Hi Bryan,
Thanks for your question. I hear that the V33 is a cool amp that many players have modified over the years, but let’s address your particular changes and questions.

I see you have begun swapping preamp tubes. This is a good place to start to make changes to the gain structure and tonal characteristics of most amps. You mentioned installing a 5751 in the V1 position in place of the stock 12AX7. This is a good way to lower the gain a bit in the front end and would generally allow the amp to clean up better when a guitar with lower output pickups is used—or better yet, when the guitar’s volume control is turned down. You can achieve so much sonically by subtly adjusting a guitar’s volume control, yet so few players take advantage of this.

If you feel the need to lower the gain even more, try a 12AT7. There are also other fun tubes to try, such as the ECC832 and ECC823, which are essentially one half of a 12AX7 with the other half being equivalent to a 12AU7. The 832 and 823 are actually mirror models of each other, so feel free to try each and see if either achieves a result better than the 5751 or 12AT7.

As far as your V2 selection of an EF86, I’m sorry but this tube will simply not function in this amp. The stock tube for this position is a 12AX7, which is a dual triode. An EF86 is a pentode style tube and has a completely different pin out configuration that will simply not work if installed in this socket. Besides that, even if you were to rewire the socket to accept this style tube (which is not an easy task in an amp with circuit board-mounted sockets), you would be losing an entire gain stage, since the triode has two but the pentode only has one. This tube serves as a sort of mixer for the clean and dirty channels, as well as a tone stack driver. While these functions could possibly be accomplished by one gain stage with some modification, I would recommend against it. If you wish to experiment with alternate tubes in this position, a 12AT7, 12AU7, ECC832, or 823 would work.

With respect to the V3 position, you mentioned you installed a 12AU7. That tube will certainly function in this position, but being it is the lowest gain of all styles of tubes that will work here, you will have the most reduced signal level driving the output stage. This would translate into more signal level required from prior gain stages in order to drive the output tubes to the same level. Not a bad thing at all, but just know that it definitely changes the feel and touch sensitivity of the amp. This, of course, is all personal taste, so experiment with a 12AX7, 12AT7, and 12AU7 in this position, but steer clear of the ECC832 and 823 here. The two different triode sections would substantially unbalance the output stage making for an asymmetrical output, which is not very good for quality clean tones.

Moving on to your output stage and speakers, you mention you’ve removed two of the EL84 output tubes so the amp would be running at 18 watts. First, I hope you have removed the correct tubes, which would be one of the tubes from the left pair and one from the right pair. Removing two from only one side would result in no load on one half of the output transformer— a condition I’d advise against.

Also, with a pair of output tubes properly removed from the amp, you have now changed its output impedance. Since you have in essence halved the load on the primary of the output transformer, the secondary of the transformer expects to have half the load on it for everything to be balanced. This means that your output impedance is now theoretically 16 Ω, so I would recommend installing two 8 Ω speakers wired in series for the correct 16 Ω load.

Now let’s finish up with your tone stack question. Looking at the schematic, I see there are a few components at the bottom of the tone stack that are not very typical. These components serve to lift the tone stack from ground, rendering it less useful. I assume the FET and associated .001 μF capacitor and 68.1k resistor are used in a “boost” function, which in the boost state would partially lift the tone stack from ground, thereby increasing gain while making the tone controls less functional. This is not unusual. What is unusual is that the remaining two components— the .047 μF capacitor and 220k resistor—serve to keep the stack lifted from a direct ground connection during normal operation. This may be causing the tone controls to be less active.

I would try replacing the 220k resistor with a jumper wire, or if possible, simply installing a jumper wire across the resistor. This would give the tone stack a proper ground reference, hopefully returning some additional functionality to the controls. As far as enhancing the “British” sound of the amp, the capacitor values in the tone stack are pretty typical British values, so no change needed here. Just FYI, you will lose some gain as a tradeoff. If this proves to be too much of a loss, try going back to a higher-gain tube in the V2 position, if you’ve already installed a lower-gain tube.

Hope your Crate crushes!