strange bias

[j.christopher]

I have been checking out various tube in my jtm45
and for some reason the bias is always different by 10mv
V4 is44.1 & V5 is 34.4
and no matter what tube i put in it it's always 10mv difference
any thoughts anyone?
thanks

[PCollen]

I have been checking out various tube in my jtm45
and for some reason the bias is always different by 10mv
V4 is44.1 & V5 is 34.4
and no matter what tube i put in it it's always 10mv difference
any thoughts anyone?
thanks

So , it is position (socket) dependent and not tube dependent, and you're using the same type tube in all cases : KT66, or ?
It seems you have a problem surrounding one particular tube socket, and it looks to me like it's the V5 socket.
1) What is the plate voltage measured on pin 3 of each tube ?
2) What is the screen grid voltage measured on pin 4 of each tube ?
3) What is the bias voltage measured on pin 5 of each tube ?
4) Are you using 1-ohm cathode resistors to derive bias current ?

[j.christopher]

V1
P1---173.3
P2
P3---1.21
P4---3
P---3
P6---316
P7---174
P8---192
P9---3
V2
P1---173.3
P2
P3---1.21
P4---3
P---3
P6---316
P7---174
P8---192
P9---3
V3
P1--- 232
P2
P3--- 41
P4---3
P5---3
P6---227
P7---
P8---41
P9---3
V4
P1---
P2---3
P3---438
P4---441
P5---neg 37
P6---440
P7---3
P8---


V5
P1---
P2---3
P3--- 440
P4--- 441
P5---neg 36
P6---440
P7---3
P8---


V6
P1---
P2---290,
P3---
P4---AC=322
P5---
P6---AC=322
P7---
P8---326,


P2&8===4v ac

[spaceace76]

screen voltage (pin 5 of V4/5) seems a little low but they match each other. are you sure the reading follows the tube? switch them and test, then try testing them individually. if you still get the reading, look over the bias section of the circuit, especially the pin 5 connections and the 1 ohm bias resistors.

your amp will still work fine, but it is preferable to have the tubes more closely matched. does it sound alright?

[j.christopher]

well the amp sounds fantastic if you ask me
I have a mesa mkIII
and a real 66 fender black face twin
and neither of them come close to this amp
esp after I put the golden lions in it

as far as the bias resistors go
all the pics i have seen show these gray 2 watt 1 ohm resistors
but my kit had these little royal blue ones
but they were the only brown, black, gold; coded ones in the kit
so i used them
they still make me nervous

[spaceace76]

you can search around the forum as this has been discussed already, you don't need anything high wattage for that spot. take note that you are measuring very low currents here, in mV's.

and if the amp is sounding good... don't worry!

[PCollen]

V4
P1---
P2---3
P3---438
P4---441
P5---neg 37
P6---440
P7---3
P8---


V5
P1---
P2---3
P3--- 440
P4--- 441
P5---neg 36
P6---440
P7---3
P8---

Do you have your screen grid resistors between pins 4 and 6 on the tube socket, with the voltage feed to pin 6 ? If so, the voltage at pin 6 will typically be HIGHER than at pin 4 because the screen grid resistor will drop a volt or two. Your voltages on pin 4 and 6 , measured across the screen grid resistors, are reversed from what I would expect to see. Also, that extra -1V bias voltage on pin 5 of V4 , and 2Vdc LESS plate voltage, would tend to make it conduct LESS than V5 and so it's pretty strange that your bias currents are so far apart with V4 having 10mA MORE current (44mA) vs. V5 (34mA).

[j.christopher]

V4
P1---
P2---3
P3---438
P4---441
P5---neg 37
P6---440
P7---3
P8---


V5
P1---
P2---3
P3--- 440
P4--- 441
P5---neg 36
P6---440
P7---3
P8---

Do you have your screen grid resistors between pins 4 and 6 on the tube socket, with the voltage feed to pin 6 ? If so, the voltage at pin 6 will typically be HIGHER than at pin 4 because the screen grid resistor will drop a volt or two. Your voltages on pin 4 and 6 , measured across the screen grid resistors, are reversed from what I would expect to see. Also, that extra -1V bias voltage on pin 5 of V4 , and 2Vdc LESS plate voltage, would tend to make it conduct LESS than V5 and so it's pretty strange that your bias currents are so far apart with V4 having 10mA MORE current (44mA) vs. V5 (34mA).

at the time of the readings i had the 5.6 resistors in but I have taken them out
just to see if it might make a difference
but it did not
I reinforced the connection between the bias pot at the board and moved the pin 5 connections away from other wires and the situation improved
now only 6volts difference between V4 and V5

[j.christopher]

Ha ha
I found it
One of the bias resistors was bad
now sits at 40.1 on both tubes!
thanks for your help guys
cp

[PCollen]

Ha ha
I found it
One of the bias resistors was bad
now sits at 40.1 on both tubes!
thanks for your help guys
cp

Bad, or simply the wrong value ....

[j.christopher]

Ha ha
I found it
One of the bias resistors was bad
now sits at 40.1 on both tubes!
thanks for your help guys
cp

Bad, or simply the wrong value ....

no it was bad
it had no resistance when tested
replaced with identical marked resistor