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Old 10th July 2008, 11:59 AM   #1
Maniac is offline Maniac  Taiwan
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Default The Rebuilding of Theta KT-88 Tube amp.

I have acquired this pair of mono block PP-KT88 (circa 1985? or sometime close) amp about a month ago. With most of the replacement parts arriving, I've finally gotten off my bum and started to copy the schematic off the PCB into some scribbling on paper that is slightly easier to comprehend.


Below is the photo of the amp in the condition that I had gotten. Power supply filter caps were bad, and thus taken out by the previous owner.

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


There are other electrolytic caps (Axial) that still remains on the amp, and I also plan to swap those out in favor of BHC's ALC40 cap that is the same capacity but higher voltage rating. For both performance and safety reasons, as the original cap is over 20 years old now.



HV Power supply, CRCRCRC filtering.
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AC powered filament
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Bias Power
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The rest of the circuit will follow soon, as I copied them down and clean it up
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Old 10th July 2008, 01:01 PM   #2
Maniac is offline Maniac  Taiwan
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Forgot to take a pic of the replacement parts that I have aquired for the fix/update/rebuild of this amp, Russian remake of the GEC Gold Lion KT88, second hand BHC Aerovox 470uF/450V caps, brand new BHC 100uF/400V caps.

Click the image to open in full size.

The darn tube socket on the amp are mostly old and tired, and the tube are only loosely held if the tube clamp wasn't used. Thus those will have to go too.
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Old 10th July 2008, 10:28 PM   #3
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Looks like a fun project. Best of luck.
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Old 11th July 2008, 08:12 AM   #4
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Thanks mashaffer, here's the circuit that I just copied down.

There are one error that I found with this version, the bias pot does not go directly to ground, but through a resistor then ground. I'll fix that in the next write up. :P


Click the image to open in full size.

Enjoy, please let me know if you see any other errors that I have not noted. The burnt marking on one of the resistor is to denote the actual resistor on the machine was dead and with visible burn mark.
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Old 11th July 2008, 03:53 PM   #5
kevinkr is offline kevinkr  United States
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I am assuming that the 4 ohm tap on your output transformer is grounded in order to provide a reference point for your cathode feedback.

There is probably another feedback loop around the entire amplifier as well.
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Old 11th July 2008, 03:56 PM   #6
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Quote:
Originally posted by kevinkr
I am assuming that the 4 ohm tap on your output transformer is grounded in order to provide a reference point for your cathode feedback.

There is probably another feedback loop around the entire amplifier as well.

4 ohm tap is not touched, it is there but appear to be completely unused.

It is very similar to Dyanco MKIII, but somewhat different in how the bias is done.
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Old 11th July 2008, 04:07 PM   #7
kevinkr is offline kevinkr  United States
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As drawn it's not correct then - there has to be a path for the output stage cathode current to flow through. Grounding the 4 ohm tap would also assure that the cathode feedback is symmetrical.
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Old 11th July 2008, 04:14 PM   #8
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Quote:
Originally posted by kevinkr
As drawn it's not correct then - there has to be a path for the output stage cathode current to flow through. Grounding the 4 ohm tap would also assure that the cathode feedback is symmetrical.

ah, you have reminded me a section that I may have accidentally missed, there is another few components that I missed while copying. I'll see If I can redraw it to reflect that in the next couple hours before I got too sleepy... (midnight here...)
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Old 11th July 2008, 04:59 PM   #9
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After inspecting further with a bright flashlight, I've found that indeed the 4 ohm is grounded. It is grounded with a dark green Teflon wire that runs at the corner of the chassis, it is so well hidden that in all my inspections, I missed that completely.

There are two definite failures that I've found, the bias to ground resistor is 47K (yellow/violet/orange/gold), but measures 5M, and one of the 50K shown in the graph is also blown. Not sure if the three Malloy axial electrolytic are live or dead, but sure as hell I ain't gonna leave them there when I test fire the thing. The whole thing looks more and more like one of the most common failure mode in Star Trak shows... Cascade failures... :P


Anyways, I'll go back to do a more spaced out drawing, so it won't be so cramped like the ones above.
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Old 11th July 2008, 05:42 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Maniac
After inspecting further with a bright flashlight, I've found that indeed the 4 ohm is grounded. It is grounded with a dark green Teflon wire that runs at the corner of the chassis, it is so well hidden that in all my inspections, I missed that completely.

There are two definite failures that I've found, the bias to ground resistor is 47K (yellow/violet/orange/gold), but measures 5M, and one of the 50K shown in the graph is also blown. Not sure if the three Malloy axial electrolytic are live or dead, but sure as hell I ain't gonna leave them there when I test fire the thing. The whole thing looks more and more like one of the most common failure mode in Star Trak shows... Cascade failures... :P


Anyways, I'll go back to do a more spaced out drawing, so it won't be so cramped like the ones above.
You're on the right track.. Make sure you replace resistors with ones with at least comparable power rating, higher in the case of ones that actually failed would be advisable.

Yeah, definitely replace all of those old Mallory caps.

Better rectifiers like HexFreds with the appropriate PIV ratings would be a nice improvement as well.

Paint those transformer lams - will look a heck of a lot better.
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