Regarding diodes for protection, there are also R3000 'Fly Back' diodes on the Output Transformer connection to the plate. Your thoughts on these would be appreciated.
When a coil has current-flow, then shuts off that current, the coil's inductance causes a reverse-voltage spike in an attempt to sustain the current.
When you play your push-pull amp loud, it moves into the "Class B" part of its operation where one side shuts off while the other continues conducting. Those diodes at the OT-primary/tube-plate are there to prevent voltage spikes when that side of the power section shuts off.
Are they needed? Well, normally transformers are hi-pot tested to a few-thousand volts because the manufacturer knows they will experience those spikes. But some amp manufacturers (even back in the 60s and 70s) included those voltage-spike diodes in their amps. I dunno if the got a batch of bum transformers that prompted the addition, but it's cheap insurance and doesn't affect the amp's sound.
Oops, I overlooked that query.
I don’t like such diodes. Although they’ve got a really high voltage rating, if they do fail (which seems super rare but is very occasionally reported), silicon power diodes tend to fail short, which puts a big stress on things.
Also I find that even with my knackered hearing, they do affect the amp’s tone, or at least when it’s overdriven.
To check whether they affected the sound, I fitted a dpdt switch in their return path to circuit common, to enable instant AB comparison. They definitely acted to roll off some high frequencies. It was apparent on the scope too. This was on an AC30, and a 5E3 build.
I suppose that in some applications, the smoothed tone resulting from the diodes might be perceived to be a good thing.
Topic: Output Tube Cathode Diode on 68 PRRI (Read 3040 times)