Another Valve Junior mod schematic

[bluefoxicy]

Here's another one I just made up.

 

 

Highlights:

 

Power Supply

[*]Lowered C6, C9, and C7 to 10k to mimic this amp's (Fender Champ 5F1) power tube anode supply. Note C6 and C9 make up the 16uF anode cap on that amp, while C7 fills the role of the 8uF Screen Grid cap.

[*]Left C8 at 22uF because I like the VJR's response in the preamp.

[*]AC filaments, twist the AC leads.

 

Input stage

[*]DC offset filter, 0.1uF filters something like everything below 3Hz? Not a problem. 0.47 works here too.

[*]Moved R1 (now the entire DC offset correction system)

[*]Effectively lowered R1 to 500k. A 6V6 amp I built uses two 1M R1 in the DC offset correction, it sounds good to me.

[*]10k R2

 

Preamp stage

[*]Voltage divider is 470k and 1M. I had better luck with 1M and 1M than 100k and 100k, so I used high values.

[*]The 1M R7 effectively makes volume 500k (a bit more complex than this, but overall...).

[*]Used 470k R6 to raise gain and encourage preamp overdrive, but not a lot.

[*]Gain stage 1 uses 820R R3 to fatten/warm the signal a bit more. Might want 1.2k.

[*]Gain stage 2 uses 1.2k R4 instead of 1.5k; 820R or 680R here isn't so great, but I want a little overdrive here so I want a slightly warmer bias.

 

Power amp stage

[*]R14 bias uses a trim pot

[*]C5 kept low (as in Fender Champ 5F1)

[*]1k/3W screen resistor.

 

 

I'll need a little more consideration and experimentation...

 

[*]For R6, not sure if I want it higher/lower but 500k/1M seemed good to me.

[*]Not sure on R2, might want it higher with that 820R R3; possibly 680R R3 and 68k R2

[*]The sag should push the power amp into a little dirty response to sudden hard, loud notes. Of course, I'm not using a slow-response 5Y3 to compliment, so the power might just be there too much for me to get the effect I'm aiming for.

[*]I'd actually like to implement a source follower into the power amp and a standby switch as per this document; though the standby switch would have to also disable the screen grid (B+2)

 

Any comments?

[RelicStrat]

How did you add the schematic to your post?

[bluefoxicy]

[Eracer_Team]

Not sure why you want to drop all the filter cap's lower in size in the power supply, you'll get more noise that way, granted the caps will not be able to support the bass notes as well which might give you a perceived 'sag' sound.

But considering an SE amp doesn’t sag, I think you’ll just end up with an amp that hums more because of less filtering.

With C3/C4 still at 22uf you are passing a lot of bass that makes these little amps a little flubby at full volume; most of the VJ modding community has dropped to 2.2uf for single coils and 1uf for humbuckers, I have stuck with 3.3uf on mine.

I think you might have a hard time finding a 350ohm 2w POT for R14 as well… you won’t need that much variation on R14. 300ohms is probably max for an EL84 biasing and once the EL84 is bias’ed it’s pretty much locked in no matter what EL84 tube you use.

The R10 at 220 ohms listed at 340v on B+ according to your schematic is far too high for an EL84 you should have no more than 315vdc hitting pin7 of the tube. You would be spending more money in replacing burnt out tubes from running them way too hot.

My suggestion is to read about all the mod’s everyone has done for the past few years on these little amp’s

This is one of the first VJ forums and its still running and has some good info:

http://vj.minuteboard.com/m/b/go.html

Beside this forum (some of which has been lost to the server switch at Gibson) don’t for get to sign on to www.sewatt.com where most of the gang has now moved to.

[bluefoxicy]

Not sure why you want to drop all the filter cap's lower in size in the power supply' date=' you'll get more noise that way, granted the caps will not be able to support the bass notes as well which might give you a perceived 'sag' sound.

But considering an SE amp doesn’t sag, I think you’ll just end up with an amp that hums more because of less filtering.

[/quote']

 

Damn. Thanks. This is a frankenstein monster trying to mimic an amp I'm designing right now (push-pull, 2x6V6 + 2xEL84, recto tube, light power caps to cause sag). Didn't know an SE would do that.

 

With C3/C4 still at 22uf you are passing a lot of bass that makes these little amps a little flubby at full volume; most of the VJ modding community has dropped to 2.2uf for single coils and 1uf for humbuckers' date=' I have stuck with 3.3uf on mine.

[/quote']

 

I'll keep that in mind.

 

The R10 at 220 ohms listed at 340v on B+ according to your schematic is far too high for an EL84 you should have no more than 315vdc hitting pin7 of the tube. You would be spending more money in replacing burnt out tubes from running them way too hot.

 

The 12AX7 should only run at like 150V too, and it gets 293 in the stock VJ. Really that whole power supply would do good on a rectifier tube like a 5Y3GT because it'd drop the power down low enough for B+1 and then you could use a Zener and a resistor to cool off B+3 for the 12AX7.

 

Of course, then 8uF caps are the max in the power supply, because you'll arc the rectifier tube with more (read the specs on the tube, they tell you this stuff); plus you suddenly need to find a center tapped power transformer, in which case, just use the 4 diodes there already and buy a lower power PT.

[Eracer_Team]

I hate to sound like a pain... but... I don't know were you are getting some of your info from:

12ax7 spec's

Specification and max ratings(*)

Filament Voltage 6.3-12 V

Filament Current 300-150 mA

Plate Voltage (max) 330 V

Plate Current (max) 6 mA

Plate Dissipation (max) 1.1 W

 

All you have to do with the valve junior's current power supply is change R10 to something between between 820ohm and 1.2k ohms 3~5 watts. That will tame the whole B+ rail. The reason I say 820 is good if your house runs standard NA 115vac but you will need as high as 1.2k if your house voltage is near 124vac.

 

The schematic at the top of the page is a VJ schematic with a few changes not a PP.

 

The 5Y3GT has been done by a few, you may want to use the search function to see what their results were.

[bluefoxicy]

Cool. I was on some site that had people complaining the 12AX7 should only have 150V but I forget where.