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yet anther fisher bias question

kymers

kymers

Active Member
hey all

I replaced the ground leads to pins 5 with on my 500b with 10 ohm resistors, so I can check bias. Do I now measure milliamps from one side of this resistor to the other side of same? If not where are measurements taken?
thanks
 
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You will measure DC volts from the tube pin side of the resistor to chassis ground. Using ohm's law if you want to bias your tubes at 32-35ma you would want 0.32 to 0.35 volts on each tube. It's a good idea to measure the actual ohm value of each resistor so you can take that into account. Ignore this if you installed 1% tolerance resistors or don't have a decent ohm meter.
 
ok
they measure from outside to inside, .342,.325,.315,.301
the resistors were checked and measured 9.9 0hms ea. on a good fluke multimeter. Bias voltage is about -15v at 117v from variac.
these are new EH 7591s.
Is it normal for the voltage to drop this much from tube to tube?
I tested with fm on at low volume through ads L-520s.
seems bass heavy with everthing set flat.
 
Looks OK to me - you have them paired right too... two lowest and two highest ones together (each pair should be as close as possible - these are within 5%).
 
You will want to check them as they burn in. I usually check them several times the first day, then once a day for the first week and then once a week for the first month. Sounds like a bit much but better to check than have a tube go ballistic.

Some may note the similarity to a hand rubbed finish, no coincidence.
 
thanks for the responses, I read 35ma "idle current", is this no volume? current increases with volume or tone boost.

thanks again
 
kymers said:
thanks for the responses, I read 35ma "idle current", is this no volume? current increases with volume or tone boost.

thanks again
Click to expand...

Yes, it is "idling current" we are measuring. The best way to measure it is to connect a load (speaker or resistor) to the outputs and let it idle with no input for about 30 minutes.
 
For me new JJ 7591 tubes seem to be happier at about 300mah (measured with a 10 ohm resistor) If I run JJ's at much more than that I see a bit of red plating after a while. I have not used EH at this point.
 
Dave C said:
For me new JJ 7591 tubes seem to be happier at about 300mah (measured with a 10 ohm resistor) If I run JJ's at much more than that I see a bit of red plating after a while. I have not used EH at this point.
Click to expand...

Watch the bias on those JJ's carefully. They have a tendency to change bias value dramatically as the tubes burn-in. The first hours are the most critical.
 
kymers said:
ok
they measure from outside to inside, .342,.325,.315,.301
the resistors were checked and measured 9.9 0hms ea. on a good fluke multimeter. Bias voltage is about -15v at 117v from variac.
these are new EH 7591s.
Is it normal for the voltage to drop this much from tube to tube?
I tested with fm on at low volume through ads L-520s.
seems bass heavy with everthing set flat.
Click to expand...

Since you're using new production tubes, I'm curious if you also remembered to change out the four 330K ohm grid leak resistors with 200K's, and increased the four coupling caps from .047µF to .1µF

Many Fisher and Scott vintage tube amp owners have toasted up their new production power tubes pretty fast without these changes. They just don't seem to be as robust as the older production tubes, and can runaway pretty quickly.

You should touch base with Jim McShane to get his insights on this issue, and why it's important to make these adjustments when using new production power tubes.

. . Falcon
 
Dave C said:
For me new JJ 7591 tubes seem to be happier at about 300mah (measured with a 10 ohm resistor) If I run JJ's at much more than that I see a bit of red plating after a while. I have not used EH at this point.
Click to expand...
That's what I'm running my JJ's at also, no red plating so far (~10hrs). Overall I'm pretty happy with them. I was a little concerned after all the negative comments I've heard, maybe their quality has improved a bit?
So far so good.

BTW: Does EH publish their tube envelope sizes anywhere? I may look in to getting a quad just to see if there is any difference in sound. The tube sockets are very close together on my 800B, as well as being very close to the transformer heat shield.

Thanks
Glenn
 
vfr800fiman said:
That's what I'm running my JJ's at also, no red plating so far (~10hrs). Overall I'm pretty happy with them. I was a little concerned after all the negative comments I've heard, maybe their quality has improved a bit?
So far so good.
Click to expand...


As FalconEddy noted you need to change the resistor/coupling cap combination. Don't go thinking that reducing the bias to 30ma/tube will negate the need for this change on new production tubes.

By the way the original combo is hard on NOS tubes also. Their lifespan is shortened significantly, not very important when they were a buck apiece. This also explains why you almost never see a virgin unit without worn out power tubes. The main difference is that it was hard on the original 7591's but is fatal to the new production tubes.
 
I didn't mention it, but I've already done that modification.
The unit has been totally re-capped and gone through for out of tolerance resistors, etc.
Glenn
 
What to measure? My 800c list the bias voltage is being -17v at pin 6 on the 7591 tubes. I was told to aim for -19 to-21 for better tube life. I have added the 10ohm resistors and have done the resistor and cap change as well.
Right now I have the fixed bias resistor, but plan to install a pot to make it adjustable. Should I focus on the voltage or the current across the 10 ohm resistors? Which is more crucial? Also if I want to add a test point for a meter to check this voltage, do I just check it on one tube's pin 6?
John
 
The idle current of the tube is the important thing, not necessarily the negative bias voltage. I'm running a bias of -17.5 with 374v on the plates. I have idle current readings of 32.4,32.7,32.2,28.6. This has changed slightly since I added the inrush current limiter to around 29mA for all tubes.
Any guitar tube amp that I've built I'd never really payed much attention to the bias voltage. I always just set the idle current of the tubes using the trim pot. I do set the trim pot to the most negative voltage when first powering the amp on just to be safe though.

EDIT:
Just to clarify, this is the bias voltage at the power tubes, I still have the -22.5 volts feeding the heaters for the preamp 12AX7 tubes.
 
I have changed the grid resistors to 220k, coupling caps 0.1uf.
changed bias rectifier with a new bridge rect. and changed the 1000uf filter caps. Today I swapped in new diodes for the B+ and changed the 2 0.01caps. The B+ was 365v, and is now 390v, schematic shows 395v.
Highest tube now measures 36ma a little high but certainly not red plating. The filament voltage to the 12ax7s is also a little high at 24.68v.

these measurements are at 117v through a variac but I would like to eventually run straight line voltage. Line voltage in my shop is around
119v, but this is a single 20amp circuit on probably 100' of 12-2 uf(detached garage) and may vary according to how many lights etc... are on. I'm guessing house voltage is 122-124.
what cap and resistors can I use to drop the voltage to the 12ax7s,as I don't want to shorten the life of these tele's?
 
sloober said:
What to measure? My 800c list the bias voltage is being -17v at pin 6 on the 7591 tubes. I was told to aim for -19 to-21 for better tube life. I have added the 10ohm resistors and have done the resistor and cap change as well.
Right now I have the fixed bias resistor, but plan to install a pot to make it adjustable. Should I focus on the voltage or the current across the 10 ohm resistors? Which is more crucial? Also if I want to add a test point for a meter to check this voltage, do I just check it on one tube's pin 6?
John
Click to expand...

Focus on the volts (millivolts, actually).

If the ground lifting resistors are exactly 10Ω, your millivolt reading should be 320-350mV at each cathode (pin 5). This translates to 32-35mA (milliamps).

When installing test points, they must be connected to the cathode (pin 5), not the bias grid (pin 6).

Also, I'd think about running the new production 7591's in the 30-32mA area to extend life.

kymers said:
I have changed the grid resistors to 220k, coupling caps 0.1uf.
changed bias rectifier with a new bridge rect. and changed the 1000uf filter caps. Today I swapped in new diodes for the B+ and changed the 2 0.01caps. The B+ was 365v, and is now 390v, schematic shows 395v.
Highest tube now measures 36ma a little high but certainly not red plating. The filament voltage to the 12ax7s is also a little high at 24.68v.

these measurements are at 117v through a variac but I would like to eventually run straight line voltage. Line voltage in my shop is around
119v, but this is a single 20amp circuit on probably 100' of 12-2 uf(detached garage) and may vary according to how many lights etc... are on. I'm guessing house voltage is 122-124.
what cap and resistors can I use to drop the voltage to the 12ax7s,as I don't want to shorten the life of these tele's?
Click to expand...

Good to hear you already made that change out.

But, those figures will all rise (possibly substantially) if you start putting 124VAC into the primary.

. . Falcon
 
ok, so I put a 5k pot in place of the 5.6k resistor, now I have 34ma in its max position with no variac, so I can adjust for lower draw only and i'm happy with that but my 12***s have -25.45v, how should i lower this?

thanks everyone,

oh yeah- EH 7591s measure 1 3/8" dia., 3 3/8" tall, fit my 500b fine
 
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