Valve Junior mods

[ChewingAluminumFoil]

I had posted a while back about some mods to reduce the hum in the fun little $120 Epiphone Valve Junior all tube guitar amps.

 

I had some free time this weekend and wrote up a fairly detailed web page detailing those mods and also delving a bit into my grand theory about Mixed-in-air Stereo Chorusing.

 

http://www.coolpick.com/way/cool/about/valvejunior.html

 

I've included circuit diagrams of both the amp stock and with my little mods. These are darn fun little amps if you're prepared to bust out the ol' soldering iron. Don't buy one unless you either have a high tolerance for hum or are prepared to do some fixing, but the end result is a fun sounding amp you can crank in a bedroom or small rehearsal space. And for me, they're an ideal solution to the problem of finding a stereo pair of amps you can turn up to the point of real tube distortion without adding to that certainly most undeserved reputation us guitar players have of always playing too loud.

 

Enjoy.

[Guitarzan]

cool.

[Ricochet]

Pretty cool!

 

I've had mine apart, but haven't done anything with it yet. I established that the source of the 60 Hz hum on mine was the printed circuit traces for the 6.3VAC heater power closely parallelling the trace to the V1B grid. The only fix for that is to build a little DC heater power board and put it between the power transformer and the plugs for the heater power on the board.

 

The only time hum bothers me is if I've got to leave the thing turned on for a while when I'm not playing. No standby switch. One could be added, of course.

 

Light bulbs in series or series-parallel combinations do make great attenuators! Series light bulbs are volume compressors, parallel ones are volume expanders.

 

The extra power transformer primary leads for different supply voltages are still there inside the amp if you ever need to move to another country, and more importantly the 8 ohm speaker leads are hanging right there, too, in case you want to rewire the 4 ohm output to 8 ohm or install an extra 8 ohm jack. Doesn't hurt to run 8 ohm speakers from the 4 ohm jack, though, it just adds distortion. I've run a 4x12 Fender cab from mine, cranked wide open, and it sounded great. Loud!

 

The stock speaker is a very nice one. I've been told it's a Weber Ceramic 8. I put a Jensen MOD in my Electar Tube 10, but I have no intention of ever changing the stock speaker in the Valve Junior unless it craps out.

[picker]

How about an effects loop similar to what was on the old Mark II & III series boogies? Can it be done?

[Ricochet]

Adding an effects loop is a fairly tall order. This is an extremely basic little amp, nothing more than two stages of voltage amplification in a single 12AX7, with a volume pot between them, and a single EL84 for output. If you want bells and whistles like an effects loop, it's better to start with an amp that has them or build one from scratch, because that's pretty much what you'd have done by the time you added it.

[Ricochet]

Back to the modded Epis:

 

Have you checked the heater voltage on that DC rectifier setup? As shown, if the power transformer's got sufficient output it should run up to about 9VDC. May be enough resistance in the 6.3VAC secondary winding on the transformer to keep it down if it's really marginal in current capacity. Else, you'll likely have to add some series resistance to get the voltage down to 6.3V or so. 6.3V tubes aren't happy being run at 9V and won't last long. 7.0V is really pushing them and is about the max you can get away with for reasonable tube life.

[EmptinesOf Youth]

sorry that i cant really contribute, but how hard is it to put in a standy by setting?

[ChewingAluminumFoil]

Else, you'll likely have to add some series resistance to get the voltage down to 6.3V or so. 6.3V tubes aren't happy being run at 9V and won't last long. 7.0V is really pushing them and is about the max you can get away with for reasonable tube life.

I measured the resultant filament voltage at almost exactly 6 volts DC, a tiny bit lower than normal. Should be totally fine.

 

And in response to the standby switch question, to add a standby switch, just put a switch in series with the wire going to the T2 connector on the circuit board. Of course don't work on high voltage amps unless you know what you're doing and always discharge the caps. This amp has no bleeder resistors so those caps stay charged for a LONG time after you turn off the amp.

[Ricochet]

Originally posted by ChewingAluminumFoil:

I measured the resultant filament voltage at almost exactly 6 volts DC, a tiny bit lower than normal. Should be totally fine.

Yeah, it should be fine. I guess there's enough voltage drop in the silicon diodes to keep it from rising to the peak voltage of about 9V. That's encouraging. I think I'll rectify my heater supply, when I get a round tuit.

[ChewingAluminumFoil]

Yup, you should, lots less hum with the volume up. It's two parts, both avail at Radio Shack.

 

The caps on the mains supply really are worth it tho too, altho to get the 450v rating you'll need to order from Digi-key.com or the like. You wouldn't even need to remove the main circuit board to make both those changes.

 

Oh, careful pulling the filament supply leads off the lugs. They're stuck on pretty good and I broke one loose from the PC board trace trying to wiggle the connector free. Easy to repair but a nuisance.

 

Always discharge the main caps with a screwdriver blade. I can't emphasize enough there are no bleeder resistors in the amp and those babies stay fully charged for quite a long time.

[Ricochet]

The volume doesn't affect the hum level on mine. Like I said, it comes from the heater supply traces being right next to the V1b grid lead, that's the return from the volume pot.

 

Good point about discharging the filter caps. I do that with something like a 1K resistor, so it won't arc.

 

The larger the caps you put in, the higher the peak current through the rectifiers. Not usually a big problem with silicon diodes, but you can and will fry rectifier tubes if the caps get too big. The biggest you can get away with on a 5U4GB, for example, is about 50 uF. Less often taken into consideration is that the same effect works on the power transformer, with the volt-ampere rating required going up rather markedly as the input capacitor goes up. The transformer will heat more. I think the low rating of this power transformer is part of why the heater voltage didn't go too high with the simple bridge rectifier and no added resistance. There's a table somewhere back in the rectification chapter of RDH4 showing the ripple voltage with different size filter caps. The gains in ripple reduction from increasing the first filter cap much past 12-16 uF start dropping off pretty rapidly for typical power supplies. On my amp, I'm sure it'd be quite acceptably quiet if the heater hum (60 Hz, vs. the ripple's 120 Hz and higher harmonics) were hushed. I expect a little hum out of a tube amp, anyway.

 

Fact is, it'd probably be just as satisfactory to me and a whole lot easier if I just put in a standby switch to mute it.

[MILLO]

bump

[ChewingAluminumFoil]

Another guy has done a Valve Junior mod page:

 

http://www.euthymia.org/DIY/VJmods.html

 

Also linked to from my page:

 

http://www.coolpick.com/way/cool/about/valvejunior.html

 

If you've never modded an amp before, this is a fun one to cut your teeth on. And what are you out if you mess up? Hey it's just $120. And really it's almost impossible to seriously harm a tube amp. It *is* a bit easier to harm yourself so always discharge those filter caps with a screwdriver blade or something before working on the amp, and observe the one-hand-in-pocket rule: if you keep one hand far away from the amp chassis you'll reduce the probability of running some high voltage across your chest. I've taken 550v across my chest and been fine, but I wouldn't recommend it to everyone.

[Ricochet]

Discharge those caps with a resistor of 1K or so. It's a lot easier on the equipment. No arcing.

[Joe Bratz]

Quuestion about amp modding: Most of the mods you have on those 2 pages seem pretty simple and well-explained with the pictures and stuff. I have never modded an amp before. Can/should i try to do some of these myself? What should a novice like me watch out for?

[svokke]

Can anyone tell me what the plate and supply voltages are in this amplifier? I've got some idea's to increase gain and decrease hum, but I need some measurement of how much voltage I can work with. Can't measure it myself, my valve junior hasn't arrived yet. Shipping is taking soo long here, can't wait to heat up the soldering iron,...

[Ricochet]

On my Valve Junior, it's about 360V supply voltage, and just a hair less at the EL84's plate and screen. I don't have my notes handy, but that'll get you in the ballpark.

[svokke]

That's quite high for a single ended EL84. Anybody tried to stick a resistor between the diode bridge and the first filter cap? That would improve filtering and bring down B+ a few volts.

[Caevan O’Shite]

Originally posted by Joe Bratz:

Quuestion about amp modding: Most of the mods you have on those 2 pages seem pretty simple and well-explained with the pictures and stuff. I have never modded an amp before. Can/should i try to do some of these myself? What should a novice like me watch out for?

Hmn, electrical safety is paramount, both while you're working on the amp, and how the results of your work affect safe and dependable operation later when you're playing through it on your guitar.

 

How are your soldering and wiring skills? Ever done ANYTHING even vaguely related?

 

Much of this is not rocket-science, but it does need to be done right, and done right the first time. Mistakes and poor workmanshiop can result in anything from mildly poor tone or noise problems, to your amp becoming a worthless pile of smoking remains, or even YOUR DEATH.

 

I'm not trying to talk you out of it, in fact I want to encourage you. But I don't want you to suffer at all, in the least, due to any oversights.

 

If you're not totally comfortable with a soldering iron (no solder "guns"), or working around potentially live circuits (capacitors store electricity, even when the amp's unplugged), get with a friend who is familiar enough with soldering/de-soldering and electronics work, and have them work with you, showing you how to do things safely, and correctly, with good workmanship.

 

A number of us here can answer many specific questions for you, or give you all kinds of good information on safety and soldering and electronics, but I'm never 100% comfortable telling someone what to do along these lines via the 'net; it feels like those scenarios in old movies where someone's doing surgery on an island while a medical Doctor directs them step-by-step over a radio!

[AnthemBassMan]

Newbie here. I've been checking out the mod site for my Valve Junior. I love the tone, but now it's time to lessen the humming. I've already done most of the mods to it, but was wondering one thing. What size is the bridge rectifier used? I can't find any reference to the size, or does it even matter as long as it bolts to the chassis? Nice page by the way. And I too was going to buy it just to rip it apart and recab it with a different speaker. But if the stock one is the Weber that I've read before, it sounds pretty sweet. Thanks again.

 

Matt D.

[unclearty]

I just picked up one of these little beauties and am planning to do a few of the hum mods this weekend. It was mentioned here that there is a mod to add a standby switch. Would someone be willing to post a diagram of exactly how to add this switch?

[fuzzfoot]

Hey, couldn't you just use a spiral filament preamp tube like the Sovtek 12ax7LPS, Electro Harmonix 12ax7EH or the JJ/Tesla ECC83S to deal with hum? I haven't tried it yet, but these are marketed as being designed to reduce hum in AC filament circuits. This might be an easier plug&play-style fix for less experienced modders. I'm gonna start looking for good prices online for these and I'll let you know what happens. At least it'll provide me with some new tonal options and backups in case I blow something. Any expert opinion would be great, I've never opened a tube amp before. Just a lot of pedals.

[ChewingAluminumFoil]

I tried the EH tubes and it made no appreciable difference before the mods. Sorry. Gotta solder on this one.

[matt_s]

Just read your page on Valve Jr mods and thought of a couple of things.

 

I don't know how the transformers are located/mounted, but I was wondering if you could do any of the following to reduce hum:

1) Move one or both of them so that the cores are 90 degrees from each other in all 3 dimensions (reduce sensitivity to magnetic fields from power transformer)

2) Construct some sort of shield for the power transformer from ferrous (iron-based) metal, e.g. steel, and ground it

 

Also it may be an idea to extract the signal at various points along the signal path and see where the hum is being introduced, so you know where to focus your efforts.

[fuzzfoot]

Hey folks -

 

I just spent my saturday night installing a bridge rectifier and capacitor as per that section on your site, and that annoying high-pitched buzz is gone.

 

I didn't find the hum that noticable before, but now that the buzz is no longer masking it I know I need to get rid of *that* too. If only the local Rat Shack carried higher-voltage electrolytics. I guess I'll have to do some shopping. Maybe get some new tubes while I'm at it.

 

Amp sounds amazingly "quieter" now and *might* have a little softer distortion sound than I remember. But that's probably all due to being able to hear more of my guitar through the noise. Not to mention being in a different room. Though didn't someone mention that the filaments are running a little lower voltage than usual? Could that affect the sound? If it does I like it.

 

Anyway major props to my electrician buddy Doug, who let me use his drill press, high-powered solder gun (anyone notice that the bridge rectifier leads take forever to heat up with a 15w solder pencil), and skilled and steady hands. Not to mention many a BH from the BOD.

 

- Chris

[monkey-boy]

Greetings,

I just got my JR this weekend, and like the sound...hate the hum.

I am planning on doing some of the mods listed here, but have a question for anyone who knows:

 

Can you explain how to do the DC-Bridge Rectifier thing? I have looked at the pics, but since this is the power I don't want to mess it up and I am not experienced with this type of mod.

 

I have not issues with discharging caps, or soldering etc...but I have not had to use a bridge rectifier before.

 

This might be an obvious question, but my other option is to return it and get the valve special, doesn't have the hum, and does have a gain knob. I figure the cost is 189 +40 for a new speaker so 239, so I guess just about $100 bucks more than this one.....I got mine from GC for 99.

 

Thanks

[Broddi]

ChewingAluminumFoil,

Nice job on the mod page. I picked up one of these Valve Juniors about a week ago. Loved the Tone hated the hum. I found your site and one or two others. I'm no stranger to electrical circuits and soldering irons so I did the mods and one of my own for a switchable input impedence.

 

The hum is definitly gone. I think the tube saturation comes a little later and softer.

 

I did a mod page of my own that I can contribute. I go into pretty good detail , I am hoping people out there can use our pages to make their Valve Junior even more fun to play. Here is the link to my mod page.

 

http://www.klugemusic.com/phpBB2/index.php

 

I am considering adding a standby switch, a gain mod, and adding a sping reverb unit next.

 

Nice site CHewingAlluminumFoil. Thanks for sharing. I really like your in air chorus setup. Makes me almost want to go out and get another valve junior. My nephew has the Valve special on order so maybe I will "borrow" his to test that out.

[ChewingAluminumFoil]

Very nice job. Excellent pics, more detailed on how to wire stuff up. This is turning into a sort of a cult, huh? Pretty soon we'll be having swap meets and forming local users groups and of course we're have to work up a secret handshake.

 

To make it easier to find my page I moved it to:

 

www.valvejunior.com

 

And I added a link to your page as well.

 

CAF

[LPCustom]

That's a great page and it's nicely laid out, too.

 

Adding a spring reverb to this amp could cost more than the amp did (tube, socket, output tranny, reverb tank, bag, springs, cable, etc.).

[Broddi]

Thanks for the compliment LPCustom.

Yes I was thinking the same thing in regards to the cost of adding a reverb tank to this amp but the bass player in our band happens to have a spare tank , bag and cables. As far as power goes I have not given it a whole lot of thought yet but I would assume one of those un-used leads coming off the transformer may be of use. Here is the first page I found that talks about adding a reverb tank http://sound.westhost.com/project34.htm

 

I figured I could stick to the simple stuff first

so I just added a standby switch. And will probably add an 8 ohm speaker ext. jack next.

 

If I could only set this thing up to brew coffee I would work all night on it.

 

Broddi