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HELP!..can't bias JMP high enough

shooto

Member
Messages
3,031
wtf?...so, I'm trying out some JJ 6ca7s instead of the EL34- hook up my AMP-HEAD bias probe- get specs from the EL34s and with a plate voltage of 385, they're biased at 31 & 32...50%...I usually bias at 60%...whatever-

check out the 6ca7s...plate voltage same but the mA reading is in the 20s...so I turn it up (shooting for 38-40mA...60%) - bias adjustustment tops out at 33 on my multimeter...

what's going on?...any ideas?

thanks!!!

ps- got these from Doug, plate current spec on the box for the 6ca7s is 29, is that the same as watts?...if I do the math with that, they should be biased at 45mA (60%)...is that right?...or, since they are an american EL34, should I just go with the 25w dissipation of EL34s?...in that case, it's the 38-40mA I'm shooting for but can't get-
 
Last edited:

gldtp99

Member
Messages
3,875
Plate current spec on box is for supplier matching only--- it doesn't apply to any other spec, only a comparison for matching tubes on their test equipment--- so ignore it unless selecting higher/lower drawing tubes from the same supplier.
Check the bias probe--- if it has a 1 ohm resistor, make sure it's still in spec---- make sure all contacts are clean and tight.
If bias probe is ok it sounds like you need to make a resistor change in the bias circuit of your amp to get the bias voltage in range for the output tubes you wish to use--- has the bias circuit been previously modded ? Such as in a 6550 to EL34 conversion ? Sometimes the quality of work done by some people doing output tube conversions in JMP Marshalls (i'm thinking you have a PCB as in most JMP's) leaves a lot to be desired----- they often "cheat" by soldering a resistor in parallel to the existing resistor and call it good----- this is a lazy, half-assed way to do the mod but you see it a lot because you don't have to pull the board to do it this way--- pulling the board and installing proper value resistor is a better way to do it.
What is the proper value for your amp ? Can't tell because we don't know what model, what wattage your amp is and we have no idea what the values are in the rest of the bias circuit--- or if the bias circuit has been modded, and if so: modded properly/modded poorly .....................gldtp99
 

dazco

Member
Messages
14,871
All you have to do is replace the bias set resistor with something a bit lower. For example, if the most you can get is 32 Ma and you need more, if the resistor 68k replace it with a 56k. If it's a 56k, replace it with a 47k. Just dropping it to the next common value will give you the extra range you need.
 

shooto

Member
Messages
3,031
Plate current spec on box is for supplier matching only--- it doesn't apply to any other spec, only a comparison for matching tubes on their test equipment--- so ignore it unless selecting higher/lower drawing tubes from the same supplier.
Check the bias probe--- if it has a 1 ohm resistor, make sure it's still in spec---- make sure all contacts are clean and tight.
If bias probe is ok it sounds like you need to make a resistor change in the bias circuit of your amp to get the bias voltage in range for the output tubes you wish to use--- has the bias circuit been previously modded ? Such as in a 6550 to EL34 conversion ? Sometimes the quality of work done by some people doing output tube conversions in JMP Marshalls (i'm thinking you have a PCB as in most JMP's) leaves a lot to be desired----- they often "cheat" by soldering a resistor in parallel to the existing resistor and call it good----- this is a lazy, half-assed way to do the mod but you see it a lot because you don't have to pull the board to do it this way--- pulling the board and installing proper value resistor is a better way to do it.
What is the proper value for your amp ? Can't tell because we don't know what model, what wattage your amp is and we have no idea what the values are in the rest of the bias circuit--- or if the bias circuit has been modded, and if so: modded properly/modded poorly .....................gldtp99
^ thanks- it is modded for EL34s, I'll have to check if I see a cheap job as you suggested...makes sense- thanks
 

Ronsonic

Member
Messages
3,303
Couple of things here ....

What amp is it????? JMP only tells us what era it's from.
The plate voltage isn't unreasonable for a Marshall 50W amp so we'll guess it is.

Mods for EL34 are all over the place. Sometimes with a hotttish set of tubes a guy can get them in range without changing anything and .... well .... they don't. Tubes are all over the place a particular set might not bias up in a particular amp.

The piggybacked resistor is a bit of a cheat, it is also a good thing if you want to change back to 6550, ever. Makes it easy.

I do recommend ignoring the whole deal of setting bias to a percentage or precalculated value. It is well to check those things, but better to listen and look at a scope and know what you are trying to achieve.

The classic Marshall sound came from amps biased according to factory instructions which left them a bit cold compared to what you get from modern tube seller recommendations. Nobody cares if it's biased to 60%, only if it sounds good.
 

CDA

Indigenous Donkey
Messages
349
Lower voltage amps are warmer...

Most "Marshall" tones people hear and die for are amps running 70% idle..

Just my experience. Have been in allot of them.
 

shooto

Member
Messages
3,031
Couple of things here ....

What amp is it????? JMP only tells us what era it's from.
The plate voltage isn't unreasonable for a Marshall 50W amp so we'll guess it is.

Mods for EL34 are all over the place. Sometimes with a hotttish set of tubes a guy can get them in range without changing anything and .... well .... they don't. Tubes are all over the place a particular set might not bias up in a particular amp.

The piggybacked resistor is a bit of a cheat, it is also a good thing if you want to change back to 6550, ever. Makes it easy.

I do recommend ignoring the whole deal of setting bias to a percentage or precalculated value. It is well to check those things, but better to listen and look at a scope and know what you are trying to achieve.

The classic Marshall sound came from amps biased according to factory instructions which left them a bit cold compared to what you get from modern tube seller recommendations. Nobody cares if it's biased to 60%, only if it sounds good.
^ '78 JMP 2204 MkII

I don't have a scope and will probably never get one for biasing-

I did throw the EL34s in there an brought it up to 38-40 mA and it sounds MUCH better...it was raspy biased cold...sounds much more "together" now...still want to fix the bias resistor so I can check out the 6ca7s though
 




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