Reverse diodes on guitar amp output pentodes - diyAudio
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Old 19th October 2012, 01:20 AM   #1
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Default Reverse diodes on guitar amp output pentodes

On (newer) tube guitar amplifiers, I've noticed reverse diodes (two 1N4007's in series) plate-ground on each pentode O/P tube bank. For example, Marshall JCM 800 schematic
I wonder what these diodes are really about, and if they are good practice.
It looks like they would prevent back emission "... it is apparent that there is electron flow from the plate to the grid [for triodes] during the negative excursion of the plate."
But for pentodes, the screens still would have +ve potential and I'm not sure what happens with the negative plate voltage.
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Old 19th October 2012, 02:24 AM   #2
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You would do better to ask over at the Instruments and Amps section - the mods will probably shift this post there anyway.

It is often discussed on the various guitar amp forums.
In a Class AB Push pull Amp the tubes can be run into cut off (zero current). That tube then looks like a turned OFF switch and that end of the push pull output transformer is effectively disconnected. Theoretically the anode can then swing up to 2 x the High Voltage rail and down to 0V. That is for a properly loaded transformer. If you have a busted speaker cable or an open circuit (blown up) speaker then all sorts of back emf and induced voltages can occur and it is quite possible for the anode to swing negative in this case (It is probably even worse for the case of an intermittent speaker connection which is making and breaking). Those reverse connected diodes are there to clamp any negative going (below 0V) spikes from possibly damaging your output tranny by high voltage punch through of the insulation.

It could also be argued that they give some protection from extreme positive voltage spikes too, by absorbing the spike as they blow up, as their PIV (Peak Inverse Voltage) rating is exceeded. Of course the amp probably still needs repair but replacing a couple of 2 cent diodes is much preferable to replacing a $100 Output Transformer.

There doesn't seem to be any real consensus as to the best scheme to try and protect output trannies. The fact that I have seen amps where those diodes have blown up but the transformer is still intact suggests that they can help.

Personally I prefer what is called the "voltage tether" method of protection. I keep an effective (minimum) load on the primary windings at all times (even when the tube is cut off) by adding say 220K resistors from each anode to ground (so that no winding actually sees a total open circuit) and similarly add a 220 Ohm across the secondary winding so that it has some load as well in the event of an open circuit (or not plugged in) speaker or speaker cable.

That 220K must be able to withstand 2 x the High Voltage rail so is usually made up of several resistors in series.

Hope this helps,

Cheers,
Ian

Last edited by gingertube; 19th October 2012 at 02:37 AM.
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Old 19th October 2012, 04:37 AM   #3
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It protects the amp against inevitable damage when the speaker got suddenly disconnected during playing in overdrive mode. When I repaired amps with blown output transformers, sometimes even welded together pins 2 and 3 on output tube sockets, after fixing all disasters I always put such diodes. The amps with added diodes never come back.
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Old 19th October 2012, 05:41 AM   #4
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Quote:
Originally Posted by gingertube View Post
It could also be argued that they give some protection from extreme positive voltage spikes too, by absorbing the spike as they blow up, as their PIV (Peak Inverse Voltage) rating is exceeded. Of course the amp probably still needs repair but replacing a couple of 2 cent diodes is much preferable to replacing a $100 Output Transformer.

There doesn't seem to be any real consensus as to the best scheme to try and protect output trannies. The fact that I have seen amps where those diodes have blown up but the transformer is still intact suggests that they can help.

Personally I prefer what is called the "voltage tether" method of protection. I keep an effective (minimum) load on the primary windings at all times (even when the tube is cut off) by adding say 220K resistors from each anode to ground (so that no winding actually sees a total open circuit) and similarly add a 220 Ohm across the secondary winding so that it has some load as well in the event of an open circuit (or not plugged in) speaker or speaker cable.

That 220K must be able to withstand 2 x the High Voltage rail so is usually made up of several resistors in series.

Hope this helps,

Cheers,
Ian
how about 1n4007's in series shunted by these "voltage tether" resistors,? i would add 1meg resistors in shunt with those anyway....so the 220k is a better choice.....
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