Hello from BCM, thanks for stopping by my blog.
A week never goes by that I don't get a call, email, or text about the best way to dial in a vacuum tube guitar amplifier whether a Class AB fixed bias or cathode bias amp. I'll give you some pointers here in layman's terms that will make this task a lot easier for you than you think.
First and foremost, if you are running a solid state guitar amplifier, there isn't a lot you can do with this other than changing the gain structure, but feel, dynamics, and note blossoming won't be near as perceptible as it would be in a true vacuum tube amp.
*** Only qualified amplifier technicians should ever open up your amp for any reason as extremely high and lethal voltages exist in guitar amplifiers. Let the pros do it. This blog post just gives you some information to broaden your knowledge base ***
Fixed bias amps. Fixed bias, means, you can physically make the idle adjustment of your output tubes. This means there is a potentiometer i.e. bias "pot", that allows you to adjust the idle of the amp by raising or lowering the idle. Remember a colder bias or idle of the amp may sound harsh and flat while increasing output tube life, but in contrast, a higher bias or idle of the amp may be sound good to slightly mushy while wearing output tubes out much faster. Output tubes could be EL34's, KT88's, 6L6's, 6V6's, etc. The perfect bias for your amp is somewhere in between these settings and only YOUR ears know where that is. Marshall, Diamond, Soldano, Bogner, Diezel, Splawn, etc.. all make fantastic fixed bias amps.
Cathode bias amps. Cathode bias, means you just pull the output tubes out and replace with a similar duet or quad. The circuitry of the amplifier will handle the actual biasing of these tubes. 65 Amps, Bad Cat, etc., make some fine Cathode biased amps. Some mistakenly call these amps Class A versus Class AB, but that is rarely correct.
Now that you have your amp biased, serviced, and checked out by your amplifier tech, now we can get down to the meat and potatoes of dialing your amp in.
1. Be for real about where, how loud, and what gain structure you want in this amplifier. If you are already opting for an attenuator, power brake, power scaling circuit, 1/2 power switch, 1x12" cabinet, or turning the speaker cabinet around, YOU have too much amp. Dialing it in won't help. Find a better amp choice for your gigs, recording, or home use.
2. REGARDLESS of what you read via online or in magazines, I've proven the following MANY times in a daily working recording studio. You will ALWAYS get more tone changes in your amplifier by changing the speaker cabinet or the speakers than you will ever get by changing preamp or output tube types. At lower volumes there are some perceptible differences with tube changes, but once you open the amp up, there just isn't that much difference as GAIN IS NOT developed in the output section of a master volume amp. Gain IS ALWAYS developed in the preamp section. Don't get creative in swapping different tube types either as your amp MAY NOT allow such changes. Contact your amp manufacturer for more info.
3. The secret of a MASTER volume amp is the fact that your gain IS ALWAYS developed in the preamp section of that amp. This circuit uses some version of a 12AX7 or ECC83S type preamp tube. If all you do is run the master way down and preamp way up, sure you can quietly shred for days like this, but you'll NEVER cut through in a full band setting. You have to get the Master volume up high enough to let the amp breathe. That is where your tone lies as well as sustain for each note or chord, feel, dynamics, and overall blossoming of notes.
If your amp has a GLOBAL master volume, that means it allows you to set the gain and master volume of each individual channel, and then run the finishing or global master volume up for your total volume. These work great when overall volume needs to be down some. Just remember to turn the individual channel masters up to mid way or so otherwise your amp will sound asleep. On amps like the Soldano Decatone 3 channel amp with a global master, if you don't run the channel masters up, you may not even hear the amp, why? Answer - That is because running the amp on 11 is 100 watts or total amp output where at 8, you may only get 25 watts. Then if you roll the global master volume back down too, who knows how much output you have, but it won't be much.
If you are running a NON-MASTERED amplifier like a 4 input Marshall Super Lead, JMP, or Plexi, then do you know where the best 2 sounding dial in points are of this amp? Answer - Amp OFF or WIDE OPEN on the master. Why, because the gain structure of this amp is best when the amp is pushed really hard. Hendrix played on 10 and many other 60's and 70's major label artist's did as well. I personally have a custom 2x12" ISOLATION speaker cabinet for my own all original 1969 Marshall Super Lead Tremolo 100 watt head, and can vouch for this very well.
4. Pros do the following on a routine basis, so you should all try this as well. Roll your guitar volume back to 7-8 or so and leave it there when you dial your amp in for your rhythm licks or clean but over driven tones. Then when you want to play that stinging lead riff, roll it wide open.
5. Start with the clean channel first. Put all of the EQ, i.e bass, mids, and treble at 12:00 o'clock. Set the gain if available where you like it, and then turn the master volume up. Once you get a good tone even if way too loud, move each dial of the EQ from its lowest to its highest setting several times to hear the differences. USE YOUR EARS, not the numbers under each knob, but just because you run the bass all the way up to 10 doesn't mean you are getting that. The amp may be mushing out on you and unable to handle that much bass.
When you feel good about your EQ, bring up the Presence control if installed for how much BITE you want your amp to have. This will really help with how your amp "Cuts" or sound in the overall mix if in a band setting. Then if you have a Resonance control or Depth control, bring that in as well if you choose, but remember most of these 2 controls use frequencies below the standard bass dial setting. You may not always want that.
Always remove the casters on any speaker cabinet if you can. If not, you are losing bass frequencies. Consider going to a semi open backed cabinet if you want less compression of your sound and a more open dynamic feel. Vintage amps love cabs like this just like my own 1948 RCA 10 watt Tweed Bassman styled amp. I had Soldano build me a custom one off semi open 1x12' cabinet for this amp and now that rig sings.
5. Now we come to the over drive, lead, or hi gain channel whatever you want to call it. You can basically do the same thing we did with the clan channel, but this time you can really spend that extra time on the gain structure. Make sure you aren't over saturated or have too much gain so as you don't cut through the mix in a band setting. You'll find that once you start getting good tube breakup and the grit is starting to come alive in your tone, you'll stop right there. Remember a guitar is a MID RANGE instrument. Remove the mids or presence and you'll fall out of this mix to where you may not even be heard. I ran my own Soldano SLO-100 next to a buddy's Mesa Dual Rectifier with both of us at arena volumes using 4x12" cabinets at a gig. He had his mids almost non existent. My SLO tone with mids on 6 and presence at 7 put his amp to sleep. I never even heard his amp.
Now that you have the basics down, stop looking at the numbers under the dials of your amp, and always use your ears. Put the settings where you like best, but NEVER cut the mids unless you are looking for that metal or hard rock tone from the 80's and 90's. Cranking the volume to get back into the mix in this setting won't work either because any good Front of House sound man is going to back you right back down in the PA system anyway. Then you'll look silly with way too much on stage volume.
Give me a call any time you need some help dialing in your amp. We'll get you dialed in right every time.
Now, TURN IT UP with a fabulous tone, and make some great music.
Blues City Music - Owner