Epiphone Valve Junior - best mod designs?

You may want to check out this site: http://www.valvejunior.com/

Dan

OK, as to attenuators, there are 2 basic types: resistive and reactive. The resistive types simply use a big power resistor (or 2 or more) as a load to replace that normally provided by the speaker to absorb some or all the power. Reactive types use some form of inductance and capacitance along with pure resistance to more closely mimic what the amp 'sees' when pushing an actual speaker.

The reactive types tend to have a more natural sound and playing 'feel' than the purely resistive types and tend to be more expensive. The resistive types are simpler and are cheaper. Weber has some good info on attenuators as well as some inexpensive quality kits that would be cheaper than going out yourself and buying all the parts.

http://www.tedweber.com/atten.htm

As to modifying your amp, you're a bit limited firstly by the fact that there's just not much there in the first place. The EVJ-type amps are a very simple single-ended (one power tube) Class A cathode-biased amp. They don't get much simpler. There are a few components whose value you could change to play with preamp gain, voicing, etc.

Besides the empty spot discussed on the valvejunior.com site for a high-frequency 'brightness' cap, if you look down at the schematic, look at the cathode of each tube (the bottom-most tube element in the drawing) and notice the resistor and capacitor in parallel going to ground. Now, for the power tube, the value of the resistor I wouldn't change much, as this controls the idle current of the tube and reducing it by much will cause the tube to burn out quickly, and possibly damage the amp. The cathode bypass capacitor, however, might be a good candidate to increase in value. 50uF is a typical value in EL84 Class A amps. This will have the effect of increasing the gain of the power stage and lowering the frequency where the gain peaks.

In the preamp stages, the cathode resistor values can be played with a bit, keeping in mind that approx. 800 ohms would be a minimum value, and 3.3 would be about maximum. Also, the value of the capacitors may be played with to alter gain and frequency response, or even eliminated.

If you decide to change cathode-bypass capacitors, make sure to pay attention to both voltage ratings, and *especially* polarity! Electrolytic caps go **BOOM!!!** if installed backwards!

As to the power supply mods shown at the valvejunior.com site, the DC filament mod would be worth doing if your amp has hum problems and has an AC filament supply. The addition of two 100uF filter capacitors in the high-voltage supply I have a slight problem with. A total of 220uF is massive overkill, firstly. That high a value adds very little additional noise reduction, and hammers the transformer and rectifier diodes when the amp is powered up, as with that much capacitance, there will be a high inrush current drawn until the capacitors charge up. I would change the stock 20uF first capacitor to a 47uF, or 80uF at maximum. It might also be worth changing the last filter cap to a 33uF, as this is the leg that powers the first preamp stage.

The other place where some experimentation can be done is with changing the value of the .022uF inter-stage coupling capacitors. They're the ones connected from the junction of the preamp tube plate and 100K ohm resistor going to the next stage. This will have an effect on the amount of signal coupled to the next stage and also the frequency response. The 100K ohm plate resistor from each stage going to the power supply can also be modified, anywhere from about 82K to 220K, although at higher values the amp may become unstable. Adjusting a combination of the values of coupling caps and plate resistors along with cathode resistor and cathode bypass cap values may yield some desirable results depending on your tastes.

Please keep in mind that even though the amp is very small, we're still talking about **LETHAL HIGH VOLTAGES** being present, even after the amp has been turned off, as the power supply capacitors can store a charge for surprisingly-long times! As a matter of fact, I would recommend adding some 220K ohm resistors across each of the power supply filter capacitors as 'bleeder' resistors to safely discharge the capacitors once power is removed. Even with bleeder resistors, wait a few minutes after cutting power and unplugging the amp and short the capacitors to ground with a clip lead to make sure. If one simply *MUST* make a measurement or adjustment on a live amp, *PLEASE* keep one hand in your back pocket or in your waistband behind you! This will prevent a circuit being made across your arms and through your heart!

Remember, it only takes *ONE* mistake when dealing with lethal voltages to become a statistic! NEVER EVER work on a live amp ALONE!!!.

Cheers and Merry Christmas!

Strat

Thanks for linky Dan!

Strat . . .wow, thanks for all that! I can see what you're saying with all that - I shall check out the Weber kits too. As for working on the amps, yeah I know what you mean! I once stripped down and serviced a high powered flashgun for a camera (I was and still am into photography) and in the process gave myself a number of pretty hefty shocks, one of which sent me off the chair I was sat on :shock: so I learnt the hard way about the charge that caps store! I also had a number of small circuits I built with electolytic capacitors in them and was made very aware from the beginning about the importance of correct polarity and voltage ratings, so you can rest easy I'll be ok on that count! :)

I shall do some more investigation now I have a few leads to follow up. Thanks again for the information!

Those flash caps are useful for amp experimentation on the cheap. There are a lot of things one can get out of dumpsters or from curbside that can be useful, like transformers from old TVs and microwaves. The high voltage warnings need to be multiplied several times for them!

I'd a lot rather just build a little amp and tinker with the component values and circuit than to start modding a VJ extensively.

It's possible to do a master volume. I've done that in mine .. I have to look around for the pointer to the site I used. I also changed the resistances on the input stage and some bypass capacitors.

The only thing I did to mine was a quick and dirty DC filament mod to reduce the hum. (I have an early combo with impressive built-in hum.) But I could live with the humming. I don't want to mod anything else. I love the straight 1-knob concept. This is a retro amp, just like the little lap steel and practice amps of 60 years ago. Only with more gain.

It's a great little amp. I used to have the combo and now have the head and cab. The only mod I do is a non mod. I run a pre-amp / processor into it. That gives me my flexability and I let the amp provide the tube tone.

Thanks for the replies everyone. :-)

Cripes! I've just been playing some lead stuff with it cranked all the way around . . . the volume is incredible! I've not really played it cranked properly before now! The tone is really quite beautiful!

I think an attenuator of some sort is probably my primary mod. This is a new amp so there's no hum I can detect, so my guess is it's a mk2 version.

What a cracking piece of kit though, even at our UK prices (this was £79 for the Grainger, about $160 USD). :D

Yeah, it sounds great cranked. Pretty loud in the house, though.

Yeah you're not kidding! I'm playing in my bedroom because that's where I keep my best sound system, and that couldn't keep up with it on backing!!!

If this is what a 5W valve amp is like I shudder to think how loud a 100W stack is cranked!! :shock:

I have been running mine through a vintage alnico Jensen 12 speaker cab and along with the added volume, it makes the tone a bit more open.

If this is what a 5W valve amp is like I shudder to think how loud a 100W stack is cranked!! :shock:
Slightly more than twice as loud, with equal speaker efficiency.

If this is what a 5W valve amp is like I shudder to think how loud a 100W stack is cranked!! :shock:
Slightly more than twice as loud, with equal speaker efficiency.

Ha ha, yep true, I'd temporarily forgotten about the logarithmic laws of decibels! I'm blaming this rather nice Australian red wine . . . . 8)

Yep, they crank. 8)