Tube amplifier output tubes cold, no sound

[Levente]

Thanks Lawrence, i think all the tubes are glowing normally. The only tube I can not check as the red Jan 5693 preamp tube, which has some moving particles inside..

Attached a picture of the output tubes glowing in daylight.

Now I have sound and music is coming through the speaker.

Also, i am experiencing a 60 cycle hum after few minutes.

I have measured the voltages now with my third DMM and it is the same 42 VAC.

Elevated heater voltage for some reason on purpose?

[Station X]

Please confirm that you have one meter lead on pin 2 of the a 6V6 and the other meter lead on pin 7.

Valve pins number clockwise from the gap or spigot when looking from UNDER the chassis.

If you had 42V across the heater pins the valves would have been destroyed long ago.

[Herald1360]

Glowing like that, there's no way those 6V6 heaters are getting 42V!

Are you sure it's not 4.2V ac?

Sound going down and hum coming up suggests something, possibly a smoothing or reservoir or grid coupling capacitor is breaking down and over loading the HT supply.

[Levente]

Thanks so much ...i think i misunderstood Lawrence. I thought I should measure each heater pin to chassis? not against each other the heater pins...?

if I measure pin 2 against pin 7 on both output tubes I am getting 5.5 VAC

I am sorry I misunderstood this.

[Levente]

Originally Posted by Herald1360

Glowing like that, there's no way those 6V6 heaters are getting 42V!

Are you sure it's not 4.2V ac?

Sound going down and hum coming up suggests something, possibly a smoothing or reservoir or grid coupling capacitor is breaking down and over loading the HT supply.

Sound is steady but also the hum and the hum going up and down with volume pot..

[Diabolical Artificer]

What amplifier is it? It looks like a projector amp or similar, a schematic would help us help you as would some good quality pictures of the underside of the amp. If it were mine I'd take the valves out and measure the voltages of the mains tfmr, if heater voltages are ok, with the amp plugged into a lamp limiter, valves in measure HT/B+ both AC and DC ( ewven better scope it), then measure each valve in turn, pins 1 - 8 or 9.

I wonder if you have a heater to cathode short or partial short? That or maybe a partially conductive valve base.

Andy.

[Levente]

Originally Posted by ms660

Picture says a thousand words....'scooze my spider writing and drawing.

Lawrence.

Thanks Lawrence...yes, i did fail. I did " NOT OK" according to your drawing..sorry.. but lesson learned again.

[ms660]

Originally Posted by Levente

Thanks so much ...i think i misunderstood Lawrence. I thought I should measure each heater pin to chassis? not against each other the heater pins...?

if I measure pin 2 against pin 7 on both output tubes I am getting 5.5 VAC

I am sorry I misunderstood this.

Picture says a thousand words....'scooze my spider writing and drawing.

Lawrence.

Post re-submitted after spelling error correction in the drawing.

[Boulevardier]

"Sound is steady but also the hum and the hum going up and down with volume pot.. "

Have you got anything connected to the input, or is it just open circuit?

Mike

[Levente]

Originally Posted by Boulevardier

"Sound is steady but also the hum and the hum going up and down with volume pot.. "

Have you got anything connected to the input, or is it just open circuit?

Mike

Hi Mike

i do...tested with my laptop stereo plug... but this unit is mono.. maybe the hum is there because of that...just hooked it up for a quick testing...

[Levente]

by the way i just wanted to say a big thank you to you all and to this forum You guys are really helping and such a wonderful bunch of people.

Thank you .

[Boulevardier]

Have you tried short-circuiting input to ground? Perhaps best through a 0.1uF capacitor to be safe. Do you get the same hum when you do that? Might also be worth trying giving the amp a bit of careful but firm physical "disturbance" - tap it hard a bit in various places with something insulated.

Mike

[Levente]

Originally Posted by Boulevardier

Have you tried short-circuiting input to ground? Perhaps best through a 0.1uF capacitor to be safe. Do you get the same hum when you do that? Might also be worth trying giving the amp a bit of careful but firm physical "disturbance" - tap it hard a bit in various places with something insulated.

Mike

i have changed the cable and the hum has gone... there is a cap to ground at the input a veeeery old black cornell capacitor..i think (but unsure) around 0.012uf or sthing like that...

it is lacking bass...a bit, could be my laptop tho...

[Robert Gribnau]

Now I have sound and music is coming through the speaker.

I am a bit lost in this thread.

What was wrong with the amplifier in the first place? Did you connect the amplifier different to the source and/or the loudspeaker after you posted your first question?

But maybe it was a bad contact of one of the valve pins. Or who knows, maybe you have shaken the 5693 back to life.

Greetings,
Robert

[Levente]

Originally Posted by Robert Gribnau

I am a bit lost in this thread.

What was wrong with the amplifier in the first place? Did you connect the amplifier different to the source and/or the loudspeaker after you posted your first question?

But maybe it was a bad contact of one of the valve pins. Or who knows, maybe you have shaken the 5693 back to life.

Greetings,
Robert

Thanks Robert...

I have no idea to be honest with you... could be either bad contacts at the light bulb going to the heaters or the 5693...it is a mystery... i might re solder that...

just one more thing.... might want to replace the output transformer...what I have is an universal low-fi Stancor push pull.... i do have a nice R.F. Gilson which I had in an EL84 push pull design...

The Stancor is 4000 ohms to 14k Ohms primary to VC

The Gilson is 5000 ohms to VC both approx 4W 35m.a.

Would this fit in this scenario?

[vidjoman]

Perhaps further details about the amplifier would help. What make/type is it so we can look at a circuit diagram etc. Just guessing is not helpful to you or us. There are amplifiers that have a dc voltage as well as the normal ac on the heaters. These usually have a ‘hum’ pot connected across the heater supply and the centre connects to the cathode of the output valves. With 6V6 valves I wouldn’t expect this to be more than 20 volts but there could be exceptions.

[Levente]

Originally Posted by vidjoman

Perhaps further details about the amplifier would help. What make/type is it so we can look at a circuit diagram etc. Just guessing is not helpful to you or us. There are amplifiers that have a dc voltage as well as the normal ac on the heaters. These usually have a ‘hum’ pot connected across the heater supply and the centre connects to the cathode of the output valves. With 6V6 valves I wouldn’t expect this to be more than 20 volts but there could be exceptions.

Someone made this in the 40s or 50s in the US

Unsure of make...its so packed with stuff and was rusty inside that it is was difficult finding out what exactly it is...posted this last year with great help from you guys...it is a push pull...and made a rough drawing ( although i think i have made a mistake at one heater pin 7 as left unconnected) but I hope the rest is correct..

could be the similar in the popular electronics.... i've also attached a pic how it looked like when i received the unit before and how is it looking today.....

[Herald1360]

I don't think it's like that Popular Electronics job- that's got a self phase-splitting output pair whereas yours looks to have a more conventional double triode amplifier / concertina phase splitter arrangement.


There seem to be quite a few bits missing in your sketch around that 6SL7.....

[Heatercathodeshort]

The heater connections of the 6SL7 are pins 7&8 not the usual octal configuration, pins 2&7. J.

[Diabolical Artificer]

I'd be wanting to check the 40v you measured on the heaters and try and trace any faults, they might come back to bite you later.

You could change the OPT, one might have a better frequency response though those Edcor OPT's have a good reputation.

"there is a cap to ground at the input a veeeery old black cornell capacitor" try unsoldering one end.

Andy.