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Just aquired a pair of futterman H3aa's.
Just starting to put them back in working condition. The old caps still work but probably need replacing.
I need a schematic and biasing instructions.
If anyone here could help it would be much appreciated.
Looking foward to hearing these amps!
Thanks
Mike
Follow Ups:
Hi Mike !
Check on this link :
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Enlightened Evolution-Astral Projection
thanks very much!
That will help alot
I think you would be better off to rewire the amp to the otl-3 schematics, that will allow you to do a proper alignment, if you look at the zener's on the H3aa schematic you will notice it doesn't tell you what the proper value is. Julius used to do the alignment with a box full of zeners testing them one by one until he was happy with the alignment. If you use the otl-3 schematic you will be able to dial them in even if your zeners are slightly off. Also Julius recommended that if you retubed the unit to send it to him so he could make sure the voltages were correct, some of the 6lf6's are very hot and would end up self destructing if dropping too much current, that was a thing of the past when the otl-3 was produced, reguardless of how strong the tube were you could still get them aligned properly.
Good Luck to your rebuild.
Very interesting post from rhino, thank you very much. When my OTL3 needs retuning, I may call on you. It's been something like ten years now.....the bias and balance meters still seem to be spot on.
Thats amazing the tubes have held up that long, if I remember right you are using Stax electrostatic speakers.
I have 2 of the otl-4's that use the same bias/balance meter that your amps use, one of the first things I noticed was that the meters were out of callibratition to the tune of 3 to 5v on the balance section and the same on the bias, there are pots on the board under the meter to callibrate the meter so that's a pretty simple fix. I was up at Ted's in NY a few months ago and he had a otl-4 that the meter was out over 10 volts from true center, something like that would surely become a proplem at sometime down the road.
Dear Rhino,
I'd love for you to come check out my amps at some point. Please let me know if you
are every anywhere nearby.
Hi everyone,
I am servicing a pair of H3aa and I see that indeed a test point is not shown on the schematics. But the most important is that test procedure is wrong if someone follows it exactly! All bias test points are wrong. I believe that the text refers to another schematic.
The idea of balancing is to connect a DVM between upper bank's g1 and lower bank's g1. Then to adjust upper bank's pot to make 2 bias voltages equal so DVM will show ZERO volts.
Step 2 is to measure each bias and see if indeed are equal plus minus 2 Volts. It is performed by connecting DVM between one upper bank's cathode and g1, and then similarly at the lower bank.
Note that lower bank has a not adjustable bias (-42V) because it originates from rectifying 35Vac while the upper bias comes from a voltage divider pot fed by a stabilized ps. That ps voltage depends on it's Zener diode selection. It is fed by lower bank's plate voltage. Am I correct?
The (zero) balance reading refers to getting exactly the same voltages on the upper and lower screens, ie no difference in readings (zero)
upper 167.3v - lower 167.3v.
the bias should be set at 85v.
you will not be able to adjust or ballance the upper and lower grids on that amp, unlike the OTL-3 there is no provisions in place to do that, you will have to ballance the same way Julius did, with a handfull of zener diode's.
Hi rhinohifi714,
I am sure you are looking at a different schematic, not the H3aa's.
As I wrote before, the balance pot does adjust the upper bias only, while both screens are powered by non adjustable stabilized power supplies where selecting the right zeners does make sense.
The lower bias can be adjusted by replacing a series resistor, as stated in the text.
Also, the bias value you mention (85) is not correct as the stable lower bias comes from rectification of 39Vac (not a voltage doubler).
good luck...
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