December 14, 2011 - 10:32 pm|
|I noticed that the RC networks from plate to grid of both drivers won't act in a balanced way since one side is coupling to a high impedance input, the other to a grid at AC ground. Besides unbalancing the load on driver plates at higher frequencies, there's also the effect of the driver input impedance (feed back components magnified by the Miller effect) rolling off the output response of the V5A. I doubt that a double-whammy was the intent. I think the intended pole-zero in the response for stability could be achieved with a single network placed across R33 or between the driver plates but not both (values to be recalculated). Keeping the drive stage fully balanced will reduce distortion at high frequencies. (Of course the value of the series resistor in the RC compensating network must not be so low as to starve the stage for current / available high-frequency voltage swing. That was the cause of many early solid state amps and op-amps developing slew rate limiting.)|
Having a portion of the V5A cathode resistor above the feedback point unbypassed would provide local degenerative feedback to reduce distortion due to V5's transconductance varying over the operating current range. That matters because the global feedback helps with everything else in the power amp except for the V5 input characteristic.
Another idea - Since any a.c. hum on supply V feeding R33 is divided by R33 and the plate resistance of V5 and is added to the signal, it might be reduced by introducing the same hum (as a common mode signal) to the grid of V7B (pin 4). Essentially make a capacitive divider using C37 and a newly added capacitor between V and C37. I read that space is tight, since electrolytics are usually too poor on tolerances and have poor long-term stability, the capacitive divider formed by C37 and the newly added capacitor would probably be better done adjacent to the filter for V using non-electrolytics, with a wire from there to V7B pin 4. Low leakage caps are better anyway since leakage would upset the grid bias. The ratio of the size of the caps could be calculated, but the dial in a value with a decade box could work as well. Beware that connecting/changing any caps to the part of the circuit when live carries both a shock risk, and a certainly of large brief output spikes. Have a dummy resistor on the amp output when testing to avoid arcing and speaker/ear damage.
April 08, 2011 - 07:49 pm|
Metal film resistors will be fine for those resistors. R41 provides negative feedback around V3A. So R33a and R41 determine the gain of the inverting amplifier formed by that. One problem with the original design was that negative feedback around the stage was shunted to ground when the level control was set low. That left the stage gain wide open, causing extra noise (and distortion) at low level settings. Let me how it goes!
I enjoy listening to an HF20 system too, by the way.
April 08, 2011 - 06:55 pm|
|Still working on it as I have time; I'm changing all the electrolytic caps and doing the loudness control mod at the same time, as well as working on one of my HF20s and my HF81. Just a quick question: Are R33A, R34A, R41, and R42 grid stoppers or what is their function? Reason I ask is that I've heard the suggestion that metal films should not be used for that purpose because of their inductance, so I was wondering if I need to use carbon comp at those locations? |
March 20, 2011 - 09:50 pm|
Thank you for your thoughtful comments. Addressing them individually:
1. You are quite right that you could replace R27,28 instead of paralleling. The only reason for paralleling would be to give finer resolution in setting it, given the available resistor values. Since we are only reducing the existing 68K by a factor of about 1.2X, a parallel resistor has an effect "geared-down" by about 5:1. The ideal net value varies with the tube, so the suggested alternative fixed value is pretty rough anyway. Your choice of replacing them with 56.1K should be fine.
2. It is true that good metal film resistors produce less excess noise than do carbon film ones, when there is appreciable DC voltage drop. I'm not sure which resistors in the first stage you are referring-to but R7,8 would probably benefit from being metal film. Since R27,28 don't have much DC drop, there would be less to be gained there. Metal film certainly can't hurt, though.
3. Adding the terminal strips seems like a good idea. The "flying lead" works well, given that it is properly insulated. Certainly the terminal strip looks classier. Thanks for the suggestion.
4. Good points.
5. I guess you actually mean C37,38, which are 1uF 450V (can be 250V). They were changed when the value was increased from 0.25uF. (Required by low frequency stability considerations.) A film cap can certainly work well there too but will be much larger. Things are pretty cramped there if you add the AC balance pots. As far as reliability, good quality 105C electrolytic caps should have a decent service life. My unit still has its original power supply filter caps; going strong after nearly 50-years!
6. C35,36 is NPO ceramic because NPO implies the excellent linearity (low distortion) that we want there. Mica is also an excellent choice but is more expensive. Hence, I don't tend to stock as many of them.
7. Yes, the value of the feedback resistor (R85,86) and the value of the compensation cap (C35,36) would need to be changed, if a different transformer tap is used for feedback. Working out the optimum compensation was the most difficult part of this project, involving many hours of measurement, experiment and simulation. Those components work in concert with the RC networks added around V6,7, to achieve the desired stability and performance results. Unfortunately, one can't always predict what compensation will be needed on a different output tap, based on behavior measured at another tap, so I would recommend leaving the feedback on the 8ohm tap, if at all possible. That also results in lower distortion on that tap. If you must move it to another tap, here are the theoretical values for R85,86 and C35,36. 16ohm tap: 6.08K, 85pF. 4ohm tap: 3.04K, 170pF. High frequency stability into a capacitive load should be checked if you try one of those.
Regarding the question about 120pF versus the original 125pF: The fact that the new value came out close to the original is pure coincidence. With the new values of the other feedback components and the networks added to V6,7, the compensation strategy is very different from what it was before.
I hope that your rebuild turns out well and will be happy to help if you have further questions. Would love to know how it goes.
All the best, Steve
March 20, 2011 - 07:44 pm|
|I'm surprised no one's commented so far, but I appreciate you sharing this. I've always like the sound of my ST-70 but the hiss has always bothered me so I'm rebuilding mine with the exception of the AC balance and phono pots. A few comments and questions.|
1.) If using a fixed resistor, why parallel R27 / R28 instead of replacing it? I don't use the phono section but decided to rebuild it anyway for completeness. I figured about 56.2 K would be a standard value that would be about right.
2.) Although carbon film resistors might be more "correct", it would seem metal film, especially at the first stage, would lower hiss further. I'm using Vishay/Dale 1% 1/2 watt 500 volts. These are brown so they don't clash visually like the standard blue ones do, if one cares about such things.
3. Rather than having them flying loose, there is (barely) room for a two junction terminal strip for the new grid resistors adjacent to pin 2 of V3/4.
4.) My capacitors C31 and C32 were also low, so it might not be a bad idea to simply recommend replacing them. I used my standard cap, 715P orange drop. For people that like these, the .047 will fit very nicely between the 6SN7s and the 7591s, and, if you get creative, the .1s can fit in the phono amp.
5.) It's not clear from your instructions why C36, C37 were changed to electrolytics, which will go bad in time.
6.) Is there any particular reason why the frequency comp and feedback caps are EPO ceramic instead of silver mica?
7.) If I move the grounding to common but retain feedback from the 16 ohm tap or move it to the 4 ohm tap does the value of the feed back resistor need to be altered? And was the change in the feedback cap from 125 to 120 for any particular reason or just because that value was close and available?
Again, I appreciate you taking the time to write this up.
February 13, 2010 - 08:11 pm|
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