I am rebuilding a Scott 222c , PP 7189 integrated amp. The amp is working good and i have replaced all the power supply caps and have rebuilt the power supply with new resistors that were out of spec, i kept the old WW which tested good. I have a set of tested 7189 tubes which are fairly closely matched.
In an effort to reduce B+ i have replaced the 80 ohm 10W resistor in the CRC filter with a choke of 6H x 165ohm. The resulting PS filter is CLC of 22uf x 6H , 165 ohm, x 100uf. I also installed a CL80 on the input AC before the switch. Even so, the B+ is still around 450 at the CT of the output trans. On the schematic the voltage should be 420. I tried a choke of 10H and 320 ohms which brought the voltage down to 430 maybe or lower but the sound quality may not have been as good. I say maybe because this was tried early on and not as much of the circuit was rebuilt. Should i put the other choke back or try other means?
Any comments or suggestions? thanx in advance. 808
In an effort to reduce B+ i have replaced the 80 ohm 10W resistor in the CRC filter with a choke of 6H x 165ohm. The resulting PS filter is CLC of 22uf x 6H , 165 ohm, x 100uf. I also installed a CL80 on the input AC before the switch. Even so, the B+ is still around 450 at the CT of the output trans. On the schematic the voltage should be 420. I tried a choke of 10H and 320 ohms which brought the voltage down to 430 maybe or lower but the sound quality may not have been as good. I say maybe because this was tried early on and not as much of the circuit was rebuilt. Should i put the other choke back or try other means?
Any comments or suggestions? thanx in advance. 808
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Change from a 5AR4, as the rectifier, to a "potato masher" 5R4. The substantial increase in forward drop will tame the B+ rail voltage. You might have to use a lower DCR choke, lest the rail voltage be too "short". "Soften" the fast B+ rise of the directly heated 5R4 by inserting a CL-130 between the rectifier and PSU filter.
Before you worry about fine tuning B+ voltage for the 7189. I just notice from the schematic that the 222 model doesn't have any easy mean of measuring or adjusting the output tube bias. I have a 233 and it has both. Yes, you can somewhat adjust bias via B+ but that is an odd way of doing it especial the bias voltage depend of the filament current of the 2 12ax7 on the phono stage.
I would consider adding 10 ohms resistor in series with the 7189 cathode and ground. This way you can know (with a simple volt meter, I = E/R) if you are in the right ball park before changing thing. Also, it would make future tube changing so much easier if you add a trim pot and a cap to adjust bias voltage. One way to do it (and keep the change somewhat vintage correct) is look up a 233 or 299 schematic and use the circuit as a reference.
IMHO, after that, I would tinker with B+
I would consider adding 10 ohms resistor in series with the 7189 cathode and ground. This way you can know (with a simple volt meter, I = E/R) if you are in the right ball park before changing thing. Also, it would make future tube changing so much easier if you add a trim pot and a cap to adjust bias voltage. One way to do it (and keep the change somewhat vintage correct) is look up a 233 or 299 schematic and use the circuit as a reference.
IMHO, after that, I would tinker with B+
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Dunno…
I think my reply / advice would be “why are you worried?”
450 ÷ 420 = 1.071 … → + 7%
7%? That's easily within the nominal working voltage of the circuit's components. But either follow Eli's advice, or just use a –30 volt string of 1N4007 diodes to drop B+ … in series with the rectification output. Cheap, easy, bomb-proof. Did I mention cheap?
They have a voltage drop of about 0.85 v/ea at 200 ma. So… 30 ÷ 0.85 = 35 of them in series. Too many? Somehow bothers you? Well … then just use Zener diodes in a similar way, since they're also good at dropping voltage a fairly reliable amount. Consider the [ 2EZ11D5-TP Micro Commercial Components (MCC) | Mouser ]. 2 W dissipation ea. 39¢ ea. 11 volt Zener drop. For a buck you can make a 33 volt dropper. 6 W. 180 ma pass current. Plenty of "overcurrent capacity" during startup. I'm sure you can find more.
GoatGuy
PS: Then I found these... https://www.mouser.com/ds/2/258/1n5338b-1n5369b(Do-15)-349568.pdf ... almost perfect! 5 watts each nominal, and able to give you 400+ ma output current. And only 47¢ ea, so a buck fifty for your voltage dropper.
I think my reply / advice would be “why are you worried?”
450 ÷ 420 = 1.071 … → + 7%
7%? That's easily within the nominal working voltage of the circuit's components. But either follow Eli's advice, or just use a –30 volt string of 1N4007 diodes to drop B+ … in series with the rectification output. Cheap, easy, bomb-proof. Did I mention cheap?
They have a voltage drop of about 0.85 v/ea at 200 ma. So… 30 ÷ 0.85 = 35 of them in series. Too many? Somehow bothers you? Well … then just use Zener diodes in a similar way, since they're also good at dropping voltage a fairly reliable amount. Consider the [ 2EZ11D5-TP Micro Commercial Components (MCC) | Mouser ]. 2 W dissipation ea. 39¢ ea. 11 volt Zener drop. For a buck you can make a 33 volt dropper. 6 W. 180 ma pass current. Plenty of "overcurrent capacity" during startup. I'm sure you can find more.
GoatGuy
PS: Then I found these... https://www.mouser.com/ds/2/258/1n5338b-1n5369b(Do-15)-349568.pdf ... almost perfect! 5 watts each nominal, and able to give you 400+ ma output current. And only 47¢ ea, so a buck fifty for your voltage dropper.
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Actually, i am more worried about the G2 voltage which should be 345vdc at 420 CT voltage but is already 370 with my variac adjusted so that CT voltage
is 420. If i let it go the B+ is 450 and G2 is almost 400. I don't think my power tubes will be hanging around too long with that.
I would consider adding 10 ohms resistor in series with the 7189 cathode and ground. This way you can know (with a simple volt meter, I = E/R) if you are in the right ball park before changing thing. Also, it would make future tube changing so much easier if you add a trim pot and a cap to adjust bias voltage. One way to do it (and keep the change somewhat vintage correct) is look up a 233 or 299 schematic and use the circuit as a reference.
IMHO, after that, I would tinker with B+
I really can not do that in a simple way. In the stock config. the cathode is part of the ground and other components , like the grid to ground resistor is attached to it. So, i am going to leave it be for now.
I did rebuild the bias circuit with all new resistors. The 18 ohm voltage dropping, were all chucked as they were more than 10% over. This did help lower the negative bias voltage but maybe not enough. I am think of putting an adjustable resistor in the bias input line. I have some 200 ohm VR which i can put inline if i find a place for it. But, i would also need to add the 10 or 1 ohm voltage sensing resistors. Doing all that would be quite a bit of work since i would also make a new ground for the tubes.
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Hi Kev, i always thought that 5y3 would be too weenie for the Scott. You think that it would ok. Have you known anyone with a 222 that uses it? If the 5y3 fails will it take out tubes or transformers? I would rather not have that happen to me. I have already put many hours into this rebuild.
Kevin, no way is a 125 mA. rectifier going to support a Scott 222C. The 250 mA. 5AR4 is close to or at its max. capability.
Sser2, the "potato masher" 5R4 is cylindrical. At least that part of its look is "correct". If the bottle is "too tall", leave the cover off or make an opening in the sheet metal.
All, functionality (IMO) trumps appearance. The unit has to be a reliable performer, in association with today's higher average house current voltage. A 5R4 has the necessary large forward drop, 250 mA. capability, and 2 A. filament draw.
I tried a 5r4gya, and the B+ was 385. It sounded ok. But, if i use it i would have to change out the choke for something with lower DCR.
So, next i changed the C1 cap to 10uf and plugged in the 5v4ga. I realize that this rectifier will be overtaxed in this use but i figure i would check it out. with the GE 5v4ga the B+ is 430 to 425 when under load. The sound was better than the 5r4 i guess because the higher voltage.
So, next i changed the C1 cap to 10uf and plugged in the 5v4ga. I realize that this rectifier will be overtaxed in this use but i figure i would check it out. with the GE 5v4ga the B+ is 430 to 425 when under load. The sound was better than the 5r4 i guess because the higher voltage.
Nah, it sounds better as is with 5v4ga. Plus i get no voltage overshoot on warm up.
I dropped the B+ voltage by 100V on my Scott 299 by going to a choke input power supply. When I selected the choke, I made certain it was short enough to clear the case if one was used and also, naturally, that would meet critical inductance for a choke input supply. I prefer the operating points for the 6P14P-EV to be low voltage and high current. This constrains power, but also gives an operating point closer to AB1.
I was kinda of thinking to do that but , i am getting tired of playing with this unit. What are the voltages that your amp finally settled in on? Also where did you mount that input choke, and what are the specs for the unit? thanks for letting me know.
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