Tremolux 6G9-B questions

[stephenhartley]

I've had my Tremolux for over 20 years and used it regularly for gigs and rehearsals.

It's been re-capped and revalved.

It started smoking the other week, with guitar going through one channel and vocals through the other.

I've built my own 5E3 based amp recently so I have a basic idea what to look for.

Inside there's an acrid burning stench which seems maximum around the PT

All the caps looked fine. All the transformer coils and resistors had continuity. The B+ was 348v and the 6L6s had been replaced with Sovek 5881s.

The tube rectifier had been replaced with solid state and there's no standby mod.

The choke had been replaced with a 100 ohm resistor.

The PT although circa 1960 with paper bobbin was not original.

The OT is a modern replacement.

I decided to restore it to original spec with the following:

Full set of Hammond Transformers including choke:

Mullard GZ34 rectifier.

Pair of Russian military grade 6L6s.

I switched on with the DMM hooked up to measure the B+ which was 410v.

Once the rectifier warmed up, the amp gave out a rhythmic high pitched oscillating sound so I unplugged it straight away and decided to take it to the repairman who I've been using for years.

Sadly, he's retired and there's no-one else local so I'm going to have another go.

I suspect that the older components may not like the new higher voltages.

I'm going to replace the few older caps and check all the resistors for drift.

Specific questions:

1. Does anyone have any tips on what else I should be specifically looking for?

2. What is the best method to bias this model? - I've been studying some of the blackface circuits with bias pots and I'm confused because on the 6G9-B the equivalent wire seems to lead directly to the tremelo intensity pot - if I insert a bias pot where is the best place to put it?

Thanks in advance

Stephen

[GregS]

With regards to the oscillation perhaps you got the OT primary wires reversed at the output tubes when you installed the new OT. This assumes the power amp has a negative feedback loop. I'm not familiar with that amp so am not sure on that.

[stephenhartley]

Thanks - I'll check

[stephenhartley]

I've checked the OT wiring and it's fine.

The high pitched oscillation is coming from the tremelo intensity pot - it goes away with the pot turned down. What's left is a fairly loud variable rumbling and hissing.

The amp itself is working, but there's a shrill overtone when a guitar is played with any volume.

A couple of the resistors have more than 10% drift.

I've prodded all the contacts and components with a wooden chopstick and I've tried swapping all the tubes. No difference.

I'm pretty sure that the original prob was the PT frying because there are brown marks around where I took it out.

I'd be very grateful for any suggestions where to look next?

[52 Bill]

I've checked the OT wiring and it's fine.

Could I ask, how did you test this? Did you reverse one set of leads or did you disconnect the negative feedback loop?

[Jazz P Bass]

You beat me to that one Bill.
To clarify.
If the output transformer is wired correctly, assuming a feedback circuit is present, if you disconnect the NFB, the amplitude of the output signal will go UP.
( This is also assuming a proper test setup with a signal generator, a scope & a DMM & a dummy load to match the output transformer)
If the NFB circuit is disconnected & the amplitude goes lower, then the OPT primary wires should be reversed.
This is called positive feedback & will most definitely make the amplifier squeel.

[stephenhartley]

Thanks for your replies. I've just looked at the circuit diagram in Weber's book and checked that the correct colour wires are in the correct place. I've used Hammond transformers and as far as I know, they are the same colour coding as the originals.

OT primary : Blue to terminal 3 V6. (2nd 6L6) Brown to pin 3 V5. (1st 6L6) Red to power caps and choke.

My knowledge is limited - I thought the OT primary is just a winding with a centre tap, so shouldn't each half of the winding be the same?

Is it worth reversing the wires and see what happens? Is it safe to do this?

[52 Bill]

Is it worth reversing the wires and see what happens? Is it safe to do this?

Yes.

There is a phase relationship between the primary and secondary of all transformers. Sometimes the color codes work and sometimes they don't. Most amplifiers use a negative feedback loop from the secondary back to the driver stage to clean up the signal. If your transformer is phased wrong, what should be a negative signal being mixed back into the driver stage is a positive one and all sorts of problem can arise like oscillation, etc. just like what you are describing. Try switching the blue and brown primary wires and see what happens.

[stephenhartley]

Thank you Bill

I'll give it a try and let you know what happens.

Stephen

[stephenhartley]

It worked! Thank you so much for your help.

There's still a bit of rumbling and fizzing which seems to clear up once the amp is warmed up and played through.

The bias voltage is -46v. and the B+ voltage is 444V. My next job is to work out which resistors to change to hot it up a bit.

[stephenhartley]

I've soldered in a bias pot and biased the tubes to about 20w using the transformer shunt method - plate voltage 415v. Plate current ~ 50 mA.

Sounds great. Really nice shimmering Fender tone with neat distortion and volume rise when played hard.

Thanks again for the help on here.