Bell 2122 volume drop off - 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 March 2009, 12:06 AM   #1
diyAudio Member
 
Join Date: Jan 2009
Question Bell 2122 volume drop off

I have a Bell 2122 Phono/Mic amp with a problem: after about 3/4 volume, the output level drops off. Any suggestions for a novice, hobbyist-type would be greatly appreciated. Thanks
  Reply With Quote
Old 27th March 2009, 08:08 AM   #2
m6tt is offline m6tt  United States
diyAudio Member
 
Join Date: Aug 2007
you either have a bad pot, or a parasitic oscillation that is being picked up at higher gain settings.

Does the amp have a resistor in series with the first grid?
Try 50-100k between input jack and first grid. Eventually you may want to put 1k series resistors on the power tube grids, but that's only if the oscillations continue. These resistors acts like a filter using the tubes internal capacitance to AC ground (B+)...they form a low-pass at fairly high frequencies (generally above hearing, def above guitar amp range).

Have you tried the "chopstick" method of pocking around with an insulated chopstick to see if you can affect the frequency/onset of the problem? This checks to see if stray coupling or capacitance is introducing a signal back into earlier stages leading to a positive feedback loop.

Try removing all cathode bypass caps, they're electrolytic, way old, and they're not always good for tone when overdriven. They will also tend to help the valve amplify radio frequencies spuriously as they get wonky.

Make sure all internal wiring is short, and isn't all running parallel with other wires, long parallel wires can couple better than short, perpendicular ones.

Experiment with different feedback cap/resistor combinations...you probably need less high frequency response somewhere.

If this thing is as old as I'm guessing, I would at least consider replacing all the resistors in the power supply with newer, quieter resistors, replacing the power supply caps, and possibly the coupling caps into the power tube grids. This will ensure quiet, hiss-free operation, making a much better guitar experience.

This is honestly the hardest part of design for me, since I always do point-to-point. It takes some practice to find the culprits...make sure you are using a big soldering iron, it makes all the tube stuff (esp. old tube stuff) easier to work with. The radioshack 30w is the worst thing ever done to newbies!
  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
Intermittent drop-out and crackle at low volume Sophie424 Solid State 6 25th February 2009 06:30 PM
Marshall volume drop off help 4lf Tubes / Valves 3 7th February 2009 08:26 PM
Bell 2122-B integrated amp - tube suggestions Lockerpuzzle Tubes / Valves 0 30th April 2007 10:41 PM
Bell schematic hurdy_gurdyman Tubes / Valves 0 15th April 2003 01:21 AM


New To Site? Need Help?

All times are GMT. The time now is 03:13 AM.


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