6V6 ?

lovedat800 · Post by **lovedat800** » Sun 04/20/08 6:00 pm

Anyone know of a schematic for a amp with a quad of 6V6? And how would a 36 watt OT do for an amp like this?

Post by **zaphod_phil** » Mon 04/21/08 10:17 am

You found your killer tone with a pair of 6V6s in an Marshall 18W power stage. I'm not a 36W builder myself, but my recommendation would be to repeat that success, by putting a quad of 6V6s into one of the 36W power amp designs from this site. You'll get all those same juicy harmonics, but with more headroom. GDS/Heyboer has a good 36w transformer set, and there may be others as well.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 1:29 pm

Ok. I just thought there might be a version thats already been tested and somewhat popular. If i have to build it w/o a schematic i'm sure i'll run into a lot of issues. Someone posted one at ax84 tho so that may be a good start.

Post by **zaphod_phil** » Mon 04/21/08 1:36 pm

Here's your schematic - http://18watt.com/storage/36wTMB5.pdf Now just put in the 6V6s, with the same changes you did for your 18W. Easy!

JohnnyCrash · Post by **JohnnyCrash** » Mon 04/21/08 1:59 pm

Hey, I've been away from my cascaded 6V6 TMB, and after be-booming it (or so I thought), I have recently come to realize the same thing you have...

The 6V6 conversion (especially cascaded) still has a Bass/Low-Mid issue.

TONESTACK
I would suggest changing your existing 6V6 build's tonestack to a more BF Fender style... same for the 6V6 36w you're planning on. This may resolve many of these woof issues.

I already changed the 0.022uF tonestack couplers to 0.01 and the 47k slope resistor to a 33k, but I'm about to go further and simply go with the BF Twin setup 0.1uf, 0.047uF caps. I've already changed the treble cap to a 500pF and will likely keep that as well as the 33k resistor... then I'll see how that sounds and go from there.

I'll also go with a 250k Bass and Treble pot as well as the 10k Mid pot and report back my results for you (need to order parts hehe).

Now that my 6V6 18w has a full on, fixed bias BF Deluxe power section, I use it for GLORIOUS clean tones, but I have the feeling that the big warm mids that cause the overly fat/woof will thin out a bit with these tonestack changes... the tradeoff will be a better sound for my JCM800 front end when dirty, but perhaps loss of fat cleans.

PI:
Obviously you'll have tp mod the PI as you probably did for your 18w - Change the grid stoppers from the 8.2k to 1.5k, the tail resistor to 22k.

6V6 CATHODE BYPASS:
I'd also recommend changing each 6V6 pair's 100uF cathode bypass to a 50uF (the 18w's 500uF to 50uF or even 25uF as well).

I am having great resuslts with dropping the cathode bias entirely and going fixed bias. A cascaded JCM800 front sounds better with a big, clean, stiff power section... power amp distortion may increase mud and woof. I went with a simple Brown Deluxe non-adjustable fixed for now (didn't have any extra bias pots).

For this reason, if you plan on cascading again, I'd recommend avoiding the dual-gain Hi-Octane setup most folks simply convert to, go with a stock JCM800 scenario... also go with SS rectification and massive filter caps.

JohnnyCrash · Post by **JohnnyCrash** » Mon 04/21/08 2:27 pm

Oh yeah, and perhaps a negative feedback loop too. I used a fixed style instead of a Presence control.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 3:03 pm

Keep me updated ! As to mine, i'm not using the hi octane pre exactly. It's basically a TMB i cascaded then modded from there. it's pretty bright too, so 1.5 grid stoppers scare me. Might try em anyways tho because every amp i see with el84's have them, and the 6V6's seem to like the same things so who knows.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 4:49 pm

Johnny....check this out. Might be just what you're looking for...

http://www.nyquist-plot.com/displaygif. ... bias_trick

JohnnyCrash · Post by **JohnnyCrash** » Mon 04/21/08 5:04 pm

lovedat800 wrote:Johnny....check this out. Might be just what you're looking for...

http://www.nyquist-plot.com/displaygif. ... bias_trick

Mine is essentially already built, I'm only tweaking the tonestack now.

The 33k and the 500pF have made some noticeable differences, but the 10k mid and 250k bass (perhaps along with different caps hehe) is my next step.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 5:10 pm

I think that 10k mid might be better. I tacked one on for a minute and it seemed good. But the pot is funky and won't fit either. it's the only 10k i had or i would have put one in. 250 bass pot didn't do much for me, nor did any value of pot.

Post by **zaphod_phil** » Mon 04/21/08 5:33 pm

Personally, I'm into using 50k for the Mid pot in some of my amps.

Lower values than the stock 25k will produce more scooped tones, while larger values IMO give more control range.

JohnnyCrash wrote:I would suggest changing your existing 6V6 build's tonestack to a more BF Fender style... same for the 6V6 36w you're planning on. This may resolve many of these woof issues.

IIRC those Fender tone stacks are heavily scooped. I would have guessed that F*nder BF tone stack would possibly make the woofy bass issue worse. Please keep us posted. Also, lovedat800 was only complaining of some remaining brightness with his 18W, in the other thread, and not woofy bass.

JohnnyCrash wrote:Obviously you'll have tp mod the PI as you probably did for your 18w - Change the grid stoppers from the 8.2k to 1.5k, the tail resistor to 22k.

I don't see any need to put in 1.5k grid stoppers, unless you find the amp too dark, which isn't what lovedate800 is reporting with his 18W. They may even need to be increased to something like 22k or 47k.

lovedat800 wrote:....it's pretty bright too, so 1.5 grid stoppers scare me. Might try em anyways tho because every amp i see with el84's have them, and the 6V6's seem to like the same things so who knows.

My own recommendation would be not to use 1.5k grid stoppers. Try 22k or maybe even 47k, if you're finding your amp is too bright. Also Marshall 18W amps as we know use 8.2k grid stoppers on their EL84s, while the EL84 design guides say not to use anything less than 10k. Amps like AC15s and AC30s use the good ol' Cut control to compensate for the bright tone, due to using low value grid stopper resistors.

FWIW....

lovedat800 · Post by **lovedat800** » Mon 04/21/08 6:22 pm

Marshall 18W amps as we know use 8.2k grid stoppers on their EL84s, while the EL84 design guides say not to use anything less than 10k.

Actually what i did just recently was to look at all the schematics i could find of EL84 amps. And literally every one of them i saw had 1.5k's ! Kinda confusing since they're such a bright tube. Maybe they use some design in the OT's that darkens the tone a lot and then design the circuit bright or some such thing. Might be a way to fight the brightness.

Also, lovedat800 was only complaining of some remaining brightness with his 18W, in the other thread, and not woofy bass.

No, it's back. I have the flab again. Not sure what happened, but it seemed to sneak up on me as i was trying to remove the brightness. It never ends i guess.

JohnnyCrash · Post by **JohnnyCrash** » Mon 04/21/08 7:48 pm

lovedat800 wrote:
Also, lovedat800 was only complaining of some remaining brightness with his 18W, in the other thread, and not woofy bass.
No, it's back. I have the flab again. Not sure what happened, but it seemed to sneak up on me as i was trying to remove the brightness. It never ends i guess.

HAHAHA

Same thing happened to me... after you started talking about it, I dug up my 6V6 TMB out of the pile of amps and played around with it.

I fully blame you

I suspect the Bass issue is due to either the 6V6 (or the conversion of an EL84 design to 6V6's), and/or the cascading. I think the ENTIRE circuit must be revised for 6V6's... instead of trying to make a square peg fit in a round hole.

In either case as you know, the usual cathode stuff hasn't been helping (whereas it has been of great help in my EL84 based amps)... so I have made it my mission to redesign an 18w cascade TMB for us freaks here who like em like that.

As mentioned, the fixed bias reduces cathode biased power section warmth. Likewise the negative feedback has helped in a similar way... PA headroom, although it sounds counter-intuitive - but this is part of the JCM800's sound.

I am now going through the whole tonestack and revising some things. The Marshall plexi-style tonestack mods I made to it didn't help much, so now more "drastic measures" are required.

I actually suspect some of the bass woof is also in the midrange, that is why I'm aiming for a more "scooped" BF Fender type approach. A low resonance can come from a lot of places, but a harsh Treble can be pulled back a little easier...

Also, for what it's worth, I'm not just swapping pot values - I believe you MUST change the caps as well.

I'll get back to you guys once I've made the changes.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 8:10 pm

i think it's a matter of a cascaded pre just being a very tricky thing that as you eluded to has to be accounted for in the entire design. then again i can help but wonder why i can reduce gain, even bypassing a stage, and still have the same issues. It's very tricky. i also think you're correct in that the midrange is part of the woffy bass. the low mids, and they are hard to remove w/o removing the rest of the low end. try this tho....it's worked pretty good for me at times and even seems to make the tone more bouncy. Put a cap across the V2A cathode, the stage just before the follower. It's a 820 on most 18 schematics i think. Try anywhere from a .47 to a 10uf and watch what happens. It can remove the low mids or with higher values do other things that seem to help that i can't put my finger on. But generally it seems to help.

anyways, i'm glad someone else here is on the same mission. hopefully one of us will find the key.

JohnnyCrash · Post by **JohnnyCrash** » Mon 04/21/08 9:29 pm

Hmm, I think you're in the right direction there... one I haven't even considered yet.

The PI on a JCM800 and a BF Deluxe are a little different than the 18w TMB...

I was looking right at it while putting in a NF loop... I think you're onto something...

I think I'll be looking at the PI before I change any tonestack stuff.

lovedat800 · Post by **lovedat800** » Mon 04/21/08 9:33 pm

By the way. Messing with the fender TS with the duncan TS program, this stack seem to be hugely superior to the marshall stack at least in the way the program shows. Of course i have already found what you see isn't always what you get with that thing. But the fender stack really lets you not only flatten the bass w/o dumping it completely like the marshall, but as you increase treble the mid notch gradually moves lower towards more low mids, the very frequency i think we have to much of.

i'm on jury duty call all week, but if i don't get picked for tomorrow i'm going to try the fender TS and see what it does.

Post by **zaphod_phil** » Tue 04/22/08 10:07 am

Sounds like you may be onto something there, after all...

lovedat800 wrote:Actually what i did just recently was to look at all the schematics i could find of EL84 amps. And literally every one of them i saw had 1.5k's ! Kinda confusing since they're such a bright tube. Maybe they use some design in the OT's that darkens the tone a lot and then design the circuit bright or some such thing. Might be a way to fight the brightness.

Some people actually like bright amps. But obviously the Marshall and Watkins amps don't have 1.5k grid stoppers. AFAIK most of all the other EL84 designs are either Vox amps or derivatives of Vox amps. So you'll probably find a Cut control lurking around somewhere.

And I also think you're right that the OTs may be specially voiced, or just simply lo-fi enough to roll some highs off.

lovedat800 wrote:i think it's a matter of a cascaded pre just being a very tricky thing that as you eluded to has to be accounted for in the entire design.

I have a hunch you're right about that. 6V6s should, at least in theory, provide a more even and tighter bass response, just simply because they have a bigger damping factor, being true beam tetrodes. The extra gain of the boost stage seems to be changing the equation of how the whole amp sounds. But you're the guys who're actually working on this, and I'm learning along with you.

JohnnyCrash wrote:The PI on a JCM800 and a BF Deluxe are a little different than the 18w TMB...
I was looking right at it while putting in a NF loop... I think you're onto something...

Let's break this down. Here are the main differences in the two LTP PIs:
- Tail resistor value. This has to be higher in EL84 amps than with octals. With NFB, the Presence pot (if you have one) faoms part of the tail resistor. Despite what the theory says the actual value of tail resistor has little effect on the PI's balance.
- Bias resistor - 820 ohms or 470 ohms
- 100k/82k vs 100k/100k load resistors. The odd values are to help balance the PI. 100k/91k would have been even better, but Fender was using 20% tolerance resistors, which you couldn't get in 91k. Personally, I prefer the PI to be unbalanced to help sweeten the tone, and not sound too hi-fi. But you guys may like to do things differently.
- NFB on the "B" input. This is the primary point NFB gets injected.
- NFB injected into the PI tail. This is optional for NFB, and purely to compensate for PI imbalance. You can completely disconnect the NFB from the PI tail if you want, and just use a fixed 15k tail resistor.
- Presence Pot and cap in the PI tail. This is also optional, since it's just filtering off some high frequencies, so they don't get attenuated by NFB. If you don't want the top end brightness, you could leave out this part. The Presence circuit also doesn't need to connect to the PI tail. It will work fine without that, and you would then just use a fixed 15k tail resistor.

Hopefully, this will help you guys work out which changes are the really important ones you need to get the sounds you're looking for.

mickc · Post by **mickc** » Tue 04/22/08 10:27 am

Hi all, first post, sorry if this ramble's a bit, I've been following all the threads about boomy bass, just of late it seems to be a popular topic. I have an 18watt JCM800-ishy thing with an Extra valve in it for more gain (switchable), I had nothing to do with the building of this amp nor do I have a schematic for it. Anyhow I seem to have the bass boom problem with it as well, I did try the tone stack fix but after a while of not using the amp I fired it up and realised that it still had way too much bass.
Now the other thing I've been thinking about lately was 6V6's (funny how these two topics seem to be colliding a lot recently), I thought (when money allows) about putting together another amp with a pair of 6V6's using my GDS 18w OT and installing the VVR control. If this work's out well a friend of mine would like me to build a 36w with 6V6's for him.

So now you can see why I've been following a lot of the threads lately, anyway I've seen this question asked before (here and other forums) without an real answer, has anyone tried to build the Marshall 4001 pre ? it's just that looking at the schematics for the pre-amp and tone stack, they did things quite differently and I was just wondering if they too ran into similar problems ?

Mick

Sorry if this seems off topic but it just looks like I'm might be heading in the same direction as Lovedat800 and Johnnycrash

JohnnyCrash · Post by **JohnnyCrash** » Tue 04/22/08 10:50 am

Thanks for that tip!

Wow, looking at Marshall's 6V6 powered 4001 is a bit strange...

http://www.drtube.com/schematics/marshall/4001m88.gif

JohnnyCrash · Post by **JohnnyCrash** » Tue 04/22/08 10:58 am

What's up with that Bass control?

Hmm, the PI has a a 470ohm cathode (the 18w has a 820), the two 470k's (like the 18w), a 22k, and then a 4.7k?

Hmm...

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