silicon diodes as 'failsafe' feature on tube rectified amp?

[R.G.]

thanks for the help everyone,

I've just taken the GZ34 out, and soldered a pair of 1N4007s directly on the valve base pins.

There does appear to be more hum now - is this 'self noise' from the diodes? maybe some caps accross them would sort it out (?)

If you have put in ONLY the 1N4007's, then yes, you may be hearing the turn-off transient on the 4007's. You can snub this with a 0.01uF 1kV ceramic disk or a custom snubber for a better job across each diode. Even better, get UF4007 diodes. They're 1A, 1000V like the 1N4007, but are fast recovery diodes and don't have the slow recovery and hard snap-off of the slower ones.

If you have the two diodes in series with the GZ34, then no, the GZ34 turn on/off will be slower than the 4007's and so the 4007 transient is masked and it's something else - probably. Mother Nature is a ... um... mother about some of these things and we have to play the chances.

[R.G.]

R.G.,
You are correct.
Are you a lawyer? Of course my reply was limited to this situation. I just don’t have the time or the inclination to qualify every response with a list of possible mistakes people could make that would cause a problem.
Cheers,
Tom

I don't have the inclination either; but sadly, I have been taught by the school of hard knocks. I have very clear opinions about technical matters, and I've learned to make sure that everyone who reads them have a sense of where the gray areas are and not just that I'm putting in a black and white situation.

Plus, I hate to mislead any beginners who have not yet developed the experience to realize where they are setting a trap for themselves in a few years when they have forgotten why they did that. I have some scars from doing that to myself.

As for calling me a lawyer... I'll get you for that....

[HTH]

I've put some UF4007s in the amp - no difference.

Maybe I'm just being a bit anal about the noise - these Bluesbreaker reissue amps ARE noisy right from the start.

Once the volume is cranked it's a moot point anyway. Strange thing is that the hum (a high hum rather than a low-pitched one) is cancelled out if I turn up the normal channel volume to around 6 - wierd!

Lastly, thanks for all the responses - much appreciated.

[HTH]

If the amp is still working at all with the GZ-34 removed then you have installed the diodes in a different manner than we have been discussing. To which pins are the diodes connected? I suspect that you effectively "converted" the amp to a SS rectifier.

Tom, yes I converted the amp over to SS rectos - it just sounds better overall in the bass end for rock playing.

I've just soldered to UF4007s directly onto the socket and removed the 5v heater wires from the socket.

[TD_Madden]

I just install Ted Weber's CopperCaps.....best of both worlds, I think.

[Bruce / Mission Amps]

Strange thing is that the hum (a high hum rather than a low-pitched one) is cancelled out if I turn up the normal channel volume to around 6 - wierd!

Lastly, thanks for all the responses - much appreciated.

There's a good example of an audio ground loop issue.

[dai h.]

I guess that's something to do with reaching the halfway point between DC and AC ground?

[HTH]

There's a good example of an audio ground loop issue.

I would personally have converted this whole amp to a star ground system but my friend said the hum wasn't that bad. It's just using Marshall's stock grounding scheme, nothing changed.

[catnine]

I have a pair of 3 amp diodes here but they are only rated at 400 volts so they might not be high enough voltage wise they are 1N5404 maybe they are 1N5408 I need to look ,and much larger so I guess I need some 1N4007 diodes to do the job.