New Build Mystery

[jazjamr]

I am doing startup testing on my new build. 6V6 Marshall JTM in a Hoffman Stout chassis. With No Power tubes voltages seem relatively normal all around. I set bias voltage at max. neg.-70 installed power tubes. Voltages now 439 B+ 1st node filter but 470 at pin 3 ? -70 at pin 5 with 50 milivolts at bias test point . I checked resistance across output transformer center tap to Brown, Blue 36 ohms on one side 39 on other. I am using a Classictone Vibrolux Power transformer 325-0-325, and a Classictone Trainwreck style Output Transformer with 6.6K Primary. I have repaired my own Fender amps for years and recently scratch built an AB763 Vibrolux 1X12 that works wonderfully. Question, how can pin 3 voltage be higher than B+? How am I getting -70 volts Bias when Bias tap is +52 volts? All Voltages are checked with meter- to ground. Bias has 1 ohm resistors from pin8 to ground. I like a good mystery and this certainly qualifies. Amp is built to Hoffman 6V6 plexi schematic with JTM shared V1 cathode 820 ohm- 25uf 25v cathode cap. .1 phase inverter caps. Thanks

[R.G.]

Could you supply a link to a schematic I don't have to mentally correct as I think about it?

If it's the schematic here: https://el34world.com/Hoffman/files/Hof ... 6V6-V2.pdf, check the voltages to ground across caps C15 and C16. Then check whether the grid voltages on the output tubes are the same as the C16 voltage.

As to why one pin 5 voltage is higher than the other, OT operation is that one tube pulling down from B+ toward ground forces the other tube's plate the same amount above B+. If the thing is oscillating, one side can measure higher than the other.

Put your oscilloscope on it and see if it's oscillating.

[jazjamr]

I've been talking to Slucky over at the Hoffman Forum. He suggested I remove the NFB wire. After doing so I've been able to get amp more stabilized. 430 Volts B+ 430 at pin three. Bias is around 40 millivolts idle rising to upper 60s when strumming chord. Now that I'm applying signal to Preamp I've discovered output is extremely low. Before removing NFB amp would idle for a couple minutes at proper voltage and Bias , but then voltage would start climbing to 470 V. range. I've installed a couple of used 6L6GC tubes for time being as I feared I would destroy the new tubes. Tomorrow I'm going to go through the circuit again to check wiring and component values. The most I've had to do before on a start up is reverse Blue and Brown plate feeds and adjust bias range resistor, so this is very interesting and a learning experience. Thanks

[pompeiisneaks]

Sluckey posts here too. If the NFB fixed it your output secondary may be out of phase and you may want to swap the leads on your power tubes, and restore the NFB and see.

~Phil

[jazjamr]

Haven't posted in a few days. I've been very busy. I had the extension speaker output jack wired incorrectly. Now I have volume where it should be. I have spaceship noises and motorboating as I turn up the volume. I'm going to swap O.T. leads today. See what happens. Do some more checking. Thanks to ALL

[jazjamr]

I got the amp up and running yesterday! Two issues caused all the instability and low volume. The extension speaker jack being wired improperly allowing signal to go to ground. I also forgot to install the chassis ground, which caused all the other issues. The aluminum chassis is not used for a ground plane. A board mounted ground buss with all grounds connected as close as possible to sockets, jacks, transformers etc. Isolated input and output jacks. I guess the chassis was acting as an antenna until I grounded it . Circuit is more a JTM45 than plexi. Bass is very prominent. I may reduce V1 cathode cap from 25uf to 1uf after trying it with some different speakers. Amp is pretty quiet on the bench. Planning the circuit to fit in the 12 x 6 1/2 inch chassis was very interesting, I guess I was so absorbed in planning that I overlooked 2 simple but essential details. Thanks to all I will try to post pictures and possibly a video if I can figure out how. Thanks

[jazjamr]

Amp chassis 1

Amp chassis 1.png

[jazjamr]

Amp chassis 2

Amp chassis 4.png

[FourT6and2]

Your photos are too large, can you shrink them more please