7868

[gary sanders]

What would be a good OT for a quad of 7868 tubes? The datasheet says 6.6k for a pair but would anyone recommend anything besides 3.3k?

[martin manning]

What plate and screen voltages are you planning on using? Fixed or cathode bias?

[renshen1957]

What would be a good OT for a quad of 7868 tubes? The datasheet says 6.6k for a pair but would anyone recommend anything besides 3.3k?

Hi,

You could use a quad on 6.6k OT and have the same power as a pair with the tonal sound of the 3.3K OT. It depends on what you are building, as martin manning pointed out. Specification sheets are second guesses as to how the tube might be used in a circuit. The AC30/4 had 6K6 OT, the AC30/6 had 4k OT using EL84 tubes.

The 7868 is a tube more frequently associated with Hi Fi Amps like the Fisher lasts tube amps (a nine pin version of the 7591), ergo more linear less prone to break into distortion, although I recall it being used in by Ampeg for amps (not known as a company that built rock and roll amps until an outside group developed the SVT patterned after Traynor's Super Custom Special).

Best Regards,

Steve

[Andy Le Blanc]

I flopped out a pair in an ampeg... its a novar 9 pin... the EH tubes had a slightly larger wire diameter pin ... all the design centers are for 6.6k loading, it has a high transconductance about twice that of a 6l6

[renshen1957]

Hi,

Resistance load for a particular tube depends on voltage on the plates, how the tube is used SE, PP, class of operation, and bias (yes you can have fixed bias class A).

These are notes for KT-66 but it will illustrate my point:

http://www.shinjo.info/frank/sheets/086/k/KT66.pdf

Note that voltage changes R-L from 2k5 in one application to 8k in another as noted above.

In the operation notes from RCA

http://scottbecker.net/tube/sheets/049/7/7868.pdf

The 6k6 is listed consistently for fixed bias but changes to 10k in cathode bias in one AB1 application and 6k6 for both AB either fixed or cathode bias on another page.

But these are based on 2 tubes.

4 tubes (two in parallel) are in essence a pair of two composite tubes with different properties.

I have to back to work, so I must continue this at another time

Best Regards,

Steve

[Andy Le Blanc]

jeez its not that complicated... you should be able to run in the ball park of 2-4k for the quad, but you need to mind the current for the PT and the OPT.
in and about fender voltages the design center say a pair will have a max signal plate current around 145 ma., so a quartet will be around 290 ma. and at 450v thats around 130w dissipation so your gonna want to use 100w transformers at least.

[martin manning]

If I got to pick I would choose 2k4, and run them in fixed-bias at around 400-430V like a typical Fender. Nothing like that (OT) available off-the-shelf comes to mind, but a set of 100W Fender Twin transformers with a tweaked OT would be the way to go.

[renshen1957]

If I got to pick I would choose 2k4, and run them in fixed-bias at around 400-430V like a typical Fender. Nothing like that (OT) available off-the-shelf comes to mind, but a set of 100W Fender Twin transformers with a tweaked OT would be the way to go.

Hi,

If you wanted the maximum power out of the set you could use a Edcor 100W OT

http://www.edcorusa.com/p/455/cxpp100-ms-2_5k

You could use a lower wattage OT transformer and have narrower bandwidth.

For power transformer, the following has 400 ma, plus 6V3 is rated for 7.5 amps, enough for a sea of preamp tubes

http://www.edcorusa.com/p/622/xpwr029_120

That is if you want to use design center suggestions (most people want all a tube is capable of).

You could use a smaller transformer set such as Hammond 1650N OT and a PT such as Hammond 270HX you won't get more than 25watts of power. The impedance of the OT restricts current flow in the tube. Even with a lower impedance OT the limits of the PT will be insufficient to supply more power. The plate voltage will be 388V DC at idle but will with transformers losses be a little lower, and will sag down when under full load of the tubes.

Which gets back to my earlier post, using off the shelf transformers, use a 6k6 transformer such as Hammond's 1650G or a 1650P and a 272JX PT for lower power amp (about 30-32 Watts) or a Hammond OT 1650k (3K5) and 272JX (about 55-60 Watts). The later combination compared to the Edcor combination, the Edcor will be a little bit louder.

The management in any case will ask that you turn down the amp.

Best regards,

Steve

[martin manning]

That's a nice OT for a quad of 7868's, Steve. The PT has 350VAC, though, and that would put them right on the edge. I vote for the Edcore OT and a Hammond Fender Twin Reverb PT.

[VacuumVoodoo]

7686, 7591 being high Gm tubes (over 8.000) are sensitive to load impedance if you are looking for maximum output power matching. 6k6 cited in data sheets is the optimum I verified empirically again and again.
A 3k3 to not more than 3k5 for 2 pairs in push-pull or available output power will drop considerably.
The above holds true for NOS, I have not tested EHX 7686 yet.
BTW Edcor is good stuff, I'm glad to recommend them.

[VacuumVoodoo]

empty page...

[renshen1957]

7686, 7591 being high Gm tubes (over 8.000) are sensitive to load impedance if you are looking for maximum output power matching. 6k6 cited in data sheets is the optimum I verified empirically again and again.
A 3k3 to not more than 3k5 for 2 pairs in push-pull or available output power will drop considerably.
The above holds true for NOS, I have not tested EHX 7686 yet.
BTW Edcor is good stuff, I'm glad to recommend them.

Hi Aleksander,

Always value your insights here and back in the day over at powerscaling.com.

Best Regards,

Steve.

PS Running a Quad of the 7591 NOS in a "The Fisher" X-202-C. Was my main amp until my wife discovered she could see her "drama shows" on computer with the "Hi-Fi" for the sound system. She lets me listen to her amp when she is gardening.

[gary sanders]

Thanks for all the advice.I have the old Sylvania 7868s in an amp I converted to guitar and love them.Asked this because I want to do a total new build using them.Guess I will have to try the new EHX tubes cause the NOS 7868 is way too much for me.Its a really clear and clean tone and thats what I like.Sounds great with a tube screamer too