2A3 get*set*go docs

2nd harmonics for the masses

2A3 get*set*go docs

Postby Shannon Parks » Sun Aug 05, 2012 6:44 am

Here's a stab at a 2A3 get*set*go. I've been getting more and more requests for a 45 & 2A3 compatible amp over the last year, which I attribute to 6B4Gs getting more expensive and modern 2A3s - particularly Chinese ones - are much more abundant. I also guess people want to use part of their tube stash, which I can't blame one for that. But the get*set*go was very much designed around the 6B4G. It uses a just two filament taps from a single power transformer, whereas a 2A3 amp usually requires four.

Anyhow, I think I have a nifty scheme using a regular GSG PCB. The filaments will be AC and the DC circuit gone. The power transformer will be an Edcor XPWR067-100/120. Three small socket jumpers on V2 turn our rectifier into a 5U4/5V4/5Y3. And a voltage divider and jumper wire is added for the 6SL7 AC heater reference (using D2 & D4 footprints, so it doesn't get messy).

get*set*go 2A3 manual
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Notes:
1) I haven't built this, so there could be some documentation errors. If anyone sees something goofy, let me know.
2) B+ voltage will probably be different than the 6B4G design due to the new power transformer (640VCT instead of 660VCT). The 5V rectifiers allow some adjustment due to their different voltage drops. The Hammond 302AX is a second source for those that can't wait 6 or 7 weeks for Edcor, but it is double the price and only 600VCT. But hopefully the 5V4 will accommodate that.
3) The voltage divider resistors are now all 24 ohms across the 2.5V filaments. Since we have a perfect sine wave, we want a perfect voltage divider. You can even match your four resistors up. The 6B4G scheme is 47 and 51 ohms to compensate for the 120Hz ripple in the rectified filament supply.
4) I assume that using 45s would mean a slightly higher filament voltage due to the lower current requirements. You might want to have some 0R1 power resistors on hand to drop extra voltage if needed. This might even be needed on the 2A3s. Symmetry won't matter, I think. Just put them in series with the filament.

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Postby TerrySmith » Sun Aug 05, 2012 10:35 am

That should work perfectly since there's no real change in the circuit.

I too can't believe the price of 6B4G / 6A3 tubes. I paid $59.95 for a matched pair of Sovtek 6A3's and now that's more than doubled in price!
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Postby kku » Sun Aug 05, 2012 10:41 pm

True, $140 for Sovtek tubes is ludicrous.

What resistor changes should be done to use 45 tube to accomodate for lower plate current(30mA vs. 60mA). Also, grid voltage for 45 is -50V vs. -45 for 2A3. Is that critical as well?

71A tube is kind of like 45 with 5V/0,25A filament and 20mA plate current. Would it be OK to just keep DC filament circuit and change R 19/20 to 1.5 ohm to get 5V like we did with 300B mod? Or there's more to it?
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Postby Shannon Parks » Mon Aug 06, 2012 6:43 am

kku wrote:What resistor changes should be done to use 45 tube to accomodate for lower plate current(30mA vs. 60mA). Also, grid voltage for 45 is -50V vs. -45 for 2A3. Is that critical as well?


It would all be about the cathode resistor changes, which I haven't research yet. I think Ed Brown recently mentioned some values for a 45. FWIW, my single 45 from my stash turned out to be a dud.

kku wrote:71A tube is kind of like 45 with 5V/0,25A filament and 20mA plate current. Would it be OK to just keep DC filament circuit and change R 19/20 to 1.5 ohm to get 5V like we did with 300B mod? Or there's more to it?


I'd have to look at this more closely. When I tested my 71As a couple weeks ago, I seem to recall thinking that 1626 tubes basically were the same tube, but IDHT and cheap.

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Postby kku » Mon Aug 06, 2012 11:30 am

Which is the cathode resistor in GSG?
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Postby Shannon Parks » Mon Aug 06, 2012 11:41 am

kku wrote:Which is the cathode resistor in GSG?


R15a/R15b/R15c (three resistors in parallel) for one side and R16a/R16b/R16c for the other. They are equivalent to a 900 ohm, 9W resistor. The 45 would basically need twice the resistance (half the current), so it would be around 1800 ohms.

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Postby kku » Wed Aug 08, 2012 4:33 pm

Thanks Shannon. Preparing order for Mouser. 2 x 6.8K and a 5K to get 2023 ohms..it should keep 45's happy.
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Postby EWBrown » Wed Aug 08, 2012 8:01 pm

IIRC, the 2A3 (Correction: 45) would need a cathode resistor of 1600 to 2000 ohms, depending on the B+ voltage.

Also, the 45's plate resistance is nearly twice that of the 2A3, so a higher OPT's primary Z may be necessary, use 5K to 7K, instead of 2.5 to 3K.

Expected output power, 1.75 to 2 WPC. [:)

HTH

/ed B
Last edited by EWBrown on Fri Nov 25, 2016 1:47 pm, edited 3 times in total.
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Postby Shannon Parks » Sat Aug 11, 2012 5:32 am

EWBrown wrote:IIRC, the 2A3 would need a cathode resistor of 1600 to 2000 ohms, depending on the B+ voltage.

Also, the 45's plate resistance is nearly twice that of the 2A3, so a higher OPT's primary Z may be necessary, use 5K to 7K, instead of 2.5 to 3K.

Expected output power, 1.75 to 2 WPC. [:)


Hold on, Ed. 2A3s and 6B4Gs are functionally identical, other than the filament voltage, right? So the stock cathode resistor is 900 ohms (three paralleled 2.7K, 3W).

kku, I think you can just get a handful of 5W resistors pairs to zoom in on that perfect bias point for 45s, instead of juggling with paralleled resistors which can make this initial work a little more difficult.

Everything else is on the mark. I forgot to mention the change in output transformer Z. Do people really have amps where they swap between 45s and 2A3s?

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Postby battradio » Sat Aug 11, 2012 12:15 pm

I think ED ment 45's instead of 2A3's , 2A3's are two 45 in parallel in one tube envelope . 1800 ohms would be fine for the 45's and 2A3 would use the same 900 ohm as the 6B4's
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Postby Shannon Parks » Sat Aug 11, 2012 12:42 pm

Ahhh - that makes sense!

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Postby kku » Tue Aug 14, 2012 1:25 am

kku, I think you can just get a handful of 5W resistors pairs to zoom in on that perfect bias point for 45s, instead of juggling with paralleled resistors which can make this initial work a little more difficult.


Do you suggest putting on one 5W resistor or two 5W resistors in parallel to get 10W?
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Postby Shannon Parks » Tue Aug 14, 2012 5:25 am

Since you'll be doing low current with a 45, a single 5W is fine.

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Why don't we keep 6AX5GT rectifier?

Postby scalardia » Sat Aug 25, 2012 11:18 pm

Hi all, I'm just wondering if we can keep original 6AX5GT rectifier in 2A3 mod design for cost reason? I know 6AX5GT runs 6.3v, 5Y3 runs 5v, is it possible to change some resistor values to make it possible? Thank you very much!
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Re: Why don't we keep 6AX5GT rectifier?

Postby Shannon Parks » Sun Aug 26, 2012 11:13 am

scalardia wrote:Hi all, I'm just wondering if we can keep original 6AX5GT rectifier in 2A3 mod design for cost reason? I know 6AX5GT runs 6.3v, 5Y3 runs 5v, is it possible to change some resistor values to make it possible? Thank you very much!


Sure! You just run jumpers from from J4-1&2 to J3-1&2 since there's only one 6.3VAC tap on the suggested transformer. The main reason I added the 5V rectifier (other than we now have a 5V tap with that tranny), is that it gives us flexibility to change the rectifier according to what we need for B+ (i.e. we can choose a lower voltage drop rectifier since we have a 640VCT power tranny).

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