12AT7/46 Direct Reactance Drive Amp - diyAudio
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Old 28th April 2009, 06:57 PM   #1
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Default 12AT7/46 Direct Reactance Drive Amp

I built this last weekend and am very pleased with the sound.

Click the image to open in full size.

I had initially used a paralleled 6N1P as the driver but had found it a bit sugary sounding for my liking.

The only other valves in my box that were really suitable were a pair of Mullard ECC81 (12AT7).

Parallelled up and run without cathode bypass caps they have proven to be excellent sounding drivers for the amp. It only puts out around one watt but my 100dB open baffles have no trouble making the windows rattle.

The 10K output loading might raise a few eyebrows but it sounds excellent at that load; better than at 5K, which would be the obvious choice.

The 46 seems to be a great tube; not powerful but beautiful-sounding nonetheless. The amp is giving my PX25 DRD some serious thinking to do.
Click the image to open in full size.
Has anyone else made anything out of a 46. There does not seem to be much DIY action anywhere with this tube.

Steve
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Old 28th April 2009, 08:12 PM   #2
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Looks pretty nice. Have you tried bypassing or LED biasing the driver? As is, rp is ~ 15K ((11k/2) + 150*mu) and the choke only presents a ~18K load at 20Hz. Workable maybe, but not ideal. The cap or LED drops rp by almost two thirds.
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Old 28th April 2009, 08:15 PM   #3
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I'm utilizing a remarkably similar 27/56 > 46 direct-reactance front-end in my Shishido-style A2 203A amp.
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Old 28th April 2009, 08:42 PM   #4
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Quote:
Originally posted by dsavitsk
Looks pretty nice. Have you tried bypassing or LED biasing the driver? As is, rp is ~ 15K ((11k/2) + 150*mu) and the choke only presents a ~18K load at 20Hz. Workable maybe, but not ideal. The cap or LED drops rp by almost two thirds.
Yeah I too was a bit dubious about the raised Ra caused by not bypassing the driver and the problem of not enough henries in the choke that it would expose. The amp is a work in progress as it was built from bits I had spare.

It does sound very good despite all that.

I have some caps to bypass the driver and will stick them in there.
OTOH the bias is just about ideal for an LED...decisions decisions

Steve
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Old 28th April 2009, 09:04 PM   #5
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Quote:
Originally posted by dowdylama
I'm utilizing a remarkably similar 27/56 > 46 direct-reactance front-end in my Shishido-style A2 203A amp.
Well I'm glad someone else is doing something similar.
There is very little information out there.

How does your setup sound?

BTW I have just put in a pair of bypasses. I had been afraid there would be too much gain but that fear seems to have been unfounded.

Steve
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Old 29th April 2009, 12:20 AM   #6
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Quote:
Has anyone else made anything out of a 46. There does not seem to be much DIY action anywhere with this tube.

I want to execute a "super-duper" variation on "El Cheapo", with PP Class "AB2" # 46es as "finals". The 12AT7 LTP would feed IRFBC20 source followers that are DC coupled to the O/P tube grids. Damn and blast, the money to build is simply unavailable.
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Old 29th April 2009, 06:12 AM   #7
rdf is offline rdf  Canada
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Consider more capacitance across the output cathode resistor. While it's true the corner frequency as-is will be very low, the impedance 20uF presents from cathode to ground won't be at low frequencies. An impedance magnitude in the range of 400 ohms at 20 Hz if my quick draw calcs are right. Monitor the voltage across the cap with scope and play something with strong low end content.
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Old 29th April 2009, 07:39 AM   #8
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Quote:
Originally posted by rdf
Consider more capacitance across the output cathode resistor. While it's true the corner frequency as-is will be very low, the impedance 20uF presents from cathode to ground won't be at low frequencies. An impedance magnitude in the range of 400 ohms at 20 Hz if my quick draw calcs are right. Monitor the voltage across the cap with scope and play something with strong low end content.

Hi RDF

I have increased the bypass cap to 47uF and have noticed a bit more control at the bottom end. Not night and day but useful. IME with these direct coupled amps you have a bit of a balancing act to perform, due to the fact that you need a higher voltage cap than you would with cap-coupling, due of course to the elevated cathode voltage. This has a knock-on effect in terms of the physical size of the cap, influencing, the sound as well as the actual capacitance presented at the cathode.

With previous DC coupled amps I have built using 45s and 2A3s I have usually arranged things so I don't have to go any higher than 47uF.

Strange thing is 47uF seems to be a good value, no matter what valve I have used.

Steve
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Old 29th April 2009, 02:01 PM   #9
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Quote:
Originally posted by Steve Cresswell


How does your setup sound?

Steve
Sounds great - likes to play loud!
I've been considering replacing the 27/56 front-end with a DHT to warm things up just a bit.
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Old 2nd May 2009, 05:36 AM   #10
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Quote:
Originally posted by dowdylama
I'm utilizing a remarkably similar 27/56 > 46 direct-reactance front-end in my Shishido-style A2 203A amp.
I'm using another similar front end... 56 DC 46. This IT coupled to a 50.
I learned about the 46 thru Gary Kaufman and his "convertable" amp. I had built the Bugle convertable with 45 and 46 only - the 45's saw little play.
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