5F6A low volume

[Helmholtz]

Perhaps I could identify the voltage drop. While I moved the V3 tube, there was a pop and and the amp was silent. Then I measured pin's tube and I detected 0V at pin2 and pin7 grid, 0V at pin8 cathode, while the voltages at pin1 and pin6 plate are increased from normal 255V to 420V. What could be the cause?

Heater wires seems right, also jumper wire from cap to ground.

Before we continue, are you sure about the 420V? I would have expected 320V or so from your chart. 420V seems almost impossible.

[vinceg]

Before we continue, are you sure about the 420V? I would have expected 320V or so from your chart. 420V seems almost impossible.

Well, I had marked the value but frankly now I do not remember and I do not think the same situation occurs. Surely I had 0v at grid and cathode. At the plates... voltage it was however higher than normal

Anyway, I'm going to lab for re measured... stay in tune

[Chuck H]

Perhaps I could identify the voltage drop. While I moved the V3 tube, there was a pop and and the amp was silent. Then I measured pin's tube and I detected 0V at pin2 and pin7 grid, 0V at pin8 cathode, while the voltages at pin1 and pin6 plate are increased from normal 255V to 420V. What could be the cause?

Heater wires seems right, also jumper wire from cap to ground.

The plate voltage rising when there is no cathode voltage seems right. The voltage reduction at the plate to below the supplied voltage happens because of current through the tube. When there is no cathode connection there is no current through the tube so the plate voltage rises closer to the supply voltage. But Helmholtz is right that it should not be as high as 420V

I think you have a bad connection of one or both V3 cathodes. It might be the socket. But it might also be a broken component lead or a bad solder connection or some other less likely problem. Look up how to tighten tube sockets and also poke and move any wires and components on pins 3 and 8 of V3 to see if that can also cause the problem.

[Helmholtz]

Well, I had marked the value but frankly now I do not remember and I do not think the same situation occurs. Surely I had 0v at grid and cathode. At the plates... voltage it was however higher than normal

Anyway, I'm going to lab for re measured... stay in tune

Sorry, but this kind of remote repair depends on reliable information from your side.

Let's assume that plate voltage went up to the supply voltage. This indicates that the tube completely lost conduction. The most probable explanation is a disruption of heater supply, as mentioned before. Seems that you have a tube pins to socket contact problem. You may try to clean contacts. I use Deoxit on a Q-tip to clean the tube pins and small dental brushes with Deoxit to clean the socket contacts.
Some tubes (especially Chinese made ones) have smaller than standard pin diameters. In this case the socket contacts need to be (re)tightened using a needle or similar tool.
You will find more info on socket contacts using the forum search function.

Depending on the socket quality used, it may be better to replace the socket with a good one.

[vinceg]

Ok, I extract the tube and I measured voltage at grid and it is 405V, without the tube. Obviously voltage at pin 8 is 0V. Then I checked for unwanted contact at pin8 and I simply folded the pin down, then insert the tube and turn on standby. There is not pop, seems work fine. I touched with wooden stick at solder joint, baseboard and some components and there is no pop. Only small microphonic sound somewhere. But I have two JJ 12ax7. So, should be a socket problem.

[Reader]

Sorry, but this kind of remote repair depends on reliable information from your side. - - -

Ok, I extract the tube and I measured voltage at grid and it is 405V, without the tube. - - -

This makes no sense at all. I am thinking that you meant "plate" instead of "grid." Also, as previously mentioned there is no logical way for a voltage as high as 405V to exist at that point.

I suggest you slow down with the posting of information until you have double checked your data and your written facts. This slower methodical approach will get you to the solutions faster because the remote troubleshooting will be more clear and focused on the correct path.

Sincerely,
Bill

[Helmholtz]

The plate voltage rising when there is no cathode voltage seems right.

Yes, but I'd only expect a rise of around 15V.

[Chuck H]

Ok, I extract the tube and I measured voltage at grid and it is 405V,

Did you mean plate (pins 1 and 6) or do you actually have 405V on one or both grids (pins 2 and 7)???

Then I checked for unwanted contact at pin8 and I simply folded the pin down, then insert the tube and turn on standby.

Do you mean to say you folded pin 8 on the tube down and inserted it?

[vinceg]

Did you mean plate (pins 1 and 6) or do you actually have 405V on one or both grids (pins 2 and 7)???



Do you mean to say you folded pin 8 on the tube down and inserted it?

Sorry, I mean plate, and no, I do not folded pin 8 on the tube, I folded pin 8 of the socket.

[vinceg]

Since I have to buy new noval sockets from tube-town, can you advice me a good preamp tubes from this site? @Helmholtz

[Helmholtz]

Also, as previously mentioned there is no logical way for a voltage as high as 405V to exist at that point.

405V seems possible, if his plate supply voltage was ~390V with normal tube operation. V3 cutting-out decreases the voltage drop across the 4k7 PS resistor by around 15V.

[Reader]

405V seems possible, if his plate supply voltage was ~390V with normal tube operation. V3 cutting-out decrease the voltage drop across the 4k7 PS resistor by around 15V.

OK. I understand. Thanks.

[vinceg]

Update

I read new voltages:

AC: 328+328 VAC
HEATERS: 2.8+2.8 VAC
B+: 431VDC

V1
PIN1 195
PIN3 2.3
PIN6 189
PIN8 2.3

V2
PIN1 186
PIN3 1.2
PIN6 326
PIN8 186

V3
PIN1 218
PIN3 25
PIN6 384 This is not normal
PIN8 25

V4
PIN3 430
PIN4 425
PIN5 -48
PIN6 431

V5
PIN3 430
PIN4 425
PIN5 -48
PIN6 431

The amp sound fine, non popping, no strange hum

[pdf64]

...V3
PIN1 218
PIN3 25
PIN6 384 This is not normal ...

Maybe the tube or a contact of the tube socket is bad? Are the plate resistors correct (82k and 100k)?
What Vdc are the V3 grids pins 2 and 7? They won't measure correctly, but they should measure very close.

[Helmholtz]

Since I have to buy new noval sockets from tube-town, can you advice me a good preamp tubes from this site? @Helmholtz

Sorry, I don't need many new tubes these days, so I can't comment on average quality of current brands. I might go with JJ as they have a long tradition of ECC83s and E83CCs.
I generally don't trust manufacturers who don't publish comprehensive, own specs.

If you search the forum for "best preamp tubes" you will find some opinions.