RE: bias
You can put your 75 ohm back in and put various 2 watt resistors that you may already have in your stash in the 220 to 330 ohm range in parallel with it. This will allow you to dial in quickly without having to pony up for a single 10 watt for experimenting. The way the current splits in parallel the 2 watt will be perfectly fine in the amp.
You can even put the larger value resistor on a switch like some modern AC 30s have. Your 75 ohm would be employed on hot stages or outdoor gigs. You can switch a 330 ohm in parallel for roughly 62 ohms (which seems to be the modern compromise for torturing EL84s just shy of death at 120Vac+ from the wall in AC 30 style circuits). Current splitting is roughly 80/20 with that config, so 2 watts power rating is as good as 10 watts.
RE: Screen resistors
It seemed like not much technical decision making went into your choice of 470 ohm.
This isn't a knock on you, but an opportunity to put some more rigor into your choices on this in the future. Datasheet max screen voltage is laughably below where just about any guitar amp runs them. Study the function of the screen in a power tube and you'll quickly see why this is a huge area of concern. Tubes aren't cheap anymore, not like even just 5 years ago. So you can attempt to extend the life of your tubes at the expense of a bit of perceived increased compression "under the fingers" (and measurable on a scope if you're ever curious) by reducing the voltage on the screens, even at elevated plate voltages. There are multiple schools of thought on how this is done best: increasing the 4 screen resistors at each tube, replacing the 4 screen resistors with a single larger value and power rated resistor, or a bit of both, a larger value resistor of higher power rating feeding 4 individual screen resistors at the tubes. I prefer the latter. I like this because I can do significant voltage cutting with the resistor up front and retain the benefits of discrete screen resistors. For more info there are discussions at length at the 18watt forum. Those guys live and breathe EL84s. All that to say, if you find yourself wanting to experiment with other screen voltages it's perfectly valid and very easy to put a 5w resistor between the existing screen resistors and the screen node. On AC 30 style circuits (read most Dr Zs, Matchless, Bad Cats) I find 1.2k before the split (@100 ohm) gets me screen voltages around 320v, the common schematic value for vintage Vox. Play around with this to determine if the change in tone is worth replacing your tubes every 6 months instead of every 4--assuming a busy gig schedule. Combine this with the hot/cool bias switch detailed above and you have an amp that is capable of prolonging tube life and growling with the best of the AC30s at the flip of a switch.
RE: coupling and cathode bypass caps
This is valuable to understand if you're going to be seriously working on and voicing tube amps. Think of cathode bypass capacitors as high pass filters with gain and coupling caps as passive low cut filters. The bypass cap allows you to select which frequencies you want to boost and a coupling cap sets which frequencies you want to allow to pass to the next stage. There is a lot more going on technically including destructive feedback, frequency dependent impedance, DC coupling, and phase shifting... But not necessary to fully understand to make useful voicing changes in an existing circuit. Both capacitors have the effect of reducing low frequency content, but they aren't doing the same thing. Also know that these are essentially flat filters, but the rolloff isn't instant. It gently rolls and extends over more of an octave in the low frequency range than the high. See log frequency plots. This is why you'll see factors of 10 or even 100 differences in the values of these capacitors. In lowish gain vintage amps like AC30s and Black/Silver Fenders I prefer to do my low frequency shaping by cathode bypass capacitor value tweaking and leave my coupling capacitors as full frequency if they were initially selected to be so. I find this leaves more of the character of the amp intact and gives me just the shaping I was looking for--just my opinion/preference though. But try a 3.3uF in place of the 22uF on v2a of your Vibrolux sometime and be amazed at the ability to turn up your bass knob and not flub out--but it still sounds like a Fender Vibrolux. Just remember you're shaping/cutting the low end and this gives you the impression of increased brightness. Tweaking the values of either or both of these capacitors alone will not add brightness. For that you'd have to increase the gain of the triode along with the capacitor changes or add a cathode bypass cap where none previously existed.
Topic: Another 1st, Another Big Project - JMI Vox AC30 (x2) (Read 22588 times)