Very strange problem with a 5e3

[chironome]

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

[wyatt]

So, you built a dozen working 5E3's and now two in row that don't work?

Troubleshooting these things is always easier with a scope or at least a probe that can send the signal to another working amp.

Oh, and fuse should always go on the Hot/Line.

[chironome]

So, you built a dozen working 5E3's and now two in row that don't work?

Troubleshooting these things is always easier with a scope or at least a probe that can send the signal to another working amp.

Oh, and fuse should always go on the Hot/Line.

Yes that's the reason why I changed the fuse's place.
I still watched what is happening with a scope and a signal generator.
The signal is amplified after the first triode and lost just before the Pi.
This is in correlation with the strange voltages.

Any idea ?

[wyatt]

Before the PI (pins 6,7,8 of V2)?

Or before the 2nd gain stage (pins 1,2,3 of V2)?

What about the Volume/Tone controls?

[chironome]

Before the PI (pins 6,7,8 of V2)?

Or before the 2nd gain stage (pins 1,2,3 of V2)?

What about the Volume/Tone controls?

The signal is weaker than normal after the coupling cap just before the Volume pot.
I believe (i try to remember) that it almost disappears after the first triode of the PI.
I remember also that I have a "Poc" when I check the voltages with the probe around the second triode of the PI. Not before (I mean no "poc" around the first tube and the first triode of the PI).
Usually you have "pocs" anywhere in the signal chain.

[chironome]

Oh, and the volume and tone pots dont do anything.

[Milkmansound]

Did you use shorting switchcraft type jacks for the speaker connection?

Very common for those to remain shorted even with a cable plugged in. You can look and see if the contacts spread open or not - and if not bend the tip a little bit until it makes an open connection with a cable inserted.

[martin manning]

What are the voltages on the grid pins (2 and 7) of the first two tubes?

Notice that the anode voltages on the tubes you labeled V1, V2, and V3 are all just 1 volt lower than the supply voltage at D. This means that they are barely conducting any current at all. V4, which I assume is the PI, is conducting and there you get the pop when you probe it.

FYI, the second tube (12AX7) contains a gain stage (the triode with the 100k anode and 1k5 cathode resistors) AND the PI (the triode with the 56k anode resistor).

[frankdrebin]

What are the voltages on the grid pins (2 and 7) of the first two tubes?

Notice that the anode voltages on the tubes you labeled V1, V2, and V3 are all just 1 volt lower than the supply voltage at D. This means that they are barely conducting any current at all. V4, which I assume is the PI, is conducting and there you get the pop when you probe it.

FYI, the second tube (12AX7) contains a gain stage (the triode with the 100k anode and 1k5 cathode resistors) AND the PI (the triode with the 56k anode resistor).

exactly,check out grounds on the preamp tubes and coupling caps for leaking(voltages on grid pins).
If you have the same results in the two amps with different components most probably its a wiring mistake.

[jelle]

Scope it.

[chironome]

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

[groovtubin]

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

ALWAYS METER OUT PARTS, CAPS AS WELL..I GOT A BATCH FROM CHINA CHEAP ( 470PF) THAT READ HORRIBLY LIKE 292 TO LOWER 300`S! CANNOT STRESS THAT ENUFF!

My best, jim@Omegaamps