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Old 26th July 2006, 01:27 AM   #21
poobah is offline poobah  United States
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What seems odd to me is that the voltage at output stage grids is -35 Volts but it is -43 at the bias pot... that implies grid current flowing and I don't think that should be the case. See what George has to say.

It also seems that the bias was meant to be adjusted simply by measuring the voltage at the bias pot... other amps use a resistor in the cathodes so that an idle current can be dialed in... here you have no such resistor.

To hell with the blue glow for now... I'll bet that goes away when you have an 8 Ohm speaker. Do you have another speaker to try?
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Old 26th July 2006, 02:20 AM   #22
Trout is offline Trout  United States
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Quote:
To hell with the blue glow for now... I'll bet that goes away when you have an 8 Ohm speaker. Do you have another speaker to try?

The Blue was just a secondary concern, The primary concern was that when I hit a cord or note on the guitar, The Screen gets brighter orange. I had originally thought that I was using was a 4 ohm speaker, and had the amp set on the 4 ohm tap. Turns out I had a 16ohm speaker.

So, I am limited to the 16 ohm speaker on the 8 ohm tap until Friday. (UPS Tracking says Friday delivery )

BTW, when the amp is set to the 8ohm tap, the screen glow/pulsations are not as severe though it is still a mis-match.

I was reading another thread here today, A guy has a picture of an EL34 with an isolated blue glowing point, Very much like what I had got.
Tubelab commented that this is common also.

Quote:
Every one of the new production EL-34's that I have has some blue glow inside them. The JJ's (I have tried 4)
Stands to reason my KT77's are JJ and basically being pushed as an upgrade over the EL34. The point of blue glow is different, but just as isolated.

Quote:
What seems odd to me is that the voltage at output stage grids is -35 Volts but it is -43 at the bias pot... that implies grid current flowing and I don't think that should be the case. See what George has to say.
I will re-check those readings again tonite and post them in the morning.

I have never measured the other working amp at the tube socket, I just set the voltage to -40V and ran the heck out of it,(on correct speakers).

Beats me on that one, I have no experience with this type set-up. I had only built cathode biased amps prior to these.

Maybe I should set it aside until the speakers get here Friday and see if the problem goes away lol
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Old 26th July 2006, 02:32 AM   #23
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Voltage drop across the 220K resistor means grid current, or leaky coupling caps. The -35 volts when you are expecting -43 can be the cause of glowing grid wires. I would put 10 ohm resistors in the cathode leads of the output tubes, so that you could measure the true tube current.
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Old 26th July 2006, 02:52 AM   #24
Trout is offline Trout  United States
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Quote:
Originally posted by tubelab.com
Voltage drop across the 220K resistor means grid current, or leaky coupling caps. The -35 volts when you are expecting -43 can be the cause of glowing grid wires. I would put 10 ohm resistors in the cathode leads of the output tubes, so that you could measure the true tube current.

Voltages confirmed, -44V at pot, -36 at tubes.

Seeing as this now has an entirely new set of caps and resistors, Would it be worth the effort to replace the coupling caps yet again? or just raise the voltage at the pot?

I have plenty of .1 600V coupling caps, but they are all the same brand CDE's from Justradios.com

The transformer is supposed tohave a 50V tap, But, I have no idea what ma rating it has at that tap.
The rectifer used in the bias circuit is an el cheap silicon rect rated 1000V 2.5A

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Old 26th July 2006, 02:58 AM   #25
poobah is offline poobah  United States
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Gene,

The thing to do at this point is find out why there is grid current, or something that looks like it, flowing in the first place.

Pull out the output tubes, and get measurements at the bias pot and the grid pins again... need to track down that current.

Just for giggles... does these tubes glow when you put them in another amp?



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Old 26th July 2006, 03:16 AM   #26
rpapps is offline rpapps  Antarctica
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Hi
Your anamalous grid voltage can be accounted for entirely by meter loading as I think Poobah mentioned earlier.
If your meter has an input resistance of 1 Meg, as most do, the voltage divider created by it and the 220K grid resistors and 1K5 stoppers gives (1/1.2215)*44=35.64 Volts.
Taking the same measurement on the other amp would prove the point.
Cheers
Rob
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Old 26th July 2006, 03:18 AM   #27
Trout is offline Trout  United States
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Quote:
Just for giggles... does these tubes glow when you put them in another amp?

Thats the interesting thing, all things being equal(production tolerances set aside)
These 2 amps are identical. I can move the tubes from one to the other, 1 is fine, the other is not.

I have logged no less than 50 hours on the good one. That was basically why I built this one, I figured I had a winner.

OK,
So, I will pull the output tubes, get readings, Then additionally pull preamp tubes and re-check.

Then I will add the 10ohm resistors to the cathodes and get readings.

Maybe I better add the screen resistors also.

BTW It is possible that the other unit is on the edge and not exposing iteslf.

I will pull the voltages off the bias circuit on that one also just to be sure and for comparison purposes.

Back to the basement LOL
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Old 26th July 2006, 03:24 AM   #28
poobah is offline poobah  United States
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Good point Rob!

How easy it is to forget one's own preaching!





Gene, probably no point, going through this exercise, Rob's explanation is perfect enough.

That is weird about switching the tubes/amps though...

You should really put in those screen stoppers... could be just that simple.
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Old 26th July 2006, 03:27 AM   #29
Trout is offline Trout  United States
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Quote:
Originally posted by rpapps
Hi
Your anamalous grid voltage can be accounted for entirely by meter loading as I think Poobah mentioned earlier.
If your meter has an input resistance of 1 Meg, as most do, the voltage divider created by it and the 220K grid resistors and 1K5 stoppers gives (1/1.2215)*44=35.64 Volts.
Taking the same measurement on the other amp would prove the point.
Cheers
Rob

You may indeed be correct, I just pulled the output tubes and got the exact same readings at the tubes, -44 at the pot, -36 at the tubes.

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Old 26th July 2006, 03:29 AM   #30
Trout is offline Trout  United States
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Ya know, wouldnt it be great if this forum had a chat room feature!!

I know a couple of computer forums I belong to like Windrivers has them, Oh what fun we could have in the evenings!! LOL
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