5F6A low volume

[vinceg]

No problem.
Now you connect Red_T and Red_B to the rectifier, and turn amp pn.
Still with lamp limiter and remember to connect some speaker to the output.
All controls set to 0.
Do you get high voltage?
Check main filter caps, plates, screens, and follow the line which feeds the preamp tubes.

You will have less than normal, probably around 300V DC.

Post results.

Do you mean with "follow the line wich feeds the preamp tubes"?

[Reader]

- - - Before proceeding with the next step I want to point out that I replaced the rectifier valve with diode like picture below, in case this could be a problem for our work

Now would be a good time to discuss why you choose that mod. It does change the response of the classic 5F6A amp. Therefore, if it turns out that it may not give you the result you expect then now would be a good time to switch back to the original tube rectifier circuit before Juan Fahey leads you through the remaining steps he has planned.

[vinceg]

Now would be a good time to discuss why you choose that mod. It does change the response of the classic 5F6A amp. Therefore, if it turns out that it may not give you the result you expect then now would be a good time to switch back to the original tube rectifier circuit before Juan Fahey leads you through the remaining steps he has planned.

A friend had suggested this change, arguing that the anodic tension is more stable. I do not know how true it is and what effect it can give on amp response.

[eschertron]

"stable" = "stiffer", less sag. good or bad, depending on what you want from the amp.

the other issue is your voltage is low, due to line voltage. What did we say, 8% low? So the SS rectifier could help make up for the lower high tension voltage. Just a thought.

[J M Fahey]

Do you mean with "follow the line wich feeds the preamp tubes"?

The whole amplifier has a "string" or "line" of cascaded filter stages, from main diodes and main capacitors and of course highest voltage available to a series resistor and a capacitor to ground which feeds next stage to .... to ... to a series resistor and a capacitor to ground which feeds last stage (counting from the supply) or the first stage, countiong from the input jacks.

I am asking you to verify the presence of reasonable voltage "along that line".

It would be sad to waste 35 posts guessing why the amp does not sound to find that some preamp tube is not being fed +V ... or filament.

I sm not too worried today about "tone" , we have a patient with a heart attack or massive stroke, let´s leave cough or bloating for a later day until we solve first the main problems.

Which "looked" like a dead power transformer, and now looks more and more like miswiring or similar problem..

[vinceg]

Goodmorning everyone!

I connected Red_T and Red_B to the solid state rectifier and turn ap on.
In the pictures below you can see the voltages at plates, screen and grids of the tubes.
The B+ voltage is ~290VDC. Main filter caps have ~290VDC to 284 VDC from B+1 to B+3
Also, I checked voltages in some points of the circuit by comparing the values measured with the original schematic and they seem correct, considering the voltage drop by lamp limiter.
Amp seems very silent.
If these measurements are insufficient, or you need some reading in particular, because I did not understand, please let me know and I will immediately.
With respect, Vincenzo

[eschertron]

This is all with the light bulb limiter in place? Looks good. I would hope your B+ high tension voltages rise considerably without the limiter.

But don't do that until Juan gives you the go ahead

[vinceg]

Yes, I checked voltages with lamp limiter, how Juan suggest. I have doubts about voltage at pin3 of V3. In the original schematic this is 34V. Mine, with bulb limiter should be smaller. Anyway, waiting for Juan response and instructions.

[Reader]

- - - I have doubts about voltage at pin3 of V3. In the original schematic this is 34V. Mine, with bulb limiter should be smaller. Anyway, waiting for Juan response and instructions.

I was having trouble visualizing how pin 3 if V3 was wired so I referred to the original schematic. Then I realized that the schematic had never been posted in this thread so I have attached a copy.

[Chuck H]

The schematic does not indicate V#'s. I have to assume Vince is using a schematic that does. Regardless... I figure V3 in his reference is the phase inverter.

[eschertron]

The schematic does not indicate V#'s. I have to assume Vince is using a schematic that does. Regardless... I figure V3 in his reference is the phase inverter.

Going with that assumption, the tail resistor or presence pot - on the drawing that Vince went by when building the amp - could be a value that explains the higher cathode voltage on the PI.

[vinceg]

I'm sorry. Yes, I mean the phase inverter. Tail resistor is 10k. About the presence control pot, two lug broken and I replaced with a 25K pot

[eschertron]

...broken and I replaced with a 25K pot

I must ask: replaced before or after the voltage readings in post #111 above.

[vinceg]

I must ask: replaced before or after the voltage readings in post #111 above.

Before. I do not have at home a 5K pot, can I left a 25K?

[Helmholtz]

Before. I do not have at home a 5K pot, can I left a 25K?

The 25k presence pot changes operating conditions and voltages of the phase inverter. I strongly recommend to get an original value 5k pot. In the meantime you may wire a 6.2k resistor across (in parallel to) the 25k pot to get voltages (and gain) back to normal.