Vespasian,
A stem to stern rebuild of the LK72A (6GH8) is a worthy project. However, as another poster pointed out, you have an excellent transformer set and a chassis that MUCH can be done with.
Scott and Fisher took advantage of the fact that the 7591 is as easy to drive as "12" W. O/P tubes. What they did, you can do too. The 12AT7 splitter/driver in "El Cheapo" will be FINE in front of PP 7591s. "El Cheapo" schematic here.
Even though power O/P is 1/2 that of pentode mode, I strongly advise you use triode mode. Linear operation of pentode mode "finals" requires regulated screen grid B+. The 15 WPC you get from triode wired 7591 pairs is plenty for speakers rated 91 dB. efficient or better.
Make CERTAIN to obtain your father's blessing before undertaking the redesign.
A stem to stern rebuild of the LK72A (6GH8) is a worthy project. However, as another poster pointed out, you have an excellent transformer set and a chassis that MUCH can be done with.
Scott and Fisher took advantage of the fact that the 7591 is as easy to drive as "12" W. O/P tubes. What they did, you can do too. The 12AT7 splitter/driver in "El Cheapo" will be FINE in front of PP 7591s. "El Cheapo" schematic here.
Even though power O/P is 1/2 that of pentode mode, I strongly advise you use triode mode. Linear operation of pentode mode "finals" requires regulated screen grid B+. The 15 WPC you get from triode wired 7591 pairs is plenty for speakers rated 91 dB. efficient or better.
Make CERTAIN to obtain your father's blessing before undertaking the redesign.
Reviving an old thread ......
My LK-72 has been in storage for a long time. I wanted to fix it but the time was never right . Too many things to do.
It's in pretty bad shape with some corrosion but it does work. It doesn't sound "nice" but just OK.
It appears to have used a 5AR4 tube for the rectifier as that's what is printed on the chassis. I replaced it with a silicon bridge.
All tubes are working.
I felt that it might be better to use it as a power amp and bypass the preamp section as I almost never use a preamp in my system.
So a rebuild of the power amp section is worth it and I hope Eli or someone from the forum can suggest a schematic which can improve upon the original power amp circuit using the same tubes.
I'm guessing that the tube sockets will have to come out . I have modern replacements but am not very happy about removing rivets etc. just to get the old ones out ! I havew a few spray contact cleaners . That couldn't be good enough I guess !
I have a new 5AR4 . Should I use that or a solid state rectifier ? That will raise the B+ voltage compared to the tube rectifier.
Any improved schematics for the power stage anyone ?
How many watts did an LK-72 produce ? Mine says LK-72 kit . I do not see an A or a B on it.
Thanks.
Ashok.
My LK-72 has been in storage for a long time. I wanted to fix it but the time was never right . Too many things to do.
It's in pretty bad shape with some corrosion but it does work. It doesn't sound "nice" but just OK.
It appears to have used a 5AR4 tube for the rectifier as that's what is printed on the chassis. I replaced it with a silicon bridge.
All tubes are working.
I felt that it might be better to use it as a power amp and bypass the preamp section as I almost never use a preamp in my system.
So a rebuild of the power amp section is worth it and I hope Eli or someone from the forum can suggest a schematic which can improve upon the original power amp circuit using the same tubes.
I'm guessing that the tube sockets will have to come out . I have modern replacements but am not very happy about removing rivets etc. just to get the old ones out ! I havew a few spray contact cleaners . That couldn't be good enough I guess !
I have a new 5AR4 . Should I use that or a solid state rectifier ? That will raise the B+ voltage compared to the tube rectifier.
Any improved schematics for the power stage anyone ?
How many watts did an LK-72 produce ? Mine says LK-72 kit . I do not see an A or a B on it.
Thanks.
Ashok.
Ashok,
Scott claimed 32 WPC for the LK72, which is reasonable. My prior remarks about a triode wired ECG stand. Do you want to get involved with the regulated g2 B+ pentode mode requires?
The same small signal tubes? I don't think so. You'd be using pentode voltage gain into "concertina" phase splitter, just like OEM.
SS B+ rectification does raise the rail voltage. Those extra Volts can be useful in a combination bias scheme.
Scott claimed 32 WPC for the LK72, which is reasonable. My prior remarks about a triode wired ECG stand. Do you want to get involved with the regulated g2 B+ pentode mode requires?
The same small signal tubes? I don't think so. You'd be using pentode voltage gain into "concertina" phase splitter, just like OEM.
SS B+ rectification does raise the rail voltage. Those extra Volts can be useful in a combination bias scheme.
When it comes to tubes I think it is safe to say that you would be hard pressed to go wrong by following Eli's advice. If I were in your shoes that Scott would be in for a resto-mod without a moments hesitation.
If you can restore it to approximately original spec you have a decent amp that will drive any reasonable speakers. Even if you end up building a nice SET one always has a few less efficient speakers around that need an amplifier. And there is always bi-amp service possibilities.
The suggestion to convert to triode mode makes a lot of sense to me (switchable?). Other improvements can be added later.
In the end if you decide that it is not of use to you then you will have learned a lot, had fun, and now have an amp that you can sell to support other projects. Win, win situation as I see it.
Good luck.
mike
If you can restore it to approximately original spec you have a decent amp that will drive any reasonable speakers. Even if you end up building a nice SET one always has a few less efficient speakers around that need an amplifier. And there is always bi-amp service possibilities.
The suggestion to convert to triode mode makes a lot of sense to me (switchable?). Other improvements can be added later.
In the end if you decide that it is not of use to you then you will have learned a lot, had fun, and now have an amp that you can sell to support other projects. Win, win situation as I see it.
Good luck.
mike
Hi Eli,
Thanks for your reply. I do not mind going with the triode strapped 7591 as long as it is sonically better than the pentode push pull.
I'm looking for the best sound I can get with these power tubes and existing OPT and power transformer .
Everything else doesn't matter as I can get other small signal tubes for the front end, preferably of current manufacture. I have 6DJ8,6922,6C45pi,12Bh7,6GM8,ECC81,ECC82,6N1p !
No problems about making a regulated supply for the bias etc.
Let me know what circuit you have in mind.
My current speakers are 90dB+ per watt ( 6 ohms min). So that should be OK. Bass is driven by a dedicated ss amp with electronic equaliser.
Thanks,
Ashok.
Thanks for your reply. I do not mind going with the triode strapped 7591 as long as it is sonically better than the pentode push pull.
I'm looking for the best sound I can get with these power tubes and existing OPT and power transformer .
Everything else doesn't matter as I can get other small signal tubes for the front end, preferably of current manufacture. I have 6DJ8,6922,6C45pi,12Bh7,6GM8,ECC81,ECC82,6N1p !
No problems about making a regulated supply for the bias etc.
Let me know what circuit you have in mind.
My current speakers are 90dB+ per watt ( 6 ohms min). So that should be OK. Bass is driven by a dedicated ss amp with electronic equaliser.
Thanks,
Ashok.
Ashok,
Triode wired PP 7591s should be OK "upstairs", in a multi-amped system. You'd have to go pentode to run full range. Scott used the negative rail for both preamp heater power (150 mA./"50" V.) and O/P tube bias. That winding on the power trafo is quite competent. If pentode mode becomes your choice, voltage multiplying and regulating a dedicated g2 B+ supply energized by that winding should be "a piece of cake".
"El Cheapo" topology will do very nicely for a triode wired amp. However, I am not at all comfortable with the amount of loop NFB needed around the O/P trafo to obtain an adequate damping factor out of a pentode wired EC. If you decide the power pentode mode yields is necessary, nested NFB loops seems best.
In any event, let's SS rectify the B+ and employ combination bias. The big gain from combination bias is being able to safely use 7591 grid leak resistance > 300 KOhms. OS 7591s were tolerant of oversized grid leak resistance, but current production 7591s are not.
I'm rather fond of "cockeyed bridge" B+ circuitry. Construct 2X series connected pairs of UF4007s. Parallel each pair with a high WVDC/10 nF. snubber cap. Connect the cathode ends of the assemblies to the ends of the rectifier winding. Ground the anode ends of the assemblies. Connect the CT of the rectifier winding to the anode of a 1200 PIV Schottky diode. Connect the Schottky's cathode to a CL150 inrush current limiter. Connect the CL150 to a CLC filter. Size the 1st filter cap. large enough to keep the rail voltage up and no greater. The idea is to hold stress on the power trafo down.
Triode wired PP 7591s should be OK "upstairs", in a multi-amped system. You'd have to go pentode to run full range. Scott used the negative rail for both preamp heater power (150 mA./"50" V.) and O/P tube bias. That winding on the power trafo is quite competent. If pentode mode becomes your choice, voltage multiplying and regulating a dedicated g2 B+ supply energized by that winding should be "a piece of cake".
"El Cheapo" topology will do very nicely for a triode wired amp. However, I am not at all comfortable with the amount of loop NFB needed around the O/P trafo to obtain an adequate damping factor out of a pentode wired EC. If you decide the power pentode mode yields is necessary, nested NFB loops seems best.
In any event, let's SS rectify the B+ and employ combination bias. The big gain from combination bias is being able to safely use 7591 grid leak resistance > 300 KOhms. OS 7591s were tolerant of oversized grid leak resistance, but current production 7591s are not.
I'm rather fond of "cockeyed bridge" B+ circuitry. Construct 2X series connected pairs of UF4007s. Parallel each pair with a high WVDC/10 nF. snubber cap. Connect the cathode ends of the assemblies to the ends of the rectifier winding. Ground the anode ends of the assemblies. Connect the CT of the rectifier winding to the anode of a 1200 PIV Schottky diode. Connect the Schottky's cathode to a CL150 inrush current limiter. Connect the CL150 to a CLC filter. Size the 1st filter cap. large enough to keep the rail voltage up and no greater. The idea is to hold stress on the power trafo down.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.