when measuring a SF champ cathode bias ?

[catnine]

Since when checking the actual plate voltage you are supposed to subtract the voltage drop across the bias resister from the plate pin 3 to ground . So if I have 371 on the plate and subtract 26.0 cathode + 345 plate actual . Now many talk about the screen being higher than the plate on 6V6 champs so what plate voltage is refered to the plate pin 3 to ground or after the cathode voltage is subtracted from the plate , if the later is true then the screen see's much more than the plate.

One other thing I noticed . On a SF champ there are two bypass caps on the 12ax7 the one for the preamp across the 1.5k cathode resister is 25 uf but the one on the second gain stage after the tone stack is 2uf . I think 25 uf lets more low freqency through than a smaller value was this do to tame the bass a bit because of the stock champ speaker or will changing the 2 uf cap to a 25 uf make no real difference?

[Chuck H]

Since when checking the actual plate voltage you are supposed to subtract the voltage drop across the bias resister from the plate pin 3 to ground . So if I have 371 on the plate and subtract 26.0 cathode + 345 plate actual . Now many talk about the screen being higher than the plate on 6V6 champs so what plate voltage is refered to the plate pin 3 to ground or after the cathode voltage is subtracted from the plate , if the later is true then the screen see's much more than the plate.

The plate should ideally have more volts than the screen so the +/- relationship is consistent. The actually working voltage (Vp - Vk = working voltage) of the plate doesn't figure into this. Only the actual voltage (Vp). The actual voltage (Vp) should probably also be the number taken into account for the maximum specification. That is, if a tube has a max Vp of 400V you probably shouldn't run it at 420Vp with 20V on the cathode. But I'm just assuming this since it seems this spec is probably relative to internal spacing and meant to prevent arcing. The working voltage (Vp - Vk) is the number to use when figuring the actual work that the tube is doing. Example: For a bias figure you should subtract Vk from Vp to determine the working voltage. Then multiply Ik times the working voltage for the actual wattage dissapation. Of course, even this figure doesn't account for the screen current that's inhearently rolled in. I think failure to figure for these two things (working voltage and screen current) is part of the reason you read about a lot of class A amps, Especially those that use el84's, running well over 100% dissapation at idle. They're actually not.

[Chuck H]

One other thing I noticed . On a SF champ there are two bypass caps on the 12ax7 the one for the preamp across the 1.5k cathode resister is 25 uf but the one on the second gain stage after the tone stack is 2uf . I think 25 uf lets more low freqency through than a smaller value was this do to tame the bass a bit because of the stock champ speaker or will changing the 2 uf cap to a 25 uf make no real difference?

The smaller bypass cap on the second gain stage cathode is probably chosen to tame low end or to tune the NFB circuit to prevent oscillation or both. On a typical cathode biased preamp stage (1.5k resistor) you can only hear the difference in values up to about 4.7uf. That doesn't mean that larger values shouldn't be used. Linearizing a gain stage is important for certain circuits and a full bypass should be used for this. But for tone shaping, where a linear stage isn't required, you can experiment with values under 4.7uf to audibly change the knee frequency. So increasing that 2uf cap will increase gain on lower frequencies for better or worse. No harm in experimenting.

It should be mentioned that the actual impedance of the circuit does affect the knee frequency. The larger the cathode resistance the lower the knee frequency will be for a given cap value. Also, the larger the cathode resistance, the more gain increase you can expect from a bypass cap.

[catnine]

The smaller bypass cap on the second gain stage cathode is probably chosen to tame low end or to tune the NFB circuit to prevent oscillation or both. On a typical cathode biased preamp stage (1.5k resistor) you can only hear the difference in values up to about 4.7uf. That doesn't mean that larger values shouldn't be used. Linearizing a gain stage is important for certain circuits and a full bypass should be used for this. But for tone shaping, where a linear stage isn't required, you can experiment with values under 4.7uf to audibly change the knee frequency. So increasing that 2uf cap will increase gain on lower frequencies for better or worse. No harm in experimenting.

It should be mentioned that the actual impedance of the circuit does affect the knee frequency. The larger the cathode resistance the lower the knee frequency will be for a given cap value. Also, the larger the cathode resistance, the more gain increase you can expect from a bypass cap.

here is the diaghram of a SF champ. I built the exact same circuit using the same values for all the componants even 1/2 watt carbon composition resisters other than the filter resisters those are 3 watt metal oxide.



So you see the 2uf cap on the 2nd tride of the 12ax7 . it looks like it may affect bass response yet it also looks like it plays a part in the NFB loop as well.

[catnine]

The plate should ideally have more volts than the screen so the +/- relationship is consistent. The actually working voltage (Vp - Vk = working voltage) of the plate doesn't figure into this. Only the actual voltage (Vp). The actual voltage (Vp) should probably also be the number taken into account for the maximum specification. That is, if a tube has a max Vp of 400V you probably shouldn't run it at 420Vp with 20V on the cathode. But I'm just assuming this since it seems this spec is probably relative to internal spacing and meant to prevent arcing. The working voltage (Vp - Vk) is the number to use when figuring the actual work that the tube is doing. Example: For a bias figure you should subtract Vk from Vp to determine the working voltage. Then multiply Ik times the working voltage for the actual wattage dissapation. Of course, even this figure doesn't account for the screen current that's inhearently rolled in. I think failure to figure for these two things (working voltage and screen current) is part of the reason you read about a lot of class A amps, Especially those that use el84's, running well over 100% dissapation at idle. They're actually not.

I am a bit confused here. So let me see if I have this right. Vp is plate voltage to ground off pin 3 on a 6V6 and VK is voltage across the cathode resister. yes? So when we talk about screen voltage compared to plate voltage are we reading the plate off pin 3 (plate) to ground and ignoring the cathode resister voltage ?

I understand the last part meaning if one ignores the cathode resister voltage and the screen current when figuring bias the bias ends up being colder when calculated , yes/no?

[Austin]

Plate voltage is the voltage measured from the cathode to the plate, you could measure it from pin three to pin 8 if you want to, the point of measuring it from ground and subtracting is so you can clip the ground to the chassis and use only one hand because its safer, less risk of taking a shock across your heart. Keep one hand in your back pocket. Screen voltage is measured from the cathode to screen too, so you would need to subtract the amount the cathode is elevated there as well.

[Chuck H]

So you see the 2uf cap on the 2nd tride of the 12ax7 . it looks like it may affect bass response yet it also looks like it plays a part in the NFB loop as well.

Which is why I opened with:

The smaller bypass cap on the second gain stage cathode is probably chosen to tame low end or to tune the NFB circuit to prevent oscillation or both.

[Chuck H]

I am a bit confused here. So let me see if I have this right. Vp is plate voltage to ground off pin 3 on a 6V6 and VK is voltage across the cathode resister. yes? So when we talk about screen voltage compared to plate voltage are we reading the plate off pin 3 (plate) to ground and ignoring the cathode resister voltage ?

I think that is correct.

I understand the last part meaning if one ignores the cathode resister voltage and the screen current when figuring bias the bias ends up being colder when calculated , yes/no?

Not phrased exactly right. More like this:

"if one ignores the cathode resister voltage and the screen current when figuring bias the bias ends up being colder than calculated"

[catnine]

Plate voltage is the voltage measured from the cathode to the plate, you could measure it from pin three to pin 8 if you want to, the point of measuring it from ground and subtracting is so you can clip the ground to the chassis and use only one hand because its safer, less risk of taking a shock across your heart. Keep one hand in your back pocket. Screen voltage is measured from the cathode to screen too, so you would need to subtract the amount the cathode is elevated there as well.

Plate voltage is the voltage measured from the cathode to the plate, you could measure it from pin three to pin 8 if you want to, the point of measuring it from ground and subtracting is so you can clip the ground to the chassis and use only one hand because its safer, less risk of taking a shock across your heart. Keep one hand in your back pocket. Screen voltage is measured from the cathode to screen too, so you would need to subtract the amount the cathode is elevated there as well.

That's what I was trying to find out . I asked somewhere but forget where or on what forum. I never knew about subtracting the cathode from the plate and didn't know you measure the screen from the plate either. I always looked at the fender schematics and at the right top they tell you all voltages are measured to ground using a electronic meter assuming that means a DVOM didn't know they had those back in the 50's and 60's. I have seen many posts asking why on their SF champ the plate VDC is higher than the screen yet it was never mentioned how they took the reading . It makes sense now since I was blasted on one forum that I was doing it all wrong and my bias was lower or cooler than I thought.

I have looked at tube charts yet most I find are from the olden days when I was just a kid , the new tubes out now like JJ and some others EH can take more plate voltage than an old RCA 6V6GT yet there are no tube charts for them.

To be honest I have not been building amps for that long. I got a few older fender champs and a music master bass amp all from the 70's and looked up amp forums and the one that popped up in 2004 when I got these amps was the fender amp forum and a fellow named vintage jon jumped in and helped me out with parts and how to bias them all cathode biased amps but he never mentioned where to measure from or I was not listening or reading his posts proper. I had new 1967 fender amps like a bandmaster and super reverb but back then I just plugged and played and didn't know a thing about amps other than to take tubes in and get them tested.

I got into building my own amps because I have two old champs one hummed and the other was newer and fine then in 1994 fender came out with their pro jr and blues deluxe so I had two champs and pro jr and a SS park and took them all in to trade for a fender 1994 tweed blues deluxe wanting a larger amp . Then when I saw how they had printed circuit boards and tubes mounted to printed circuit boards I sold the deluxe in 2003 and got the musicmaster bass amp and another champ because I wanted nothing to do with the issues of ribbon wires and PC boards still don't . It was not until then in 2004 that I decided to recap the two amps I had and then build my own. Then on a bug I got a crate classic I think VC15 a push pull 15 watt with cathode bias and drive and gain and another 2001 pro jr and I liked them but I sold them because I liked the idea of building my own. I only built a few amps from 2004 to 2007 and then kept the two I have now . I don't have the knowedge of how a tube really works and what relates to what to design an amp but I can read a schematic or wiring diaghram and build one or change a few parts like splicing a preamp and tone section from one fender amp to another and can see how that works. I have done a few mods too. But I get lost when it comes to the technical part and the language used. I'm 64 and I can learn but all I really want to do is just get the best out of the two amps I have so I ask questions and try to understand it all . I spent 35 years working on cars and trucks for a living and I could diag an electrical issue just using the schematics and started with the simple things first like a fuse of ground in a certain circuit it was easy for me . I could find a short and know where it was without even taking a panel off because I knew how it worked . But with amps there are things I will never know , it's not just wires and fuses and switches and such and separate circuits .
I thought today I might get another OT for my champ so I could run a 6L6 and I know what one to get the PT in it can handle a 6L6GC can't say the OT can because there is not enough info of this weber OT to tell . it says 6 to 12 watts and a 7K primary and has a 4 and 8 ohm secondary tap . Yet I am fine with the 6V6 it sounds like a champ and to spend $60 on a OT with shipping is a bit out of my range right now. I used weber PT's and OT's on other builds and they worked just fine and sounded great to me. The P-P 6V6 amp I have built has allen amps tranny's with a lot more power and a lot more money yet they do sound much better so I do have that build. Year back I bought all sorts of speakers to see which one sounded best in each build and sold the ones I did not like but I can't afford that these days. I feel if my champ had a 10 inch alnico it would sound better than the jensen 10" ceramic . I had a lot of 12's in my P-P build but the weber sig 12S alnico sounded the best to me. I tried most of the eminence speakers and weber ceramics all 12's and out of 15 I liked two . I found the weber silver bell ceramic sounded great with a fender 12 alnico with the P-P build I have now with the 6L6GC's but not so good with one 12 and 6V6's they were to bottom heavy where the tone control could not cut the bottom enough so it all works together that I do know. I just didn't need that much amp with 6L6's and two twelves in this tall cab I made , if I still gigged fine but here in this apt the p-p and the champ are loud enough I don't use the P-P build much because it's real loud on 2 . I play blues with three strats I built years ago and I like a good bottom and pick attack and a bit of breakup if I want it so since I can't crank the amps to get the breakup it's easier for my to use an old OD pedal and that does the job but I don't want the amps to breakup at low levels or med levels if they do they lose the tone I like. So there you go my long story .

[Enzo]

COntext is everything. In one sense, plate voltage is the voltage on the plate. We usually mean with respect to ground when we just ask what voltage is on something.

When you look at a tube itself, rather than an entire circuit, then we need to know what the voltage is across the tube. And that means plate to cathode. A tube with 400v on the plate and 40v on the cathode has 360v across it. A tube with 380v on the plate and 20v on the cathode also has 360v across it. The tube has no idea what the voltages are to ground, all the TUBE knows is what there is across plate to cathode.

When we calculate dissipation, we multiply the voltage across the tube by the current. The actual voltage to ground doesn;t enter into it. Obviously, we can take two readings to ground, and subtract to get the voltage across the tube.

When a tube data sheet says plate voltage, they are talking about the tube itself, and they mean the voltage across the tube. If we ground the cathode, well, then the voltage across the tube and the voltage on the tube will be the same.