Problem with a Vibroverb

[Marcfrom]

Hi Guys,
I recently build a Vibroverb, it was working perfectly but now when i turn it on there is a buzz and the tubes (6L6GC) show 3mA. Sound is small and saturate.

After some test I replaced a spare OT, problem still the same.

I tried to unsolder the lead of the choke and put a wire instead, and like this the tubes show the good value 35mA. There is a hum but it seems normal without the choke.

So i thinked the problem is the choke i have replaced it by a new one but the problem come back again !!

So please everyone has an idea?

Thanks
Marc

[Blackburn]

Not sure which Verbrovibe you're talking about, but I recently repurposed a chassis and very flawed 5B6 into the 6G16 circuit, so if it's that one, maybe I can help you. I didn't use a choke, simply because I didn't want to spend any extra money. It's a clean amp, so I'm not cranking it. I used two 1K 5W cermics in parallel, in place of the choke. I didn't build it with the verb, just vibe. I used two 100K plate resistors on the first stage. I think everything else is stock to the schem. Such a fantastic clean amp with great chime, and of course, the wiggle wiggle.

[Marcfrom]

It's a AB763 Schematic.
I want to debug and use the choke as the original.

[JazzGuitarGimp]

With the choke in-circuit, what are the voltages on the plate side and the screen side?

[Marcfrom]

With the choke:

Plate (Pin 3) 404 V
Screen (Pin 4)403 V
Dissipation 3mA

Without choke (wire instead):
Plate (Pin 3) 480 V
Screen (Pin 4) 480 V

18mA

I have under biased the amp for the test.

Do you see something?

[JazzGuitarGimp]

Not yet. What is the bias voltage at the junction of the 220K grid resistors, both with the choke and wire a wire?

[Marcfrom]

It's -57 V (negative)

The same with the choke and the wire.

Does a problem of the Filter caps seems possible?

[JazzGuitarGimp]

It's -57 V (negative)

The same with the choke and the wire.

Does a problem of the Filter caps seems possible?

The amp is a recent build. Assuming you used new, rather than NOS filter caps, I wouldn't expect that to be the problem. Unless they were stressed with overvoltage, but then I'd expect to see signs of physical evidence, such as bloate body or electrolyte leaking out of the pressure release vents.

Not sure where to go next on this one. I was going to ask about the bias voltage as it looked a bit too negitive. But after looking at the schematic, and seeing -55V, I don't see that as an issue either.

Were there any symptoms or strange behavior leeding up to the failure?

[JazzGuitarGimp]

Oh, wait a minute, I do see something. The difference in the plate voltage with and without the choke seems more divergent than I would expect. 480V vs. 404V, I think this is pointing to the problem. Schematic says the HT should be 468V. Did you measure your HT when the amp was working?

[Marcfrom]

I think the voltage (480V) without the Choke (just Wire) it's correct. Because i have under biased 16mA.

When the amp worked, the voltage was around 460V, with 45mA biasing.

The filter caps doesn't show any loate body or electrolyte leaking. It,s new TAD caps, no NOS.

The only thing I done between the problem, there was a NOS 6L6GC GE in this amp.

I just take it to try in the Bassman Tweed, after that I puted it in the Vibroverb and when I turned on, the problem appear.

I tried the amp with another new 6L6 pair and it's the same.

So until now I have replaced the OT, PT, choke, 6L6GC. This is the situation.

Now I think about a wrong contact or a socket, but the strange thing is that work when i replace the choke by a wire.

[Leo_Gnardo]

Plate V 480 without choke, 404 with?

Is this choke in series with the entire power supply?

Usually B+ is taken from the first filter node (to output transformer center tap), then a choke/cap filter to second node (supplying screen grids).

If the choke was in series with the whole supply, and got roasted because it's not rated for that much current, that would explain why the B+ is failing and plate currents low.

[Marcfrom]

Here you can see a clear schematics and layout.

http://www.tubeampdoctor.com/images/Fil ... 050712.pdf

I exactly done this wiring, but with Mercury Transformers, choke, reverb driver, etc...

Yes, Plate V 480 without choke, 404 with.

But the choke is not burned, it show 100R as it have to be.

[JazzGuitarGimp]

Can you measure the DC resistance of the choke (out of circuit)? Also, can you disconnect both of the choke leads, connect them together, then measure the DC resistance from there to chassis?

[Marcfrom]

Yes, out of circuit it's around 100R and it's the value noted by Mercury.

Both lead and chassis is infinite resistance.

[JazzGuitarGimp]

This one is a bit of a head-scratcher. Maybe someone else will chime in with more ideas. I'll continue to think about it. I really don't think the HT should drop all the way down to 404V with the choke in the circuit, especially considering there is only 3mA flowing through the tubes. I really think this points to the issue.