Replacing 6L6 with 6V6S

[BackwardsBoB]

I took the 6L6s out of my beloved Carvin XT112, a delightful amp with a single pair of power tubes, class AB, LTPI. I believe the transformer is the same as for their 100W quad 6L6 model. Schematics are here:
http://www.carvinmuseum.com/pdf/amps..._schematic.pdf
I added a bias pot long ago.

I installed JJ's 6V6S tubes, in spite of a 475V B+, and biased each of them at 25ma of cathode current. The goal was to get a bit closer to the Fender clean of my Deluxe Reverb re-issue, though the 6L6s do a pretty good job of warming things up. I powered up with a Variac at 100V, and the tubes didn't complain. I've since removed the Variac, and the amp's been playing happily for weeks. Apparently the JJ's 6V6S's reputation for taking high plate and screen voltage is deserved.

I note that under maybe 1W, I get the creamy tone with strong, tight bass that I crave. Initially, I set the output impedance selector to 4 Ohms for my 8 Ohm speaker. This worked well, but as I turn up the volume past "bedroom jazz" to "solo jazz" and "jazz trio" levels, the bass gets loose and boomy, and going louder, I get output tube overdrive sounds, including ghost notes in the bass, much, much below the point where the 6L6s would do it (in fact, they wouldn't overdrive on the clean channel). Surprisingly (at least to me), turning up the output impedance switch to 8 Ohms to match the speakers increased the "clean" range of volume.

So... I've got a hot B+, an output transformer with an impedance that's too low to match the circuit, and an NFB loop set up for EL34s. I can drop the B+, and I can change the output transformer, but I have a feeling that the feedback loop needs a boost and tuning to counteract the crossover distortion I'm getting at higher volume once the circuit passes from class A to class AB. I think the effect of the output impedance switch argues in favor of this. Does that sound reasonable, or should I just go and buy a transformer and some zeners?

At low levels, with the 6V6Ss, the amp really does a good job of producing "jazz to fall asleep to" in a classic jazz sort of way, with wonderful harmonic enrichment in the not-too-highs.

[stokes]

Sounds to me like you want more headroom.If thats the case go back to the 6L6's.The 6V6's break up sooner than the 6L6's,just the nature of the beast.The circuit isnt "passing from class A to class AB",and you arent getting "crossover distortion".If using the 8ohm impedance switch increases the headroom or as you say the "clean range",then I would say that is a closer or "more proper" impedance for your set-up with the 6V6's and that OT.If as you say you are looking to get a cleaner more Fender tone,using 6V6's in an amp designed for 6L6's is not the way to go.You are trying to get orange juice out of apples.

[BackwardsBoB]

My goal is to eliminate my Fender DRRI by getting functionally equivalent tone from the XT112. The 6V6S change worked wonders for the tone. The 6L6s do indeed provide what seems like infinite headroom, but I'd be happier with an amp that does jazz at low volume and gets a bit raunchier when you turn it up for blues.

I hooked up a signal generator doing a sine wave at 120Hz, as well as higher frequencies, and monitored the speaker output with a scope. As you suggest, I don't see any evidence of crossover distortion, all the way up to power tube saturation, which looked pretty good too, with a clean, rounded top and bottom. Given what I saw with a square wave, It's pretty clear that the speaker isn't the best place to look, but there aren't alot of alternatives, and I didn't see or hear any problems with sine waves.

I actually have a good fix to increase headroom. Soldering in two more tube sockets and two screen resistors, and punching two holes in the chassis will let me fit 4x6V6S in the design, which should let me get a better match with the transformer.

[BackwardsBoB]

I've been digging further into my problem. I've found that even at low volume, low notes dominate chords. There's simply an imbalance in the frequency response, at least for guitar,with low frequencies dominant, at least with 6V6S tubes. The amp has a graphic EQ and active tone controls before the power amp that lets me vary the response greatly, and the amp simply sounds better at any volume with the low end cut somewhere between 12 and 18 dB.

To experiment, I changed the cap that couples the pre-amp output into the PI from 0.1uF to 4700pF, and things got much better, with less bass "bloom" at high volume, and the mud is gone at low output, but it sounds like two 4700pF caps in series to make 2350pF will be just about spot-on. I'll probably want to boost the bass a bit at low volume using the tone/EQ controls, and turn the bass down a bit at high levels.

I think I'm getting close to my personal Nirvana of an amp that gets great traditional jazz tone at low to moderate volume, and turns blues at higher volume. I still love 6V6s. Gotta add a second pair.

It sure is fun listening with your ears, coming up with a mod with your brain, making the change with your fingers, and testing with your ears.

I'd characterize the 6V6 performance in my amp, as compared to 6L6s, as "added warm harmonics in the high-mid/low-treble range and enhanced bass, even at low output, with a musically useful overdrive tone with earlier onset marred by bass flatulence at high levels." Anybody else get this impression running 6V6 fixed-bias push-pull?

[Steve Conner]

Hi BoB,

6V6 have less transconductance than 6L6 or EL34. So there will be less gain inside the NFB loop, hence less damping of the speaker resonance. This may explain the extra bass.

Also, because of the lower allowable plate dissipation and your high B+ voltage, you have to bias them colder than a larger tube, and that means more crossover distortion.

I think adding a second pair of tubes might actually be a pretty good fix for the above, if you can be bothered with the metalwork.

[txstrat]

Also, because of the lower allowable plate dissipation and your high B+ voltage, you have to bias them colder than a larger tube, and that means more crossover distortion.

I bought a matched pair of those 6V6s and aside from the 500v they can handle they came in boxes with IP:44 and GM:4.
As far as I know the tubes are tested at around 430v. From what I know these values would even be high for a 6L6. Any thoughts? The amp runs perfect, though.

[Steve Conner]

Well, for Ip, you can get any value you want depending on how the test rig biased the tube. As for gm, if we assume that "4" is in units of mA per volt, then that's a little lower than the 6L6, which has 5-6mA/V.

I wouldn't be surprised if these heavy duty 6V6s are made out of the same guts as the tube maker's 6L6 product, if they offer one. It could be a smaller plate to get the look of a 6V6, with the same cathode/grid/screen/beam plate assembly inside.

An EL34 has 11mA/V, which is one louder, innit.

Seriously, this is one of my pet hates. People say that EL34s have a different frequency response to 6L6s, when they really mean that the lower gm of 6L6s reduces speaker damping through the NFB loop, and that brings out the bass resonance and mitigates the HF rolloff from voice coil inductance.

It's not like 6L6s have filters inside that boost bass and treble, and EL34s have a built-in midrange booster like Clapton's Strat, but from the way people talk about it, that's what they seem to think.

[octal]

which transformer tap is the NFB taken from on this amp? If you're trying to get a grip on loose bass, take the NFB from whichever tap you're driving the speaker from, then adjust the value of your feedback resistor as needed.
Just a thought.

Nathan

[BackwardsBoB]

Thanks for your kind responses. I have it on good authority that adding a pair of 6V6s will dramatically improve the situation. This will effectively double the feedback, and when I add the parts, I'll see if doubling the feedback signal has the same effect on a pair of tubes.

[octal]

Quad 6v6 amps are extremely cool, but won't adding a second pair of 6V6s get your power output back up to just about where it was with the 6L6s? (should be about 40-45 watts for a quartet of 6V6s.) I wonder if you'll be able to get the gentle overdrive of the power tubes at the volume level you like.

re: NFB levels- comparing a duet of 6v6s to a quartet- the same voltage in at the grids will give the same swing at the plates. The only difference will be the ratio of output transformer...i.e. a duet of 6v6 need 6.6-8K load, a quartet will will need 3.3-4k, so the voltage reduction/turns ratio would be 31 for an 8K/8ohm trans and 22 for a 4k/8ohm transformer. Taking an example of 100 volts plate-to-plate will equal 3.2v at the speaker output with the 8k/8ohm trans tap versus 4.5 volts on the 4k:8ohm setup. So, yes, you will have more NFB, but if increasing NFB is your goal, you'd do far better to just reduce the value of the NFB dropping resistor coming from the speaker output.

This is just my 2 cents- I'm not saying that you won't get the sound you like with a quartet of 6V6s... I'd just go with an easily reversible resistor value change before punching holes in the chassis and spending more money on tubes. Good luck with your project and let us know how it turns out for you.

[Steve Conner]

As 2 cents worth go, that one seemed pretty valuable.

[BackwardsBoB]

I agree with what you said, but an XT112 is a good-sized combo. I'd like to get it back up to 40W. It'll still be good clean at bedroom volume, though I expect that some of the 6V6 charm will be reduced. Later on, I want to use a pair of 6V6s for tone, and use a pair of 6L6s as gm multipliers. That will get me into the 70W range, and make for a simple powerscaling platform and a "goto" amp.

On the other hand, I have quartets of the old X60 and X30 amps, and a pair of 6V6s will be appropriate for them, so I need to get a handle on taming just a pair. I think that increasing the NFB and perhaps cutting the bass at the PI signal input will get me where I want to go.

I've got the punch, sockets, screen resistors and the grid leak resistors, and I want to replace the existing sockets and make a few other changes anyway. The board layout already supports the additional tubes with the components depopulated.