"Fade control" for quad power tubes

[tubenit]

GlennJeff posted this comment on the D'Mars ODS EL84 thread:

I noticed on one commercial amp site that has a quadEL84 design that they don't switch out 1 pair of valves but somehow FADE one pair out claiming to thereby combine "hard driven output tube sounds" with "clean output tube sound".

I personally have NO interest in ever doing that with an amp, but in the interest of creativity ............. I thought I'd post these responses for critique or ridicule.

This would only make sense to me to attempt IF someone was mixing different power tubes like EL84's and 6V6's. Then maybe it would be useful?   You could PPIMV or VVR both sets of power tubes.  Using two OT's would allow to have two different types of speakers such as maybe a Celestion for EL84's and a Jensen for 6V6's.

Sluckey, HotBluePlates ................ or anyone else that might actually know if this works .......... can you weigh in on the feasibility of this, please?

With respect, Tubenit

[sluckey]

I'd suggest using a separate PPIMV dual pot for each pair of tubes.

[tubenit]

I'd suggest using a separate PPIMV dual pot for each pair of tubes.

I think that is a good idea.

So, ............... I am presuming the way I drew that up will work using the .1 cap between the dual pot PPIMV's?  Correct?  I was concerned that the PPIMV's could be interactive and wanted to prevent that?

Thanks for the help and info!  With respect, Tubenit

[sluckey]

I am presuming the way I drew that up will work using the .1 cap between the dual pot PPIMV's?  Correct?  I was concerned that the PPIMV's could be interactive and wanted to prevent that?

You don't need the .1µF caps. You only have one PPIMV so there is nothing to be interactive. I know you're using a dual pot, but it functions as a single MV.

In the first circuit, The top PP pair will be on full blast all the time. The single MV only controls the lower PP pair. You can blend in the level of signal produced by the lower pair with the uncontrolled upper pair. Not what I would call fade.

My definition of fade may be different from yours. Think of a car stereo system with front speakers and rear speakers. You use the fader to vary from full signal up front with no signal in rear at one extreme of the pot. Crank the pot to the other extreme and you have full signal in the rear and no signal up front. Put the pot somewhere between extremes and you have some signal in front and rear. The pot controls how much signal goes to the front and/or rear.

In the car stereo example you use the volume control to set the overall music level and you use the fader to control the amount of music in the front and/or rear. I'm not gonna talk about left/right balance except to say balance works just like the fader, only left to right. You could get the same volume and fade control effect by using separate volume controls for front and rear.

In the second circuit, you still only have one MV. But this time it simultaneously controls both PP pairs. Turn the knob to zero and both speakers are silent. Turn the knob to 10 and both speakers are loud. You cannot 'fade' anything with this arrangement. All you have is a single MV controlling two power amps.

[tubenit]

they don't switch out 1 pair of valves but somehow FADE one pair out

I didn't do a very good job explaining "fade".  But you did a great job giving me a useful answer!   Thanks!

I was trying to key off of GlennJeff's quote where a master volume "fades" out one pair of power tubes (vs. switches off)

I am understanding the below schematic will work OK.

As always, I appreciate the help!  Thanks for weighing in.

With respect, Tubenit

[sluckey]

Yes. #3 will give you total control. The .1µF caps are still not needed.

[tubenit]

Thanks, my friend!   Appreciate the help!

Note to myself to delete the .1 caps.

With respect, Tubenit

[Glennjeff]

That will be very useful for "combining different output valve" experiments.

[jazbo8]

Did a bit of research on the "power tube blend control":

From ampgarage about the Egnater version: "Yes, the tube blend circuit is interesting. The PI has two sets of coupling caps to output to each pair of tubes (6V6 and EL84). The blend circuit is mounted on its own board and uses a 500kB quad ganged and tapped pot with some other resistors and caps working in conjunction. This is sandwiched between the PI output coupling caps and the grid leak resistor pairs for each set of power tubes. The EL84 grid leaks are 220k/220k and the 6V6s are 100k/100k and both are running in fixed bias. "

Directly from Mesa Boogie (patent holder): "Mixing the two different types of tubes (EL-34 and 6L6) is only possible in the Roadking and some Simul-Class amps – NOT the Dual or Triple Rectifier. That said, I doubt you would hear much of a difference with the mix of tubes if you could. The majority of the tone of the amp comes from the preamp and your settings. The tubes will only provide subtle differences, especially when blended like that. As romantic as it sounds to do tube blending and as much as people suggest that it’s the power tubes that make the sound of the amp, Mesa tends to find that it’s the overall design, quality of parts and the preamp which shapes the majority of what the amp sounds like. The power tubes do the job of making all that other stuff loud!" - This is interesting, even the patent holder does not think it makes that much of a difference!

Anyway, it appears that you need a special quad-ganged pot (as already suggested by sluckey), which might be hard to find and likely somewhat expensive... So I thought of another way to do it, here is the idea, use a dual-ganged pot wired in reverse and use them to control 2 VVRs, with each VVR setting the voltages applied to the output tube pairs. Worth a shot?

Jaz

[Glennjeff]

jazbo8

I think two output transformers would be needed to make the VVR's work. Could their secondaries be wired in series to power one speaker?


tubenit

That GJ-idea#3 does not need the 0.1uF for 2 pairs of EL84's, and looks very usable for experimentation.

One pair of EL84's with one pair of 6V6's may however need different coupling caps  so two caps directly to each PI leg.
I don't know, just responding to what I read from jazbo8's research.

[jazbo8]

I think two output transformers would be needed to make the VVR's work. Could their secondaries be wired in series to power one speaker?

Good catch, but if we just adjust the screen voltages with the VVRs, then only one OPT is needed.

Jaz

[sluckey]

I wasn't suggesting a quad ganged pot. I was suggesting two each dual ganged pots to be used as two separate PPIMVs, one for each PP pair.

[HotBluePlates]

Directly from Mesa Boogie (patent holder): "Mixing the two different types of tubes (EL-34 and 6L6) is only possible in the Roadking and some Simul-Class amps – NOT the Dual or Triple Rectifier. That said, I doubt you would hear much of a difference with the mix of tubes if you could. The majority of the tone of the amp comes from the preamp and your settings. The tubes will only provide subtle differences, especially when blended like that. ..." - This is interesting, even the patent holder does not think it makes that much of a difference!

I haven't personally tried it, but I think there's one possible significant exception: if you use 2 tube types of very different power output and/or bias levels, you have a better chance of hearing an interesting blend. More so if you crank the amp to the point the small tubes are distorting and the big tubes are clean.

For example, 6V6/6L6 or better yet EL84/6L6. In the latter case, EL84's bias up with much less voltage for a similar dissipation than 6L6's. For example, for hot class AB at some lowish plate voltage, the EL84 bias might be around 12-15vdc while the 6L6 bias might be more like 30-40vdc. If the same 14v peak drive signal is applied from the phase inverter to both output tube types, the EL84's will be distorting heavily while the 6L6's are amplifying cleanly. The implication is you might get a clean/distorted tone at once in one amp.

The downside of the above is there is a narrow volume range (without using VVR/powerscaling) where the interesting sound occurs.

[kagliostro]

The Rebel 20 used something like that

http://www.el34world.com/Forum/index.php?action=dlattach;topic=11436.0;attach=22915

Plexi50 has build an amp with 6v6 & el84 tubes

http://www.el34world.com/Forum/index.php?topic=11436.msg105922#msg105922

K

[Tone Junkie]

This has many interesting possability,s think of the differant combinations you could get with 2 output transformers . Even maybe going so far as too have 2 totally differant types of PI feeding them . Opens up hug variables in tonal idea's . If 2 output transformers were used you would definatly have to make sure there was a big 8ohm resister when no speakers is plugged in on one side to save tranny but thats easily done.
Thanks Bill

[Tone Junkie]

Way to think outside the box Jeff this really got my brain turning . I guess now I will have to make phase inverter box to try (LOL) differant ones in a circuit, Crao this bo will to have 2 tubes.  :think1
Bill

[jazbo8]

I wasn't suggesting a quad ganged pot. I was suggesting two each dual ganged pots to be used as two separate PPIMVs, one for each PP pair.

Sorry about that, so two knobs instead of one to play with with the dual PPIMVs.

The Rebel 20 used something like that

Thanks for the Rebel schematic, K. BTW, does anyone have the Mesa schematic that shows the tube blending circuit? All the Mesa schematics I have seen only show the Simul-Class switching circuit. Also interesting to note that the Mesa Simul-Class amplifiers utilize OPT with multiple primary tabs to provide B+ to the different tube sets, which is another higher cost item.

TIA,
Jaz

[kagliostro]

Phase Inverter "madness" from Mr. Merlin


http://i81.photobucket.com/albums/j207/merlinblencowe/SwitchablePI-1.jpg




---

Mesa Simul-Class amplifiers utilize OPT with multiple primary tabs

The member of an italian forum, some time ago, told me he dismounted one of those OT and he say that the primary is a common CT winding, no additional intakes or multiple primary

K

[jazbo8]

The member of an italian forum, some time ago, told me he dismounted one of those OT and he say that the primary is a common CT winding, no additional intakes or multiple primary

May be he has seen another model, but on the Mark series and the Roadking II schematic, all of them have similar OPTs as shown below, there are taps to give lower plate voltage to one set of tubes. Then again, the schematics could be wrong just to mis-lead people.



Jaz

[HotBluePlates]

... on the Mark series and the Roadking II schematic, all of them have similar OPTs as shown below, there are taps to give lower plate voltage to one set of tubes. ...

There won't be significantly different voltage on either pair of tubes, because the resistance of the primary is low enough that voltage drop will be negligible (maybe 5-10v difference).

However, you could present different primary impedances to each pair of output tubes in this manner. Likely impedance difference would be 20-40%. I've never considered hooking up an ultralinear OT in this manner before, but that might be a possibility.

[jazbo8]

... on the Mark series and the Roadking II schematic, all of them have similar OPTs as shown below, there are taps to give lower plate voltage to one set of tubes. ...

There won't be significantly different voltage on either pair of tubes, because the resistance of the primary is low enough that voltage drop will be negligible (maybe 5-10v difference).

However, you could present different primary impedances to each pair of output tubes in this manner. Likely impedance difference would be 20-40%. I've never considered hooking up an ultralinear OT in this manner before, but that might be a possibility.

Very good point, although I wonder about this whole blending idea... What about the actual usage in the field? Perhaps in a studio, you can use the blend control to dial in a particular tone, but I can't imagine fooling around with it on stage, since the effect is said to be subtle (at least according to Mesa). It seems a multi-amp set up a la Eric Johnson would be more workable, to get specific tones from different amps, via volume pedals and switches vs. doing it all within a single amplifier... I guess I got to play one with a blend control to really appreciate it.

Jaz
 

[Glennjeff]

However, you could present different primary impedances to each pair of output tubes in this manner. Likely impedance difference would be 20-40%. I've never considered hooking up an ultralinear OT in this manner before, but that might be a possibility.

I'll let you know how that works out in a couple of weeks. If you want to watch the experiment progress see this pictorial / sound clip thread http://www.el34world.com/Forum/index.php?topic=15650.0 (Once I figure out how to post sound clips)