Strange Things with a 5e3

[Strat76]

Hello guys,

I built at least one dozen of 5e3 and I am very happy with the results : no hum, pretty gound tone, good overdrive etc …

But I have an issue with my new build and I must confess that I am lost now, after spending many many hours to try to debugg the amp.
So every comment is desired. I feel like Dr House which is waiting for new and pertinent ideas

Here is the issue :
1) The amp has no sound, just a little hum which comes (from my point of view) from the PT and goes to the OT.
The tensions are OK in the power section area but after the second drop resistor, in the PI and preamp areas, they are odd.
B+ : 355
After the first drop resistor : 300
After the second drop resistor : 184 (instead of 230V)

The voltages are jointed in a picture tab. On the left : the voltages I use to have. On the right the odd voltages.

Of course I checked everything : connections, grounding, …
I changed the tubes several times.
Changed the board with new components and the preamp section with all the pots. I changed the OT and the sockets of the preamp and PI tubes. The only things that I didn't changed were the PT, the switches and the 6V6 sockets.
So I believed it was a failure in the PT accordingly that I was surprised that during the light bulb dim test the 25watts bulb was lighting very dimly without the rectifier tube. And I believe to remember that even without the rectifier, the 25watts bulb is enlightening much more.

So I decided to build another amp (I already had the stuff for that) to be sure.
And I have exactly the same problem. And the light bulb is lightening just when I put the amp on and fades after one or two seconds.

The voltages are the same and the behavior of the amp is identical : no sound !

I have to say that every components are tested before to be mounting.
For information the PT is a 40-18078 Classic Tone. It is wired for 240 V (European voltage wall) and I prefer to use the orange taps (330-0-330). I never had a problem with this transformer.

The only thing that I changed in relation to the last 5e3 that I built is that I put the Fuse before the power switch rather than on the neutral of the transformer.

FYI : the layout is very close to the original layout and I only changed the values of some coupling caps.

I feel lost, so every comment is welcome.

Many many thanks in advance
Voltages issues.pdf

[gui_tarzan]

Are you using the same transformer in the new amp?

[Strat76]

Are you using the same transformer in the new amp?

Not the same transformer but the same model. Classic tone 40-18078.

I wondered if the colors of the taps have changed but I tested the taps with an ohmmeter and it's apparently ok.

[pdf64]

I suggest that you check resistor values with a meter. It may be that you've mis-read the colour codes.
eg the triode sections of V1 are drawing current, as there's voltage at the cathodes, but there's almost no voltage drop across the plate resistors.
Pete

[Strat76]

I suggest that you check resistor values with a meter. It may be that you've mis-read the colour codes.
eg the triode sections of V1 are drawing current, as there's voltage at the cathodes, but there's almost no voltage drop across the plate resistors.
Pete

Thanks Pete for your reply. I 'd still checked but I can do it again. It 's difficult to believe that I made this mistake three times on three different boards. But, now, I must try anything and any idea

[Mick Bailey]

Check for DC on the preamp grids. If you're getting a voltage on the cathodes and very little drop across the plate resistors you could have a conductive board.

[Strat76]

Check for DC on the preamp grids. If you're getting a voltage on the cathodes and very little drop across the plate resistors you could have a conductive board.

Good idea
I hope no. The 3 boards come from the same place. Could be a bad batch. I 'll check. Thanks

[g1]

the triode sections of V1 are drawing current, as there's voltage at the cathodes, but there's almost no voltage drop across the plate resistors.
Pete

Yet V2 PI side shows less current at cathode and more at plate? And node D voltage indicates excess current?
Please recheck your voltages and wiring of V1 and V2.
Also, try swapping V1 and V2 with tubes from one of your working units that gave your "ref" voltages.

[Strat76]

Yet V2 PI side shows less current at cathode and more at plate? And node D voltage indicates excess current?
Please recheck your voltages and wiring of V1 and V2.
Also, try swapping V1 and V2 with tubes from one of your working units that gave your "ref" voltages.

Always done unfortunately. The wiring has been checked at least 10 times as well as the tubes.

[Strat76]

It's becoming logical there is an issue with a board which can lost his insulation. I 'll try tomorrow if there is any dc currents on the grids when the plate resistors are unsoldered.

[Strat76]

Fixed : I bot a bad batch of 100K resistors. They have the good color code but measure only 100 Ohms. With the Ohmeter I read 100 Ohms but didn't take care about the little "Kohms" which was missing on the right.


My three board now, are working very well. I must confess that it made crazy.

Thanks for your ideas