2000 Vibrolux Reverb

[Randall]

Oh man, I had one of those amps dog me for weeks once. I never did get it to sound good. And it cost me a new OT.

[Chuck H]

Oh man, I had one of those amps dog me for weeks once. I never did get it to sound good. And it cost me a new OT.

FWIW I used that trem circuit in a build because the "vintage" circuits don't recognize the cathode to filament restrictions in the data (which can be a problem with some newer tubes). It wasn't horrible :-/ ,.. I was hoping for better based on reviews. I made some modifications that helped a little but I'm still considering an improvement if that amp ever makes it back to my bench. The owner doesn't use the trem much. We'll see.

[J Luth]

Took a lot of voltage readings need to check them against with the schematic when I get a chance.

Checked a lot of resistors and caps on the board, NP, solder joints look OK.

He called today and may come and pick it up without repair should know shortly.
As he puts it sounds like a bad 5 or 10 Watt amp and doesn't want to put much into it.
He also told me today that this was not a new issue, it started a few years ago. He put it away and has not used it for a few years.

I'll keep looking at it until I hear from him.

[Dave H]

Before checking anything else I'd measure the output power as suggested in post #2
The schematic says the output should be 11.3V rms at the onset of clipping into 4 ohms with 83mV rms at the input grid and tone controls at 50%

[J Luth]

Sorry, couldn't get back on until today.

The initial testing I had done was using the scope.

When checking it with the scope it would start to show clipping when the volume on either channel would get to around 3 ½ and bad distortion the rest of the way up. On the 2 and 5 volt scale of the scope it was just a shade over 11 volts. Using the Fluke VM it read 10.98 volts.

I did some tap testing to see if anything changed but it did not. Still was acting the same.

Yesterday I tuned it on to do more testing and now the normal channel can be turned up to just over 7 before it clips and the voltages are the same as listed above, just a shade over 11 on the scope and 10.9 on the Fluke. The Bright channel has not changed, it still can’t be turned above 3 ½. I would think that 6 ½ to 7 would be about right before clipping.

[g1]

. The Bright channel has not changed, it still can’t be turned above 3 ½. I would think that 6 ½ to 7 would be about right before clipping.

That is determined by the size of input signal. Are you using the settings from post #19 ?

[Dave H]

And what is the power output at the onset of clipping?

[Dave H]

When checking it with the scope it would start to show clipping when the volume on either channel would get to around 3 ½ and bad distortion the rest of the way up. On the 2 and 5 volt scale of the scope it was just a shade over 11 volts. Using the Fluke VM it read 10.98 volts.

How did I miss that? If the load was 4 ohms then the power output is as it should be.

[pdf64]

When used to 8 and 16 ohm rigs, the Vac across 4 and especially 2 ohmers can seem deceptively low

[Helmholtz]

In a lot of cases a tube amp problem can be narrowed down by the DC voltages. Other methods often take longer.

[J Luth]

Was using 100mV input and 4 ohm load. Yes the output is a shade over 11 V.
Owner is coming by around noon today.

[Pedro Vecino]

I have not followed the topic completely but has anyone noticed that the amplifier does not have a feedback string?

[vintagekiki]

Not have, and most vintage Vox series AC .. no feedback string.

[Jazz P Bass]

I had one of these "Custom Shop" Vibrolux amps in for repair awhile back. (pre pandemic)
The client was a jazz guitarist.
He hated this amp.
Wanted it modded.
I declined.
He sold it & got a 'real' Vibrolux.
Happy. Happy.

[J Luth]

He is going to put it back in the closet. Not going to have it fixed right now.

He is thinking about taking out the circuit boards and having it hand wired with eyelet boards.