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Old 2nd September 2004, 06:09 PM   #1
Masami is offline Masami  Hong Kong
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Default 6as7 se amp

Hi,

I am take interesting to built a se amp with 6as7.
I prefer to use the Svetlana circuit.

http://www.svetlana.com/graphics/TB/No.26fig1.jpg

but I don't like the driving stage use 700v.
Can I just use a EF86 to drive the 6as7?

http://www.svetlana.com/graphics/TB/no.23fig1.jpg

how many p-p voltage to drive the 6as7 to full output?
What is the function of the 10ohm resister which connect to 6as7's plate?
Can I remove it?

Thanks.
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Old 2nd September 2004, 09:59 PM   #2
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Hi,

Quote:
Can I just use a EF86 to drive the 6as7?
Not as a penthode, even trioded I wouldn't recommend it.
There are plenty of other tubes that can manage it easily though.

Quote:
how many p-p voltage to drive the 6as7 to full output?
Depends on how it's been biased.

Quote:
What is the function of the 10ohm resister which connect to 6as7's plate?
Current sharing between the two triodes.

Quote:
Can I remove it?
I'd leave it and insert some testpoints so current and voltages can be measured easily.

Cheers,
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Old 5th September 2004, 09:11 PM   #3
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It looks like the Svetlana circuit has the 6AS7 idling at a little over 100mA. Thats a bit low for a SE design since they can handle 125mA per triode, for a total of 250mA as idle current.
Anyhow the amp no doubt sounds great, that tube is very good. The 500ohm reostat lets u adjust so the triodes have equal Ia, but how to measure each triode's Ia? Thats where the 10ohms come in. They can be removed,and are only to help adjust equal Ia.
I think you can drive the 6AS7 with a 5687 either tranny connected or with active load(CCS) from the same 400V rail.
I'd shoot for a drive signal of +-125V and go from there.

I did a SE amp with a 6AS7 but had 130V rails and cathode resistance of only 10ohms (only to check Ia which was 100mA/triode). The grid was at -45V. I drove it with +-50V, but should have had more grid drive. I didnt have the correct OT and was busy designing OTLs so I never fixed it. It really sounded good tho, but only gave me 2W because of output tranny was wrong.

I love that tube, and plan to make a super simple stereo 2W SE amp with only one 5687 and one 6AS7, one triode half for each channel. Just something for the office.

Best regards
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Old 6th September 2004, 04:34 PM   #4
Masami is offline Masami  Hong Kong
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Hi SemperFi,

Thanks for your reply.

If I use EF86 for the driver stage,
will it not enough current to drive the 6as7 (output impedance too large) ?

If I connect a voltmeter to the both cathode of the 6as7 and adjust the reading to 0v, then the Ia of both section is same, isn't it?
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Old 6th September 2004, 05:55 PM   #5
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The problem with only the reostat is that you dont know the resistance of each leg. So when u adjust for 0V the Ia may be different since the resistance on each cathode is different. The grid voltage is slightly different for the triodes, so having equal voltages at the cathodes do not guaranty equal Ia. But I dont think it matters much if one triode idles at 80mA and the other at 60mA so I think u can skip the whole reostat business. Try and if not convinced, just add the reostat later.

I'm sorry but I have no experience with the EF86. Here again its such a simple circuit that I think u should try it and tell us about it, then change driver if not happy.
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