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Any good mods for the Pro Jr?

2/13/1999 12:41 PM
Steve A.	Any good mods for the Pro Jr? I was just looking over the Pro Jr schematic and noticed a few things: The voltage going to the preamp and driver tube plate resistors is a bit on the "brown" side: 232vdc. The preamp plates are running at 152vdc, which is great if you are looking for a distorted sound, but I'd think that you'd more clean headroom (a common complaint) if you were to raise the voltage from the power supply a bit. They use a 22k/1 watt resistor (R33) which could be replaced with something like 10k or 12k to increase the voltage to the preamp plates up to maybe 190 or 200vdc. (Of course, if you want the amp to distort easily you'd skip this mod. And with the higher voltage to the driver/PI you might blow out the stock speaker if you cranked up the volume control.) Neither V1B nor V1A is bypassing the 1k5 cathode resistor with a cap. I'd think that adding something like a 0.47uF to 22uF cap added to the first stage (V1B) would give a nice boost to the preamp. You might even want to add something like 0.47uF to 2.2uF to the second stage (V1A). I haven't taken apart a Pro Jr so I don't know how feasible it would be to add switch(s) for the Ck cap(s). If you boost the gain of the first stage you'd probably want to tone down the input a bit by replacing the 10k series resistor (R1) with something like a 33k resistor. Someone was asking me about adding a main amp input jack to the Pro Jr so that they could use an external tube preamp. I was thinking that the 0.01uF coupling cap going into the driver/PI could be replaced with a pair of 0.022uF caps, with a switching jack inserted between the two caps. Or you could go crazy and add 2 jacks for an unbuffered passive fx loop. Any feedback on these ideas from someone who has worked on a Pro Jr? Steve Ahola

2/13/1999 12:41 PM

Steve A.

Any good mods for the Pro Jr?

    I was just looking over the Pro Jr schematic and noticed a few things:

    The voltage going to the preamp and driver tube plate resistors is a bit on the "brown" side: 232vdc. The preamp plates are running at 152vdc, which is great if you are looking for a distorted sound, but I'd think that you'd more clean headroom (a common complaint) if you were to raise the voltage from the power supply a bit. They use a 22k/1 watt resistor (R33) which could be replaced with something like 10k or 12k to increase the voltage to the preamp plates up to maybe 190 or 200vdc. (Of course, if you want the amp to distort easily you'd skip this mod. And with the higher voltage to the driver/PI you might blow out the stock speaker if you cranked up the volume control.)

    Neither V1B nor V1A is bypassing the 1k5 cathode resistor with a cap. I'd think that adding something like a 0.47uF to 22uF cap added to the first stage (V1B) would give a nice boost to the preamp. You might even want to add something like 0.47uF to 2.2uF to the second stage (V1A). I haven't taken apart a Pro Jr so I don't know how feasible it would be to add switch(s) for the Ck cap(s).

    If you boost the gain of the first stage you'd probably want to tone down the input a bit by replacing the 10k series resistor (R1) with something like a 33k resistor.

    Someone was asking me about adding a main amp input jack to the Pro Jr so that they could use an external tube preamp. I was thinking that the 0.01uF coupling cap going into the driver/PI could be replaced with a pair of 0.022uF caps, with a switching jack inserted between the two caps. Or you could go crazy and add 2 jacks for an unbuffered passive fx loop.

    Any feedback on these ideas from someone who has worked on a Pro Jr?

Steve Ahola

2/13/1999 1:01 PM
Greg	What about this mod that Jonathan Whathisface from Mill Hill (you know, the $25000 amp guy) posted on alt.guitar.amps? What do you guys think? Greg Hello Pro Jr owners , I have passed this modification on to a few other clients of mine with great results. I must warn first and foremost that electronic circuits carry voltages that can kill you! If you do not know what you are doing hire a tech! Second, any modification to your amplifier will void your warranty! But realize this is a cheap amplifier and tone is the name of the game!!! The two modifications I am suggesting for the Pro Jr. are raising the bias to run in class A and removing the negitive feedback circuit. These two mods will result in a rawer tone with added harmonic content. First , unplug your amplifier from the wall. Second remove the back panel protecting the circuit. The first thing we need to do is to discharge the filter capacitors. Failure to do so may end up with your death! Discharge C15, C16, C17, C18 these are the big electrolytic capacitors. Ok, now we have a safe amplifier to work with. The first modification will be to raise the bias resistor from 56K ohm to 75K ohm. This resistor is a 1/2 watt unit and should be replaced with the same rating. Looking at the back of your circuit, the bias resistor we are changing is R28. It is under and to the left of center of C15. Remember you discharged these big electrolytics! There are two ways to replace this resistor. The professional way is to remove the circuit card, desolder R28 from the pads and install the new 75K ohm 1/2 watt unit. Remember to follow the "two second" rule when dealing with printed circuit cards. We do not want to lift any traces! Only touch the solder connection with your iron in two second intervals, allowing time inbetween for cooling. Be sure to use solder wick and flux as these two items will make life easier. Be mindfull and not a butcher, take your time! The quick way of doing the same mod would be to clip R28's legs as close to the resistor body as possible. Then bend the legs straight creating two turrets to solder the new 75K ohm 1/2 watt resistor legs to. Solder in the new resistor and trim the excess lead from the new component so as to not interfere with the rest of the circuit! Wow that was easy! Now your tubes will be drawing 35 ma of current ea. This is Class A! At this point I would recommend installing a matched set of EL84's. A 75 K ohm resistor will get you into the ballpark of class A, if you really want to dial in the circuit, check the bias and adjust R28 for the tone you like! Now to remove that Negitive Feedback circuit. Find R27, it is to the right of C17. (This is another one of those large electrolytics you have discharged!) Clip the legs of the resistor at the base of the circuit card. You have just disconnected the feedback circuit! Reinstall the rear protection panel, and plug in the amplifier. After letting the amplifier warm up for one to two minutes, plug in and play! You will find the tone is much fatter and full of harmonics! ***Once again, I must warn you to discharge the filter caps C15, C16, C17, C18 before you attempt any of these modifications! Pay attention to what you are doing, work in a clean space, and work with a smile on your face! If any of these steps confuse you, either take it to a professional technician or email me for further clarification! Jonathan, Please practice peace... Toll Free 1.877.Mill Hill or email me at www.millhillaudio.com

2/13/1999 1:01 PM

Greg

What about this mod that Jonathan Whathisface from Mill Hill (you know, the $25000 amp guy) posted on alt.guitar.amps? What do you guys think?

Greg

Hello Pro Jr owners ,

I have passed this modification on to a few other clients of mine
with great results. I must warn first and foremost that electronic
circuits carry voltages that can kill you! If you do not know what
you are doing hire a tech! Second, any modification to your amplifier
will void your warranty! But realize this is a cheap amplifier and
tone is the name of the game!!!

The two modifications I am suggesting for the Pro Jr. are
raising the bias to run in class A and removing the negitive feedback
circuit. These two mods will result in a rawer tone with added
harmonic content.

First , unplug your amplifier from the wall. Second remove the
back panel protecting the circuit. The first thing we need to do is
to discharge the filter capacitors. Failure to do so may end up with
your death! Discharge C15, C16, C17, C18 these are the big
electrolytic capacitors.

Ok, now we have a safe amplifier to work with. The first
modification will be to raise the bias resistor from 56K ohm to 75K
ohm. This resistor is a 1/2 watt unit and should be replaced with the
same rating. Looking at the back of your circuit, the bias resistor
we are changing is R28. It is under and to the left of center of C15.
Remember you discharged these big electrolytics! There are two ways
to replace this resistor.
The professional way is to remove the circuit card, desolder R28
from the pads and install the new 75K ohm 1/2 watt unit. Remember to
follow the "two second" rule when dealing with printed circuit cards.
We do not want to lift any traces! Only touch the solder connection
with your iron in two second intervals, allowing time inbetween for
cooling. Be sure to use solder wick and flux as these two items will
make life easier. Be mindfull and not a butcher, take your time!
The quick way of doing the same mod would be to clip R28's legs
as close to the resistor body as possible. Then bend the legs
straight creating two turrets to solder the new 75K ohm 1/2 watt
resistor legs to. Solder in the new resistor and trim the excess lead
from the new component so as to not interfere with the rest of the
circuit! Wow that was easy! Now your tubes will be drawing 35 ma of
current ea. This is Class A!

At this point I would recommend installing a matched set of
EL84's. A 75 K ohm resistor will get you into the ballpark of class
A, if you really want to dial in the circuit, check the bias and
adjust R28 for the tone you like!

Now to remove that Negitive Feedback circuit. Find R27, it is to
the right of C17. (This is another one of those large electrolytics
you have discharged!) Clip the legs of the resistor at the base of
the circuit card. You have just disconnected the feedback circuit!

Reinstall the rear protection panel, and plug in the amplifier.
After letting the amplifier warm up for one to two minutes, plug in
and play! You will find the tone is much fatter and full of
harmonics!

***Once again, I must warn you to discharge the filter caps C15,
C16, C17, C18 before you attempt any of these modifications! Pay
attention to what you are doing, work in a clean space, and work with
a smile on your face!
If any of these steps confuse you, either take it to a
professional technician or email me for further clarification!

Jonathan, Please practice peace...
Toll Free 1.877.Mill Hill or email me at www.millhillaudio.com

2/13/1999 8:10 PM
Bruce	I think it is mostly a lame mod. I've worked on a few of these now and the first thing to do to make them sound better is get rid of the tiny speaker and use an extension speaker! Pretend like it's a little 15 watt kick butt head instead. As for the mods as written...for one thing, stock, the amp is about class A as it is and with less then -10.5v stock bias voltage on it it sounds like it too. Why he says this mod will make it class A is odd. Just because it is drawing 35ma doesn't mean it's stuck in class A. The amp is running full blast and removing the feedback loop makes it way too mushy and woofy at anything louder then 1/3 to 1/2 the way up. Keep some FB in there. And, as for the bias supply rail, wouldn't raising the 56K resistor to 75K, probably move it closer to allowing class AB since there will be more raw AC voltage delivered to the bias diode and consequently more negative voltage... so the EL84s will bias up colder... which by the way, is a good thing with this amp. I have been setting them at about -11.5 volts for a really good sound. But not by doing it his goofey way. The right way, if your going to mod this amp, is to cut out the 15K resistor (R29) and solder a 20K trim pot there in series with a 6K8 1/2 watter so you can have an adjustable bias pot on the board. The next thing to do is use a little piece of shielded cable from the input jack to the 10K series resistor( sort of like a grid stopper). Then remove the 22K voltage dropping resistor on the power supply rail and use a 12K instead. This will raise the preamp's supply voltage up to about 265vdc instead of 235vdc so you can hit the preamp a little harder. Then install a 1uF cap across the 1K5 cathode resistor of V1b. When tuning I've used between a 22K to 56K feedback resistor instead of the 100K. Get rid of the stock EL84s and use some muscular sounding Teslas and replace the preamp tube with a killer one and these amps turn into a killer screamin' monkey! Driving a big bottom is fantastic with this little powerhouse. Bruce Mission Amps

2/13/1999 8:10 PM

Bruce

I think it is mostly a lame mod.
I've worked on a few of these now and the first thing to do to make them sound better is get rid of the tiny speaker and use an extension speaker!
Pretend like it's a little 15 watt kick butt head instead.
As for the mods as written...for one thing, stock, the amp is about class A as it is and with less then -10.5v stock bias voltage on it it sounds like it too.
Why he says this mod will make it class A is odd. Just because it is drawing 35ma doesn't mean it's stuck in class A.
The amp is running full blast and removing the feedback loop makes it way too mushy and woofy at anything louder then 1/3 to 1/2 the way up. Keep some FB in there.
And, as for the bias supply rail, wouldn't raising the 56K resistor to 75K, probably move it closer to allowing class AB since there will be more raw AC voltage delivered to the bias diode and consequently more negative voltage... so the EL84s will bias up colder... which by the way, is a good thing with this amp.
I have been setting them at about -11.5 volts for a really good sound.
But not by doing it his goofey way.
The right way, if your going to mod this amp, is to cut out the 15K resistor (R29) and solder a 20K trim pot there in series with a 6K8 1/2 watter so you can have an adjustable bias pot on the board.
The next thing to do is use a little piece of shielded cable from the input jack to the 10K series resistor( sort of like a grid stopper).
Then remove the 22K voltage dropping resistor on the power supply rail and use a 12K instead.
This will raise the preamp's supply voltage up to about 265vdc instead of 235vdc so you can hit the preamp a little harder.
Then install a 1uF cap across the 1K5 cathode resistor of V1b.
When tuning I've used between a 22K to 56K feedback resistor instead of the 100K.
Get rid of the stock EL84s and use some muscular sounding Teslas and replace the preamp tube with a killer one and these amps turn into a killer screamin' monkey!
Driving a big bottom is fantastic with this little powerhouse.

Bruce
Mission Amps

2/16/1999 5:22 PM
Mook	My 3 fav mods to the Pro Jr.: 1) New speaker - Weber VST 2) Disconnect NFB (I don't think removing the Negative Feedback makes it any more "mushy" than stock). 3) Install Decent NOS tubes. Mook

2/16/1999 9:03 PM
Steve A.	Re: How about adding a standby switch? Thanks, Greg, Bruce and Mook! Any thoughts on adding a standby switch to the Pro Jr? Interrupt B+ after the first filter caps and before it goes to the OT and plates? Kill the cathode connection to ground from the output tubes? Thanks! Steve Ahola

2/16/1999 10:19 PM
Aaron Flynt	I agree W/Bruce about the NFB, but I also kind of like the sound of the amp with a 25uf bypass cap on the cathode of the first gain stage. Gives it alot more gain without sounding buzzy. Of course you lose the clean sound then, and putting it on a switch causes a loud pop. Aaron.

2/17/1999 1:42 AM
Steve A.	Aaron: ...putting it on a switch causes a loud pop. Putting a 54k to 220k resistor across the switch contacts will eliminate any pops when switching in a 25uF cap. (220k to 470k will handle any pops from a smaller cap like a 1.0uFm,and 470k to 1M for something like a 0.47uF cap.) I'm not sure if it makes any difference, but I usually run the cathode to the cap and put the switch (and resistor) between the cap and ground. BTW if you get too much gain from a cathode resistor bypass cap (especially on the 2nd or 3rd gain stage) insert a 470 to 10k resistor in series with the connection to ground. Steve Ahola

2/17/1999 1:42 AM

Steve A.

Aaron:

...putting it on a switch causes a loud pop.

    Putting a 54k to 220k resistor across the switch contacts will eliminate any pops when switching in a 25uF cap. (220k to 470k will handle any pops from a smaller cap like a 1.0uFm,and 470k to 1M for something like a 0.47uF cap.)

    I'm not sure if it makes any difference, but I usually run the cathode to the cap and put the switch (and resistor) between the cap and ground.

    BTW if you get too much gain from a cathode resistor bypass cap (especially on the 2nd or 3rd gain stage) insert a 470 to 10k resistor in series with the connection to ground.

Steve Ahola

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