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2A3 cathode resistor

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I have build the Shishido 2a3 amp and now I am in the process of tweaking it. Currently I have 3k "coffin box" resistors, those white cermet resistors in the cathode.

Were you to optimize parts, what brand/type of resistor would you use in place of a cermet.

The amp sounds pretty good now, however I still have it on a piece of plywood until I get it just the way I want it. Looking for input.

Oh, I am using low esr caps for the bypass caps. Unless there is something better than those in that position, I had intended on leaving them. Any input on those as well?

Thanks in advance.
 
I am using Arcol/Ohmite AP101 Series resistors with good results: thickfilm resistors (non-inductive) in an isolated TO-247 package. You just have to mount them somewhere on your enclosure.
You may select another manufacturer (e.g. Vishay) or series with lower power handling or rated voltage as you like.
 
Hello,
Hiraga always wrote to use non inductive wirewounds high power resistors. The resistor in aluminium housing are good quality but people say the Mills are better. But Mills are just 12 watt so maybe put 3 in parallel. They will get hot but they have no problem getting hotel. Just take care they don't heat up the capacitor! The mills don't need wires like the aluminium resistor do. So try to use the wires that are attached to the Mills. I think that can be done. Have some distance between the resistors to get some airflow. Greetings, Eduard
 
Ok, thanks
 

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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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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
 
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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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 ?
 
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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