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previous: A.Colman The reason for the conversion is av... -- 10/17/2000 10:46 PM

Re: 7868 to 6L6GC conversion

10/17/2000 11:21 PM
jon	Re: 7868 to 6L6GC conversion FWIW, I've done a few of these swaps. I'm not familiar with the exact Bogen you have...if you're new to this stuff, probably a good first mod to tackle. Things to consider: have you changed the preamp/ voicing? What voltages are on the plates of the 7868's? What do you want the amp to be when you're finished? 7868's are lower in power output than 6L6's, higher than 6V6's, given similiar circuit conditions. Think more clean headroom with the 6L6's, more "singing sustain" with the 6V6's. More of the "singing" with either, if set up cathode biased. Your output tranny will probably be happy with either the 6L's or 6V's. You might even be able to use either with the right choice of cathode resistor in a C-B setup. Hopefully, you'll have enough reserve 6.3 heater voltage to run the 6L6's as they want .9A, whereas the 7868's want .8A if I'm not mistaken. Shouldn't be a problem unless you've added more tubes eg to the preamp. Some of those Bogen power trannies, especially on the SE amps and on some of the Lear era had marginal trannies with voltage doubling schemes. In those things, you need to add an extra filament tranny just to add a 6.3V pilot lamp! What I do is decide what I want the amp to be first, then use the output tranny and shoehorn in a PT that fits my design, trying to work within the chassis constraints. If you just want to keep everything else stock, and change the tubes in a cathode biased scheme, try to keep between 30-35 mA for 6L6's or in the high 20's for 6V6's. The exact resistor value (10-20W) will probably range anywhere from 250 to 600 ohms, depending on plate voltages and other circuit conditions. Also, add big (5W preferred, you can get away with 2-3W) 470-1K screen resistors, Fender style. If there arent bleeder resistors in the power supply, they're nice to have.... You'll have to play with matching the right output impedance tap of the OT depending on your tubes/speakers....hope this helps. cya, Jon

10/17/2000 11:21 PM

jon

Re: 7868 to 6L6GC conversion

FWIW, I've done a few of these swaps. I'm not familiar with the exact Bogen you have...if you're new to this stuff, probably a good first mod to tackle.

Things to consider: have you changed the preamp/ voicing? What voltages are on the plates of the 7868's? What do you want the amp to be when you're finished?

7868's are lower in power output than 6L6's, higher than 6V6's, given similiar circuit conditions. Think more clean headroom with the 6L6's, more "singing sustain" with the 6V6's. More of the "singing" with either, if set up cathode biased.

Your output tranny will probably be happy with either the 6L's or 6V's. You might even be able to use either with the right choice of cathode resistor in a C-B setup. Hopefully, you'll have enough reserve 6.3 heater voltage to run the 6L6's as they want
.9A, whereas the 7868's want .8A if I'm not mistaken. Shouldn't be a problem unless you've added more tubes eg to the preamp. Some of those Bogen power trannies, especially on the SE amps
and on some of the Lear era had marginal trannies with voltage doubling schemes. In those things, you need to add an extra filament tranny just to add a 6.3V pilot lamp!

What I do is decide what I want the amp to be first, then use the output tranny and shoehorn in a PT that fits my design, trying to work within the chassis constraints. If you just want to keep everything else stock, and change the tubes in a cathode biased scheme, try to keep between 30-35 mA for 6L6's or in the high 20's for 6V6's. The exact resistor value (10-20W) will probably range anywhere from 250 to 600 ohms, depending on plate voltages and other circuit conditions.
Also, add big (5W preferred, you can get away with 2-3W) 470-1K screen resistors, Fender style. If there arent bleeder resistors in the power supply, they're nice to have.... You'll have to play with matching the right output impedance tap of the OT depending on your tubes/speakers....hope this helps.
cya,
Jon

Replies:

A.Colman Thanks to J.Burt,K.Bull,Jon & SG fo... -- 10/18/2000 1:04 AM