The Amp Garage

Thx guys! Colossal - the 6SL7s are old, one is likely 70s-80s production Sylvania w/red-orange lettering and flashing at the top. That’s the super microphonic one. The other is probably from the 50s-60s, it’s RCA I think, and shorter w/Raytheon logo and flashing around almost the entire envelope. I ...

Colossal - I think cathode bias would be my choice, but I like the fixed quite a bit as well. pdf64 - Thanks for the notation! Would the calculation for cathode bias output be considered inaccurate for audio output watts too? I'm going to mess around with different tubes and see if I can get the amp...

OK! Finally got a chance to test “at volume” https://youtu.be/a6GX9vdGXHY In the video above, I’m using a semi-matched pair of 6V6s. Measurements below are from a separate pair very tightly matched pair of 6V6s: Fixed bias Plate 337VDC Screen 330VDC tube 1 current 24.0 mA tube 2 current 23.9 mA Appr...

Ok! Got it wired up exactly as Sluckey’s layout with the illustration of the Carling SPDT switch. Bias pot is now working fine and plate voltage between cathode/fixed is not as dramatic - around 375 in cathode bias and 390 fixed. No chance for a sound test but these are encouraging results & numbers...

Sluckey! Thank you so much for the sheer poetry you have bestowed upon me (I'm talking about the layout with the Carling SPDT illustration). I have learned a lot from your posts on the Hoffman boards. I am honored! I was fortunate enough to catch a glimpse of what you wrote earlier this afternoon. I...

thx in advance to any suggestions!!

Happy Sunday everybody! I'm kind of cruising through a little mod to an homebrew experimental amp here and adding a cathode/fixed bias switch. I dunno if any of you guys are familiar with the Hoffman EL34 world boards but a big wig over there named "sluckey" who has mucho knowledge and experience ha...

Extremely delayed conclusion: it turned out to be a “bias feed” or “grid bias” or plain ‘ol “grid” resistor. I replaced both, and boom the amp sounds great.

Thanks for your help as always everyone!

Howdy y'all! Been a while since I've been here, what with the pandemic and all. Losing my job, finding a new one, etc it's been crazy as I'm sure many of you have experienced. Anyhow I've got a friend's modern Deluxe Reverb on the bench and it has an interesting issue. One of the power tubes is draw...

Fantastic, thanks guys!!

Nothing is out of the question provided it will work! Do you call it a scratchy pot because it will be scratchy sounding while being adjusted? (But hopefully quiet once set?)

Howdy y’all Pardon my ignorance if you will, but I’m wondering about installing a switch in a new build which would enable the cathode resistor of a preamp tube to be changed from say 1K to 11K without any loud popping, etc. I know cathode bypass caps can be put on a switch, but in this case it’s th...

Wow, ages later but I am now seeing I didn’t respond to the last post. FWIW the speaker jacks are Switchcraft Type 11 mono jacks. They are grounded at their mounting points on the chassis (not isolated from chasss) but I also run a ground wire from the sleeve (?) conductor to a separate ground bolt/...

Thanks guys - I figured it would be ok but wanted to see if there were some secret knowledge I wasn’t privy to. As you can see in the stats it’s quite large and weighs a substantial amount as well, at least as big as your average 50 watt OT.

Gonna go for it!

I recently acquired an old Thordarson OT, type T-6753 (see stats in link below) it’s a nice healthy size and looks like it’d be a great candidate for an EL34 or 6L6 pp amp (possibly w/upwards impedance mismatch to reflect 5.8K to the primary). The only strange thing in the description is that it is ...