JTM45 Low output

[TommyT]

Hey all,
I finished up my first build, a JTM45, and it powered up and KT66’s biased at 39mA . I started playing and it sounded pretty good at low volume, I cranked the volume and the amp just doesn't shake the walls...I could easily turn it up to 5 or 6 and the amp is not loud at all and not much bass...something definitely wrong

I put it back on the bench and took measurements. Voltages seem good on the tubes, I can't remember exact readings but I can update the post later, I'm getting about 480v plate and bias is -52v. The weird thing is, with no input, if I turn up the bright channel volume the voltage on V4 pin3 drops considerably. I checked the same measurement on V5 pin 3 but the voltage stays around 470. If I turn up the normal channel with no input I don't get any volume drops. I replaced all the tubes (including preamp) checked component values (and yes, it's a 470 to the PI not 470k )but same results.

Since I’m getting a voltage drop on one side of the primary OT with no input, does this suggest a blown primary winding? I thought I had some weird oscillation that is causing the voltage drop but if that was the case, shouldn't I get the same reading on V5?

I was reading the geofex on simple OT testing and I'm starting to think it's an OT issue because I measured the resistance from all the pins to the chassis and I got a very low reading. I also measured the primary resistance of the OT as it sits in the amp and on one primary to CT I'm getting 120ohms (the side that gets the voltage drop) and 80 ohms on the other side to CT.

I thought I would hit up the forums for any other suggestions since I'm new to amp building and need to bounce some ideas off people. Thanks for the help and sorry for the long post!

[OnTheFritz]

Check that your 470 ohm between the two 1 megs isn't a 470K, very common mistake.

[toner]

If you're sure the 470 R is correct (yellow-violet-brown, NOT yellow-violet-yellow), post some pictures or at least your tube pin voltages. From everything you've described, I doubt the OT is the issue. It's normal for one side of OT to have higher DC resistance.

[TommyT]

I confirmed and measured: 470R

Pin Voltages are:
V1:
1. 226
2. 0
3. 1.93
4. 3.3 AC
5. 3.3 AC
6. 237
7. 0
8. 1.93
9. 3.3AC

V2:
1. 195
2. 0
3. 1.28
4. 3.3 AC
5. 3.3 AC
6. 344
7. 195
8. 197
9. 3.3AC

V3:
1. 256
2. 28
3. 46
4. 3.3AC
5. 3.3AC
6. 258
7. 30
8. 46
9. 3.3 AC

V4:
1. 38mV
2. 3.3AC
3. 474
4. 474
5. -54
6. 473
7. 3.3AC

V5:
1. 39mv
2. 3.3AC
3. 474
4. 474
5. -54
6. 473
7. 3.3AC

When I turn the high treble channel volume up to 10, the voltage on V4 pin 3 drops to 380V. I will take pin measurements with the volume turned up if that will help, pictures too, let me know what will help. I'm baffled and very, very grateful this forum is so supportive!

[toner]

Voltages look okay except the plates are high, at least in the preamp, especially with power tubes installed, which I assume they were since you gave a bias current reading. ?

That's an unusual voltage drop on the V4 plate with no input signal. This is just a guess but the only thing I can think of that would cause that is oscillation, even if inaudible (high frequencies). Do you get any "motorboating" or unusual sounds through the speaker with no input and the volume(s) up?

Sorry, I realize it's not much help. Pictures may help at this point.

[TommyT]

Thanks Toner I really appreciate the help!
Measurements are with the tubes installed and no input. With the amp plugged into speaker with no input and volume cranked, i get a little hiss but theres no motor boating at all, the amp is really quiet, no hum no hIss except at max volume (then hiss).
I will get some pictures up as soon as I can.
At this point I can only guess I'm getting DC leakage from a bad cap but I only have a DMM to test with. It's strange the voltage drop only happens on V4. I would think if it's something in the signal path it would happen on both output tubes in a push/pull...but I've been wrong a couple times before. I even swapped the output tubes around to verify the issue and the voltage drop stays with v4 and doesn't follow the tube.

[mathd]

I can't be of much help but how are the screen resistor? All connection for v4 socket are correct? Maby it is just a bad socket?
Am no tech but i would check that and the PI output/bias section if it was my amp.

[TommyT]

Hello Internet, here's my build...
Screen resistors are good. I was thinking maybe the pi coupling cap was leaking DC...I would be ticked if it was a bad socket!

I'll swap the cap and socket to see if that fixes the issue unless anyone can spot something wrong from the pics.