Switching 845 tubes - do they have to be matched precisely?

Hi folks,

I have some Ming-Da 845 monoblocks and want to switch the stock "wood & twine" Shuguang 845's that came with the amp with some of the new Full Music/Golden Dragon Premium 845's. Problem is, I don't know if this is a simple swap, as the GD 845's probably have different electrical characteristics to the Shuguangs.

Does anyone have the MC845-C's? Would appreciate some guidance here.

Rgds,
DT

Will also get round to rolling the 6SN7 & ECC83's at some point...

 

it depends if your new amp biases automatically, or if it has fixed bias.

You definately want to check this one!

Fixed bias means your amp needs setting with a multimeter before you can change power tubes.


What are your initial impressions of the amp?

Cheers
Chris

 

Not sure tbh bottleneck - I assume autobias requires some kind of control circuitry? If so - this is definitely fixed bias! No PCB's in this baby! I'll double check with the distributor though. There's no way I'm poking round this thing with a multimeter - that would be asking for trouble. Might have to get someone round to help in that regard.

I'm reserving judgement on overall sound quality until I replace the stock chinese tubes (which as you can imagine, are hardly the last word) with something more worthy of the amp. It sounds very promising in its current form though; listening to the Living Stereo/Offenbach/Gaite Parisienne SACD last night was a religious experience.

Rgds,
DT

 

Self bias doesn't require control circuitry- its done with a resistor between cathode and ground. Basically your creating a voltage drop with the resistor which gives the cathode a positive potential, so it is positive with respect to the grid (the thing you bias when you do it manually). Usually theres a cap involved too so the AC signal can still see ground. I'm afraid other than that i don't know any specifics about your amp

 

INteresing - thanks anex. Its just that most autobias boards I've seen are PCB based, hence my original thoughts...

 

what amp was that in? I'd guess there may have been something else involved.

 

I've seen Prima Luna, Shanling and AudioValve amps with PCB autobias boards. An exercise in cost-saving perhaps?

 

Primaluna use 'Adaptive Auto-Bias' where they monitor voltages etc. around the circuit and adjust on the fly. That requires control boards and what not, its far more complex than standard self-bias.
AudioValve use 'smart bias' which I think is a similar arrangement.
Not sure about Shanling though- I thought they were manual bias with pots?

 

OK

Pretty sure the Shanbling spc-80 blocks are autobiasing.

DT