adapter for 7868 tubes?

[gogofast]

i had no idea how expensive the 7868 tubes are! holy cow!

is there any tube out there that i can make adapter to run it in place of 7868's? i mean other than obvious 7591 or 6gm5 which are more expensive and doesn't make sense to do it.....

i have a few guitar amps that i run those tubes really hard and need replacements soon. thanks for any idea.

 

[Tom Bavis]

Yeah, even the Russian ones are expensive! And use the E-H ones and the sockets will be stretched so that nothing else will fit.

Have never tried it but 6GT5 is a sweep tube with exactly the same pinout. The screen voltage has to be lowered, so you need to add a screen dropping resistor and bypass cap (one of each, wired to all the screens, or one of each per tube). But they're not SUPER cheap... $14 each at findatube.com

 

[gogofast]

thanks for the suggestion.

so, for each screen (pin 7 for 6gt5?), i should add a bypass cap and and resistor after that?

what voltage should i shoot for? approximate values of the cap and resistor?

thanks!:thmbsp: looks like they can be bought for under $10 a piece. much better than $40 a piece.

 

[Tom Bavis]

Absolute max screen voltage is 220V. 250V plate and 150V screen, it will drawi 70 mA at -22V bias (about what the 7868 needs) so I would say screen voltage has to go lower yet. I would put in a VR tube socket and then could select from 75, 85, 105V. Or use a string of zener diodes to supply bias. Draws 2 mA each at 150V screen, but could peak at 25 mA or more. Say we could put 30 mA through a VR tube or zener. Assuming 300V drop in the resistor, that's 10K Ohms at 9W (I'd use a 25W resister). Resistor B+ to screens, Zener string or VR tube to ground. I think I have a reel of 15V zeners, a string could give you 60, 75. 90, 105V. And I have power resistors too - let me know if want to try this.

 

[tube-a-lou]

Hi,
There not as bad in price as the 7591 tubes, I think they go for higher. I got
some nice sets of 7868 tube off ebay that weren't that bad in price, you just have
to wait a little to see what pops up on there.

Tube

 

[gogofast]

Hi,
There not as bad in price as the 7591 tubes, I think they go for higher. I got
some nice sets of 7868 tube off ebay that weren't that bad in price, you just have
to wait a little to see what pops up on there.

Tube

i remember seeing some of 7868's pulled from tv's before, but i didn't think much about them until i started using guitar amps with them. maybe i'll snatch some used ones and take my chances.

Absolute max screen voltage is 220V. 250V plate and 150V screen, it will drawi 70 mA at -22V bias (about what the 7868 needs) so I would say screen voltage has to go lower yet. I would put in a VR tube socket and then could select from 75, 85, 105V. Or use a string of zener diodes to supply bias. Draws 2 mA each at 150V screen, but could peak at 25 mA or more. Say we could put 30 mA through a VR tube or zener. Assuming 300V drop in the resistor, that's 10K Ohms at 9W (I'd use a 25W resister). Resistor B+ to screens, Zener string or VR tube to ground. I think I have a reel of 15V zeners, a string could give you 60, 75. 90, 105V. And I have power resistors too - let me know if want to try this.

wow, you lost me in the middle somewhere. i've used zener diodes before and kinda see the logic behind your explanation....let me read what you said a few more times.

 

[Tom Bavis]

wow, you lost me in the middle somewhere. i've used zener diodes before and kinda see the logic behind your explanation....let me read what you said a few more times.

Since I can't find any curves for the 6GT5, I can't really pick a good operating point. But if you can adjust the screen voltage, I'm sure you can get close. I thought of the zeners since I have a few hundred... free is always good.

 

[TerrySmith]

If you have a voltage doubler type power supply you will have a proper screen voltage at the capacitor junction.

I have a Fisher 400 that I could try it with, but I hate to use it as a guinea pig as it's a really nice unit.