class a

Turning a Crate V33 2x12 combo into a more powerful little tone machine.

Hello Jeff,
Thanks for all the useful information each month. I’m hoping you may have some insight that would help me. I have Crate V33 2x12 combo—a 33-watt, class A amp running EL84 power tubes. It’s not a bad little amp—especially considering the deal I got on it. But I am looking to turn it into a more powerful little tone machine. I have swapped the preamp tubes from the three stock 12AX7s that came in it to a 5751 in the V1 position, an EF86 in V2, and a 12AU7 in V3. I also pulled two of the EL84 power tubes, so it is running at 18 watts. This allows me to drive the power section harder and that contributes more to the tone.

Next I plan to change the stock speakers to a couple of Warehouse Guitar Speaker models. The amp is currently looking for an 8 Ω load. Would you suggest I get two 8 Ω models and run them in series in order to run the power section even harder? I’m also wondering if you could give me a few mods to make the tone stack more effective. In particular, there doesn’t seem to be much response coming from the mid control. I am looking to enhance the “British” feel, which this amp already has.

Thanks for the suggestions.

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!

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The Underground Custom is an evolution of the Underground 30 that incorporates different capacitors and resistors, adds a Mid control, and includes a negative feedback loop that re-voices the amp significantly.

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Clips recorded with Gibson ES-335 Historic, Planet Waves Custom Pro Cables, Shure SM57, and Apogee Duet into GarageBand.
While many collectors acquire guitars and amplifiers, Tony Bruno is constantly on the search for capacitors and resistors, both new-old-stock (NOS) and modern. Bruno keeps a library of these parts in the upstate New York shop where they become the guts of some of the most coveted boutique tube amps in the world today. Some of these amps—like the Underground 30, a mid-powered machine, and the Cow Tipper Pro II 22, a 22-watt, 6V6 blackface Fender-inspired model— have already become legend. Bruno’s reputation even led to a gig working with Vox on the design of new amps.

Bruno’s latest offering under his own name is the Underground Custom, an evolution of the Underground 30 that incorporates different capacitors and resistors, adds a Mid control, and includes a negative feedback loop that re-voices the amp significantly. The amp is the product of Bruno’s restless need to refine his work, and that drive has won over more than a few high-profile guitarists. One of Bruno’s most prominent fans is country superstar Brad Paisley, who set aside his beloved Trainwreck in favor of a new Underground for recording the leads on his forthcoming album, This is Country Music. Given this endorsement and Bruno’s work to date, we were super-psyched to review the amp.

Gorgeous and Stone Solid
The Underground Custom is one handsome amp. Our review model arrived clad in black vinyl (though the amp can be ordered in a variety of coverings), which contrasts nicely with its silver grille and white vintage-style chicken-head knobs. The Bruno also manages to combine Fender, Vox, and old Marshall styling cues without looking derivative. On the grille you’ll find an especially tasteful flourish, a simple Bruno logo in an art deco–style font that underscores the amp’s vintage aura and classy, upmarket feel.

Bruno spared little expense in building the impressively solid Underground Custom. The head’s chassis is made from heavy-duty steel and the cabinet is constructed from finger-jointed solid pine and 13-ply Baltic birch baffles, which discourage rattling and unwanted vibration. Both the head and cabinet have thick, padded handles for comfortable hauling.

Inside the Underground Custom, the handwired, point-to-point circuitry incorporates CTS pots and other high-end electronics. The tube sockets are made of ceramic, and everything is held together with stainless-steel screws and bolts.

The non-master-volume Underground Custom has a somewhat unconventional tube configuration—five EL84s and four 12AX7s. However, that extra EL84 isn’t for power, it’s a reverb driver. Like the Underground 30, the Custom features about 36 watts of class-A power. But unlike its predecessor, the Custom has a solid-state rectifier in place of a GZ34.

The Custom’s front panel is elegant and simple. At the far left is a single 1/4" input, followed by three controls for the built-in reverb—Mix, Tone, and Dwell—that will be familiar to anyone who has used a standalone Fender Reverb unit. To the right of the reverb controls are the amp’s Treble, Mid, Bass, and Presence knobs, followed by a pilot light and the standard standby and on-off toggle switches, plus a ground switch. On the back of the amp are 1/4" outs for 8Ω and 16Ω speaker configurations.

Positively Killer Sounds
I auditioned the Underground Custom head with a half-open Bruno cab equipped with two 12" Celestion G12-65 speakers. Plugging a recent Gibson 1963 ES-335 Historic with factory Burstbuckers directly into the Custom, I was blown away by the amp’s power. Set on 2, it seemed as loud as other 30-watt models do at much higher volume settings.

Bruno says he conceived and voiced the Custom for lead playing. So I first tried some single-note lines in a variety of idioms, including blues-rock and modal jazz. The Custom did indeed impress in all these contexts, delivering a warm and spongy sound filled with blooming harmonics and uncommon definition and sustain.

The Custom offers a great deal of clean headroom too. Pushed past 5 or so, it growled richly, yet the sound was smooth and liquid. The amp was thrillingly responsive in this setting. And when I eased up on my pick attack, the sound cleaned up dramatically. It’s a touch-sensitive amp, to say the least.

Since Underground fan Brad Paisley is known to wield a Tele, I plugged in a 2001 Fender Custom Shop 1963 NOS model. On the bridge pickup, the Custom added warmth that negated the pickup’s tendency toward shrillness. And countrified bends and chicken pickin’ sounded especially robust when emanating from the Custom.

Switching back to the ES-335, I came to respectfully disagree with Bruno’s characterization of the Custom as being primarily a lead amp. Triadic harmonies sounded plush and full-bodied, while more complex chords rang out in perfect agreement, with none of the murkiness sometimes encountered on lesser amps. For chord work, the Custom responded equally well to flatpicking and fingerpicking, arpeggios or block voicings. And the amp would willingly range from cutting to mellow, depending on pickup selection and how I manipulated the guitar’s tone and volume controls.

As the control set for the reverb section suggest, the Custom’s reverb is inspired by Fender’s legendary stand-alone units and their awesomely echoic sounds. Whereas a standard built-in reverb has only a level control, the Bruno has additional dials for adjusting reverb tone and duration. This lush reverb is perfect for adding anything from subtle depth to super-wet surf insanity—all without the noise often encountered in vintage units.

The Verdict

Tony Bruno’s Underground Custom may be an evolution of the Underground 30, but it has the potential to become just as lust-worthy on its own merits. It’s made from super-premium components, it’s all handwired, and it has an uncommonly solid build. While Bruno conceives of the Underground Custom as being voiced for lead work, the amp’s warm, lush sound lends itself to an impressive variety of contexts. If you have the dough, this amp can fill many roles and is capable of everything from biting Vox-like lead tones to spongier and cleaner Fender sounds. It’s built to last, too. Which is a good thing: Once you play through the Underground Custom, you may never let go.
Buy if...
you’re looking for a boutique, handwired tube amp with a powerful lead voice and gorgeous rhythm tones.
Skip if...
you’re a high-gain player, or only vintage will do, or you’re short on cash.

As reviewed $3299 (head) and $979 (cabinet) - Tony Bruno Custom Amps -