Quicksilver Bias Problem - 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 28th March 2004, 03:33 PM   #1
diyAudio Member
 
Join Date: Nov 2002
Default Quicksilver Bias Problem

Having intermittent bias problem with one of my 60watt 8417 converted mono block amps. With the bias set at 100mA hot it will sometimes change to 50-55mA. The bias pots were changed when I converted it a while back. A small hum is also heard from the speaker when this happens. I can turn it off wait 2-3 minutes and turn it back on and have it behave ok or it can rear its ugly head and change its bias again after its been on a while.

Any ideas?

Thanks
J
  Reply With Quote
Old 28th March 2004, 03:45 PM   #2
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default bad tube?

Is one of the output tubes bad? That would cause loss of half of current and hum from resulting loss of 2nd harmonic cancellation of power supply ripple.

Michael
  Reply With Quote
Old 28th March 2004, 04:28 PM   #3
diyAudio Member
 
Join Date: Nov 2002
Michael

That is surely worth a try. I will switch the output tubes and see if the problem goes to the other amplifier and report my findings.

Joe
  Reply With Quote
Old 28th March 2004, 04:36 PM   #4
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default intermittent

Could be an intermittent solder joint in the amp circuit as well. Any crackle when it drops back?

M.
  Reply With Quote
Old 28th March 2004, 05:05 PM   #5
GaryB is offline GaryB  United States
diyAudio Member
 
Join Date: Sep 2002
Location: Silicon Valley
I'm not sure I know whats meant by a converted monoblok. Does it still run 8417s or have you switched to another tube? If its still running 8417s, then stability problems are a well known phenomena and are caused by 8417 tubes that draw too much grid current. Certain 8417 tubes work well (Sylvanias are the ones I remember) while others cause the instability you mention. I think I remember GE's had this problem.
You can fix it by modifying the amp to be more tolerant of higher grid current. All you really need to do is lower the grid resistor and optionally to increase the value of the coupling caps to keep the same LF roll off. Details are documented on the Triode Electronics website, along with other possible mods.

http://www.triodeel.com/8417.htm

If its not running 8417s, then I'd try swapping tubes between channels and see if it moves with the tubes. Could be a leaky cap or a a bad solder connection as others have mentioned.

---Gary
  Reply With Quote
Old 28th March 2004, 05:08 PM   #6
stormy is offline stormy  United Kingdom
diyAudio Member
 
Join Date: Mar 2004
Location: Scotland
Hi,
is 1 of the valves starting to glow red when this happens,
could be runaway on a bad valve.

if the amp is more stable when biasing neerer cutoff then I
think u need to get new valves.

just a guess



Regards stormy
  Reply With Quote
Old 28th March 2004, 06:15 PM   #7
diyAudio Member
 
Join Date: Nov 2002
GaryB

The amps are running KT88's now instead of the original 8417's. I still have enough 8417's but I save them for special listening

Stormy

No glowing red, both valves look normal.


audiobot

Michael, the problem followed the valves to the other amplifier. I installed another pair of KT88's and the problem was resolved. Never saw this problem before. Its one I will remember from now on.

Thanks to all.

Joe
  Reply With Quote
Old 28th March 2004, 06:24 PM   #8
diyAudio Member
 
Join Date: May 2003
Location: Colorado
The internal steel leads of the tube are soldered inside hollow brass pins on the tube base. Sometimes this connection is poor (intermittent) and you need to re-solder this connection. It just takes a minute or two.
  Reply With Quote
Old 29th March 2004, 07:20 PM   #9
diyAudio Member
 
Join Date: May 2003
Location: Colorado
Quote:
Originally posted by burnedfingers
Never saw this problem before. Its one I will remember from now on.

Joe
I guess I wasn't too clear in my previous post. I've seen this happen before, and it is a bad solder joint inside the tube pin that is causing the problem. The internal steel lead exits through the bottom of the brass sleeve, and is trimmed flush after being soldered. Steel is not the easiest thing to solder, so it's not that uncommon to see a bad connection here.

Instead of throwing the tubes away, you can simply re-solder the connection. Since you probably don't know which pin is bad, add a little bit of solder to each one. Be sure to touch the tip of the iron only to the tip of the brass sleeve -- you want to avoid getting any solder on the side of the brass sleeve, where it would change the dimensions of the pins.

(This procedure only applies to tubes with an external base like most power tubes. Miniatures like the 12AX7 have a different system whereby the internal wire is the same piece of metal as the external pin.)
  Reply With Quote
Old 29th March 2004, 11:45 PM   #10
diyAudio Member
 
Join Date: Nov 2002
Thank you Charles I will give it a shot and see what happens.

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
Bias problem- 1 Socket shows extremely low bias! otokomae Tubes / Valves 12 31st December 2008 11:01 PM
Quicksilver 8417 bias runaway? popeye4 Tubes / Valves 37 13th January 2008 01:18 AM
bias adjust installation, bogen mo-100a mo 100a quicksilver apehead Tubes / Valves 2 27th December 2006 03:13 PM
Problem with Quicksilver amp Original Burnedfingers Tubes / Valves 82 21st March 2003 12:20 PM
auto bias on a Quicksilver KT-88 amp Original Burnedfingers Tubes / Valves 4 15th February 2003 03:40 PM


New To Site? Need Help?

All times are GMT. The time now is 12:09 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