'Nother Nifty Heathkit Amp

for the DIY ST35, the Dynakit and every other PP EL84

'Nother Nifty Heathkit Amp

Postby Shannon Parks » Sat Jan 31, 2004 1:01 pm

Just got an email from Terry with some pix:

Terry wrote:Hi Shannon,
Thanks for a quality product! Here are some pics of the completed amp I got last year. I used opt's from a Heathkit/Daystrom DA-282, same as AA-151, and a Hammond 270HX pt.

Being the tube head that I am, I used a tube rectifier.

It's built on a 12x10x2 chassis painted glossy gold Imron. (automotive paint)

Added a 100k vol pot, no pre amp, hooked to my pc sound card, and using KLH speakers that cost less than 100 bucks and sounds really great!

You may use these pictures on your website.

Terry


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Update on this amp

Postby TerrySmith » Tue Jan 24, 2006 6:56 pm

This is my first DIY-35 project. It has been used daily for 2 years now, and works flawlessly! It's now the main amp for the living room.

The only thing I have done to it is roll some different tubes, and like the old stained mullards the best.
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Postby TerrySmith » Sun May 27, 2007 4:37 am

Vince, here is my '151 amp. This is the rev B board, the D is basically the same only smaller.
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Postby Shannon Parks » Sun May 27, 2007 6:18 am

Terry, how has that paint job held up with holdhold nicks and scratches? Did you put a clear coat on it?
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Postby TerrySmith » Sun May 27, 2007 9:15 am

The paint has held up perfectly, no scratches even after a 550 mile move. Imron is an industrial type paint, a true two part polyurethane.

Also the amp is still holding up perfectly!! Yellow_Light_Colorz_PDT_03
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UPDATE

Postby TerrySmith » Sun Jul 27, 2008 1:38 pm

After 4 1/2 years of daily use I decided to give it a good check-over. The old Mullard EL84's have gotten very weak and one was noisy, replaced with a set of 6P14P Russian tubes. The el-cheapo volume pot from AES (scratchy) replaced with an Alps pot.

Then I modified it to a fixed bias arrangement similar to the Ike circuit. On the board the 470 ohm cathode resistors removed and jumpered. The 470k grid resistors were changed to 270k and the ends usually grounded now connect to neg bias voltage. Bias is set to 25ma for each tube. It seems to have a lot more power, and runs cooler.
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Postby Ty_Bower » Sun Jul 27, 2008 2:12 pm

I'd like to see some new pics of the bottom. I'm trying to figure out where you stuck the extra parts for the fixed bias circuit. Can you trim the bias voltage for each output tube individually?
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Postby TerrySmith » Sun Jul 27, 2008 3:07 pm

Here's some pics of what I did. I only had two 10k pots, so one adjustment per channel.

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Postby Ty_Bower » Sun Jul 27, 2008 5:28 pm

Neat trick. I ought to try that on mine - it has a bias winding on the PT. I like the new knobs, too. Why is the bias current down at 25mA? Seems a little on the low side. Where's your B+?

Shannon, if there's ever a Rev E of the ST35 board, you ought to put in a couple pads for a pair of 10 turn PCB mount pots, like on the Poseidon boards. Probably easier said than done, huh?
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Postby TerrySmith » Sun Jul 27, 2008 7:08 pm

With fixed bias you don't need a high idle current. It will go well above 35ma, enough to turn the plates red, I found that out first hand.

Since you've got a bias tap most of the work is done. Get a couple of 10k pots and wire it up like a ST70 circuit except the two 10k resistors. Make the top one 15k and the bottom 5.1k, that will get the bias voltage in range. EL84's will bias at a low neg voltage. YMMV
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