I'm at least as confused as you are!
https://milleraudiollc.com/pa-060-70-power-supply-transformer/
B+ starts much higher than 360 but falls to that under full load...
https://milleraudiollc.com/pa-060-70-power-supply-transformer/
B+ starts much higher than 360 but falls to that under full load...
Ons would expect a surge in the startup B+, up to 510Vdc in the standard Dyna but then it settles down to about 415-430 Vdc. That is why Miller’s “ideal” B+ of 370-380 Vdc is surprizing to me. In your build it then falls further below that. The fact that it drops to such low B+ typically is an indication of excessive current draw, once the power tubes start conducting.
But you have found that each 10 Ohm cathode resistor in the auto-bias kit shows a voltage of 400 mV, which would indicate a current of 40 mA. That is within acceptable limits, so one wonders where the excessive current is drawn.
But you have found that each 10 Ohm cathode resistor in the auto-bias kit shows a voltage of 400 mV, which would indicate a current of 40 mA. That is within acceptable limits, so one wonders where the excessive current is drawn.
If those 1n4004 diodes are damaged they may still be working to a degree but not well and can't support the voltage under load. Have you measured the power supply resistance to ground? As the caps charge you should see a high resistance value ultimately. If it doesn't climb to a very high final resistance reading you may have a high resistance short to ground or faulty power supply caps.
I just ordered some of the diodes you recommended. I'll look to replace all three.
Can you tell me how it's possible to have +57 VAC at the cathode of a diode and -68 VDC at the anode? I mean, it must be a function of the circuit it's in, obviously, but I just don't understand why there would even be a diode in a position like that.
Can you tell me how it's possible to have +57 VAC at the cathode of a diode and -68 VDC at the anode? I mean, it must be a function of the circuit it's in, obviously, but I just don't understand why there would even be a diode in a position like that.
That 57VAC is the bias supply. The bias voltage has to be (-)volts so a diode is placed in-line in reverse so only (-) sine volts passes through it. The capacitor on the output side raises the votage as it charges toward the peak voltage of the sine. So (-)57VDC becomes (-) 68VDC.
Positive and negative is really a reference to the direction of current flow...relative to the source. You know that an AC voltage sine wave has a half cycle (-) to 0? Yes?
Yes! I just hadn't really thought about it that way but it clicked as soon as you said it. So is the -57 bias just the other half of the +57 sinewave that's at the cathode?
But what I'm still struggling with is how an apparent +57 volts at the cathode becomes -68 at the anode.
Learning as I go here so thanks for helping me understand!
But what I'm still struggling with is how an apparent +57 volts at the cathode becomes -68 at the anode.
Learning as I go here so thanks for helping me understand!
Yes the (-) 57V is the bottom half of the sine. A single diode is a rectifier. It only allows current to pass through it in one direction. Flip it for the reverse polarity. Also forget about calling the VAC (+). It's just AC of a certain level. There is no (-) ACV. So your meter gets to decide the polarity of the DCV relative to the grounded lead.
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I'm with you on all that. I'm just trying to understand how the voltage at the anode seems "more negative" than the voltage at the cathode. Because I thought that a diode would simply prevent a lower (or more negative) voltage from crossing from the anode to the cathode. I mean, I thought the anode had to be positive with respect to the anode for current to flow.
But now I'm thinking about what you said and maybe it's sinking in...the AC voltage is positioned at the "wrong end" of the diode so it passes negative DC instead of positive DC? AND there's also a cap on the DC end of the diode which then makes the voltage at that end appear higher than at the AC source? I think it's clicking in what passes for my brain.
But what I don't get is why current would flow (from AC to DC since that's the purpose, to create the DC bias voltage) if the DC voltage is higher than the voltage of the AC's half-sinewave. Or does it not matter which side has a higher (absolute value of) voltage for whether current flows through the diode?
But now I'm thinking about what you said and maybe it's sinking in...the AC voltage is positioned at the "wrong end" of the diode so it passes negative DC instead of positive DC? AND there's also a cap on the DC end of the diode which then makes the voltage at that end appear higher than at the AC source? I think it's clicking in what passes for my brain.
But what I don't get is why current would flow (from AC to DC since that's the purpose, to create the DC bias voltage) if the DC voltage is higher than the voltage of the AC's half-sinewave. Or does it not matter which side has a higher (absolute value of) voltage for whether current flows through the diode?
The voltage you read as VAC on your meter is RMS volts, not peak. After the current passes through the rectifier the capacitor charges all the way to the peak voltage.... with consideration for any load on it that may bring it down. Bias circuits are low load and they will be filtered to a higher voltage so that they can be adjusted without loading down the tranny source voltage, .
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aaaah....so that makes sense. Is it right to say that peak should be approx 1.41 (square root of two) times the displayed RMS voltage?
Yep. All day. Wall voltage 120 RMS, 170 peak on your scope..
Then the ST-70 reduces the bias voltage with resistors and control pots to reduce the voltage on the grids to the needed voltage for whatever idle current you want on the outputs.
Then the ST-70 reduces the bias voltage with resistors and control pots to reduce the voltage on the grids to the needed voltage for whatever idle current you want on the outputs.
I assume you measured these voltages at D3? It appears that D3 is the single diode that serves as halfwave rectifier, and it rectifies only the negative half of the AC wave to provide the negative bias supply. Since -68 Vdc is close to spec this diode seems intact and if it is a 1N4004 or better it probably does not need to be replaced at the low voltage it is working.
Mr Miller suggests that the amp hum (somewhat reduced from tighter wire twisting) could be significantly caused by having wires that are running in parallel or very close to each other that shouldn't be. I'm going to try to work on that "lead dressing" because I think the odds of a bad part are so low and I do believe in the Miller Audio circuit. If that's the problem, then it's such a rookie mistake that I'm not surprised experts didn't mention it to me because you wouldn't even think of someone making that kind of mistake.
OK, a crazy thing just happened: When I turned on the amp, B+ moved up to 470 VDC (normal). When green autobias lights came on it was down to 390 VDC. Starting about 10 seconds after green lights come in, it started dropping but held at about 360. All that is basically normal behavior as I understand from the manual, and better than the past few days when B+ was dropping to 355-356. Then out of nowhere, after the amp being on for a minute or two, I heard a loud noise like a rush of air and I saw B+ plummet into the 200s and now it’s climbed back to about 358.5. OK, it’s now 3 minutes later and it just happened again…rush of air sound and B+ dropped to 191 and now within 10 seconds back to 358.
Seems to me that is a problem tube or capacitor. I don’t know what a bad diode sounds like. That was crazy. But maybe an interesting clue! (I still think I have rookie wiring errors as far as wires running parallel that shouldn’t be, etc, but there’s something else weird here.)
Tomorrow I will try swapping out all tubes. They all tested good when I started this process but maybe one has failed? Or do you experts think that sounds more like a cap or a diode? And if a cap, where's the most likely part of the circuit to cause such a thing? So weird!!!
Seems to me that is a problem tube or capacitor. I don’t know what a bad diode sounds like. That was crazy. But maybe an interesting clue! (I still think I have rookie wiring errors as far as wires running parallel that shouldn’t be, etc, but there’s something else weird here.)
Tomorrow I will try swapping out all tubes. They all tested good when I started this process but maybe one has failed? Or do you experts think that sounds more like a cap or a diode? And if a cap, where's the most likely part of the circuit to cause such a thing? So weird!!!
Was the noise from the amplifer hardware, or from the speakers? Could you monitor current draw by the amplifier? Or power consumed? If the current does not increase when B+ drops then it points to a PSU issue. If the current increases so B+ drop is due to increased load, then that points away from the PSU to something that would affect the bias of the output tubes. What is the fuse rating on the Primary side of the PT?
Or I suppose you could monitor the bias voltage, but is trickier with 4 tubes. If that goes to zero then the tubes will be full 'on' and will consume a lot of power, red plating quickly. But the fuse should blow in that case.
Or I suppose you could monitor the bias voltage, but is trickier with 4 tubes. If that goes to zero then the tubes will be full 'on' and will consume a lot of power, red plating quickly. But the fuse should blow in that case.
Hey @OldHector, the noise was DEFINITELY from the speakers but I don't know for sure if it was one speaker or both. I only had a meter set to DC volts connected to the B+ test point at the time. Are you suggesting that when I test again to see if it happens again, I leave the meter connected there but switch to measuring amps instead of volts? Or are you saying I should test amps at some other point? I could try to measure bias voltage but as you note I don't have a way to measure more than a couple of points at one time.
There's just one fuse...it's 3A slo-blow, in the usual place in the circuit (between the wall power and the transformer input.)
There's just one fuse...it's 3A slo-blow, in the usual place in the circuit (between the wall power and the transformer input.)
BTW, as I get started with current measurements, @OldHector, on startup I get DC amps of about .335 on the B+ test point as the amp starts warming up. Then about 10-seconds after the auto-bias lights come on (the same time frame during which the B+ voltage drops from maybe 370 to 358), the DC amps drop from .335 to .312. Waiting now to see if that weird thing happens again and if so I'll try to catch a glimpse of the meter reading.
No! You will at best blow your meter fuse if you turn the meter from measuring Volts to Amps with the probes unchanged.
Where exactly are you measuring the current mentioned - “amps drop from .335 to .312” . Is it across R B+? What is its value? The 0.312 amp current you measured is much higher than I would expect for a Dynaco ST70 at idle. And the relatively high value (2k-4k) of R B+ makes me think that it is only serving the PCB and NOT the power tubes. (For other readers- R B+ is shown upper left of the PCB photo on page 11 of the Mille Audio Manual previously referenced)
I am concerned about damaging something valuable in your amp. Random actions without the benefit of a schematic is not getting us anywhere. I suggest a slowdown, careful studying, planning your steps and going about it systematically.
Looks like your next step is to redress the wiring. Watch some videos on how tight to twist thr AC heater wiring and tuck them into the chassis corners. Also watch for AC carrying wires near the signal path. I expect it will not be the entire problem but might help.
Or, take it to a knowledgeable friend, or pay a tech to go over it. I’m sure there is help available somewhere in Denver.
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