That is the conventional way, isn't it? It's better actually, since most use half wave rectification. Just a bridge into a CRC into the bias system.
Clearly other things are powered from the supply too though (4) otherwise why 7 watts for a 200R resistor for bias?
Clearly other things are powered from the supply too though (4) otherwise why 7 watts for a 200R resistor for bias?
Thanks, uh, if ~350 is too high for the screens, what's a better value that's "way" lower?
The tubes seem to be "content" with their individual 470 Ohm cathode resistors. I wanted to run fixed with the cathodes essentially grounded, which led to the red plate run away on the one tube. Unsure if I dare go back (I could) even after replacing the two grid resistors in that socket.
Cathode bias takes some of the heat off the tube, so generally it’s safer. And any increase in current causes the bias to automatically back off. Maybe some combination bias, where it’s adjustable to balance out unmatched tubes but still drops voltage on a cathode resistor?
IMO 350 is too high a vg2 for any EL84 type. G2 ratings need to be respected, even if you‘re running everything else down a bit. Most tubes will take higher plate than you’re supposed to use if you bring g2 down below its maximum. If you brought it down under 300 they probably wouldn’t red plate at 400V with fixed bias. But it does lower the current capability. Those *86’s I bought to play with will in no way take 350V on G2. But they show a decent amount of plate current at 170 on G2. I was thinking 170/340 volt supply, which is easy to make on a budget, rather than having to dial G2 into something more difficult to make and control. With tubes that are only going to be rising in price.
IMO 350 is too high a vg2 for any EL84 type. G2 ratings need to be respected, even if you‘re running everything else down a bit. Most tubes will take higher plate than you’re supposed to use if you bring g2 down below its maximum. If you brought it down under 300 they probably wouldn’t red plate at 400V with fixed bias. But it does lower the current capability. Those *86’s I bought to play with will in no way take 350V on G2. But they show a decent amount of plate current at 170 on G2. I was thinking 170/340 volt supply, which is easy to make on a budget, rather than having to dial G2 into something more difficult to make and control. With tubes that are only going to be rising in price.
Be careful if you try combination bias. The last time I tried it, I had nasty crossover distortion and never quite got it clean.
WHY?
You don't get any more power within ratings. It's 17 Watts at 4% THD either way. It is 18W at guitar-amp rating, and the major EL84 "18Watt" g-amps run cathode resistors. If they got "louder" in fix-bias, they would... but it doesn't so they don't.
If you treat/beat them like 7189 you get a 24W claim at 400V fix-bias. Leaving aside the question of which new EL84/6BQ5 tubes will survive 7189 abuse (it can happen), the difference 17 vs 22 is not amazing. And the over-bias to make 7189 live a whole day at 400V ensures crossover distortion.
I beg to differ. If you have cathode bias and you waste 30V on the resistor, that 30V can be used for power in fixed bias... You just need to find the 30V for that somewhere else. The extra voltage can be used for power...
OTOH, cathode bias would be the same if you boost B+ by 30V, right?
OTOH, cathode bias would be the same if you boost B+ by 30V, right?
It's a refurb of an old Fisher amp. All the cathodes were already connected together, on their way to the now non-existent heater string in the preamp. So I figured a connection to ground therein with negative grid bias is the same as a little + grid bias with a bigger than 12V drop to ground across the original heater string. So I started this thread to find out what some techniques were for generating the negative bias voltage.
Figured I could even make use of the original "output bias" control - how cool would that be to still have in place and functional?
It was more about using as much as the amp as wired, to avoid "molesting" the original factory connections. Especially to the tube sockets. I've since taken the stops off that idea. Now it's got additional terminal strips soldered to the chassis to carry extra circuitry. I expect by the time I'm done with it, it'll be "Fisher" only in OPTs, the PT and chassis hardware.
Received the replacement 6BQ5 yesterday, makings different (Nestone) versus the other 3 (Sovtek), even though all came inside Nestone boxes, with "25" written on the box. This new tube sets up the same cathode voltage drop across 470 Ohms, about 14V, as the others. So that part's done.
Now, to drop the screen voltage, which will require more drop across the OEM resistor, the first in the B+ string. I was hoping to use that, but clearly Fisher's screen voltage choice isnt the best, so I'll be replacing it with a bigger value. When I do, I'll conclude this thread with a summary of what I've ended up with.
Now, to drop the screen voltage, which will require more drop across the OEM resistor, the first in the B+ string. I was hoping to use that, but clearly Fisher's screen voltage choice isnt the best, so I'll be replacing it with a bigger value. When I do, I'll conclude this thread with a summary of what I've ended up with.
It turns out I had 30 - 40 24V zeners, 0.5 W. I calculated the power dissipation at 25mA; close enough; figuring if I drop the voltage by 50, the current will drop some too. A string of 4 didnt make it past initial turn-on; 3 shorted. Ended up with a 5K resistor, to take the screens to 300V. Now, to fix the voltage level on the input and driver 12AX7s, as they dropped 50 with the screens, being downstream in the power supply B+ scheme.