6L6 with NFB vs EL34 with NFB?

[iknowjohnny]

I have a scratch built somewhat JCM style amp that i recently found i prefer with 6L6. The OT is a 3.2k mag comp for marshall. The output from PI on back is pretty much like a 2204 extent i have a 100k put and 33k resistor in series to make the NFB variable.

That said, when i went to 6L6's i found that i soon noticed the NFB didn't work the same. Usually as i turn the NFB up it gets less open and crunchy and sorta compressed feeling. Turning it away from nfb it's brighter and harder sounding and more open. I think you all know the difference, and my description may be off because i have a hard time describing it. But in any case, do 6L6's not work the same with NFB somehow? I noted that now when i turn the variable NFB knob to get the most NFB, the volume lowers as usual, but if i compensate for that by turning the master up a bit so the loudness is the same as before i turned the knob, the tone is almost the same. In other words, it's almost as tho the variable NFB just drops the amp's volume but doesn't change the way the amp sounds or feels much. It does, but only maybe 10-15% as much as it did with EL34's. I don't think i made any other changes in the time i've been using 6L6 so it looks like somehow they don't work the same with NFB, but i can't imagine that. Any thoughts as to what may be going on here? By the way, the presence control work normally as it did before, and is of course less effective as i lessen NFB with the variable knob. So thats mostly mostly what attributes to the difference i hear when i say theres maybe 10-15% difference in tone.

[Cliff Schecht]

There are a couple of things you may have read here at TAG that IMO hold true. First off I think you're finding out that the circuit will have the biggest effect on the sound. If you build a Marshall close to the schematic, load up the preamps with good AX7's and swap around power tubes, the differences are pretty subtle (compared to circuital changes).

The second thing you're hearing is how changing the output tubes will change the overall gain of the PI-->PA-->NFB setup and therefore directly relates to how effective that control is. By switching output tubes, you've changed part of the equation and now much compensate for the gain change in the NFB loop.

[Reeltarded]

Yeah it's different. The impedance changed and the voltages changed and the operating curves all changed. You changed tube types. Lke changing output transformers. Everything moves.

133k on 4 ohm tap? If you are the 8 move it to the 4, at 16 move to the 8, at 4 change NFB resistance to about +1.5 times your existing total or move to the eight and use about .7 what you have now. Not all arithmatic just gross approximations for clues.

[iknowjohnny]

I did that. It's on the 16 but moving it (tried 8 and 4 for a short time) just lessens the amount of NFB and at full rotation it already goes pretty far. I suppose you're right tho, i should have left it on the 8 ohm tap but i didn't feel at the time the extra cut of less NFB was necassary. Maybe i DID have that little before with the knob maxed and just didn't realize it. I'll put it back on the 8 and give it more time. But still, it just seems odd that 133k down to 33k does so little in the way of tone. Yet it seems to lower volume as much as it did with the EL34's. Well, ill give it a go.

[gingertube]

El34's have a gm of 10.5 while 6L6 have a gm of 5.6 (book value).
Actually at typical guitar amp operating points it will be more like 7 and 3.5

Regardless, the power amp circuit will have twice the loop gain with EL34 that it has with 6L6. That would suggest that you want to drop the feedback level for 6L6 compared to what you run for EL34. That take s care of the gain side of the equation.

Now if we look at the output impedance, with lower loop gain the 6L6 will already present a higher output impedance than the EL34. If you then decrease the feedback to get back to the same effective gain you will increase the output impedance again. Higher output impedance means less speaker damping

Cheers,
Ian

[iknowjohnny]

El34's have a gm of 10.5 while 6L6 have a gm of 5.6 (book value).
Actually at typical guitar amp operating points it will be more like 7 and 3.5

Regardless, the power amp circuit will have twice the loop gain with EL34 that it has with 6L6. That would suggest that you want to drop the feedback level for 6L6 compared to what you run for EL34. That take s care of the gain side of the equation.

Now if we look at the output impedance, with lower loop gain the 6L6 will already present a higher output impedance than the EL34. If you then decrease the feedback to get back to the same effective gain you will increase the output impedance again. Higher output impedance means less speaker damping

Cheers,
Ian

I can't quite wrap my head around all that. I get some of it, but can you tell me what it all means as far as what i should to to approximate the way the NFB worked with the EL34's? You seem to be saying just decrease NFB like reeltarded said, but there are other things that made it seem like theres more to it when i read it all.

[Reeltarded]

Oh, Ms.! I speak jive!

Johnny, bigger resistor value is less NFB. Higher tap is more. Remove the presence for a minute. Listen, then try straight 100k/4R and then 27k/16R.

The 100/4 is almost as raw and loud as none.

Try a 15k+250kL pot on the 8 tap for range. 100+33k on the 16 is not much more or much less at either end of travel.

Too tired to fake math. Try it!

[gingertube]

.... Usually as i turn the NFB up it gets less open and crunchy and sorta compressed feeling. Turning it away from nfb it's brighter and harder sounding and more open.

Ths is describing the interaction between the amplifier output impedance and the speker.

More NFB means lower Zout and more speaker damping ("less open and crunchy and sorta compressed feeling").

Less NFB means higher Zout ("brighter and harder sounding and more open").

That is an absolute classic description of speaker damping by the amplifiers output impeedance.

With 6L6 you aleady had much higher Zout due to the lower loop gain. this is made worse by the fact that 3K2 Primary OT Impedance is way low for 6L6GC. It means that you are always at the low feedback end of the "EL34 control range".

What you can maybe try - Wire your speaker to the next lowest output tap, e.g. put an 8 Ohm Speaker on the 4 Ohm tap. That will increase the speaker damping by reflecting a higher impedance back to the primary.

Cheers,
Ian

[iknowjohnny]

I'll have to try it more tomorrow, but i gave it a quick try just now and the same thing seems to happen.....turn it from min NFB to max and adjust volume to compensate and tone is almost the same. On the OT thing, i've asked in the past about whether i'd be better off using a OT for 6L6 and was told that would only be about 4k and that it shouldn't matter much. Do you think that answer was not accurate or too subjective? I DO have a 4k 50w heyboer, but i've used it with el34's in this amp and the tone seemed rather thin. Maybe with the 6L6's that would change and it may be a better match? I can tack it in w/o installing it and give it a test. Thoughts? And thats much for your replies by the way, i DO appreciate it !

[Reeltarded]

4k would make a great OT primary for the EL34s as well.

[iknowjohnny]

I felt it was too thin with the EL34's. Maybe a 4k would be brighter and that would account for it? I'm using a mag comp 3.2k now and it sounds notably fuller than the heyboer. Never the less i will be giving it a go with the 6L6's. I just can't leave any stone unturned. I'm like that when it comes to amps and guitars. theres always that "what if it sounds better" and the feeling like i might be missing a bit better tone by not trying it. It's a curse really ! Amp sounds better than anything i've ever owned as is but i just can't stop looking for the pot of gold.