Yes, one should always look carefully at the rated voltages for caps AND chokes, especially when using military surplus. This also furthers the argument for using delay circuits.
John
> working out the total current drawn ... looks like about 5mA for the phono stage (both channels)
Can't be. Using the worst-case guess, that the tubes are totally shorted, we have (nominal) 305V across 275K+2K. 305V/277K= 1.1mA per tube, times 4 is 4.4mA total, not quite 5mA. But the tubes can't be working as dead shorts. As a lazy-guess, their plates are around half the supply voltage, so total current is half of the shorted-tube value, or 2.2mA. "Plates sit about half B+" is very rough and open to wide variation. If it really-really matters, plot the bias on the tube curves. If you just need a good-guess: on a hi-Mu tube, multiply the cathode resistor by the Mu and pretend the plate-cathode DC resistance is about that number. Cathode resistors here are ~2K, 12AX7 is Mu=100, the tubes are sorta-like 200K resistors. Adding the resistances 275K+200K+2K= 477K, and 305V/477K= 0.64mA per tube, times 4 is 2.6mA total.
> the line stage. Around 15 for both?
The line-stage happens to say "7.15mA" next to the cathode follower; maybe true. If the other (input) tube is shorted, it draws 250V/48.5K= 5.25mA. Using the "half" approximation for a working voltage amp, it would be 2.6mA. Using the Mu approximation, the tube is 1.5K*20- 30K, 47K+30K+1.5K= 78.5K, 250V/78.5= 3.2mA.
Just as a cross-check: applying the Mu approximation to the cathode follower stage gives 820*20= 16.4K, 16.4K+0.82K+10K= 27.22K, 250V/27.22K= 9.2mA, not the 7.2mA noted on the schematic. The Mu approximation is not exact, worse for medium-Mu, anyway two different 6SN7s in this circuit could bias at 7mA or 9mA. That 2-decimal-place notation is bogus: tubes aint that exact.
Best-guess is 2.55 in the phono, 2*(3.2mA+7.2mA)= 21mA in the line-stage, total 24mA.
> The coupling cap from the sch. of the first post should be 0.33uF not 0.033uF!
As you wish. However, the load on that cap is not the 220K that you see. This is boot-strapped by the cathode follower. Using a rough approximation, the CF gain is 0.86. 1/(1-0.86) is 7. The load is bootstrapped up to about 220K*7 or 1.5Megs. 0.01uFd is a low-Fi value for coupling to 1Meg (-3dB at 17Hz, -1dB at 34Hz) so 0.03uFd and 1.5Meg is not an implausible value for hi-fi: -1dB at 7Hz, -0.5dB at 14Hz, etc. The input R-C is about the same point, so overall we have -1dB at 14Hz, -14dB at about 1.7Hz and 12dB/oct below that. My experience is that anything with a phono input should have a 12dB subsonic filter. Yes, it "should" be in the phono stage but sometimes it is expedient to shorten the line stage. This one gives about 22dB down at LP-warp frequency, a useful value.
I was taught that with choke-input, you either have enough bleeder to keep the choke working before the tubes warm up, or you design the caps and other parts to take nearly full 1.414*VAC for many seconds many times (or some compromise like the bleeder alone holds voltage down to 1.2*VAC, which may be sweet for this case).
Chokes sometimes have higher inductance at low current than at rated current. This reduces the need for bleed. When designed for that feature, they call them "swinging chokes". But even a swinging choke has a minimum current below which it won't choke and voltage soars to 1.4*VAC.
> that's quite different from the 0.5 RMS available with cap. I/P filtration.
A strict derivation suggests Idc = 0.6*Iac for RMS heating. I do agree with using 0.5*Iac in most cases, because small transformers sag and DIY is not about shaving every nickel.
> a benefit of choke I/P filtration... is a cooler running power trafo when compared to cap. I/P filtration.
The 0.6*Iac for C-in and ??*Iac for L-in factors give the same heating.
> If the penalties in cost, size, and weight are acceptable
Sure. And I agree that 350VAC L-C-R-C is an elegant design. But Hamish is declining to buy a different transformer. I would assume that buying a choke (which is generally not a lot cheaper than the corresponding transformer) is also not "acceptable" for the present goals and budget.
Can't be. Using the worst-case guess, that the tubes are totally shorted, we have (nominal) 305V across 275K+2K. 305V/277K= 1.1mA per tube, times 4 is 4.4mA total, not quite 5mA. But the tubes can't be working as dead shorts. As a lazy-guess, their plates are around half the supply voltage, so total current is half of the shorted-tube value, or 2.2mA. "Plates sit about half B+" is very rough and open to wide variation. If it really-really matters, plot the bias on the tube curves. If you just need a good-guess: on a hi-Mu tube, multiply the cathode resistor by the Mu and pretend the plate-cathode DC resistance is about that number. Cathode resistors here are ~2K, 12AX7 is Mu=100, the tubes are sorta-like 200K resistors. Adding the resistances 275K+200K+2K= 477K, and 305V/477K= 0.64mA per tube, times 4 is 2.6mA total.
> the line stage. Around 15 for both?
The line-stage happens to say "7.15mA" next to the cathode follower; maybe true. If the other (input) tube is shorted, it draws 250V/48.5K= 5.25mA. Using the "half" approximation for a working voltage amp, it would be 2.6mA. Using the Mu approximation, the tube is 1.5K*20- 30K, 47K+30K+1.5K= 78.5K, 250V/78.5= 3.2mA.
Just as a cross-check: applying the Mu approximation to the cathode follower stage gives 820*20= 16.4K, 16.4K+0.82K+10K= 27.22K, 250V/27.22K= 9.2mA, not the 7.2mA noted on the schematic. The Mu approximation is not exact, worse for medium-Mu, anyway two different 6SN7s in this circuit could bias at 7mA or 9mA. That 2-decimal-place notation is bogus: tubes aint that exact.
Best-guess is 2.55 in the phono, 2*(3.2mA+7.2mA)= 21mA in the line-stage, total 24mA.
> The coupling cap from the sch. of the first post should be 0.33uF not 0.033uF!
As you wish. However, the load on that cap is not the 220K that you see. This is boot-strapped by the cathode follower. Using a rough approximation, the CF gain is 0.86. 1/(1-0.86) is 7. The load is bootstrapped up to about 220K*7 or 1.5Megs. 0.01uFd is a low-Fi value for coupling to 1Meg (-3dB at 17Hz, -1dB at 34Hz) so 0.03uFd and 1.5Meg is not an implausible value for hi-fi: -1dB at 7Hz, -0.5dB at 14Hz, etc. The input R-C is about the same point, so overall we have -1dB at 14Hz, -14dB at about 1.7Hz and 12dB/oct below that. My experience is that anything with a phono input should have a 12dB subsonic filter. Yes, it "should" be in the phono stage but sometimes it is expedient to shorten the line stage. This one gives about 22dB down at LP-warp frequency, a useful value.
I was taught that with choke-input, you either have enough bleeder to keep the choke working before the tubes warm up, or you design the caps and other parts to take nearly full 1.414*VAC for many seconds many times (or some compromise like the bleeder alone holds voltage down to 1.2*VAC, which may be sweet for this case).
Chokes sometimes have higher inductance at low current than at rated current. This reduces the need for bleed. When designed for that feature, they call them "swinging chokes". But even a swinging choke has a minimum current below which it won't choke and voltage soars to 1.4*VAC.
> that's quite different from the 0.5 RMS available with cap. I/P filtration.
A strict derivation suggests Idc = 0.6*Iac for RMS heating. I do agree with using 0.5*Iac in most cases, because small transformers sag and DIY is not about shaving every nickel.
> a benefit of choke I/P filtration... is a cooler running power trafo when compared to cap. I/P filtration.
The 0.6*Iac for C-in and ??*Iac for L-in factors give the same heating.
> If the penalties in cost, size, and weight are acceptable
Sure. And I agree that 350VAC L-C-R-C is an elegant design. But Hamish is declining to buy a different transformer. I would assume that buying a choke (which is generally not a lot cheaper than the corresponding transformer) is also not "acceptable" for the present goals and budget.
> The idea is "Electronic Design", not "Electronic pot-luck".
To argue with myself: actually there is a long DIY tradition of "Try it and see". When done with reckless abandon, this usually leads to poor performance, or smoke (and no performance). But studying the situation can lead to different values (i.e. new parts) which work better (or work smokelessly). Usually a few iterations are needed to approach semi-optimum performance, and the cost of major changes (like power transformers or main power caps) can be discouraging. IMHO, pot-luck electronics is for people with well-stocked parts-piles, and possibly modest expectations. But some good things have been born that way. I cringe, because I am a real cheapskate, and I want to "know" that the major parts will do what they need to do instead of burn-up or have to be exchanged for others.
To argue with myself: actually there is a long DIY tradition of "Try it and see". When done with reckless abandon, this usually leads to poor performance, or smoke (and no performance). But studying the situation can lead to different values (i.e. new parts) which work better (or work smokelessly). Usually a few iterations are needed to approach semi-optimum performance, and the cost of major changes (like power transformers or main power caps) can be discouraging. IMHO, pot-luck electronics is for people with well-stocked parts-piles, and possibly modest expectations. But some good things have been born that way. I cringe, because I am a real cheapskate, and I want to "know" that the major parts will do what they need to do instead of burn-up or have to be exchanged for others.
- Status
- Not open for further replies.