Stromberg ASR-880 Biasing
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 16th January 2017, 06:47 PM #11 Lawrk diyAudio Member   Join Date: Jan 2017 yea, it looks like I'd need two more diodes to drop 1.6 volts. Do you have a first estimate of how much to increase it by, 20% 50%? Is it just a ratio, that is, if 10K drop from -60 V to -45 Volts, just calculate the the extra? Last edited by Lawrk; 16th January 2017 at 06:59 PM. Reason: clarify
 16th January 2017, 07:42 PM #12 Mooly   diyAudio Moderator     Join Date: Sep 2007 Its not the easiest thing to calculate. If the wiper of the pot is at -46.3 (turned to the end that gives the lowest i.e. most positive voltage) then you probably have around -63 volts rather than the indicated -60. If the balance pots are midpoint then you have 25k +150k to ground, repeated four times. That gives a parallel resistance of 43.75k loading the bias control wiper. So if you move the bias control to midpoint we end up with around -48.5 volts. Now increase the 10k to 18k and you would get a swing on the bias wiper from around -45 to -42 volts. That might be sufficient. However reducing the load on the -60 volt rail will cause it to increase slightly negating the effect a little. Another trick would be to add a high value resistor from the wiper to ground. Something like 100k would pull enough to just lower the voltage a little. The downside of that approach is adding to the wiper current and marginally loading the supply more. __________________ Installing and using LTspice IV. From beginner to advanced.
 16th January 2017, 08:24 PM #13 Lawrk diyAudio Member   Join Date: Jan 2017 Thanks very much. One more question, is the 500 Volts left well enough alone once bias is corrected to what it should be for 470 Volts?
 17th January 2017, 12:55 AM #14 6A3sUMMER   diyAudio Member   Join Date: Jun 2016 Make sure that you have all the preamp filaments in place, and running. The -60V runs resistor and filaments in series. See the schematic at the lower left corner. Without that preamp filament load (with only the power amp tubes in place), the -60V supply will be essentially unloaded, and so the negative bias will be unusually high (larger negative).
 17th January 2017, 01:51 AM #15 Lawrk diyAudio Member   Join Date: Jan 2017 No, all of the tubes were in place when I made the bias measurement. So it is a Volt and a half more negative than it should be. What I was attempting to say/ask was that if I followed Mooly's procedure to restore the bias to -45 Volts, was it ok to maintain the raised B+ as well. I said this because the -45 value is associated on the schematic with a plate voltage of 470 Not the 500 that I have now. So, is -45 volts ok with 500 plate? And is 500 plate ok (or should it be adjusted also, performance, tube life etc..)? thanks to everyone who has helped me with this!
 17th January 2017, 03:46 AM #16 6A3sUMMER   diyAudio Member   Join Date: Jun 2016 The 7355 has ratings: Plate 500V, screen 400V, Plate Watts 18. The 7591 has ratings: Plate 550V, screen 440V, Plate Watts 19. 500V * 34mA = 17W Plate. So, you may be OK here, but may have a shorter tube life than you would if all the voltages in the amp are corrected. The -60V supply may be high too. That will make the preamp filament voltages too high. So, it appears that all the problems you are having with voltages may be mostly related to what you power line voltage is. You said the amplifier was made for 117VAC. What is the voltage on your power mains? Even if you solve all the voltages on the B+, and Bias, and filaments, you may end up with a hot power transformer if it has more volts on the primary than it was built for. What about solving all this with a power resistor in series with the transformer primary. You might pursue this solution. It is probably the easiest to implement. But some people will say that now your power supplies will sag slightly on musical transients. I would not worry too much about that, as long as there is lots of filter caps with enough capacitance to take care of the short period of most musical transients (we are not talking about trying to max out a guitar amp for a Rock Party are we?). Just get a guitar amp for that (and don't wonder why there is distortion when it is maxed out). The only other solution is very expensive, and requires some real exact measurements, calculations, design, etc. A new power transformer ... but trying to get the primary voltage rating right is only the start. You would also need to have the same effective secondary voltages under the correct loads (the output impedance of each winding), and the same core losses. Don't even go down this path. If you want to do that, just design and build a new amp from the ground up.
 17th January 2017, 05:40 AM #17 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA FWIW- the "1.2meg" R91 at the bottom of the bias-adjust can not be correct. From inspection of 60V vs 45V, 10K and a 5K assumed centered, R91 is 47K. Not a meg. A 5K pot would make "no" difference (0.5%) in a circuit with a Meg in it. This is probably a SAMS typo (they did that). What do you really have there? What if you tack-solder a 10X larger resistor across it? If you really have say 50K, then a 500K across it makes 45K and a few-volt change in extreme-hot G1 bias. I would not over-fret 117V vs 125V wall voltage. Many tube amps thrive on modern walls. As this fixed-bias amp draws variable power depending on signal, a simple resistor would drop variable voltage. A 120V:6V 2A filament transformer wired "bucking" would drop 125V to solid 119V, and put you closer to 1963. > VTVM vs modern meter? Same thing. First the "470V" line would tolerate huge meter loading. Second, most (not all) DMMs on DCV show 10Meg input loading, and the most common VTVMs showed 11Megs, same-as for most purpose. The old alternative was passive "VOM" meters. While 1K/V meters are still with us, most electronics techs would have a 20K/V instrument. On the "100V" range this is a 2Meg loading. On a 500V range it is 10Meg. So high-volt systems hardly care 20K/V VOM, VTVM, or DMM. Last edited by PRR; 17th January 2017 at 05:43 AM.
 18th January 2017, 12:54 AM #18 Lawrk diyAudio Member   Join Date: Jan 2017 I checked R91 and its Br,R,Gr 1.2M Ohm. I guess that means that their voltages are wrong..
Lawrk
diyAudio Member

Join Date: Jan 2017
Quote:
 Originally Posted by 6A3sUMMER What about solving all this with a power resistor in series with the transformer primary.
Line voltage is 121 Volts.

So I could add a series resistor after M11.
(480-360)/6k=20 mA. Add to the 140 mA on 480 volt leg get 160 mA total, so 20 Volt drop can be made with a 125 Ohm 5 watt resistor (dissipating 3 Watts). You agree?

Last edited by Lawrk; 18th January 2017 at 01:23 AM.

 18th January 2017, 01:49 AM #20 6A3sUMMER   diyAudio Member   Join Date: Jun 2016 You have several alternatives as to how to get the voltages correct. There were several suggestions. Just pick one. The circuits will probably work as is, except for the current in the output tube is low. As long as the filament voltages, screen voltages, and plate voltages are within the rated tube limits, all that is left are 2 things: 1. You need to get the bias correct so that there is enough current on the output tubes. starving it by about 10 mA per tube will not likely give optimal results, both measured power and distortion, and sound (listening results). 2. The Plate and Screen dissipation (The tube plate current times the plate to cathode voltage, and the tube screen current times the screen to cathode voltage. This needs to be checked after you get the proper current.

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