After reading your articles over the past year, I’m convinced your advice will help me make some good decisions. I recently did some “horse trading” and ended up getting a Carvin Belair 212 with Celestion Vintage 30s for $40. It is stock and the clean channel [channel 1] sounds great, but the dirty channel  is muddy.
I practice in my basement, rehearse with my band in the drummer’s basement, and gig everything from small bars to large private clubs. Here’s what I’d like to do: Clean up the dirty channel and have a way to switch or reduce output to allow for use in small rooms. I’ve done some research and found some mods that promise to do this, but I want the opinion of an expert. I’ve attached the mods in PDF format for your convenience. (Also, does altitude have an effect on tubes? Most of the bars here are at about 5,000 feet, but we have some clients that have a bar at almost 10,000 feet.)
PG reader Joseph Centeno scored this Carvin Belair 212 with Celestion Vintage 30s for $40. Here, EAST Amplification’s Jeff Bober guides Centeno through some of the Belair mods he found online.
Thanks for reading, and for writing. Hopefully, my column and the rest of Premier Guitar has given you some insight, enjoyment and useable information. I hope to be able to do that here, as well.
I’ve read all the modifications you’ve supplied me with, but lacking the space to print them all along with my comments, I will simply address each style modification one at a time and give you my take on them. Let’s get to it!
Clipping Diodes. Your first concern was to alleviate the muddiness of channel 2, and for this you submitted a mod that would remove the clipping diodes (D1, D2, D3, and D4) from the circuit. This is probably a good idea. These diodes are placed at the grid of a preamp tube—in this case, the third gain stage V2A—to artificially cause the waveform to clip at a predetermined level. This technique is used to give the illusion of more gain or overdrive, but it can also affect the tone and responsiveness of the amp. A similar technique was used in a particular manufacturer’s Silver Jubilee amps, which are rumored to have given a particular top-hat-wearing guitarist his unique tone, but that’s probably not what you’re hearing or interested in hearing here. Remove them, and the dirty channel should sound more natural and open. However, do not forget to install the recommended resistor in their place, or the tube will not operate properly. Be aware that this channel may now appear to have less gain, and you may need to increase the drive control setting to compensate.
Tone-Stack Modification. By cleaning up some of the darker and probably overly compressed distortion in channel 2, you are more than likely looking to make it an overdriven extension of the clean channel. This mod should re-contour the tone stack and help make the dirty channel sound more like the clean one, because the schematic reveals that both tone stacks are identical, though each uses somewhat different component values. What’s immediately noticeable in the mod you’re considering is the difference between the values of the R15 and R16 resistors. In the dirty channel, resistor R15 is a 47k Ω and R16 is a 22k Ω, while the corresponding resistors in the clean channel, R4 and R15, are 150k Ω and 100k Ω, respectively. Initially, it appears that changing these two dirty-channel resistors to match their counterparts in the clean channel should result in identicalsounding tone stacks. However, as the modification later suggests, this may not be the case. This is partly due to the fact that the channels aren’t identical—the clean channel’s tone stack comes immediately after the first gain stage, but in channel 2 it is placed after the third gain stage. The other difference is that the second halves of the tone stacks are presenting different loads on the signal, causing a different frequency response. While the clean channel utilizes a 120 pF treble capacitor and a 1M pot, the dirty channel uses a .0022 μF cap and 100k pot, respectively. My recommendation would be to start by using the suggested values of R15 (100k) and R16 (47k) and see how you like the results. If channel 2 still sounds dark, try changing the value of C21 from a .0022 μF to a 100 pF cap. If you want to go a step further, change the treble pot from 100k to the same 1M value found in the clean channel.
Cathode Resistor Bypass Caps. This modification deals with increasing the gain of the second and third gain stages in the dirty channel. Adding a bypass capacitor in parallel with the cathode resistor of a triode gain stage will increase the AC gain of that stage, so this should help make up for any apparent gain lost by the removal of the clipping diodes. But be careful here—you can overdo it! The modification suggests installing 4.7 μF capacitors across R10 and R13. The larger the capacitor value, the more low frequencies are amplified, and a 4.7 μF cap is a pretty full-frequency cap. Some amps use cathode caps from 25–250 μF. This will increase the low-end response of the particular gain stage, but it is only useful up to a certain point. Unless the amp is capable of reproducing considerable low end—which is determined by other stages in the amp, as well as its output configuration— the effect tends to manifest itself in a “flubby,” loose sound. Generally, the more gain stages in the circuit, the more this becomes apparent, so don’t go overboard here. If adding these caps muddies up the sound at higher gain settings, you may want to consider smaller values, such as a 1 μF, a 0.68 μF, or even a 0.47 μF. The other option would be to eliminate one of the caps. Try each approach and see which result you like better.
That’s it for this month. Next month we’ll finish up with a couple more mods, as well as a power-reduction suggestion. Till next time!
Jeff Bober is one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff recently launched EAST Amplification, and he can be reached at firstname.lastname@example.org.