Fixed to Cathode Bias switching problems...

[liquidair]

Hey

Maybe it's a coincidence that your Rc-b =330k and the screen voltage sampling resistor in the bis reg is 330k? The two are actually unrelated as in fact is the case for the Rs around the BJT bias switches. You have to think of the output of the bias reg as a voltage source and the BJTs simply as switches, tying that voltage source to the bias-set network, or isolating the voltage source from the bias-set network.

The Rs around Q9,10 are selected to allow sufficient base current that the BJTs can pull the bias pots as close to the bias reg output as possible.

There are lots of ways to have mixed bias. TUT4 shows swept fixed to cathode bias using RmX methods. Another approach is to have a variable Rk and a tracking bias reg with adjustable tracking ratio. You could also make things completely manual but safe by using much higher Rk values. Older STUDIO amps had such a feature controlled via the Fiixed-Cathode switch and a Heat switch, later combined on one 3-way switch as Tweed-Mix-Modern.

My apologies for taking so long to get back on this. Every time I wanted to come tell you and the forum that the problem was solved, the relay would stick again. I started with a 15V zener in series with a diode on the coil and that worked for a few days. I increased the zener to 33V and it worked for a while more, and a 50V zener kept it from sticking even longer, but ultimately it always stuck. I gave up on it for a while and then recently decided to just modify the existing PCB so that instead of shorting the Rk/Ck combo for fixed bias, the relay's center is grounded and the relay selects Rk/Ck or the tube cathode. This way we shouldn't have to worry about Ck's discharge current as Rk will bleed it off quickly. This has worked ever since, even with 100uF caps!

Re: Rc-b = 330k, merely a coincidence. Once I saw the ratio was ~100:1, I was concerned (unfounded) about Rb-sw being small so I just multiplied the usual 33k by 10 to allow for Rb-sw to be a bit bigger. And actually the 330k was causing an issue with power scaling where once I would dial down even just a little the distortion would sound pretty bad. Going with the 33k/jumper combo got rid of a lot of that, so thank you for mentioning that....I might not have found that as easy!

The only problem I have left in this build is this raspy distortion that comes about when driving the output tubes any higher than a mild overdrive and is most apparent when notes are ringing out. It's as if it stacks on top of the natural overdrive from the tube and has this odd swirl as it decays, and gets worse when power scaling. The output stage is a 4 tube where 1 pair can be dialed out (like in the G-M-X chapter of TUT4) and I've found that physically removing EITHER pair gets rid of it, OR the 2nd pair must be a full volume or that raspy distortion is there. I see no oscillations on the scope, no odd voltages or signals, but simulations appear to show that the "on" pair can drive the "off" pair through the plates. Peak power in the "off" tubes appear to be a considerable % of dissipation (16W), and the current waveforms appear to be FW rectified...Any clue what that is or what can stop it?