• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

want to do PP-GM70

Status
Not open for further replies.
Jump to Latest
#21
I noticed a big mistake in the design of the output bias circuit. At the minimum you really, really need some high quality capacitors from the bias pot wipers to ground. Better still do what I suggest a couple of lines further along in this post. The transformer secondaries are floating off the 22K pots and the source impedance as well as ac signal level seen at the grids will differ depending on the position of the wiper. This is meant to be a dc adjustment only as well. I would recommend inserting a 50K resistor between the bias pot wiper and the secondary of the interstage transformer, in addition I would recommend installing about a > 0.33uF film cap from the cold end of each winding to the cathode return of the output tubes. This will provide a very well defined ac ground at the cold end of the driver transformer. Should sound a lot better too as the non-linearity of the wiper will not be an issue.. Your measured phase shift is related to the current configuration as well.

Kevin

Edited to improve clarity
 
#23
I have new bad thing . ITS get very hot just 50 ma bias same the circuit on the top.
the old once has no ITS work very good.
 

Attachments

  • new1.jpg
    new1.jpg
    38.3 KB · Views: 1,682
Status
Not open for further replies.
Top Bottom