Hi Doug h,DougH wrote:Those are good points. Matchless does (did?) the same thing- lowered the screen voltage to keep the out-of-spec voltage-wise EL84's from going up in flames. My Peavey Windsor has 450V B+ but IIRC the screen supply voltage was reasonable. Then they only put 400 ohm screen resistors on them (?). I changed those to 1.5K's I had lying around. But then, they connect the EL34 supressors to the negative bias voltage and also put diode protection on the output transformer. They engineered a lot of smart stuff into that amp to keep it durable. Off topic, but another misconception I always found funny was the thing about Marshall having bad output transformers when it was actually poor screen regulation that was killing those amps. Something that could have been fixed by a four 25 cent resistors...phatt wrote: One thing I'd like to add is some of these amps have a rather high working voltage.
There maybe merit in dropping the HT if you want to stay below the flashover issues and also the punch through in the OTr windings.
Dai h, The .22 Amp schematic shows 390Volts HT which is highish. To Messa's credit they have at least lowered the screens by a good 100 volts or so, that will help as the higher the supply the more you have to drop the screen voltage,, if you want it to survive for a long time.
Quite obviously you have your finger on the pulse and not on Advertizing proppaganda.
I wish someone would shot a hi quality Vid of a screen grid disentergrating they light up like a chrismas tree.
Good entertainment but also might help Get the message out to the masses that screens ARE the weak link and one should care for them.
For those that wish to know why Doug lowered the screen Voltage?
The Plate is is exactly what it says, *A plate of metal* whereas the screen is tiny wires.
With only a couple of volts between plate and Screen, under extended load the PSU sags and the screen can actually float more positive than the Plate. All the electrons pass through the very fragile Screen,, Not the Plate. ouch!
Laws of phyiz will tell you the free Electrons from cathode have a negitive charge and they will pass to whatever is at the highest Pos potential,, Electrons don't give a hoot either way.
Under load The first filter cap is draining but the second will drop less so although only a momentary thing over extended periods you flog the crap out of your Power tubes.
If you keep a safety margin this is never going to happen and tube life will be greatly extended.