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.
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.
AC powered filament
Bias Power
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.
============
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"...
The transformer is relatively clean, considering the age and the humid condition in Taiwan.
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.
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.
Newer ver's top view, ugly, ugly, ugly, and dirty too...
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.
Newer ver's angle shot, age and dirt...
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.
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...
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.
Well, I'll post some more when I made more progress.
===================
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
Right channel
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~~
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.
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.
AC powered filament
Bias Power
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.
============
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"...
The transformer is relatively clean, considering the age and the humid condition in Taiwan.
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.
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.
Newer ver's top view, ugly, ugly, ugly, and dirty too...
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.
Newer ver's angle shot, age and dirt...
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.
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...
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.
Well, I'll post some more when I made more progress.
===================
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
Right channel
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~~
Last edited:
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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...
Anyways, I'll go back to do a more spaced out drawing, so it won't be so cramped like the ones above.
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...
Anyways, I'll go back to do a more spaced out drawing, so it won't be so cramped like the ones above.
Maniac said: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...
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.
Hopefully this one is correct... Whew...
It is really similar to Dynaco MKIII for sure, but appears everything is done the other way around. Interesting.
An externally hosted image should be here but it was not working when we last tested it.
It is really similar to Dynaco MKIII for sure, but appears everything is done the other way around.
Interesting.
Actually it is only superficially similar to the Dynaco amplifier in that it uses the same tubes, in many ways it is quite different. (There are only so many ways to do it.) The global feedback component values seem to imply very low closed loop gain - only about 9dB or so from the 0 to 8 ohm tap if the 0 ohm tap goes to the other binding post. Are you sure that all values listed are correct - I suspect this low gain level would prevent the amplifier from achieving full power output if true due to overloading the first stage.
Note that the outputs are floating from a signal standpoint.
Note that the outputs are floating from a signal standpoint.
Since this is a mono block, I have another one that is "less" damaged, and I have replaced the rectifier diode and the caps. It appears to fire up just fine and here's what I've measured:
Transformer's HV AC output:
405V AC (Main's voltage is 115V)
CRCRCRC's each stage's voltage
1st stage: 531V
2nd stage: 530V (Power A)
3rd stage: 447V (Power B)
4th stage: 330V (Power C)
The bias' voltage drop across 0.5 ohm resistor is 0.036V, or 36mV when adjusted to meter's "green" zone.
Bias DC voltage at the capacitor is -66.666V
Transformer's HV AC output:
405V AC (Main's voltage is 115V)
CRCRCRC's each stage's voltage
1st stage: 531V
2nd stage: 530V (Power A)
3rd stage: 447V (Power B)
4th stage: 330V (Power C)
The bias' voltage drop across 0.5 ohm resistor is 0.036V, or 36mV when adjusted to meter's "green" zone.
Bias DC voltage at the capacitor is -66.666V
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"...
The transformer is relatively clean, considering the age and the humid condition in Taiwan.
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~~~
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"...
An externally hosted image should be here but it was not working when we last tested it.
The transformer is relatively clean, considering the age and the humid condition in Taiwan.
An externally hosted image should be here but it was not working when we last tested it.
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~~~
Maniac said:
When I inquired about rusty old transformers, the advice I received was to refinish the end bells (covers) but leave the core laminations alone. If you start trying to grind off the rust on the laminations, the resulting damage is going to be worse than how they are now.
You can probably clean up the end bells with some sandpaper, steel wool, and spray paint.
Ty_Bower said:
Thanks, I was thinking more in the line of using wool wheel or the like to "rub" the rust off and then paint it. I won't try to grind it for sure.
just clean the uneven surface rust off then I'll paint it over with some black (and hopefully anti-rust paint).For the end bells, well fair game I guess...
powered tools, here I come. 
I'm also considering dipping the screws in paraffin wax before putting it on, hopefully the blued steel screw will last as well.
Anyhow, it should be interesting to see how much effort was actually required to clean one of these amps inside out
I generally do as little as possible to disturb the laminations and just apply something like rustoleum that is rust tolerant.. The endbells are another matter, anything you can do here will be worthwhile.
Brass hardware is a nice touch, and one that some claim is audible. (I am not sure about this, but it can't hurt.) It is non magnetic if that matters. Some vendors ship with brass hardware by default. (MQ, and at least some transcendars as well.)
Hopefully the paper insulation is relatively dry, if they have sat around in a very damp environment for a long time I would consider baking them in a moderate oven at 60 - 70 degrees C or so for at least a few hours just to be sure.
Brass hardware is a nice touch, and one that some claim is audible. (I am not sure about this, but it can't hurt.) It is non magnetic if that matters. Some vendors ship with brass hardware by default. (MQ, and at least some transcendars as well.)
Hopefully the paper insulation is relatively dry, if they have sat around in a very damp environment for a long time I would consider baking them in a moderate oven at 60 - 70 degrees C or so for at least a few hours just to be sure.
Maniac said:
Obviously using the wrong tools and very cheap hardware if you are having problems with damaging phillips head screws. Not like you should have to do anything with them once they are properly installed. Use washers with lock washers, keps-nuts or lock nuts and you should have little problem with them loosening over time.
Don't over torque - a tendency I notice in many audiophile types..
kevinkr said:
Actually, it is more like the screw is rusted, and becomes rather "crunchy", when this happens, the screw's edge that grips the screw driver will often snap and the whole thing just go to hell. I try to find the right size philips screw driver all the time (which is why I have like half a dozen of them), but often it is someone else that did the damage for me and made the machine nearly impossible to take apart without breaking out the drill.
I have picked up some interesting nuts that have a built-in rubber ring that will hold onto the thread and dampens any vibration. I have seen those on customer's equipments (my previous employment, industrial type electronic stuffs...), and had no idea where to find them until today. I feel those are extremely reliable, likely only to be bested by using locking wire or some methods that actually glues or locks the screw.
I'll take a photo of those nuts next time, its a rather interesting design.
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