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I have been using AC on filaments for so long .......

173.189.217.15
Posted on February 8, 2016 at 12:04:36
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 that I have forgotten how to hook up a cathode resistor and cap if you are using DC. You don't still need a hum pot then do you? So where and how do you hook it up?
Decided to give DC a try on the 26 preamp as it finally dawned on me that any hum I get in the preamp from ac on the filaments will be amplified in the amp, right?

 

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RE: I have been using AC on filaments for so long ......., posted on February 8, 2016 at 14:01:22
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Let's say you would use a 100 ohm hum pot across the filament for AC, just use two 50 ohm resistors across the filament with the point between the two as the cathode connection (just like the wiper when you use a pot).

And don't ground the DC supply anywhere else.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I have been using AC on filaments for so long ......., posted on February 8, 2016 at 14:07:42
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Well, that sounds easy enough even I can do it.....
Thanks Tre'

 

I think it is FINALLY all gone....., posted on February 8, 2016 at 15:11:45
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I thought I had gotten rid of the preamp ac filament hum before by going to 1 turn pots with resistors on the ends, but I think that the freezer in my shop was masking it. I shut everything off in there and I could then hear a low level but audible hum. I tried every trick I could think of and every trick everyone suggested, but no luck.
I finally decided to try one channel with dc using a bridge and a 10k uf cap on each side of a small resistor. The preamp is now quiet. I think I will have to concede that for preamps, dc filaments are probably the way to go. I will still keep ac on my amp filaments however, as I have been able to get that hum very very low.
It has been quite an education. Thanks to all for your help!

 

RE:Now that we have you on the DC path...., posted on February 8, 2016 at 15:56:21
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 lets contact Rod Coleman and get you a couple boards, they are cheap and perfect for the 26

 

RE:Now that we have you on the DC path...., posted on February 8, 2016 at 18:30:43
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 One step at a time deaf, one step at a time..... : )

 

Vinnie my 26-26 DC 45 amp is the quietest ever ....., posted on February 8, 2016 at 19:22:40
drummerwill
Audiophile

Posts: 965
Location: St Louis Mo.
Joined: January 7, 2003
 ... Every since I added a hum-pot to the second Type 26 input tube for each channel.
.... I have DC heated filaments on the 26's with a pair of 50ohm resistors to the Rk's. ..... Also now I have humpots , the center tap of the humpot goes to the top of the Rk also.

The 45's are AC heated with a 2.5volt tranny using the C.T. as to Rk and Bypass caps then ground.

The amp takes a little time to warm up ( start 72mvAC ) and both input 26's reach a harmonic equilibrium. About 10mins and hum drops to about 32mvAC. On my GPA 604 MLTL's 3ft away I hear barely a hum.

It has as little hum as my Reference Stereo 2A3 DC amp and much quieter than my Stereo Universal DHT amp......these all have IDHT input tubes.

So Vinnie, at this point for me the question is whether to pursue the AC heated inputs and put up with some inevitable AC Hum .... or just stick with a very quiet DC heated setup ?

....I still need to hear AC Heated inputs ...... crazy !

Have fun
Willie

 

RE: Vinnie my 26-26 DC 45 amp is the quietest ever ....., posted on February 9, 2016 at 02:40:49
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Hey Willie
At this point I can only say for sure that the dc works best on the filaments for the 26's in the PREAMP. From what I heard when I had the 45 amp bread boarded, and from what I hear with my 91A clone, I think the amp will be fine with ac on the filaments, but I won't know until I have the 45 amp put together in final form in a month or so.
I am curious about how a hum pot works with dc; there is nothing to balance out is there? Isn't 32 mV kind of high for a hum level? I always thought hum should be less than 8 mV. I have not measured mine yet; guess I should. Just been going by ear.
You really owe it to yourself to try all ac filaments in the amp. I would think that if any dc is needed in the amp it would only be the filaments of the input 26, but I am hoping not even that.
If you get there first let me know what you hear.

 

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