Vox AC30 V1 biasing

[Colossal]

Hello all:

As many of have seen and built Trainwreck Rockets, ZWreck/Rocket variants, Big Bottle Rockets, and maybe even the Brian May minimalist variant, you know V1 (single triode) has a 220k plate load and a 1k5/22uF cathode bias in these amps. If we assume a 290VDC preamp supply (which you can see on many Vox schematics), a 1k5 resistor will give a plate voltage of 128VDC and cathode voltage of 1.1VDC (0.73mA across 1k5).

Now, if you are replicating a true AC30, V1AB is a 12AX7 with two separate 220k plate loads and a shared 1k5/22uF bias arrangement. So with the aforementioned AC30 variants, we see a single gain stage with a 1k5/22uF, yet to maintain the same operating point, the 1k5 should really be 3k. 3k results in a plate voltage of 167V and bias point of -1.6, and this is shown on the AC30 schematics.

220k/1k5 yields a warmer bias than 220k/3k which is more centered. The Songwriter 30 uses a paralleled triode with a single 220k and 1k5, but this is equivalent to 440k/3k for a single triode. Both the Dave Petersen and Greg Fryer Brian May amps use 220k/1k5 on a single triode, again with warmer bias.

I am wondering if the 1k5 bias resistor in the single triode preamp variants was just left by the designers intentionally selecting for the warmer operating point, or it's just a mistake propagated through time when designers separated the V1AB triode in favor of a single triode and kept the same parts in place?

[Littlewyan]

Very late to this thread but I recently was playing around with the first stage bias in my Vox style amp. Mine is simply the ‘normal’ channel but with no 220k mixer resistors and I was amazed that the bias of the first stage actually affected the tone a lot. 220k/1.5k was very ‘bold’, good for distortion. 220k/2.7k was much smoother and much more ‘Vox’ in my opinion, much better for clean and edge of breakup tones. So that’s what I went with.

I suspect designers purposely go with 1.5k for slightly more gain and perhaps it sounds better with a boost pedal (in case of Brian May). Or they may prefer its tone, but with my single stage preamp I preferred the colder bias sound.

It’s worth noting that Vox did mess with the bias a few times in very early AC30s and the AC10 even ran 220k/820ohm!

[Roe]

vox used a shared 1k5 for two gainstages, something that amounts to 3k for a single gainstage w 220k anode resistor. I'm using 3k3 personally