bias question

[chris61]

i have a metro amp jtm 45
Now they tell you to put bias resistors on pin 1&8 on v4 & v5
this i understand is to allow you to use mV as a proxy of mA
to make it easier to bias the tubes
but shouldn't those resistors come out when your done?
don't they influence the operation?
thanks for any opinions
cp

[Tom Phillips]

Most people use 1 Ohm resistors for this application. Some people use 10 Ohm resistors so an acceptable measurement can be made with lower quality volt meters. In either case, the resistors can be left in the circuit with no apparent effect on the amp performance.
Cheers,
Tom

[chris61]

Yes I belive they are 1 ohm
thanks for the reply

[Jazz P Bass]

Generally 1% tolerance resistors are used.
This will insure a valid reading from side to side.

[chris61]

Generally 1% tolerance resistors are used.
This will insure a valid reading from side to side.

yes i know why they are used
I was just wonderijng wether they should be left in
I can't seem to find any schematis where they are present as designed

[trevorus]

Yes, you can leave them in. I often put these in any fixed bias amp so I can easily bias it and just leave it that way. You will notice no difference as long as you aren't using a really tiny resistor.

[R.G.]

yes i know why they are used
I was just wonderijng wether they should be left in
I can't seem to find any schematis where they are present as designed

Yes, leave them in. They cause no problems in normal operations.

The reason you can't find them in any production schematic is an economic one. Even one added part on thousands of amps is a lot more money which will probably not ever be of value **to the seller/manufacturer**. A manufacturer ceases to have any interest in what they've made when the warranty period is over in most cases. The MBAs that permeate industry today will ruthlessly look for parts that can be left out.

Put another way, manufacturers have no interest in making it easy for a possibly-nontechnical user to work on the insides of the amp. In fact, this adds only legal liability and no perceived value.

I only know of one commercial amplifier which put bias sensing resistors in and also made it easy to use them, and I designed it. The Workhorse series of amps had bias sense resistors along with a bias pot for each tube and an indicator LED circuit that was dark for too cold, red for too hot, and green for just right. The bias pot shafts were recessed in a hole that accepted a #2 Phillips screwdriver and prevented that screwdriver from getting to a dangerous voltage. To rebias, you let the amp warm up, then looked at the indicator while twiddling the bias pot. Green = good, and you're done.

[vintagekiki]

I can't seem to find any schematis where they are present as designed

Dynacord
http://www.peel.dk/Dynacord/Dynacord_Eminent_2_T.pdf
http://www.peel.dk/Dynacord/Dynacord_Bassking_T.pdf
0.3 V on 10 Ohm (R418/419) or 0.03V on 1 Ohm = 30 mA

Ampeg SVT Series
http://dbl.musicweb.ch/schematics/Music%20Instruments/Amplifier/Ampeg/SVT%20Series/svt6550a.gif

... I only know of one commercial amplifier which put bias sensing resistors in and also made it easy to use them, and I designed it. The Workhorse series of amps had bias sense resistors along with a bias pot for each tube and an indicator LED circuit that was dark for too cold, red for too hot, and green for just right. The bias pot shafts were recessed in a hole that accepted a #2 Phillips screwdriver and prevented that screwdriver from getting to a dangerous voltage. To rebias, you let the amp warm up, then looked at the indicator while twiddling the bias pot. Green = good, and you're done.

SVT-VR
http://music-electronics-forum.com/t8440/

Framus Cobra

http://www.framus.de/media/manuals/Amps/Cobra/Cobra_V2.pdf

[Phostenix]

I only know of one commercial amplifier which put bias sensing resistors in and also made it easy to use them, and I designed it. The Workhorse series of amps had bias sense resistors along with a bias pot for each tube and an indicator LED circuit that was dark for too cold, red for too hot, and green for just right. The bias pot shafts were recessed in a hole that accepted a #2 Phillips screwdriver and prevented that screwdriver from getting to a dangerous voltage. To rebias, you let the amp warm up, then looked at the indicator while twiddling the bias pot. Green = good, and you're done.

The Ampeg VL-503 had a bias adjustment setup like that, too.

[Jazz P Bass]

Framus Cobra.

[Enzo]

And all those Marshalls with the three little pins sticking out.

CHris, that 1 ohm is invisible to the amp circuit, no one would ever hear any difference were it there or not, in fact you could use some other method of current measure and never measure any difference with it in or out.

[vintagekiki]

... all those Marshalls with the three little pins sticking out ...

Marshall DSL100 JCM2000 Double Super Lead
http://www.drtube.com/schematics/marshall/jcm2-60-02.pdf --> DSL100 power amp
http://www.drtube.com/schematics/marshall/jcm2-64-02.pdf --> DSL bias board

Marshall DSL401 JCM2000 Double Super Lead
http://www.drtube.com/schematics/marshall/dl40-60-02-iss9.pdf --> DSL401
http://www.drtube.com/schematics/marshall/dl40-68-00-iss2.pdf --> Biasing instructions for DSL401

Marshall TSL100 JCM2000 Triple Super Lead
http://www.drtube.com/schematics/marshall/tl10-60-02.pdf

Fender The Twin
http://www.el34world.com/charts/Schematics/files/fender/the_twin.pdf
Setting the bias described in the schematics.
(40 mV was set for two tubes)

[Amp Kat]

They also act as fuses and do realize that as with any faulty tube a Cathode to grid short could either open them all the way up or partially open them up which could cause thermal runnaway. Many use metal film so they will burn open like a fuse.

[chris61]

well that seems to be the consensus
thanks guys
by the way could I install these in my 66' twin reverb?

[vintagekiki]

... by the way could I install these in my 66' twin reverb?

View schematics Fender The Twin.
http://www.el34world.com/charts/Schematics/files/fender/the_twin.pdf

Uninstall wire between pin8 (cathode) and the chassis.
Between pin8 (cathode) and the chassis install resistor 1 Ohm / 1W