Low filament voltage on st-70

Hello, first post here, I'm not a native english speaker so please forgive some mistakes in the text.

I have a trouble with my dynaco st-70 (triode elec ef86 driver board, the rest is stock) biasing, when I try to set bias on output tubes the weird thing happens - one channel reaches a maximum of 1.3v and the other channel stuck at 1.2-1.3v and doesn't change the bias voltage when I change the bias pot position.

I've checked all voltages and noted that the filament voltage on all of el-34 tubes is about 3.3v ac - half of it's normal voltage.

Could it be a wiring error? Transformer issue? What to check next?

I would appreciate any ideas, thanks!

The voltage at pins 5,6 of el-34 is about -45v
The voltage at pins 3,4 is about 445v.

Oops, my bad. The filament voltage is OK.
I've replaced that selenium rectifier indeed.
I'll try to change the resistors value and report back.
Thank you very much for your help!

I've changed the resistors to 5k and now both channels stuck at 1.25v with both potentiometers turned fully clockwise. Both channels bias almost doesn't change as I change the position of a corresponding potentiometer.

The potentiometers are Bourns 3590s-2-103l 10-turn 10k pots.

I can leave it at this bias but I'm worried that non-changing bias may be a sign of some other troubles i.e. miswiring or something else.
I didn't try yet to listen to this amp with speakers attached, maybe I should now to test if this amp work as it should.

Any truth to the rumor that you should never ever power up a tube amp without speakers attached?

Well, i've tested the amp in my system and it works OK, no hum, no hiss so far. Both channels have the same volume.

So the one issue left is the bias, I can't set the bias voltage higher (or lower on one channel) than 1.25v even I want to.

I did the test and on the right channel the voltage stuck at -38v no matter in what position the corresponding pot is.
On the left channel the voltage changes from -38v to -43v depending on the position of the pot.

I've checked both bias capacitors and they both seemed OK ~45uF, 1ohm esr.

So is it some miswiring?

Test the bias pot?

Reason I ask is I just had an issue with a runaway tube ... went thru the usual then noticed the pot just didn't feel right. Tested bad, replaced it, and all is well ...

Sorry, I just redone the test and the results are different.
On the right channel the voltage goes from -38v to -50v
On the left channel the voltage goes from -38v to -42v.

Well, I've just unsoldered the Bourns pots from the circuit and temporary soldered the old original Dynaco pots. This fixed the issue - both channels bias voltage can be set from -26v to -50v no problem.
I suspect that the bourns pin numbering scheme is different, and I effectively swapped the connection points on the 7-lug terminal strip. Does it make any sense?
Or maybe that Bourns pots just broken, I'll have to check it.

Anyway, thank you very much for your very helpful ideas, you saved my day! )

Yes, the pins on that Bourns pots come in order 2,1,3 not 1,2,3 so i'm an idiot.
That will be a lesson for me.

Glad you found your problem.  Usually makes sense once you focus on the problem area.  

Any truth to the rumor that you should never ever power up a tube amp without speakers attached?

Very true.   Countless number of amps have been ruined in short time operating without any speaker load.  Blown power tubes and fried output transformers are the common results.  This should be at the top of the list for Do's and Don'ts for every new tube amp owner.