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Old 27th February 2006, 02:04 PM   #1
sgerus is offline sgerus  United States
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Default What is the best input/driver for 2A3 SE?

Hello All,

I just built my first tube amp based on the JE Labs 2A3 2001 edition.

Its a 6sl7 grounded cathode - cathode follower capacitor coupled to the 2A3. I drive the amp from the pre-amp output of my Stereo receiver into 100db/w/m horn type speakers.

The amp sounds OK. But just OK

Im looking for ideas for a better front end.

The first choice is to stick with a tube that fist into the 8 pin socket.

However Im open to switching to a 9-pin tube as a last resort.

So, of the following configs, what do you like.

A. grounded cathode (just use of the tube)
C. U-Follower
D. grounded cathode - cathode follower (as built)

Thanks in Advance
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Old 27th February 2006, 02:49 PM   #2
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I am a bit confused why you used a cathode follower cap coupled . I think I'd try parallelling both halves of the 6SL7 in a grounded cathode, plate coupled via capacitor to 2A3 grid. That would be very easy to obtain with minimal rewiring. You have not complained about a lack of sensitivity so obviously you have enough external drive available. This won't change much. It will get slightly better actually but not enough to hear. You may need to reduce R plate of the 6SL7 to as much as 1/2 original value as well as the common cathode R if you want the same tube current and operating characteristic as originally had with the first stage.

I am curious about the speakers you are using with it. Are they full range front horns?
"There are more worlds than the one you can hold in your hand." Albert Hosteen, Navajo spiritual elder and code-breaker, X-Files TV Series.
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Old 27th February 2006, 03:07 PM   #3
sgerus is offline sgerus  United States
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My speakers are dual horn type 3 - ways... Klipsch KLF-20

Yes, the input sensitivity and output power are fine for my setup.

If I rememder correctly, 1.25V input = the 54V I need to drive the 2A3
(as built)

It's the preformance of the "horn sounds" I'm looking to improve.

Here is the link of the amp:


Thanks for the info

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Old 27th February 2006, 03:30 PM   #4
Gluca is offline Gluca  Italy
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My pick-up:

B. Or you might consider a choke loaded 6C45pi (you'll need to change the socket).

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Old 27th February 2006, 04:48 PM   #5
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Default Re: What is the best input/driver for 2A3 SE?

Originally posted by sgerus
I drive the amp from the pre-amp output of my Stereo receiver
That's where I'd look if you don't like the sound.
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Old 27th February 2006, 05:30 PM   #6
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Hi Scott,

for simplicity, and to stay with two octal driver sockets, I would have a try at two paralleled 6sl7 sections per channel. Have a look at the EL34SE amp also shown at the Angela site to get the picture.

A problem is, that available B+ for the drivers in that 2A3 amp is a bit on the low side for 6SL7 anyway. 6SL7 sounds best with about 200V at plate, which would leave only 150V or so across the plate resistor in your circuit.

But at least it is worth a try, and a simple one to do, too.


P.S: You could also try something along the DC-coupled 6SN7 driver shown at that site implemented in the 300B SE amp. Again, no socket type change required. On second though, that even might be the better option, since it gives better driving capabilities (less driver Zo). The PT must be able to deliver 300mA more heater current at 6,3VAC, if you want to try this.

If in doubt, just measure.
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Old 27th February 2006, 05:42 PM   #7
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another 2a3-

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Old 27th February 2006, 07:00 PM   #8
sgerus is offline sgerus  United States
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This amp started out as the JE Labs 6sn7-300B
That amp sounded great but I gave up due to hum Problems which I thought where heater related.
I have since fixed the hum, changing to a 6sl7-2A3 amp in the process.
(Had nothing to do with the 300B heater).

So yes, changing to a 6sn7 is an option.

Here is what Im thinking based on the replies so far:

1. Test with 6sl7 as SRPP, thats easy and I have the parts.
2. Test as a U-follower even thought nobody has recommended that to this point!
3. Convert to 6sn7 per your suggestion
( which would be a good exercise for my next project ....)

I would still like to keep the input sensitivity around 1V.
Can I do that if I convert to the 6sn7?
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Old 27th February 2006, 07:43 PM   #9
ToE is offline ToE  United States
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I built the JE labs 2a3 srpp version which I enjoy very much (though I have some hum issues of my own I am chasing down...)

I agree with dsavitsk, your amp can't sound better than what your reciever is feeding it. A potentiometer on the input, I used a 100k on mine, feed it directly from your source and see how it sounds.

The JE labs deluxe 2a3 with a single 6sn7 for input and driver is another option. ( I plan to try this one myself...)

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Old 27th February 2006, 08:09 PM   #10
sgerus is offline sgerus  United States
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For what it's worth, here are some of the steps I went through to fix the HUM with my amp. This is a letter I sent to Derek Walton, who was very helpfull with the 300B amp.

Hello Derek,

Good timing, I was planning on sending you an update.

I got the hum under 2mV.... but this is no longer a 300B amp.

After testing the 300B amp with my receiver as a pre-amp and on the 98dB/w/m speakers, I came to the conclusion that:
1. I did not need all the power of a 300B
2. I did not need the low input sensitivity level of the 6sn7 input/driver.
3. Getting the HUM out became my top priority.

I returned the 300B tubes and changed the amp to a 6sl7 2a3
(JE Labs SIMPLE 45) with the PSU based in your deluxe amp.

I'm happy with the amp I ended up with... but I think the 300B sounded better!

Right now I'm planning my 2nd amp for next winter. Right now I'm thinking of a 6sl7 2A3 PSE.... or a 6sl7/300B.

Here are the steps I went through to get rid of the HUM

1. when I switched to the 2A3, I had to by a separate transformer for the 2A3 heater, I believe this helped a lot. My next project will go this way.
By the way, hum pots didn't help. I have the R/C on the heater transformer ct.

2. I re arranged the tube's to the the GZ34 away from the input tube.
The first amp had the GZ45-6sn7-300B in line.
The current amp has the 6sl7-2A3-GZ34 in line.
My next project will keep all the PSU components- all the AC and all the single-path far away from one another.

3. I re did the grounding... I will still send you photos in a couple of weeks.

4. Now I heat the 6sl7 from T1, grounding the 6.3vac dropped the sound level (hum?) a LOT, but didn't change the amount of mV I was seeing at the speaker

5. PSU changes: with the 6sl7 I only need 1.5 mV. So I figured out I was not drawing enough current through L2. My PSU is now choke input:

THIS CHANGE LOWERED THE HUM A LOT. Before the psu changes, I was about 4.3mV. With the psu as described above, I'm under 2mV.
The meter reads 4.4 mV, but I read 2.7 mV with the amp off, so I guess the hum would be 1.7mV

Update 2/2/2006
Add 80v dc to 6sl7 heater ct.. Hum is now arounf 1mV
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