I'll probably try it anyway, what should I expect? (OT impedence question)

[nandrewjackson]

I've got a 4-6L6 (4-EL34) P-P amp built and running fine. I'd really like to stick 4 6V6 tubes in there for funsies.


I'm waiting on parts to install a high/low power switch, using zeners on the power transformer center tap. There's currently 480VDC at the OT center tap, and down from there along the B+ rail.


I've got 20V zeners, I'm planning on using 6 to get roughly 100VDC lower. 


My OT is Weber 022889, 2K ohm primary, single 4 ohm output.  I switch back and forth between 6L6 and EL34 by changing the negative VDC bias, both types of tubes sound good to my ears with this OT.


Should I try to drop more than 100V for the 6V6's?
Is the negative VDC enough to prevent tube meltdown?
Should I raise speaker cab impedence to compensate for OT mismatch?


I have searched a bit here and Google but haven't found quite enough info so I'm putting this post up here, you guys aren't going to steer me wrong!

[sluckey]

I'm assuming you ain't gonna be swapping tubes between sets while on stage? If so...

I would install a VVR circuit to change B+. Scale the power amp only. Sounds like the bias voltage adjustment may already have enough range for 6V6s. If not, a slight mod to the circuit would widen up the range enough to cover all three tube types. I would use an 8Ω speaker load for a better match for a quad of 6V6s that would be happy with a 4K primary load.

[nandrewjackson]

Thanks Sluckey.


Definitely not changing types of tubes on the fly.


My main concerns are plate voltage, negative bias voltage, and OT impedence mismatch.


If this amp can't get all of that together, I'll look into getting a new build started, purpose built instead of trying to piggy-back ideas into one box.

[sluckey]

I believe the suggestions I made will do this... On the bench, not on the stage.

[jjasilli]

easiest:  just pull 2 power tubes (1 from ea pair).  I do this w/ my Ampeg V4. Same basic result.  No re-wiring needed. This will also double impedance, as you mentioned.

[sluckey]

easiest:  just pull 2 power tubes (1 from ea pair).  I do this w/ my Ampeg V4. Same basic result.  No re-wiring needed. This will also double impedance, as you mentioned.

It's not the same. He wants to use a quad of 6V6s in a quad 6L6 amp. 6V6s need a different bias. And they probably would like a lower B+ than 480V.

[jjasilli]

My main concerns are plate voltage, negative bias voltage, and OT impedence mismatch.
If you don't want VVR or to pull tubes, then then there's one more thing.  The iron will be way over-spec for the lower current draw of the 6V6's.  That means a stiff, well-regulated PS, uncommon to guitar amps. No sag & no OT saturation.  It's a subjective preference that you may like.  Just wanted to point it out.

There's currently 480VDC at the OT center tap. . . Should I try to drop more than 100V for the 6V6's?
420 plate volts for 6V6 seems high, but the Fender does it in the Deluxe Reverb.  There's also a brand of 6V6's that can supposedly take about 500V (JJ's ???).  Anyway you can shoot for 420V or less.

To guesstimate, I would measure your amp's unloaded plate voltage.  Check the Deluxe Reverb's specs for power tube current draw & multiply by 2.  Figure the voltage drop in your amp with 4X 6V6's.  The measured voltage drop w/ 6L6's may be helpful.  Or, maybe use the Duncan PS calculator.  Then you'll know how may additional volts to drop with zeners.  I think a 100V drop is the practical limit recommended for zeners.

Personally, I would build another amp for a quad of 6V6's, with the right iron.  I like to hotrod guitar amps and overwhelm the iron, but not vice-versa.

[92Volts]

Screen voltage is usually a bigger issue than plate, for reliability at high voltage. High plate voltage does mean lower bias which can impact sound.

It's also easier to drop screen voltage... say a quad of 6V6s draws 200ma (at full output). If you reduce plate voltage by 100v, your regulator is eating 20W. A string of multiple zeners or a large MOSFET may have that rating, but SS parts NEED great cooling for full dissipation... sometimes a pain in the ass and sometimes not achievable.

Screens draw roughly 1/7 the current for most tubes, less if you run them at low voltage. Much easier to regulate that.

Especially if you use a tough brand of 6V6 like JJ 6V6S (jjasilli is right, these are actually rated for 500v) or newer EHs, you can get away with lower screen voltage only.

This gives 2 bonuses:
1. Unlike plate regulation, you can build something that easily handles screen current for 6L6s too, so you can adjust power or tweak tone with those installed too.
2. Low screen voltage reduces the negative bias voltage you need, likely enough that you can avoid other modifications to reach the needed bias voltage.

EDIT: Also, a doubling of impedance is about right. This can be accomplished with a higher impedance cab or plugging the same one into a lower-impedance tap.

[Willabe]

If it were my amp, I would not use zeners to drop a 100vdc.

I would use a VVR/Power Scaling circuit, on the power tubes only, in a heart beat instead.

You can use a VVR/Power Scaling circuit for just the screens. It could be an option, but for that amp I'd do the plate and screens on the power tubes.

[jjasilli]

Summing this all up, what I'm getting is that VVR is the best way to implement the stated goal of a quad of 6V6's in this amp. Pulling 2 big bottle power tubes is the easiest way to implement a similar result.  Other methods lag far behind.

[nandrewjackson]

The votes are in : VVR wins 


Thanks everyone. It's not the wattage level I'm after, I have pulled two 6L's and 2 EL's, to see what it sounded like,  I think it sounds different with each combination. (4 or 2 6L6, 4 or 2 EL34).


My favorite combo so far is 4 EL34, and now I'm itching to hear 6V6 coming out the speaker cab.


I have worked on a VVR circuit before. It was a 20 Watt VHT head. The customer bought it used for a song, the power scaling was not working. Luckily the full schematic was available to us, it was a fairly simple VVR on the output tubes. New transistor replaced, worked fine. Good sounding amp.