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Old 17th January 2005, 08:41 PM   #1
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Default Looking for KT88 circuits.

Hi there,
I have just purchased for 50Euro an old MiniMitter AM transmitter.
This is a beast, with a power transformer weighing in at about 10Kg, and two enormous chokes. One choke would be worth the purchase price.
It uses two TT21 (need to confirm this) which i understand are equivelants to KT88's. It use's one 6Y6G as a driver tube !!
I was thinking that everthing points to a choke loaded parafeed KT88 SET. Any pointers as to suitable circuits.
I would be more than happy with 10Watts of power, but need something with clean deep bass. I once had a Leak stereo 20 which I loved, but had to get rid of it in the end because the bass was so flabby.

Shoog
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Old 18th January 2005, 11:11 AM   #2
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The TT21 is indeed a KT88 with a topcap.

If you're looking for something with good bass, the "RH88" by Alex Kitic (a member here) might be just what you're looking for, and will give around 10W output. I saved the schematic round here somewhere.... Hmm... you'd better just ask Alex for it.
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Old 18th January 2005, 06:15 PM   #3
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Looks promising. Nice and simple and probably about the correct voltage for the transformer I have (haven't powered it up yet). I emails Alex asking for the schematic.

Shoog
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Old 19th January 2005, 07:44 AM   #4
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.....try steve bench's website......& triode electronics ; duncan amps sites........ tons of info

rich
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Old 19th January 2005, 05:37 PM   #5
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Hi there,
Tested the main transformer today 780V ac, a bit to much of a beast for my likeing - I will have to see if i can swap it for something a bit more managable. It just scares me thinking about those sorts of voltages.
I had a crazy though, how about a STC circuit using the 6Y6 as the driver tube. Should get around the overhead problems of this circuit.

Shoog
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Old 19th January 2005, 06:22 PM   #6
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Just a thought...this trafo you got, with 780AC.., isn't there a centertap, so you get 390 - 0 - 390?
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Old 19th January 2005, 06:57 PM   #7
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Good point on the centre tap. Didn't see one, but without stripping it out its a bit dangerous to test. I will have another look.
To be honest i'am a bit scared of this beast and don't trust myself with it. The friend who sold me it is competent in these things so i'll probably take it round to him to do a full spec test.

Shoog
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Old 19th January 2005, 08:19 PM   #8
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I just looked at the transformer and the tap that I tested looks as though it is supplying the low level circuits. The other main tap (with heavy duty wires) blew a fuse when I tested it across the silicone bridge on the 600V AC setting of my digital multimeter.

Shoog
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Old 20th January 2005, 08:22 AM   #9
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Default Re: Looking for KT88 circuits.

Quote:
TT21
My sheets on this tube quotes 1.2kV anode operation @ 28-130mA ......200W AB1...... screen grid 300V @ 71mA; g1 =-45V

this is a pokey tube

rich
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Old 20th January 2005, 08:31 AM   #10
316a is offline 316a  England
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Default Re: Looking for KT88 circuits.

Quote:
Originally posted by Shoog
Hi there,
I have just purchased for 50Euro an old MiniMitter AM transmitter.
This is a beast, with a power transformer weighing in at about 10Kg, and two enormous chokes. One choke would be worth the purchase price.
It uses two TT21 (need to confirm this) which i understand are equivelants to KT88's. It use's one 6Y6G as a driver tube !!
I was thinking that everthing points to a choke loaded parafeed KT88 SET. Any pointers as to suitable circuits.
I would be more than happy with 10Watts of power, but need something with clean deep bass. I once had a Leak stereo 20 which I loved, but had to get rid of it in the end because the bass was so flabby.

Shoog
Hello ,
Get your TT21 tested before committing to a project , that transmitter may have been caning the things for years , decades even . These valves are also getting scarce and are very expensive , but at least you can also pin the sockets for KT88 easily . Also if you're going to build a stereo amp you will have to go single ended with just the pair of valves . May be an idea to launch the things on Ebay and use the cash for output transformers .

cheers

316a
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