I apologize in advance if this question shows my inexperience, or comes across as asking for a free education without putting in the work. I'm hoping someone might be able to tell me what is going on here, or at least point me in the right direction so I can figure it out myself. Anyway...
The attached schematic is for the preamp/power amp section of my Lafayette LA-226C stereo receiver, a rebadged Trio WX-400. I recapped it and cleaned it up, and now it sounds very nice, and generally speaking I'm very happy with it. I just want to know more about how this circuit works. Only one channel is shown in the schematic, of course. The rest is mostly not pertinent.
My question stems from what's going on with the cathodes of the output tubes. As you can see, the two cathodes are tied together--the other two 6BQ5s have their cathodes tied into the same node as well. I realize that this is not a particularly unusual practice, especially for amps of this time period. I have a matched quad of JJ EL84s in it, since this means they are all biased together.
What I can't figure out for the life of me is why there are two resistors seemingly forming a voltage divider, with a 9V output going to the grids of the output tubes through those 470kOhm resistors. Is this intended to add a positive bias to the control grid? Or something else? Maybe loading the cathodyne phase inverter?
OK, if by some miracle I got that part right, now comes the really confusing part (for me, anyway). On our way down to the cathode resistors, we also see two series tube heaters in parallel with the cathode resistors. So, I assume that with the resistance of the heaters in parallel with the cathode resistors, they are also an important part of biasing the output tubes. Furthermore, I guess this means that the 150V 40uF capacitor serves both to smooth the DC for the filaments of V13 and V14, as well as a cathode bypass cap... I think??? Since it seems like the heaters of V13 and V14 are important for the output tubes' bias, I have made sure that I put healthy tubes here. More on them later.
Finally, to top it all off, maybe everything I just said is basically moot, or not, because where you see the little "K" with a circle around it leads to a little humdinger-type potentiometer (100k, linear) between the +/- of a 5V 0.3A secondary of the power transformer. So I don't know how that affects the bias, if at all. Interestingly enough, the hum balance knob doesn't do anything. The receiver is already absolutely dead silent. (woohoo!)
So how does one calculate the cathode resistance/current through the output tubes?
The reason I ask is because, in addition to my own curiosity, I may eventually like to modify this part of the circuit. It is my understanding that with the tubes biased this way, a matched quad is necessary, and if one of them goes down, the others go with it. For the sake of reliability, it might be nice to change this so each tube, or at least each p-p pair, gets its own cathode resistor (and cap, if necessary). Also, again for the sake of reliability, it would be preferable imo to have the heaters of V13 and V14 out of the circuit. In fact, I've noticed the power transformer runs pretty hot, so maybe in the long run the best move would be to pull V13 and V14 entirely. V13 and V14 comprise the RIAA preamp, which I don't ever intend to use--this receiver doesn't even have a ground screw, and I've got other, better phono preamps anyway. I should also note that I'm not concerned about the resale of the unit, so any (reasonable) mods don't bother me, and shouldn't bother you either 😉
Sorry for being so long winded. Can anybody help??
The attached schematic is for the preamp/power amp section of my Lafayette LA-226C stereo receiver, a rebadged Trio WX-400. I recapped it and cleaned it up, and now it sounds very nice, and generally speaking I'm very happy with it. I just want to know more about how this circuit works. Only one channel is shown in the schematic, of course. The rest is mostly not pertinent.
My question stems from what's going on with the cathodes of the output tubes. As you can see, the two cathodes are tied together--the other two 6BQ5s have their cathodes tied into the same node as well. I realize that this is not a particularly unusual practice, especially for amps of this time period. I have a matched quad of JJ EL84s in it, since this means they are all biased together.
What I can't figure out for the life of me is why there are two resistors seemingly forming a voltage divider, with a 9V output going to the grids of the output tubes through those 470kOhm resistors. Is this intended to add a positive bias to the control grid? Or something else? Maybe loading the cathodyne phase inverter?
OK, if by some miracle I got that part right, now comes the really confusing part (for me, anyway). On our way down to the cathode resistors, we also see two series tube heaters in parallel with the cathode resistors. So, I assume that with the resistance of the heaters in parallel with the cathode resistors, they are also an important part of biasing the output tubes. Furthermore, I guess this means that the 150V 40uF capacitor serves both to smooth the DC for the filaments of V13 and V14, as well as a cathode bypass cap... I think??? Since it seems like the heaters of V13 and V14 are important for the output tubes' bias, I have made sure that I put healthy tubes here. More on them later.
Finally, to top it all off, maybe everything I just said is basically moot, or not, because where you see the little "K" with a circle around it leads to a little humdinger-type potentiometer (100k, linear) between the +/- of a 5V 0.3A secondary of the power transformer. So I don't know how that affects the bias, if at all. Interestingly enough, the hum balance knob doesn't do anything. The receiver is already absolutely dead silent. (woohoo!)
So how does one calculate the cathode resistance/current through the output tubes?
The reason I ask is because, in addition to my own curiosity, I may eventually like to modify this part of the circuit. It is my understanding that with the tubes biased this way, a matched quad is necessary, and if one of them goes down, the others go with it. For the sake of reliability, it might be nice to change this so each tube, or at least each p-p pair, gets its own cathode resistor (and cap, if necessary). Also, again for the sake of reliability, it would be preferable imo to have the heaters of V13 and V14 out of the circuit. In fact, I've noticed the power transformer runs pretty hot, so maybe in the long run the best move would be to pull V13 and V14 entirely. V13 and V14 comprise the RIAA preamp, which I don't ever intend to use--this receiver doesn't even have a ground screw, and I've got other, better phono preamps anyway. I should also note that I'm not concerned about the resale of the unit, so any (reasonable) mods don't bother me, and shouldn't bother you either 😉
Sorry for being so long winded. Can anybody help??
Attachments
> intended to add a positive bias to the control grid?
Look again. It makes 12V -negative- of cathodes.
They wanted clean 20V-24V DC for the preamp heaters, but the EL84 at this condition only wants about 12V bias. So they split the -21V down to size.
I know it looks reckless. I have no idea how we survived 1960 with wacky amps like this.
Look again. It makes 12V -negative- of cathodes.
They wanted clean 20V-24V DC for the preamp heaters, but the EL84 at this condition only wants about 12V bias. So they split the -21V down to size.
I know it looks reckless. I have no idea how we survived 1960 with wacky amps like this.
Yeah, as PRR says, this is an artifact of braver times, when men were men, and, well, you know. It was possible then, but difficult, to get clean DC for heaters, and four EL84s idle at about the right current, and need to be biased anyway, so for the day an elegant solution.
It depends, to work right, on everything running close to spec and inexpensive replacements available at the corner drugstore. Today, we might keep the DC filaments, but add a fat 24 Volt Zener across them, for safety, add separate 10R resistors in each EL84 cathode (to measure cathode current across) and install separate bias trimpots for each valve, replacing some of R306 and R309.
I'd guesstimate that four 1K Ohm trimpots in parallel, in series between R306 and R309 would be close, but 2K Ohm pots would give a better range. And allow for my sloppy guess. Expect a bias of 11 > 16 Volts for cathode currents of 38 > 40coupla mA per each.
All good fortune,
Chris
It depends, to work right, on everything running close to spec and inexpensive replacements available at the corner drugstore. Today, we might keep the DC filaments, but add a fat 24 Volt Zener across them, for safety, add separate 10R resistors in each EL84 cathode (to measure cathode current across) and install separate bias trimpots for each valve, replacing some of R306 and R309.
I'd guesstimate that four 1K Ohm trimpots in parallel, in series between R306 and R309 would be close, but 2K Ohm pots would give a better range. And allow for my sloppy guess. Expect a bias of 11 > 16 Volts for cathode currents of 38 > 40coupla mA per each.
All good fortune,
Chris
Haven't seen the whole schematic, but are you sure this is correct? A 5V 0.3A secondary seems pretty useless, and a humdinger for the output valves also seems useless. Don't have a schematic, so can't really say.
WRT your future plans for this, I'd just like to mention the two biggest issues I see restoring these old parties. You're probably already aware of both but: Modern mains voltages in the US (and, anecdotally, in other parts of the world) are considerably higher than when these old gals were built. Everything is now half a century old, and now faces a maybe 10% higher supply voltage. Valves were better then and cheaper. And nobody then, in their wildest dreams, expected these old parties to still be running, let alone coveted, in Pandemic Year 2021. So, as conservative an approach as possible seems in order.
Second, safety standards then were what's now called Class 0, meaning without chassis-to-protective Earth ground connection. Or often worse, with now-ancient capacitors between mains and chassis. This really, really needs to be brought up to Class 1, meaning no (ancient, or even non-code) capacitors mains-to-chassis and a 3-wire mains cord. Green wire is bolted to chassis with a dedicated screw, lug, and locking nut.
All good fortune,
Chris
The Bypass cap for the cathodes, C307, is a 150V part.
That means if you pull out V12 or V13, or if V12 or V13 filament opens up, the cap voltage can rise to a very large voltage, but then the output tube will be almost cut off, so there is very little current in the cap to destroy it.
A lower voltage rating on that cap (C307) could have been a problem.
That means if you pull out V12 or V13, or if V12 or V13 filament opens up, the cap voltage can rise to a very large voltage, but then the output tube will be almost cut off, so there is very little current in the cap to destroy it.
A lower voltage rating on that cap (C307) could have been a problem.
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Thanks to everyone for the answers thus far. If you wish to peruse the whole schematic, do a search for "trio wx-400 schematic" and you should find it easily enough.
It is quite useless. They seemed to have put it there as a matter of best practice. I also accidentally omitted the info that the 5V secondary is used for the heater of the first preamp/driver for treble & bass knobs, V15, half of which is shown in the pic of the schematic. So maybe not so useless after all? Anyway, the setting of it doesn't matter, at least. No change in noise.
I'm planning on adding a CL-60 current limiting thermistor the next time I crack it open. I have two on-hand, should I add them in series? The transformer primary wants 117V, so not far off the 119.7V I just measured out of the wall--I'm very lucky to have good, stable AC here.
Before I ever turned it on, I pulled a nasty old 0.02uF cap from line to chassis ground out of there and threw it right in the trash. Interestingly, the schematic does not call for this, instead denoting it should go from line hot to neutral. So I put a 0.022uF X2-rated cap across the line there. So that at least should not be a problem now, and I suspect is one of the reasons this piece is so nice'n'quiet.
I should install a three-prong power cord, you're right, but I will need to do a touch more research on avoiding creating a ground loop. As it sits, it's already much safer than when I first got my hands on it.
I suppose maybe my verbose first post was too roundabout to ask what seems the clear question(s) now: Can't I just run a single resistor to ground from the cathode of each output tube? How do I calculate the resistance I need there to maintain the same bias current that's on the tubes now? I assume then I should get rid of those 470kOhm resistors? Finally, do I need cathode bypass caps (I imagine circa 47uF/35V)?
Thanks in advance for your help!!
It is quite useless. They seemed to have put it there as a matter of best practice. I also accidentally omitted the info that the 5V secondary is used for the heater of the first preamp/driver for treble & bass knobs, V15, half of which is shown in the pic of the schematic. So maybe not so useless after all? Anyway, the setting of it doesn't matter, at least. No change in noise.
I'm planning on adding a CL-60 current limiting thermistor the next time I crack it open. I have two on-hand, should I add them in series? The transformer primary wants 117V, so not far off the 119.7V I just measured out of the wall--I'm very lucky to have good, stable AC here.
Before I ever turned it on, I pulled a nasty old 0.02uF cap from line to chassis ground out of there and threw it right in the trash. Interestingly, the schematic does not call for this, instead denoting it should go from line hot to neutral. So I put a 0.022uF X2-rated cap across the line there. So that at least should not be a problem now, and I suspect is one of the reasons this piece is so nice'n'quiet.
I should install a three-prong power cord, you're right, but I will need to do a touch more research on avoiding creating a ground loop. As it sits, it's already much safer than when I first got my hands on it.
I suppose maybe my verbose first post was too roundabout to ask what seems the clear question(s) now: Can't I just run a single resistor to ground from the cathode of each output tube? How do I calculate the resistance I need there to maintain the same bias current that's on the tubes now? I assume then I should get rid of those 470kOhm resistors? Finally, do I need cathode bypass caps (I imagine circa 47uF/35V)?
Thanks in advance for your help!!