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Old 13th January 2013, 02:10 PM   #11
banaca is offline banaca  Canada
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Quote:
Originally Posted by jrenkin View Post
My guess, since you CAN bias to 20mA, is that the bias pot is the problem, or one of the other resistors in the bias circuit. Maybe check those.
I bias tubes individually with a meter on my amp ( meter integrated in the face plate) Normally I bias my kt88 to 45ma. Now all tubes bias 20ma max.
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Old 13th January 2013, 02:16 PM   #12
kevinkr is offline kevinkr  United States
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These are ultra-linear amplifiers based pretty closely on the old Williamson design and usually with ridiculously huge electrolytic caps in the supply (tube rectification by 5AR4 which usually don't live that long in these amplifiers.)

Most likely the screen stopper resistors are fried if fitted, check them first (pin 4 of each KT88) if present.

Quicksilver does not provide schematics for servicing any of their products as best I have been able to figure out. Many years ago I attempted to get schematics for a pair of monos I was servicing and was unable to do so. They aren't particularly complex, but there have been a number of variations. I no longer remember all of the details but this particular pair utilized 8417 output tubes and I converted them for KT88. (Note that I no longer service anything! )
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Old 13th January 2013, 02:25 PM   #13
banaca is offline banaca  Canada
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[QUOTE=kevinkr;3323848]These are ultra-linear amplifiers based pretty closely on the old Williamson design and usually with ridiculously huge electrolytic caps in the supply.
Most likely the screen stopper resistors are fried if fitted, check them first (pin 4 of each KT88) if present.

Great info! Thanks!
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Old 13th January 2013, 02:38 PM   #14
kevinkr is offline kevinkr  United States
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jrenkin's comments about the bias supply are worth heeding as well, and I have just looked at that amplifier on line and realize it differs in significant ways from the much older ones I have worked on. You might also want to check the power supply voltages.

Note that the fact that you can't set the bias any higher than 20mA could indicate that bias supply voltages are too high for the tubes used. It may also indicate problems with screen resistors if indeed it is UL or with the screen supply if tetrode mode is employed.

Take the bottom cover off and snap some very detailed pictures which might give us all a clue as to what might be going on since you can't get the schematic.

I would recommend you find a good local tech unless you have test equipment and are comfortable working on tube amps which I suspect you may not be.
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Old 13th January 2013, 11:59 PM   #15
banaca is offline banaca  Canada
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Originally Posted by kevinkr View Post
jrenkin's comments about the bias supply are worth heeding as well, and I have just looked at that amplifier on line and realize it differs in significant ways from the much older ones I have worked on. You might also want to check the power supply voltages.

Note that the fact that you can't set the bias any higher than 20mA could indicate that bias supply voltages are too high for the tubes used. It may also indicate problems with screen resistors if indeed it is UL or with the screen supply if tetrode mode is employed.

Take the bottom cover off and snap some very detailed pictures which might give us all a clue as to what might be going on since you can't get the schematic.

I would recommend you find a good local tech unless you have test equipment and are comfortable working on tube amps which I suspect you may not be.
I will try to post some pictures this week ( the inside) I'm bringing the amp to a tech monday night and I will give him a copie of this discussion..:-)
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Old 18th January 2013, 12:46 AM   #16
banaca is offline banaca  Canada
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The amp was checked by my tech.... I destroyed the meter... It can not be fixed... I will have to order another one from Quicksilver. ( got some help from them too. they are a good company) It is only 40 dollars for a new one. Here is a pic of the amp ( bottom) . I will be happy when it is all fixed....
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Old 18th January 2013, 03:24 AM   #17
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Those broken key posts can be a real pain in the a*r*s*e. I bet you'll be a little more careful next time!
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Old 7th February 2013, 02:29 AM   #18
banaca is offline banaca  Canada
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My amp is fixed now. We put the new meter in.... I'm happy now....
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Old 7th February 2013, 03:00 AM   #19
tim614 is offline tim614  United States
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is that a Christmas light i see in there? what is that for?
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Old 7th February 2013, 11:46 AM   #20
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if one channel is working correctly, it can provide an easy way to check voltages and resistance. Start measuring and something will be bound to pop up.
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