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Old 28th July 2011, 03:30 AM   #1
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Default 2A3 Guitar Amp Project - in need of guidance.

So a friend of mine recently introduced me to the 2A3, which is the coolest thing ever in my opinion. I want to build a call AB cathode biased guitar amp with it. I have a preamp designed, I just don't understand how to bias the 2A3. I mean, I have one idea. and that is to ground one lead of the filament tap, and send the hot wire to one of the heater pins on the 2A3, and then have a 780 ohm bias resistor to ground on the other heater pin. I honestly like my amp to hum a little, so i'm not too worried about that. I just want to make sure this will work. any help, advice, etc will be greatly appreciated. I'm used to building amps with indirectly heated tubes, so this has never been an issue until now. Thanks again!
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Old 28th July 2011, 03:36 AM   #2
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What you've described won't work, the filament needs to float on top of the cathode voltage, grounding one end of the cathode clearly prevents that.

You can use a center-tapped 2.5V filament winding or created a pseudo center tap with a pair of resistors which the cathode resistor and bypass cap are connected to, the filament is connected across the winding. You'll need one for each tube.

Note that the 2A3 in PP AB will give you about 15W, it is also incredibly fragile and should not be placed in or on the speaker cabinet.

Since this is really a guitar amp I'm going to move this thread.
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Old 28th July 2011, 04:08 AM   #3
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Ah, I saw a design like that. Simple enough I suppose. I think I might build a class A with one 2A3 first and just use the same preamp for the class AB. I understand this will result in two different tones, but I'm cool with that. Is it just that the 2A3 is a fragile tube to begin with, or is it just the NOS models? Because I saw a JJ 2A3 that was rated for 40W of plate dissipation, so if the old 2A3s are fragile, surely the JJ will be a little more shock resistant? Either way I had planned on mounting it in an aluminum chassis and mounting that inside a little wooden cabinet. I had planned on placing it on top of a speaker cab, but it's not required. I don't like open back models with the chassis inside. And thank you for moving the thread and for helping me understand how to bias directly heated tubes.
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Old 28th July 2011, 04:20 AM   #4
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The JJ would probably be a great choice in terms of electrical ruggedness and you could some power out of it, up to 10W or so making this an SE screamer..

All tubes with filaments are mechanically fragile, the JJ 2A3-40 is no exception. Shock mounting is probably advisable as well as building the amp as a head separate from your speaker cabinet.

The same pre-amplifier circuit should be suitable, note that the driver should be able to swing 200Vpp to drive the 2A3-40 to the maximum output of which it is capable, supply voltage with cathode bias can be as high as 500V also allowing you to use a simple RC coupled driver circuit.. Lower powers are attainable with lower B+, or you can use fixed bias in the output stage and get roughly the same power on a 400V B+..

Opt probably 2.5K primary and built as would generally be the case for one of the little Fender amps.. (Champ or similar?)

Probably overwhelming you.

Hopefully George (tubelab) will weigh in here..
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Old 28th July 2011, 05:02 AM   #5
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Nah, I actually understood everything you said. I built a 5F1 Champ as my first build. If I remember correctly the Fender Champ OT is 7K/3.2. I found a nice 5K/8 OT, so I'll just hook it up to a 4 ohm load. I had read a few threads previously on the subject of the preamp throwing enough voltage to drive the 2A3. I'm not worried about that; I'm using a 4 gain stage preamp with 9 pin dual triodes. I plan on using a relatively low power supply for this thing (don't need a lot of watts, just a different sound. class a studio amp) 250V secondary with a 5U4 rectifier with a 40uF cap input. Plate characteristics in the RCA tube data sheet for the 2A3 show it operating as low as 100V! I know I'd make a lot of the radio guys and old skool tube guys mad about this since i'm actually trying to design something that is not very good at being an amp. I believe that when you're using an amp strictly for guitar, the more out of wack your throw it, the better it sounds (with limitations of course!) Anyways, thank you for all your help. I'll post some pictures and keep this thread updated on the progress of this thing.
Oh, and any opinions on Chinese 2A3s and 5U4s? They're super affordable and the China 5U4's have the ST shape I so adore. I understand it probably won't be premium quality tubes, but like i said. SO AFFORDABLE!

Last edited by searon386; 28th July 2011 at 05:04 AM.
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Old 28th July 2011, 06:00 AM   #6
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There's always fixed bias, that's a no brainer to make and sounds good.
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Old 28th July 2011, 06:09 AM   #7
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oh, quick question. i was thinking it would be cool to put a 5U4 and a 5AR4 in the same power supply for this amp, but only using one at a time. The idea is to tie the plates tied together in parelell with the 250 tap, and have individual 5V heater transformers for each tube. then to have the filter section tied to the main pole of a DPST, with the B+ pins on the tubes tied to the other poles, so that by flipping the switch, it attatches the heater either to the 5U4 or the GZ34. The idea is that by changing rectifier tubes, you get a different tone and voltage supply to the tubes, almost like having two amps in one. My only question is that will the rectifier tube have problems with not having a filter tied to it? do i need like a dummy filter load when it's not tied to the filter? (and could there be other problems with this circuit?) Thanks again for all the advice, this forum is an amazing resource for me.
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Old 28th July 2011, 06:12 AM   #8
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For some reason I just do not like using fixed bias for a class A amplifier. I will probably use fixed bias when I put together the amp with two 2A3s though.
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Old 28th July 2011, 07:22 AM   #9
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Quote:
Originally Posted by searon386 View Post
then to have the filter section tied to the main pole of a DPST, with the B+ pins on the tubes tied to the other poles, so that by flipping the switch, it attatches the heater either to the 5U4 or the GZ34.
What I meant to say was that by flipping the switch, it attatches the filter to either the 5U4 or the GZ34. sorry.
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Old 28th July 2011, 02:08 PM   #10
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Quote:
Originally Posted by searon386 View Post
For some reason I just do not like using fixed bias for a class A amplifier. I will probably use fixed bias when I put together the amp with two 2A3s though.
This actually works better in HIFI applications IMHO than cathode bias because you don't have what is generally a low quality cathode bypass capacitor in the signal path - even film caps I have experimented with in most cases add audible colorations to the sound.. (Almost all of my SE designs use fixed bias)

Noting that this is a guitar amp it could well be that cathode bias will give you exactly what you need, you're not looking for hi-fi obviously. The one possible advantage of fixed bias in this application is it would allow you to tune the operating point to suit, and it is more efficient.

Operating on what is effectively a 200V rail (roughly 50V burned across the cathode resistor) you will have about 2W on tap - hopefully this is sufficient for your purposes.. If not boost the plate voltage to 300V and/or use fixed bias - this will get you to around 3.5W out..

Since you are not pushing for higher power output I would start with an inexpensive Sovtek 2A3, rather than the JJ 2A3-40 which will cost a great deal more..

I think you have an interesting project..
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