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Old 8th July 2009, 01:29 AM   #1
ixe13 is offline ixe13  Canada
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Default Weird Bias on Simple SE

Hi,

I get these funny values on the KT-88 tubes on my Simple SE.

Left channel: plates 384 volts, cathode current: 67Ma
Right channel: plate 388 Volts, cathode current: 87 Ma

What's weird about it, is that if I switch the tubes, I get the same value??!!? it doesn't follow the tubes...

What would cause both tubes to draw more current on one side?


Thanks

Yves
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Old 8th July 2009, 01:52 AM   #2
chrish is offline chrish  Australia
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Have you checked that the cathode resistors are the same value?
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Old 8th July 2009, 01:55 AM   #3
ixe13 is offline ixe13  Canada
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Hi Chrish,

Yep they are the same value,

Bizzare!
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Old 8th July 2009, 02:56 AM   #4
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The channel with the highest current has the highest plate voltage. This doesn't make sense. More current should equal more voltage drop across the OPT.

Does the amp work OK otherwize?
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Old 8th July 2009, 03:34 AM   #5
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Quote:
Originally posted by tubelab.com
More current should equal more voltage drop across the OPT.
Unless something is wrong with one of the output transformers. He said the cathode resistors are identical. Try measuring the DC resistance of each output transformer's primary.
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Old 8th July 2009, 04:38 AM   #6
ChrisA is offline ChrisA  United States
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Default Re: Weird Bias on Simple SE

Quote:
Originally posted by ixe13

I get these funny values on the KT-88 tubes on my Simple SE.
The voltages are within 2% of each other. Are you using 5% tolerance resisters? If so that might explain it. Even with 1% resisters you could see this. Also I wonder if the two sides of the OPT are really identical.

The good news is that your ears work on a logarithmic scale and I doubt you could hear even a 20% voltage unbalance
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Old 8th July 2009, 05:52 AM   #7
fredex is offline fredex  New Zealand
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1. Check to see if there is any positive voltage on the grid, you may have a leaky coupling cap.

2. When you say the cathode resistors are the same, did you measure them with the tube removed from the socket?
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Old 8th July 2009, 05:58 AM   #8
chrish is offline chrish  Australia
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OK, I am not an electronics engineer, and I often need help with the basics on this forum from the "Gurus".

You have a difference in current between channels through output tubes. Problem does not change channels when changing tubes. How have you deduced this? Is it from measuring the voltage drop across the cathode resistor? Assuming this is yes, the problem cannot be the tube, as problem remains fixed when swapping tubes. The only other components in the current chain are:

The power supply, which is common to both channels - so problem cannot be here.

Cathode resistors, which you have measured as identical (you have measured them haven't you...) so problem not here.

Output transformer. There may be a problem with the transformer, but I would guess it is more likely that you have mis-wired them. Check wiring thoroughly, including for shorts from small strands of wire from screw terminals. If you have not done this already, wire up for simple triode mode only with no cathode feedback to help eliminate chance of mis-wiring.

Experts may be able to offer advice here, but maybe a faulty cathode bypass capacitor may cause problems too. Is it wired correctly (polarity?), has it failed?

Best of luck,

Chris
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Old 8th July 2009, 10:40 AM   #9
ixe13 is offline ixe13  Canada
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Thanks guys,

I'm off to work, will check it tonite

Bye for now...
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Old 9th July 2009, 12:28 AM   #10
ixe13 is offline ixe13  Canada
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STOP THE PRESS! STOP THE PRESS!

It's following the tubes. My mistake... dunno what I did yesterday, must have been zombie for a while there...

I redid the test tonite.

Here's what I'm doing. Once in a while I borrow this gadget from a friend of mine who own about 6 guitar amps :

Bias Rite

and use it to check the Bias on my Quad II.

Now I used it on the simple SE to check the KT-88 that I bought a month ago.

What's Happening is that both tube get to around 73 mA pretty fast and then as one goes slowly up to 90 mA , the other one drop a bit to 69 and stay there. I let the amp on for 10 minutes for the test.

When I first put the KT-88 in a month ago, they did draw the same amount of current. They were perfectly matched.
Not the case anymore...

I use the 560 Ohms cathode resistor with the Hammond 374BX power transformer and a pair of Hammond 1628SAE

And to answer you question George, yep, it's working fine.

I must say this amp has a pretty good sound. Goes deep in the low end and I dont find it lack Hi-end either.

But then, I like an amp with a darker sound. that's why I wanted to try the simple SE,
after reading George's "report" on how it sound with KT-88 and the huge Hammond OPT, I decide to go for it. I figured, it's a good way of trying an SE amp and still have the deep bass I like.



So I replaced the cathode resistor for the 620 Ohms one and tried 2 old JJ 6l6GC for 10 minutes, they both sit steady at 54mA. They get there pretty fast, (less than a minute) and then stay there...


So, Is it that one of the KT-88 in already dying?

That'd be a bit disapointing... considering the price.
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