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 Post subject: 6AH4 preamp
PostPosted: February 26th, 2023, 4:20 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 489
I wanted to try a single-tube preamp so I put together a breadboard for the purpose. I started with 1626's but didn't really care for them and they were extremely microphonic, so I switched to a triode-wired 6V6. This was better but still not quite what I was looking for. Then I found a pair of 6AH4's in my stash and this is exactly what I was hoping for--very transparent and clean, a good tonal match to my Aikido, but faster and with amazing bass. I like it so much I think I'll build it. I'm using about 350 B+, 8K load on the plates and 500 ohms bypassed on the cathodes, with the tubes running at about 20mA each.

The problem I'm having is getting it quiet. The power supply is 5AR4 > 47uF > 15H > 47uF > 30H > 47uF > 30H > 47uF. You'd think this would be enough but it still hums a bit. I'm thinking about a regulated supply.

John Broskie sells his Janus board. It's not regulated but designed to shunt all ripple. I could set it for 300VDC and drop the plate load a bit, no problem.

https://glass-ware.stores.yahoo.net/jashrekit.html

Then there's this Neurochrome Maida regulator board, which isn't cheap but it would be easy to implement:

https://neurochrome.com/products/21st-c ... -regulator

Do folks think either of these might be effective? Would there be a better way? There's a 6V6 preamp design out there that uses a 6L6 as a shunt tube, would that be an easier way to do it?

https://wtfamps.com/sofrito-preamplifier/


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 5:49 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 923
Grover,

Try DC heaters. Either that or something on the grounding is a problem. You have a heroic amount of filtering, so do not think it could be B+, but who knows. Btw, not sure if the Janus reg handles the current level, Broskie has another regulator that is a Maida design that works well, and Pete Millett has a Maida regulator board for sale ( you have to source the parts). Finally, there is the Salas SSHV shunt regulator. Teabag on Reverb sells kits, I am using a version of the regulator designed by Kevin Carter of the former K&K Audio that he offered in a kit in a phono preamp. Works great, extremely low noise. If you want to keep it simple, a stabilized Zener type reg may work. And there are always series regulator options out there. But first try DC heaters.

David


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 6:03 pm 
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To follow up, the Merlin Blencowe book, “Designing Power Supplies for Tube Amplifiers” is a good read and goes thru a lot of different designs and options. Worth picking up, lots of ideas.

David


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 6:25 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 489
David McGown wrote:
Grover,

Try DC heaters. Either that or something on the grounding is a problem. You have a heroic amount of filtering, so do not think it could be B+, but who knows. Btw, not sure if the Janus reg handles the current level, Broskie has another regulator that is a Maida design that works well, and Pete Millett has a Maida regulator board for sale ( you have to source the parts). Finally, there is the Salas SSHV shunt regulator. Teabag on Reverb sells kits, I am using a version of the regulator designed by Kevin Carter of the former K&K Audio that he offered in a kit in a phono preamp. Works great, extremely low noise. If you want to keep it simple, a stabilized Zener type reg may work. And there are always series regulator options out there. But first try DC heaters.

David


Thanks, David. DC heaters don't make any difference. Any decent preamp uses a regulated supply, probably for just this reason. A lot of amateur builds like this use a volume pot at the output, which alleviates but doesn't eliminate the PS noise. The Neurochrome board appeals, despite the cost, because it's so versatile and could be used for other projects. Plus it's prebuilt. Words alone cannot express how bad I am at working with anything smaller than a noval tube socket. ;-)

I'll see if I can tighten up the grounding, that may be part of the problem.


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 6:30 pm 
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Posts: 489
David McGown wrote:
To follow up, the Merlin Blencowe book, “Designing Power Supplies for Tube Amplifiers” is a good read and goes thru a lot of different designs and options. Worth picking up, lots of ideas.

David


Thanks, I'll take a look at that.


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 6:56 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 923
Grover,

Hard to tell the grounding scheme from your picture, but where is the power supply grounded? On my builds, I bring the signal ground and the power supply ground together right at the input jack. I am sure you do the same, but again hard to see.

David


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 7:18 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 489
David McGown wrote:
Grover,

Hard to tell the grounding scheme from your picture, but where is the power supply grounded? On my builds, I bring the signal ground and the power supply ground together right at the input jack. I am sure you do the same, but again hard to see.

David


There's an L-shaped buss running through the circuit. From the inputs there are twisted pairs to the pots, the outputs are grounded to the buss. Maybe there's a loop in there. I haven't tried to build a preamp since forever. I'll play around with that.


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 Post subject: Re: 6AH4 preamp
PostPosted: February 26th, 2023, 7:33 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 923
Grover,

You positively want to keep all your signal grounds and power supply grounds separate. Ideally, the decoupling capacitors should ground to the power supply ground as close as possible to the input jack, not the signal ground. This works for me, and my builds tend to be extremely quiet. Basically a variant of star grounding.

David


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 Post subject: Re: 6AH4 preamp
PostPosted: February 27th, 2023, 12:19 am 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 289
Hi Grover,

Just a few small suggestions.

Try choke loading the rectifier and then double up two of the 47uF sections at the end of the filter string: L-C-L-C-L-CC....? (I think I got the parts count right, based on your text.....)
Put two small(er) decoupling caps physically right at the B+ end of the plate resistors.
Shorten that "L".
Reroute that final leg of the B+ line, it's millimeters away from the ground bus and the lead on one of the output caps. Fly it in from the top.....

FWIW, I vaguely remember Roscoe fighting to get the ripple eliminated on one of his preamps, with 5-6 sections of C-L-C filtering....it never got to inaudible IIRC.


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 Post subject: Re: 6AH4 preamp
PostPosted: February 27th, 2023, 1:21 am 
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Joined: February 28th, 2013, 10:38 am
Posts: 1698
Did you try turning the power transformer 90 degrees?

Roscoe

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