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I recently built a 6BG6G audio amplifier, first of a stereo pair, loosely based on a 1950s Grommes 216BA amp. Sounds fantastic, no distortion. My question is, the cathode voltage doesn't really measure.
I have 395V on plates, 340V on screen (pentode mode), -49V on grid, but I measure .222VDC on the cathode. There is a 50ohm balancing pot between the cathodes, and I just adjust to make them the same for now. Bias is adjustable. It's odd. No red-plating; I'm inclined to leave it alone but wondering if anyone willing to comment on the cathode voltage.
Thanks
Pete
Follow Ups:
...in case anyone's interested.
The OP transformers are small, what I have for now. They are *claimed* to be near full-range (30Hz-18kHz) as long as you don't try to push more than 25W thru them. So far, no disappointment, seem to be performing ok - deep bass, cymbals sound great, not rolled off. Very musical. 6.6 Kohm.
P
Excellent work, nice color! How about a pic of the innards too?
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Buy Chinese. Bury freedom.
nt
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Have Fun and Enjoy the Music
"Still Working the Problem"
My brother saw them and said "With the hot rod styling all you need now is flame decals."
:0)
Edits: 06/14/17
Higher numbers for plate and screen. -45 VDC on grid. And 1.2 VDC on cathode. Seems you are running your setup a bit too cold.
Yes, it's dialed in now. The schematic showed -45V at the bias source AND -45V at the grid which must be wrong, after passing thru resistors.
Now have 396V on plates, about -32 to -33 on grid, and 1.17V at both OP tube cathodes. Sounds better than ever (naturally). Around 42-44mA. Will do more minor tweaking. And gotta drop filament V down from 6.6V AC.All tubes made 1950s.
Thanks for input.
Edits: 06/14/17 06/14/17
I did the math and this makes way more sense. Don't go by grid voltage quotes, go by cathode current and setting a near 70% plate dissipation. At 0.22v, assuming 25 ohms with the pot centered, that's only 8.8ma, very low! Now you have near what the datasheet claims for the 6BG6 of 48 ma.
"The schematic showed -45V at the bias source AND -45V at the grid which must be wrong, after passing thru resistors. "The grid draws no current. The two voltages should be the same. The factory simply had this amplifier set well below what would later become more typical for AB1. Many so-called AB1 amplifiers are in fact closer to Class A. This one really is AB1, where only a small percentage of total current flows without excitation. I would recommend caution before increasing the current with stock components, as the power supply and output transformers might not be up to the task.
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Buy Chinese. Bury freedom.
Edits: 06/14/17 06/14/17
I think you meant Class AB2. More like solid state. With the tubes barely on.
The "2" has nothing to do with where you bias the tubes.It has to do with how hard you plan to drive them.
When the driver stage is up to the task you can drive the output tubes into grid current (grid positive WRT the cathode). That's the "2" part.
"With the tubes barely on" is Class B (or at least what they call Class B).
The "barely on" part is just to keep the notch distortion down.
True Class B (rarely used for audio) is when the tubes are both in cutoff (not just in the cutoff region but in real cutoff ie; no current flowing) in the absents of input signal.
Now, just to start a flame war, in true Class A the tubes are never allowed to enter the cutoff region (the region approaching cutoff where the linearity of the tube goes to sh_t or the non-linear region close to saturation).
Class of operation is more than just the bias point. It's how the tube is biased, how the tube is loaded and how hard the tube is intended to be driven.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/16/17
Nothing to do with SS, have no idea what you mean. What I said is accurate.
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Buy Chinese. Bury freedom.
nt
So about 16 watts per tube. Equal to around 60-65% of max, in that topology.
I want to get that higher. Still tweaking.
PS: 395-0-395 > 5V4 rect > 20uf 500V film > 5H choke > B+ 403VDC > 50Uf > 900ohm 5W > 50uf > 4Kohm > 334V screen, etc etc
Maybe try a 5U4GB rectifier, if PT can handle extra current needs? A bit more B+. Readjust bias to desired output.
Edits: 06/14/17
Thanks for advice. I have both 5U4 and 5V4, and 5V4 gives slightly higher B+. I have GZ34 to try as well.Now at 405v plates, 1.20V at cathode, -32V at grid. Pretty close.
Adjusting bias, I can get the B+ to go up, but cathode voltage drops to around 1V.Rough calculation of current: 1.21v / 26ohm (each side of 52ohm balance pot) = 46.5mA ?
Anyway, not worried about it much.
Edits: 06/14/17
Odd, the 5U4G should make a bit more voltage than the 5V4G. At the cost of drawing 3-amps from the PT (versus 5V4's 2-amps).
With -49V between grid and cathode, the tubes are very near cutoff. Check the published curves and you'll see what I mean.
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Buy Chinese. Bury freedom.
There shouldn't be any red plating.
Let's assume the 50 ohm pot is in the middle. That means each tube has 25 ohms to ground. .222/25=.0088ma. If anything the tubes are running way too cold.
Dan Santoni
Thanks, I was wondering if that was the calculation. The spec is supposed to be 1.2V at cathode, which would be 1.2 divided by 25ohm = .048, about right.If I turn my bias adjust all the way to the other extreme, I had -26.6v on the grid, the cathode read .660v and the plate voltage plummets down to 294V. But cathode reading was moving in the right direction
Solved it mostly, by bypassing a dropping R in the pi filter which bumped up the B+ and now I can adjust for 1.2V at the OP tube cathode. I can raise B+ further if I go from 5R4 to 5V4 rectifier.
Thanks to responders.Edit: The Sams Photofact schematic said -45V on grid which must be wrong, but it was what I had to go by. -33V is dialed in nicely.
Pete
Edits: 06/13/17 06/14/17
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