Scratch build valve power amps

NickK said:
Not tempted to 'ultra linear' the long tail pair you have there? (http://www.tubecad.com/january2000/page11.html) .... #nowresisttheurge :p (actually I'm not entirely sure what they're doing at that point.. I think it may be witchcraft)
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I think that is a bit too complicated for my experience level and the additional valves, whilst pretty, only serve to use more power and produce a lot more heat. It would be nice to achieve it using a hybrid with solid state parts.

NickK said:
Interesting you have issues with the 6SN7 - the TungSol datasheet states that the cathode should be attached to the heater and that the voltage difference between the heater and cathode should be kept as low as possible. Sheet here:https://frank.pocnet.net/sheets/127/6/6SN7GT.pdf

Interesting as I've never heard that about other SN7s.
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Not sure I've had any issues with 6SN7, though I use the equivalent noval 6CG7/6FQ7. The only valve I've had problems with is the input pentode. One EF806 and a single 6059 both of which produced large amounts of hiss.

I finished the Mk IV prototype and have now tested it. I made the usual mistake of reversing the phases leading to positive feedback, but I've managed to do it 3 times now so know how to quickly fix it. Takes 2 mins with a soldering iron to reverse the outputs from the phase splitter. Tested on the bench, the performance appears to be very similar to the other amp which is excellent.

1KHz square


5KHz square


10KHz square


It's a good looking amp, which would be even better with some decent wood panelling. (But I'm not wasting a good piece of oak or similar on a prototype)


Working together with the Mk III. First time having both speakers driven by my valve amps. Doesn't sound half bad. I've currently not got anywhere to put the second amp so it had to go on top of the speaker. (the Kef isn't in use)
 
Bought some higher voltage rated capacitors with some matching capacitor mounts and more octal sockets. The F&T 500V rated capacitors may be reused to replace one of the 10uF capacitors. The additional belton octal sockets will replace the old finder sockets I've used on the rectifier as the pins are a bit loose.
 
Impressive stuff. One day I fancy building a simple solid state amplifier. Not from scratch though, beyond my skills or inclination to learn all the skills required!
 
Thanks Kei
Neat work. My expertise is power electronics, mostly variable speed drives and most of that is industrial. But some are a bit unique. The Orient Express, Rutherford Appleton Laboratories, some 30,000 aircraft machine tools...........and other stuff
 
Troezar said:
Yep.
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In the dim and distant past (in my Student youth) I have built "HiFi" amplifiers with a few transistors from designs in "Practical Wireless". These could probably be called "simple solid state amplifiers" but I doubt if they were really hi fidelity. Now, if I look inside my Sony "scala" hifi amplifier (bought a fee years ago) it looks highly complex, with ICs as well as power transistors. Presumably (?) this complexity is there for some reason??

I do not know, but the "best" (i.e. the most expensive) kit, which is meant to provide the purest sound replication (?) does not look like "simple solid state" gear to me. I am not doubting your desire, just wondering whether it is achievable in practice?
 
Mel_P said:
In the dim and distant past (in my Student youth) I have built "HiFi" amplifiers with a few transistors from designs in "Practical Wireless". These could probably be called "simple solid state amplifiers" but I doubt if they were really hi fidelity. Now, if I look inside my Sony "scala" hifi amplifier (bought a fee years ago) it looks highly complex, with ICs as well as power transistors. Presumably (?) this complexity is there for some reason??

I do not know, but the "best" (i.e. the most expensive) kit, which is meant to provide the purest sound replication (?) does not look like "simple solid state" gear to me. I am not doubting your desire, just wondering whether it is achievable in practice?
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I'm no expert at all but the modules you can buy for building your own amps look like solid state to me but I may be wrong. I can probably assembled components, like a PC but with a bit of soldering, but no way could I make one from all the individual components.
 
Mel_P said:
In the dim and distant past (in my Student youth) I have built "HiFi" amplifiers with a few transistors from designs in "Practical Wireless". These could probably be called "simple solid state amplifiers" but I doubt if they were really hi fidelity. Now, if I look inside my Sony "scala" hifi amplifier (bought a fee years ago) it looks highly complex, with ICs as well as power transistors. Presumably (?) this complexity is there for some reason??

I do not know, but the "best" (i.e. the most expensive) kit, which is meant to provide the purest sound replication (?) does not look like "simple solid state" gear to me. I am not doubting your desire, just wondering whether it is achievable in practice?
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Before I moved into the realm of vacuum tubes, I had done some heavy leg work on building a Quad 606 clone. As amps go, for the power output, the quad current dumping design is relatively simple in terms of size and component quantity. The Naim NAP250 is commonly cloned and that seems to be a very simple amp with a very good rep.

I finished off doing the second one this morning.


Doesn't look too bad considering the cobbled together nature of these prototype builds. Once I've figured out how I'm going to get the proper aluminium chassis made, I can get them built properly.


I've also invested in some DC blocking PCB's and all the necessary components to build them. I've always had some residual hum on all five of my amplifiers which has made me suspicious of there being some slight DC offset.


I've made a start on building one of them. I got 3 PCB's so I can do other parts of the house.
 
I've now built the box for the DC trap. I simply need to wire it up and solder in the diodes.
 
Thanks, I finished fitting the diodes this evening. Just the box wiring left to do now.


Those large tracks absorb a heck of a lot of heat and are take a while to solder even with a 70W iron.
 
I finished the DC trap this evening. All the wiring seems to check out ok. Will test it properly tomorrow.

 
Kei said:
I finished the DC trap this evening. All the wiring seems to check out ok. Will test it properly tomorrow.
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I see diodes and caps, essentially a smoothing network. The issue I also see is there's no bleed resistor for the caps, so when you switch off it holds charge.
 
NickK said:
I see diodes and caps, essentially a smoothing network. The issue I also see is there's no bleed resistor for the caps, so when you switch off it holds charge.
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Doubt it matters in this case as they are only 16V capacitors. My valve amps don't have bleeders either. When switched off, the caps discharge completely. I built a Carlson's Lab style capacitor discharge tool to be sure when dealing with capacitors anyway.

I can report that it has made the transformers quieter on all of the amps which is nice.
 
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