Bassman 50 red plate and bias - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 17th September 2012, 02:31 AM   #1
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Default Bassman 50 red plate and bias

I'm working on a friends Fender BASSMAN 50 (Schematic here : http://ampwares.com/schematics/bassman_aa165.pdf) and it is running what I believe is a bit hot.

B+ is 465V
Grid bias is -45V

There is no bias adjustment, but there is a balance pot.

I can adjust it so both tubes are just slightly red at the seam of the fin (best seen with the lights out).

I'm considering increasing the value of the 15K resistor from the wiper of the balance pot to ground. This will increase the bias and reduce the cathode current.

So my basic question is "Is this how guitar amps run (which contributes to short tube life)? Or is the amp running too hot.?"
  Reply With Quote
Old 17th September 2012, 03:01 AM   #2
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Gimp,

Are you sure the schematic is for that amp? If it is then the bias is adjustable at that pot. It's not a "balance" pot. Are you getting the full -45v? Can you not adjust it higher than that? May have a bad filter.

Is the bias rectifier a diode or is it a selenium type?
  Reply With Quote
Old 17th September 2012, 03:07 AM   #3
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Well Mr. Gimp, you're throwing us a curve. If the schematic provided is correct, there is a bias adjustment but no balance pot. And the 15K resistor is not from the center of the pot, but from the low end.

With the plate voltage stated, and the negative grid voltage given, a 6L6GC should not red plate. If they do, I would guess that they are out of specification. What kind of tubes are they and how do they test? With today's poor quality tubes one can expect "OOT". So yes, raise the 15K to 20K or so and see if that helps. And no, I don't think amplifiers are designed to run their tubes excessively hot.
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
  Reply With Quote
Old 17th September 2012, 04:31 AM   #4
diyAudio Member
 
Join Date: Sep 2005
Smile Amplifiers and aging capacitors.

Hi

I studied the schematic, and if it appears as if the adjustment potentiometer in the grid circuit is a "balance pot" there is something wrong in the circuit. I suggest that one or both coupling capacitors to the output stage are leaking so that the grid voltages of the output stage are not what you think they are, but something much more positive.

In aging amplifiers the first thing to check are those old coupling capacitors if they leak, which they often do, replace them with new and more modern caps. I suggest a 630V rating, same value, maybe a 1000V rating to be on the safe side.

Hans J Weedon
  Reply With Quote
Old 17th September 2012, 05:55 AM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
All guitar amp threads belong in Instruments and Amplifiers so I'll move it there. Plenty of troubleshooting horse power over there too..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 17th September 2012, 11:36 AM   #6
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Oops, sorry. Wrong version of the schematic (amp is from 1962). Try this one:

http://ampwares.com/schematics/bassman_aa270.pdf

The 15K resistor goes from the wiper on the pot to the pot body, so it is to gnd.

Rectifier appears to be silicon not selenium (given that all selenium I've e ver seen were stack type).

I guess one option is to change to the other bias adjustment circuit, although I hate to make more changes than necessary.

Tubes are 6P3S (multiple tubes trying to select a pair), 6P3S-EB and TAD 6L6GC. That the 6P3S red-plated is not surprising. I was surprised that the 6P3S-EB and TAD tubes did the same.

All tubes are new as the tubes that were in the amp were damaged (broken bases) and I did not trust using them. When the amp was purchased, it was turned on to demo it and smoke came out.

I suspect the smoke was the AL electrolytic caps venting, as three had done so. I replaced all caps before powering up the unit (light bulb test).

Last edited by TheGimp; 17th September 2012 at 11:39 AM.
  Reply With Quote
Old 18th September 2012, 02:10 AM   #7
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Man, is this amp overbiased!

I added a 10K pot in series with the 15K resistor to gnd.

I dug out my bias adjustment socket and even at the minimum setting (-45V) it is still over 50mA per tube!

I'll crank it down below 40mA/Tube and let him try it and see how he likes it. (40mA is as low as I can go without changing something else.)
  Reply With Quote
Old 18th September 2012, 04:13 AM   #8
diyAudio Member
 
Join Date: Sep 2005
Hi with the new and corrected schematic, some parts must be missing there is over 100% negative feedback because there is not an attenuator to gnd in the feedback path. If the amplifier is really built like the schematic shows, I believe the reason for the "over-biased" condition is that the output stage is oscillating at supersonic frequencies. An oscillating amplifier does what it wants to do relatively independently of adjustments. Please either investigate what is wrong with the schematic, or correct the circuit so that the amplifier stops oscillating, and then adjust the balance or the bias condition.

I will be glad to estimate a bias-point based on circuit parameters for you. The proper way to bias a high quality amplifier is to set the bias at about 75% of maximum spec-sheet dissipation in the anode when the amplifier is idling. This is easily determined from the manufactures data sheet. Overbiasing the output stage of Push-Pull amplifiers does not buy you anything except repeated trips to the tube-store for new tubes. Personally I like to bias tubes like the 6L6, EL34, KT66, KT88, or 6550 at 1/2 the spec sheet maximum allowable dissipation. The bias point has very little to do with the amplifier performance in a push-pull amplifier. In a single ended output stage, however, you gain significant dynamic range by biasing the tubes close to maximum.

Hans J Weedon
  Reply With Quote
Old 18th September 2012, 08:38 PM   #9
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Thanks Hans,

I'll check it out.

I was trying to adjust to 38mA which should be about 75% of where it was with the plates starting to glow.

The RCA datasheet shows 50W out in class AB1 at 450V B+ (close) with 95mA per pair (also close), but at -30V bias. So the bias certainly does look like it is off.

Ill check for the potential oscillation.
  Reply With Quote
Old 18th September 2012, 08:57 PM   #10
diyAudio Member
 
Join Date: Sep 2005
Hi

While you are checking: The plate current in a pentode is more a function of the potentials on the two grids rather than the plate voltage. The mu of the screen-grid to the control grid is about 8. That means that to a first approximation if you divide the screen grid voltage by 8 and apply that negative voltage to the control grid that should be the voltage that should cut the anode current to zero. Since the tube is somewhat non linear the anode current will only drop to like 10% of max, about 5 to 10mA or so.

Good luck.

Hans J Weedon
  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
Red plate KT88 h00hbt Tubes / Valves 32 13th September 2012 12:47 PM
Why does red plate spell early death? Ty_Bower Tubes / Valves 19 28th September 2008 03:37 AM
antique fender bassman 50 retubed HELP firefart_1st Tubes / Valves 1 26th May 2008 11:06 PM
Jansen Bassman 50 guitargeek Tubes / Valves 15 17th June 2007 04:18 PM
Heathkit W-5M Red Plate Issue Trout Tubes / Valves 7 28th May 2006 02:18 AM


New To Site? Need Help?

All times are GMT. The time now is 07:35 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