Oldchen 845-A trim pots - Hifi Exquis answered my questions

[opnly bafld]

This amplifier has 3 pairs of trim pots and yet I can't find any mention of them on the internet, do those that own this amplifier not know about them or just ignore them?
A friend bought one of these new (from Jeff B's place) and of course did not get an owner's manual (or schematic).

Hum balance? Bias? Which is which?
Oldchen 300B amplifier has one pair of the blue pots for hum balance, no other adjustments.

Any thoughts from looking at these pictures I swiped from the sale site?
I know, I know, more info would be helpful, but I do not have the amp to look at.
6SN7 - 300B - 845
I'm hoping to get an owner's manual from a Chinese reseller.

Thanks for any input,
Lin

 

[BmWr75]

Bias pots or hum pots that are only accessible by removing the bottom plate I've seen before, but never understood why somebody would build a tube amp like this. Good luck.

 

[opnly bafld]

Hum pots I understand because it's preferable to have a tech use equipment vs listening for hum and it should not require adjustments very often.
The blue ones are multi turn, so on the 300B I had I could turn and turn and not notice any difference in hum by ear.

 

[BmWr75]

Hum pots I understand because it's preferable to have a tech use equipment vs listening for hum and it should not require adjustments very often.
The blue ones are multi turn, so on the 300B I had I could turn and turn and not notice any difference in hum by ear.

I've only got a few amps that have hum pots (best I can recall right now) and all are accessible without removing any covers. The newer ones have hum pots on the top plate where the tubes are inserted.

First, I'd never buy an Oldchen amp or any other the other brand names they sell under. But, if I did in a weak moment, would never leave the room while it was on and would keep my eyes on it.

 

[opnly bafld]

Only information in the owner's manual I received today:

·This device has an internal resistor to adjust the value of the tube bias, there are two adjustments to the
midpoint of the 300B electron filament, you are not recommended to adjust by your own.

 

[BmWr75]

That was lots of help wasn't it. Sounds like it is a cathode biased amp based on the first sentence. Second sentence sounds like hum balance maybe.

 

[opnly bafld]

HiFi Exquis (amplifier was not purchased from them) confirmed that the blue multi-turn pots are for hum balance and the larger black pots are for bias measuring across the 2 blue resistors next to them.

 

[opnly bafld]

This amplifier is running again after adjusting hum pots and bias, hopefully the new tubes last longer than one of the original 845's.