Can the output make this much difference?

[daz]

I have two builds, one a former 18 watt turned into 2X6v6 we'll call A, the other a 2xel34 similar to a JCM we'll call B. I have redone the A's preamp so it's exactly the same at the same as B all the way thru the PI. Every imaginable difference has been tried to recreates A's drive tone in B. The reason is this. A's drive tone is very intense and rich and totally clear and in your face. B's drive ton by comparison is like the proverbial blanket has been put over the speaker. Maybe a better way to describe it is the difference between using a small NFB resistor compared to a large one, lets say a 33k compared to a 100k off the 8 ohm tap. A is like the 100k, B like the 33k.

The thing is, i have at one time or another tried different parts/circuit details in B that if responsible for the blanketed tone should have changed that to some degree.

Both have the same filter caps

tried 3 different OT's in B

same preamp tubes in both

tried 4 different 34's and 2 types of 6550's in B

layouts are very close

tried different PT just last nite with much more voltage (PT thats going into new build of B)

Tried a resistor in place of choke as in A

both have nfb and fixed bias

tried with same speaker, cable, guitar

made pre voltages the same

I may have missed some things, but the point is the only differences left are the different output, IE 6v6, and i don't know if it could matter but steel vs aluminum chassis. Any other differences have been change if only for a minute to test it and see if they were responsible. So far the tone remains the same on B and while some things have indeed changed it, nowhere near in a way that brings it close to A. the difference remains nite and day. vey very different amps.

What gets me is this....the nite and day difference is evident at any volume level, even 2AM bedroom levels. A has that super alive grindy organic unmistakable as TUBE sound at any volume. B by comparison has a much harder treble that doesn't have the aliveness and give in the drive tone. And as you know, the preamp which as i said is the same in both is a fairly high gain one, so all the drive tone comes from there at low levels.

I'm not clueless about how different a 34 is from a 6v6. Output sections are critical to great tone. But what i don't get here is how they can make for such a huge difference in the tone and feel of the preamp OD. I've even tried chopsticking B thinking maybe theres some sort of issue there. But a friend has the first example of this amp i built, and it's tone is basically the same but with wires run differently, so it can't be that.

So, i'd like to hear thoughts on this because i am starting a new build of B and if i could get a touch of A's alive drive tone in there i would be one extremely happy guy ! Maybe i'm missing something and someone here can enlighten me. I've found this forum seems to be home to some of the smartest amp guys on the internet when it comes to tube amps, and this is a tough one so have at it fellas.....

[Steve Conner]

6V6s have less than half the transconductance ("gain") of EL34s, so if you keep all other component values the same, switching from EL34s to 6V6s more than halves the amount of NFB. That gives you a resonance and presence boost that you can hear at any volume, not just cranked.

Maybe a better way to describe it is the difference between using a small NFB resistor compared to a large one, lets say a 33k compared to a 100k off the 8 ohm tap. A is like the 100k, B like the 33k.

Funnily enough Why don't you try making B's NFB resistor twice the value.

[daz]

I have. well, not twice the value, but higher and from the 16 tap too. it just makes it harder whereas the 6v6 amp you can add as much top as you want but it stays smooth and squishy w/o getting harder like the 34 amp. In other words, i've played with the nfb forever and it doesn't change the amp's basic character....that blanket over the speaker feel. I can drop the nfb resistor in the other amp and it doesn't lose that cut and aliveness at all. In fact, i just put a 47 the other day and it still has no blanket over the speaker. Not in the least. It's inherent in that amp's tone. But i will try a 220k and report back. Don't see it happening, but who knows.

[defaced]

What happens when you remove NFB from each amp as a test?

[Steve Conner]

Going to the 16 tap would cancel out the effect of using a higher resistor. So try a higher resistor but leave it on the 8 tap.

EL34s and 6V6s have different distortion "flavours", to be sure, but this shouldn't come into the equation. At 2AM volume neither type of tube is generating any distortion.

[jrfrond]

When it comes to power amp saturation, the power tubes make a HUGE difference. It's key to point out that, not only are there major differences between power-pentodes (e.g. EL34, EL84) and beam-tetrodes (e.g. 6V6, 6L6), but also subtle (sometimes NOT-so-subtle) microflavors between different manufacturers and/or vintages of the same tube type. Some of these are major, others are splitting hairs, but it IS a major contributing factor to the overdriven power amp sound.

[daz]

When it comes to power amp saturation, the power tubes make a HUGE difference. It's key to point out that, not only are there major differences between power-pentodes (e.g. EL34, EL84) and beam-tetrodes (e.g. 6V6, 6L6), but also subtle (sometimes NOT-so-subtle) microflavors between different manufacturers and/or vintages of the same tube type. Some of these are major, others are splitting hairs, but it IS a major contributing factor to the overdriven power amp sound.

Right, but if you re-read my post you'll see i said this happens at any level including 2AM bedroom levels. In fact, it has nothing to do at all with power amp distortion. The preamps are configured the same but sound totally different at levels you could talk over easily.

Steve...i tried all the way up to 440k and still the same thing. Yes, it of course becomes bighter and more in tour face, but nothing like amp A. It's the character of the brightness that makes the difference. You could add enough brightness to B to tear your head off but it will still sound like a blanket is over the speakers. I know that doesn't seem to make sense but i can't find the words to explain it any clearer. It's like the high end in B has not squish or sizzle in it, or at least thats part of it. It has a hardness to it so that if you increase the highs they get harsher than A, and no matter what you do it doesn't have that cutting yet pleasing articulation like A. Thats as good as i can do to describe it at the moment.

[rocket]

Have you tried A's preamp into B's powerstage?

[daz]

No, but i'ce tried it into a classic 30's, and thats a pretty dull sounding amp. I still got that intensity, just a little less cutting.

[daz]

I'm wondering whether lead dress can make this much difference. Does anyone know of a good site that addresses this issue in depth?

[loudthud]

You seem to have gone to great lengths to make the preamps identical. What I haven't read is anything about the biasing, tube types and output transformers. From my own experience I can say that 6CA7s can sound very thin if the bias is too cold. Also, the headroom of the PI (B+ and tail resistor) can play a role in the overdrive character of an amp. Have you tried any changes in this area?

[daz]

It's not the PI voltage, tail, or the PA bias. I've endlessly messed with all that and it yields light variations of the same character of tone. It's gotta just simply be the trannys and 6v6's. Either that or some sort of phase cancelation due to lead dress or layout or such. in fact, thats what it sounds like to me....it sounds like the original signal is being attenuated in the VERY top end....the chime and sizzle area where theres a lot of aliveness. I never notice that playing the amp till i play the other one and i hear that top end that the big amp doesn't have. It's alive and rich and intense, and thats in large part a product of top end. So i'm thinking thats getting canceled in the layout. lead dress, maybe, tho i've been chopsticking the he|| out of it trying to hear a difference. Could be wiring under the board or something that can't be chopsticked, but at this point i think it's either that or the PT/OT/6V6's that are everything. Then again, i've heard marshalls with a lot more sizzle, so that makes the former seem the likely reason.

[Steve Conner]

Have you tried swapping the power tubes between amps A and B? I'm sure 6V6s would take the higher voltage for long enough to do a test, as long as you don't crank the volume too high. If the "aliveness" doesn't follow the 6V6s, then you know the power tubes are a red herring.

Maybe 6V6s have a different plate impedance to EL34s even without any feedback, and this test would reveal it. All pentodes and beam tetrodes are supposed to have a plate impedance so high that you can neglect it and assume them to be current sources, but maybe that's not good enough for a really critical ear.

Or if you run your 6V6s at a high voltage, then you have to bias them at a low current so as not to overheat them. This would generate a lot of crossover distortion, and that's known to spice up the sound of high-gain master volume amps. Peavey intentionally biased the 5150 very cold, and in other amps they added a diode network to generate the same kind of distortion.

[daz]

I've run the 6v6's in my amp many times, and i'd run them on the 4 ohm tap with my 8 ohm speaker as i was told to do here. I won't be able to once i build the new one because it puts 475v at the plates. (tacked it in to see) But i did like the 6v6 in the el34 amp. But i don't recall noticing the kind of tone the small amp gets now. Of course i haven't run them in the big amp since i've had this latest preamp circuit. I suppose i should try.

[loudthud]

The output transformers are about the only thing left. There are people who claim their transformers will make a huge difference, what are you using?