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Heathkit SB-220

Bassbug

Member
Apr 30, 2005
19
1
11
Springfield,Ohio U.S.A.
Hello, all. You know of anyone that is able to convert the Heathkit SB-220 to a five tube amplifier. That is with an external Peter W. Dahl transformer? My local Teck dose not has the time and is swamped with work. Thanks.
 

Hello Bassbug:

This is a aggressive Mod!

The SSB220 was designed for two each 3-500Z Tubes. Ok no biggie.

Yeah you need a transformer that will output 2400 Volts AC at around 2 amps or more. About 500 dollars from Peter Dahl Transformers. The shipping will be for a 65 pound transformer, wow! And you need a bigger filiment transformer also, the cost vs the performances start to grab ya here. I can see a couple of thousand dollars buying the good stuff, already.

Anyway this mod would probably best be done by a experenced Tech who has done stuff like this before. This is not a begginer mod. Maybe why some Techs do not want to tackle this type of Mod!

It maybe better to invest in a newer amateur amp that uses a pair of 4CX800 tubes that requires a lower drive level than the 5 ea 3-500Z Tube Modified SSB220. That will probably become a White Elephant type of amp.

Suggest you price out the differeance and performances between the two amps.

Hope this helps.

Jay in the Mojave
 
Four tubes can be done. At least there is room, if you mount the four sockets to a new deck plate, to squeeze enough air across all four. One thing the 'hacks' usually seem to overlook is the need to blow air across the UNDERSIDES of the sockets. The stock fan does this. That's why the deck is cut out where the fan blades are. So some of that air blows across the UNDERSIDES of the sockets. One after another, we've seen a 'converted' SB-220 with fans that blow ONLY on the tubes above the deck. Strangely enough, the complaint is that the tubes "drop solder" out of the cathode pins. Kinda like this.

burnedzbase15pw.jpg


Or this:

fourgoodpinsleft7zb.jpg



The new HV power transformer will have to be twice the size of the old one, making an external cabinet necessary for the power supply. This leaves plenty of room for the larger filament transformer, once the old one is removed. Sticking to four tubes also permits you to save a few bucks using a ten-Volt center-tapped transformer to run the filaments. One pair of tubes goes on each side of the ten-volt winding's center tap. This permits you to use the stock cathode choke, since it's built to carry the 29 Amps of filament current for two tubes.

If you put five tubes in that cabinet, you'll end up starving one (or more) of them for air flow. What it really needs is a large (8-inch wheel) low-speed blower. A high-speed motor will be too loud. A quiet blower will have to have a large wheel to deliver the air flow.

And any custom work that gets quoted at a cheap price is not being done well. If you cut corners on this setup, the thing will fail, over and over. Gets expensive to own, even if it was cheaper to acquire.

For the same 3 thousand or more bucks it takes to do this right, you could have a Henry or Alpha that will run for years without breakdowns.

I'm with Jay on this proposition, but I'll go one better. If you need more than two tubes, what you REALLY need is BIGGER tubes. The only reason to use more than two would be if they were REALLY cheap, in the Watts-per-dollar department. If so, this would make up for the high cost of custom work. Don't know where you'll find 3-500Z tubes, or all the other parts you'll need cheap enough to justify what you'll end up with.

73
 
Hello Bassbug and Nomadradio:

Very impressive reply there Nomadradio, with hands on photos.
Great info to know. Especially with the high cost of a pair or even 4 each, 3-500z Tubes.

Yeah I can see 4 each 3-500z Tubes, with a can do heavy duty external power supply. The one I seen had a power supply chassie just as big as the SSB220 amp.

Another good point was made on the tube count being over 2 each, to get bigger tubes. Unless other tubes can be gotten for real cheap. Another thing I would like to suggest, is the biasing of multible tubes. As the bias can be adjusted indivualy allowing the same plate current to flow in a Tetrode. But I don't know if it can be done with Triodes in grounded grid. I would think you could do that?!??!

Eimac web site "The Care and Feeding of Power Grid Tubes" show how this is done, with induvual tube bias adjustement for each tube.

http://www.cpii.com/eimac/index.html

How do ya post a photo, thats neat to do!

Jay in the Mojave
 
Hay paws264. Is that a Heathkit SB-220? Thanks Jay and Nomadradio. My father use to have a five tube Heathkit SB-220 when I was a kid. And I agree that the newer tubes like the 4CX800 for higher output is better on wattage thin the 3-500Z tubes. I know that the 3-500Z was cheaper 25 years ago then they are now. And it was cheaper using the 3-500Z tubes back then to build large amplifiers.
 
Cruises at 6k and slam dunks 10k on a Bird 43P.

It started life as a SB-220, it has 3500 volts on the tubes and the transformer will supply 3 amps continuous and 4.5 amps intermittent.

.
 
Yes Paw264. I would like to see a photo of the power supply? The one that my father had was a Peter W. Dahl transformer installed into a Winchester box. The wires ran from the Winchester box to the Heathkit SB-220. The output was something like 7000 watts. The Heathkit had five 3-500Z tubes. He used it on a six element Signal engineering beam.
 
Sharp eyes, Jay. I missed that first glance.

"The bottom tube looks like the plate is at half mast, and listing and not hooked up?!?!?!?!"

I'd say more like "capsized" than "half mast". That's one side effect (ONLY one) of running an Eimac "Z" too hot. The spot welds that join the anode to the cap connection start coming loose when you run it too hard. Kinda like what the top of a piston looks like when the nitrous button got held down a little too long.

On the other hand, it's a visual confirmation that the operator was "hot-rodding" that box before that tube's anode came loose and went "clunk". I'll bet closeup shots of the underside of that specimen would reveal some "challenged" solder in the tubes' pins.

Jay, you asked about image hosting. Easiest free host I know is Image Shack.

Click on the link and sign up. You upload your pics from the hard drive, then cut/paste the image's URL from their web page for that image, and onto the post here, using the "Img" button above the edit window.

There are more and more of these services appearing all the time, but Image Shack is pretty versatile, and always seems accessible without long waits.

73
 
Jay in the Mojave said:
Hello Paws264:

The bottom tube looks like the plate is at half mast, and listing and not hooked up?!?!?!?!

Got a photo of the power supply?

Jay in the Mojave

Yup, that was a picture of it on the way in the door, like Nomad said, consideration has to be given to the heat dissapation thru the socket connections expecially when you are dealing with 90 amps at 5.0 volts (that's 450 watts of heat just to light the tubes!).

The funny part about a well-built power supply is that it doesn't fail and if it does, you have the diodes and or capacitors on a removable board; saves your back from lugging that X-former around.
 
Jay in the Mojave said:
Hello Paw264:

Yeah good deal that amp looks like it has seen better days.

I strongly disagree with you there, that amp IS cruising TODAY at 6 grand and WILL BE cruising at 6 grand TEN YEARS from now.

The tubes have that way over driven and cooked look to them, hehehheee.

Don't let the lighting effects of bad picture-taking fool you, you really cannot get a true representation of the "Coffee-stains" on the tubes from that pic.

BTW, now I'm not telling you anything that you didn't already know; Tubes are CONSUMABLES, they don't last forever. When they "POP" you go buy another (or six).


Jay in the Mojave

One more thing, you guys are focusing on the wrong thing; forget about the tube color, this was brought in after a lightning strike.

If you want to play "Where's Waldo", find the tube that lightning blew the top off!


Mel
 

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