6GK6 power tubes

[POWDOG]

Hi all. I'm converting an old Thomas organ amp into an 18 watter. A pretty nice amp, full of the blue-molded signal caps like old Fender Blackface amps. It runs an EZ81, a couple 12AX7's and a pair of P-P 6GK6 power tubes. I see that these are similar to 6BQ5's, but have a different pinout and can take a little more plate voltage. Has anyone used these before in an 18 watt or other guitar amp application? Are they as musical and "gainy" as their 6BQ5 counterpart? And if used in an 18 watter, should any R/C values be changed or modified?

Thanks everyone. This is the greatest forum on the web.

[Plexi]

Yes, i have built amps using those tubes, and also posted about them a while back.
Seems not many picked up on it.. Its pretty much the same tubes,just different pinout. They are rated for 330v plate and screens,and i believe a watt or so more than the EL84. Plus the good thing was, you could buy NOS for about 6.00 a tube.

[kedsinger]

Wow! Thats funny Powdog, you won that lastnight right? I almost bid on it also....seems we have the same taste for conversion. I'll tell you what. Whenever I see you have already bid on something I'll try not to bid against you(unless I absolutly have to have it). My e-bay name is kedsinger67. I recently bought a couple of hammond ao-35s. I have around 100+ amps(I'm absolutly insane) Have you tried any bogens(chb-20)?

[Phil_S]

I built my TMB18W with 6GK6. They are very nearly identical to 6BQ5 except for pin out and price. Price is a big consideration and there seems to be old stock available. Further, that old stock is represented by well thought of brands like RCA and Sylvania. I've got a pair of black plate Sylvania.

Probably the biggest difference between 6GK6 and 6BQ5 is that the supressor grid (g3) is not connected internally to the cathode. Remember to make that link on the tube socket.

Frankly, I don't know why more homebrewers don't use this tube. It seems like a no-brainer to me. (Yes, I understand why they don't use it. It's a rhetorical statement. I'm not looking for trouble.)

[zaphod_phil]

Sshhhh guys, you'll bump up the price of these tubes.

With the suppressor grid not being connected to the cathode, I was just thinking maybe you could try the same trick some folks do with EL34s, of biasing the suppressor grid negative wrt the cathode. In a cathode-biased amp, you do this by simply connecting g3 to ground. In a fixed-biased amp, you would connect g3 to the bias supply, as described in http://www.tone-lizard.com/Marshall_Myths.htm Disclaimer: I've only tried this with EL34s and not 6GK6s so far, so don't blame me if your tubes catch fire or something

[kedsinger]

Sshhhh guys, you'll bump up the price of these tubes.

With the suppressor grid not being connected to the cathode, I was just thinking maybe you could try the same trick some folks do with EL34s, of biasing the suppressor grid negative wrt the cathode. In a cathode-biased amp, you do this by simply connecting g3 to ground. In a fixed-biased amp, you would connect g3 to the bias supply, as described in http://www.tone-lizard.com/Marshall_Myths.htm Disclaimer: I've only tried this with EL34s and not 6GK6s so far, so don't blame me if your tubes catch fire or something

Phil, am I correct in saying the suppressor grids job is to minimize spacecharge and keep electrons from deflecting back to the grid after smashing off of the plate?Making the tube more efficiant. What would making it more negative to the cathode do?

[Plexi]

my brother built a 2 channel 36 watt amp with channel switching,and the VVR for adjustable bias amps. It also uses 4 6GK6.

http://www.18watt.com/modules.php?name= ... c&start=60

[POWDOG]

My thanks to everyone regarding the tubes. It looks like this amp will make a killer 18 watter.

Kedsinger...I'll try not to bid against you either. I usually use these old amps when I build prototypes. It's alot cheaper than building with new parts, and the iron from the 50's and 60's is usually pretty nice stuff. I started building about 5 years ago, and I too have amps all over the place.

I haven't used a Bogen CHB20 before. I have used Challenger CH18 and CH30's, which look similar to the old Bogen metal case PA amps. They worked out nicely, and really turned into tone monsters when I replaced the output trannys.

My favorite 18watt amp conversion is a Hammond AO-43. Basically the same tube complement as a TMB, really nice iron, and the chassis has the same dimensions as aftermarket 18 watt chassis, so the aftermarket faceplates match right up. Nice and roomy. I've used them for head cabs and combos with very few hiccups.

Cheers all.

[zaphod_phil]

...am I correct in saying the suppressor grids job is to minimize spacecharge and keep electrons from deflecting back to the grid after smashing off of the plate?Making the tube more efficient. What would making it more negative to the cathode do?

I'm no authority in this, but IIRC the screen grid minimizes the space charge, while the suppressor grid helps prevent secondary electrons emitted from the anode being sucked to the screen grid. Supposedly biasing the suppressor grid more negatively makes the tube even more efficient, because it reduces the secondary emission effects even further, and helps increase the tube's life time. Tone lizards site also mentions having to rebias tubes after doing this mod, due to the improved electron flow.