Silvertone 1482 heater voltage & popping

[slapbackdaddy]

Hi all,

My Silvertone 1482 has recently been re-capped and re-tubed (except the 6X4 and the 12AU6). Has been purring like a kitten, sounding really great, switched it on today to be met with the occational popping and a hum through the speaker. The hum gets a little louder when i turn up either of the volume controls but then stays at a constant volume even if i turn the controls to max. The popping is very intermitent, have been through with my trusty chop stick and can't make it happen by touching any componants. I've pulled tubes, replaced tubes, makes no difference. The only tube i don't
have a spare of is the 6X4 rectifier which is original 1965. The tubes sockets have been cleaned and tensioned and all the remaining resistors are within spec.
I did take some voltage measurements and the heater voltage to the tubes is reading 3.15vac....should be 6.3vac. The plate on V1 is 113v...correct going by the schematic i have, the plate on V2 is 184v...the schematic gives 138v, so seems a bit high.
The paltes on the 6v6's are 358v & 358v and the cathode current 35.3ma and 35.2ma....seems a bit high for a pair of 6v6's????
Am i right in thinking that the bias is set by the 270ohm 5w resistor to ground from the cathode of one of the 6v6's (they share this?). The resistor reads 268ohms and is new and good.

I realise there's a few different questions here but to sum up.
I have popping!
I have hum!
I have low heater current!
I have possibly a high bias setting!

Could most of this be caused by a failing rectifier tube....do they tend to do this or do they either work or not.

Many thanks in advance.

[rf7]

You need to measure heater voltage across the tube to get 6.3Vac. Put your ground probe on pin 9 and put the other probe on pin 4 or 5 (for 9 pin tubes), and you will get 6.3Vac. So, I think your heater voltage is fine.

Does it still hum with the first preamp tube pulled out?

[slapbackdaddy]

Ahhhh, OK, will try that, thanks for putting me right. I see, the voltage is split where i'm measuring...hence 3.15v...half of 6.3v
Yep, still hums, but not as loud with first tube pulled, still pops too. Sounds like an type earth hum?

[Enzo]

Your 6V6s are idling at about 12 watts. Remember it is the voltage across the tube - cathode to plate - not the voltage to ground - that matters. SUbtract the cathode voltage from the plate voltage to get the tube voltage - or just measure it.

Yes, the bias is created by the cathode resistor.

Hum can come from more than one source. Grounding, shielding, power supply ripple, etc. Mismatched power tubes will hum more than matched. MAtched tubes in an unbalanced circuit will also hum. If one side of the push pull is not running - like an open screen lead maybe - wil cause hum.


Popping? SOmeting breaking down. Turn all the controls to zero. Still there? ANy control that affects the noise is AFTER the source of the noise. So if the EQ makes the noise sound different, then it is from before the EQ, for example.

Scope the circuit to see where the noise is.

[slapbackdaddy]

Thanks Enzo,

I already did calculations for the bias setting, realised that gives around 12w....12.63w to be precise, correct me if i'm wrong but isn't 12w the MAX dissipation of a 6V6GT tube? Shouldn't it be more like 70% of that? i.e. around
8-9w.
I presume that using a higher value resistor would lower the cathode current?

What do you think about the higher plate current on the PI? (184v....should be closer to 138v)

The popping happens with all controls at zero, i've been through and checked every possible earth conection i can including tag boards and rivets etc.
Sounds like something is breaking down somewhere.

The tubes were a well matched pair, sounded great before this started happening.

Any thoughts on the rectifier tube question?....do they just give up or gradually break down.....what's the usual case with these tubes?

Unfortunately i don't have a scope to check the amp.

Thanks

[Sweetfinger]

IIRC the power supply dropping resistors, which are soldered to the lugs of the can-cap, are carbon comp, and lower watt than you'd like, so they tend to roast . Have a look.

[slapbackdaddy]

Thanks Sweetfinger

The dropping resistors and cap can are new, had weber make the can, the resistors are OK, tested them yesterday, both within spec.
Sorry, for future reference, whats IIRC? Not used to these abbreviations

[danielzink]

If I Remember Correctly

[MWJB]

Max dissipation of a 6V6GT is 14W, your amp is cathode biased and can be biased anywhere from 60% up to 100% plate dissipation (sometimes a little more, but 90% is a safe max). They'll also take up to 40mA easily at the (uncorrected) voltages you mention. You don't need a higher value cathode resistor unless you are deliberately trying to alter the character of the amp.

Check for any arcing/possible shorts/solder drips around the hot leads of your new filter caps?

Pull all the preamp tubes, still popping? Then suspect power tubes & sockets. If the popping stops it's happening earlier in the circuitry, start refitting preamp tubes, one by one, working from rectifier end of the amp to narrow down where the pops are coming from. (might also help narrow down where the hum is coming from - though many cheaper, old amps do hum without extensive work).

If you used a can style filter assembly, scrap it and replace with individual filters, double the first filter cap value after the rectifier, ground this cap & screen supply cap at a PT bolt. Ground PI/preamp caps (at original value) at input jack.

I wouldn't be overly concerned about the increased voltages on their own, wall AC supply has risen over the years. Any voltages on old schematics from 50's & 60's will be wrong.

I'd be surprised if it was the rectifier (previous step will eliminate it), they usually work or they don't.