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Old 27th September 2004, 03:17 AM   #1
coffin is offline coffin  Taiwan
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Default My 6C45 DC 6C41, advice wanted

Hi
This is my recently made 6C45PE direct coupled 6C41C Single endeed amp.
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when I plug it on the speaker, it got hum and fizz. of course I have to near the speaker to hear it. Is this circiut OK?

please advise....

Thanks!

Coffin
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Old 27th September 2004, 11:34 AM   #2
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Wow finally another design with the 6C41! Nice...How do you like the sound?

The hum is most likely a grounding problem.

Looking at the size of the caps/chokes etc, you have used there should be very little psu ripple?

The fizz is probably because of the 6C45pi...You could try larger grids stoppers and/or...ferrite beads and ceramic caps on the heaters etc...to quiten it down...

PS..beautiful amp!

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Bas
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Old 27th September 2004, 11:51 AM   #3
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Hi,

Quote:
The hum is most likely a grounding problem.
My thoughts as well.
If it is build exactly as shown on the drawing then the entire amp is floating with respect to ground.

From the picture it's hard to tell whether or not the WBT RCA input grounds are connected to the topplate of the chassis (or other ground for that matter).

Is it necessary to tie the heater of the 6C41 to its cathode?
According to the Russian datasheet and allowing for the fact that my reading of anything cyrillic is basic at best, I think heater to cathode insulation is +/- 300V which should be more than ample giving the B+ used.

Quote:
PS..beautiful amp!
The woodwork reminds me of the LINN LP12 turntable.
Well done.

Cheers,
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Old 27th September 2004, 12:12 PM   #4
coffin is offline coffin  Taiwan
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Hi
Thanks for replying.

The ground is attached to topplate from 120uF. The RCA ground is attached to 120uF by a 2mm diameter wire.

But which is the best way to ground?

And yes, it is not necessary to bias filament of 6C41.....I'll detach it.

I'll report the new mods if I got time to do it.

cheers

Coffin

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Old 27th September 2004, 01:22 PM   #5
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Quote:
Originally posted by coffin
And yes, it is not necessary to bias filament of 6C41.....I'll detach it.
Is your filament just "floating"? If so, try grounding it out through a 10uF electrolytic of appropriate voltage. Just one side is OK. That cured one of my hiss/buzz/hum problems that biasing the filament did not
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Old 29th September 2004, 01:21 AM   #6
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Hi,

Unless you have a specific reason for having the input impedance at 10K, I'd suggest upping it to about 100K instead.

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Old 29th September 2004, 02:06 AM   #7
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Quote:
Originally posted by fdegrove
Hi,

Unless you have a specific reason for having the input impedance at 10K, I'd suggest upping it to about 100K instead.

Cheers,
Hi
Thanks!

I did this because I read about the Cag of 6C45 is 11pF. If the input resistor is too large, Miller effect will affect the high frequency response.

Is 100K OK for this situation? Or the 11pF was wrong?


cheers

Coffin
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Old 29th September 2004, 02:26 AM   #8
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Hi,

Quote:
Or the 11pF was wrong?
Cag for the 6C45-P is 1.1 pF not 11.

Naturally it's not just Cag and Rin that determine the frequency response but stray capacitance, stage gain and Cgk as well.

With an input resistance a little on the high side you'll also be able to control the 6C45-P's tendency towards oscillation (high Gm) much better and it will make driving the amp a much easier task especially for a tube preamp.

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Old 29th September 2004, 03:11 AM   #9
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Gee ....1.1pF......Thanks!


Coffin
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Old 29th September 2004, 04:08 PM   #10
coffin is offline coffin  Taiwan
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Hi
Today I modified the amp. First change input resistor to 100K ; then link floating 6C41's filaments to ground with 22uF ; and adjust the grounding.

But the major mod was change PSU of 6C41 from LCRC to CLCRCRCRC.....
Because the choke fuzzed in choke-in style.....
After mods, although the choke is still vibrating, but the fuzz was gone.

The result is not bad. Hum and fizz from speaker are hardly hear.

I'll take this amp to friend, ask him to test the specs for me.

The sound is really good......

Modified schema
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Some pics
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BTW, why is the choke in LC style fuzz? How to eliminate it?

cheers

Coffin
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