Favorite bias measuring and metering methods? - 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 11th March 2004, 10:24 PM   #1
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default Favorite bias measuring and metering methods?

Hi All,

I've been trying to decide the best means of measuring and/or metering bias for a DHT PP amp. Would an ammeter and a multipole rotary switch to insert the meter into the cathode circuit of each tube in turn be a good choice. It would be nice if an intelligent person with no tube experience could follow a simple procedure to check and bias the outputs. Any schemes to share? Suggested parts values?

Also, if a person needed to balance the current in two tubes (sections), but the absolute value of the current wasn't important, how to best do that? A zero centering ammeter?

Thanks,
Michael
  Reply With Quote
Old 11th March 2004, 10:46 PM   #2
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Michael,

Yes, that would work.
You could replace the rotary switch by a simple DPDT switch for a single PP.

I have an ammeter in place all the time and if I want a readout I just flick a switch, left hand or right hand side, push a button and the meter gives a readout.

Add some TPs to insert DVM probes and you have a voltage readout too.

Cheers,
__________________
Frank
  Reply With Quote
Old 11th March 2004, 11:23 PM   #3
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default Other methods?

Hi All,

Thanks Frank,

What other methods could you suggest? What differences in setup would there be for fixed bias or cathode bias?

How would you handle the balancing situation? Just flip back and forth until you get the same reading on each?

I have an old piece of equipment that has a meter that reads something like 4 current ranges, one or two voltage ranges and pressure in torr. Something like that might not be a good thing for a person who just needs to bias the outputs, but it would be kind of cool to be able to test the whole amp without a multimeter. How would you implement that? Precision resistors and an ammeter?

Thanks,
Michael
  Reply With Quote
Old 12th March 2004, 07:59 AM   #4
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
On my amplifiers, I install a 10 ohm resistor between the cathode and ground. I measure the voltage drop across the resistor.
10 ma = .1 volt.
When balancing a PP stage, I measure the voltage differential between the two cathodes (at the high side of the 10 ohm resistors) and trim the bias voltage for zero volts.
__________________
Frank
  Reply With Quote
Old 12th March 2004, 10:20 AM   #5
diyAudio Member
 
Join Date: Nov 2002
Hi Michael

I favor the Quicksilver method. The Quicksilver's have a 1/4" jack in which to plug in the meter and make your adjustment and then unplug.
While I like the looks of a meter on board some amps just don't have the room. Individual bias adjustment for each tube would be a plus.

Joe
  Reply With Quote
Old 12th March 2004, 12:12 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
An ammeter is good, especially if you have provision to switch it out of circuit. And I really mean "ammeter"; use the highest current range meter consistent with being able to read the idle current with some accuracy, since high current range equates to low series resistance. For DHTs where you have some confidence in the tube matching or don't have individual bias adjustments, consider putting the meter in the B+ feed.

If you really want to keep the circuit as pristine as possible, use a very low value resistor (like 0.5 ohm or less) in series with the filament transformer's CT, then have an opamp with a x10 or x20 gain provide a nice, easy-to-measure voltage for a voltmeter.

I very much favor differential measurements to set tube-to-tube idle current balance; nulls are easier to get right.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 12th March 2004, 01:01 PM   #7
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default methods

HI All,

Frank,
Certainly straight forward for most uses. Wonder if it would be more complicated for DHT, fixed bias, etc. I need to get some wires and connections floating around in my head so I can picture the final product.

Joe,
Do the Quicksilvers have individual tube bias? What sort of meter do you plug in?

Sy,
Opamp idea would be cool, but adds a lot of complications to implement, maybe. Would you use one opamp and vary the input connections or many opamps and vary the output connections? For an ammeter, you favor a large current as opposed to a small current with series resistance, where you are only diverting a small percent of the current? To me it seems that if a person wanted to measure currents in both the volt amp/driver section and the power amp section the small current would be easier. Am I missing something? What would be the mechanics of implementing a differential reading meter?

Anyone,
How would the meter that reads both current in different ranges and voltages be done. I dug into my pile of junk far enough to see the meter I have that does this, but there is no indication on the face even whether it is a voltmeter or an ammeter.

Thanks,
Michael
  Reply With Quote
Old 12th March 2004, 02:35 PM   #8
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Michael,

To give you some idea I dug up Bruce Rosenblits diagram for an OTL amp.

You can apply the same principle to any amp really.

BRUCE ROSENBLIT 6AS7G OTL

Cheers,
__________________
Frank
  Reply With Quote
Old 12th March 2004, 03:11 PM   #9
diyAudio Member
 
Join Date: Sep 2003
Location: iowa
Default schematic

Thanks Frank,

Would the meter be a voltmeter reading across the 1 ohm resistors or an ammeter? Do you think the 100 ohm pot is just to calibrate the meter? I guess that would mean voltmeter?

Thanks again,
Michael
  Reply With Quote
Old 12th March 2004, 03:35 PM   #10
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Would the meter be a voltmeter reading across the 1 ohm resistors or an ammeter? Do you think the 100 ohm pot is just to calibrate the meter? I guess that would mean voltmeter?
I can't remember exactly what BR used, I'm pretty sure it's an ammeter.
I use an ammeter which is built in and test points that are switched around according to the rotary switch position.

The 100R trimmer is to calibrate the meter, they're never spot on.
Whether it's an ammeter or a voltmeter that's put there doesn't really matter.
It's just that we're used to use amps for bias current and this is the reading people expect to see there, otherwise you'd need to the conversion.

Cheers,
__________________
Frank
  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
Cult movies - what is your favorite - what is your favorite part Ken L The Lounge 247 9th December 2013 02:32 AM
Bias Measuring points, pros cons Zero Cool Everything Else 4 18th November 2011 06:49 AM
Metering the IMD of EMU0404 with Multi-Instrument Pro-3 fotios Solid State 25 20th April 2009 10:26 PM
on the job teaching methods.... unclejed613 Everything Else 1 12th April 2009 09:39 PM
Bias methods? Psychobiker Tubes / Valves 8 3rd April 2008 09:07 PM


New To Site? Need Help?

All times are GMT. The time now is 03:24 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