Choke replacements

knowledge base for the classic Dynaco ST70

Choke replacements

Postby TerrySmith » Tue Jan 25, 2011 10:02 am

I was looking a the Triad C-24X and C-36X chokes as replacements in future Dyna and DIY projects. The specs on the C24X is 1hy, 240ma, 50r DC resistance, 2.375" mounting centers same as the C354. C36X is .5hy 300MA and 30r DC resistance, 2" mounting ctrs.

I like the extra current capacity but is there any reason not to use these?
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Postby TomMcNally » Tue Jan 25, 2011 10:32 am

No reason they won't work ... if you load a choke lightly, the
inductance increases. I can't see you ever getting close to
300 ma unless you're building a really big amp.

A couple times I've used two chokes, with additional caps, one
for each channel. It's a way to go with physically smaller
chokes, and be able to hide them under the chassis.

Here is a good deal on a choke from Radio Daze ...
8 hy / 125 ma / 150 ohms / 500 volts $ 10.95

http://www.radiodaze.com/RD-transformer01.htm

They have good prices on some other parts also.
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Postby tomlang » Wed Jan 26, 2011 3:39 pm

I have used both of those chokes on Dyna ST70 builds with great success. I also think Bob Latino now uses a Triad choke in his kit(s) as I saw in a picture he recently posted. From looking at the pic, he is probably using the 240 mA version as it is virually identical in size to the original choke in the ST70. The 300 mA version is smaller.

Like I said, I have used both, depending on the PS design and whether SS or tube rectified, etc.
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Postby Ty_Bower » Wed Jan 26, 2011 3:59 pm

I've used the C-24X in a couple of amps. I used one in my GetSetGo. The C-24X looks suspiciously like the Dynaco C354. The size and weight seem the same. The DC resistance is the same. Only the rated Henries differ between the two - perhaps the gapping is slightly different, or they are rated at different DC currents?

I dunno... but I like the C-24X. It's a little cheaper than the C354, and can be ordered easily from Mouser along with other parts. That can save a little on shipping charges.
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Postby corndog71 » Fri Jan 27, 2012 9:45 am

Has anyone tried replacing the c354 choke with a resistor? What value would work best? I've read on another forum that 1K-5K would work well.
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Postby EWBrown » Fri Jan 27, 2012 1:30 pm

If you were to replace the C354 with a resistor, use a 50 to 60 ohm, 10 Watts WW resistor. (Or use two 100 or 120 ohm, 5W WW resistors connected in parallel).

The DC resistance of the C354 is nominally 56 ohms, but that can vary +/- 10%. Its (AC) reactance at 120 Hz is about 1130 ohms, which is a lot more effective at reducing the B+ voltage's 120 Hz ripple content.

FX = 2PI X F X L; XL = 2PI X 120 X 1.5; (6.28 X 120 X 1.5) = 1130.

Applying this to the 1H choke, its XL = 754 ohms, and for the 0.5H choke , XL = 377 ohms.

Both of which are a better choice than the more basic resistor CRC filtering

Th resistor will work, but there will be less effective 120 Hz ripple filtering. May not be a real problem, with a PP amp like the ST70 - The ST35 does not use a choke, just a 50 ohm, 5W WW resistor, and that has no ripple issues.

THe ST35 draws about 160 mA total B+ curent, so the voltage drop across 50 ohms is about 8 VDC. The ST-70 is about 220 mA, and across 56 ohms, the voltage drop is about 12.5VDC, hence the larger sized 10 Watt WW resistor should be used in that application

Chokes are a lot more important in single ended amplifiers, as the PP output stages will cancel out any 120 HZ ripple, but there is no such hum / ripple cancellation in an SE amp.

HTH

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Postby Aljaheejus » Sun Feb 19, 2012 2:37 pm

Some spare parts came with the project chassis, and I have posted photos at the link below.

I wonder if the black choke-looking item is useful for an ST-70 rebuild. It has the following text/labeling "L1" on top and "150-G" on the side.

The quad cap out of the box is the stock-spec quad that I will try first. Inside the box shown is a smaller quad cap - and I wonder is it of any use in this rebuild?

http://home.bway.net/bogle/ST-70%20-%20spare%20parts/
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Re: Choke replacements

Postby kheper » Wed Oct 02, 2013 8:45 am

Just saw this Hammond 194G. The specs are better than the c-354.

3H @ 250ma

http://www.hammondmfg.com/pdf/EDB194G.pdf

$27 at Angela Electronics.

http://angela.com/hammondchoke194g.aspx
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Re: Choke replacements

Postby Bob01605 » Thu Oct 03, 2013 3:41 pm

Both of those chokes that you mention are too tall to fit inside a stock ST-70 chassis. Both are 2 inches tall and you only have about 1 7/8 inches of space inside a stock ST-70 chassis. Use the C-24X Triad choke (1 5/8 inches of height) in place of the stock C-354 Dynaco choke. The stock C-354 Dynaco choke was junk. It would sometimes melt out a "brown goo" from the heat that would deposit on the inside of the bottom cover directly under the choke. It was rated at 200 milliamps while the C-24X is rated at 240 milliamps. I have used hundreds of C-24X chokes inside Dynaco type tube amps. Not one has ever failed.

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http://www.tubes4hifi.com/bob.htm
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Re: Choke replacements

Postby Aljaheejus » Thu Oct 03, 2013 9:13 pm

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Re: Choke replacements

Postby kheper » Fri Oct 04, 2013 11:40 am

Bob01605 wrote:Both of those chokes that you mention are too tall to fit inside a stock ST-70 chassis. Both are 2 inches tall and you only have about 1 7/8 inches of space inside a stock ST-70 chassis. Use the C-24X Triad choke (1 5/8 inches of height) in place of the stock C-354 Dynaco choke. The stock C-354 Dynaco choke was junk. It would sometimes melt out a "brown goo" from the heat that would deposit on the inside of the bottom cover directly under the choke. It was rated at 200 milliamps while the C-24X is rated at 240 milliamps. I have used hundreds of C-24X chokes inside Dynaco type tube amps. Not one has ever failed.

Bob Latino


The only choke I mentioned was the Hammond 194G. Its size is too big for the stock st-70 chassis. It's good replacement for the c-354 in DIY st-70 clones and related projects, however.
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