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I just followed the instructions that a kind gentleman posted for me on how to test the voltage at various points on my ST-70 as I am trying to figure out why it distorts, and the tubes start to flicker at a high volume. While testing I got a reading of 801 volts AC betweet pins 4 and 6 on the 5AR4 rectifier tube socket, it's supposed to be 360, any idea what is throwing it off so violantly? Also, is there any way for me to test the filter cap?Thanks
MMMbop
> why it distorts, and the tubes start to flicker at a high volume
Hi,
It aint the case that the feedback curcuit is in the same condition as the bias curcuit was? Broken feedback would mean some distortion from lack of feedback (that one can cope with) and a lot of distortion coz the amplification is to high (all tubes driven far to much at the grid (G1)). Check at point 12.
/Pär
"While testing I got a reading of 801 volts AC betweet pins 4 and 6 on the 5AR4 rectifier tube socket, it's supposed to be 360, any idea
what is throwing it off so violantly?"The transformer is actually 720 volts *center tapped*, if
you check between pins 4 & 6, you'll get at least 720VAC
(probably more , since most people have higher AC line
voltage than when ST70's were made), with the transformer
loaded (rectifier tube in and EL34's biased up). If you are
checking this with the the 5AR4 pulled, it's unloaded and
you get a noticeably higher voltage.ST70 is not a Class A amp, the tube current changes
if you've got the amp cranked, the tubes could certainly
flicker with the music (some tubes do this more than others).
I think your distortion problem is on the board or
because of a burnt power supply resistor.You can download the manual at http://www.curcioaudio.com,
it's got voltage checkpoints you can check.
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