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Old 10th March 2003, 11:42 AM   #1
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Default Problem with Quicksilver amp

I was listening to crystal clear music only to have it interupted by some nasty sounding static and noise. I traced this down to a Mica? small value cap on pin 3 of the output tube socket. I had a small value cap rated at 600volts so I replaced it to see what happens. It was clean sounding again.

Question :

Anyone know the value of this cap, I haven't seen one of these in over 40yrs. It looks like it has been hot and I can't read the paint dot scheme that would probably tell me what the heck it was. The schematic looks like it shows 100UF or is it PF?

Also there is a 5K resistor shown in the feedback on the schematic and their is a 562 ohm in the amp.

Any help would be appreciated.


Joe
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Old 10th March 2003, 12:56 PM   #2
EC8010 is offline EC8010  United Kingdom
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100uF in mica would be enormous. It is 100pF. When you say you can't read the colour code, do you mean it can't be discerned, or that you can't interpret it?
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Old 10th March 2003, 01:40 PM   #3
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Joe,

If you can't post the schematic can you tell us what the output tube is? It would then be possible to determine the function of the cap and possible values from the pin connections...

(i.e. if pin three is the cathode, 100uf would be ok as a cathode bypass, but if pin three was the anode, then with 100uf you wouldn't be hearing anything!)
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Old 10th March 2003, 02:49 PM   #4
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Default RE:Problem with Quicksilver amp

Hi,

Quote:
(i.e. if pin three is the cathode, 100uf would be ok as a cathode bypass,
It connects from pin 3 (the anode of a 8417) to the NFB loop and EC8010 is correct 100pF is the value.

Quote:
Also there is a 5K resistor shown in the feedback on the schematic and their is a 562 ohm in the amp.
If that 562R is correct than that's a hell of a lot of FB.

Cheers,
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Old 10th March 2003, 11:11 PM   #5
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Frank

Yup its a 562ohm 1% nasty looking black resistor about the thickness of my little finger and I would assume its a 3 or 5 watt job. The resistor is going to be replaced per the 8417 conversion guide that was on the board a while back. I think I will use somewhere around 270 ohm.

I wasn't able to purchase a mica capacitor but did snag a 100pf/600volt film to replace the original.

What are the correct values of the resistors on the cathode of the 12AX7 to ground? I think I can see a 820 but the other value is blurred on my schematic.

Any suggestions for hot rodding this garage amp?

Per Mike at Quicksilver the impedance of the output transformer is Quote "Closer to 3900 ohms" not 4200ohms.

The schematic I posted looks much better than what printed off for me.

Joe
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Old 11th March 2003, 12:05 AM   #6
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Default QS.

Hi Joe,

Quote:
Yup its a 562ohm 1% nasty looking black resistor about the thickness of my little finger and I would assume its a 3 or 5 watt job. The resistor is going to be replaced per the 8417 conversion guide that was on the board a while back. I think I will use somewhere around 270 ohm.
Ouch,even more FB?..Beyond me really, why? What board you mean?

Quote:
I wasn't able to purchase a mica capacitor but did snag a 100pf/600volt film to replace the original.
Should be fine.

Quote:
What are the correct values of the resistors on the cathode of the 12AX7 to ground? I think I can see a 820 but the other value is blurred on my schematic.
820R on top and 220R at the bottom.

Quote:
Any suggestions for hot rodding this garage amp?
Depends on what you want to achieve.

Quote:
Per Mike at Quicksilver the impedance of the output transformer is Quote "Closer to 3900 ohms" not 4200ohms.
He should know, the ones I saw had 4K2 written on the OPTs but who am I, right?

Cheers,
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Old 11th March 2003, 12:09 AM   #7
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Default 5k or 562 Ohms...

Quote:
Also there is a 5K resistor shown in the feedback on the schematic and their is a 562 ohm in the amp
If the feedback resistor it was 562 Ohms the voltage gain of your amp wil be 3,5X ...so with a 1Volt input you only get 3.5 Volt at the output...

So definitively must be 5k ...for the voltage gain to be 24 X..


Quote:
What are the correct values of the resistors on the cathode of the 12AX7 to ground? I think I can see a 820 but the other value is blurred on my schematic
It seems to be 220 Ohms

Regards
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Old 11th March 2003, 12:15 AM   #8
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Default Speedy Gonzales...

Frank

You are so fast!!!!!
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Old 11th March 2003, 12:32 AM   #9
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Default RE:Speedy Gonzales...

Hi,

Quote:
Speedy Gonzales...
Wait till Summer...I am still hibernating.

You're not too bad either in the speed department...

Um abracao, (wrong keyboard settings to spell it correctly)
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Old 11th March 2003, 12:40 AM   #10
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You guys are really quick.

Ok, lets throw another twist on things here. The feedback resistor is on the 8ohm leg of the transformer. The transformer is a 3400 ohm Hammond 1650K unit. The original Quicksilver OPT transformers are gone. Whats the gain now?



Joe
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