7868 tubes - replace with 6L6's?

[PRNDL]

I have an a Precision amp that uses a pair of 7868 output tubes.
Unfortunately, these tubes are fairly expensive (at least $25 each).

The 7868 is a 9-pin pentode, 19W, 6.3V heater at 800 mA, 550V max plate.

It's possible to rewire the amp for 6L6 tubes, which would require putting in octal sockets.

I'm planning on using this as a guitar amp.

Is this a meaningful switch, or should I stay with the 7868's?
(i.e. do they sound much better?)

[7N7]

Hello.

If you don't mind top-caps have a look at 6BG6 - check the g2 voltage, as 7868 is rated at 440V whilst 6BG6GA is rated at 350; otherwise I think it should do quite nicely.

7N7

[Eli Duttman]

Stay with the 7868s. Electrically, the 7868 is identical to the Octal based 7591. Drive requirements are no different than that of EL84s and 6V6s. It's quite likely the small signal circuitry currently in place can't provide the drive voltage 6L6s require.

Jim McShane charges $46.50 for a well matched pair.

BTW, once you switch to current production 7868s, there is no going back to NOS. The Magnoval bases stretch the Novar sockets out.

[eyoung]

g'day mates...

6v6's sound wonderfull however EL34 are pin for pin and much more durable when it comes to plate current . a word of advice use a bias probe to set bias, these can be found on ebay, beware of imitations the original bias probe comes with a book that includes settings. to make a long story short all bias probe does is measures voltage drop across a 1 ohm resistor in which mv can be interpreted as mA. i got my bias probe from groove tube and it has saved me a ton of time and tears. groove tube also has a book by aspen pittman(guru owner @ groove tube) a great reference for tube guitar amps...


good luck,

Elwood

[TubeHead Johnny]

If you don't want to mod the amp, EH currently makes a 7868 that's not too pricey. If you rather stick to NOS tubes, the 7591's are a lot more easily available and almost identical electrically. The cheap way out would be replace the tubes with 6V6's (or 6L6's if they fit).

[Johan Potgieter]

Folks,

Depending on the amplifier voltages, I would venture that the 6V6 may be somewhat out of place as a replacement here. It needs about 40% higher drive signal while the Gm is only some 36% of that of the 7868. Also Pa = 14W compared to 7868 = 19W.
The 6L6 is OK as far as Pa is concerned, but also way lower in Gm and larger. Difficult then, but with circuit adjustment rather 6L6.

Do you have to replace that often, and if so why? Perhaps too much Pa? Then go for 6L6.

[PRNDL]

I decided to stick with the 7868's, even though they are quite expensive when compared with 6L6 or 6V6.

The main reason is the amp looks beautiful, and the change would mess up the wiring. Also, the 7868 has a unique sound, which is desirable.

I have a potential buyer that wants a portable tube jazz amp with plenty of headroom.

I've made a few mods so far, but would like to keep it in original condition.

The mic input connected through a .047 coupling cap with a resistor to ground. I replaced this with a 1M resistor to ground and 67K grid stopper, which is commonly used for guitar.

The first stage coupling cap was a .01 ceramic disc, which I plan on replacing with a .018 Russian teflon cap.

The ceramic disc is an oddball since the rest of the caps are .047 orange drops.

I'm fairly certain after tracing the circuit that it has enough gain for guitar using the mic input. Mic 1 is without RIAA. Mic 2 switches one in for phono.

It's a Precision 535 made by Western Electric, in case anyone has a schematic.