Vintage SVT plate resistor question

[Randall]

Got a vintage SVT in today. Guy says it is the first year they made them, and has been converted to 6550's. I don't know what that means exactly in this case.

It has a blown fuse, whitened 6550, with it's 3.6 ohm 5W plate resistor burnt to a crisp. One of the other resistors has been replaced with a 5 ohm 5W, and the other four are original 3.6 ohm. AFAIK 3.6 ohm 5W is no longer available, and Fliptops sells a 5.1 ohm 6-pack for vintage SVTs. Would this be the way to go? And shouldn't it have been done when the 6550s were put in? That's what I see on one of the 6550 equipped schematics.

[52 Bill]

AFAIK 3.6 ohm 5W is no longer available...

Mouser part # 280-CR5-3.6-RC, they have 194 in stock.

There are something like 4 different versions of original SVTs and most have been fixed in one way or another through the years. Sometimes the 3.6 ohms will all be replaced with 5 ohms, sometime only one or two will have been replaced. If this is really an original version that had the 6146B tubes, I'd go through the amp and check that all of the correct changes have been made to convert it to 6550s.

When I get one in, I try and find the correct schematic and I will try and return it to as close to the original form as I can.

[The Dude]

1.4 ohms is not a substantial difference in the circuit we are talking about here. The 6 pack should work just fine.

Edit: Sorry to cut in, Bill. We must have both been typing at the same time.

[The Dude]

Also, I'm sure you know this, but in case not, do not put the whitened tube back into the amp.

[Randall]

I think you mean do Not, eh Dude?

[The Dude]

I think you mean do Not, eh Dude?

Yes I did. Thanks for the catch. (previous post edited)

[bob p]

forgive me, but I'm not looking at the schematic as I write this...

what was the wattage rating on those original plate resistors? it wasn't 5W was it?

my simple understanding of those old SVT circuits is that the low-Z plate resistors act as a fusible link. i'm thinking that if someone replaced the original resistor with a resistor that had a higher dissipation rating, that would be a bad idea. you want the resistor to burn out under conditions that would cause it to burn out; increasing the wattage rating would prevent that from happening and move the point of failure to somewhere else in the amp.

i'm thinking that it would be a good idea to verify that whoever previously worked on the amp has not increased the power rating for those resistors, and that whoever is selling replacements hasn't upped the power rating.

[Randall]

I don't have the original schematic, and the original resistors don't carry the wattage rating on them. The one that had already been replaced is a 5 Watt. And a schematic that is close to this version lists 3.6 ohm 5 watt resistors. And these look about right for 5 watts.

[Jazz P Bass]

I would go with 5 ohm 3 watt Flame Proof.

[g1]

They were 5watt type and did operate as fusible resistors. If a power tube shorted, the "fuse" would take it out of circuit so you could continue playing, rather than blowing the mains fuse. With 6 power tubes to work with, you could lose a few and still sound ok.
Used to see amps that had several blown, you never knew how long they had been running that way, "well, it's been sounding a little off for a month or so".