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In Reply to: RE: 6BG6G mono amp sounds great but posted by PeterI on June 13, 2017 at 16:14:31
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.
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.
Buy Chinese. Bury freedom.
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.
Have Fun and Enjoy the Music
"Still Working the Problem"
Nothing to do with SS, have no idea what you mean. What I said is accurate.
Buy Chinese. Bury freedom.
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.
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.
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