diode string protection express mod

[toner]

was thinking of doing the 3 diodes the express power tubes has on my stock tweed deluxe clone… any issues doing this on a cathode biased 6v6 amp like this? i liked it in my express, tried taking it out, but liked it better with… so was expand to my other amps and see what happens

[guitarmike2107]

no should not be an issue, as far as I knew is was a safety feature, not a tone feature?

[pdf64]

They seem to 'symmetricalise' the wave a bit, and remove a lot of flyback spike when overdriven into a speaker load.

[guitarmike2107]

ok, one to try then..

[rooster]

Man, Ken talked about this way back and of course he used them in his Express. I think this adds a noise to the output, something I can hear. In fact, I removed them one day in one of my Express. Immediate improvement, this weird little edgy fizz was gone. So I started to pull them from all my amps, one at a time to see if there was a change in them, notably Fender style amps, but some Marshall examples, too. I ended up pulling all of them and do not use this diode string any longer in anything I build.

If you think about it, Fender, Marshall, Hiwatt, Vox, etc., any amp from the glory days of recorded music, did not have this diode protection in their amps. And guess what? These amps are still around, still working properly and sounding great. If there were OT failures in these amps, I would be very hard pressed to believe that the diode string would have actually prevented it, nor do I think there is any proof to the contrary.

Kenny was a brilliant guy in so many ways but I think this was a miss, particularly from a guy who apparently had the Golden Ears.

[Randy Magee]

rooster,

I had the same problem with my Express... I ended up removing the diodes as well...

[David Root]

I believe KF used the diode strings partly because he was familiar with them from his Ampeg days, but mainly because the Express OT primary impedance he used, 6k6, is approximately double the "normal" ie max output impedance for a pair of EL34. This raises the back spiking quite a bit. He did also say that only the 1N4007 was OK soundwise, and not to use later "improved" types which he believed did have a deleterious effect on tone.

It would be interesting to see if there is a similar effect in the JTM45 circuit, which also uses a 6k6 OT primary vs ~4k "normal" for a pair of KT66.

[johnnyreece]

Interesting you mention the JTM45...I actually put these in mine after repeatedly having plate to heater arcs on one socket (replaced the socket each time). Only happened when I was REALLY driving it hard. I replaced the OT (it was questionable) and installed the diodes; no trouble since. My ears are tin enough to not really notice a difference with or without them.

[Colossal]

RG Keen recommends MOVs as a sonically transparent alternative to diode strings.

Johnny, that's interesting about your JTM. I wonder if it was a dodgy OT?

[johnnyreece]

RG Keen recommends MOVs as a sonically transparent alternative to diode strings.

Johnny, that's interesting about your JTM. I wonder if it was a dodgy OT?

That's my guess. It would play fine at low volume, but crank it and ZAP! Almost every time, and within a minute. This amp was a Weber kit that I bought off of someone who couldn't sort it out, so I have no idea what damage was done beforehand. Bought a Mag Comp replacement and haven't looked back since.

[toner]

i tried the diode protection mod on my 5e3 deluxe.. it doesn't work.. it made the 6v6's sputter on and off.. so i immediately took the diodes off.. all is well again.. maybe it doesn't work on self biasing amps?

[jaysg]

I believe KF used the diode strings partly because he was familiar with them from his Ampeg days [clip] He did also say that only the 1N4007 was OK soundwise, and not to use later "improved" types which he believed did have a deleterious effect on tone.

Yes, the V series amps have these, but not strings.

Was he talking about the UF4007 ?

[pdf64]

i tried the diode protection mod on my 5e3 deluxe.. it doesn't work.. it made the 6v6's sputter on and off.. so i immediately took the diodes off.. all is well again.. maybe it doesn't work on self biasing amps?

There's no reason that the diodes shouldn't work as intended; I fitted them to my amps, cathode and fixed bias, until I tacked them on to a switch one time and noticed that they affected the amp's characteristics.