This is probably a silly question, but I lack specific experience, so I'll ask anyway 🙂
In a push pull output stage that shares a current lol resistor, I.E both tube cathodes are together, and then share a 0.5 ohm resistor to ground (A) there's no issue with changing it to 2 resistors (B) so the current can be seen individually?
That's not going to make it upset, right? If the cathodes aren't directly connected to each other anymore?
In a push pull output stage that shares a current lol resistor, I.E both tube cathodes are together, and then share a 0.5 ohm resistor to ground (A) there's no issue with changing it to 2 resistors (B) so the current can be seen individually?
That's not going to make it upset, right? If the cathodes aren't directly connected to each other anymore?
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Should work fine. You will get some differential series feedback, but with 1 ohm per valve, it will be negligible.
I use this unbypassed technique on each diff pair to apply regenerative feedback, esp. on high gm tubes that have rather low Vg/ cath margins to lessen grid distortion effects. The thingy to watch out is the often undesirable reflected increase of anode impedances, esp with output stages which in most cases the anode impedances should be recalculated. There is no rule of thumb to the amount but if there is sufficient gain then there is more to play with.
These are 6550/kt88 and maybe kt120, if I can figure out how to fit a couple of dedicated filament trafos in the chassis. (edit: Or I might just go for it with the stock trafo because the list of trafos killed by kt120 filaments is still pretty much 0)
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This is pretty common in push-pull amps using fixed bias. Individual 10 ohm unbypassed resistors are pretty often used to monitor cathode current to make it easier to measure (10mv/mA).
So not sure what your dealing with on powering filaments. As I said I have a build right now that uses KT90s (4) and a couple of octals and 9 pin tubes. Total current load is bigger than my traffo (6A). My total current draw for filaments is 8.3A. I am using a DC switcher for the filaments rated at 10-12 A. Smaller than comparable traffo for the filaments and cheaper. Do not here anything from the switcher and works great.
Oh, I'm still messing with this arc vt60. It was in sorta rough shape, so I'm going through it and upgrading / replacing stuff as I can. The filament issue is simply that I'm not sure what the filament winding rating is, and arc says "not recommended" with kt120. Not sure if that's because of the filament winding, or if it's just something silly, like the cage won't fit back on. I have the kt120s that I was going to build something with, but if I can get away with them in this amp, then I will. Probably try it for a few days, and if the trafo starts getting too warm, I've got a couple of 6v triad torroids that will fit in the bottom of the chassis.
Is all of the voltage regulation working as it should be in the power supply stage? Also the current ratings of the filament 6550 vs KT120 are very close and it probably is worth trying. I would monitor the current through the filament winding and compare between the two.
Yeah, everything seems to be working as it should. I powered it up when I first got it and everything checked out, except for a 7808 regulator that they're using for the filament on the first input tube that wasn't working. Working on that led to reflowing a bunch of solder, which led to measuring and subsequently replacing the main filter caps that were all reading 15-20% low. Then I realized the tube sockets were pretty loose, so they're getting replaced. The 8 pin sockets arc were using in the 90s are hot garbage, anyway. There's also a couple of 6.9v references getting replaced, they were cheap, and the new parts have way better temperature characteristics, it's something like 15ppm/c vs 100ppm/c.
Then, I started to have nightmares about the two tubes sharing a bias adjustment and measurement point, mostly because one of my other tube amps, a Cary sla-30 started acting up with a couple of the tubes running away. It's cathode biased, at some point I'll work on it, but I need to get 8 new 7189a tubes for it, so it's a shelved project for now. I may end up just re-using the iron and build something different in it's chassis, but if it lives, it's going to need a complete re-thinking of the bias.
The vt60 though, I'm trying to do all the mods to it so it's bulletproof, everything is getting meters, everything is getting adjustments, everything that can be updated is getting updated. I'm a bit of a Tim Taylor, except it's not hotrods and power tools, it's audio gear lol.
Then, I started to have nightmares about the two tubes sharing a bias adjustment and measurement point, mostly because one of my other tube amps, a Cary sla-30 started acting up with a couple of the tubes running away. It's cathode biased, at some point I'll work on it, but I need to get 8 new 7189a tubes for it, so it's a shelved project for now. I may end up just re-using the iron and build something different in it's chassis, but if it lives, it's going to need a complete re-thinking of the bias.
The vt60 though, I'm trying to do all the mods to it so it's bulletproof, everything is getting meters, everything is getting adjustments, everything that can be updated is getting updated. I'm a bit of a Tim Taylor, except it's not hotrods and power tools, it's audio gear lol.
Maybe I can stick another question in here without starting another thread, but I think the wisdom surrounding bypass caps may have changed over the last couple of decades since this thing was built, but it's littered with these 0.01uf "wondercap" bypass caps, (c8, c5, c6 and about 7 more in the power supply sections), which I think are reacting poorly to the ipa I've used to clean the board.
Are these values critical? Should I order new fancy ones, or can I use other values of poly caps I have laying around?
Are these values critical? Should I order new fancy ones, or can I use other values of poly caps I have laying around?
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I call it a shunt resistor because it's there primarily to measure current.
You can call it what you like, but if you want to be understood, you have to use the same terminology as everybody else. It’s still a series resistor: and you can’t use a shunt resistor to measure current.
Tektronix: "A shunt resistor is a precision component inserted within a circuit to measure current."