Hum increase with modded EV Jr.?

[KrankKall]

Hello people!!
I have a version 2, EV Jr. head amp.
Since day 1, it always had a very slight hum....to be expected I suppose from an SE tube amp?
After doing a few modifications (changing resistor values of R1 and R2, plus replacing the stock OT with a Hammond 125CSE), the amp is significantly louder.
The very slight hum from the stock amp has increased slightly (still not significant though) in the modded amp.
I'm assuming this is normal with the increased gain.....but I could be wrong?
Any comments will be greatly appreciated!
THANKS!!

Steve

[ZehQuait]

Hey Steve!

Do you have schematics for this amp?
Then I could take a look, cause I'm not familiar with the EV jr and don't even know what R1 and R2 are all about

[mooreamps]

I think this could probably be normal. With the normal R1 , R2 mod, if it's increasing the input impedance from 64K to 1 M, yes the pre-amp is going to be more sensitive to ambient noise, and probably the 60 cycle magnetic field being radiated by the power transformer.

-g

[ZehQuait]

+1
after looking the schematics I also think it's normal since you're making the amp more sensitive...
maybe I would try a 68K/1M pair for R1/R2, like the normal Marshall inputs... but expecting a little more 'hum' anyway

perhaps you could try to improve the filtering on the power supply?
what do you guys think?

[KrankKall]

Seems to be many different values for R2?

Epiphone in its Version 3 has: R1 = !m and R2 = 68K
However, if you dig up the many EV Jr. mod threads....some have R2 = 10K.....or R2 = 20K....or even R2 replaced with a jumper wire?
So I guess the value of R2 is....... "whatever" works for an individual?

Steve

[tubeswell]

Don't particularly know what an Epi Valve junior layout is - my guess is its a fender champ clone of some sort. In which case if it hasn't had any of these things done, you might want to consider them (if isn't laid out with PC boards)

If your heater winding is 6.3VAC, you can wire the heater circuit in parallel to each side of the heater filaments of each bottle, and attach the Heater winding CT to the Output tube cathode pin (to raise the ground above the other grounds in the output section). If there is no heater winding CT, you can emulate one by putting 100R from each end of the heater winding to ground (or to the output tube cathode pin). Keep the parallel heater wires twisted together to cancel out the 6.3VAC EM hum.

You can also ground the amp with a split grounding scheme. Group the preamp filter ground, the vol ground, preamp cathode grounds, grid leak resistor grounds, and input ground(s) all at one input jack. Run the other filter cap grounds along with output tube cathode ground, and grid leak ground, and mains AC ground and HT CT to one of the PT chassis bolts at the other end of the chassis. This reduces the chances of voltage differences inducing noise into the more sensitive input part of the amp.

To further reduce noise being picked up in the sensitive input circuit, you can put the 68k (or whatever size) input resistors right on the input jacks and run shielded cable for the other end of the input resistors to the grid pin of V1. Ground the shield at the input jack end only.

[mooreamps]

I have done quite a few Valve Junior mod's, so I'm pretty familar with it's layout. The version 2 does have dc filaments, but it's a small filter cap. Here is what I would try first.

1. Install a bigger filter cap for the dc filament power supply ; up to 6800 uF.
2. Replace the first high voltage filter cap with 100uF @ 450 volts. The stock value is 22uF.
3. Don't mess with the grounding. They already use star grounding inside the chassis.

Give is a go.......

-g