Metropoulos Forum

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

j.christopher wrote: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

No noticeable influence; the voltage drop across them is negligable.

PCollen wrote:

j.christopher wrote: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

No noticeable influence; the voltage drop across them is negligable.

ok that seems to be the consensus
I just notice on almost all schematics i look at, that they are not present

some say they affect the tone

Roe wrote:some say they affect the tone

Ahhh, but are you some people ?

well, I haven't done real testing on this myself. I'm using 1ohm resistors, or even .1ohm resistors

j.christopher wrote:

PCollen wrote:

j.christopher wrote: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

No noticeable influence; the voltage drop across them is negligable.

ok that seems to be the consensus
I just notice on almost all schematics i look at, that they are not present

Because most commercial amps are not built to be biased by the owner but by the repair tech (i.e., no user-serviceable parts inside); they are typically a mod to most amps, added by the hobbist/boutique builder (i.e. Komet 60).

Roe wrote:some say they affect the tone

...and most say they don't. You state you use 0.1 ohm, vs. 1.0 ohm. Do you have a meter that can measure accurately to
4 decimal places, which is what you would be reading across 0.1 ohm resistors during a typical bias set-up; can your measuring equipment accurately differentate between, for example, 0.0032 vs. 0.0036 volts, or 0.0040 vs 0.0044 volts ?