D.I.Y. valve biasing .

jpfamps

Boromedic said:

Direct quote from the message he sent: 

"I recommend a new set of power tubes if they have 40 or more playing hours on them."

I don't want to embarrass the guy as he's been really helpful, but even I in my limited knowledge had to scoff at that............

That would be good for valve vendours!

Most valves are rated for 5000 hours use, although a guitar amp may well be regarded as abuse....

DJH83004

As per ICBM that is a very low figure, the problem is no one will ever fully commit to valve life as it depends on so many factors. If pushed I would say that 160 hours for an output valve, not before failure but when they start to 'dull off' and sound a bit lifeless and more than double that for pre-amp valves. I guess the amp builders on the forum are the guys to speak to as they may have to provide some form of warranty on them, I just have to fix them when they break  

ecc83

jpfamps said:

ecc83 said:

All B's fixed biased amps were biased to 25mA cathode current per valve and I never had a punter ask for it to be made hotter.

The "fashion" for biasing very close to the limit seems to have passed? Not only did it put the valves on the edge but HEAT is the enemy of all things electronic so traffs and caps suffer as well, needlessly IMHO. Then, an amp could be biased to 95% of max Pa but with mains in at 230-240V and it CAN get to 253 and still be legal!

Dave.

Of course Blackstar are the last word in valve amplification, so of course that's the way to do it..............

Not my intention and see no need for sarcasm? I am merely pointing out that I did not have any request for a hotter bias.

I do not promote the company and certainly have no financial interest. I post in two other forums ( HR and SoS) and mention the company very rarely unless I feel it is relevant. No one at those forums has ever given me the slightest indication that I am out of order.

I also do not run my own company making or repairing amplifiers.

Dave.

clarkefan

martinw said:

Guitarse said:

OK, it's a Marshall 4210, a JCM800 combo. 

As you're handy, instead of buying a probe, desolder the earth connection at pins 1&8 on the output valves and solder a high quality 1 ohm resistor in there instead.

Then measure away, do the maths etc. Usual safety rules apply.

Did this for years with old school Marshall heads, went for 65% at idle, had a spreadsheet with the calc, all good.  

Be wary of any post that says the correct figure is 38 ma or whatever,  they don't know what they're talking about, you need to measure the plate voltage as well, eg Marshalls are all over the shop.  Meaning the required bias current will vary accordingly.

Like @martinw said, do the maths, it's a simple a X b = c thing. Oh yeah and adjust per @ICBM.  And of course,  Safety First in there.  Get your head around it first before trying it.

ecc83

Anyone who has ever monitored bias current over a few hours will see it vary by several mA up and down. Va will of course vary with mains voltage.

Quiescent Ia will depend upon the program history (the problem with many early hi fi transistor designs) . New valves will invariably have a high initial grid current which can vary by several 100% depending upon brand and quality (they don't "burn" them you know!) Running the bits off the OP stage for an hour will drop grid current by 2 or 3 times..THEN check bias.

Depending upon how negative bias volts are derived, tracking with mains voltage changes will be close or poor.

Dave.