Quicksilver 8417 bias runaway? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st September 2004, 07:16 AM   #1
popeye4 is offline popeye4  United States
diyAudio Member
 
Join Date: Aug 2004
Location: Boulder Creek, CA
Question Quicksilver 8417 bias runaway?

I recently picked up a couple of Quicksilver 8417 amps (the originals, not the GE version). One of them is exhibiting runaway bias current after warming up for a few minutes. I've backed the bias pot all the way down, but using the ammeter supplied with the amps, I hear a crackle then the ammeter starts to climb quickly (and pegs). The amp blew a fuse the first time this happened (I was listening, not monitoring the bias current). I swapped the 8417 tubes between the amps and the problem did not go with the tubes.

Does this sound like a bad cap? Or do I have more expensive problems to resolve?

Thanks for any info,
Doug
  Reply With Quote
Old 1st September 2004, 11:01 AM   #2
diyAudio Member
 
Join Date: Nov 2002
Lets see if I can post a schematic
Attached Images
File Type: jpg qs8417amp.jpg (46.8 KB, 554 views)
  Reply With Quote
Old 1st September 2004, 11:10 AM   #3
diyAudio Member
 
Join Date: Nov 2002
Doug

I own several versions of the Quicksilver 8417 amplifier. There is NO difference other than the selection of the feedback resistor and several other resistors. The feedback amount remains the same between the different vintages. As far as their being a difference that allows you to use either the GE or Phillips tube I will cry BS.

I would suggest never changing tubes when there is a problem of this nature. The 8417's are costly and hard to come by. Just my .02 I will move aside while Frank answers your question. The fix is easy.
  Reply With Quote
Old 1st September 2004, 11:21 AM   #4
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Sounds like one of 0.180µF coupling caps is leaky.

Try a new one_or swap them around_ and see waht that does.

Cheers,
__________________
Frank
  Reply With Quote
Old 2nd September 2004, 05:12 PM   #5
popeye4 is offline popeye4  United States
diyAudio Member
 
Join Date: Aug 2004
Location: Boulder Creek, CA
Thanks for the info, guys. It goes to the shop today, I just wanted an idea of what it might set me back.

Thanks, Doug
  Reply With Quote
Old 2nd September 2004, 09:21 PM   #6
diyAudio Member
 
Join Date: Nov 2002
Doug

There is no need to send her to the shop. Go to your local parts store and purchase 2) caps and replace them yourself. When your done treat yourself to your favorite bottle of scotch.
  Reply With Quote
Old 3rd September 2004, 01:14 AM   #7
popeye4 is offline popeye4  United States
diyAudio Member
 
Join Date: Aug 2004
Location: Boulder Creek, CA
Well, I did get the caps and put them in myself. The amp now works fine. The guy at the shop I frequent had them (0.22 uF rather than 0.18, but he assured me that was OK); no one else had them.

Thanks for the help and encouragement. I just wasn't properly motivated. (I like Scotch!)

Doug
  Reply With Quote
Old 3rd September 2004, 10:43 PM   #8
diyAudio Member
 
djmiddelkoop's Avatar
 
Join Date: Feb 2003
Location: Noord-Holland
Quote:
The 8417's are costly and hard to come by
I still have some NOS and good used ones.
If someone is interested, please send me a mail.

Dick.
__________________
“Information is not knowledge.Knowledge is not wisdom.
Wisdom is not truth.Truth is not beauty.Beauty is not love.Love is not music.Music is THE BEST.”
  Reply With Quote
Old 3rd September 2004, 11:23 PM   #9
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Well, I did get the caps and put them in myself. The amp now works fine. The guy at the shop I frequent had them (0.22 uF rather than 0.18, but he assured me that was OK); no one else had them.
Forgot to mention it...
Ideally the caps should be 600V MKP types or better and matched within 1% for both monoblocks.

If you can get hold of reasonably priced polypropylene film & foil cap such as MIT, RelCap or TRTs give them a try.
No doubt in my mind there are other good ones but at least I know these are excellent.

Best policy is to change the caps in both blocks, you don't want to end up with two disparate channels.

In case you plan on modding the amps to take the GE 8417s and change the bias scheme, don't forget that, since the gridleak resistors are substantially lower, you'll need to use higher value coupling caps (0.47µF to 0.68µF/600VDC).
It doesn't hurt to put in these values now even if you plan on using the GEs later on.

Instructions on how to adapt the amp for use with the GE8417s can be found here:

UNCLE NED'S ADVISE.

The complete schematic can be found here:

QS 8417

Cheers,
__________________
Frank
  Reply With Quote
Old 3rd September 2004, 11:36 PM   #10
diyAudio Member
 
Join Date: Nov 2002
Frank


Please explain the differences between the Phillips and the GE version. I have both versions and they are the same in the bias section. There are different values used in the feedback loop. I have found only two resistors that are different in the amps.

Joe
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Quicksilver 8417 conversion Original Burnedfingers Tubes / Valves 25 18th December 2011 10:33 AM
Quicksilver 8417 question john65b Tubes / Valves 1 24th October 2008 07:40 PM
Quicksilver 8417 Monoblocks dshortt9 Swap Meet 0 5th December 2003 03:57 PM
quicksilver 8417 hum dreaded 42 Tubes / Valves 4 11th September 2003 02:45 AM
Quicksilver 8417 dreaded 42 Tubes / Valves 4 4th September 2003 11:14 PM


New To Site? Need Help?

All times are GMT. The time now is 02:11 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2