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81

Tube Amp Building - Tweaks - Repairs / Re: I just had to mess with it...

« Last post by tubeswell on June 02, 2026, 11:59:53 pm »

V1:
pin 1=293V
pin 3=20.3
pin 6=292
pin 7=0
pin 8=19.9

V2:
pin 1-292
pin 3=0
pin 6=240
pin 7=23.4
pin 8=56.3

I can see V1 voltages are out of whack.You should be reading under 2V for pins 3 and 8.V2 it looks like one triode isn't conducting.

Use your ohm-meter to heck the shared V1 cathode resistor value on V1 = Should be about 820 ohms. If this is in the ballpark, then you may not have the V1 plate resistors hooked up properly. You should be seeing about 100V dropped across each plate resistor (between each plate and the B+ node for the pre-amp filter cap). If you have somehow connected the plates directly to the B+ rail, that could explain a 20V across the cathode resistor


On V2 Pin 3, check you have the requisite 1,500 ohm cathode resistor hooked up between the ground and Pin 3. If this is wired properly and you are still getting 0V on Pin 3, it could be a loose socket pin clamp for Pin 3 - or it could be you have a dud 12AX7 (which seems less likely).

82

Tube Amp Building - Tweaks - Repairs / Re: I just had to mess with it...

« Last post by AlNewman on June 02, 2026, 09:59:43 pm »

I can see V1 voltages are out of whack.
You should be reading under 2V for pins 3 and 8.

V2 it looks like one triode isn't conducting.

83

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by sluckey on June 02, 2026, 09:42:00 pm »

The trimmer has 3 legs. One outer leg and the center leg (wiper) are soldered to the back of the balance pot. The other outer leg has the 7.5K resistor soldered to it. The bias circuit is a critical circuit so if you do a sloppy solder job your output tubes will likely burn up. Take your time and make a good solder connection to the back of the balance pot. Use enough heat to make a good connection but not so much that you destroy the trimmer.

Either of these two bias circuits will work fine with any tubes you have.

There are many styles of trimmers. Post a pic of your balance pot and your trimmer and I'll see if I can make a better drawing.

84

Tube Amp Building - Tweaks - Repairs / I just had to mess with it...

« Last post by slim66 on June 02, 2026, 09:33:19 pm »

I'm a newbie, and I built a standard 5E3 circuit.  It sounded glorious through the P10R in my original 5E3.  But then I decided to switch out the wire to pin 2 of V2 with a shielded wire, just to try and make it a little quieter, and the amp has been dead ever since.  I looked under the board to make sure I didn't accidentally pull out a jumper.  All components measure correctly.  I put the original wire back to pin 2.  I am getting crazy voltages to both 12ax7s.

V1:
pin 1=293V
pin 3=20.3
pin 6=292
pin 7=0
pin 8=19.9

V2:
pin 1-292
pin 3=0
pin 6=240
pin 7=23.4
pin 8=56.3

I have no idea what to do, other than rip it up and start over.  I've changed tubes, I've re-flowed solder joints.  Not sure how I blew this thing up so badly.

Any insight would be appreciated.

Thanks,

Slim

85

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by rud85ny on June 02, 2026, 09:18:25 pm »

Thank you for the diagram @Sluckey! 
So the trimmer is just physically placed onto of the pot. Copy that sir, I will try my best to study the circuit, but will this work just fine if I have 2 NOS good tubes?

86

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by Mongrel714 on June 02, 2026, 06:17:42 pm »

That's got to be a pretty massive PT, and it will certainly have the current to run all the tubes you have planned. Since it used a 5U4, the 5V tap should be rated for at least 3A.

Looking back at your photos, it looks like the amp model is AO-27, not 37.

Naw, its AO-37, The chassis was so long I cut it in half! I got some goodys in, this was for JCM 800 build, but Im sure we can use some for the 6V6!

87

Tube Amp Building - Tweaks - Repairs / Re: Oscilloscope question

« Last post by astronomicum on June 02, 2026, 06:13:24 pm »

OK, well, I've never built an amp with those kind of voltages.

With these voltages and the James tone stack, reminds me of the Univox 1236. Univox had a number of models with similar voltages.

88

Tube Amp Building - Tweaks - Repairs / Re: Oscilloscope question

« Last post by Banjan73 on June 02, 2026, 03:11:41 pm »

OK, well, I've never built an amp with those kind of voltages.

Me neither😉

89

Tube Amp Building - Tweaks - Repairs / Re: Oscilloscope question

« Last post by acheld on June 02, 2026, 02:38:05 pm »

OK, well, I've never built an amp with those kind of voltages. 

90

Tube Amp Building - Tweaks - Repairs / Re: Oscilloscope question

« Last post by Banjan73 on June 02, 2026, 02:04:18 pm »

Some mess.
Two threads about "almost" the same. Well..
Some updates.
I have changed the grid stoppers on thise EL34s from 68k to 220k. I also adjusted bias from 22mA to 27mA.
And.. the crossover distortion i almost gone.(see attached picture).
Wattage is now 42W.
And as requested I did some extra measurments.
Anode at idle: 749V. Just before clipping: 666V🤘.
Screen at idle: 340V. Just before clipping: 269V.
The amp has been tested a bit, and sounds pretty powerful. On thursday I will test it fully with a band.
Think I am getting there now..