scope tests on a Heathkit A7E

[PRNDL]

I got a used Tektronix T932A scope to test a Healthkit A7E amp that I'm converting to guitar. Here's the schematics
http://www.naturdoctor.com/Chapters/...eathkita7e.gif

It worked fantastic for about a day, and then the volume dropped off considerably, although the sound is still great.

The tube amp repair web site said that this might be due to failure of one side of the phase inverter tube, so I ordered a 12SN7 (plus the rest of the tubes just in case). They should arrive next week.

In the meantime, I thought I'd test with the scope.

I applied 150mV at about 500Hz.
2VAC at pin 4 of ther 12SL7 (1st gain stage)
70VAC at the top of the volume pot (2nd gain stage)
35VAC at the middle of the volume pot (set at 5)
20VAC after the .05µF cap (third gain stage)
1/2VAC + 5VDC on pin 4 of the 12SN7 (after the tone stack)
the same was on pin 1 of the 12SN7 (part 2 of the phase splitter)
1/2VAC on pin 5 of both 12A6's (output tubes)

I tried measuring the treble pot with an ohmeter, but got odd results, which may be due to the capacitors. The DC voltage may be due to the .002µF cap to ground. It's a James Baxandall tone stack, which I'm not very familiar with.

I only have a few spare parts to test with, such as a 250K pot (instead of the 500K treble and 1M bass). I could get them pretty quickly from the local tube guy.

Here are my questions.

Does this look like a problem with the tone control, or is it the phase splitter?

How much insertion (gain) loss would you expect from a Baxandall tone circuit?

I've also got some homework to do -- calculating the gain for the third gain stage and phase splitter.

Also, are there any web pages with info on how to test tube guitar amps with an oscilloscope?

Thanks!!

[EC8010]

I usually find that checking the DC voltages is the fastest way of locating a fault, but you need to use your new toy. A capacitor is a short-circuit to AC, so I'd check that any capacitors with one end connected to ground don't have any AC at the far end.

[janneman]

Originally posted by PRNDL
[snip] It's a James Baxandall tone stack[snip]

It's a Peter Baxandall tone stack..

Jan Didden

[PRNDL]

Sorry about the mistake in Peter's name.

Here are the DC voltages.

12SL7
1 = 0
2 = 96
3 = 0.92
4 = 0
5 = 97
6 = 0.9

12SQ7
2 = 0
3 = 1.57
6 = 185

12SN7
4 = 4.18
5 = 4.19
6 = 2.91
1 = 4.18
2 = 330
3 = 23

12A6
3 = 397
4 = 400
5 = 0
8 = 23

12A6
3 = 399
4 = 402
5 = 0

A+ = 404
B+ = 387
C+ = 186

I'm wondering if pin 5 and 1 of the 12SN7 are out.

[kevinkr]

You might want to take a close look at everything connected to pins 1 & 5 of that 12SN7 - no way the dc voltage there should be 5V if I understood your measurements correctly. Should be about half of the available supply..

[PRNDL]

There's a small 200pF brown ceramic disc cap from pin 1&5 to ground.
The schematic calls for 200 MMF.
There's another one on top of the treble pot.

The ceramic discs stick out since the rest are high quality Sangamo reds and Astron yellow caps.

Could it be the ceramic cap has blown with age and fairly high voltages?

I could try replacing it, since I have a bunch of those on hand.

Is it possible to remove that cap completely?
It does look like something to stabilize and limit treble.

[kevinkr]

No you can't remove that capacitor it is there for loop compensation to keep the amplifier from oscillating due to phase shift in the output transformer.

It is more probable that the 150K resistor is open, but it could be that the 200pF is shorted. I would get in there with an ohm meter and do a couple of simple resistance measurements, anything that is off by more than 20% is suspect. (all pins) I ran into one instance where the grid and cathode of a 6SN7 in one of my amps shorted and that caused somewhat similar problems. Start by checking the measured values of all of the resistors in the 12SN7 voltage amplifier stage and the cathodyne phase splitter.

IMO the ceramic cap is less likely to be the cause of problems than those nice looking film caps..