Wem Dominator Bass amp Problems

[blindboybenton]

Hi

Im pulling my hair out on this one and i don't know why. Should of been easy

The amp owner said the output tx was smoking and then it was dead, I believe he brought it from ebay faulty

When i tested it fuse gone and it was the power tx. I sent it away to be rewound. The company did say that it was a problem one and they made me another.
Dropped right it wired it up all powered up and should of been boxed up in 20 mins
watkins-dominator-mk-iii-amplifier-schematic.pdf

Noticed the tubes were red plating on checking the bias via the TAD bias master tubes were over 100ma. I had tested the tubes prior to the repair and all were good
Check and changed the 100r 5watt and cap. Even if i increase the 100R too 150R + still red plating. Tried a new set of tubes and older used set still red plating
Tried without TAD bias master still the same. Output tx has been swapped out at some point for a partridge T69876 ( H2623H) looks like a 30-50 watt

Ive been told the power tx is rated at 150ma
EL84 pin 7 -370v without tubes
EL84 pin 9 - 364v without tubes
EL84 pin 3 across the 100R is 24v. I have changed the wires connecting from the tube bases to the 100R bias resistor
318v at the 32uf
Under load with the tubes red plating the B+ is down to 290

I'm at a loss on this one. I have just had the power tx and psu Tube bases and o/p tx with the main pcb removed

I'm beginning to think its the power tx------ is the 370v too high ??????????

Can anyone help me

Bassman

[Enzo]

Yu checked everything but the grids. Is ther zero volts DC on the signal grids?

How did you test the tubes? Do you have another amp to try them in? Tube testers are great for telling you bad tubes are bad, but they are no good for telling you any tube is good. bad tubes pass tube testers all the time.

[g1]

Also, have you done basic checks on the OT ?
He did say it was smoking. You found the PT bad, but that does not mean the OT is not also bad.
Unless he has actually pointed which one was smoking, and it was the PT.
Bad OT > blown fuse > oversize fuse > blown PT is not all that uncommon.

[blindboybenton]

Yu checked everything but the grids. Is ther zero volts DC on the signal grids?

How did you test the tubes? Do you have another amp to try them in? Tube testers are great for telling you bad tubes are bad, but they are no good for telling you any tube is good. bad tubes pass tube testers all the time.

Ive run them with the pcb removed . 0v on the grids. I have a orange divo tester and ive tried a new set of EH el84

Bassman

[blindboybenton]

hi I think the customer thought the tx was smoking was the o/p tx
Ive checked the o/p tx. it meters ok. and you can play guitar though and it over driven but seems like a correct vol

[g1]

Try disconnecting the cap that is across the 100R cathode resistor (50uf).

[nickb]

Have you checked for HF oscillation?

[Enzo]

With the circuit board removed, do the grids still have a resistor to ground?

[lowell]

Check DC resistance from grids to ground. To confirm Enzo's hypothesis. May measure 0v, but that doesn't confirm the grids have a path to ground via the grid leaks

[blindboybenton]

Hi

Thanks for the help. The amp is done I had not touched the tube bases and soldering looked un molested from new, But the grid was connected too pin 1
The amp is all working fine now, Bias at 50-55ma

many thanks all i have learnt a lesson today. My knowledge on tubes is ok well so i thought . Next time i get an issue like this i know where to look I just repair for fun really and a little extra cash

Bassman