Low volume tube stero power amp

[Willabe]

 

The owner of the used record store that I sold my LP's to bought up close out back stock of several Eastern Electric tube stereo amps, China. The chassis are pretty thick/heavy gauge, they look nice, wired up well.      The big amps are 60lbs, Pentode and/or UL switched, K bias, 4 x EL34, 2 x 5AR4/GZ34.

I fixed 1, 22uF/450v 1st B+ cap died,  but he gave me 4 more, 2 like the 1st 1 and 2 little MiniMax
2 x  8w/6BM8 stereo power amps.

One of the MiniMax amps has very low output, about 1/10th of the good MiniMax.

I've swapped the 6BM8 tubes from the good amp to the low output amp, no difference.

Except for the plate/K on the split load PI, all voltages look good/right. And the wiring seems correct too.

Here's the schemo below.     

[Willabe]

Here's the voltages. Amp is UL. 4 x 6BM8, 1 x 5AR4/GZ34. No other tubes.

Pentode side;
                                      V1b        V2b           V3b        V4b
Plate, pin 6;               1. 281dc, 2. 282dc, 3. 281dc, 4. 281dc

Screen/UL tap, pin 7;  1. 283.5dc, 2. 283.6dc, 3. 283.5dc, 4. 282.5dc

K, pin 2;                    1. 25.7dc, 2. 24.7dc, 3. 25.2dc, 4.24.8dc

Control grid, pin 3;     dcv just bounces around in dcmV.

Triode side;
                                   Left V1a        V2a       Right V3a       V4a
Plate, pin 9;              1. 110.7dc, 2. 143.2dc, 3. 111.1dc, 4. 143.0dc

K, pin 8;                   1. 2.3dc, 2. 110.5dc, 3. 2.36dc, 4. 110.5dc

Grid 1., pin 1;           1. 0.0dc, 2. 110.7dc, 3. 0.0dc, 4. 111.1dc

The split load PI plate and K R's are 47K, yellow/purple/orange. 

Why would the split load PI plates and K dcv's be ~33dcv apart?
     

[SILVERGUN]

The good news is, there ain't much there. 

Check the input jack ground (grounded by contact to chassis?)...same goes for the speaker jack

Got a signal generator?

Divide and conquer.

[Willabe]

The good news is, there ain't much there. 

Yeah, but it's a heavy little bugger, 21.5 lbs.

Check the input jack ground (grounded by contact to chassis?)...same goes for the speaker jack.

No, their wired grounds with jack insulators, input and speaker outs.
 
Grounds all read 0 ohms and continuity buzzes. 

[SILVERGUN]

Any shielded wire inside?...check conductor to shield.

[Willabe]

Any shielded wire inside?...check conductor to shield.

Yes, I did check that.

Their good, no shorts.

[sluckey]

Show us some pics.

[Willabe]

Show us some pics.

Charging the battier.

What setting for pic size on camera?   

[PRR]

First stage plates should be 71V, not 111V. Why the difference? Wrong parts? Bad parts?

Open cathode resistor is suggestive, but does not quite fit. But meter is cheaper/surer than thinking.

[Willabe]

First stage plates should be 71V, not 111V. Why the difference? Wrong parts? Bad parts?

I think you nailed it PRR. 

2 of the few I hadn't measured yet. 

Their hard to see because they stood them up vertically from the tube socket pin to the ground rail buss. 

I just took them out and can't get a reading on them, they must have shorted open?

They look a little burnt, I can't tell the color bands, middle 2 bands are the worst. 

On the 1 working amp, they read 1.2K and I can see/read the color bands just fine, brown/red/red.

[Willabe]

Here they are.

[Willabe]

The OT secondary -FB gets injected at the junction of that 1K2 R and K.

I can't tell if they put in the wrong R's.

Could the -FB take those K R's out?

The -FB series R is 62K/2w, they measure and colors read correct.

[Willabe]

Here's some pics of the amp.

[Willabe]

....

[Willabe]

....

[Willabe]

I don't have any at 1K2, will 1K work? (For the burnt triode K R's.)

I also have 1K5 in metal oxide, 2w.
Here's the schemo again, 6BM8's; 

[Willabe]

I gator cable clipped in a 1K5 R, 1 in each channel, volume came way back up, on both channels. Plate and K dcv's came back where their supposed to be. 

But, it was very noisy/hum, almost like feed back, might be the long cable leads? 
 

[PRR]

1K or 1.5K will work, though you want 1.2K to match the other amps.

Long leads will suck-up all the hum and buzz in the room. (Specially since these are set for awful high gain.)

No obvious fault would burn those resistors. In this case, I think the parts were just bad.

[Willabe]

No obvious fault would burn those resistors. In this case, I think the parts were just bad.

Thank you very much PRR. 

[Willabe]

Here's a pic of where that K R was hiding.           

[shooter]

design interruption;
Since there's a dc coupled stage and, well 1/2 the PI is a CF, would the Valve wizard's diode mod for dccf apply? 
thanks
dave

[Willabe]

Good question.   

I was reading that today.

If the amp was mine, I think I'd try it.

[Willabe]

I replaced the 1K2 K R's and the volume is back ,  ~ BUT ~  now it's motor boating, very strong.

Bad B+ filter cap?

I'm going to swap the power tubes just to see what happens.
 

[Willabe]

I swapped the tubes, same motor boating, it's on the right channel.

Only happens with CD source signal applied.

Power supply B+ feeds both channels.

[SILVERGUN]

Motorboating could come from a shared cathode connection stage to stage...did you do anything goofy when you replaced those resistors?

or

Did you reconnect the NFB to the FIRST stage?

[Willabe]

Motorboating could come from a shared cathode connection stage to stage...did you do anything goofy when you replaced those resistors?

    No, I didn't do anything goofy, well, at least not today. 

Did you reconnect the NFB to the FIRST stage?

I just disconnected the -FB on the right channel, no more motor boating.

[SILVERGUN]

I just disconnected the -FB on the right channel, no more motor boating.

Did you have it connected to the first stage as it should be, or did you accidently have it on the PI?

[Willabe]

Can I just swap the wires on pins 8 and 9 on the split load PI?

Instead of swapping the OT primary leads?

[Willabe]

Did you have it connected to the first stage as it should be, or did you accidently have it on the PI?

No, I never moved the -FB R. It's right where it was, injected into the K on the 1st gain stage.

[SILVERGUN]

Did you have it connected to the first stage as it should be, or did you accidently have it on the PI?

No, I never moved the -FB R. It's right where it was, injected into the K on the 1st gain stage.

That's weird...what happened?

[Willabe]

I don't think this amp ever worked.

The used record store owner bought several of some Eastern Electric tube amps and preamps on a deal.

He's having me go through them so he can sell them.

[SILVERGUN]

Can I just swap the wires on pins 8 and 9 on the split load PI?

Instead of swapping the OT primary leads?

I missed this as we were posting at the same time....yes you can

I would still make sure that the NFB goes to the actual first stage since it came in not working

[sluckey]

I don't recall motorboating ever being a symptom of wrong NFB phase. Do you really suspect a phase issue with the NFB? If so, just compare the OT wiring for both channels. If it looks the same I would be looking for something else.

[Willabe]

I don't recall motorboating ever being a symptom of wrong NFB phase. Do you really suspect a phase issue with the NFB? If so, just compare the OT wiring for both channels. If it looks the same I would be looking for something else.

I disconnected the -FB series R on the right channel 1st gain stage K and the motor boating stopped.   

So I just flip/flopped pins 8 and 9 wires on the split load PI and it works. 

He gave me 2 of these, the good 1, both channels pins 8 and 9 are wired the same.

But to get this 1 working pins 8 and 9 are now wired opposite of each other.   

Thanks guys.     

[SILVERGUN]

I don't recall motorboating ever being a symptom of wrong NFB phase. Do you really suspect a phase issue with the NFB? If so, just compare the OT wiring for both channels. If it looks the same I would be looking for something else.

We may have been expanding the definition of the term motorboating.

Willabe, for the record, the common use of the term motorboating refers to a low frequency oscillation that kinda sounds like a putt, putt, putt, putt, etc...

NFB phase issues usually sound like a constant squeal or howl.

[Willabe]

...did you do anything goofy when you replaced those resistors?

Did you have it connected to the first stage as it should be, or did you accidently have it on the PI?

We may have been expanding the definition of the term motorboating.

Willabe, for the record, the common use of the term motorboating refers to a low frequency oscillation that kinda sounds like a putt, putt, putt, putt, etc... NFB phase issues usually sound like a constant squeal or howl.

     Yes, I know. SG, you must think I've lost my mind or something.

It was a low pitch oscillation.

[SILVERGUN]

We may have been expanding the definition of the term motorboating.

Willabe, for the record, the common use of the term motorboating refers to a low frequency oscillation that kinda sounds like a putt, putt, putt, putt, etc...

NFB phase issues usually sound like a constant squeal or howl.

    Yes, I know. SG, you must think I've lost my mind or something.

It was a low pitch oscillation.

No Sir...and no disrespect intended.
I just know that descriptive words can mean different things to different people.

Motorboating has multiple meanings 

[shooter]

NFB phase issues

I just flip/flopped pins 8 and 9 wires on the split load PI

that solves this question, but if you swapped OT primary's, wouldn't you also swap G2's since it's a UL?

[Willabe]

NFB phase issues

I just flip/flopped pins 8 and 9 wires on the split load PI

that solves this question, but if you swapped OT primary's, wouldn't you also swap G2's since it's a UL?

Yes, I believe that's right.