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 Post subject: National Kits Stereo STA-24 amp
PostPosted: Jun Tue 30, 2009 6:27 pm 
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Location: Farmington , Michigan
I got a National Kits Stereo STA-24 amp at an estate sale last weekend. I haven't been able to find any information on it. There's a joker on epay that has the same model listed for $999.99. He previously had it listed for $99.99, so I'm not sure if it's a typo or the guy is just crazy.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0410300082

He's calling it a National Radio Kits amp. Were they part of National Co.? It has four 6V6's, a 5Y3GT, a 12AT7, two others with all the lettering gone that are the same size as the 12AT7, and one empty socket. I'd like to find a schematic and some other specs on this. Any ideas?
Thanks,
Sam


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 Post subject:
PostPosted: Jun Tue 30, 2009 6:45 pm 
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There was a post on AudioKarma that was just about the same question. (I will try to find it and post a link.) This amplifier uses push-pull 7408 (6V6GTA) output tubes connected in an ultra-linear mode. Each grid 2 is connected to an additional tap on the output transformer. It is supposed to cause less distortion with the same amount of output. Kinda like a triode connected amplifier with a pentode output power.

All of that being said, I had one of these and now have recently acquired another one. It is also the same circuit and output transformer as the Merrell mono integrated amplifier. They sound rather flat but that is just my opinion. Both the National and Merrell amplifiers were offered as kits in the 60s. They were made up of a lot of surplus parts and were made to sell inexpensively. I am not saying that they were junk, just that they were inexpensive, entry level integrated amplifiers.

As I said, I had a Stereo National amplifier and back in the 70s, disassembled it and re-assembled it with a Motorola design. It sounded to me much better. I also got another Merrell mono amplifier recently. I had one of those back in the 60s as well but gave it away years ago. The reason I have gotten another Merrell and another National amp is just for nostalgia. Heathkit, HH Scott or EICO, they are not. But still they are a tube amplifier.

Unfortunately, I don't have the build directions any longer so I don't have the schematic. Hope that helps?

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Last edited by Don Cavey on Jun Tue 30, 2009 6:54 pm, edited 1 time in total.

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 Post subject:
PostPosted: Jun Tue 30, 2009 6:53 pm 
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You may be interested in this thread:

http://www.audiokarma.org/forums/showth ... p?t=232643

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 Post subject:
PostPosted: Jun Tue 30, 2009 7:26 pm 
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Thanks Don. That's great information! ARF members rule.
Regards,
Sam


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 Post subject:
PostPosted: Jul Tue 21, 2009 8:33 pm 
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throwing out random bits of useless information...

there was a battle i believe between national and mcintosh over an amp design. something about the transformer design.


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 Post subject:
PostPosted: Jul Tue 21, 2009 10:25 pm 
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Location: Hawthorne, Ca
I have a National Kits integrated amp that I built for my father in the 60's. It has two 6V6's a 12AX7 a 12AT7 and a 5Y3. It came with decent instructions that are still around here somewhere. It used mostly surplus parts with tubes labeled by National Kits. It sounds decent, but not in the same league with an Eico HF12 or the smaller Heathkits. Harry


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 Post subject: National Kits Stereo sta-24
PostPosted: Aug Mon 10, 2009 2:14 am 
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I built one of these about 30 years ago and still have manual / schematics. Available upon request.


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 Post subject:
PostPosted: Oct Thu 07, 2010 8:58 am 
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grz_mky seems to have made only the one post on this forum, so... Big Harry, do you still have the instructions for the National STA-24 amp? I'm trying to track down why the B channel is only a fraction the level of the A channel. A schematic or the original instructions would be great.


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 Post subject:
PostPosted: Oct Thu 07, 2010 1:59 pm 
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I have the monaural version of the amp and at this time I can't remember the model number. I have the assembly instructions, but at this point in time, I'm not sure where they are. I'll look through my schematic files asap. When I find the info, I'll send you a PM. If I remember correctly, it isn't a particularly good sounding amp as it sounds rather flat. Harry


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 Post subject:
PostPosted: Oct Thu 07, 2010 3:14 pm 
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Big Harry wrote:
I have the monaural version of the amp and at this time I can't remember the model number. I have the assembly instructions, but at this point in time, I'm not sure where they are. I'll look through my schematic files asap. When I find the info, I'll send you a PM. If I remember correctly, it isn't a particularly good sounding amp as it sounds rather flat. Harry


Harry is right, these sounded rather flat and shallow. And you all know that I am not at all subjective when it comes to Tube Audio :wink:

The mono version was often marketed as a Merrell brand. I have two of them and they are very different than each other. And they have lots of different components. I guess the common item is the smallish output transformers. But mind you, it is a part of tube audio history. I lost the assembly manuals many, many years ago. If they are available, I too wouldn't mind having a copy as well.

Thanks,

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 Post subject:
PostPosted: Oct Thu 07, 2010 9:59 pm 
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The amp is a sentimental project for me as my late Grandpa made it in, I suppose, the early-mid sixties. Thus far, what I've heard from it has been pretty decent. There is certainly a really nice mid-range to it that handles female jazz vocal particularly well. It's paired with PSB Atoms.

At this time, the A channel is ok, but the B channel is probably at about 5% the output level of A. I've been working through it trying to find loose connections and bad caps and resistors. I've changed a few parts and improved some connections, but I haven't solved the problem yet. I'm eye-ing up the four .05 MFD waxed-paper caps that run from the 12AT7s to the 6V6s next...

Regarding the output transformers: if I get both channels working, I may buy some upgrades. Seems like a good area to toy with.

thanks for the replies


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 Post subject:
PostPosted: Oct Thu 07, 2010 11:32 pm 
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Now I understand. So let's start by replacing those wax capacitors with new film caps. Replacing the .05 ufd with the modern .047 ufd is the right thing to do. Make sure they are at least 400 WVDC. That is where most of the problems will be located. Then, you can start fine tuning the rest. Resistors may have drifted but those caps are the main issue. You also may have to replace the electrolytic capacitors.

And, if you are not into more bass, those output transformers will be OK. They are just a little small. But quite adequate (and original!) if you prefer midrange.

Please let us know how you make out and we will help.

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 Post subject:
PostPosted: Oct Fri 08, 2010 12:27 am 
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Don Cavey wrote:
Now I understand. So let's start by replacing those wax capacitors with new film caps. Replacing the .05 ufd with the modern .047 ufd is the right thing to do. Make sure they are at least 400 WVDC. That is where most of the problems will be located. Then, you can start fine tuning the rest. Resistors may have drifted but those caps are the main issue. You also may have to replace the electrolytic capacitors.

And, if you are not into more bass, those output transformers will be OK. They are just a little small. But quite adequate (and original!) if you prefer midrange.

Please let us know how you make out and we will help.


I agree with Don, but I'll suggest that the coupling caps feeding the 6V6 control grids be 600 volts at least. I have seen a lot of these caps fail through being underrated.

To arrive at the voltage value they must withstand, you must add the B+ on the driver plates and the signal voltage peaks they see. In a higher-power amp especially, the sum may be as much as 350 to 400 volts, or even more.

And then you must add the bias voltage on the 6V6 grids, regardless of whether it's fixed bias or cathode bias. Suddenly you're looking at perhaps 425 or even 450 volts of total stress on those caps.

Good luck with your restoration project. As Don says, we'll be happy to help out where we may.

:wink: Larry

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 Post subject:
PostPosted: Oct Fri 08, 2010 7:41 am 
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Thanks guys, much appreciated.

So, I soldered in a pair of new .047 uF caps on the B channel side - both old caps measured ok, but I replaced them anyway. As I suspected, no change in the level. Poking around for other possibilities for bad caps, I disconnect the multi-cap canister. The readings I get for the two sections that are used are high ~19 uF (spec'd at 16 uf) and 182 uF (spec'd at 125 uF). Does this seem like a likely candidate for the low signal in the B channel? I'm going to see if I can find a replacement tomorrow.

BTW, I've used 600V replacements, thus far.


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 Post subject:
PostPosted: Oct Fri 08, 2010 12:31 pm 
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That "multi-cap canister" sounds like a multisection electrolytic. It will be used for filtering and decoupling along the B+ line. If the amp has cathode bias on the output stage, the canister may contain a low-voltage section used as a cathode bypass for that stage.

As with all electrolytics in sets that old, it should be replaced on principle. It is almost 100% certain to be faulty at that age. However, you likely won't find a direct replacement, or even a close approximation. Most likely you'll have to replace each element of the canister with single caps, installed under the chassis.

This may or may not be difficult to do, depending on the space you have available. The old canister must be cut TOTALLY out of the circuit, even if you don't pull it off the chassis. Any resistors or other parts mounted on its lugs must therefore be moved. You may have to install one or more terminal strips to mount them (and the replacement caps).

The important things are to be patient and careful. This needn't be a touchy job, but it may be, and you can't be too careful with B+ lines and voltages involved.

Good luck :),

Larry

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 Post subject:
PostPosted: Oct Sat 09, 2010 6:51 am 
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Capacitor uf values are not that critical for electrolytics. Even when new the supply caps in these old amps and radios might rated at +/- 20%, 30% or wider tolerances yet. Working voltage is the more important spec to meet or exceed for the modern replacements. Leakage is what usually fails the old caps, though you will find open or shorted ones from time to time or a cap that's not too leaky, but way off its uf value.

-Ed


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 Post subject:
PostPosted: Nov Wed 03, 2010 6:59 pm 
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Well, I replaced the caps in the multi-cap can using a 100uF + 20uF in parallel and another 15uF at 350WVDC to replace the 125uF and the 16uF. Sadly, no change to the very low signal in the B channel.

As another check, I jumpered the inputs to the balance pot to by-pass it. No improvement, so I can rule out that component. I'm wondering if I can do something similar to test the 2, stacked, volume pots...

Meanwhile, I put a meter across the new '120uF' cap and read 180V. When I measure across what I believe to be the equivalent cap on the A side, I get IIRC 320V. This seems odd, but there is visual confusion in the wiring on the A side as it's sitting near/amongst the power transformer wiring - that is, it's hard to see if I'm comparing the same sections.

I might try to check the output transformer next.


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 Post subject:
PostPosted: Nov Mon 29, 2010 9:41 pm 
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I have the schematic and complete parts list for the Merrell SA-30 integrated amplifier. I beleive it is similar to two MA-12s on one chassis and may be of help with your project. Email me you snail mail address and I will be happy to forward a copy to you. My email is klcb@frontiernet.net


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 Post subject:
PostPosted: Nov Tue 30, 2010 12:02 am 
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Thanks broadcast, e-mail sent...

Back to the amp, does anyone have some advice regarding:

Quote:
Meanwhile, I put a meter across the new '120uF' cap and read 180V. When I measure across what I believe to be the equivalent cap on the A side, I get IIRC 320V.


I feel like I'm running out of things to replace!

-dave


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 Post subject:
PostPosted: Mar Mon 14, 2011 5:54 am 
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This might be where you might want to do a little comparing between channels....you got 2 identical channels....one working and one not so well.
The fastest way to do this is to hook up a signal source...your tuner is ok and follow the signal thru and see where you lose it.

While its great that you're replacing parts and renewing this amp, it might be awhile before you happen upon the problem that way though.

Then after you find the actual fault then you can continue renewing the amp, replacing a couple parts at a time and trying it again.

It sounds like this little amp is a nice starter project and its always a plus that you have one good channel to compare to.


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