Tweedle Dee build

[andresound]

Ouch! That's hot! What is the measured voltage on pin 5?

I will install old tubes and recheck. Should I go back to the recommended 250 ohm cathode resistor? One thing I did notice was, the B+ would go to 380v once the rectifier starts to conduct (about right for a 315-0-315 PT after FWB). Then as the 6v6,s start to conduct, the B+ slowly starts to decrease. If I leave it long enough, the B+ drops as far as 300 ish. Something must be drawing too much? I have gone over the wiring 100 times . Slucky..., you used the same iron with a 6v6 plexi build? Could it be a faulty OT?

BTW, if I plug an LP in... it works and sounds great for a few minutes, then starts to run out of steam

[martin manning]

Leave the cathode resistor as it is; you don’t want it running any hotter! Power up with the output tubes removed and measure voltage on pin 5. Should be zero or very close for a cathode biased power stage.

[andresound]

Leave the cathode resistor as it is; you don’t want it running any hotter! Power up with the output tubes removed and measure voltage on pin 5. Should be zero or very close for a cathode biased power stage.

Will do. However, I have already reverted back to the 250 ohm cathode resistor. I did briefly check pin 5 and saw 25 ish volts (with all tubes in). Will report after a QUICK test. I will put in a 5Y3 (Mullard NOS) for the test.

[andresound]

Leave the cathode resistor as it is; you don’t want it running any hotter! Power up with the output tubes removed and measure voltage on pin 5. Should be zero or very close for a cathode biased power stage.

Will do. However, I have already reverted back to the 250 ohm cathode resistor. I did briefly check pin 5 and saw 25 ish volts (with all tubes in). Will report after a QUICK test. I will put in a 5Y3 (Mullard NOS) for the test.

With only the GZ34 loaded, I get 470v B+. The voltage on pin 5 starts out at just below 1v when I first put my probe on it, and then reduces rapidly to about 100mv, then fluctuates up and down between 50mv and 150mv

[martin manning]

Both pin 5’s? It sounds like you have a leaky coupling cap(s), but check resistance to ground from both pin 5’s.

[andresound]

Yip Martin, on both pin 5’s. One is slightly higher than the other.will check the caps for leakage. They were all NOS 6PS from a set meant for an express build. I think I got the whole set from one of our members here.

[martin manning]

Just to be sure the grid leak path is correct, measure resistance to ground from both pin 5’s. You should see 221.5k.

[andresound]

Thanks Martin, I think I can “see” where I *^%*@# up! I will check when I get home tonight and report back. Just looked at the schematic again. I bet you, I left out that 220k junction to ground un-grounded

[andresound]

Thank you Martin!!! The junction of the 2x 220k resistors was not properly grounded. I had not stripped the wire (under the board going to the earth bus) properly. Redone connection and amp sounds fantastic! Really chuffed . Thanks to all here on TAG for taking the time to reply.

Plate to cathode of 348 (will try different recto tubes)
Voltage drop across the 250 resistor is 21.1
Current per tube is 39.9ma
Dissipation is roughly 13.9w per tube

[martin manning]

Great, good to hear it’s solved! That’s pretty hot for plate dissipation, though. I’d cool it down to 100% (12W) or less for 6V6GT; JJ 6V6S can take 14W.

[andresound]

I am running NOS NEC 6v6GT’s and I haves another matched set available. I thought the consensus with this build was to run them on the edge of a cliff? . Some have posted 17-18w dissipation!! I could buy a set of JJ’s . If I “cool” it down..., must I do so with the cathode resistor value or just lower B+?

[mojotom]

I am running NOS NEC 6v6GT’s and I haves another matched set available. I thought the consensus with this build was to run them on the edge of a cliff? . Some have posted 17-18w dissipation!! I could buy a set of JJ’s . If I “cool” it down..., must I do so with the cathode resistor value or just lower B+?

A 5Y3 could help lowering plate voltages.

At 400V the NOS should hold well but will wear quicker, EH 6V6 will work too but yes the JJ will be fine for that amp and will last a while if you find a good pair (somme rattle due to the speaker/cab vibrations).

Always nice to have a new set of current production tubes at start-up in case something is wired wrong (better keep the NOS for later once everything is ok) and you will be able to know if your NOS pair is still powerful (lots of mine are weaker than new tubes but not all).

[andresound]

I am running NOS NEC 6v6GT’s and I haves another matched set available. I thought the consensus with this build was to run them on the edge of a cliff? . Some have posted 17-18w dissipation!! I could buy a set of JJ’s . If I “cool” it down..., must I do so with the cathode resistor value or just lower B+?

A 5Y3 could help lowering plate voltages.

At 400V the NOS should hold well but will wear quicker, EH 6V6 will work too but yes the JJ will be fine for that amp and will last a while if you find a good pair (somme rattle due to the speaker/cab vibrations).

Always nice to have a new set of current production tubes at start-up in case something is wired wrong (better keep the NOS for later once everything is ok) and you will be able to know if your NOS pair is still powerful (lots of mine are weaker than new tubes but not all).

Tried a NOS MULLARD 5Y3 and liked the “brown” sound. However there is too much sag for my liking. Also this tube rolling thing needs to be heard in a band situation. We wont be playing for the next 2 weeks.. so cant wait to try in a band situation. I dont use pedals, so have to rely on the right amount of crunch vs clean volume. With a 5Y3, I don’t believe the amp will have enough clean headroom?

[martin manning]

I was thinking try a larger cathode resistor. 400V on the plates is ok, and will maximize the headroom. For life it's the dissipation that matters.

[norburybrook]

I am running NOS NEC 6v6GT’s and I haves another matched set available. I thought the consensus with this build was to run them on the edge of a cliff? . Some have posted 17-18w dissipation!! I could buy a set of JJ’s . If I “cool” it down..., must I do so with the cathode resistor value or just lower B+?

A 5Y3 could help lowering plate voltages.

At 400V the NOS should hold well but will wear quicker, EH 6V6 will work too but yes the JJ will be fine for that amp and will last a while if you find a good pair (somme rattle due to the speaker/cab vibrations).

Always nice to have a new set of current production tubes at start-up in case something is wired wrong (better keep the NOS for later once everything is ok) and you will be able to know if your NOS pair is still powerful (lots of mine are weaker than new tubes but not all).

Tried a NOS MULLARD 5Y3 and liked the “brown” sound. However there is too much sag for my liking. Also this tube rolling thing needs to be heard in a band situation. We wont be playing for the next 2 weeks.. so cant wait to try in a band situation. I dont use pedals, so have to rely on the right amount of crunch vs clean volume. With a 5Y3, I don’t believe the amp will have enough clean headroom?

Put 6L6's in and do the NFB mod on a footswitch and you can switch from clean to OD with that and your gtr volume.


M