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Trainwreck pages..............diodes from plate to ground?

8/3/1999 4:06 PM
Benjamin Fargen
Trainwreck pages..............diodes from plate to ground?
I was glancing through the the Trainwreck pages and noticed a diagram for running 3 X in4007 diodes from each plate to ground to protect the power tubes and output tranny from damage.  
Can someone elaborate on how this works and if it is so beneficial, why don't more amps do not come stock with this setup?  
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8/3/1999 4:18 PM
Jim S.

The diodes act as voltage spike protectors. Three 1N4007 diodes in series creates a net PIV (peak inverse voltage) of at least 3000V. If the voltage on the output tube plate spikes to a value greater than this, it will get shunted to ground through the diodes rather than arcing across the tube socket or within the output transformer.  
I've heard conflicting reports of whether or not these diodes cause any changes in amp tone. If there is any, I would think it would be very subtle. Maybe someone else has had some experience with this.
8/3/1999 9:49 PM
John Stokes
There is some misinformation here. The diodes are in series to develop a 3000 PIV to keep them from zener breakdown during normal operation. If you look at the plate waveform with an o-scope, you will see that when the tubes on a side are cutting off (tubes on the other side are turning on hard), the plate voltage rises to double (or more, depending on leakage inductance and load quirks) the center tap voltage. The diodes are there to conduct if the plate voltage goes negative (clamping the voltage at ground to prevent it from going negative), again, due to leakage inductance and load quirks. One of the problems with this is if things are misbehaved, the diodes get whacked with fairly high peak currents and eventually fail to shorted condition. IMO, if you were going to do this, I'd put a 100 ohm current limiting resistor in series with the diode string.
8/3/1999 10:03 PM
When the diodes are in series, the first diode donīt draw the whole spike? , like the capacitors in series  
Plossible damage occur?
8/4/1999 2:41 AM
Steve A.

When the diodes are in series, the first diode donīt draw the whole spike?  
    You are probably right, and I think that is why they use 3 of them in series: if the first one fails as a short circuit then there are still 2 diodes for backup. Of course if the first diode was to fail as an open circuit you'd probably never know unless you tested it. I've put them in amps to make them more "bulletproof" and didn't notice any change in tone.  
    BTW I've seen quite a few factory schematics that include a single diode instead of the 3 in series.  
Steve Ahola
8/3/1999 10:26 PM
Reid Kneeland

They may also serve that purpose (preventing  
Va from going negative), but I know from  
personal experience that Jim is right. While  
I had my V4B on the bench running full tilt  
one day, my load resistor opened up. The main  
fuse blew, because those diodes had shorted (a  
kind of crowbar protection). The tubes and OT  
survived. Added series resistance might have  
prevented that from happening, so I think the  
circuit is correct as is.  
If you think I missed something here, please  
tell me, but this is consistent with what I've  
read elsewhere about this circuit.  
8/3/1999 11:15 PM

There is an inevitable leakage inductance between the ende of the half-primaries. On transients this will cause spikes on the high side that can't be clamped by the low side diodes. As the transient spikes die out, the low side can then clamp the high side. Both effects occur, depends entirely on the electrical conditions.

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