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Old 30th May 2010, 01:12 AM   #1
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Default Tube preamp build help-heaters

Hey guys!
Pretty sure this is my first post.
Im building a tube preamp for bass guitar and Ive got it all wired up basically but Im having trouble with the tube heaters/filaments. This is my first attempt at anything with tubes.

I just want to check that I am on the right track. The preamp has two 12ax7's wired in parallel off one dc out, and two 12au7's wired in series, each 12au7 being powered by one dc out, so there is 3 12.6v dc outs on the power supply/rectifier board. For the DC3 out for the 12ax7's, I have two wires coming off the positive, each wire going to pin 4 on the two 12ax7's, i then have a wire connecting pin 4 and 5 together on each 12ax7. I then have two wires running back to negative on the DC3 out. Each negative wire coming from pin 9 of each 12ax7. Is that the right way to do it for parallel 6.3v wiring? The tubes light up nicely exept that I still read 12ish volts, even though the two 12ax7's are running off one 12.6v output. Also the heatsink is getting really really hot, like you can only touch it for a few seconds, is that normal?

Now the 12au7's. These are not meant to be in parallel and are meant to have 12.6v to each tube. So I have two DC 12.6v outputs for each 12au7. On each, I have a wire from positive of one DC output to pin 4, then a wire from pin5 to negative on the DC output. The problem is, they are not glowing barely at all, they get slightly warm aswell as its heatsink.

Now for the rectifier tube (6x4). 6.3v does go into the filament, and I measured pin 7 at 337v (red from multimeter to pin7 and black from multimeter to ground on power supply board, think that may be wrong) but the tube isnt glowing and Im not reading any high voltage at the B+ out. No idea whats going on.

Help will be greatly appreciated!
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Old 30th May 2010, 06:50 AM   #2
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Myself and maybe others here got lost halfway thru your descriptions...perhaps you could post a schematic of your 'pre' so we could take a gander at it. I would guess if you follow thru the schematic as we do you might very well get that "A Hah" reaction.....Yourself realizing your mis-wire.
The 12XXX series tubes with the 12.6 & 6.3 wiring "options" can really get you turned around sometimes. Walking away for awhile & returning later with fresh eyes usually will reveal your mistake....rather obvious one at that.

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Old 30th May 2010, 07:03 AM   #3
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Ok sweet, attatched is the schematic for the preamp and schematic for the rectifying power supply (pdf).
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Old 30th May 2010, 07:08 AM   #4
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I got lost

But for the 6X4 connections:
Transformer high voltage secondary wires should connect to pins 1 and 6, and the center tap of the transformer winding should connect to the common negative/ground of the power supply. If you missed the center tap connection, that will probably be why you don't see any B+ output.
Transformer 6.3v secondary (if that is what you are using for the heater) should go to pins 3 and 4
Output DC to filter choke/capacitors is on pin 7 (as you measured 337v which seems reasonable).

Without a schematic we are shooting in the dark as to the other problems, sorry.

Last edited by rotaspec; 30th May 2010 at 07:11 AM.
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Old 30th May 2010, 07:21 AM   #5
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Quote:
Originally Posted by rotaspec View Post
I got lost

But for the 6X4 connections:
Transformer high voltage secondary wires should connect to pins 1 and 6, and the center tap of the transformer winding should connect to the common negative/ground of the power supply. If you missed the center tap connection, that will probably be why you don't see any B+ output.
I think I did miss the center tap going to ground. The high voltage is 280v, so Ive got the 280v pin and the 0v pin from the transformer connected to the power supply board, by center tap do you mean the 0v?
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Old 31st May 2010, 07:59 AM   #6
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I just copletely re wired the whole thing, made everything neater aswell, still got the same problems. Schematics are in previous posts. Im desperate for some help, this is my major work for yr 12 at school. Thanks for the help so far!
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Old 1st June 2010, 10:02 PM   #7
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I found a diagram that explains the wiring of the tubes so i went with that, seems ok except that the tubes barely light up, they do get fairly hot though, the heat sinks dont burn your hand either. Attatched is that diagram.

So the only problem I have is figuring out why Im not getting any B+ and why the rectifying tube isnt heating or lighting up. Faulty tube?
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Old 2nd June 2010, 01:09 AM   #8
chrish is offline chrish  Australia
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OK, if I understand, you are saying that the problem is no B+ out at the output of the power supply board? You are, however, getting 337 at pin 7 of the rectifier? It would appear that the rectifier is working then.

Rotorspec mentioned the centre tap of the transformer to get a voltage. I think the two silicone diodes on the anodes of the rectifier valve form a hybrid bridge giving the ground. If you measured between pin 7 of the rectifier valve and ground and got 337 volts I am assuming that this part of the circuit is OK, ie silicone diodes ok. The problem would therefore be 'downstream' of the part of the circuit that is demonstrated as working. I would suspect a problem with the voltage regulator section of the power supply. This is the section including the MR856, C9013 and the two zenner diodes and D3. I would confirm by attaching the ground (black) lead of your multimeter to the ground of the power supply and work 'downstream' with the positive lead. To do this safely, I would make sure that the multimeter lead is equipped with a clip of some type, so you can safely have your hands out of the way when you are applying power, as the voltages are quite dangerous. So, I would probe pin 7 of the rectifier and re-confirm you got voltage there, then probe the + side of C1, then the + side of C3, the junction of D4 and R2 (this is the top of the zenner string that sets the 280 volts, you should see 280 volts here), then the + side of C7.

I am a newb too, and will not be able to help with the failure of the regulator too much, but someone here will be able to help once we zero in on the problem!

Did you solder the components in to the circuit board of the power supply, or was it supplied completed?

BTW, at first glance, your wiring of the heaters looked ok. Lets sort out B+ problem first.

Good luck!
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Old 2nd June 2010, 05:42 AM   #9
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Quote:
Originally Posted by chrish View Post
I think the two silicone diodes on the anodes of the rectifier valve form a hybrid bridge giving the ground.
yep that is correct.

ok sweet Il have a crack at that!

I bought it complete so Ive only been soldering wires for tubes, transformer and so on.
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Old 2nd June 2010, 06:18 AM   #10
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Ok, I got out my mulitmeter again and got about 337 V ac on both diodes on the ends that connect to the H. AC input (the diodes labelled D1 and D2 on the power supply schematic attatched in a previous post). But on the other ends of the diodes that connect to the rest of the board basically, I get nothing at all. I also measured all the caps and other components after these two diodes and i get nothing ofcourse.
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