Marshall MA50H bias puzzle

[Stratfordade]

While investigating the current drain of low voltage circuit referred to in earlier post, thought I'd check the bias current of the EL34s. It's the 50W version so only V5 and V6 present in attached schematic. The amp has 1R resistors soldered in and test points for checking bias current. At CT of output transformer I get 490V.

V6 was running 42mA at idle, V5 32mA. This equates to 82% and 63% of the 25W rated max respectively.

There are two bias trim presets. The minimum V6 could be set to was 40mA. But when set to the minimum, the bias current of V5 drops to about 15mA!

I haven't checked the resistors involved in setting bias voltage for any faults, but why would the bias of one of output pair affect the other in this way?

Thanks for advice.

[sugeda]

Change the location of the tubes and test.
V5-V6 -> V6-V5

[Helmholtz]

Need to see the bias supply circuit.

[Stratfordade]

The bias supply is lower section of schematic posted. The only thing missing is showing the secondary of the mains transformer but that secondary connects directly to the ‘sec3’ points at bottom right corner. Good call on swapping valves will try that later. It’s odd as I thought as valve aged it ran less bias current for given bias voltage, but I’m seeing a very high current compared with nominal 35mA set at factory.

[Helmholtz]

The bias supply is lower section of schematic posted.

Sorry, missed that.
As the bias trim pots vary the bias supply loading there will be some interaction.

To exclude a leaking coupling cap, lift one end of C61 and C62 each and see if idle currents change.

[Stratfordade]

Thanks Helmholtz. To lift components I’ll need to remove main board so will report what I find tomorrow. The amp actually sounds good, but I’d like to get it all correct.

[Delta362]

I've found when biasing Marshalls with two sets of bias pots, that when one side is turned hotter, the other side goes a little cooler due to the uneven loading. I will adjust one, check/adjust the other. Then go back and tweak the first one/recheck the 2nd until they are where I want them.

[Stratfordade]

Change the location of the tubes and test.
V5-V6 -> V6-V5

Interesting results doing this. I took a new baseline reading first and got V6=40mA, V5=31mA.

Swapped the positions and gave plenty off time for current to settle, and now got V6=34mA, V5=38mA so one valve is running a bit hotter than the other in the same position.

But with the currents now closer together, I was able to adjust the trim pots so that they could both run a bit cooler. I chose 60% dissipation so set both to 31mA. As Delta362 said it needed a bit of back and forth but it was possible whereas in the original position they were very far apart and I couldn't adjust them to same idle current.