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Old 3rd August 2015, 09:24 PM   #1
Celsius is offline Celsius  Spain
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Default voltage drops HELP

I use one 6sn7 both triodes per channel in cascade coupled by cap, in one channel of the the second stage begin with P 220v RG 220k K 6V with resistor load or CCS at 9mA but after 20 or 30 minutes the voltage drops 2v por minute to P 100v.

Different tubes give the same result
I change the the K resistor R750, same result.


I am going to welding again all pin of the socket, but I would like to know why, I feel really curious.

Any ideas.
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Old 3rd August 2015, 10:29 PM   #2
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Circuit?
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Old 3rd August 2015, 10:55 PM   #3
gofar99 is offline gofar99  United States
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Yes please post the circuit. A guess from the clear blue sky would be the one CCS is heating up and becoming non-linear. Usually they go in the opposite direction in my experience.
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Old 3rd August 2015, 11:22 PM   #4
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Check the voltage not with a digital meter but by a scope! if your smoothing cap is damaged, you will see a huge ripple, and a DVM reads less DC voltage, when infact you have a lot of AC.
Also check your AC at transformer taps, when DC voltage has dropped considerably, to make sure it is not your transformer.
Also you could put a 100R resitor in series with supply line, so you could measure current through it when DC has dropped to see if you are drawing too much current.
Circuit definitely helps.
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Old 4th August 2015, 05:57 AM   #5
Celsius is offline Celsius  Spain
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This schematic is very similar to mine.

only the the fist stage load by CCS 10m45 and the second I tried both resistor and CCS with the same problem. this problem begun after I rewire the socket, solution is easy rewire again, but I feel really curious to know why.


Click the image to open in full size.
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Old 4th August 2015, 06:36 AM   #6
Celsius is offline Celsius  Spain
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Ha ha ha the couple cap between 6sn7 is the guilty, begin to don't stop the DC from the first stage plate to the grid of the second stage. the bias run from 6v to 0v and the voltage plate run in the same direction reducing the voltage to give the 8mA.
Resolved thanks
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Old 24th August 2015, 08:07 PM   #7
Celsius is offline Celsius  Spain
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Default The problem follow

After holidays I have checked again the 6sn7 grid of second stage and I have again the same problem, the voltage of the plate drop because the grid of the second stage have DC voltage.
Begin wit 0,3v after 30 minutes have 4V only in one channel.
I have change the 10m45s of the plate, the cap between the stages and the problem follow.

Ideas why?
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Old 25th August 2015, 01:09 AM   #8
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Another bad cap? Same batch? But first swap tubes between channels to eliminate them as the problem's source.

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Chris
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Old 25th August 2015, 08:33 AM   #9
DF96 is offline DF96  England
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What caps are you using? Voltage rating? (Need over 400V for that circuit). Dielectric? (Avoid paper, as it absorbs water and becomes leaky). New? NOS?
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Old 25th August 2015, 09:14 AM   #10
Celsius is offline Celsius  Spain
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Never is only one guilty.

Wax oil caps 1000v 0.22uf when begin to heat begin to give same DC 0.4v to 1.5v. To check the amp the position is upside down and the temperature inside the amp is higher that in the standard position.

the old Raytheon 6sn7, one of them, has a problem and the grid have 3v dc more in the grid that the other channel.

I was crazy to find the problem, when I put the Teflon 400v 0.1uf K72 cap and the problem following don't understand what happened. After replace the Tubes and the Teflon cap all is (seem) running perfectly with 0v ( at the moment )
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