Help diagnosing Tube Amp PLEASE

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sonolink said:
Ok, so WITH the tubes and new 5w screen resistors these are the readings:


V1 ECC83

1: 175
2: 0
3: 1,6
4: 39
5: 39
6: 227
7: 0
8: 2
9: 39		V2 ECC83

1: 170
2: 0
3: 1
4: 39
5: 39
6: 313
7: 170
8: 170
9: 39		V3 ECC83

1: 226
2: 43
3: 67
4: 39
5: 39
6: 227
7: 37
8: 68
9: 39		V4 6V6GT

1: 0
2: 40
3: 422
4: 422
5: -40
6: 0
7: 39
8: 0
V5 6V6GT

1: 0
2: 39
3: 422
4: 422
5: -40
6: 0
7: 39
8: 0


That seems more like it, right?
Cheers
Sono
Click to expand...
Injecting and Tracing the signal helps find the point where the signal stops.
Static DC voltages can only tell so much.
If you dont have a tone generator you could possibly use a cellphone with a 3.5mm jack and get an app, inject the signal.
Very basic stuff.
 
The fact that the Output valves' Anode and Screen Grid voltages are identical throws up a question, usually these are not the same.
Screen grid voltage is almost always lower than Anode voltage.

At this point I would measure the Output transformer primary windings' DC resistance.
Measure from the centre tap to both outer taps.
Make sure all HT voltages are discharged first, of course.
Then with the amp powered up and valves in place, you can measure the voltage across each OPT primary winding, to work out what kind of bias current you have through the output valves.
This will tell you if the output stage is actually working, or not, and also whether the OPT is faulty, or not.
 
Hi guys,

Thanks a lot for all your input as usual. I appreciate :)

So as I mentioned in my earlier post, I replaced both screen resistors and plugged in new tubes and measured voltages. There's a pic of the new 5w resistors ;)

IMG_20240324_132113.jpg

I was sort of in a hurry so I just posted the voltages. Later this evening I plugged my guitar in and I'm happy to report that the amp is now fully working. It's a really good sounding amp BTW...

I asked the owner when was the last time he had changed the tubes. He told me "I can't remember really.... probably 5 years ago". 😱😱😱
This guy is an active musician btw...🙄

So my guess is that one power tube went naturally faulty because of its usage, and blew the screen resistor...

Thanks again for all your help, time and patience! ;)
Cheers
Sono
 
NoisyIndividual said:
Well far be it from me to say "I told you so" but I told you exactly this in posts 2 and 3 of this thread. :)

Anyway, a happy ending, and I guess we all learned something.
Click to expand...

You did indeed and that's what I checked ;)

At first I was a bit unsure about putting new tubes without being sure the amp wouldn't toast them.

Thanks again for your help 😉
Cheers
Sono
 
sonolink said:
I asked the owner when was the last time he had changed the tubes. He told me "I can't remember really.... probably 5 years ago". 😱😱😱
This guy is an active musician btw...🙄

So my guess is that one power tube went naturally faulty because of its usage, and blew the screen resistor...
Click to expand...

For what it’s worth, I also don’t change tubes preventively, because I feel that tossing out working tubes is a waste :)

The idea that a typical user/player should periodically change all tubes in an amp (like we change guitar strings) probably dates to 1990s tube retailer/relabeler marketing.

At least I’ve never seen evidence that the word/concept “retube” existed before then.

I typically replace tubes when they fail (this includes subtle failures like noise and microphonic behavior) or (particularly in the case of output tubes) when I notice degraded performance.

A shorted tube that takes out a screen resistor is not a the failure mode of a tube that’s “tired”—it’s the failure mode of a tube that’s broken or defective

Every rule has exceptions. If you’re a touring artist who employs a touring tech, and you run your amps really hard and your amps are naturally the type to have poor thermal management (AC30), and the amps simply cannot ever fail—there can be some benefit to a preventive tube replacement schedule focused on output tubes

But that’s an edge case. I tour with two Super Reverbs. They get sent on a semi truck between tour stops every night (in their flight cases)

Our touring tech of course has comprehensive spares, but in five years of fairly-grueling touring we have not shotgun-replaced tubes in them. There’s no degradation in performance!

IMO, you can rest easy running quality tubes for a very long time

I regularly run across 50s and 60s gear with 60- and 70-year-old tubes still sounding great
:)
 
Last edited:
sonolink said:
Hi guys,

Thanks a lot for all your input as usual. I appreciate :)

So as I mentioned in my earlier post, I replaced both screen resistors and plugged in new tubes and measured voltages. There's a pic of the new 5w resistors ;)

View attachment 125308

I was sort of in a hurry so I just posted the voltages. Later this evening I plugged my guitar in and I'm happy to report that the amp is now fully working. It's a really good sounding amp BTW...

I asked the owner when was the last time he had changed the tubes. He told me "I can't remember really.... probably 5 years ago". 😱😱😱
This guy is an active musician btw...🙄

So my guess is that one power tube went naturally faulty because of its usage, and blew the screen resistor...

Thanks again for all your help, time and patience! ;)
Cheers
Sono
Click to expand...
Normal usage would not short a screen to a plate, but mfg defect or shock could.
There may be other problems with tubes besides internal shorts, like low emission, gas leaks, etc. A tube tester is handy.
Excessive negative grid voltage could make the amp a hard to drive class-C, maybe a fuzz box replacement ;-)
 
NoisyIndividual said:
The fact that the Output valves' Anode and Screen Grid voltages are identical throws up a question, usually these are not the same.
Screen grid voltage is almost always lower than Anode voltage. 0
Click to expand...
It's not very surprizing. Consider that the voltage drop in the DCR of the primary is not too different than that in the screen grid resistor.
 
Last edited:
Having Anode and Screen Grid voltages identical or near identical would tend to suggest there is no current flowing through the output valves at all.
As happens if you just pull the valves and measure these voltages at the valve socket.
 
NoisyIndividual said:
Having Anode and Screen Grid voltages identical or near identical would tend to suggest there is no current flowing through the output valves at all.
Click to expand...
No. Just consider the case where the DCR of a half primary is 100 ohms and the anode current 50mA, and the screen grid resistor is 470 ohms with 10mA of screen current. The voltage drop would be very similar.
 
I'm going by what I see on the bench.
With the output valves removed, Anode and Screen Grid voltages are pretty much always very similar. With output valves in place they are noticeably different.
This happens often enough that if I see these voltages are very similar with the valves in place, I would investigate further.
 
NoisyIndividual said:
I'm going by what I see on the bench.
With the output valves removed, Anode and Screen Grid voltages are pretty much always very similar. With output valves in place they are noticeably different.
Click to expand...
It depends on so many factors (primary DCR, screen resistor, bias...) that you can't make a rule of it.
If you look at the Fender schemos, e.g. Pro Rever AA165, you'll see that the screen and anode voltages are exactly the same at 440V.
 
Last edited:
Sonolink, please measure the Anode and Screen Grid voltages again, with all valves in place.
If they are all 422v I will donate some money to a local charity!
 
amplexus said:
If it’s connected that way then it’s a big departure from an ab763 deluxe. That configuration doesn’t occur in a deluxe reverb and definitely not on V2 which should be the first two gain stages of the Vibe channel.
Click to expand...
Measurements in post #36 cleraly suggests both V2 is connected that way, with V2b as a voltage follower.
 
  1. Ok here goes then:​

V1 ECC83
1: 176
2: 0
3: 2
4: 40
5: 40
6: 228
7: 0
8: 2
9: 40		V2 ECC83
1: 172
2: 0
3: 1,5
4: 40
5: 40
6: 315
7: 175
8: 175
9: 40		V3 ECC83
1: 230
2: 46
3: 68
4: 40
5: 40
6: 230
7: 38
8: 67
9: 40
V4 6V6GT​
1: 0
2: 40
3: 430
4: 431
5: -41
6: 0
7: 41
8: 0		V5 6V6GT
1: 0
2: 40
3: 430
4: 432
5: -41
6: 0
7: 41
8: 0

.....

Cheers
Sono
 

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