Bare Knuckle Pickups Forum

Forum Ringside => Tech => Topic started by: MDV on January 08, 2011, 10:39:46 PM

Title: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 08, 2011, 10:39:46 PM: 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

?
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Transcend on January 08, 2011, 10:58:31 PM: wouldnt it be the last two?

3/4 maybe?

im just guessing here though
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 08, 2011, 11:40:13 PM: That would be my guess, but I also am just guessing.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: shobet on January 09, 2011, 12:14:43 AM: 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
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 09, 2011, 12:19:43 AM: Hmm. That was one combination I didnt expect! Doh. I have no idea at all now.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 09, 2011, 12:20:12 AM: Oh, cab impedence - 8 ohms.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Dmoney on January 09, 2011, 01:36:50 AM: 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.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MrBump on January 09, 2011, 08:38:50 AM: When I reduced my GRO100 down to 50 watts, I pulled the outside pair.

Mark.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 09, 2011, 08:41:03 AM: Thanks lads. Especially dmoney for going to the trouble of trying to find a pic/diagram.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Dmoney on January 09, 2011, 10:02:33 AM: http://jvmforum.com/phpBB3/viewtopic.php?t=551

(http://img.photobucket.com/albums/v48/MikeyJazz/Amplifiers/inside3.jpg)

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.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Twinfan on January 09, 2011, 11:06:49 AM: 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 :)
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Transcend on January 09, 2011, 11:08:12 AM: is it possible to do this on a 60 watt 2x el34 amp?
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Twinfan on January 09, 2011, 11:22:00 AM: Nope, only from quads to pairs of output valves...
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Dmoney on January 09, 2011, 11:35:25 AM: Quote from: Twinfan on January 09, 2011, 11:06:49 AM
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!"
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 09, 2011, 12:31:50 PM: 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
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: dave_mc on January 09, 2011, 01:24:52 PM: yeah I always heard inner or outer pairs too, but i haven't tried it myself, so wait for mjw, jpf or someone like that.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: blue on January 09, 2011, 01:54:03 PM: i'm fairly sure that's the right way too, but you should go look on the JVM or Marshall Vintage Modern forums. there's plenty of information on both those sites, and the VM forum has Steve Dawson, Marshall designer, as a regular contributor.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: HTH AMPS on January 09, 2011, 07:07:00 PM: inner pair or outer pair, but I usually take out the inner pair so the two outside valves are left with more air round them.

set the amp to half the cab's impedance, job done.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 09, 2011, 11:16:07 PM: Kickass, that'll do me,thanks.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: James C on January 10, 2011, 10:45:03 AM: Mark

From (hazy) recollection i think the outer pair were pulled when i had the amp

hope that helps

James
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 10, 2011, 04:42:47 PM: Thanks. Did you run it at half impedence? I've heard on the marshall forum that the JVM doesnt need to be half impedence when running one set of valves?
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: James C on January 10, 2011, 06:14:27 PM: I ran it at standard impedance and had no probs whatsoever with the JVM
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: Ibanez2570z on January 10, 2011, 06:21:06 PM: Going off on a tangent as ever what are the Marshall JVM410H tubes that come with it like? and what is the amp like on the whole? Not that I am getting one but just being a nosy so and so.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 10, 2011, 06:23:40 PM: They svetlanas. The JVM is marshalls latest attempt at making a JCM800 with more versatility and the ability to do very heavy music without mods or boosts or anything, basically. They did a good job.
Title: Re: Dumbass question you'd think I'd know - which tubes to pull?
Post by: MDV on January 10, 2011, 06:27:35 PM: And cheers james. I think I'm set. Pairs are inner and outer, no need to use half impendence, should probably rebias, but I'll be silly and go by ear (sounds a little hot at the moment and frankly I'm fine with that; new valves every 8 months instead of every year is still far less than I spend on strings)