Dumbass question you'd think I'd know - which tubes to pull?

[MDV]

JVM410H, 4xEL34 - to make 50W, which tubes to pull, as I look in from the back, 1&2, 3&4, 1&3 or 2&4

?

[Transcend]

wouldnt it be the last two?

3/4 maybe?

im just guessing here though

[MDV]

That would be my guess, but I also am just guessing.

[shobet]

If the new ones are like the old the outer pair or the inner pair. That's what I remember on my 1970s head anyway. I think it also changes what you have to set the impedance as for speakers. I followed what was in Aspen Pitmans tube amp book

[MDV]

Hmm. That was one combination I didnt expect! Doh. I have no idea at all now.

[MDV]

Oh, cab impedence - 8 ohms.

[Dmoney]

it is usually the outer or inner pair.
by doing that you normally remove 1 tube from each side of the Output Transformer Primary.
You do also need to change the impedance on your head.
If the impedance of the cab is 8ohm, you need to set your head at 4ohm I think.

that is the most common way when people talk about it.

however... in some amps it is possible to have the tubes paired differently, so pulling 1+2 out of 4 rather than 3+2 for example would be the way to go. I think that is rare though. I have a feeling the Uberschall is that way inclined.

I can think of one way you could figure it out by removing the tubes and using a multimeter, but I'm not 100% sure its safe and involves taking out the valves and poking around with a multimeter.


someone will have better advice than me... surely. I tried to google for pics of JVM guts but didn't find any. soz.

[MrBump]

When I reduced my GRO100 down to 50 watts, I pulled the outside pair.

Mark.

[MDV]

Thanks lads. Especially dmoney for going to the trouble of trying to find a pic/diagram.

[Dmoney]

http://jvmforum.com/phpBB3/viewtopic.php?t=551



I found this.
The fat red and brown wires look like the OT primary.
I'd hazard a guess that the white cable running across the back is the centre tap of the OT primary going off to the +HT. Big voltage on that!

I think its going to be inside or outside pair. You can see that if you pulled valves 1 + 2 then you have no output valves on one side of the OT.
Does this have dual bias? looks like it.


EDIT: might be best to wait to someone more experienced look at that pic.
actually... looking at the top pic, i think this guy pulled the outside pair and rebiased. you can see 2 power valves through the gap.

[Twinfan]

Pull inner or outer pair, halve the impedance setting on the amp.

So for you, pull the inner or outer pair and set the amp to 4 ohms.

Job done  :)

[Transcend]

is it possible to do this on a 60 watt 2x el34 amp?

[Twinfan]

Nope, only from quads to pairs of output valves...

[Dmoney]

Pull inner or outer pair, halve the impedance setting on the amp.

So for you, pull the inner or outer pair and set the amp to 4 ohms.

Job done  :)

Just wanted to say again... for anyone reading this thread... its not true 100% of the time (as in with all amps)  that the inner or outer rule works. However it is the most common case.

"60% of the time... it works ALL the time!"

[MDV]

Thanks guys!

OK, I'm pretty happy now that thats the way, but the risk factor means I'm gonna hope that one of the amp gurus (HTH, MJW, JPF etc) chimes in with a quick "Yep, that", and then I'll do it.

Thanks for the help