RobRob Stereo

[robrob]

We're going to need photos.

 

[DestroXVII]

Hey that's a cool project.

IMO, 324V on the plates is plenty to give you sufficient volume. Definitely more than a 1/4W!

Back to your original problem. The signal should not cut out when one and only one tap of the OT secondary is grounded. This indicates a problem if you cannot ground the secondary. If you disconnect the grounds and measure resistance from each binding post to ground, you should get essentially 1.9kΩ. (This assumes NFB is connected. With no NFB, you should read OL--open circuit.) Can you verify that please?

324 is B+ to the OTs. The plates receive 288. I’ll look into the other stuff when I can

 

[andrewRneumann]

324 is B+ to the OTs. The plates receive 288. I’ll look into the other stuff when I can

Something is odd here. You are using the Hammond 125DSE OTs... each has a primary DCR of 126.5Ω or thereabouts. If you are dropping 36V over the OT primary (324 minus 288), that is 285mA of current per tube, which is insanely high. I think it is time for us to see voltages (to chassis) on all pins (heater pins measured in AC), plus send us the voltages (to chassis) at B+1,2,3. If you can put them all into a nice table format, that would be awesome.

 

[DestroXVII]

We're going to need photos.

I haven’t decided if I’m going “shabby chic” or if I want to finish it. I soldered the Jack/pot plates in with a propane torch, which ruined the authentically distressed look in those spots. Oh well. It sounds good and is pretty quiet for homemade high-gain. Surprising, given some long wire runs that were necessary because one of my goals was to not hack up the chassis or cabinet. Oh, and all the iron is inside the original AK cans. That was another goal.

 

[DestroXVII]

Something is odd here. You are using the Hammond 125DSE OTs... each has a primary DCR of 126.5Ω or thereabouts. If you are dropping 36V over the OT primary (324 minus 288), that is 285mA of current per tube, which is insanely high. I think it is time for us to see voltages (to chassis) on all pins (heater pins measured in AC), plus send us the voltages (to chassis) at B+1,2,3. If you can put them all into a nice table format, that would be awesome.

Yes, current is stupid-high. I measured over 100ma at about half volume. I’ll get to the voltages soon…

 

[andrewRneumann]

Yes, current is stupid-high. I measured over 100ma at about half volume. I’ll get to the voltages soon…

When you measure the voltages, we are only interested in the "idle" voltages with no signal applied. If you can get those to us, I think we will have a good idea where to look for problems.

I would limit the "on" time to whatever is necessary to take the readings. Check both 6V6s for red-plating and also check the PT and OTs for excessive heat.

 

[DestroXVII]

Line AC: 121
Filament AC: 6.1
PT AC: 263

B+: 322
B+1: 280
B+2: 278
B+3: 266

6v6 plates: 283
6v6 screens: 279
12ax7 plates: 183

No red-plating, OTs are cool as cukes.

 

[andrewRneumann]

Line AC: 121
Filament AC: 6.1
PT AC: 263

B+: 322
B+1: 280
B+2: 278
B+3: 266

6v6 plates: 283
6v6 screens: 279
12ax7 plates: 183

No red-plating, OTs are cool as cukes.

Ok we need to see the other pin voltages too, pins 1 through 8 around the clock on both power tubes and 1 through 9 on the preamp tubes. If you’ve got some resistors mounted to unused pins, don’t worry about it—we’ll figure it out. Just report each pin.

I’ve given you a couple instructions to completely disconnect NFB at the binding posts, but you haven’t responded to that idea. Has that been tried?

 

[robrob]

We have to figure out why you have a huge voltage drop between B+1 and the power tube plates. Fix this and most likely everything works as intended.

 

[DestroXVII]

Ok we need to see the other pin voltages too, pins 1 through 8 around the clock on both power tubes and 1 through 9 on the preamp tubes. If you’ve got some resistors mounted to unused pins, don’t worry about it—we’ll figure it out. Just report each pin.

I’ve given you a couple instructions to completely disconnect NFB at the binding posts, but you haven’t responded to that idea. Has that been tried?

Removing NFBs results only in hum. I did notice that the shielding doesn’t seem to be necessary.

Here are the complete voltages:
V1 - 12ax7
1: 183
2: 0.1 mv
3: 16
4/5: 16
6: 183
7: 0.3 mv
8: 16
9: 16

V2 - 6V6
1: 279 (tied to screen by 470 ohm)
2: 16
3: 283
4: 279
5: 20 mv
6: 3 mv (nc)
7: 16
8: 16

V3 - 6V6
1: 279 (to screen - 470)
2: 16
3: 283
4: 279
5: 80 mv
6: 3 mv (nc)
7: 16
8: 16

 

[andrewRneumann]

Removing NFBs results only in hum. I did notice that the shielding doesn’t seem to be necessary.

Here are the complete voltages:
V1 - 12ax7
1: 183
2: 0.1 mv
3: 16
4/5: 16
6: 183
7: 0.3 mv
8: 16
9: 16

V2 - 6V6
1: 279 (tied to screen by 470 ohm)
2: 16
3: 283
4: 279
5: 20 mv
6: 3 mv (nc)
7: 16
8: 16

V3 - 6V6
1: 279 (to screen - 470)
2: 16
3: 283
4: 279
5: 80 mv
6: 3 mv (nc)
7: 16
8: 16

That's a lot of 16V where it doesn't belong in V1! Give me some time to look this over along with your photo.

 

[DestroXVII]

Yeah, I wondered about that…

Do you want more close-up pics?

 

[andrewRneumann]

Do you measure 16V at V1 pins 3, 8, and all of these circled areas? These should be 0V.

 

[andrewRneumann]

Side note:
For safety reasons, you should remove the plastic wire tie from the safety ground. This bolt shouldn't have anything on it except safety ground. Make sure that the cake pan coating has been ground away so that your bolts/star washers get the best contact possible. I'm starting to wonder if your cake pan is causing other grounding issues.

 

[andrewRneumann]

Yeah, I wondered about that…

Do you want more close-up pics?

Yeah can we see this from an optimal angle(s)?

 

[DestroXVII]

Side note:
For safety reasons, you should remove the plastic wire tie from the safety ground. This bolt shouldn't have anything on it except safety ground. Make sure that the cake pan coating has been ground away so that your bolts/star washers get the best contact possible. I'm starting to wonder if your cake pan is causing other grounding issues.

View attachment 999194

I’ve wondered about the coating. All ground points have continuity, though. Could it still be an issue even with my meter showing continuity?

I’ll get some more pics up when I hit the workbench tonight

 

[DestroXVII]

Yeah can we see this from an optimal angle(s)?

View attachment 999196

Safety ground saftied. Now that I fixed my NFB wiring error, all of those points read 0v.

I ground away the coating at all ground points. The amp is much louder now with a predictable sweep through the volume control—BUT, I’m still getting low voltage:-/

B+ remains ~325v. HV a/c is only ~260. That said, plate current is normal…

 

[DestroXVII]

Safety ground saftied. Now that I fixed my NFB wiring error, all of those points read 0v.

I ground away the coating at all ground points. The amp is much louder now with a predictable sweep through the volume control—BUT, I’m still getting low voltage:-/

B+ remains ~325v. HV a/c is only ~260. That said, plate current is normal…

 

[DestroXVII]

I think it’s functionally ok. Thanks for all the help, guys. I’ll post some pics when it’s loaded into the cabinet.

 

[andrewRneumann]

I think it’s functionally ok. Thanks for all the help, guys. I’ll post some pics when it’s loaded into the cabinet.

Are you able to ground each OT secondary without losing signal?