Will 2 6v6 or el34 work with a 65 twin reissue OT?

[Clinkous]

Hello all,

I want to build an amp with two 6v6 or el34's in a push pull configuration. Loosely based on a TW Express except I'm wanting to use the PT and OT from a recycled 65 Twin Reissue. I can't seem to get my head around figuring out the impedance matching deal. Sometimes it seems clear and then ...... I don't what I'm doing yet.

I'm not trying to clone a TW, just want to build a working amp that I can experiment with and use at some of my louder gigs.

Any advice or thoughts would be greatly welcomed.

Thank you,
Chris

[Reeltarded]

The PT will be ok but too much voltage for 6v6.. I can't remember what the voltage was last one I pulled apart though.

OT is kind of low for a pair of 34s by like 50% ish and just the single tap.

I don't think it's a good idea, but I am not a guy who spends lots of time shoe horning anything about the power and output. Those should be pretty close or pick another.

[Firestorm]

You'll have to use JJ 6V6S to handle the voltage if you do it. Run the OT into an 8 ohm load to get closer to the proper impedance.

[Clinkous]

Thanks,

I know I would have to bring the voltage down for 6v6's. So do you mean low primary impedance for the el34's because of the 4ohm tap being the only one? What do two el34's want to see versus two 6v6's as far as primary on an OT?

[martin manning]

What do two el34's want to see versus two 6v6's as far as primary on an OT?

Anode voltage is going to be high since the PT is designed for 4x6L6GC.

You have a nominal 2k:4R OT there, so you could have 4k on the primary with 8R on the secondary. For typical plate voltages two EL34'susually work into ~3k7, so probably close enough there if you connect an 8-ohm load.

6V6's like 6k6-8k, with anode voltage at ~400 or lower. If you connect a 16-ohm load and show them 8k, you'd be pushing the limits of plate dissipation at the plate voltage you are going to get with a Twin PT.

[JD0x0]

Have you considered 5881's? Kind of like a the child of a 6L6 and a 6V6 if they mated. They should handle more voltage than a 6V6 and they get more compression than a typical 6L6

[Clinkous]

Thanks Martin,

I was planning on dropping the anode voltage from what the twin PT has via large ample watt resistors.

If I can get a B+ of ~380 then I could show 6v6's an 8k primary by using 16R secondary and use JJ6v6's then it looks ok on paper, right?

Looking at the TW Express schematic on the Ron Worley build guide it looks like that PT started at 585V and was dropped considerably with two 100k 3W resistors.

Thanks for the help and advice.

Chris

[jazbo8]

Looking at the TW Express schematic on the Ron Worley build guide it looks like that PT started at 585V and was dropped considerably with two 100k 3W resistors.

No, the 100k's are bleeder resistor, they do not drop any voltage. The 585V is the secondary AC voltage, and 395Vdc (schematic) or 420Vdc (voltage chart) is the B+1 voltage.

[Stevem]

You also need to think about the heater voltage soring with only two 6v6s in the amp, I would stil leave the two 6l6s in there with no drive signal just to tame the heater voltage and not pop the filaments.
Two 34s pull more heater current than two 6l6s so you can just run it that way.

[Clinkous]

Ok, thanks for the info on the B+ and the heater voltages.

I didn't know you had a problem with heater voltage until you exceeded the current limit of the PT filament winding (adding more heater draw than designed for). I don't understand how the voltage can rise above the design of the transformer.

Care to explain?

Thanks
Chris

[martin manning]

If the heater winding is designed to hit 6.3VAC at 5A say, it will be a bit higher at lower current draw. You could measure it unloaded to see what you have.

A Hammond Twin Reverb PT is spec'd at 7.02VAC unloaded, which is just over the nominal +10% (6.93VAC) upper limit for filament voltage.

[Clinkous]

I see now, thanks everyone!