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Old 5th July 2005, 01:31 AM   #11
beamnet is offline beamnet  Netherlands
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Now that the thread is on a roll, why not ask something else?

I have a loud, nasty hum in my right channel.

No obvious reasons to me. All kind of things could go wrong, but it is the exact mirror of the left channel

(i have been tinkering with this for weeks)

I am really on the verge of despair.

any tube switching is useless, pulling wires, touching wires, shielding wires, nothing works. DC is as clean as the other channel, input noise is comparable.

I had the same problem when it was running with kt88's.

I am about to jump to the conclusion that the wood i used is hummy or my right ear, as i can not find anything wrong in the amp.

I can measure the hum on the coupling caps. the waveform is like__/|__/|__/|_

Both "hums" are in opposite phase after the phase splitter.

As my first stage is dc coupled, i cannot disconnect it to see if the hum is from there (or can i?)

Could i disconnect the coupling caps?

I need a way of eliminating to find the spot where it goes wrong..

BAs
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Old 5th July 2005, 07:37 AM   #12
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How are you powering the heaters? 7193s don't seem to like heater - cathode potentials much. I had a similar problem which i solved by grounding one side of the heater supply (not sure why only one channel was affected - put it down to slight variations between tubes).
Things won't be helped in your circuit by the fact that the ECC99 cathodes are at at fairly high voltage. You need to check the maximum heater - cathode potential allowed with these tubes. (you might need to use a separate heater supply for the ECC99s)
Whatever you do, the heater supplies need to be tied to either ground or something at a similar potential to the cathode. Don't just leave them floating or you will get hum problems
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Old 5th July 2005, 08:30 AM   #13
beamnet is offline beamnet  Netherlands
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Ok..

Swapping the 7193 with a 6sn7 yields the same problem, and it' s heaters are referenced to ground with a center tap.

Th ecc 99's are referenced to gnd with a 1uF cap on the centertap, dunno why, but thatis what the schematic said. Tying it directly to ground made no diference. Both channels come fom the same winding..

The GU50's all have their seperate 12,6 volt dc supply (couldn't get 3A regulators, so used a 1,5per tube)

they are run from the same winding. How do i reference those? Would i get problems if i would hook em to gnd with a voltage devider? Or is grounding the centertap before rectifying enough?

Bas
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Old 5th July 2005, 04:18 PM   #14
beamnet is offline beamnet  Netherlands
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Tried elevating heater voltage references, but no audible difference ( i used a 2.2meg resistor with a 470K pot in series over the B+ and connected the centertap to the middle, ajusting the pot does not make a difference.

Still the question, can i decouple the coupling caps safely?


Bas
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Old 22nd July 2005, 02:34 PM   #15
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this tube is very famous in our country,for it was for military use.it's called ry50/fu50.i believe the gu50 is not as good as ry50/fu50, wihch is one of the best tubes china had built.the other is fu5(=805),which is almost the best 805 in the world.
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Old 23rd July 2005, 08:58 AM   #16
Colt45 is offline Colt45  Serbia
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Quote:
Originally posted by VanguardVillage
this tube is very famous in our country,for it was for military use.it's called ry50/fu50.i believe the gu50 is not as good as ry50/fu50, wihch is one of the best tubes china had built.the other is fu5(=805),which is almost the best 805 in the world.

I'd assume the chinese version is made on (perhaps modified) russian tooling anyways.

The soviet version is indeed a duplicate itself.

German army was using it already in 1935 (?)

datasheet
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Old 28th May 2006, 03:38 PM   #17
gemby is offline gemby  Croatia
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Quote:
Originally posted by beamnet
By the way, i bodged a schematic together for the interested persons.

This is my personal adaption of a kt88 circuit, please feel free to comment
beamnet, what are those yelow spots in schemtics?
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Old 28th May 2006, 07:27 PM   #18
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Quote:
beamnet, what are those yelow spots in schemtics?
Obviously test points for biasing / cathode current.

Tom
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Old 28th May 2006, 08:24 PM   #19
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Quote:
Originally posted by beamnet
thanks!

I got it working now


I read somewhere that somebody made these tubes run in ultralinear. What would be the consequence if i hooke the g2 to the ultralinear tap?
I connected the g3 to the cathode.

I'm still very fresh on pentodes, but i'm excited i got them to work! (plus they look ultracool in their cast aluminium enclosures)


Bas

I wouldn't advise getting to UL taps until everything else in the amp works the way it should and the expected power is achieved and is clean......The reason is connecting g2 to UL can stir up more trouble with an RF tube like LU50. I've done several designs not using this tube but other high gm tubes which often oscillated...(and that was without global feedback) .....The solution is a g2 to anode snubber..... but estimating the correct R/C values requires experience.


richj
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Old 8th June 2006, 03:45 PM   #20
gemby is offline gemby  Croatia
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Hi beamnet, is it posible to replace 7193 with 1/2 ecc82? What trafo have you used for output?
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