A tale of two Vibro Champs

[cbass]

An barely loaded B+ winding may be running probably 10% high, and the 5Y3 won't be dropping any volts.
The combination of those could cover the 80 to 100V excess on the B+.
With good tubes in place, all could be fine.
A good 5Y3 means one that was made in the USA.

That's what I was thinkin but more like a 70 to 80 volt drop.I think the b+ needs to get below 400 for any kinda tube life.

[xtian]

Are you running it via a light bulb limiter?
It may be a good idea to do so, until probs are resolved.

Tru dat.

[xtian]

Here's the report on the 1973 model (that's the date stamped on the inside of the chassis).

Recap went perfectly. Voltages are just as high as the other. 750vac unoaded. With recto, 475vdc at B+. Fully loaded, 6.6vac on heaters, 432vdc plate, 29.2vdc cathode, giving me a bit over 90% dissipation.

Sounds great, no issues. Tremolo ticks a little, but not badly.

Of course I will report all the warnings to the owner about the high voltages, but he doesn't want to pay any more than he needs to get it sold.

Now back to the 1964 unit...

[xtian]

I tried a different 6V6 in the 1964. First, with the bulb limiter. Worked OK with the limiter. Didn't run away. Although I was seeing 33vdc on the cathode, 180vdc on the plate. Which, when you scale it up....see next:

Testing without the bulb limiter now. Same problem. Current thru the 6V6 wants to run wild. Was seeing 80+ vdc on the cathode. I have an infrared temp sensor, and I saw the resistor quickly rise to 150 degrees before I switched it off.

So, why the runaway?

[xtian]

Answer! Because! The grid reference to ground thru the 220K is broken.

[xtian]

I put a 1K, 25W resistor between the recto and the first reservoir cap. It drops the plate down to 347vdc, the cat to 28.1, giving me 70% dissipation!

[cbass]

1k might be a bit extreme

[xtian]

1k might be a bit extreme

In what respect?

My voltages now closely match Fender's spec:

http://vintagefenderamprepair.com/wp-co ... ematic.pdf

[pdf64]

Well done identifying the bad grid leak resistor!
But with a 1k cathode resistor, won't the cathode current and plate dissipation be rather different to Fender's design?

I put a 1K, 25W resistor between the recto and the first reservoir cap. It drops the plate down to 347vdc, the cat to 28.1, giving me 70% dissipation!

I don't see the relevance of 70% to a cathode bias SE amp? It seems likely to result in an over biased condition with consequent compromised performance, eg limited power output.

[matt h]

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[Stevem]

Your lucky the OT in one is still good, or the owners lucky I should say!

[cbass]

1k might be a bit extreme

In what respect?

My voltages now closely match Fender's spec:

http://vintagefenderamprepair.com/wp-co ... ematic.pdf

Power dissipation and voltage drop. Your dissapating nearly 7.5 watts constantly that thing is gonna get way hot. Your droipping more voltage than you really need to. just get the b± under 400 . that should get you in the 370 or so range for your plate voltage. Measuring plate to cathode .
If you lower you rk that 1k 25watt will be dissipating even more
A couple of 135ohm 10 or 15 watt on the transformers secondaries before the rectifier might be a better route .
Maybe someone else will chime in on that?

[matt h]

(deleted)

[cbass]

Matt knows a lot more than I do for sure.which isnt saying much but I have a single ended amp that had the same problem. I used a 100ohm 15watt resistor on each secondary it helped and didn't get to hot.I ended up building a bucking transformer for it and it worked great.I think I left the resistors in there. It had a 330ohm ck stock and the 6v6 was dissipating almost 20 watts!the wall voltage is typically 122-123 around here

MOSFET zeener probably would be the best most cost efficient way but it still won't address higher heater voltages.

I know it's the cusdtumers amp but I bet if the the input voltage was about 115 with the stock rk the voltages would be pretty close.
Id probably just leave it make sure its not redplating or other problems.and give it back to the owner with a warning that vintage amps need to variaced or something to run safely and tube life may be decreased.

[xtian]

Really appreciate all the advice and insight. I would certainly keep tinkering if it was up to me, but I was given a mandate: CHEAPEST.

Great experience, though. Have not heard an 8" speaker for ages, and these two Vibro Champs sound great! They don't flub out, the distortion is very nice, the tremolo is nice, the tone stack is nice.