Beltone Teisco Valco Lifco 630 amp repair

[omnitek]

Hi everybody,

I'm trying to repair a Beltone guitar amplifier.
Apparently, this amp was made by Teisco and the circuit is similar to a Lifco 630 amplifier. It might be similar to another Valco amp also.
There is four 6AV6 preamp tubes, two 6BQ5 power tubes and one 5Y3 rectifier tube.  This combo has two 10" speakers.

So far, I replaced the two 6BQ5 power tubes with two new EL84 power tubes (equivalent). I also replaced the 5Y3 rectifier.

The amp is working but the B+ seems to be low for that kind of amp. I'm measuring 260Vdc. I'm also measuring 23V on the 150 ohms resistor which also seems to be high for that kind of amp. When I remove the power tubes, the B+ goes up to 400Vdc. I replaced the filter capacitors and the coupling capacitors on the power tubes but that did not help.

The voltage on the power transformer is 2 x 300V @ 80mA. So I think the B+ should be higher than what I'm measuring.

Anybody knows those kind of amps and could help me?

Thank you!

Nicolas
 








 

[sluckey]

I'm also measuring 23V on the 150 ohms resistor

That equates to 153ma flowing thru that resistor. That's why your B+ is so low. And a PT that can only supply 80ma @300v will likely slowly roast. I assume that resistor is connected to the cathodes of the output tubes? Those tubes will roast too.

Measure and report the voltages on all pins of the EL84s. Grids (pin 2) should be zero. A positive voltage on the grid will cause the tubes to conduct hard. Coupling caps would be a likely suspect. (I know you replaced them) Don't leave it on very long until this is resolved.

[stratele52]

Bias are not too hot too drop so many voltage  ?

I think the problem is a bigger than I read because you are a pro in fixing amp .

[PRR]

Yeah, leaky grid-caps or open grid resistors.

[omnitek]

I'm also measuring 23V on the 150 ohms resistor

That equates to 153ma flowing thru that resistor. That's why your B+ is so low. And a PT that can only supply 80ma @300v will likely slowly roast. I assume that resistor is connected to the cathodes of the output tubes? Those tubes will roast too.

Measure and report the voltages on all pins of the EL84s. Grids (pin 2) should be zero. A positive voltage on the grid will cause the tubes to conduct hard. Coupling caps would be a likely suspect. (I know you replaced them) Don't leave it on very long until this is resolved.

I just found out that the grid resistors and capacitors were connected on pin 1 instead of pin 2.
I think that is because those pins were internally connected in the original 6BQ5 tubes but not in the EL84 tubes.

I connected pin 1 and pin 2 together and now all the voltages are back to normal:
Plate: 330Vdc
Screen: 285Vdc
Cathode: 10Vdc

Thank you for you help!

Nicolas

 

[stratele52]

Salut Nicholas ,
Pin 1 and 2 are not internally connected in 6BQ5 like in EL84 if where refer to RCA receiving tube manuel .  This mean your EL84 have no  negative  bias voltage on their grid on original amp.


Like I write before ; bias where too hot to drop so many voltage .

[sluckey]

Well, having the signal grid open was definitely causing the bias to be undetermined and allowing the tube to draw excessive current. Glad it was a simple fix. Strange though. I don't remember pin one ever being connected to the grid on a 6BQ5.

I'm curious about something else though... You originally said...

The amp is working but the B+ seems to be low for that kind of amp.

And later said...

I just found out that the grid resistors and capacitors were connected on pin 1 instead of pin 2.

So how could the amp have been working? The signal was not geting to the grids of the output tubes?

[sluckey]

Like I write before ; bias where too hot to drop so many voltage .

But you originally said...

Bias are not too hot too drop so many voltage  ?

Seems to be a direct contradiction, or am I misinterpreting???

[stratele52]

Bias are not too hot ?

Look at the " ?  " Whith this  this " ? "  I mean "are you sure the bias is good  ? " . For me it is sure there is a broblem with bias.

[sluckey]

OK. I just didn't understand.

[stratele52]

No problem Steve, myself I do that too many times.