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Old 5th March 2010, 09:31 AM   #91
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Herr,
Are all of your grid stoppers right at the base of the tubes?
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Old 5th March 2010, 09:37 AM   #92
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Not realy, but there isn't a giant amount of wire between them and the socket either. I know that they are supposed to be as close as possible and with the output tubes I acutaly soldered them directly to the socket. But the one for the input stage has aprox 2" of wire. I can correct that though if rendered necessary.
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Old 5th March 2010, 10:33 AM   #93
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STOP!
DOUBLE CHECK the o/p stage tubes are correctly connected; screen grid to screen grid tap and anodes tap on each respective side of each half primary. Check you haven't got anodes and screens swapped. If it is wrong, this is the quickest way to ruin a set of good o/p tubes by destroying the screen grids.
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When powering up for the first time even if correctly wired, temporarily use 470R 0.25W resistors in each screen grid (g2).....this will limit the screen current and will go o/c if something is wrong.
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Old 6th March 2010, 11:13 AM   #94
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I checked everything three times, all the values are right and everything is connected correctly, there are no short wires. The one thing I could possibly think of is that I didn't connect Pin 1 to anything. It's connected to the metal base of the tube. Should that be grounded? That really can't be it.
I don't understand why this blue glow thing should happen. The two sides of the OPT should be absolutly equal. Although of course I only use the 4 Ohm tap and leave the other ones unconnected, but that is really the last thing I could think of and it shouldn't matter either.

Last edited by Herr Grau; 6th March 2010 at 11:15 AM.
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Old 6th March 2010, 05:36 PM   #95
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Quote:
Originally Posted by Herr Grau View Post
Pin 1 to anything. It's connected to the metal base of the tube. Should that be grounded? That really can't be it.
I don't understand why this blue glow thing should happen. The two sides of the OPT should be absolutly equal. Although of course I only use the 4 Ohm tap and leave the other ones unconnected, but that is really the last thing I could think of and it shouldn't matter either.
Pin I is normally grounded. However, I've seen many designs which is just ignored. You need to ascertain that the output stage is drawing the correct current, as I suspect the blue glow could be high quiescent current. You need to measure the volts across the cathode resistors with no signal. For basic measurements I always start with quite a low value 40-50mA just to start off with. It can be increased later.
I've never known this kind of amp with 6SN7's to start oscillating; there should be no sign of oscillations, without global nfb.
Make sure you are working with a dummy load. The quiescent current should be quite steady.

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Old 6th March 2010, 06:57 PM   #96
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The B+ is between 505 and 510V and I have set the quiescent current to 65mA running the amp in pure Class A. I can swap the phases again and measure what happens when the blue glow appears.
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Old 7th March 2010, 01:46 PM   #97
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Btw... I have redrawn the schematic, so it's correct and more readable:

Click the image to open in full size.

Last edited by Herr Grau; 7th March 2010 at 01:57 PM.
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Old 7th March 2010, 03:06 PM   #98
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Herr, can you post critical voltages? Bias, Plates, Cathode, etc?

EDIT - whoops, just saw you posted some of the voltages on previous post...

"I have set the quiescent current to 65mA running the amp in pure Class A"

What power are you at in Class A at that bias and B+?
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Old 7th March 2010, 05:58 PM   #99
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Looking at the Genelax Datasheet, drawing a load line from 500V 0mA to 0V 250mA (Raa of the OPT is ~4k, so every tube should see 2k, right?) and looking at 65mA takes us to a bias voltage of -47V and a power out of around 24W per tube. But as always: I can be wrong.
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Old 7th March 2010, 09:15 PM   #100
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What is the voltage across R32 and R33 respective. Tube sheets are not accurate enough due to tube variation to set up by. Must measure to set it up correctly.
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