Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: toddm7 on April 01, 2016, 01:20:34 pm
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Title almost says it all. I bought the Twin Reverb in the 70s (was manufactured in 74), have black-faced it, and committed the abominable act of making it more Marshall-like with tone stack, speaker choice, etc. (I have Weber Silver Bell ceramics in it.)
Right now have jj 6L6s. I intend to pull the two outer 6L6s and disconnect one speaker for home use. I have done the EL34 mod to accept either 6L6 or EL34, but I haven't as yet tried EL34s. I know I'd have to re-bias for them, but I'm concerned about blowing out the PT with excessive current draw.
I've read about various amp techs who have done this EL34 mod for decades on Twins ... supposedly without any problems. (Unless it was planned obsolescence.)
I'd appreciate any helpful suggestions!
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4 EL34s want 6 amps filament current. 4 6L6s want 3.6 amps filament current, so you'll be gambling that the PT can do it without damage. I don't know. You can plug 4 EL34s in while in standby mode and measure the filament voltage ACROSS pins 2 and 7 of a big socket. If the voltage is no less than 6VAC I'd think you're safe. Let it idle like this for a couple hours and see if the PT gets too hot to touch.
You may also consider permanently running with just two EL34s if you're nervous about the extra filament current to run four. It won't be much quieter.
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Personally, I'd try Sluckey's suggestion but only with 2x EL34's first, and then move to 3x EL34's if the amp seems fine with 2.
I say this because I once tried to put EL34's in a 2x 6L6 Fender. The extra heater current made the stock heater wiring so hot it instantly burned the insulation of the heater wires on power-on (this was very early in my amp-tinkering days, and there likely was additional wiring faults).
... I intend to pull the two outer 6L6s and disconnect one speaker for home use. ...
... You may also consider permanently running with just two EL34s ... It won't be much quieter.
Agreed.
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Thank you for the helpful suggestions. I was just wondering if there was a bullet-proof way of having 2 EL34s with one 8 ohm speaker hooked up. I've read about getting a separate Xfrmr for just filament … which seems like a less expensive way than replacing the PT with a beefed-up version. But … if the beefed up version would affect the sound in a good way, I might bite the bullet. (Sorry, too many bullets.)
I know … some would say to just dump the amp and get a Marshall.