Model 530 amp, 6L6 has no screen or grid voltage

[mauro]

Ok, I got the short straightened out and now am starting to check voltages.
6L6 plates and heater are fine but I have no screen or cathode voltage and resistor 69A (1650 ohms from screens to center tap of output transformer) is getting way too hot!
I've checked resistors and associated capacitors and all seem fine.
I've concluded since I'm getting good plate voltage the output transformer is fine as well.
Any thoughts on this one?
Thanks, Mauro

[mauro]

Sorry title is incorrect, I meant to say no Screen and Cathode voltage.

[mauro]

Sigh....
The short to the screens of the 6L6's is back.
Dont get it? Thought I found it.
I'll keep poking around but could the output transformer put out voltage and also have center tap internally shorted to ground?

[robnyc]

Mauro, the schematic shows that dual section resistor as being voltage divider that supplies both the 6L6 screens and after further filtering, the lower current voltage amplifying stages.

The output tran can not cause this problem.

Since the short to ground appeared to go away after the amp was moved around, you likely have a simple short somewhere downstream of the divider.

With power off, and meter set of resistance, use clips to attach it and poke around and see what/if anything changes. this sort of thing is common after a rework.

RobNYC

[mauro]

Rob,
Thanks for chiming in here.
You were correct, that's exactly what I did and found that one of the wires at the filter caps was in fact touching something it shouldn't have been.

I have another general question, seems that whomever was in this amp previously replaced many of the original .01 or .022 and perhaps other caps with ceramic caps in series.
I finished the cap job but left the ceramic caps in place. I always believed "a cap is a cap" but could this affect the sound or performance of the amp in any way?
It seems to be working well now but I always think it could be better as I think it's just a bit shy on the bass output.
Thanks again,
Mauro

[Ron Rich]

Hi Mauro,
I have heard arguments on this before--seems to me there is no consensus of opinion on this subject ! I tend to leave ceramics alone--after checking that they are NOT the "Solar" brand, which I instantly will replace with another ceramic, as I have had lotza problems with them. Ron Rich

[robnyc]

Jan 18, 2019 1:15:38 GMT mauro said:

I finished the cap job but left the ceramic caps in place. I always believed "a cap is a cap" but could this affect the sound or performance of the amp in any way?
It seems to be working well now but I always think it could be better as I think it's just a bit shy on the bass output.
Thanks again,
Mauro

There are several schools of opinions regarding capacitor types and their effect on audio.  In general ceramic caps are considered less desirable in that they supposedly exhibit an unusual reactance that mushes-up highs in relation to other frequencies -again, supposedly. I don't hear that effect.

In any case, if the caps you mention are in the cobra oscillator tank, they'll have no effect on frequency response. In those setups audio is derived by "slope detection". This is a process where the oscillator is shifted in frequency by the vane acting against the coil in the cart. Shifting the frequency changes the DC consumed by the circuit and since those changes occur at a rate resembling the audio on the record, once the DC and RF components are blocked and filtered out, you have audio.

If the damping silicone in the cart is dried up or missing, the vane will resonate and cause an awful screech. This is why these setups often seem weak on bass. Try putting some silicone gel or lithium grease into the cart. Try just a small blob that touches both the metal vane and the inside of the cart. Add more or remove to get the desired sound.

Rob

[mauro]

Thanks guys for all the info and helping me understand the effects of these caps.
Also picked up that interesting tidbit from Ron re: solar brand ceramic caps.