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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.
Click the image to open in full size.


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


For some reason I can't edit my other old messages, I'll just post them here I guess.:

====================

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 Horrific Truth...


The transformers were rusty, and is really kinda yucky... Little did I know that it was even worse inside.

Thankfully the damage seems to be happening on the cover only. The silicon steel core is mostly with little cosmetic damage. I went to a local customized transformer builder and asked if he could refresh it for me. The cost was about 1/2 of what he would charge for building a new one... Thus, I gave up on the idea of letting someone else do it, and started taking the transformer apart.


The "Horrific Cover"...

Click the image to open in full size.


The transformer is relatively clean, considering the age and the humid condition in Taiwan.
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To refresh the look of the transformer, I have gotten some hex screws and nuts to replace the old and tired screws. I like hex nut better due to the wear resistance and resistance to attack from the screw driver. Philips or flat just don't cut it if you want it to last and endure the constant attack of the tools.

As for removing the rust, I'm currently looking into some dremel like tools, hopefully I can get them fast to start working on it.


On the other hand, a friend had VOLUNTEERED to do a professional layout job of the board for me... YIPEE~~~


===============


Update, with LOTS of photo...


Transformer, repainted and somewhat refurbished, there's a few wires replaced due to insulation had failed. Was thinking about replacing all of the wires, but it would take too much time.
Click the image to open in full size.
Click the image to open in full size.


New and Old, the newer version is not yet refurbished, and thus looked really old. Because the newer version have better setup, thus the older version is tore down and being rebuilted first. It will be updated to similar configuration as the not yet refurbished newer version amp.
Click the image to open in full size.


Newer ver's top view, ugly, ugly, ugly, and dirty too...
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Older ver's top view, under the influence of powerful cleaning agents, it is basically clean, but there are scratches and other marks that witness the history of this machine. That I'll not try to remove.
Click the image to open in full size.



Newer ver's angle shot, age and dirt...
Click the image to open in full size.



Older ver's angle shot, you can see that there are a few missing connectors, but it is for sure much cleaner and neater than the other one. Older ver's got an extra volume pot that the newer ver does not have, thus the pot was removed and the hole plugged. The original grounding binding post was removed and replaced with a sparkling new terminal, just for the looks. As for the Bias pots, as they are made by AB, I feel they might still last, so they will stay until they fail. While the pots stayed, the bias meter switch was already losing its touch, and thus was replaced as well.


Because of so many different parts between the two machines, it is impossible to build two almost identical machine. So I throw out most of the resistors and replace them with units from Philips and Dralorics, while Cement resistor are used in non-audio parts like RC circuit and discharge resistor. BHC caps are used when electrolytics are needed. While the coupling caps are donated by a local boutique cap maker UCC, his MKP2000 have quite a following and is very reasonably priced.
Click the image to open in full size.




Newer ver's internal shot, you can see that the age hasn't been kind to it, nor did the modders/owners who came before me. Lots of solder marks and stuff...
Click the image to open in full size.



Older ver's internal shot, you can see that the discharge resistor is now about doubled in value at 390K, it is reduced to that because the total capacity of the new BHC setup will not be even close to original design's 660uF/330uF. It should compensate for some lost capacity and produce less waste heat.
Click the image to open in full size.




Well, I'll post some more when I made more progress.



===================

Click the image to open in full size.

99% done and working, if you want to see the ultra-large pic, please click the photo above.

Main mod is IeGO gold plated copper IEC socket (used to be power cable+fuse socket, fuse is now relocated to an internal location.), UCC MKP2000 caps, newer connectors and etc. What else needs to be done?

RCA connector will be replaced with a higher end CMC RCA connector (Factory original one looks kinda bad).
Ground loop hums, the machine is deathly quiet when not connected to my DAC, and my DAC to my solid state system is also deathly quiet. But once the tube amp and the DAC are connected, it hums quite a bit. I think all I needs to do is put a ground loop breaker in the grounding line of the power input. Hopefully that's all I need to do.


==============

Everything is DONE!!! Whew!!!


Left channel
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Right channel
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Some chance since last photo, I've upgraded the amp's RCA socket with a pair of CMC RCA sockets. The original ones looks like switchcraft, but I couldn't see any mark stamped on it. Since the whole socket looked rather tired, the plastic insulation appears to be going as well. Might as well upgrade it to something better while I'm at it.



Some of you might want to know what's still original on this amp?

Well here we go:
Case (all screws are updated with new ones, old ones are too tired and rusted.)
Bias Pots and meter (these works like new, no reason to replace them.)
Transformer (rust removed, repainted, some parts rewired)
4 power resistors per amp (two 0.6 ohm resistors and two R/C filter resistors per amp)

That's it, everything else is updated, replaced, upgraded.




Final setup is with:
Sony CDP-X5000 as source (Belden digital cable to D/A)
DA-131 as D/A (IeGO golden analog I/C to pre)
Proton AP-400 Pro as pre (Monster balance cable with Neutrik connector)
Two Theta KT-88 amp (IeGO L80520 heavy gauge speaker cable)
Acoustic Energy AE-120SE speaker


The amp compared to my original Ashly FET-1500 amp, this tuby setup does not have nearly enough bottom end thump, but the mid and highs are quite gracefully done. We find it not as suited for complex stuff like symphony or passages with lots and lots of stuff, but it does it quite nicely when you have vocals and musics with not as many instruments. For example, one of my favorite, Alison Krauss did quite nicely on this amp, the amp adds a little bit of sweetness into the tone that makes her already sweet voice even more enjoyable.

I feel this amp is superbly suited to most musics except the more complex ones(well, it is using KT-88 PP and was tested on scope to produce over 60 watts RMS), and perfect for relaxing at night with a bit of soft music and some drinks... Ahhhh....




Is it all done? Well, not yet, this is for my dad, and that's like one pacific ocean away from me. Thus I'll have to pack and ship it out with some cables. Final weight of the box with two amps, two power cable and two speaker cable is 29.8KG, just one hair below the max weight limit of 30KG.



Off it goes~~
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Last edited by Maniac; 5th January 2016 at 05:22 AM. Reason: updating image links
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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
Maniac is offline Maniac  Taiwan
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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
Maniac is offline Maniac  Taiwan
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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
Maniac is offline Maniac  Taiwan
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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
Maniac is offline Maniac  Taiwan
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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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