I'd love to see someone bias a Blackstar Series One 200.
No different to biasing any of the Serie Ones*. Same bias system on the power supply/power control board and the same balance trim pot on the middle valve PCB.Originally Posted by deathbunny
Yes, the KT88 anodes are fed from a "takes no prisoners" 700V+ supply but all connections are well booted and you woul have to be a suicidal ***t to find it!
Same plug for I sense and you set for 100mV =100mA total thru the valves giving a very conservative ~18W dissp' per valve giving long life to what is QUITE a wad of lager vouchers to replace. Other measures to extend valve life are very low grid leak values with an appropriately low impedance driver design.
*ONCE this old bugger got it on the bench!
Dave.
Hi,
I have been trying to bias my HT-60 but the readings have me puzzled. With my mulitmenter set to 200mv and with a matched pair of Tung-Sol EL34bs fitted. The initial reading off the 2 test point pins was 12.9 and the maximum amount on tap seems to be 31.4. Am I missing something as I was expecting to be able to tweak it up to 50?
:s
Fanning the flames of burning GAS for over 30 years
I think the optimum for HT Series is between 11-12 (or this could just be the HT20, although I assume not) so if you're up to 30 you're biasing way too hot!
With the amp cold and drained of volts, croc' clip a meter across R207/D27. Click meter to Ohms and you should get 1.0Ohm (maybe 1.2-1.4, meter error) if not, pack it up and Ring Northampton!
'Sright? Ok, plug in the ORIGINAL valves, I assume they weren't forked? Set the bias pot for, as you say, 50mV....DO NOT FORGET you need a speaker plugged in AND a jack plugged into the input!
OK?
Replace the valves with the boutique pair and try again. If those valve will not bias up the same they are way out of spec'. This is not unknown with "modern" valves and they all tend to "err" on the cold side.
If all is well, put ear by speaker and set the balance pot for minimum hum. Let things cook for 10mins at least then check bias and balance again. With new valves I strongly suggest a good hours' belting at max power to try to bust 'em!
HT-20:Find 7W cement 220 Ohms (don't have C ref handy but you can't miss them) and clip meter across one of them. Set for about 11.5V . Set balance the same as for the 60 but you won't get a 20 quite as quiet as a 60.
NOTE! You are perforce working inside a live chassis! Take all precautions, one hand, no kids, no pets, don't do it alone. The high voltage points are booted and sleeved to a large extent but you CAN find hurty stuff if you try!
Dave.
Cheers Dave,
I'll be sensible and will check back later.
Fanning the flames of burning GAS for over 30 years
Good. Pretty sure you would be but it costs nothing to bang on about it!Cheers Dave,
I'll be sensible and will check back later.
BTW if that is the 60 with the op valves on a separate pcb do check that the grid lead is securely pushed home and secure it with a couple of small cable ties. (same for the HT-100)
Dave.
Hello all
I have an ht-100 head and have been swapping out various el34's and 6ca7's and re-biasing with no problems with much thanks to this thread, how "difficult" would it be to install a set of 6550's? Or is this a whole different animal?
Thanks
Tony
From the curves I have found the 6550 will need a fair few more bias volts, -60 and up, to get the factory setting of 100mA total Ik. I strongly suggest that you do not go for a higher current since you would already be pulling an extra 3watts or so off the mains traff for heater power. If you have a set, try them but I would not buy any!
On the other hand a set of 6L6s might be an idea? I know their use was mentioned in the HT-20, can't see any harm in putting a set in the biggy!
Dave.
Thanks Dave, will give the 6l6's a try. I'll keep you posted.
Tony
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