OT protection diodes SR-2873?

[tedmich]

whats the best replacement for the Peavey SR-2873 which I believe is an old Motorola MR250-2, 2KV 250mA fast recovery diode?

2 series UF4007?
Vishay RGP02-20E-E3/54 ? maybe discontinued...
Rectron R2000F-B
http://www.rectron.com/data_sheets/r1200f-r2000f.pdf
more voltage?
http://www.rectron.com/data_sheets/r2500f-r5000f.pdf

on the Rectron's is 200mA enough?

[Enzo]

You could always call PV and have them send you the exact part.

[jrfrond]

I usually use the Rectron R3000, which is a 3kV diode, but not fast recovery, which is REALLY not necessary.

Then again, transient suppressor diodes usually aren't necessary at all, unless you are protecting cheap output transformers as seen on many new tube amps.

[Enzo]

Or when the customer uses cheap speaker cords, or guitar cords for speaker cords, or has loose wiring in his cabs, or forgets to plug his cabs in, or plugs them into the FX return...

[oskar]

I use always for this application , the GP02-40 , on europe it's available thru RS components and others.


oskar.

[Steve Conner]

I think three series 1N4007s is a common bodge. Two to get your 2kV PIV rating, and a third one for luck.

I agree that they don't really need to be fast recovery, since they shouldn't normally even be conducting at all.

I'm sure I remember using those GP02-40 diodes in a FWCT rectifier. I wanted fast recovery because I believed that thing about FREDs sounding better at the time.

[tedmich]

I I believed that thing about FREDs sounding better at the time.

its not true?


FRED Durst, as an example, does sound terrible...no one ever recovered quickly from hearing him for the first time...

audiophiles do love their Stealth diodes...

[gui_tarzan]

I'm resurrecting this old thread because I have a XXX on the bench right now with both of these diodes blown. If (according to the schem) these are to protect the OT or tubes and only have 400v or so on them, why do they need to be 2Kv?

[Enzo]

Because what they are protecting is the transformer, and they are not protecting it from 500v. If a signal is cranked through the amp without a speaker load, the inductance of the transformer can kick the voltage up to very high levels, which can arc inside the transformer. Those diodes conduct any time the voltage at the end of the transformer tries to go below zero, which would mean the other end was trying to go over twice the B+ voltage.

[gui_tarzan]

Ah, ok. That makes perfect sense. Someone has already replaced them with single diodes but I can't see any printing other than "250" on them. I wonder if they replaced them with something insufficient.

[Alan0354]

I think it's easier to get 2 1KV diodes in series rather than single 2K or 3KV diode. I just cut all the pcb and stuff out of the Marshall Dual Reverb, I saw they use 3 1N4007 in series to protect the OT. That is not in the schematic!!!!

[gui_tarzan]

So I went the triple 1K route in place of the GP1250s which are apparently rated at 2500v, and I still have no sound even with a completely new set of tubes. At this point I wonder if it took out the OT?

The original diodes measure .999/open and .970/open.

[Jazz P Bass]

It does not appear that there is anything incorrect about the original diode readings.

As far as troubleshooting a 'no output' goes, I would start as the output tube grids.

- is the bias voltage correct
- do the output tube plates have there proper connections & voltages. (plate, screen, bias & cathode)

If all is good, insert 1 100mv/ 1K signal into the amp while monitoring one of the output tube grid pins.
Volume & tone maxed, read the Vac signal voltage on the grid pin.
If you have a nuce healthy Vac voltage on both grids & the plate, screen & cathode connections & voltages are proper, Then you can consider the OT.

[gui_tarzan]

V5

3 - 423
4 - 422
5 - -43.3

V6

3 - 420
4 - 421
5 - -43.2

The bias circuit on this is weird, it is switchable for 6L6 and EL34 and doesn't have an adjustable pot. -44.2 on the switch.

[Jazz P Bass]

Alright making sure Pin 3 is grounded.....

EDIT: Pin 8 is Ground.

Run a signal through.
100mv's/ 1K.
Set your meter to read Volts AC.
Black probe on Chassis Ground.
Red probe on any grid.
Crank all of the controls & tell us the Vac reading.
(I would expect 50 some volts/ac).