I would certainly consider it if I had better access to CNC. I used to, but since I moved off the shop floor and into engineering I make all my little tube project parts on a prototrax enabled knee mill. With no active Z axis drilling all those holes is a chore. I made a couple one afternoon, but haven't really been looking forward to making more lately.
Building a little CNC router is on my bucket list, that would make turning boards like this out into a trivial task indeed.
Building a little CNC router is on my bucket list, that would make turning boards like this out into a trivial task indeed.
Worked up a concept for a fancy enclosure today.
I designed it in sheet metal, which isn't my forte, so I might need to retool it for some off-the-shelf sheet metal box or make it from U channel and a single plate on top…
Toroidal transformer and caps in the back, tubes in a row in the middle, Sowters up front.
Brian
An externally hosted image should be here but it was not working when we last tested it.
[/URL]An externally hosted image should be here but it was not working when we last tested it.
[/URL]I designed it in sheet metal, which isn't my forte, so I might need to retool it for some off-the-shelf sheet metal box or make it from U channel and a single plate on top…
Toroidal transformer and caps in the back, tubes in a row in the middle, Sowters up front.
Brian
I had, mistakenly, assumed it was a PCB. It would be quite easy to get such a PCB made although there would not be much copper on it. You could add traces for 0V and HT+ along the top and bottom and include PCB mounting tube sockets with traces brought out to turrets to ease connecting them. Do you think it is a worthwhile project? I can do the PCB layout.
Cheers
Ian
Ian,
I use 1/8" FR4 material for my boards... the same kind used for turret boards in guitar amps - that's where I picked up the technique. It would be pretty simple to make a PCB version, but one would need to decide if to use turrets or not. The turrets make the board good for prototyping because it can be reworked repeatedly without traces lifting. And some determination about whether to hardwire the tube pins to certain points or ad buss lines for B+ and ground.
Brian
I use 1/8" FR4 material for my boards... the same kind used for turret boards in guitar amps - that's where I picked up the technique. It would be pretty simple to make a PCB version, but one would need to decide if to use turrets or not. The turrets make the board good for prototyping because it can be reworked repeatedly without traces lifting. And some determination about whether to hardwire the tube pins to certain points or ad buss lines for B+ and ground.
Brian
I agree. Turrets are the way forward. I was thinking the PCB would provide the holes for most of these in the normal way. There would be some across the top and bottom wired together for HT and 0V plus those tracked to the tube pins. For noval tubes I use 1mm gold plated pins the fit directly into holes in the PCB. You then plug the tube straight in. Works very well.
The only issue I see is making the countersink on the component side. I don't think PCB manufacturers can do this so the user would have to do this themselves.
Cheers
Ian
Ian,
I've never actually taken the time to countersink the backs of the holes. The swages hold just fine without them.
I was waiting until some orange drop caps showed up to finish wiring the board - but I found out I had them shipped to the wrong address and will need to wait a bit longer for them. I decided to finish wiring everything up using parts I actually had, and it looks like everything is working.
The closed loop gain is closer to 60 or 65, I'd expected a bit more. Also, the gain doesn't change with or without a bypass cap on the SRPP and the frequency response seems flat down to 20 hz with only a 1uf poly cap. Interesting.
Can't listen to it yet, still waiting for the transformers.
Brian
I've never actually taken the time to countersink the backs of the holes. The swages hold just fine without them.
I was waiting until some orange drop caps showed up to finish wiring the board - but I found out I had them shipped to the wrong address and will need to wait a bit longer for them. I decided to finish wiring everything up using parts I actually had, and it looks like everything is working.
The closed loop gain is closer to 60 or 65, I'd expected a bit more. Also, the gain doesn't change with or without a bypass cap on the SRPP and the frequency response seems flat down to 20 hz with only a 1uf poly cap. Interesting.
Can't listen to it yet, still waiting for the transformers.
Brian
OK, if that is the general approach then a regular PCB should be OK. I'll give the layout some thought.
You would normally expect the closed loop gain to be close to 100 but 65 is only just over 3dB down. Normal NFB calcs assume infinite open loop gain and zero output impedance. Here, our open loop gain is only about 20dB more than the closed loop and the output impedance is about 1500 ohms so the closed loop gain turns out to be a bit less. With the loop closed, the output impedance drops to about 150 ohms. There is probably only a dB or so difference in gain with the SRPP cathode bypassed or not. However, every dB of extra open loop gain is a dB less distortion and a dB lower output impedance. I would keep it in but of course it is your choice.
Cheers
Ian
Ian,
That makes sense, thanks. I wonder if I'm misinterpreting the measurements because everyone else seems to use a big electrolytic cap to bypass the lower cathode. If I can get away with a small film cap (Even if my my -3db point were higher than 20hz) I think I'd prefer that.
I can email you a detail drawing of the turret board if you like. It has the dimensions and spacing that I used.
Brian
That makes sense, thanks. I wonder if I'm misinterpreting the measurements because everyone else seems to use a big electrolytic cap to bypass the lower cathode. If I can get away with a small film cap (Even if my my -3db point were higher than 20hz) I think I'd prefer that.
I can email you a detail drawing of the turret board if you like. It has the dimensions and spacing that I used.
Brian
Death trap v2:
An externally hosted image should be here but it was not working when we last tested it.
[/URL]
You should be using a big electrolytic to bypass the SRPP cathode. I thought I had already mentioned this but it is possible I forgot. It should be 220 microFarads - its my poor hand writing that makes it look like picoFarads.
That would be good. My email is ianbellATukfsnDOTorg.
Cheers
Ian
OK, now I understand. No you will not see any effect of the bypass cap at 20Hz in the closed loop response because the NFB will largely compensate for it. The difference between the gain of the SRPP with the decoupling cap and without it is only a small number of dB, probably no more than 3 so that is the maximum change in response you could see anyway. 1uF and 300 ohms turns over at 530Hz so a decade below that the cap is effectively not there i.e. around 50Hz. From there onwards the NFB is just 3dB less than it was, which will make very little change in the response, until the other time constants kick in. There's one between the stages due to the 100nF coupling cap and the 470K grid resistor which begins at 3.3Hz and then there's one due to the output cap and the load reflected to the primary of the transformer which you don't see right now. Assuming the reflected load looks like 10K the the 10uF output cap -3dB point will be at 1.6Hz. None of these should have any material effect on the 20Hz response. With the specified 220uF, the 3dB point at the cathode is around 2.4Hz.
Cheers
Ian
Aaaah, of course. Tough stuff it is working with things inside the loop - seems alien to me. So even if the feedback flattens the frequency response at low frequencies, I take it that giving up the low frequency open-loop gain will come along with a proportional increase in low frequency distortion. Even at 3db, I think a big electrolytic will be the way to go.
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Exactly. If this was an op amp with 100dB open loop gain and 40dB closed loop then it would hardly matter. But we have 60 something dB of open loop and 40dB closed loop. We need a good 20dB NFB to get the distortion of the SRPP down to acceptable figures.
Cheers
Ian
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