Valve Junior intermediate results -- Anyone do amp mods?

[BluefoxIcy]

More of my obsessive modding; this is all pretty much in planning. I'll tell you how it goes when I get all the stuff put together right. If you got any good ideas, lemme hear it.

(Yes guys, I'm still planning on buying that Roland Cube 15X. And I've got a couple of components I have to get around to soldering into my guitar.)

I got a Valve Junior Combo amp, but I think it's a V1. It's a great sounding amp but it's got some nasty buzz; nothing I can't fix. Anyone here into modding your own amps?

A little history on the Valve Junior. First off it's a tube amplifier, so... yeah. If you've never had a tube amp, you can try to find a Valve Junior (head or combo -- remember, combos like mine put stress on the tubes; you may want the head if you have a speaker cabinet!) but make sure it's got a 15 digit serial number because it will sound MUCH better; these are the V3, they have better circuitry, better stock tubes, and the OT is matched to the tubes properly.

Valve Jr. V1 used Sovtek tubes (like mine). Horrible sounding crap. They also use star grounding for the input jack (more on this later). They ALSO use an OT Transformer with 7.5k impedance (WRONG! More on this later). Cruddy tubes, bad grounding, loads of hum.

Valve Jr. V2 used a Sovtek 12AX7 (preamp) and an Electro Harmonix EL84 (power amp) tube. The grounding I'm told is better. The transformer still has 7.5k impedance.

Valve Jr. V3 uses an Electro Harmonix 12AX7 and a JJ EL84. The grounding is fixed, and they use a 5k impedance transformer that sounds close to a Hammond 125DSE (more on this later).

Case in point: The V3 isn't a $1600 Marshall tube amp, but it sounds much better than the V1 and V2. Too bad I'm stuck with the V1's Sovtek tubes, bad grounding, and mismatched OT, right? Not to mention all the Valve Juniors are a little overbiased and muddy the sound coming out of the EL84.

POINT 1: RETUBING AND REBIASING

Eurotubes.com sells JJ tubes, tested for gain and harmonics and microphonics and all kinds of things. These guys get the stock, pull each tube OUT of the box, throw it into one of their amps, and play with it for a little while to determine how it sounds. (Hey an hour jam session isn't going to kill something that's supposed to last a year)

They then rate the power tubes based on where they start to break up; from here they'll recommend specific tubes for specific amps and playing styles, so you can tell them you're a bluesy king of guy and have a VJ or Fender or Marshall or whatnot and they'll pick the tube that'll break up at the right volume and give the right tone. Or you could just ask them for an EL84 #36 or #28 or whatever, if you know the grade you want.

I hit this place up for a JJ EL84 and a matched and balanced JJ ECC83S Gold Pin 12AX7, and pretty much told them where I want the 12AX7 to start to distort. The 12AX7 is the hard part; I'm probably going to go through a few different grades before I find the "right" sound (which is mostly personal opinion anyway), but I'll let them pick a good starting point.

When the tubes get here, I'm going to have a technician down the street rebias my amp. I could do the work myself but... it involves doing some measurements with the amp on (HIGH VOLTAGE! WOO! ...k amperage kills, not volts), jamming the leads of a meter into the power tube's socket (so, the exact area where all the really high power current goes through), and then doing a bunch of calculations and trying to match something to a specific voltage based on E=I/R (Ohm's Law). Soldering in the right resistor is the easy part-- the only part I can do.

Edit: When retubing, remember to use contact cleaner on each of the pins of the NEW tube before inserting (it'll clean the contacts on your amp). Also make sure all the contacts in the socket are tight; if not, re-tension them using a tiny eyeglass screwdriver or such to bend them inward a bit so they'll grab and hold. Bad contact causes all kinds of nasty stuff; in amps, it can cause tubes to crackle, overheat, or blow once in a while (wisdom I gleaned from some guy who's been servicing amps and has swapped tubes in thousands of them over the years; these may be things you never see, but they do happen).

POINT 2: BAD GROUNDING

Star grounding. In electronics, star grounding involves taking each ground point and running a jumper from that point to a common ground. You can solder the ends of all these together, and you have what looks like a star. Think star topology in networking.

In signal processing circuits, you typically don't star ground the parts of the circuit that the signal travels across. Specifically, you wouldn't ground the input jack directly to the case; you'd ground it to the ground point on the next part of the signal circuit, which would get grounded to the next ground point, which would then go to the case (ground).

For example, in a guitar (all signal electronics), the ground on the pickups goes to the back of the vol pot. The vol pot has a pin that grounds (voltage divider; part of the signal goes to ground, the other part goes out to the circuit), which also grounds there. You then jump that to the back of the tone pot. The output jack ground also jumps to the back of the tone pot. Somewhere here (usually the output jack's ground pin) you jump ONE wire to ground (usually the bridge). This ground circuit follows parallel to the hot lead; hot exits via the output jack's hot lead, and ground exits via the output jack's ground.

In the Valve Junior, they grounded the input jack's ground pin to the case. This is wrong. It has to ground to the next ground point in the signal processing circuit, so that the ground path runs parallel to the hot path; the hot path from the input jack (i.e. the signal) should not reach any gain electronics (for example, TUBES) very far from the electronics' ground point. Epiphone created a nasty ground loop and this creates all kinds of nasty buzz and hum.

Easy enough fix. I found something online where somebody modded this one out; he didn't insulate the input ground from the case though (wrong), so I'm either cutting the appropriate trace or desoldering the appropriate wire, depending on what I see when I get in there.

POINT 3: INCORRECT OT IMPEDANCE

Those tubes should be hooked up to an OT with an impedance of 5k (don't ask me why, I don't know; I read up on it and found this out). Epiphone's V3 Valve Junior uses an OT with an impedance of 5k; this sounds very close to the Hammond 125DSE that most people replace the original OT in the V2 and V1 with.

I ordered a Hammond 125ESE (better frequency response) from Angela.com; this was not a fun process, I phone-ordered it because they don't have a secure way to pass credit card information to them (i.e. plain text e-mail was the primary method). $36 plus around $10 shipping; I could drive to the place (it's in Annapolis, not too far away) but it'd cost me more than $10 in gas.

I'll replace the existing OT before I have the amp rebiased, and drop in the new tubes like right before taking it to the tech.

Closing

So what do we have here?

• Valve Junior - $130
  
• JJ 12AX7 Gold Pin Matched/Balanced - $23
  
• JJ EL84 - $10
  
• Hammond 125ESE - $36
  
• Rebiasing - $48
  
• Shipping - $20
  

So $130 amp with $69 of mods, $48 of labor, and $20 of shipping costs. $199 if you can do it all yourself, $247 if not, $267 in the end for me.

I still want to mod a 3-band equalizer into this thing but I'll have to research that first; that's probably going to be some $17 a pot + $4 of resistors and capacitors and jumper wires for each knob, so $63? (It's a tube amp; messing with the tone pot on your guitar drives it nuts, an EQ is going to give you a LOT of control).

References

Do your own damn research. Here's some of what I got.

• Epiphone Forums
  
• http://www.valvejunior.com/
  
• http://www.thegearpage.net/board/archive/index.php/t-185780.html
  
• Google
  
• Some people in ##electronics
  
• Some random people I know that know electronics
  
• a little luck
  

[zircon]

All music gear or tech questions go in Remixing... again.. moved.

[BluefoxIcy]

Well I fixed the ground, added a rectifier to the tube filaments, got the Valve Jr. V3 5.2k impedance OT, and threw in JJ tubes. No rebiasing on the EL84 yet.

Results are great. I fixed the ground first, which made minimal difference in hum. As soon as I threw the DC rectifier on, however, the hum dropped off massively! It was like getting a whole new amp

With this problem solved I moved onto dropping in a JJ tube. I actually threw in the JJ EL84 before changing the OT, a #34 from Eurotubes. A bit more of the hum dissappeared, enough to get my attention (I was only expecting tonal changes). The tone was only a bit better from this though.

The V3 OT I threw in made the real difference. The overdrive went from a weird muted-ish muddy sound to a crisp, clear kind of noise; I think I get what people mean when they say their amp has a "crunch" to it now. A JJ 12AX7 gold pin matched/balanced from Eurotubes went in to replace the Sovtek; the clean sound is clean, the overdrive still has great tone, and I like it.

I'm seriously considering redoing this with a straight V3 in the future. I could clean up the rectifier (it's not bolted to the chassis, needs longer connectors); add 8 and 16 ohm outputs; add connectors to my filter cap mod (so you can unplug the filter caps and more easily remove the board); and ebay it after picking up a V3. I want to go on and add a tone stack (and gain mod the amp since the tone stack kills gain) eventually, but would rather invest that kind of time into a shiny V3 since the board's more stable for modding.

(Mind you, a V3 Valve Junior includes a rectifier exactly where I put mine, and includes the updated output transformer).