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2A3 cathode resistor
2A3 cathode resistor
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Old 30th December 2017, 10:45 AM   #11
soulmerchant is offline soulmerchant  Switzerland
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You can go to mouser.com and look up this part:

NH0253K000FE02

Chassis mount using a little bit of thermal conductive compound. keep spaced well from other components. These are rated 25 watt, so you are not going to have any problem.

Best regards
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Old 30th December 2017, 04:25 PM   #12
lexx21 is offline lexx21  United States
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2A3 cathode resistor
Would there be any issues with sound quality with those? I have read conflicting posts to that regard.
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Old 30th December 2017, 05:50 PM   #13
crazyfrog is offline crazyfrog  Canada
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2A3 cathode resistor
Default Ultrapath

Did you consider ultrapath? My 2A3 tubes in my amp have 30uF ultrapath caps (Solen bypassed with K40y). However, the power supply has to be VERY quiet.
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Old 30th December 2017, 08:47 PM   #14
lexx21 is offline lexx21  United States
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2A3 cathode resistor
Right now the 2a3 filament supply is dc as is the supply to the two 12ax7s. The only noise that I am hearing I have traced back to an old Stancor power transformer. Its vibrating. Probably needs to be revarnished.
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Old 30th December 2017, 10:32 PM   #15
soulmerchant is offline soulmerchant  Switzerland
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Hi Crazyfrog - with ultrapath you still need a cathode resistor. You also don't need to have a very quiet PSU if you do L-W noise cancelling.

lexx21, there is no need to use DC on indirectly heated valves/tubes unless they are intended for a microphone or phono stage... but since you already did it then ok. I have used AC as well as DC filaments on 2a3's. Frankly, for 2a3 I would rather do AC than poorly done DC filaments. If you go "old-school" for DC, then the best way is to use filament chokes. Otherwise its best to use a well considered regulator circuit. I find that even a regulator circuit similar as in Merlin Blencowe's book (using LM317T) can work nicely for 2a3.

If you are using 12ax7, then a zvn0545a mosfet "source follower" is also worth trying out. Its so very inexpensive, takes up almost no space and is dead simple to try. No heater requirements either.

Lastly, those vishay non-inductive wirewound resistors I noted are really decent, durable and not expensive, and are a far better choice than any inductive ones you might consider. If you want to spend more money, go for Mills, but I doubt you will hear any difference. 3x mills in parallel (as noted by one poster) would seem more expensive than necessary to me though.

Ian
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Last edited by soulmerchant; 30th December 2017 at 10:56 PM.
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Old 31st December 2017, 01:49 AM   #16
eduard is online now eduard  Netherlands
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Hello,
The Rod Coleman regulators seem to work well on these tubes. I use them with a choke input power supply in my line amplifier with indirect heated tubes. But I had the idea that adding a choke input to a very standard lm350 regulator was a bigger difference than replacing the lm350 with the Coleman. Of course my tubes were not directly heated.
The original poster is living in the USA so getting a few Mills does not break the bank. With the aluminium ones you will need to drill holes and they need a large area to dissipate the heat. I never used the Mills in that "" position "" used them in anode circuit of a line stage where the original resistor would get hot but we'll within their limits but their the Mills were a real big improvements considering the cost.
You just have to give it a try. I remember the French people in the eighties did use non inductive 50 watt aluminium housing resistors so guess with Mills you need 3 in parallel. I think they are about 5$ a piece. Greetings, Eduard
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Old 31st December 2017, 09:35 PM   #17
soulmerchant is offline soulmerchant  Switzerland
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Mills non-inductive resistors in a power supply? I wouldn't consider this a good way to spend my money. Usually I am happy to have a little more inductance in my power supply....

The 25 watt non-inductive vishays are about $4.50 from Mouser (even where I live in europe) while the 12 watt MRA-12 mills cost $6-7 apiece from partsconnexion. I stick by my claim that you will hear no difference between the two. I have used them both. Here is yet another idea - heat sink the 12 watt Mills. I have done that as well, and it also works great but it is a pain to do, and does not sound any better than the 25 watt NH vishays.

I have never tried Rod Coleman's regulators, but have built more elegant regulator circuits for other valves/tubes. The simple LM317T circuit I previously mentioned works very well with 2a3, which does not tend to have 'stringy' or microphonic filaments. Of course the builder may decide that purchasing Rod's excellent solution is more convenient.

I would again assert that with this design the builder should seriously consider addressing the whimpy driver problem.
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Last edited by soulmerchant; 31st December 2017 at 09:45 PM.
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Old 31st December 2017, 11:18 PM   #18
Shef is offline Shef
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It seems the little inductance is not a bad thing for:
- PSU, for dropping the B+, indeed, by a common sense;
- grid stoppers, to make it a bit better against oscillation;
- the auto bias: some people use the huge chokes there;
- khe-m. .. as a plate load too? even more huge chokes spotted in very high-end implementations;
- same for grid leaks. ..

Being said, _where_ the wirewound R (with some inductance) is NOT applicable then? like ultimately, e.g. NFB ? else ?
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Old 1st January 2018, 12:21 PM   #19
lexx21 is offline lexx21  United States
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2A3 cathode resistor
I actually want to keep this all tube. The dc heaters that I have implemented are setup as follows:

Driver tubes:
Schottky rectifiers -> .2R resistor -> filter cap -> common mode choke -> .2R resistor

That took out the hum coming from the drivers. I had tried creating a fake center tap to HT ground, and also elevating the heater voltage but neither helped. DC heaters cleared up that issue. The amp is currently on a sheet of plywood so I routed the heater wires as carefully as I could within the space that I had. Wire twisting was done with a drill, and there was no way that I could have made the twist any tighter.

Power tubes were done in a similar way except that I used a voltage doubler since I was using 2.5V @6A transformers. Pretty much the only noise now is coming from the vintage stancor transformer that I mentioned. I might just replace that and be done with it as I will need to get a vacuum pump to re-varnish it properly. The pumps aren't expensive, only about $120 on the bay.

I will give the aluminum clad resistors a shot and see if I can hear any difference.
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Old 1st January 2018, 05:12 PM   #20
50AE is offline 50AE  Bulgaria
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2A3 cathode resistor
Quote:
Originally Posted by Shef View Post
- grid stoppers, to make it a bit better against oscillation;
Uh, doesn't series inductance form a series resonant LC filter with the input tube capacitance? Keep in mind that increasing it in such cases, decreases the resonant frequency and increases the Q factor. Are you sure you still want inductance in your grid circuit?
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