Tube amps, part 1

After my guitar, I got myself a fancy… okay, I got myself a cheap $120 amp. I considered at a fancy $100 Roland Cube 15X (a good amp), but decided to go for an Epiphone with just a volume dial. I sorely need an equalizer for this, seriously; even just a three-band would help loads.

So why chose a 5 watt amplifier with just one dial over a 15 watt modeling amplifier with lots of gain, built-in effects, and a 3-band equalizer? One word: Tubes. My new amp runs on vacuum tubes instead of solid state silicon electronics; specifically, a 12AX7 preamp and an EL84 power amp. I just wanted to try out a tube amp, so I went with this nice little Epiphone Valve Junior Combo over the Roland Cube.

I love the tubes. This amp emits a lot of loud noise when you crank it. It acts different at different volume levels. It responds vastly different depending on tone knob, pickup settings, or guitar volume settings. An equalizer would let me cause all kinds of interesting sounds to come out, whether I put a box in line or open the thing up and solder a couple more dials onto it. I threw down $15 for a pair of Hearos high-fidelity earplugs so I can hear it clearly but not kill my hearing, just so I can crank it to where I want.

The amp didn’t have the best sound at first, but it did have good sound. I found out that Epiphone likes to bias the EL84 too hot, causing it to sound muddy and not give the clean overdrive I wanted; I decided to get that fixed just as soon as I could get new tubes. I also decided to look inside the circuit to check for common ground path; I heard that the input jack has its own ground path straight to the case. I also wanted to get some extra power filtering in from the input power supply (i.e. the wall), and replace the OT transformer with a 125ESE Hammond to match the impedance to the EL84 and in general get better sound.

For tubes I ordered a JJ EL84 from Eurotubes, and a matched and balanced JJ ECC83S Gold Pin 12AX7. These guys test their tubes for all kinds of stuff, make sure they sound good, they’ll even pick specific tubes for you if you i.e. order a JJ EL84 and tell them it’s for a Valve Junior to play Heavy Metal and you want a good crunch halfway up.  I’ll get the amp bias fixed when these come in; if possible I want to have the tech replace the resistor with a pot for dial-adjusting the bias, and then get a bias testing tool from Eurotubes to help me check over the bias on my own periodically.

I cracked open the amp and slid the head right out to get to work on that ground connection. The ground lead for the input jack goes nowhere on the circuit board; instead, the lead where the wire to the board comes from also has a second wire going to the body of the jack, for grounding to the case. I broke that away and created the jump shown at; mind you, leaving the original ground in place creates an even worse buzz. The changes worked pretty well, with a good deal of hum vanishing right off.

The next change required a run to the electronics store. I picked up two 450V 400uF capacitors ($11 each!) and did the power filtering mod from, though with four times the capacity. This raised the filter circuit from 60uF to 860uF, contrast also with the original mod’s 260uF capacitance. Note that most power supplies use a 200V capacitor, so the stock electronics already exercise healthy overkill.

Later I want to order a Hammond 125ESE from Radio Daze or Tubes and More.  This OT gives better frequency response for the power tube, and also has 5K impedance to match the EL84 instead of the 7.5K of the stock transformer.  That will ready my amp for a fresh bias adjustment and new tubes, and hopefully give me the best sound I can get.  I considered doing the DC tube filament rectifier but I heard that changes the tone a little and I’d rather not do that right now; perhaps with a switch later?


~ by John Moser on February 4, 2008.

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