Diodes used WITH rectifier tube......thoughts.....

[Paultergeist]

Building a Rocket clone, I am scratching my head on the use of diodes mounted at the rectifier tube socket......yes, I realize this is a common mod, done for the safety of the amp -- should the rectifier tube ever have an internal short plate-to-cathode......

But.......if I add these diodes, am I not then really using a solid-state -- as opposed to tube -- rectifier? The diodes would be the enitities really rectifying, correct? Also, how would I be certain IF the rectifier tube was functioning correctly in such an implementation? In fact, won't the amp still play even if rectifier tube is pulled if these diodes are in place? (and maybe that is the point)?

Some of the circuits which employ these diodes add a small cap across the hot legs at the rectifier tube, presumably to reduce diode switching noise -- this almost seems to me like another potential (albeit unlikely) failure point: if that small cap ever failed in the closed position.....?

I guess I need to first really hear the amp with the tube recto.....

Am I way off-base?

[martin manning]

Building a Rocket clone, I am scratching my head on the use of diodes mounted at the rectifier tube socket...

...if I add these diodes, am I not then really using a solid-state, as opposed to tube rectifier? The diodes would be the enitities really rectifying, correct? Also, how would I be certain IF the rectifier tube was functioning correctly in such an implementation? In fact, won't the amp still play even if rectifier tube is pulled if this diodes are in place? (and maybe that is the point)?

If the silicon diodes are in parallel with the vacuum rectifier diodes then yes you will be solid state rectified since the resistance of the Si diode is much less than the vacuum diodes. To use them in a safety mode, put the Si diodes in series with the vacuum diodes (between the PT secondaries and the vacuum diode anodes), so they will function as back-up.

Place them on the socket from pins 3 to 4 and 7 to 6 (cathodes to pins 4 and 6), then connect the transformer secondaries to pins 3 and 7.

[Paultergeist]

Thank you for the reply, Martin.

Since the silicone diodes would get "first crack" at rectifying the HT, does such a configuration (PT scondaries -- to silicone diodes -- then tube rectifier) still provide whatever effect is produced (softer attack, sag) by a standard tube rectifier set-up?

[martin manning]

No problem. Both types of diode only conduct when forward biased, and the silicon diode will only drop a volt or so. The following vacuum rectifier will exhibit its usual 10-60V drop depending on type and current flow.

[Paultergeist]

Okay then.....do you have a thought regarding the practice of bridging across the diodes -- if used -- with a small value capacitor?

Thank again for the support.

[martin manning]

Feel free to add a capacitor across each silicon diode if you like. Some people say you can hear the switching noise, but I don't think it is much of an issue in a guitar amp. If multiple silicon diodes are used in series, then placing caps across them will help insure that they share voltage equally, and that is worthwhile.

[tubeswell]

I put the SS diodes (in series with the tube rectifier anodes) without snubber caps and can't hear any switching noise. I daresay I think if there was any, it'd get mushed by the tube rectifier

[Paultergeist]

Feel free to add a capacitor across each silicon diode if you like. Some people say you can hear the switching noise, but I don't think it is much of an issue in a guitar amp. If multiple silicon diodes are used in series, then placing caps across them will help insure that they share voltage equally, and that is worthwhile.

"across each silcone diode....?"

NOT bridging across the two legs of the HT leads at the diodes?
(a al Ceriatone "Rock It" kit layout?)

[tubeswell]

Feel free to add a capacitor across each silicon diode if you like. Some people say you can hear the switching noise, but I don't think it is much of an issue in a guitar amp. If multiple silicon diodes are used in series, then placing caps across them will help insure that they share voltage equally, and that is worthwhile.

"across each silcone diode....?"

NOT bridging across the two legs of the HT leads at the diodes?
(a al Ceriatone "Rock It" kit layout?)

.01uF 1kV snubber cap goes in parallel with each SS diode

[Paultergeist]

Martin, Tubeswell,

I think I am getting a handle on this.

Thanks very much for the assistance!