Pro Junior to Princeton Reverb delete tremolo & reverb conversion help

[mattmayhem]

So I recently decided to gut my Fender Pro Junior and convert it to a Fender Princeton Reverb without the trem or reverb based on this project on Rob Robinette's site and this thread here on the forum. I had to modify the EQ section to work with the single tone control so I swapped it to the tone circuit used in the Harvard and Tweed Princeton. Finished this morning and after walking through Rob's startup procedure and checking voltages along the way (nothing unexpected based on the Princeton Reverb schematic), there are a couple issues:

1. The amp is crazy gainy! It has even less headroom than the original Pro Junior circuit. Already starting to overdrive when it's barely cracked on.

2. It also has a high noise floor. Doesn't really sound like 60hz hum to me, but more like something is acting like an antenna in the circuit? Dead quiet with the volume knob down, but comes in as the volume is brought up and is shaped by the tone. Does that mean the noise is coming between the input and the volume? I used shielded cable to the grids of V1 with the shield grounded on one end to the input jack.

All that being said, it sounds great with the volume on the guitar rolled wayyy down (doesn't change the noise floor, but otherwise). Wondering if anyone has insights as to why this modified circuit would have so much gain compared to the original Princeton Reverb, or if there are any suggestions for reducing the gain. I know I can swap the negative feedback resistor to a 2.7k, but I'm pretty sure that isn't going to do enough. I tried a 12AY7 in V1 and it sorta worked. dropped the noise floor significantly, but was still overdriving really early on the dial and lacked the punch and snap of the 12ax7 when the guitar volume was rolled down. Anyway, would love to hear any thoughts or suggestions on troubleshooting or reducing the gain. Thanks!

 

[printer2]

You delete a tone stack that has a 20 dB loss (gain of -100) and you wonder why you have noise?

 

[schmee]

First: Follow the Princeton non reverb schem. That gets you much of it.
Need to see your layout inside for noise reduction ideas.

Consider the 5F10 Tweed Harvard scheme. No tremelo in that, fixed bias and 2 x 6V6. Known for a clean robust sound just like the non Reverb Princeton has. Easy to change the tone from single pot to Treble and Bass.

 

[dan40]

As Printer2 mentioned, removing the tonestack has left you with a crazy amount of gain. The strange noises you are hearing are likely a result of the extra gain. The amp is probably oscillating and creating the antenna effect that you are experiencing.

 

[mattmayhem]

I knew I could count on you guys to point out my stupidity... I’d heard so much about modding the silver face champ with the tweed Princeton tone circuit but never heard anyone talk about it ramping up the gain. So I guess now the question is what’s the best solution to make things work. Do I try and send some signal to ground and simulate the original eq circuit’s gain drop or do I cut out a gain stage and try to bring it more in line with the Harvard. Again, any suggestions? Or just roast me for learning while I go.

 

[sds1]

Before anything else I would just throw a voltage divider in there to try and compensate for the missing tone stack. It's an easy solution to insert, tweak, undo.

 

[printer2]

Try put a lower resistance across the 470k resistor, maybe 49k, really depends on how hot or tame you want it. Or get rid of the second stage and then get rid of the circuitry around the 3.3M resistor.

 

[Mexitele Blues]

The tweed tone knob doesn't do much with the volume up. You may get more use from a standard TMB stack with a resistor replacing the bass pot.

 

[Lowerleftcoast]

You could also try removing the cathode bypass caps. I would start with the one before the PI and work my way back. I imagine all three can be removed.

 

[jonrpick]

I'll point out that your schematic shows THREE bypassed gain stages in the preamp, where as a Princeton NON-reverb only has two. There's a reason why the extra gain stage wasn't needed in the non-reverb version. This is very similar to something I built. Cool amp, but doesn't sound anything like a Princeton, or any other Fender, honestly.

I ended up removing the bypass cap on V2a and raising the resistor value to like 10k. It made it useable...

 

[SerpentRuss]

Good ideas above.

Paralleling the first gain stage, like a Matchless spitfire would lose one gain stage AND lower the noise floor.

One other thing I would consider is dropping the voltage delivered to the plates of the first two stages by putting another dropping resistor and cap between the two 12AX7s. It's bad form to have that many stages hanging out on one filter section.

 

[tubeToaster]

You can make a single potentiometer TMB tone stack that would be similar to the Princeton Reverb tone stack.
Like this

 

[mattmayhem]

The TMB circuit with a single treble knob would probably have been a good idea if I'd thought of it to begin with, but this chassis is small, and I already made a cramped custom eyelet board for my current layout so it'd be tough to add that much circuitry in at this point. May be worth attempting a new layout at a later date if I'm feeling adventurous though.

I would like to experiment with adding a voltage divider off the output of the volume pot since that is something I could do point-to-point, but given the cramped space, I'll have to pull the pots out to have enough room to experiment.

As it stands, I tried several lower values in place of the 470K voltage divider resistor and lots of combinations of pulled bypass caps, but ultimately what sounded the best to my ears was leaving the bypass caps in place, dropping the 470K to 220K, and running a 12AU7 in V1 and a 12AT7 in V2. Removing the bypass caps killed the "sweetness" of the amp. It just sounded kinda bland and the OD felt harsh. Similarly, further lowering the resistor and using higher gain tubes also didn't sound quite a rich. Maybe it's just the tubes I have, but they were all new JJs.

The new tube line up sounds really nice to my ears. Aside from perhaps being an inelegant solution, is there any downside to using tubes to compensate for the gain in this way that I am unaware of?

Thanks everyone for the suggestions. Next time I decide to try modding a circuit I'll post the schematic before I commit my mistakes to solder.

 

[tubeToaster]

@mattmayhem you can build the circuit on a small circuit board like the kind used for building pedal circuits on. Then attach it to the back of the tone pot with double sided tape. I’ve done a similar tone stack this way when I didn’t have room on my board. It was very easy to add.

https://www.tubesandmore.com/sites/default/files/uc_products/p-pc-pad11_1.png

 

[printer2]

I did a little add on board with a tonestack for bass and treble in this amp.

 

[mattmayhem]

@mattmayhem you can build the circuit on a small circuit board like the kind used for building pedal circuits on. Then attach it to the back of the tone pot with double sided tape. I’ve done a similar tone stack this way when I didn’t have room on my board. It was very easy to add.

Nice, I think I can find a place for that little board somewhere in the chassis. Great suggestion, thanks!

 

[mattmayhem]

Thanks everyone for the help. I converted to the TMB tone stack with a set 56K resistor for the bass and it's sounding pretty good! Was able to plug a brown princeton clone into my speaker/cab the other day to compare and I think I'm digging this princeton reverb style circuit better. Now I'm wondering about the princeton non-reverb circuit with a long tail pair phase inverter...