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Blue Collar's Hot Rod Tube Amp Mods, Volume #1!!!!
By Mick Polich - 04/16/2008 - 10:29 AM EDT

Hot Rod Mods – we love ‘em for ‘classic’ muscle cars, and tube-type amps!

Internet blogs are full of homebrewers, tinkerers, and repair techs droppin’ knowledge and stakin’ a claim to offers services and ideas on how to make our gear sound better and different!

I wanna pass on some stuff I learned as a tech along the way – some of it doable for the novice, some on a tech level. But first, the All –Impo’tant Disclaimer:


O.k., much better   - felt like my dad, wife, and my high school shop teacher Mr. McCoy, all at once!

We hold a certain mystical fascination with old and new tube type amps – the digital stuff is the future, and will one day be the norm, but like a Mustang Shelby Cobra Ed “Big Daddy” Roth’s “Rat Fink”, we dig the crazy old stuff. It’s fun, it makes sense, and it’s a reminder of our Kollective Kustom Kulture Past that we draw on for inspiration. From tweed era Fenders to the high-gain Mesa Boogies, Soldanos, and Bogners of the ‘80’s, ‘90’s, and into this new millennium, it’s the stuff of music videos (gawd, the wall o’ Marshall stacks in ANY hair –band video from MTV back in the 1980’s) and countless books on the subject of collecting, restoring, and modding tube amps.

So, let’s start with a relatively simple mod for folks who want to back off on some of the gain of the amps such as the Boogie Rectifier series and the new Marshall JVM four-channel amps. This mod involves changing out pre-amp tubes up in the first, second, and perhaps third pre-amp tube slots in the amp’s front end. Most tube amps today have ‘standardized ‘ – meaning, they have gone with the classic 12AX7 tube compliment up front for gain staging. Occasionally, you’ll find a 12AU7 or even a 12AY7 tube for staging, but these tubes are usually in designs that want to emulate lower gain/ wattage amps. But, those two tube types – the 12AU7/12AY7 – are going to be used for our experiments in lowering gain (and changing tone some what) in high-gain gear.

So, grab two or three of each tube types, and start a search-and-destroy (or replace-and-listen) by locating the first two preamp tubes closest to the input jack. No need to take the amp out of the cab, but when you’re pulling tubes out, make sure the amp is off, o’ course (obvious, but you would be surprised….). I would recommend brands such as Electro-Harmonix, Tungsten, and J.J. – different for tone, and reaction in circuit. The amount of gain decrease, or ‘mu’, should be significant. The more tubes you can afford, the more you can experiment with, but that switch from 12AX7 to 12AU7 and 12AY7’s makes a big dif…..

O.k, please take these next couple o’ mods to a qualified tech – there is my disclaimer, so throw the puck on the ice and skate with it!

One signal increasing mod that I picked up from Gerald Weber (he of Kendrick Amp fame) is decreasing the series resistance in a signal path in amp designs. I’ve tried this on both tube and solid-state amps – works well once you can hear what results that you want. Usually, the place to find series resistance is either before or after a pre-amp tube stage – going into the grid or coming out of the plate of the tube, after the coupling capacitor and plate load resistor. You have to figure out what you want - a strong, overdriven signal going INTO the tube, or more amplified signal coming OUT of the tube going into the next stage. One of my favorite mods is reducing the 470K ohm resistor after the first stage 12AX7 tube on early Marshall designs –just boo-koo gain comin’ outta that first stage, really beefs up the sound!

Another disclaimer: I’m going ahead with explaining less of the terminology as I go along – the good thing is, along with a trusted, qualified tech, there are several great books out that explain tube amp design. Of course, Gerald Weber is my tone king, and I have all three of his books, along with publications from Dave Funk and Dan Torres.

There is a lot more technical information out now than the old days – one, maybe two books on the subject of musical instrument amps when I graduated from United Electronics in 1979……

The cathode by-pass capacitor in a pre amp gain stage makes a world of difference. Both Fender and Marshall set the standards, and then it seems amp companies emulated from there. Fenders usually have a 25uf@25VDC (it’s a rating –25 microfarads at 25 volts direct current) while Marshalls, depending on the channel, bright or normal, could have 220 to 330uf to .68uf. Changing the capacitor values can change the sound CONSIDERABLY….

Did I mention the POTENTIAL of LETHAL voltages in tubes?? Man, I’m more paranoid than a foreign national looking for a wire tap, I tell ya…..

Capacitors value changes, at anywhere along the circuit, can greatly affect amp tone (so again, get an amp tech for this one). Usually, I have learned in a lot of Fender and Marshall circuits, a great deal of gain and tone is changed in the first two stages thru the coupling caps off the plate load resistors and the cathode by-pass caps in parallel with the cathode resistor. Many modders and newer amp designers have incorporated capacitor changes as one of many ways to color the amp’s tone.

Talkin’ about Fender and Marshall quite a bit, eh, Polich? Well, yeah, those guys were the biggest and the first – BUT, kind reader, there are OBVIOUSLY others!!! Ampeg, Silvertone, Hi Watt, and a host of some smaller ‘mom-n’-pop’ brands have made music history. Recently, a good customer brought in a converted phonograph (the black slab of petro, ya know, old school, album cover art, and – HEY! SHUT UP, GRANDPA, AND MOVE ALONG WITH THE STORY!!! Poopy! O.k., you youngsters, always in a hurry, I tell ya….) amplifier that needed a re-tube and cleaning. This amp was a Webster model from the late 1940’s – Webster having made amps and phonographs out of Racine, Illinois. Well, I did my ‘Net research, couldn’t come up with a schematic, but thought, what the hell, clear the deck and launch the jets, so I jumped in, took a few notes, and modified the amp for more signal gain and interactive channel controls (microphone and phono – just in case, back then, you wanted to sing along with yer tuneage or start a primitive DJ gig – “Hey everybody, WHAT UP?? DJ Cornpone is in DA V.F.W. HALL!!! WHAT UP, DAAAWWGS???!! “ Er, ah, sorry, just a little old-school ‘antiquity’ humor….).

A Tip O’ The Hat And A Shameless Plug: Mojo Musical Supply makes some excellent amplifier kits, but I stress, ONLY IF YOU’VE BUILT SOME AMPS BEFORE!!! This isn’t like going to Hobby Lobby and buying an AMT model to put together on your kitchen table, but I have seen the results of the 45 watt “Marshall” clone head: A good amp repair customer of mine back in Atlanta put together a kit  - VERY NEAT AND METICULOUSLY, I might add – and the unit rawked, dudes and dudettes!!! Fun stuff, but again, not for the faint of heart (or inexperienced of soldering iron technique…). Mojo’s catalog is on-line, and you can download it for the paper experience, but I would venture to sat between them, C.E.B. Distribution in Tempe, and New Sensor Corp (home of the infamous “Electro Harmonix” pedals and such…), you can cover all your parts and cabinet needs for mods, re-builds, or amp builds…..

There are a few manufacturers of replacement transformers, output and power, that can help change an amp’s sound and performance. You can get reasonably priced trannies from new sensor, Antique Electronic Supply, or Mojo, or you can shell out a few more clams for the ‘custom’ stuff from such companies as Mercury Magnetics and Obsolete Electronics. A good, after-market transformer can upgrade and stabilize your sound – kinda like a Hurst shifter kit on a ’72 Barracuda.

I have been amazed by the quality of replacement speakers these days, especially the low-cost units. Eminence has expanded their line with great clarity and attention to detail for not a lot of money. I do have some favorites in ‘new modern’, though –Tone Tubby and Weber SVT  - but the Eminence line pulls through if you want quality for low-cost. But every time I’ve done a speaker replacement with a Weber (especially good on Fender stuff) and Tone Tubby (I’ve heard ‘em make great high-end amps sound even better, plus they can be ordered as standard-issue on models such as Crate’s V50 amps….), I’ve never been disappointed with the outcome (o.k., Shameless Plug Number TWO!!!).

Unfortunately, to get the details of all the mods here, I would need every column space from last year to make it happen – in other words, a book – and there lies the final piece o’ info I wanna lay on ya, pilgrims: some book recommendations!

#1. Any of Gerald Weber’s books or DVDs  - Gerald, who is owner, prez, and chief cook n’ bottle washer of Kendrick Amps, has written an excellent series of how-to books on tube amp mods and repair. Plus, we get bonus material from the late, great Ken Fischer of Trainwreck Amps fame! His column in “Vintage Guitar” magazine dispenses useful knowledge each month to all us amp mod maniacs out there……

#2. Dan Torres and his book on amp topography and modding – ol’ Dan has been around the block a time or two (I used to get his newsletters with great mod tips and promo for his shop back in the early 1990’s), and his book – in an easy-to-read-easy-to-understand format, is extremely helpful!

#3. Dave Funk –ah yes, the Funkster! Dave’s book covers mostly Fender circuit design topography (big, EASY to see schematics, too – love it, Davey Boy! The first things to go on a tech are the eyes, then, er, I dunno what the next thing is….). But the old ”Thunderfunk” amps are Dave’s design, and his enthusiasm regarding his subject –nee amp design and mods – is paramount in the book!

There are many candidates for amp modding on eBay – some are excellent, some ain’t so hot. Besides the industry stalwarts such as Fender, Marshall, and Ampeg, other brands such as Valvco, Silvertone, Webster, Peavey, Gibson, Magnatone, Supro, Hiwatt, and Alamo can be had for sometimes beyond reasonable bidding (and there are still NOS tubes around the country that could help them out – not saying that there will be supplies for everything, but I’m just sayin’, o.k.?).

Again, this is a fascinating area to explore – digital amps make it comfortable to have a lot of sounds programmed into an amp that sound damn close to the real thing. But tube amp nuts are like old school dudes tinkering with hot rods – always under the hood in the garage, looking to see what can make their ride BETTER!!! And, for the umpteenth time, if you don’t feel comfortable or feel safe working around high voltages - DON’T. Take my advice –it ain’t worth the potential to get thrown on your butt, or WORSE……

But have some fun, and appreciate the pioneering spirit that still exists in musical instrument amp design and engineering – Hot Rod Modders, UNITE and CRANK UP!!!!

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