that sounds anyhow.
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The amount of noise would depend on the quality of the pot. Just go for 220K, and if that's not enough, add a Ck of maybe 1uF, and if that's not enough, try a 470K Ra 🙂
How much gain are you trying to get? How much does it have now?
How much gain are you trying to get? How much does it have now?
I'm not really after a specific amount of gain, more so the alleged improvement in "sound/tone" made by adding the "gain mod" I've seen spoken about but not actually detailed in numerous forum posts around the place. It may not turn out, but I love to tinker so I'm keen to try it, crank out some tunes, and have some fun! 8)
I plugged some figures into the triode calc you linked above, but to be honest I'm not entirely sure what to put into the Rl (following stage impedance) field. Based on the Schematic above, I used the following:
Rp: 100 (or should it be R109 + R110 + R111 = 105.7K ?)
Rg: 1M
Rk: 1K
Rl: 470K <-- default
This gives 54.08 bypassed, and 32.53 Unbypassed.
using Rp=220K, gives 68.16, and 46.70
However, as per the comments below the calc, I gather there are some factors not taken into consideration, like different B+ voltages, and that Rk doesn't seem to affect the gain, which I have read elsewhere.
The first triode of the valve appears to be a pretty classic signal amp, but I'm not entirely sure what is happening with the second triode, is it some kind of cascaded gain stage?
Thanks.
I plugged some figures into the triode calc you linked above, but to be honest I'm not entirely sure what to put into the Rl (following stage impedance) field. Based on the Schematic above, I used the following:
Rp: 100 (or should it be R109 + R110 + R111 = 105.7K ?)
Rg: 1M
Rk: 1K
Rl: 470K <-- default
This gives 54.08 bypassed, and 32.53 Unbypassed.
using Rp=220K, gives 68.16, and 46.70
However, as per the comments below the calc, I gather there are some factors not taken into consideration, like different B+ voltages, and that Rk doesn't seem to affect the gain, which I have read elsewhere.
The first triode of the valve appears to be a pretty classic signal amp, but I'm not entirely sure what is happening with the second triode, is it some kind of cascaded gain stage?
Thanks.
The DC coupled cathode follower, does neat things. Read The Valve Wizard
As for Rk not affecting gain, it only affects unbypassed gain. That calculator is good for giving you a relative idea of gain changes. Different B+ values will change the bias position for a given Rk which changes the gain due to the anode characteristics curves. Still, if the calc shows you'll get about 20 more gain, that's likely pretty accurate for any B+. Just experiment, and be careful of the high voltage. Always check it (and have the amp unplugged) before soldering.
As for Rk not affecting gain, it only affects unbypassed gain. That calculator is good for giving you a relative idea of gain changes. Different B+ values will change the bias position for a given Rk which changes the gain due to the anode characteristics curves. Still, if the calc shows you'll get about 20 more gain, that's likely pretty accurate for any B+. Just experiment, and be careful of the high voltage. Always check it (and have the amp unplugged) before soldering.
aaaaaah, right you are, I did the calcs again and paid more attention. Also, when considering the cap, it makes perfect sense.
Also, cheers for the link, I'll do some reading and hopefully absorb some nice info!
Also, cheers for the link, I'll do some reading and hopefully absorb some nice info!
I find his site very good, for the most part. He has the first chapter of his book (Triode gain stage) on there for free. I recommend you download that and read it, I think it's about 40 pages. All of what I said will make lots of sense after that. 🙂
Do you have any set up for recording? I would love to hear some before and after samples of your experimenting.
Do you have any set up for recording? I would love to hear some before and after samples of your experimenting.
Yes, I agree, it is an excellent site, I dug deeper after your last post and there is some great info there. I didn't find the download to the first chapter of his book though. However I found this link: Designing Common-Cathode Triode Amplifiers by Randall Aitken, referenced on the triode calc page though. Was that what you meant? Or have I missed an obvious download somewhere?
In regards to the before and after, I'm currently thinking that I'll make the Plate resistor, Bypass cap, and Tone stack mods switchable. I think I'll neatly mount a metal jiffy box under the amp chassis at the rear, and have a switches to individually switch the 3 bypass caps mentioned, some plate resistors (100k, 220k and maybe 470k), and figure out a way of switching the different tone stack in for the clean channel. Placing it in the rear would also help with avoiding switching while turned on. In regards to the tone stack, I was considering replacing some of the original pots with some dual gang pots, to allow for the reversal in direction associated with the tone mod. All this combined should in theory allow for some relatively simple back to back before and after recordings. (and testing to see what I perceive as the best combination of mods)
In regards to actually recording these tests, I don't have a specific setup at the moment, but I do have a 4-track recorder, and shure microphones are pretty affordable these days. I'm sure I could find some kind of DI circuit for the mic (any ideas?) or find something 2nd hand off ebay or the like, so I could definately look into that if the results are positive!
Also, I've just read the article you suggested on The Valve Wizard, Cathode follower page and noticed the bootstrapping mod at the end. This apparently pushes the valve near its max gain. If I want more after the 470K plate resistor (I'm doubting I will go past this, but..... you never know) this seems an option. Do you see any issues with this mod in the valvestate?
thanks again!
In regards to the before and after, I'm currently thinking that I'll make the Plate resistor, Bypass cap, and Tone stack mods switchable. I think I'll neatly mount a metal jiffy box under the amp chassis at the rear, and have a switches to individually switch the 3 bypass caps mentioned, some plate resistors (100k, 220k and maybe 470k), and figure out a way of switching the different tone stack in for the clean channel. Placing it in the rear would also help with avoiding switching while turned on. In regards to the tone stack, I was considering replacing some of the original pots with some dual gang pots, to allow for the reversal in direction associated with the tone mod. All this combined should in theory allow for some relatively simple back to back before and after recordings. (and testing to see what I perceive as the best combination of mods)
In regards to actually recording these tests, I don't have a specific setup at the moment, but I do have a 4-track recorder, and shure microphones are pretty affordable these days. I'm sure I could find some kind of DI circuit for the mic (any ideas?) or find something 2nd hand off ebay or the like, so I could definately look into that if the results are positive!
Also, I've just read the article you suggested on The Valve Wizard, Cathode follower page and noticed the bootstrapping mod at the end. This apparently pushes the valve near its max gain. If I want more after the 470K plate resistor (I'm doubting I will go past this, but..... you never know) this seems an option. Do you see any issues with this mod in the valvestate?
thanks again!
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Aikenamps.com has a lot of great info too. There also some great info here Amp Books for Professional Builders Check out this article The Soldano Super Lead Overdrive Channel - Analysis of the First Stage
Here's chapter 1 from the valve wizard:
http://valvewizard1.webs.com/Common_Gain_Stage.pdf
Shure has a XLR to USB module so you can record directly into your computer. It's on my wish list!
I tried the bootstrapping of the DC coupled cathode follower in my amp build, near the start. I should throw it back in there now that I'm done and see how it is. It was definitely wicked gain, but I've got wicked gain right now without it (as in, enough gain for shred metal, no problem).
As for the switching, for the plate I would suggest that you put the highest value of resistor fixed in the circuit, then use a switch to add more in parallel. So a 470K fixed, then parallel in another 470K to give you 235K, then a 220K to give you about 113K. This will avoid floating the plate during switching. Depending on what you plan to do in the cathode you could likely do something similar.
Here's chapter 1 from the valve wizard:
http://valvewizard1.webs.com/Common_Gain_Stage.pdf
Shure has a XLR to USB module so you can record directly into your computer. It's on my wish list!
I tried the bootstrapping of the DC coupled cathode follower in my amp build, near the start. I should throw it back in there now that I'm done and see how it is. It was definitely wicked gain, but I've got wicked gain right now without it (as in, enough gain for shred metal, no problem).
As for the switching, for the plate I would suggest that you put the highest value of resistor fixed in the circuit, then use a switch to add more in parallel. So a 470K fixed, then parallel in another 470K to give you 235K, then a 220K to give you about 113K. This will avoid floating the plate during switching. Depending on what you plan to do in the cathode you could likely do something similar.
Thanks for the links, I'll check them out tonight!
In regards to the switching, that is a great point you make. I had considered leaving the 100k as permanent and just adding the resistance in series. However as you mentioned there is that moment during switching where it would become open circuit and no doubt have a little bounce as well. I appreciate that tip. As for the Cathode resistor, I hadn't planned to do much there, but maybe while I'm doing the mod I might as well pick some values to tinker with there also. Maybe 820R, 1K, 1.5k, and 1.8k ish values.
In regards to the switching, that is a great point you make. I had considered leaving the 100k as permanent and just adding the resistance in series. However as you mentioned there is that moment during switching where it would become open circuit and no doubt have a little bounce as well. I appreciate that tip. As for the Cathode resistor, I hadn't planned to do much there, but maybe while I'm doing the mod I might as well pick some values to tinker with there also. Maybe 820R, 1K, 1.5k, and 1.8k ish values.
Yeah, changing Rk alone won't do a whole lot until you get to high values like 33k that will be bossing very close to cutoff. This decreases gain and gives smooth distortion with compression. I would suggest switching in some series RC's in parallel with Rk, for stepped shelf filtering. But really, I would just bypass Rk with 1uF and see how it sounds.
Hi Dave, I've been flat out and haven't had a chance sadly. Hoping I get a chance soon. Interestingly though I've bought an 8280 off eBay, which sounds like it has a dodgy jack, and a dead tube. It will be another nice little project. I think If it all works out I'll hang onto that one for awhile and move some of the smaller ones on.
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