Vox AC30/6 TP Bias Question

[opacheco]

CJ and Dan,

Thanks for your comments!!.....but the Mullard GZ34 and Tung-Sol EL84 are NOT NOS tubes!! they are new production tubes for www.tubestore.com

Opacheco.

 

[opacheco]

Well, after I changed the R70/R71 to 120 Ohms each, I got a lower bias current (from 55miliamperes to 47miliamperes), everything look like ok; no red plate at all!!!

I have had it turning on (standby switch on of course) for at least 1 hour (and now is on until!!) and the voltage in the cathodes of EL84 never goes up to 11.3 DCVolts; someone have a comment for this bias mod?? Is correct change these resistors in order to low the bias instance to sacrifice the warm sound a bit???  Do someone have done this before??

Thanks
Opacheco.

 

[MagnetoSound]

It's a fairly common compromise. As was mentioned earlier in this thread, the designer of this amp was pushing the tubes past the limit of their design rating - not such a problem in the glory days of Mullard (although even then redplating was not unheard of), but much more difficult in these days of cheap Eastern manufacture.

If you can find the sonic compromise (you will lose some harmonic richness and sustain but hopefully avoid crossover distortion), I imagine your wallet will thank you for it.

 

[opacheco]

It's a fairly common compromise. As was mentioned earlier in this thread, the designer of this amp was pushing the tubes past the limit of their design rating - not such a problem in the glory days of Mullard (although even then redplating was not unheard of), but much more difficult in these days of cheap Eastern manufacture.

If you can find the sonic compromise (you will lose some harmonic richness and sustain but hopefully avoid crossover distortion), I imagine your wallet will thank you for it.

Dan, that si a good point!!.....I will follow your advise!!

Thanks a lot!
Opacheco

 

[firehazard]

Hi,

sorry to have missed your question.

If your amp has a standby switch then the information I gave you might be bad.

My amp is from 1964 or so.

I have no experience of the modern version.

So while your bias voltage seems good you have a few things to think about.

47ma seems a little close to the 50ma I seem to remember as max for an EL84. You should be able to run a lot colder before crossover distortion is an issue. 

Warm tone should not be a problem here. No negative feedback. Designed for Rock and Roll.

Take care

 

[opacheco]

Hi,

sorry to have missed your question.

If your amp has a standby switch then the information I gave you might be bad.

My amp is from 1964 or so.

I have no experience of the modern version.

So while your bias voltage seems good you have a few things to think about.

47ma seems a little close to the 50ma I seem to remember as max for an EL84. You should be able to run a lot colder before crossover distortion is an issue. 

Warm tone should not be a problem here. No negative feedback. Designed for Rock and Roll.

Take care

firehazard,

You know what?; I installed the two 120 Ohms/ 5 Watts Resistors and everything look like ok!!...No red plates and the Cathode measured voltage was 11.3 Volts ( it was taken after one hours and half turning on for established temperature I suppose!) According to my electronics analysis this is 47miliamperes per EL84; exactily to the matched value from The Tube Store tubes.

The amp is playing great now!

Opacheco.