Hello again, I'm trying to find the equivalent to the MR250-5 diodes that are on my tube sockets on my MM210/65 combo amp.
I understand that the original MR250-5 diodes were made custom for Leo Fender by Motorola Co. I believe the spec rating for these are: 250mA@5000volts.
My search so far has led me to these:
Peavey Electronics Part No. 304 0 2873 Spec Rating: 250mA@2000volts Price: 40 cents a piece.
But Peavey wants a $5.00 minimum order, OK so maybe buy a ?????
Mouser.com Part No: 583-R 5000 F-T Spec Rating: 200mA@5000kilavolts Price: 76 cents each, no minimum order necessary.
OK. so what I need to know is, does anybody, has anybody bought either of these? If so which one ? If not, what have you bought to replace these ?
I'm trying to rebuild the tube sockets on my amp, mainly because I think after 37 years of intense heat that these amps generate, the parts on these tube sockets have to be a bit tired ?
Please, can anyone advise me on this ?
Peace & Happy Holidays, Charles
ps...to all you moderators, THANK YOU so much for helping people like me, the world is a much better place for having kind and generous folks like you.
I don't have the number handy
I don't have the number handy, but I used the 3KV 250ma in my RD100. I seldom use full power, and so far they are working fine.
Purpose of those Plate Diodes
Hello,
The purpose of those diodes on the tube sockets is fault protection. When the amp functions properly, even at Hi Power setting,, those diodes are reverse biased (inactive) and do nothing. They come into play only if there is a catastrophic failure of the HV power supply or the output transformer or the speaker. Any of these type of failures can cause a huge negative voltage spike to appear at the Tube Plates. At that point, the diodes become forward biased (active) and conduct those negative spikes to ground. So what is the correct value for those diodes? Anything greater than 3 to 4 times the maximum plate voltage should be more than adequate. 700VDC x 4 is 2.8kVDC. Lars picked a good number at 3kV Peak Inverse Voltage. The next obvious question is why did MM choose 5kV PIV? Who knows? They probably got a really good deal on 5kV PIV diodes or it's what they had on hand in lab stock when the engineers designed the circuitry. In short, any diode rated at 3kV PIV at 250mA or greater should work fine. Make sure to note the polarity of the diodes when you change them out. If you accidentally put them in backwards, they will short out your HV Power Supply as soon as you switch on the power causing the smoke to get out. The cathode (the side with a line) of the diode must connect to the tube plates and the anode must connect to ground.