help with bias ?

[john t]

Fired up the 5e7. One mistake for this build, When I fed the choke and output wires through the chassis I mixed up the black wires. One fuse later and I dont think I damaged anything, The amp sounds great. just the right breakup for what I was after. Iam not sure about setting the bias for the output tubes. This is my first 6l6. Where do I take my reading and what should they run at. The schematic says -34 to -52

thanks John

[MWJB]

The negative grid voltages are really just a guide to get you in the ball park...one brand of tube might run at -40v all day, another might blow staight away! You can only really set by negative voltage if you have a supply of tubes that all have the same characteristics, in the real world individuals don't and only some manufacterers, that should, do.

You really want to be able to read your plate current in milliamps. The easiest and safest way for you to do this is to unground your power tube cathodes and install matched 1ohm 2W resistors between pin 8 (cathode) & ground on each power tube socket. Be sure to have a really good ground connection. You could ground the 1ohm resistor to a solder tag under a tube socket nut?

Now with the 1ohm resistors in place and the power tubes removed, read the negative voltage at pin 5 of your power tube socket to ground...I'd recommend that you adjust your bias pot to read -55v (or -52 if that's as high as it will go). Power down and install the power tubes, reset your meter to <200mV DC.

Install power tubes, power up, wait 30 secs minimum then throw standby to operational. Now measure the voltage at pin 8 of each power tube, this figure converts pretty well to milliamps. Adjust to 30-35mV. If your meter looks like it's going to settle above 40mA power down and adjust for more negative voltage at pin 5 before trying again. In this kind of amp 60-70% of max plate dissipation works well (therefore 17.5W divided by your plate voltage will give you your max 'permissable' current).

I'm assuming that you have already made provision for a bias pot?

If you have built this amp into a slide in/out tolex Fender style chassis, or a Marshall style chassis, you may as well buy some bias probes to save you having to take the chassis out every time you want to check your bias.

[john t]

MWJB, thanks for the help. This is a Mission amp kit. It has a 10K-L bias pot and came with JJ tubes. Will the pot adust the bias or simply balance the tubes. Iam not sure about this.

Thanks John

[MWJB]

I would imagine that the pot adjusts the bias rather than balances the tubes. To be sure (though I am 99.9% sure already), power down & remove the power tubes & turn it to see what happens to the negative voltage on pin 5. If turning it one way makes the voltage on BOTH pin 5's either rise or fall together, then it's a "bias adjust". If turning the pot one way makes one pin 5voltage rise, and the other pin 5 voltage fall then it is a "bias balance".

The bias balance as seen on SF Fenders was fitted in order that they could use mis-matched tubes, save time & cut costs. Some people like to have this facility as well as a a "bias adjust pot", but as most people nowadays have access to matched pairs of tubes the need for a "balance pot" is purely a matter of personal preference rather than a necessity.

[TD_Madden]

It is a bias-adjustment, NOT a balance.

By the way, my 5F4 (same basic circuit) sounds totally amazing biased at only 32ma, using the TAD short-bottles it came with. :)

[eruannu]

not to be off topic but i have a bias prob of my own
i am currently updating- modifiying the circut of a stromberg carlson signet 33 it has no bias pot but it has a feedback adjustment pot
i would like to know two things first what value of pots would i use to bias two 5881s individually or am i better off with my thc el34 adapters which are awesome and what the feedback control does I have a scem if that helps but i dont have a ggod enough pic to post

[R ski]

To understand bias is to know how much the power tubes are conducting,

The B+ voltage to transformer winding resistance will throw your
power tube conduction out considerable under some circumstances.
Especially when you tweek your negative bias.

When I first built tube amps I watch the negative bias, however,
the long tube life is better to know how much current is flowing
through the 6L6 tubes.

I set the current to 25 mA - 35 mAmps

Performing a current measurment is challenging. It requires
the nasty buisness of inserting the meter in series.
Risky task avoid this tact

However a voltage measurement from output transformer to
center tap and a calculator can achieve the current flow happening

First allow the amp to discharge and the power tubes cool.

Use your multimeter on ohms. Measure the output transformer center
tap to pin 3(plate power tube) Average both readings
I not sure what your output tranny is. but ohms range from 45 - 150 ohms.
for each leg are typical.

with your amp juiced up switch your meter to DC volts. Now measure the
center tap to pin 3. If you clip your probes on on cool power up,the
reading will slowly climb as the tubes warm up.

Plug the voltage reading in your calculator divided by the ohms will
result in a decimal number, muliamps is less than one.

This technique is using ohms law : I = E/R

Knowing your flow you shouldn't see tubes glow

[FiddlinJim]

Great to follow this discussion... I'm about to start stuffing the board on my 5F4 project. My only previous project was a 5F2-A (Marsh kit), so these comments are helpful. MWJB: thanks for the clearest bias adjustment explanation I've come across.

[stokes]

Rski-As per your method to check bias,and I am not saying that method is wrong,but wouldnt it be simpler to connect the meter as you described and set it to read milliamps and do away with the math?If you are using 2 power tubes the reading is what goes thru that side of the OT.If using a 4 power tube arrangement you have to divide by 2 to get what is going thru 1 tube.If there is some advantage to your method,please let me know.Thanks.

[Bruce / Mission Amps]

MWJB, thanks for the help. This is a Mission amp kit. It has a 10K-L bias pot and came with JJ tubes. Will the pot adust the bias or simply balance the tubes. Iam not sure about this.

Thanks John

The bias pot is current level setting not current balance.

I usually include a pair of precision 10 ohm 1/2 watt resistors in the 5F4 kits to be used under the cathodes of the power tubes.
In that application you'd just take a DVMM and read the hundreds of milivolts across the 10 ohm resistors and divide by 10 to arrive at the actual DC ma of current through each tube.
If you measured 320mv DC or, .320v DC, then that would mean .032 amps or 38ma of current.

I like to set my power tubes using the "Idle wattage" method.
Measuring the plate voltage from pin 3 to 8 will give you the other part of the equation.
Say you measured 420vdc from 3 to 8 on the 6L6 socket, you'd now multiply 420vdc X .032a and get: 13.5 watts.
That's cold but OK.
However, for a true vintage sound I'd like the tubes to idle around 18-20 watts each.
So, since the bias supply is adjustable in these kits, you'd turn the bias pot so the negative voltage moves closer to zero... -40v...-38v...-36v...-34v... (it's moving to zero) and the tube will draw more current.

18watts divided by the plate current will = the idle current you want to set the power tube to.

As an example:
18w/410vdc= 44ma

Set your DVMM across one of the 10 ohm cathode resistors to read .440vdc or 440mvdc.

[john t]

When the measurement is taken for the plate voltage (pin 3 to pin8) is that with the power tubes in or out.

Thanks all for the help.

John

[R ski]

Disconnecting to insert a milamp meter is risky

you need to clip your leads until the tubes warm
up. If your lead comes undone during conduction,
the flyback pulse from DC collapse is scary.

soldering and resoldering is prone to burns

dividing by two isn't going to indicate if
any tube is working harder

[MWJB]

Why disconnect anything and "insert" a meter? If you don't want to go the safer route and use 1ohm or 10ohm cathode resistors, why not just shunt the transformer with your meter (assuming that your B+ & screens aren't tied together 5E_ style)? Ohms law is often a good "rule of thumb guide" but in a lot of instances the math doesn't produce the same answer as an actual measurement (I find this quite a lot when trying to establish bias current on a cathode biased amp by using ohms law and cathode voltage).

[Enzo]

Plate voltage is ALWAYS measured with the tubes in place. The current through the tubes draws the voltage down. With no tubes, the B+ voltage will be a lot higher than when the amp is actually running normally.

[Bruce / Mission Amps]

If you are measuring plate voltage in order to derive power tube idling power, you have to do it with the tubes in because the power supply will sag a little under load.

[stokes]

Rski-I was refering to the transformer shunt method of measuring the bias not unsoldering the leads.As far as dividing by 2 to get the plate current for one tube if you are using closely matched tubes the difference should be negligable,and by closely matched I have found, and many techs agree, in a guitar amp +/- 5ma is good enough,if you feel they need to be closer then you would have to check each individually.