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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The Rebuilding of Theta KT-88 Tube amp.

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Wow. More Theta owners coming out!

I have a pair that I rebuilt a year ago. They sound fantasic, easily bettering my Copland CTA505.

It would be great to get hold of a schematic, as I just substituted old for new components where required - assuming that the 'old' ones were indeed correct.

The main problem was the old tube socket pins losing contact with the PCB. The resulting loss of bias almost cost me an output transformer!

Crank
 
Theta KT88

Crank

I have my Theta's up and running totally unmodified other that the replacing the 'lytics which had leaked.

These babies bested my Bryston 4B-ST for musicality and punch and are now my main amps.

I will however begin tweaking soon!

- pm me with yr e-mail address and I'll send you a schematic.
 
Wow. More Theta owners coming out!

I have a pair that I rebuilt a year ago. They sound fantasic, easily bettering my Copland CTA505.

It would be great to get hold of a schematic, as I just substituted old for new components where required - assuming that the 'old' ones were indeed correct.

The main problem was the old tube socket pins losing contact with the PCB. The resulting loss of bias almost cost me an output transformer!

Crank

I do have the schematics of the amp, which was kindly sent to me by Scott.

When I was upgrading, I also ditched the old socket, but the cuff that came with the machine is amazing, it locks the tube down perfectly, good enough for almost any kind of possible vibration.

As for the sound, it's a bit soft for me, as my speaker seems to like getting the kind of high current punishment that my current solid state amp can dish out. (ARX SS1200VC, refurbed myself and modified to better suit proper home audio use, including replacing the pesky fan with some quieter stuff.)

Now that I've mentioned it, I do remember that my ARX SS1200VC came with "Plumbing Store Special" power plug (I suppose sometime in its life, someone either decided to shorten the cord, or damaged the cord somehow), which really doesn't fit quite well into the scene.

Then I remembered I had a long forgotten IeGO Gen 1 pure silver NEMA plug. It is one of the classic, and most painful model for resellers. Yes, it is just PURE Silver, no coating, no plating... so all of the luster and shine will go to hell in a few month. Yes, the plug sounds fine, but it's darker than a moonless night! Yes, a lot of the users actually like the sound after the plug oxidizes more and more, but most buyers don't have experience with that, and even if they do, some still want the shiny ones...


An externally hosted image should be here but it was not working when we last tested it.


Ladies and Gentlemen, may I present the IeGO Black Magic.... (just a better name for a forgotten plug salvaged from an unused power cable... :cool: )

Yes, it's all oxidized to hell, yes, it is ugly, but it's still silver, and silver oxide is a very good conductor... Not to mention it is already used, so I don't have to spend a buck on it...


After replacing the plug, I think a felt overall "density" of sound getting better, and more of a buttery feel. However, don't quote me on that, as I didn't bother to A/B test and never will... (Err, the original plumber's special is soldered on, and IeGO is screw tightened, I don't think I really want to A/B test them...)


Ah well, all in good fun, if anyone want the schematics of the Theta 6550 amp, just let me know (my email zycamanic@gmail.com) and I'll forward it to you.


Thanks

David
 
Theta KT88 upgrade

After 2 blissful years of PP amplification and several other projects completed, I need to revisit my Theta PP amps. This is prompted my a second KT88 failure in about 10 months. It seems that the Theta is a loosely based on the Dynaco Mkiii. The major differences being a SS power supply and higher B+ (approx 550V to the KT88's).

My plan is to upgrade the existing Theta in the following sequence...

1. Replace the 6AN8 based driver circuit (too many options to choose from - I am overwhelmed!)

I have a pair of Softone RC-20 interstage transformers in hand and would like to put these to use if possible.

2. Change the heater supplies to DC usingTom Christensen's universal filament regulator (PCB's in hand)

3. Change to a regulated B+ - again I have a set of boards for Tom Christensen's 21st century Maida supply.

I plan to keep the Power and OPT Transformers but will replace the chassis, staying with a monoblock theme.

Am I on the right track in terms of upgrade sequence and any suggestions on a suitable Driver circuit? (I don't have the usual Dynaco chassis limitations and will prefer to go for a point to point wiring scheme) Also, I am not tied to any particular driver scheme.

I will use these amps amps to drive a pair of "Gunned" Magnepan MG1.6's
 

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Wow! time flies when you are having fun - my last post is almost a year old...

I just completed building my 21st Century Maida Regulators and got the top plates cnc milled and powder coated.

My next step is to test the regulators. I have also decided to stay with the original circuit and save the Softone RC20's for some other project.
 

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Wow! time flies when you are having fun - my last post is almost a year old...

I just completed building my 21st Century Maida Regulators and got the top plates cnc milled and powder coated.

My next step is to test the regulators. I have also decided to stay with the original circuit and save the Softone RC20's for some other project.

Indeed looks like fun... :)
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.