Bias reading not stable

[UltraHookedOnPhonix]

When I take bias readings on my 50W, EL34, non-HRM I see the measurements on my DMM fluctuate up and down from about 30.6mA to 31.2mA (per tube). It should be stable no?
Should I try to stick a 47uF cap from bias pot wiper to ground like the newer ODS amps to stabilize it? I’m using the older type power supply where the two main caps ground near the PT and the smaller 20uF filters ground at a lug just in front of the Standby switch.

[ayan]

When I take bias readings on my 50W, EL34, non-HRM I see the measurements on my DMM fluctuate up and down from about 30.6mA to 31.2mA (per tube). It should be stable no?
Should I try to stick a 47uF cap from bias pot wiper to ground like the newer ODS amps to stabilize it? I’m using the older type power supply where the two main caps ground near the PT and the smaller 20uF filters ground at a lug just in front of the Standby switch.

The volatage fluctuates without you playing the amp? If so, how is the stability of the negative bias voltage itslef, which should be ~ -4X VDC?

Gil

[UltraHookedOnPhonix]

It fluctuates when it's in operate mode off standby with guitar connected (guitar not playing though).
I don't have a way of measuring the bias tap from the PT right now but I can say that I did not have this problem before. I distinctly recall seing stable voltages. -40V? You mean to measure after the dropping resistor and diode?

[ayan]

It fluctuates when it's in operate mode off standby with guitar connected (guitar not playing though).
I don't have a way of measuring the bias tap from the PT right now but I can say that I did not have this problem before. I distinctly recall seing stable voltages. -40V? You mean to measure after the dropping resistor and diode?

I mean the final voltagge you feed the power tube grids, which can be measured at the junction of the two 220K bias feed resistors at the output of the PI. It should be -40 something for EL34s, or so.

Gil

[UltraHookedOnPhonix]

Ah! Thanks for the tip. That's what I'll do next time I'm inside.

If I were to see a slight oscillation there what would be a good way to proceed (besides getting one of Brandon's amps!)

Can I bypass the 220K/220K junction with a cap to stabalize?

[DonMoose]

It fluctuates when it's in operate mode off standby with guitar connected (guitar not playing though).

Is it more stable with no guitar cable connected?

[UltraHookedOnPhonix]

Don-
That is something I'll try tomorrow for sure and report back. Thanks for the nudge.

[Bob-I]

Ah! Thanks for the tip. That's what I'll do next time I'm inside.

If I were to see a slight oscillation there what would be a good way to proceed (besides getting one of Brandon's amps!)

Can I bypass the 220K/220K junction with a cap to stabalize?

If there's a change in this voltage, something's wrong. You may have a bad cap, diode, PT winding. You're better off fixing it than putting a bandaid on it.

[ayan]

Ah! Thanks for the tip. That's what I'll do next time I'm inside.

If I were to see a slight oscillation there what would be a good way to proceed (besides getting one of Brandon's amps!)

Can I bypass the 220K/220K junction with a cap to stabalize?

Bypass? No, you can hang a cap from the junction to ground... and make surer the cap is connected backwards, since you're dealing with a negative voltage. However, I guess what I am curious to find out is whether thethe bias and plate voltages are constant, while the tubes fluctuate, or whether the bias voltage fluctuates all by itself.

Gil

[gahult]

Maybe check the voltage with the tubes out for a start point.
Just a thought.

Gary

[UltraHookedOnPhonix]

If there's a change in this voltage, something's wrong. You may have a bad cap, diode, PT winding. You're better off fixing it than putting a bandaid on it.

Yeah, you're right. I should focus on getting to the root of it.

Bypass? No, you can hang a cap from the junction to ground... and make surer the cap is connected backwards, since you're dealing with a negative voltage. However, I guess what I am curious to find out is whether thethe bias and plate voltages are constant, while the tubes fluctuate, or whether the bias voltage fluctuates all by itself.

Cap from junction to ground yes! Slip of the fingers.
Good point, that's something I'll try tomorrow as well.

Maybe check the voltage with the tubes out for a start point.
Just a thought.

Thanks! Another thing to check out.

[UltraHookedOnPhonix]

Update:

Tried the amp today and without playing found that the bias is at 31mA per tube but when I crank the volume to 3.5 and strum a chord I see the bias jump to 60mA and back. Normal?

Plate voltage is stable at 462V.

So doing the bias math:
.6x25/462=32mA (at 60% plate dissipation)

Also, at 3.5 on the Master Volume I sometimes experience voltage sag. The note being played just “ducks under” and comes back. The power transformer’s heater supply is rated at 5A and with 3 12AX7s drawing about .9A and 2 EL34s drawing about 1.5A there shouldn’t be a problem at a total of 3.9A current draw?

Much appreciated guys!

[Bob-I]

Update:

Tried the amp today and without playing found that the bias is at 31mA per tube but when I crank the volume to 3.5 and strum a chord I see the bias jump to 60mA and back. Normal?

Yes, bias is IDLE current. Current fluxuates with the signal, that's how the amplification works.

Plate voltage is stable at 462V.

So doing the bias math:
.6x25/462=32mA (at 60% plate dissipation)

Also, at 3.5 on the Master Volume I sometimes experience voltage sag. The note being played just “ducks under” and comes back. The power transformer’s heater supply is rated at 5A and with 3 12AX7s drawing about .9A and 2 EL34s drawing about 1.5A there shouldn’t be a problem at a total of 3.9A current draw?

Much appreciated guys!

That sounds like the main B+ windings are not keeping up. What is the B+ supply current rating of the PT?

Put your meter on the plate and watch it as this sag happens. If the voltage drops dramatically then it may be that your PT is under spec on the main supply windings. I don't expect to see heaters change with signal, they usually remain steady.

[UltraHookedOnPhonix]

Yes, bias is IDLE current. Current fluxuates with the signal, that's how the amplification works.

That sounds like the main B+ windings are not keeping up. What is the B+ supply current rating of the PT?

I'll repost when I find out.

Thanks Bob-I.

[UltraHookedOnPhonix]

Bob-I

The B+ current rating is 345-0-345 @ 200mA