• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Why are these diodes here on the OPT?

Why are there diodes on the screen and OPT here? I suspect it's to stop back EMF if the user switches between UL and pentode mode while the amp is playing? Am I right? Are there any sonic implications to having them there? I can't remember that I've ever seen this before even in my textbooks. Since the designer has a UL/pentode mode switch why didn't he just go ahead and add a triode mode too?


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2005-11-24 3:44 pm
I have built the older version of this schematic there was no possibility to choose UL / pentode and NFB even then there were diodes in the same place. I have had it explained to me why they are there and what it was good for but unfortunately I forgot about it but it sounds good anyway.

Someone can certainly explain both the pros and cons about it.


Paid Member
2011-10-31 1:00 am
As far as I can tell, the diode on the screen is to prevent it from emitting. As far as 1% resistors, I use nothing else unless it's a metal oxide for power. 1% MF types are so cheap, why bother with 5%?

I've blown 100V caps on a cathode that was "only" 30V... If you oversize everything, a bad tube won't take out half the amp with it.
Who did this design?

Not sure, I'd have to trace it down. As a learner, I've been data-mining google images by tube number to just sit and read schematics by tube number. I couldn't explain this diode other than the fact that I know diodes are often installed across relay contacts, so I assumed it was something to do with a voltage spike caused by the UL switch there. But that's just a guess.
This diodes are completely useless there. Also, a 50VW in the cathode where there is only 15 or 16V is stupid. A 25V unit will be better there. Also, a 4.68KΩ as a grid stopper 1% resistor for a huuuuuumble grid stopper???? Who did this design? Surely someone that likes spent money stupidly.

All that, but the biggest money waster is four dedicated 6.3 volt windings, ouch.
All that, but the biggest money waster is four dedicated 6.3 volt windings, ouch.

Yes, as all them are grounded, one winding of the sum of the independent capacities is sufficient. Unless it be a toroidal trafo, where too large diameters are difficult to wind, several independent wires is easier.


2005-11-24 3:44 pm
Windcrest asked what the diodes are doing here and now some are about to throwing up on the schematic if the designer has done in a way you just have to accept this. But the site is a Japanese tube site in English and as far as I know there is a thread here with Japanese tube construction which is appreciated. But I have the link so everyone can watch EL84 Single Ended Amplifier D.I.Y


2016-06-07 6:50 am
Reasons for putting those 1N4007 diodes in that schematic:

1. Someone has too many 1N4007 diodes and wants to sell them on the web.

2. There is an approximate 0.6V drop across the diode. Gets the voltage at the exact value the designer intended.

3. The designer has seen similar schematics that uses diodes in the screen, transformer primary, and B+ positions.
He copied the errors of others.

Just my opinion . . . take the diodes out, and use a straight piece of wire (which is a very small inductance).