Dead Selmer treble 'n' bass 50 SV

[John Corson]

Hi all,

New guy here.

Firstly,I have no training in electronics,but I like to have a go and have managed to fix more than 20 dead amps in the past few years.However,this Selmer has got me stumped.
I bought it on Ebay as a working amp,but when I plugged it in,there was a lot of popping and only a barely audible signal from one channel.Then a huge humming noise with one EL34 looking as if it would burst into flames.The seller assured me it was working and claimed it must have been damaged in transit.I'm dubious.
I replaced the EL34 bases and and the 10 watt resistor,,,,,,it sounded great for 30 mins and then I switched it off.I put it back in the cab and then it wouldn't work.Now a very low volume from all 4 channels,but louder when plugged into the echo return channel.
I found wrong value resistors and replaced them (the bias resistor was 3 point 9 K).The voltages seem to be higher than the drawing and I can't get a voltage drop on 2 resistors.The big caps look all original but as I had the thing working for a while I didn't suspect them.Changed all the valves for known working ones and then smoke from a EL34 base.
Any thoughts would be well appreciated or anyone in the Glasgow area who would be willing to have a look,even better.

Thanks a lot,,,,,,,John.

[Dave Curtis, dB AudioTech]

Welcome!

Steve Connor is in Glascow. Maybe shoot him a PM, but update the group on the amp...

[Steve Conner]

You rang?

In my S.V. the big caps were all fine, but many of the resistors had drifted in value and gone noisy.

Smoke coming from the valve bases suggests that something is oscillating and generating a very high voltage, causing them to break down and arc. I wouldn't run it too long like that as it might smoke the output transformer. Are you sure you wired those replacement valve bases correctly?

[John Corson]

You rang?

In my S.V. the big caps were all fine, but many of the resistors had drifted in value and gone noisy.

Smoke coming from the valve bases suggests that something is oscillating and generating a very high voltage, causing them to break down and arc. I wouldn't run it too long like that as it might smoke the output transformer. Are you sure you wired those replacement valve bases correctly?

Yes,,,,,,,,I photographed and made drawings before I unhooked the old ones,,,,,,,,I'm thinking it's the big caps as the six of them completely lose their charge in under a minute.There is a slight vibration from the amp and I thought transformer,but the voltage is spot on at 460 after the diodes.

[John Corson]

Welcome!

Steve Connor is in Glascow. Maybe shoot him a PM, but update the group on the amp...

Thank you.

[Steve Conner]

Faulty capacitors might explain blowing fuses or a lack of sound, or smoking dropper resistors, but they wouldn't make smoke come from the valve bases. Is it actually coming from the valve bases, or is it rising through the chassis from an overheated component inside?

The T'n'B 50 SV has balancing resistors across the main filter caps that will bleed them down fairly quickly.

[John Corson]

Thank you Steve,,,,,,,,I think the base smoked because the valve went into overheat again.It started sparking internally and then glowed like a fire inside.I'm suprised it didn't melt the glass.There are no signs of any physical damage or burning anywhere in the amp.

[Steve Conner]

When a valve does that, the most likely explanation is that it is very old, worn out and gassy. Are you sure it was "known working"?

The second most likely explanation is that the grid pin is disconnected causing the grid to float and the valve to pass maximum current. Check the 220k resistors that feed bias to the grids, and related wiring. If this has happened a few times, the valve will be toast.

Missing bias voltage will do the same, but to both power valves.

[John Corson]

Thanks Steve,,,,,I'm looking at it right now.I'm using valves from a working amp.The problem must be further down the line as I've already changed the bases and valves,but it's still burning out,,,,,John.

[John Corson]

I unhooked the output transformer and the new 10 watt resistor started smoking.It's fed directly from one of the 4 main caps which measures 464v on the +ve and 235v on the -ve.

The voltage at pins 5 are supposed to be -32,,,,,mine are -14.

[Steve Conner]

I don't recall any 10 watt resistor in my amp. Which one is it in this schematic?
Selmer TnB 50w SV Amplifier Schematic

If you mean the 1.5k resistor feeding the screen grids, yes, it will smoke if you remove the anode voltage by unhooking the OT, but leave the power valves installed. This is because all of the current that would normally go to the anodes is now going through the screens.

It will also destroy the screens, so don't do it.

[John Corson]

The 1.5 k at the top right,,,,,,amended previous post about pin 5.

[John Corson]

Popping out for 2 hours,,,,,,,JC.