Sounds like you have what you were after now with 400V on the plates with full line voltage, correct?
ViperDoc wrote: ↑Fri Jun 03, 2022 8:02 amWhat confuses me most is the relationship between Ohm's Law and the fact that when I reduce the tail resistance of my bias supply to a minimum (turn the bias pot to zero), the grid voltage becomes more negative. Conversely, when I increase the resistor value on the other side of the circuit, the bias AC voltage feed for example, the grid voltage also goes down. Please excuse me a 2 A.M. unicorn WTF, but do you see how confusing this is? So when positive voltage drops across a greater resistance, it becomes less positive. When a negative voltage drops across a lesser resistance, it becomes more negative at the source?
There really is no difference, but it can be confusing. People are constantly saying things like -45V on the grid is higher than -42V, which is not correct. If the average low in February in Fairbanks is -13F, and it's -3F in International Falls, which city has the lower average temp?
Ohm's law works either way, i.e. (Vin - Vout) = IR, but the direction of conventional current flow is always from a higher to a lower voltage because resistance cannot be negative. That means that the current in the bias circuit must flow from ground and to the more negative voltage nodes, and that is probably the missing piece.
martin manning wrote: ↑Fri Jun 03, 2022 4:36 pm
Sounds like you have what you were after now with 400V on the plates with full line voltage, correct?
Ohm's law works either way, i.e. (Vin - Vout) = IR, but the direction of conventional current flow is always from a higher to a lower voltage because resistance cannot be negative. That means that the current in the bias circuit must flow from ground and to the more negative voltage nodes, and that is probably the missing piece.
Yes, more or less 400 V with 120 at the wall. What's "funny" this morning is now my wall AC voltage is 117. I can't win.
OK, thanks also for the negative voltage explanation and current flow. Makes more sense. More negative is lower, of course. It's interesting that the negative bias DC voltage is created by running an AC voltage through a backwards diode, which increases in magnitude AND switches polarity when rectified, but is still "lower" relative to ground. A bit of a mind bender.
Another thing I noticed is my bias adjustment range is now quite massive. You have a way of surgically pinpointing exact bias circuit values. How do you do that?
ViperDoc wrote: ↑Fri Jun 03, 2022 5:23 pm
It's interesting that the negative bias DC voltage is created by running an AC voltage through a backwards diode, which increases in magnitude AND switches polarity when rectified, but is still "lower" relative to ground.
Yes, but basically just a mirror image of the case with positive voltage at the output.
ViperDoc wrote: ↑Fri Jun 03, 2022 5:23 pmYou have a way of surgically pinpointing exact bias circuit values. How do you do that? :mrgreen:
You can make estimates using simple math, especially if the circuit is close to the typical arrangement, but for accuracy a simulation is by far the easiest route. Even then there are some unknowns like the winding resistances. The different ways to adjust the bias level affect the range and linearity of the adjustment. Generally its best to adjust the resistor in front of the rectifier to get the minimum (most negative) voltage you need, and then the rest of the circuit can remain more or less the same, which maintains a typical adjustment percentage.
Still working out the kinks on this amp, though it's sounding very good. I installed Merlin's DC-coupled CF circuit with the 100R plate to grid, the 10K V2B grid leak-to-cathode +1N4007 and made the V2B cathode resistor 68K. It added a nice bit of compression that really sounded good when turning up the overdrive, yet added quite a bit of sibilance that was off-putting. I just detached the circuit (except I left the 100R on the tube socket) and it sounds calmer, most of the sibilance is gone. I see in Merlin's book how to tailor the compression by reducing the V2B cathode resistor. How do you get the compression but tame the brittle added tone? Low pass filter off the grid? He mentions this DC-CF for "high gain amps", and this 5F8A surely is not that, but thought I'd try it.
Thanks to Colossal, I made some massive improvements in my PI. I previously noted better clipping when I reversed the PI plate load resistors, so I ended up putting a 68K/82K on with a 10KL balance pot and was able to get a 6-volt spread on the PI plate voltages. It sounds much better now, less strident. I even thought 4V sounded good.
I also was having trouble with low wall voltages since I installed the mosfet B+dropper, so I just removed it and was reminded why I got into it in the first place. If I drop the AC wall voltage with the Brown Box, I can get my B+ where it needs to be, but at the expense of my 6.3V heater voltage. How critical is the 6.3V supply? If I'm dropping 5-7%, that makes for a 5.85V heater voltage. I know that variable wall AC voltage is unavoidable, but how do you best tackle this? It seems that a 117v primary would be the best way to design in the future, just because I could predictably get there on any day of the week.
martin manning wrote: ↑Fri Jun 03, 2022 2:32 am
Yea, as mentioned above, the bias voltage will be reduced (becomes more negative) along with the B+, so you will have to make some adjustment to your bias circuit.
If the B+ reducer is going to stay in the amp long term, and the B+ is reduced a sufficient amount, the negative voltage at the transformer center tap can be used as the bias supply.
Using the same circuit on the center tap to do both the B+ dropping and the bias voltage supply is probably simpler than keeping the stock bias circuit alongside the new B+ dropping. That's more of a long-term thing though, sounds like there's a few things that also still need tinkering.
Last edited by nworbetan on Sat Jun 25, 2022 6:40 pm, edited 1 time in total.
ViperDoc wrote: ↑Sat Jun 25, 2022 5:51 am
Thanks to Colossal, I made some massive improvements in my PI. I previously noted better clipping when I reversed the PI plate load resistors, so I ended up putting a 68K/82K on with a 10KL balance pot and was able to get a 6-volt spread on the PI plate voltages. It sounds much better now, less strident. I even thought 4V sounded good.
I would look at the PI balance with a scope rather than guess with voltage difference.
ViperDoc wrote: ↑Sat Jun 25, 2022 5:51 am
I also was having trouble with low wall voltages since I installed the mosfet B+dropper, so I just removed it and was reminded why I got into it in the first place. If I drop the AC wall voltage with the Brown Box, I can get my B+ where it needs to be, but at the expense of my 6.3V heater voltage. How critical is the 6.3V supply? If I'm dropping 5-7%, that makes for a 5.85V heater voltage. I know that variable wall AC voltage is unavoidable, but how do you best tackle this? It seems that a 117v primary would be the best way to design in the future, just because I could predictably get there on any day of the week.
Data sheets will say +/- 10% on heater voltage, so no worries if it's above 5.7.
I took the mosfet dropper out because my wall AC all of a sudden has been averaging 117V. All I need is a 117V primary and I think life would be a peach. I’m stuck with a 120, so I’m back to dropping the 120V 3% or riding the 117. Just that my whole package drops with it.
martin manning wrote: ↑Sat Jun 25, 2022 6:37 pm
I would look at the PI balance with a scope rather than guess with voltage difference.
Data sheets will say +/- 10% on heater voltage, so no worries if it's above 5.7.
Thanks. I just discovered that my voltage spread on my PI is drastically higher when using my dummy load than when hooked up to speakers. INCONVENIENT!!! I've been using a Weber Tru-Load. With speakers, I've got a 5V spread on my PI, but with the dummy load I've got a 15+V spread. How do I get around that? I also notice my rectifier tube works harder with the dummy load in place. How do I get good scope readings? When I dime one or both gain knobs, one side of the wave form plateaus wide and flat, the other side only a bit, and on opposing sides as you would expect across the PI tube. The clip region was narrower with higher plate voltages. I might like that better. Too bad I can't A/B that very well.
The PI balance will change with signal level. You can balance it wherever you like, but the standard I've been using is to balance it at 50% of max clean output power, feeding a clean sine into the PI input to avoid any second-order harmonic distortion coming out of the preamp.
martin manning wrote: ↑Sat Jun 25, 2022 7:09 pm
The PI balance will change with signal level. You can balance it wherever you like, but the standard I've been using is to balance it at 50% of max clean output power, feeding a clean sine into the PI input to avoid any second-order harmonic distortion coming out of the preamp.
