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Old 17th April 2008, 11:39 AM   #11
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Forgot! JJ makes a 2A3 with 40W Pamax. With that tube it is of course possible reach far beyond 12W PP in class-A.
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Old 6th June 2008, 09:03 AM   #12
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Not anyone with a schematic?

Here is a picture of the amplifier
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File Type: jpg dsc00294.jpg (70.0 KB, 1451 views)
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Old 6th June 2008, 11:26 AM   #13
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Hej Maarten,

I think the link that gingertube provided is very close to your amp. Scroll down for a picture of the amp and the original hand drawn schematic.


Click the image to open in full size.

-- josé k.
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Old 13th June 2008, 03:52 PM   #14
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I have a series 300 mk2..... ECL86 amp.


I came across this thread just now and thought I'd post it - see one poster has schematics for them:

http://www.pinkfishmedia.net/forum/s...ad.php?t=47929

I intend emailing him to get one for my own amp.

Fran
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Old 14th June 2008, 10:03 AM   #15
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AI 300 power amp schematic:

http://www.drtube.com/audioamp.htm#AI
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Old 21st June 2008, 02:34 PM   #16
guyser is offline guyser  United Kingdom
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the schematic above was for the Second Audio which was slightly different in a number of ways besides being parallel push pull.

The First Audio was just a push/pull design giving 7.5 Watts per channel in Class A

It was available in 3 forms. Initially it was supplied with 6B4G's with an ECC88 input & driver stage. decent NOS 6B4G's became harder to find but the Chinese had started making 2A3's and were able to supply Octal based versions. (New production UX4 bases were also hard to find then, this was the late 1980's! No-one else was using 300B's or 2A3's in any quantities back then.) Finally, the input stage was re-designed using ECC82's and the input sensitivity was slightly reduced.

Here are the schematics for the ECC88/2A3 which is I think the amp pictured above although the LT supplies appear to have been modified.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

hope this helps.
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Old 23rd November 2008, 01:20 PM   #17
2a3geek is offline 2a3geek  United States
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Not meaning to drag up a dead old post, but after reading this months ago I got very curious about 2a3's in guitar amplifiers.
I contacted Straub since they were the only company I could find that is using them in production. He would not send me the schematic, but he did indicate that he is indeed using the JJ 2a3s to get the power up to 12 watts in class A. He was nice, but vague on the details of the circuit. I think he is onto something with the tones he is getting(his clips are on YouTube: YOUTUBE LINK ) and want to learn more. I like the overdrive tones and how it cleans up with just the guitar volume.
Has anyone gotten a chance to try one of these amps yet, or taken one apart? I would like to know what voltages he is running, and the circuit for the driver tube(6sn7).
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Old 24th November 2008, 06:13 AM   #18
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You do not need JJ to get 12W out. Maybe Straub does not know enough about DHT? The talk about Class-A is usually b-s in the guitaramp-business. What they mean is Class AB, cathode bias.

The 6B4G is a much better option as it is the same tube but octal and has 6,3V heater and the same socket layout as 6V6, 6L6 etc. Sovtek has a Uamax of 450V.

A pair of std 6B4G will gladly deliver15W in class AB fixed bias.

I recommend 10W cathode bias with Ua 300V(B+ ca 350-370V) and 5k Raa.

You ought to have a capable driver though. I would suggest an OS Gibson GA-40 paraphase inverter with at least 400V B+. For guitar use the 12AX7 will work and has high gain.

You also need to add ca 14dB preamp gain compared to a typical Gibson/Fender to compensate for the low sensitivity of the 2A3/6B4G.

Edit: Just did a quick sim in LTSpice with the proposed working point and get 11,5W with 1,5% THD 6B4G biased at 14W each.
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Old 24th November 2008, 02:48 PM   #19
2a3geek is offline 2a3geek  United States
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Thanks! I really appreciate the input and help.

As you can probably tell from my moniker, I like 2a3's. As much as the 6B4G would be easier to work with, I already have over 40 2a3's in my collection that I want to build with. Plus, is anyone making a 6B4G anymore? Last I heard the russians (sovtek?) stopped manufacturing them and they were the last that I know of?
Straub said that their amp is not cathode biased, which is what intrigued me about his class A claim. I tend to agree with them that fixed bias amps sound better, at least on triodes.
I might just build one up my test setup and see how close I can get using a variable bench supply for experimenting.
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Old 24th November 2008, 04:25 PM   #20
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No problem if you got the 2A3 and sockets. 2A3 might even have a more durable cathode due to the lower heatervoltage. The 6B4Gs(and 6S4S) are cheaper than 2A3 due to that they are not hyped. I have a lot of 6B4Gs so for me it is the other way around. But it does not matter soundwise, same same.

How do you know triode guitaramps sound better with fixed bias when you haven´t tried it yet????? Got a crystal ball?

Just remember the PI needs to be able to swing. So a typical Fenderstyle won´t do.

Let me know when you have tried it out.

I will do a special version of our reVintage Pro to try out for myself. Will substitute the 7591s for triodes and the 12AX7 PI for a 6Q11 and use the extra triodesection for gain-compensation. www.revintage.se
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