split load phase inverter

[txbluesboy]

Do you think I could drive a 4X6V6 output section with a split load inverter, or would I need an additional driver stage?

[PaisleyTube]

It's up to you!

Jim Kelly amps did use cathodyne PIs for four 6V6s as example.

[stoo]

Ya that's right. There's no gain in this type of PI so obviously you need some preamp stages ahead of it.
Stew

[txbluesboy]

Thanks, I will look at some Jim Kelly circuits.

[RB]

Split load inverters turn up in a lot of interesting places. The Kelly amps, as noted above and the Orange 120 MKII driving 4 EL34s is another example.

I have had the most split load experience driving 4 tubes with the Double Deluxe circuit. Victoria builds one and you can find a kit from Weber called the 5E3X2. This is a Tweed Deluxe circuit with 4x6V6s and a higher current rated PT and 4K OT. I have built a number of these both as heads and 2X12 combos. In the right hands these can be amazing amps but the circuit doesn’t provide for much clean head room so it can turn out to be a rather one dimensional amp.

The problem I ran into using the split load PI with theses amps was when I was asked to try and give them more clean headroom and gain. When pushing the gain up, really fast pick attack/playing would cause the PI to collapse and drop notes until it has a chance to recover. The PI just couldn’t deliver the current required to drive the 4 tubes (I think it would do 4 EL84s just fine).

I tried many work arounds, higher voltages on the PI, different PI plate and cathode resistor values, fixed bias on the 4 6V6s, different grid resistors on the 6V6s etc. I could control the blocking on all but the most extreme conditions but now the overall tone had changed and not to the liking of the user. The only satisfactory fix was replacing the PI with Long Tail PI. But then I lost the magic of the 5E3 circuit.

You may or may not experience these issues. I love Tweed Supers, Pros, and Bandmasters. They all have split load PIs. I love 4x6V6 power sections. What a marriage that would be. Have a go at it. For my playing level the blocking was never an issue.

If you are building an amp expecting a wide tonal pallet for various styles of music I would suggest using modern rather than vintage circuit design. If you are trying to capture a vintage vibe then use the period correct technology.

That brings me to asking you, Just why do you want to use the split load PI?

Regards
RB

[echuta13]

Maybe a 12DW7 could drive a quad better with the 12AU7 section of the tube running the PI duties?

[txbluesboy]

The reason for the inquiry is that I have a friend that loves his Fender Superchamp but wishes it had a little more balls. He also is a fan of 6V6's, so I thought I might build him a Superchamp with a 4X6V6 output section. I wanted to keep the rest of the circuit as original as possible. I was concerned however by the possibility of the condition you described RB. That why I thought it might require the drive section after the PI, maybe something like this:

[Andy Le Blanc]

Its a guitar amp, don't over do it with extra stages after the inverter.
The cleans will be very nice but the distortion on break up will be harsh.
You should be able to drive 4 6V6 with a split load just fine.
You can get more head room with more plate voltage, within the limits of the tube,
and you should be able to pick an operating point for the inverters drive stage
that should be adequate. You do need to watch out for and not exceed the
heater to cathode rating for the tube type you intend to ultimately use
which is easily addressed by applying a positive voltage (hum pot) to the
tube heaters.

A 12ax7 split load like you typically see will even drive KT88, KT90, and
6550 into distortion easily.

[txbluesboy]

Thanks Andy, I'm all for keeping it as simple as possible. In line with what echuta13 stated maybe I should go with this:
:

[stoo]

I have a 150 watt Univox running 4 6550s at 650V plates and 325v screens being driven by split load PI. Mind you it's 12au7..but still.
here's the 6l6 version
http://www.univox.org/pics/schematics/u1011.gif

[txbluesboy]

Interesting, So the 12au7 has more current capacity? Whats up with the 500uf cap across the plate resistor on the PI?
Terry

[Rich M]

Peavey Windsors use a cathodyne to drive four EL34's - you should be fine.

Important reading:

http://www.freewebs.com/valvewizard/cathodyne.html

[txbluesboy]

Thanks Rich M that was a very informative article by the wizard.

[Phil_S]

I think the 12AU7 is probably better for the concertina for its current handling capacity. In this type of PI, the plate voltage is not the same as the cathode voltage. In the application, there is actually a fairly big spread in voltage. It is the current (Amps) on the plate and cathode that is supposed to be the same.