GE "Dual Speaker" radio to Harmony H303 conversion : Help, please? - diyAudio
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Old 6th July 2007, 03:21 AM   #1
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Default GE "Dual Speaker" radio to Harmony H303 conversion : Help, please?

I'm trying my first "Tube Project" - in this case, a conversion of my likely lethal $4 old GE radio (which works great, aside from the rusted out antenna) into a hopefully less lethal Harmony H303 guitar amplifier.

You can see something of the sort below:

Harmony H303A

I can't make heads nor tails of of the original radio schematic, but I do recognize that all three tubes are peasant...and there's a mysterious lack of a power transformer. The radio sounds kind of lousy, and reception is awful (the lack of the loop antenna explains that), but it does seem to work.

I'd like to make the amplifier as simple as possible...but how, exactly, to do that? I figure I might as well use an isolation transformer; for the time being, I've got 750VA monster I can use for assembling it (and that's just the smaller of the two I have in the basement!) that should at least eliminate the problem of grounding.

I've never really done much with tubes before, but this seems like a simple enough schematic that even I am unlikely to mess it up too badly - and even if I do, the tubes cost less than transistors anyway.

The "Chassis" for this particular amplifier will be a Radio Shack Solo 103 speaker that I bought on eBay for $3 hoping for a FE103, but which lacks any form of meaningful treble. Of course, guitars don't really do much above 4khz anyway, and I figure I can replace the driver with another 4"er anyway.

Also, presuming I keep the impeadance of the spaker the same in both cases, can I more or less safely assume that the output transformer is of the correct variety?
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Old 6th July 2007, 01:40 PM   #2
PRNDL is offline PRNDL  United States
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The Harmony 303 seems to be simple enough.
It uses an isolation transformer (T1) with AC connected series heaters.

You may want to review some Champ schematics and layouts, which will help you understand how to simplify things.

For mods, I'd suggest removing C1 and using it as C3. This will add more bass.
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Old 6th July 2007, 05:28 PM   #3
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Quote:
Originally posted by PRNDL
The Harmony 303 seems to be simple enough.
It uses an isolation transformer (T1) with AC connected series heaters.

You may want to review some Champ schematics and layouts, which will help you understand how to simplify things.

For mods, I'd suggest removing C1 and using it as C3. This will add more bass.

A Champ is Project #2. I just don't have a power transformer for the stupid thing - I've got the tubes, though.

I'll take your advice for the capacitor swap. I don't have too many caps, and I don't want to have to buy more than I have to.
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Old 9th July 2007, 03:51 AM   #4
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you dont want to build this without a power transformer, you will kill yourself.

I built this amp, but later converted it to a 6aq5/6au6 tube lineup.

I found a transformer that works great and is dirt cheap here:
http://www.tubesandmore.com/scripts/...?item=P-T262E6

While it lists it as 87ma, mine says 93ma on it so you have enough headroom there for a good sized output tube and a preamp tube.

Here is the ouput transformer: http://www.tubesandmore.com/scripts/...FXP?item=P-T31
again, dirt cheap.

you could just build the circuit as is and buy a $20 isolation x-former from tubesandmore or some other place.
Heres the link to my amp (I'll upload some finished pics soon): http://getchellaudio.googlepages.com/modeltwo

By the time I had the cabinet done and everything, the total cost of the amp was around $120.
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Old 10th July 2007, 01:30 AM   #5
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Quote:
Originally posted by ThSpeakerDude88
you dont want to build this without a power transformer, you will kill yourself.

I built this amp, but later converted it to a 6aq5/6au6 tube lineup.

I found a transformer that works great and is dirt cheap here:
http://www.tubesandmore.com/scripts/...?item=P-T262E6

While it lists it as 87ma, mine says 93ma on it so you have enough headroom there for a good sized output tube and a preamp tube.

Here is the ouput transformer: http://www.tubesandmore.com/scripts/...FXP?item=P-T31
again, dirt cheap.

you could just build the circuit as is and buy a $20 isolation x-former from tubesandmore or some other place.
Heres the link to my amp (I'll upload some finished pics soon): http://getchellaudio.googlepages.com/modeltwo

By the time I had the cabinet done and everything, the total cost of the amp was around $120.
I'm aware of the issue with the isolation trafo - mine's far too large, but I'll get a smaller one in short order - I might have two 50ma isolation transformers on the bottom of my junk bin anyway.

The whole point of this project is to make it as cheaply as possible. The speaker cabinet is an old RadioShack single driver speaker (it's external - I'll get a better replacement soon), and all the tubes and OPT are from a radio that uses them. I'm even scrapping the potentiometer and a few of the resistors from an old Geiger counter - really!
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Old 11th July 2007, 07:25 AM   #6
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I fully understand where you're coming from! My first tube project started the exact same way with the same tubes- $350 later I have a 5 watt Class A guitar amp with a dual preamp high gain stage pre , an oak cabinet with a vintage Jensen P10


If your isolation transformers are 50ma, they won't work, as you are dealing with line operated heaters. Your output tube alone will draw about 50ma on the plate. And yes, I agree a 750va isolation transformer is a tad big

By the way, the original schematic had errors in it that caused it not to work when built. Check the schematic on my website for the revision, one of the resistors on the preamp is shown going to ground when it should be going to "B".

I salvage parts a lot. The only parts you will have to have new, are the power supply caps. If you happen to be a computer junkie, and have access to some ATX power supplys, they are a great source for big electrolytics. You can get some 100uf 200v caps out of there that will work just fine for the amp. You will put excessive strain on the 35w4 , but it wont be too bad. You could order some new also, they are very cheap.

Also, you said the radio sounded lousey, was it just because of bad reception, or was there a lot of hum? If there is a lot of hum it just means your caps in the power supply have given up the ghost. I have an old GE musaphonic that sounded horrid when I got it, and I couldnt get it to tune to a staion to save my life. I replaced the caps in the power supply and it works like a charm now.

Another reason it could sound lousey,If there is a lot of distortion at lower than normal volumes for 1 watt of power, then the output tube is most likley worn out. I have a couple of 50c5's from radios that were used a lot, and one is so bad that when you ground the cahode ( no cathode resistor) the plates don't even glow. The other tested marginal , and shouldn't really be used. See if someone you know has a tube tester before spending any money on parts
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Old 11th July 2007, 05:46 PM   #7
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Quote:
Originally posted by ThSpeakerDude88
I fully understand where you're coming from! My first tube project started the exact same way with the same tubes- $350 later I have a 5 watt Class A guitar amp with a dual preamp high gain stage pre , an oak cabinet with a vintage Jensen P10
Heh. Project #2 is hacking a PP 6v6 amplifier I got for $35 on eBay. Should be good for twelve to fifteen watts.

Quote:
Originally posted by ThSpeakerDude88


By the way, the original schematic had errors in it that caused it not to work when built. Check the schematic on my website for the revision, one of the resistors on the preamp is shown going to ground when it should be going to "B".

Link, please? I don't know what your website is!

Quote:
Originally posted by ThSpeakerDude88

I salvage parts a lot. The only parts you will have to have new, are the power supply caps. If you happen to be a computer junkie, and have access to some ATX power supplys, they are a great source for big electrolytics. You can get some 100uf 200v caps out of there that will work just fine for the amp. You will put excessive strain on the 35w4 , but it wont be too bad. You could order some new also, they are very cheap.
I actually have about three dead PSUs sitting on hand. Thanks for the tip!
Out of curiousity, why would they provide excessive strain on the 35W4? The original amplifier calls for a total of 60uf.

Quote:
Originally posted by ThSpeakerDude88

Also, you said the radio sounded lousey, was it just because of bad reception, or was there a lot of hum? If there is a lot of hum it just means your caps in the power supply have given up the ghost. I have an old GE musaphonic that sounded horrid when I got it, and I couldnt get it to tune to a staion to save my life. I replaced the caps in the power supply and it works like a charm now.

Another reason it could sound lousey,If there is a lot of distortion at lower than normal volumes for 1 watt of power, then the output tube is most likley worn out. I have a couple of 50c5's from radios that were used a lot, and one is so bad that when you ground the cahode ( no cathode resistor) the plates don't even glow. The other tested marginal , and shouldn't really be used. See if someone you know has a tube tester before spending any money on parts
Part of the sound problem is that the speakers are very old, and very cheap, and sound exactly like very low-end computer speakers. Also, the antenna had literally rusted through. However, what would come through sounded quite good.

I have a tube tester, and will run all the tubes through it, but seeing how the radio works very well (on the one station it somehow was able to pick up), I'm willing to bet they're all in fine shape.
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Old 12th July 2007, 12:46 AM   #8
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alright, just make sure your 35w4 12au6 and 50c5 are good

About the 35w4: 60uf is listed as the maximum, and yes 100 uf will put some strain on it - you 4won't be able to pass much current through it. Its only an $8 tube, so if you cant find a 47uf cap don't worry about it. http://www.mif.pg.gda.pl/homepages/f...093/3/35W4.pdf There is the link to the datasheet for the 35w4. You can see reccomended capacitances, and how different ones affect output voltage and current capabilities.

Your main problem is that there is always going to be some background hum due to the fact that the 35w4 is half wave, meaning it only has one diode. I have a radio with 100uf on the rail of a half wave rectifier, and the background hum still presists slightly.

You would be best off using a solid state bridge rectifier, and trying to to find a different way to obtain your heater voltages. Here is what I did on my first design: I had a 120v:120vCT transformer, meaning the secondary had a center tap at 60v. Rectify the 120v for your B+ and use the 60v for the heaters of your 50c5 and 12au6 run in series.

sorry, here is the link : http://getchellaudio.googlepages.com/modeltwo

You can see all the problems I had when designing an amp like this. There are far better tubes to use than these, but the reason for building it is because its a cheap fun way of diving into tubes, and you can say "hey i built an amp out of radio tubes!".

If you wanted to see my 5 watt, here is the link for it: http://getchellaudio.googlepages.com/home2
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Old 12th July 2007, 02:42 AM   #9
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Why on earth do they specify using both a 40uf cap and a 60uf cap?

They did seem rather large.

How about I just use a 40uf cap?

Also, as a side note, I've got parts for a better tube amp - most of the parts for a Fender Champ 5C1, actually. I just want to use what I've got on hand to make an amplifier so I can better make the second one. Amplifier #2 is going to use a step-up transformer (120v -> 480v) and a more "proper" dual-rectifier tube.

Also, your amplifier is very, very nice. I think I'll be borrowing some elements of your design, in fact, though I'm thinking of going for something a bit more art deco.

EDIT: On second thought, you're right: A diode rectifier WOULD make more sense. It would also mean I could use a smaller transformer, which is nice, too.
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Old 12th July 2007, 03:25 AM   #10
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Thanks I'm glad you like it! It was actually thrown together, and then later spiffed up. I never planned on it turning out the way it did . Actually my next amp is more art deco. Please be aware that I build these and sell them to people, so change the design around a bit ( IE, don make it look exactly like it. ) I took designs from a BUNCH of different amps and mixed them all into one. Look at some older gibson amps ( 1940's stuff). Also supro, valco, oahu, kay, harmony etc. They were all off-brand amps but all were very art deco looking. If you have seen Swart amplifiers design it is based almost exactly off of a late 40's harmony.

On another note: They dont specify 40uf and 60 uf, they specify 40 and 20. This adds up to a total of 60uf right off the rectifier, and then there is an RC filter for the preamp and grid. You can add as many caps as you want to a string of RC filters, as long as right off the rectifier there is no more than 60uf ( this is usually true for all tube rectifiers) and there is a resistor after that.
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