SOT: List of fixes for an Epi Valve Junio

[brett]

Hi
I just finished doing some work for a friend.  He has an Epi VJ that was humming badly and I traded him for some work on my resonator.

Looking at the schematic and inside the amp, it was clear that some things needed changing.
These were my changes:
1. Rectified (WO4) and filtered (1000uF) heater supply.  This gave 6.4V of DC with minimal ripple.
2. Isolated input socket.  This took out most of the hum, which had been very prominent.
3. Changed the pull-down resistor (R1) from 68k to 470k and fitted it at the input socket.  The extra resistance improved the tone quite a lot.
4. Added a mains filter cap (0.047uF 250VAC)
5. Reduced the grid stopper (R2) from 68k to 33k.  I don't know whether this did much, but it felt good.
6. Added an extra 2 x 100uF caps to the B+ line (it only had 22uF standard).  Not sure whether this actually did much.  An extra 22uF or 47uF would probably have been enough.
6. Added an 8 ohm output.  There's a red wire in the amp which is an unused 8 ohm tap from the OT.
7. Added a power soak.  It consisted of a 3.3 ohm 5 W resistor in series with the 4 ohm speaker socket and a 1.2 ohm 1W resistor in parallel with the 4 ohm socket. This reduces peak output power from approx 5W to approx 0.25W

These changes quietened the amp a lot.  It went from being a noiseier than average amp to quieter than average.  It also sweetened the tone, and slightly increased the available gain.
cheers

[Doug_H]

1. Rectified (WO4) and filtered (1000uF) heater supply.  This gave 6.4V of DC with minimal ripple.

This is usually overkill for a low gain preamp like the valve jr and can introduce hashing noise from the rectifier. I know a lot of people seem to like to do this but unless you are building something like a soldano there's not much returrn for your effort and you risk introducing more noise.

I don't mean to rain on your parade- thanks for the list. I just want beginners to understand what they are getting into, esp if they are still developing good layout skills.

[brett]

Good point.
Ever since I went to build my first amp and saw layout with lots of twisted, chassis screened and carefully laid heater wires I've *assumed* that AC heaters were prone to problems, and never looked for any real evidence.
Also, the AC to DC conversion for the Epi valve junior is mentioned on a few web sites, and I didn't think about the concept critically or skeptically.
cheers

[MartyMart]

Brett, some of those changes are well worth doing, I'm presuming this one was
the "Old style combo" as the newer ones have addressed the "hum" issue.
My fixes to a pair of combo's were :
Reducing B+ down nearer 310v ( was almost 355v ! ) with a 15 watt metal body r
Adding an 8 Ohm out jack
Removing R5 ( 220k to ground) pre grid stopper and wiring a master volume control, 500k log
Reducing R8/R9 to 1k2 and 680 Ohms ( for more higher gain drive )
Reducing C3/C4 from 22uf to 2u2's ( less bass gain/mud )
Made R1 1M and removed R2
Reduced R6/R7 to 220k's I think
Changed C1/C2 22n's to Spragues
Changed the OT to a Hammond 125CSE equivalent

Total cost around £30 each and they are now MUCH nicer sounding little amps, running them
through a 1x12 or 2x12 makes a world of difference, the little 8 inch webber just doesn't
cut it !

MM.

[brett]

Thanks Marty
The voltages I measured were quite high.  A bit over 360, 330 and 310V (my supply is usually 240 to 245V).
My amps mostly run the preamp 12AX7s at about 250V, so 310V and 330V seemed a bit high.
But my friend likes clean "Fendery" tones and dislikes dirty "Marshally" tones, so I didn't want to decrease the B+ in case too much  headroom disappeared.
I'll mention the 125CSE to him.  He doesn't have much money, but he's a very good player and deserves good gear.  (Maybe I'll tell him I found one at the rubbish dump and would swap it for some lessons )
cheers

[MartyMart]

No worries, I HIGHLY recommend an OT upgrade, as that stock one is a little mis-matched
with a 7.5k at the output, the CSE is also twice the size, so depth of sound is improved a LOT !
more iron

MM.