Fender Vibro Champ Amp

[Bufo Bill]

Hi all, I'm working on a Fender Vibro Champ combo amp. I'm altering the biasing of the 6V6GTA, but the plate current is resolutely at 60+ mA. I'm going through the circuit and I noticed what looks like a tant cap across two of the pins of the valve, but I can't see one on the only schematic I can find (see attached). The cap goes between pin 5 and pin 8 (cathode). It has writing on it: 1KV 330k Z59. Any ideas what it's for? Has it been added or am I using the wrong schematic? The amp is 1979 judging from the serial number.
Cheers from Bill.

[jjl]

It's a 330pF 1kV ceramic cap rather than a tant by the sound of it. If connected between g1 (pin 5) and cathode (pin 8) it may have been added to tame some HF instability, possibly by the manufacturer.

John

[Bufo Bill]

Cheers John, yes it is a ceramic disc cap, not a tantalum "blobbule" shape!
Can't turn the amp on right now, a 1k 1 watt cap just let out the magic smoke, and am short of replacements. Quite a few bad resistors unfortunately. Done the electrolytic caps, the others seem okay. Hopefully this will sort out the high current.
Cheers from Bill.

[Bufo Bill]

Does the cap filter high frequencies? How does it work?
Cheers from Bill.

[Herald1360]

It forms a low pass filter with the stray inductance of the input wiring and the series impedance of the input source.

[Aub]

How are you altering the bias? As it's auto bias, I assume you're altering the value of the cathode resistor. Just curious.

Cheers
Aub

[Radio Wrangler]

If the output bottle is creating high frequency oscillation, you might not be aware of it directly unless you've had a prod around with a scope.... or unless you happened to notice an increase in the bias current, perhaps?

Quite often, stability issues in entertainment equipment is done on a suck-it-and-see empirical basis, sometimes abbreviated to the word 'bodging'.

That capacitor is in a place which provides HF loading on the anode of the preceding valve. This creates an added zero, rolling off the gain at higher frequencies, but also rolling in phase lag. The game is to bring the gain down to les than unity before the phase lag has built up enough to reverse feedback. On top of this, valves and transistors can go unstable on their own without needing loops or much of the other stages. This is what parasitic stopper reistors are used for.

Just because a box has a famous name on it isn't a guarantee that stability has been properly sorted.

So the next step is to go looking for any hooting.

David

[Bufo Bill]

Thanks for the responses.
Aub, I was planning to alter the Bias via Cathode resistor, as the 6V6GTA (as the Americans call it) was running very hot. I have found a lot of duff resistors though so I will put it back to standard and go from there.
David, The Bias current is very high so you might be right about on the button as usual.
Cheers from Bill.

[Bufo Bill]

The Cathode current is about 50 mA when it should be mid twenties. A 1k resistor expired and I am awaiting stock, so nothing can be done for a day or two. About a third of all resistors need changing. Judging by the chrome fittings it has been in an attic for a while.
A roadie friend of my dad's (now gone to the great lock-in in the sky) once told me Fenders "Eat Valves" due to being designed to run ridiculously hot anyway, so I'm not surprised by all this.
Cheers from Bill.

[joebog1]

Correct on Fenders eating 6V6's. As you say they do run them very hot.
I am sus on that number!! 6V6GTA. Is that a Sovtek or Svetlana by any chance ?.
Joe

[Bufo Bill]

I think the 6V6GTA was branded Fender, will check tomorrow.
Cheers from Bill.

[Diabolical Artificer]

Before making changes it might be an idea to fix the present issue else you might introduce more.

How are you measuring anode current? What is Vk, should be around 12 to 15v? Ck isn't leaky by any chance is it?

Andy.

[ms660]

Originally Posted by joebog1

I am sus on that number!! 6V6GTA.

Controlled heater warm up time (11 secs. average)

Lawrence.

[Bufo Bill]

Hi, thanks for the responses.
Joe, the tube is branded "Fender USA".
Andy, I will definitely be putting this back to factory spec before making any changes.
Anode current was calculated via voltage and impedance. I measured this between pin 8 of the 5Y3GT valve which goes to the transformer and anode pin of the 6V6GTA and calculated the current that way. I can't take further measurements until I can replace the 1k resistor that burned out.
Cheers from Bill.

[Herald1360]

Originally Posted by Bufo Bill

...


A roadie friend of my dad's (now gone to the great lock-in in the sky) once told me Fenders "Eat Valves" due to being designed to run ridiculously hot anyway, so I'm not surprised by all this.
Cheers from Bill.

He's not wrong in this case!

Just taking the voltages shown in the circuit snippet, the 6V6GT is being over volted on the screen and right on the upper limit of its anode and screen dissipation ratings.

Is reliability or originality the foremost requirement?

[Valvepower]

Hello,

I’ve attached a schematic for the Champ I located whilst researching Fender Deluxe and Bassman amps in the last few days.

It shows 21V on the cathode to ground so I make that 45mA and with +20% it could be as high as 54mA, so 60mA with a very good valve isn’t out the question. This could be exaggerated by a higher mains and subsequently a higher HT voltage.

Check g1 is at ground potential to make sure the coupling capacitor isn't leaky.

As for the 330pF capacitor I agree it could have been added to stop instability. I noted there are no grid stoppers, which isn’t uncommon in Fenders, and to keep faithful to the original design they may have opted to use the capacitor instead of adding g1 stopper?

Having done time building and working on guitar amplifiers I’m guilty of solving stability issues in entertainment equipment using a suck-it-and-see empirical methods… And, I stand guilty as charged Ma Lord!

Fender took the ‘ole 6V6 within an inch of its life so the 6V6’s must be good ‘uns.

Ps. The cathode voltage reading and thus the calculated current will include the screen g2 current.

Terry

[Valvepower]

I assume your measuring just the current flowing to the output transformer and not the entire HT supply and current?

Going back to my last post the cathode voltage reading and thus the calculated current will include the screen g2 current so the actual anode current will be lower than the cathode current – I think that right... The napper is overheating in this weather!

Terry.

Originally Posted by Bufo Bill

Anode current was calculated via voltage and impedance. I measured this between pin 8 of the 5Y3GT valve which goes to the transformer and anode pin of the 6V6GTA and calculated the current that way.
Cheers from Bill.

[Bufo Bill]

Originally Posted by Herald1360

Is reliability or originality the foremost requirement?

This is a user not a museum piece, but I don't want to gut the thing entirely, unless essential.
Cheers from Bill.

[Bufo Bill]

Originally Posted by Valvepower

Going back to my last post the cathode voltage reading and thus the calculated current will include the screen g2 current so the actual anode current will be lower than the cathode current – I think that right... The napper is overheating

You've got me there Terry, I'll let you know when we're back and functional.
Cheers from Bill.

[Bufo Bill]

Postie's been and no 1k's, so it'll be Monday before things move further folks.
Cheers from Bill.