Well, latest update.
I've took apart the transformer, sand blasted the end bells and painted them black again. One of the power transformer have signs of damage on one of the filament tap to ground (short + melted insulation), and boy am I glad that I took them apart, or else I would never have noticed that.
As for PCBs, they are now ready, 2.4mm thick double sided PCB that had been made to look like single side while retaining some double side's advantage.
I'll post some more photos soon.
I've took apart the transformer, sand blasted the end bells and painted them black again. One of the power transformer have signs of damage on one of the filament tap to ground (short + melted insulation), and boy am I glad that I took them apart, or else I would never have noticed that.
As for PCBs, they are now ready, 2.4mm thick double sided PCB that had been made to look like single side while retaining some double side's advantage.
I'll post some more photos soon.
Great to see this post, i thought the 2 monobloks i have were the only ones in existence, they have been sitting in storage for the past four years after searching the net high and low for info and schematics. Keep up the good work you have inspired me to pull out the units and resurrect them.
P.S. Does anyone know what company made the outputs on these units & what quality to expect?
P.S. Does anyone know what company made the outputs on these units & what quality to expect?
susnick said:P.S. Does anyone know what company made the outputs on these units & what quality to expect?
My friend, who owns only the one Theta, is sort of a Tube Dr. and he has inspected his Theta very well. Originally fitted with mill-spec parts so it is robust. I think you can expect serious quality.
I was very impressed when I saw it the first time. Build quality etc.
D
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.
Thanks
David Wei
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
Newer ver's top view, ugly, ugly, ugly, and dirty too...
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
Newer ver's angle shot, age and dirt...
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
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...
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
Well, I'll post some more when I made more progress.
Thanks
David Wei
David,
Im very impressed
BUT
I'm a little concerned about you changing those voltage share resistors across the high voltage power supply capacitors to 390K.
Bear with me while I check this out.
OK - You have Aerovox BHC ALC40 Series caps 470uF and 450 V rated.
Leakage Current for ALC40 Series is quoted as 0.003 Cr Vr or 10mA which ever is the smaller. Cr is rated capacitance and Vr is rated voltage.
So do the calc Ileak = 0.003 x 470 x 10^-6 x 450
I get 0.6345 mA leakage current.
For the capacitors to share voltage properly then you want 3 to 5 times this current through the bleed/voltage equalising resistors. So assume we want 2mA through those resistors.
With a 530V rail you have 265V across each cap. So resistors want to be 265V/2mA = 132KOhms (maximum value)
I would strongly suggest you revert to 120K resistors across the caps.
Power Dissipated in each resistor
= V squared on R = (265)^2 / 120K = 0.585 Watts
Use 2W metal films or 3W metal oxide or the 5W wire wound as before.
Cheers,
Ian
Im very impressed
BUT
I'm a little concerned about you changing those voltage share resistors across the high voltage power supply capacitors to 390K.
Bear with me while I check this out.
OK - You have Aerovox BHC ALC40 Series caps 470uF and 450 V rated.
Leakage Current for ALC40 Series is quoted as 0.003 Cr Vr or 10mA which ever is the smaller. Cr is rated capacitance and Vr is rated voltage.
So do the calc Ileak = 0.003 x 470 x 10^-6 x 450
I get 0.6345 mA leakage current.
For the capacitors to share voltage properly then you want 3 to 5 times this current through the bleed/voltage equalising resistors. So assume we want 2mA through those resistors.
With a 530V rail you have 265V across each cap. So resistors want to be 265V/2mA = 132KOhms (maximum value)
I would strongly suggest you revert to 120K resistors across the caps.
Power Dissipated in each resistor
= V squared on R = (265)^2 / 120K = 0.585 Watts
Use 2W metal films or 3W metal oxide or the 5W wire wound as before.
Cheers,
Ian
FWIW I agree with Ian, and this could be quite an issue particularly under high ambient temperature conditions where the leakage current could be high.
120K would seem to be a safe recommendation - I generally use 100K - 121K resistors when I use large electrolytics in series such as the case here
120K would seem to be a safe recommendation - I generally use 100K - 121K resistors when I use large electrolytics in series such as the case here
An externally hosted image should be here but it was not working when we last tested it.
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:
That's it, everything else is updated, replaced, upgraded.
Final setup is with:
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.
This is how the box looked like when it is in the back of a Suzuki Grand Vitara....
Off it goes~~
Left channel
An externally hosted image should be here but it was not working when we last tested it.
Right channel
An externally hosted image should be here but it was not working when we last tested it.
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.
This is how the box looked like when it is in the back of a Suzuki Grand Vitara....
An externally hosted image should be here but it was not working when we last tested it.
Off it goes~~
This is a very interesting thread, didn't even know Mike Moffet made a power amp. I have only seen his preamp which I had the opportunity to rebuild one for a friend years ago. Someone had modified it to where it didn't work very well. I managed to track down John Beatty, Mike's partner in "crime", and we had a long conversation over the phone about it, at least two hours. Managed to figure out the original circuit with John's help. He told me many things to watch out for and what to do about it if I came across them. Oscillation was the biggest problem he warned me about, the preamp has a very wide bandwidth. Still have the schematic I drew and all of the notes I took. I think he was one of the first to make the 6DJ8 and siblings popular. Would be great to see more of his designs before he sold off to the new age Theta.
Craig
Craig
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