I have a 300W bass amp an ampeg SVT and there are diodes across the Gride resistors for protection and they read 555-2? does anyone know the cross reference?

Thanx
Bubba

:I have a 300W bass amp an ampeg SVT and there are diodes across the Gride resistors for protection and they read 555-2? does anyone know the cross reference?
:
:Thanx
:Bubba

Sorry, I ballsed up my last reply!..I'll try again...
I'm looking for the exact same equivalent in my SVT as well (555-2 with letters 'SI' or 'IS' printed perpendicular to this number)
I know this isn't much help to YOU but, can you let me know if you've found any?

Cheers
Macca (UK)

I don't know anything about this application or cross reference. However, unless they're zeners, diodes are pretty much diodes. You need one big enough to handle the maximum forward current. Unless you get better advice, compare the physical size to a 1N4007 - if the original diode is no larger, try a 1N4007.

Good luck.

Ok. This is just from observation, as I've never had any amplifiers that were powerful enough to need this. If there was a diode connected across from the control grid to ground, wouldn't this diode serve to prevent the grid from going positive? I suspect that a positive grid in such a heavy duty application might cause such a current flow that things would get blown.

If this is the case, I would think that all you need is a regular diode, and not a Zener diode, as Doug said. The big thing you would want to look for is a diode that has no leakage (which most modern ones don't have), as leakage would affect grid bias. I can't see a control grid having all that much current flow, as it doesn't take much current to make a grid operate. Seeing as this is no ordinary amplifier, I could be wrong.

Has anyone tried writing to National Semiconductor about this? They may be a big help. They are at www.national.com. You can also use www.nationalsemiconductor.com, but the former is easier to type out.

Thomas

:I have a 300W bass amp an ampeg SVT and there are diodes across the Gride resistors for protection and they read 555-2? does anyone know the cross reference?
:
:Thanx
:Bubba

Just to clarify things......want to make sure everyone knows that I said that Doug said that it was probably not necessary to use a Zener diode. I know it sounded like I said that he said you should use one, but that's not what I meant.

T.

thanks for you're speedy replies guys!
I've since discovered (via a friend) that this 555-2 (SI = american JDEC system) which is now obsolete (obviously!) had an If = 1.5 amps and was rated at (Vrm) 800V.
A safe equivalent would be a 1n5408 (If=3amp / Vrm =1000V)
All the diodes (not Zeners BTW) on the ampeg SVT circa 1970 used these 555-2's. I need to replace mine in the bridge rectifier cct (which has 2 diodes in series for each leg of bridge...the transformer secondary ac output = 535Vac giving 750VDC after rectification. So half of this voltage dropped across each diode)
unlike Bubba's, in his Grid cct which are parallelled with a 22ohm 1W resistor (if anyone can tell me why they're are used I'd be interested...I can't see the point of them?? I have the schematics for perusal, if anyone is interested)

Cheers
Wayne (UK)