Dynaco ST70 speaker pops on power down

pdxwaker wrote:My Stereo 70 causes some pops on my speakers after turning the amp off.

This is not related to the switch... it occurs well after turning the switch off (and also occurs if I use a power strip / relay).

Preamp is still powered when this happens.  I've tried it with and without source active... with source, the sound fades a bit before pops occur.  It is like the caps are bleeding off then something happens during the bleed.

Any ideas?

peterh wrote:

pdxwaker wrote:My Stereo 70 causes some pops on my speakers after turning the amp off.

This is not related to the switch... it occurs well after turning the switch off (and also occurs if I use a power strip / relay).

Preamp is still powered when this happens.  I've tried it with and without source active... with source, the sound fades a bit before pops occur.  It is like the caps are bleeding off then something happens during the bleed.

Any ideas?

Schematics  ?  I assume this is something else then a Dynaco ST70

pdxwaker wrote:

peterh wrote:

pdxwaker wrote:My Stereo 70 causes some pops on my speakers after turning the amp off.

This is not related to the switch... it occurs well after turning the switch off (and also occurs if I use a power strip / relay).

Preamp is still powered when this happens.  I've tried it with and without source active... with source, the sound fades a bit before pops occur.  It is like the caps are bleeding off then something happens during the bleed.

Any ideas?

Schematics  ?  I assume this is something else then a Dynaco ST70

It is a Dynaco Stereo 70 (1960s, cloth insulation on output transformers).  New to me project that was recapped (by me) and new output tubes.

A/B/C/D caps are mounted in the chassis.  47uF 600V, 100uF 550V, 100uF 500V, 100uF 500V.  Each of these are bypassed by 1uF 600V film cap.  I tried to keep the first cap smaller (to limit in rush stress on rectifier tube) but was very limited in available 600V caps.

Everything is working fine, stable bias, good sound.  But, I'm getting this 'pop' through speakers on power down.  I'm to the point where I may add a speaker switch box and switch in 8 Ohm power resistors at power down.  But, that won't help if there is amp/tube stress too.

pdxwaker wrote:

peterh wrote:

pdxwaker wrote:My Stereo 70 causes some pops on my speakers after turning the amp off.

This is not related to the switch... it occurs well after turning the switch off (and also occurs if I use a power strip / relay).

Preamp is still powered when this happens.  I've tried it with and without source active... with source, the sound fades a bit before pops occur.  It is like the caps are bleeding off then something happens during the bleed.

Any ideas?

Schematics  ?  I assume this is something else then a Dynaco ST70

It is a Dynaco Stereo 70 (1960s, cloth insulation on output transformers).  New to me project that was recapped (by me) and new output tubes.

A/B/C/D caps are mounted in the chassis.  47uF 600V, 100uF 550V, 100uF 500V, 100uF 500V.  Each of these are bypassed by 1uF 600V film cap.  Too large?  I tried to keep the first cap smaller but was very limited in available 600V caps.

Everything is working fine, stable bias, good sound.  But, I'm getting this 'pop' through speakers on power down.  I'm to the point where I may add a speaker switch box and switch in 8 Ohm power resistors at power down.  But, that won't help if there is amp/tube stress too.

peterh wrote:

It's not impossible that the excess amount of caps could give this problem. An original cap
might be emptied faster.  Too large caps (and no screen stoppers ) could cause
oscillations which might be heard as "pops".

pdxwaker wrote:

peterh wrote:

It's not impossible that the excess amount of caps could give this problem. An original cap
might be emptied faster.  Too large caps (and no screen stoppers ) could cause
oscillations which might be heard as "pops".

Thanks.  I'm a total newbie to tubes...

Would you recommend I investigate adding screen stoppers?  I'll do some research but feedback is welcome!

j.click wrote:Same thing happened to me. Someone far wiser than me explained that when you switch off the ST70 the magnetic field in the power transformer collapses and sends a surge/pop through the speakers - clearly audible on my Quad ESLs. Also some of these power switches are over 50 years old and a simple Dynakit replacement switch is safer. A simple AC suppression cap across the AC switch solved the pop issue.

peterh wrote:
As a start i would recommend filter caps 30+20+20+20  or similar.  Screen stoppers 100ohm2w prefereable CC  will make the amp more resistant to oscillating
See http://tronola.com/html/maximize_tube_life.html  for a discussion of how too large caps might cause instability.