Sunn 1200S - Should all tubes glow?

[Bond...James Bond]

Hello! I have a vintage 1200S that I use for bass. Looking at the back of the amp, the four RCA 6550 tubes glow but the tube on the left does not. (it's a Syvannia & hard to read the first number:  ?550 - it's the same height as the the RCA's but skinnier - no bulge) Anyway, it doesn't even get warm. Is this a problem or is that tube for something that I don't use (like reverb or tremolo)?
If I need a new tube, can I put it in or does the amp have to be biased?

Thanks! & here's hoping someone takes over the site.

[JoeArthur]

Gee... ya got me!  If you have 4 glowing 6550 tubes thats all there should be - assuming that extra one is a 6550 type.  All tubes should glow if they are to be functional.

Chances are the tremolo is all solid state and the reverb recovery would be also.  The reverb driver probably uses a 12au7.  This is all typical of the era.

Perhaps it is a modification someone made... maybe for some reason one of the output tubes needed to be relocated in the chassis?

Like I said, got me!

[AdmiralB]

Sounds like your rectifier tube.  Or at least the rectifier tube SOCKET.  Does your amp have two sockets together to the left?

Does the amp work?  Sylvania made 6550s; they were straight-sided but would otherwise look about like your RCAs (rebranded Tung-Sol, actually).

Sounds like someone stuck an extra 6550 in a recto tube socket.  Since the 6550 expects heater voltage on pins 2 and 7, but the recto socket supplies it on 2 and 8, it won't light up or get hot.  The AC would be supplied to pins 4 and 6; on a 6550 pin 4 is the screen and pin six isn't connected.

If this is what's happened, and the amp works, it must have silicon diodes installed on the tube socket internally.

[AdmiralB]

Look at this picture:

http://www.dangpow.com/~sunn/1200s/p9110002.jpg

The two sockets to the left are for rectifier tubes.  This particular amp has a solid-state module in one of them (the red thing).  What does your amp have there?

I have a '70 2000S that came with one socket installed here and the other hole left blank.  A SS module was plugged into the existing socket (which was wired incorrectly, BTW, the heaters were connected to the wrong pins).  I installed the second socket, wired it up correctly, and use a pair of tubes, but I think this was a transitional era between the 1000S/1200S/2000S and the 350L/350B - which didn't have any facility for tube rectification.

BTW, the power tubes in this pic are Sylvania 6550s.

[Bond...James Bond]

Look at this picture:

http://www.dangpow.com/~sunn/1200s/p9110002.jpg

The two sockets to the left are for rectifier tubes.  This particular amp has a solid-state module in one of them (the red thing).  What does your amp have there?

I also seem to have a Solid state module (though it's black not red) in the same position. But where you have that empty socket is where I have that Sylvannia tube that doesn't glow. So mine is incorrectly setup? Is there something that should be plugged into that (2nd from left) position or maybe the previous owner just stuck that tube in there to hold as a spare?

Yes, the amp works. It sounded a bit different the other day when I used it so I took a look in the back & saw this tube not glowing & it got me wondering.  


I have a '70 2000S that came with one socket installed here and the other hole left blank.  A SS module was plugged into the existing socket (which was wired incorrectly, BTW, the heaters were connected to the wrong pins).  I installed the second socket, wired it up correctly, and use a pair of tubes, but I think this was a transitional era between the 1000S/1200S/2000S and the 350L/350B - which didn't have any facility for tube rectification.

BTW, the power tubes in this pic are Sylvania 6550s.

[AdmiralB]

That extra tube should not be there.  It's not glowing because the socket is wired for a completely different type of tube.

[Bond...James Bond]

Ok, Admiral but what should be there?

[AdmiralB]

If you're going to use the SS rectifier module, nothing needs to be there.  

If you want to use tube rectification, you should remove the SS module and put a 5AR4/GZ34 recto tube in each socket.

[Bond...James Bond]

Thanks for the info! What sonic difference will changing it to tube rectifiers will there be? Warmer sound? More volume?  Remember I'm using this for bass & want a clean Ampeg type tone

I f I do switch it to tubes, do I have to get a tech to bias the amp or can I just out them in?

Thanks again

[AdmiralB]

There'll be a little less power, maybe a little warmer, and the amp will be  a little bit 'slower' in terms of response.  I prefer the tube sound, but it's a taste thing.

If it's properly biased now, plugging in a pair of tubes won't hurt anything.  It'll run a little cold compared to where it is now, which might not sound the best (or, it might sound just fine), but that's it.  All voltages in the audio circuit will come down a bit.

[JoeArthur]

Tube rectifiers will have a greater voltage drop than solid state rectifiers.  B+ voltage will be lower.

This might mean that the bias voltage needs to be adjusted.

If you want to use tube rectifiers, plug one in first and see if it glows.  If it doesn't then that problem would need to be corrected before removing any internal solid state diodes.

I have seen too many cases where the 5V rectifier heater winding on the power transformer was blown due to using the wrong rectifier tube and the "problem" corrected by a tech installing a solid state rectifier circuit instead of replacing the power transformer which would certainly be more expensive.  Usually when this happens the savvy tech will put something like silicone goop into the tube socket to prevent a tube from being inserted - not that it would do anything but warn an owner that something was amiss... using another tube like a 6550 in a rectifier socket might serve the same purpose until someone questions "why is this?"

Have a tech check it out and it will eliminate any guessing.