Not one but two noob questions

[headroom]

Just getting into this strange tube world
This is a great place for learning stuff!
I've built some effects so I know how to read schematics and stuff but about tube amps I know very little.
To the questions -
1. I have a tube amp (5x12ax7 and 2xel34) that has been modded so it can use tube or solid state rectifier.
When I bought it it came with SS rectifier.
Ground switch (supposedly) used for this switching.
My question is - how do I know my switch is in the right position?
Or - what would happen if the switch is in the "tube rectifier" position and the rectifier tube socket is loaded with "ss rectifier"?
2. My friend has an old sf Twin Reverb.
What I want to know is - can the amp be damaged if he pulled the wrong power tubes out for half power mod (1 and 3!?), and didn't switched the output to one speaker?
It seems his amp is working like this for years. And I must say it sounds awesome.
Thanks

[surfsup]

1. I have a tube amp (4x12ax7 and 2xel34) that has been modded so it can use tube or solid state rectifier.
When I bought it it came with SS rectifier.
Ground switch (supposedly) used for this switching.
My question is - how do I know my switch is in the right position?
Or - what would happen if the switch is in the "tube rectifier" position and the rectifier tube socket is loaded with "ss rectifier?

Take your DMM out and test continuity between the switch positions for the leads running to each rectifier. All this can be done simply testing the switch lugs themselves. Then label it on the faceplate.

2. My friend has an old sf Twin Reverb.
What I want to know is - can the amp be damaged if he pulled the wrong power tubes out for half power mod (1 and 3!?), and didn't switched the output to one speaker?
It seems his amp is working like this for years. And I must say it sounds awesome.

Perhpas a mroe experienced member will answer this but if you pull tubes out, less current will be drawn and your passive components will then deal with less heat dissipation. So that should be fine. The impedance change won't "damage" anything, I don't think. It will just make the tubes run at a different "shift" of the load line. If one side was removed completely and the other side left in, you'd have half your signal output and the load line wouldn't shift. So in my inexperienced thought process I would think it would be okay. but again, get a 2nd opinion.

[Gibsonman63]

Welcome, Headroom.

I would have to see a schematic to answer queston number 1. I would imagine that it drops B+ a bit to protect the power tubes. It may or may not be required depending on the tubes you are using.

As for question 2, those old twins seem to be able to take a lot of abuse. Basically, he is running it single ended at half power, which probably gives a bit more crunch than a twin normally would. Combined with an impedance mismatch on the output, you are thowing away a bit more power which gives a more driven sound at lower volume levels. If it has been working that way for years and he isn't running it really hard, it is probably ok.

I ran my BF twin with two tubes and both speakers for a long time with no issues, although I was always careful to pull either two inner or two outer tubes. Now I have plenty of amps that distort, so I keep all the tubes in it and use it when I want lots of clean.

[martin manning]

Welcome!

On 1), there is no way to guess without a schematic or some good photos of the wiring.

On 2), many amps can stand a 2:1 miss-match in load impedance without damage, but it depends on where it is under the normal (all tubes in) condition. You can remove either the inner pair or the outer pair, or every-other tube and the effect will be the same. Just don't remove a pair from one side, as you'll eliminate the upper or lower half of the signal and get a distorted sound. It would be correct to unhook one speaker (if they are wired in parallel, and protect against the leads shorting) to double the load impedance and correctly match the remaining two tubes. Re any damage, I think you have your answer, if the amp still sounds good after all this time.

[headroom]

This was fast

http://drtube.com/schematics/elk/twinamp50.gif

That's the link for the original schematic (without the switch).

(I'll try to upload a pic of the guts).

The ground switch is on the left below the rectifier tube socket.
I got the picture from the previous owner I didn't open the amp to see for myself.
But on the pic there seems to be no connections on the ground switch?

[surfsup]

perhpas this is my inexperience but I don't see a SS rectifier. I just see tube from that photo, with a tube in the socket. I think I see a three lug switch just below the tube recto socket on the back plate. Nothing is hooked to it as you state.

I also am curious about the first cap can there to the left of the rectifier socket. Is it 100/50uf like the schemo says? The reason I ask is that GZ recto tube datasheet shows 60uf as max.

[headroom]

http://www.mojotone.com/Solid-State-Rectifier-image.jpg

I have something looking like this loaded in the rectifier tube socket - so you can't see it on that gut shot (it says "1N5408" on it).
Also - the Japanese company that made these amps changed names and whatnot, and my amp lacks the logo and the original tolex/grill cloth so this schematic is my best bet being something 90% close to the amp than being "the" schematic.
I'll probably take it to a tube amp tech these days but i just wanted to get some info before I do.
Cap is 50uFX2.

[headroom]

Also while I'm at it - this amp is pretty weak for a 2xel34 output amp.
Output tubes are pretty old I guess so I'll replace them but could something else be the cause? Cap cans?
Thanks everyone for taking the time to reply.
Cheers

[surfsup]

After trying new tubes....

I see the schematic shows you're running fixed bias already which should be giving you the ability to squeeze out a bit more power output vs cathode bias. They can be biased differently to try and get more power, but one would have to review where you're at now with the existing bias to see where you're at on the graphs...

[David Root]

1N5408 is a 1000V 3A diode. There would need to be more than one, at least two, to make sense.

Edit: Now I see it's a SS rectifier in a tube base, that uses 1N5408s. Now it makes sense.

[headroom]

I'll open it up to see for myself in a few days, and make sure is the switch connected or not.

Thanks again.