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|12/6/2005 2:18 PM|
||Killer mods for Epiphone Valve Junior amp|
Erik Miller wrote up the mods he did to his Valve Junior in this article:
However, I decided to take a different route and try to emulate the current issue Gibson Les Paul Junior amp. This involved changing the 100k plate resistors for the 12AX7 to 220k and replacing the power supply resistors and adding a 22pf cap in parallel with the 1M resistor ahead of the volume control.
Following the suggestions in Erik's article I isolated the input jack and boosted the 22uF/450v cap at the first node of the power supply. I also tried Bruce Collins' trick of connecting the two 100 ohm resistors on the ac filament supply to the cathode of the EL84, which seems to have lowered the noise level a bit, but I still get some hum when the volume control is set higher than 12 o'clock. As someone here pointed out the filament supply connects to the circuit board right next to the 3 pin plug that goes to the volume control. So I'm going to try switching the filament supply to DC... (I'll post the results of that here.)
Thanks again to Erik Miller for drawing up the schematic and writing up the article on his mods!
P.S. So how does the amp sound (other than the hum when I set the volume above 12 o'clock)? Absolutely killer! The stock Valve Junior is a very dark amp- with the Les Paul Junior mods, the amp is very bright, but you can adjust the tone control on your guitar to take the edge off. I haven't tried a REAL Gibson Les Paul Junior amp, but I suspect that this sounds a lot like it. (I have a hunch that Gibson/Epiphone intentionally screwed up the design of the Valve Junior so as to not compete with the LP Junior- like wiring up the input resistors as the low gain input to a Fender amp!)
P.P.S. I noticed one mistake in the schematic that Erik drew up - the two resistors on the input (R1 & R2) are reversed. (In the stock amp they are both 68K so that mistake would be easy to make.) Not to sound ungrateful- Erik's schematic saved me several hours. In fact I wasn't even going to try to modify the stock board until I read his article- I was going to make up a new eyelet board and wire it up as a Champ with an EL84 instead of a 6V6.
|12/6/2005 5:11 PM|
Hmmm... I wired up the dc filament mod and the noise got worse. I used 1/4" quick connect terminals so I switched back to the ac filament supply and the noise was still there. I then snipped off the 100 ohm resistors and tried the dc filaments again but the noise was still there.
Somewhere down the line I think I screwed up one of my ground connections... And it sounded so good this morning (as long as I keep the volume under 12:00 o'clock). Darn!
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|12/6/2005 6:03 PM|
||Egg on face...|
No problem with the amp— it was 100% operator error! I had grabbed an unshielded speaker cable to plug in my guitar so it was humming like a sonofabush...
It is very quiet now with the dc filament mod. With only a single 12X7 and EL84, I needed to use a 1.0 ohm 2 watt resistor to bring the filament voltage down to 6.3vdc from ~7.25vdc. (In my Pignose G40V I used a 0.22 ohm resistor to do the same, but it uses 3-12AX7's and 2-6L6's.)
Now I'm wondering if I should have left the two 100 ohm resistors in there which were connected to the cathode of the EL84...
|12/6/2005 11:45 PM|
||Recommendations for 8" speaker|
The Valve Junior comes with a "Special Design" 8" speaker- no numbers on the speaker at all but it is very light and "airy"- with a lot of sound per watt.
I just tried putting in the 8" Celestion speaker that came with my Kustom Tube 12 amp, which makes the stock speaker sound like a transistor radio. However it doesn't have a lot of highs so if I'm going to keep it in this amp I will have to revoice the preamp a bit. Like with the 1M series resistor which is paralleled with a 22pF cap, I'll probably want to up the value to maybe 50pF. And maybe put a 50pF-120pF bright cap across the ungrounded terminals of the volume control since it does brighten up a bit when you crank it up.
This speaker is not as loud as the stock speaker, but that's okay because I think that it gets too loud if you turn it much past 12 oclock. With the Celestion you can turn it all of the way up and it still sounds sweet and mellow.
There is not a lot of details on the Gibson LP Jr speaker, except that it is an 8" Gibson Goldtone ceramic-magnet speaker. I think that I do want to stick with a ceramic magnet in this speaker. So I'm just checking to see if there is something that would work well in this amp-like the Celestion, only not quite as dark and muddy.
|12/11/2005 2:52 AM|
||Jensen P8R sounds great!|
I didn't like the way the amp sounded when I revoiced the bright circuitry so I took the 50pF cap off the volume control and replaced the 50pF RC cap with the 22pF one I was using originally.
I had some dope that Ted Weber had sent me so I figured I might try doping up the surrounds on the stock speakers. It seemed to make them a little bit less airy, but nothing to write home about.
So I tried putting in a Jensen P8R reissue and that sounded really great! Some of their reissues aren't that hot but this one sounds really nice.
|12/11/2005 8:17 PM|
|Frank De Salvo
||Breaking longest self-perpetuating thread!|
|12/11/2005 11:19 PM|
Thanks for the post but I've talked to myself a lot longer than this thread... I believe I racked up a dozen posts without getting one reply from anybody else.
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