Something I found interesting a while ago!

[M Gregg]

Some time ago I built an EL34 PP amp and as we do, tried many mods!

When reading Aspens Tube Amp Book I saw a mod where the EL34s were protected with reverse biased diodes (Across each O/P Tube) to protect them and extend life from the back EMF of the O/P Tx. (Guitar amp mod)!

I tried this mod on the Hifi Amp and the results were much cleaner sound also most of the blue electron cloud (Blue glow) seemed to reduce in the valves. I know this is dependant upon perceived listening tests and we do not like the idea of improving valve sound with semiconductors. Switching noise etc, etc. I let a few HIFI nuts myself included (LOL) listen they thought it was very good! I tried removing the diodes and the sound seemed to be muffled by comparison! Also obviously different types of diode gave different changes! (Soft recovery, Hex, Etc) Hex seemed to give best results at the time!

I assume others have tried this?

Regards
M. Gregg

[Globulator]

IIRC Bob Carver used this idea too.

[M Gregg]

Aspens mod showed 3 diodes in series I guess to allow for a diode to fail during a stage performance, I just used one hex diode per tube. This was just DIY try it and see! I was surprised at the results!

The diodes stayed on the amp!

I assume this would work with other pp pentode amps although I have not tried it!

[M Gregg]

Originally Posted by Globulator

IIRC Bob Carver used this idea too.

I will have a look for a schematic I remember in the past "Carver amps". You don't see many about these days even mentioned in forums.

[jrenkin]

would you mind showing a small schematic of this mod? I can't seem to put a picture in my mind this slow morning. Thanks.

[Globulator]

Originally Posted by M Gregg

I will have a look for a schematic I remember in the past "Carver amps". You don't see many about these days even mentioned in forums.

It was in this thread - not 100% they are the same but it did use diodes on the gates I think.

[M Gregg]

Originally Posted by jrenkin

would you mind showing a small schematic of this mod? I can't seem to put a picture in my mind this slow morning. Thanks.

The MOD involves connecting Diodes to the plate connection of each output tube from the plate to ground. That is the connection of the output transformer to the anode of the tube to ground reverse biased so the diode blocks te HT. If you were using a 1N4007 the band would point to the Anode of the tube. You can use BYV96E diodes however HEX are better!

Obviously if you connect them in forward bias they will short your HT to ground! The guitar mod showes 3 1N4007 in series per tube. The type of diode has a big impact on the sound (from a HIFI point of view!)

Here is the schematic!

[Wavebourn]

I used that long time ago in guitar amps. There were several KD-213 diodes in strings, since they were only available high frequency power diodes then.

[woodturner-fran]

So this might work on an EL84 amp too?

Part number for the best diodes for this?

Interesting....

Fran

[Globulator]

You need to bear in mind that this is considered a risky mod.

If the diode gets tired and dies it will usually form a short circuit and fry your output transformer, and possibly your power transformer too.

So while an 1N4007 is good in theory I'd be fitting something much better.