hum balance pot hf85???

[ferninando]

could I use a 1k pot in place of a 100 ohm pot for hum balance in my hf85. I think the 100Ohm pot I bought is bad and near as I have in my stock is 1k.
Or could I use 2 50ohm Rs to gnd each side of the heater lines.
or just remove the pot altogether. But what effect would that have on anything.?

 

[ferninando]

the pot was bed, open at one end of track.
bought 2 new honeywell 100 ohm pots on ebay will deliver next week.
free ship.
so Im good.

 

[BrokenHill]

The hum pot in my HF-85 is the only potentiometer that I did not replace, it was still in good shape and
is currently connected in circuit, all other pots in my Eico pre got replaced and work very nicely now.

Sounds like you are getting much closer to finishing your HF-85. :thmbsp:

 

[ferninando]

Yes it's all wired and ready to go except for the new pot shafts
I have to cut.
The orig. hum pot was destroyed is reason for the new one, cheep hum pot for git amps.
I smashed it with a hammer. But I did manage to get a signal to go thru the line stage quite nicely even with the open pot. So after new pot install, another run on the bench and if all goes well into my system for a listen to see how it sounds. the case stilll needs paint but it doesnt affect the works.

 

[BrokenHill]

Cool, if everything turns out good it should sound really good on your Magnavox PP 6BQ5 amp, it does on my Maggy 9303 amp.

I think it must of been three or four months after I had mine rebuilt before I painted the case, do plenty of test listening first.

 

[ferninando]

Id like to get some paint on before it gets cold. warm is better for painting.
It ready for prime, I have the color and can do it anytime.
I have 15 pics of the finished chassis all wired but dont know if puotobucket can do them all. Ill try soon.

 

[jaymanaa]

I used two 50 ohm metal film Rs to reference ground and eliminated the pot all together. Are you going to use the phono stage? It's easy to put DC on those tubes like Eico should have.

 

[gadget73]

I've hotwired around bad hum pots that had one broken end by tying a 47 ohm resistor from the broken end to the center wiper, and then adjusting it. It works about as well as a functional pot. Failing that, two 47 or 50 ohms to ground works too.

 

[ferninando]

good to know 50 ohm Rs will do the pots job.
thanks for that J and gadget.

 

[jaymanaa]

good to know 50 ohm Rs will do the pots job.
thanks for that J and gadget.

Yep, hum pots typically don't do much when you adjust them and they tend to end up in the center (or very close) anyhow, so the resistors just make more sense to me. You should also check your filament voltage at your wall juice. If you are in there soldering, it would be the perfect time to add a dropping R if needed. I DO NOT like to run filaments too hot. Not with what tubes cost these days. I'd rather be a hair low, than any amount high.

 

[ferninando]

where do you put the R and how big? 1K, 5k, 10K?
just in seires with a 2ndary winding?

 

[gadget73]

generally its a 47 to 100 ohm resistor from each side of the heater circuit to ground. If the heater supply has a center tap, you don't need this. I do have stuff that the hum pot makes a big difference on, but most things it does just about nothing. Typical hum pot value is 100 ohms and its usually pretty near the center, so 47 ohm resistors sub for it pretty well. 1/2 watt is generally fine, or has been in all the stuff I've used them in.

Another little trick that you can do with cathode bias rigs is to tie the center wiper of the pot, or the junction of your two resistors to the cathodes of the output tubes instead of to ground. This puts some DC on the heater circuit, reducing the H-K voltage difference to nearly zero, and makes the supply slightly quieter. It ends up referenced to ground via the cathode resistor, typically under 300 ohms, which is plenty good enough. Many amps came with this configuration from the factory.