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Old 7th October 2003, 01:10 PM   #1
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Default My first tube amplifier

Okay, after making several line amps I feel that I should be up to the task of making a amplifier. Granted I'm not quite up to the design part but if a suitable design can be found I'm ready to make it.

I'm thinking along the lines of a single ended amplifier because I already have enough Quicksilvers. My speakers are in the neighborhood of 102db 1W 1M so efficiency won't be a factor. I have a supply of KT90 and EL34 tubes so I am thinking along that line.

I have found several designs by Antique Sound that use a KT88 and a 12AX7 and another that uses a 12AX7 and EL34. Needless to say I can't post them because of copywrite laws. The only thing I would need for this would be the selection of the output transformer.

Any ideas on this or a more suitable circuit?


Thanks
Joe
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Old 7th October 2003, 01:55 PM   #2
316a is offline 316a  England
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Default SE

Hi Joe ,
If your speakers are that sensitive and you're new to power amps why not consider a small triode such as 12B4a ? They're cheap , plentiful and linear . A decent sounding 1.5watt amp can be made with one of these using a 5k output transformer and a single driver valve such as 6SL7 , 5755 or 5751 (1 section per channel) . The next step would be 2A3's but this would be far more expensive !
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Old 7th October 2003, 01:56 PM   #3
Fuling is offline Fuling  Sweden
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Personally I wouldn´t use 12AX7 in a serious amp.
Have a look at PC/EC86 instead, or maybe a triodewired pentode like EF86 or E81L. Be creative!

With 102dB speakers I guess you could aim for low distortion rather than maximum power from the power tube, like using 5k load instead of 2,5k for a triodewired EL34.
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Old 7th October 2003, 01:59 PM   #4
Fuling is offline Fuling  Sweden
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Oh, did someone mention 12B4???
One of my absolute favorites!!
Far better than EL34!!!
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Old 7th October 2003, 02:02 PM   #5
316a is offline 316a  England
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Default 12B4A

Quote:
Originally posted by Fuling
Oh, did someone mention 12B4???
One of my absolute favorites!!
Far better than EL34!!!
The things sound far too good to have lived inside TV's and oscilloscopes for all these years ! I'm using mine with battery grid bias driven by a CCS loaded battery biased 14F7 (loctal 6SL7) . Crappy old console outputs , but sounds excellent . How are you using yours ?
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Old 7th October 2003, 02:10 PM   #6
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Well, I guess I hadn't thought of the 12b4a. Do you have a schematic that you would care to share? I have several 5751's and 6sl7's. I will be building a set of 2A3's soon

There is nothing that says that I can't have all three designs. I will start out and make my progressions.

Joe
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Old 7th October 2003, 02:24 PM   #7
Fuling is offline Fuling  Sweden
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I use them in a "Free Lunch" amp (direct coupled choke loaded input stage with the B+ for the input stage taken from the cathode of the power tube).

Input stage is 6C4 loaded by Hammond 150H 8mA chokes, output stage 12B4A parafeed with Hammond 30H 40mA choke and 125C OPT coupled trough a 4uF paper in oil cap.

I have future plans of a linestage using choke loaded 12B4A.
It´s a good tube to use with inductive loads because of the low plate resistanse (about 1Kohm).
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Old 7th October 2003, 02:56 PM   #8
316a is offline 316a  England
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Default No need for a schematic !

Quote:
Originally posted by burnedfingers
Well, I guess I hadn't thought of the 12b4a. Do you have a schematic that you would care to share? I have several 5751's and 6sl7's. I will be building a set of 2A3's soon

There is nothing that says that I can't have all three designs. I will start out and make my progressions.

Joe
6SL7 or 7F7 grid to 0V , 680K grid leak no stopper
100k resistor or 1mA CCS anode load (TO92 DN2540 + 2k pot)
1.25V NimH battery from cathode to 0V
12B4a 470 ohm grid stopper , 680K grid leak connected to 3 x 9V batteries (- to grid leak , + to 0V)
0.1uF polyprop coupler
12B4a cathode connected to 0V
Driver HT decoupled by 10uF motor run-20H choke to B+
Power supply is 240VAC to EZ81/1n4007 hybrid 4uF , 15H choke to 40uF of motor runs
Crappy 5k outputs circa 1965

Hope this helps , sorry about the batteries but I hate electrolytic bypasses !

316a
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Old 7th October 2003, 04:46 PM   #9
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Default KT88, EL34 fine

KT88, EL34 would be just fine (don't know how the KT90 sounds).

And don't get all worried about having to use a 2A3, not necessary.
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Old 7th October 2003, 05:26 PM   #10
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Quote:
Originally posted by burnedfingers
Well, I guess I hadn't thought of the 12b4a. Do you have a schematic that you would care to share?


Click the image to open in full size.

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