Princeton Reverb Build Harsh Overdrive

[chriscwill07]

I finished up my kit build of a Blackface Princeton Reverb. The amp sounds fine clean but any amp breakup or overdrive pedal gives a terrible result. Very fizzy. I have switched out tubes, checked solder joints, and checked capacitors. Can’t see the problem on a scope. Please help. This is driving me nuts

[tubenit]

There have been a lot of posts over the years commenting that the Princeton Reverb sounded harsh when pushed into overdrive. 

You didn't provide any information about what speaker you are using?  So that could be a huge factor in this.

IF I were experimenting, I'd try these mods to smooth the overdrive:

1)   I'd use a 5751 in V1
2)   I'd reduce the cathode cap on the first 3 gain stages to 5uf
3)   I'd change the coupling caps after the phase invertor from .1 to .047
4)   IF you've used Orange Drops thru out the amp, I might try Doug's Jupiter polyester caps after the phase invertor
      for the .047 coupling caps.

IF those didn't get me where I wanted tone wise, then I might try some 2w 100k plate resistors on the first 3 gain stages also.

With respect, Tubenit

[chriscwill07]

Thank you for the advice. I’m going to try those. It’s not the speaker. The amp is a head and I have hooked it up to my 2x10 dr z cab and a 1x12 Celestion cab. Same results. Also all voltages are where they should be. I had already planned on changing the .047 before power tubes so that works. I am having terrible issues with bass too much bass also which should help tame that.

[Willabe]

I am having terrible issues with bass too much bass also which should help tame that.

2)   I'd reduce the cathode cap on the first 3 gain stages to 5uf

Reducing the K bypass caps will help greatly in reducing bass. Takes the mud out.

[galaxiex]

I use .068uf V1a K bypass cap in my 1974 Silverface Princeton Reverb.

Tames the "flubby" bass, but still has enough.

[tubeswell]

Put a 470k-to-1M grid stopper resistor at the pin of the cathodyne (inverter) stage. This should cure the propensity for frequency-doubling distortion and/or gain-spike distortion when the PI gets driven hard. This will keep the journey into heavy distortion smoother.

Also, consider adding 10k-to-47k grid stoppers on the grid pins of the output tubes to help to reduce the onset of blocking distortion in the output stage under heavy signal (overdrive) conditions.

[chriscwill07]

Put a 470k-to-1M grid stopper resistor at the pin of the cathodyne (inverter) stage. This should cure the propensity for frequency-doubling distortion and/or gain-spike distortion when the PI gets driven hard. This will keep the journey into heavy distortion smoother.

Also, consider adding 10k-to-47k grid stoppers on the grid pins of the output tubes to help to reduce the onset of blocking distortion in the output stage under heavy signal (overdrive) conditions.

I have already put the 470k on the inverter. No change. Currently waiting shipment from Hoffman for the grid stoppers on output tubes. 5uf cathode to ground caps for gain stages and all Jupiter caps for coupling caps and tone stack are on the way. Also going to try putting a 10k grid stopper on second half of V1 and on V3. Not sure that will do anything but worth a try. Pulled the reverb driver tube so I know that isn't the issue. Ordered some shielded cable for extra protection in preamp. The problem is much worse with humbuckers and I have tried both inputs and messed around with multiple pedals. TS9, Timmy, and OCD. All bring the amp into harsh overdrive. Thanks for the suggestions.

[chriscwill07]

There have been a lot of posts over the years commenting that the Princeton Reverb sounded harsh when pushed into overdrive. 

You didn't provide any information about what speaker you are using?  So that could be a huge factor in this.

IF I were experimenting, I'd try these mods to smooth the overdrive:

1)   I'd use a 5751 in V1
2)   I'd reduce the cathode cap on the first 3 gain stages to 5uf
3)   I'd change the coupling caps after the phase invertor from .1 to .047
4)   IF you've used Orange Drops thru out the amp, I might try Doug's Jupiter polyester caps after the phase invertor
      for the .047 coupling caps.

IF those didn't get me where I wanted tone wise, then I might try some 2w 100k plate resistors on the first 3 gain stages also.

With respect, Tubenit

UPDATE: Got the Sovtec 5751 in and it did wonders! The amp didn't like 12ax7 or a 12au7 in V1.  Overdrive tones are sooo much better.  The amp does run cleaner without a pedal. Which I don't mind at all. Sounds pretty good when only playing in bridge position but there is still way too much bass for any neck pickup stuff. So I can't wait to get the capacitors in for that fix. I definitely think this amp is going to sound great when those changes are made. I wonder though why that 12AX7 doesn't want to work? This kit from Mojotone does have a beefier power transformer that puts out more B+ voltage than a stock Princeton. Sooo changes at the plates? Which you would expect more headroom with. IDK. BTW I don't have the stokes mod installed. Maybe its just a combo of a lot of factors, tube brand, etc. Maybe the phase inverter is just adding too much gain so the 5751 tames that. I don't have to know cause this works but I'm curious. Are all these other people who are buying these kits having the same issue and just don't have the ear to recognize it?

[Willabe]

...... and all Jupiter caps for coupling caps and tone stack are on the way.

Those are very good sounding caps, IMHO. 

[Willabe]

Are all these other people who are buying these kits having the same issue and just don't have the ear to recognize it?

Or, they might like the distortion?

[Willabe]

I'd try just changing the K bypass caps 1st, then hear how it sounds, how much bass end was shaved off.

Then if still needed, I'd change the power tube coupling caps.

If you do 1 change at a time you can hear what they do or don't do. If you do them all at once, you don't know what helped and/or which did what to the amps sound/feel.

[chriscwill07]

I'd try just changing the K bypass caps 1st, then hear how it sounds, how much bass end was shaved off.

Then if still needed, I'd change the power tube coupling caps.

If you do 1 change at a time you can hear what they do or don't do. If you do them all at once, you don't know what helped and/or which did what to the amps sound/feel.

I hear you. I'll definitely be testing between each change. If you heard the crazy bass from this thing you would be saying "change every value!" but yeah it will be interesting to see what tone each one of those mods will give me.

[chriscwill07]

Are all these other people who are buying these kits having the same issue and just don't have the ear to recognize it?

Or, they might like the distortion?

It's the kind not even a mother could love. Buddy of mine who has been way more of a gear guy than me for years noticed it immediately. It was not an overdrive color. It was just plain bad.

[tubenit]

On the resistors in the kit ………….. did you actually measure the value of every resistor before you soldered it in (even if it came labeled with a value)?

Occasionally, NFB doesn't howl when hooked up backwards & instead just has a bad sounding distortion to it.  Have you tried removing the NFB & seeing how it sounds without it?

Not saying it's either of these issues, but it may be worth ruling these possibilities out.

With respect, Tubenit

[chriscwill07]

On the resistors in the kit ………….. did you actually measure the value of every resistor before you soldered it in (even if it came labeled with a value)?

Occasionally, NFB doesn't howl when hooked up backwards & instead just has a bad sounding distortion to it.  Have you tried removing the NFB & seeing how it sounds without it?

Not saying it's either of these issues, but it may be worth ruling these possibilities out.

With respect, Tubenit

I checked all values. All within tolerance. Maybe one R got damaged on install. I’ve put my meter on a lot but of course can’t check all resistors without pulling them. I did remove Neg feedback. And reversed leads on tubes to output transformer. They are set up right. Wouldn’t it still sound terrible despite tube choice in V1 if that was the problem?

[chriscwill07]

There have been a lot of posts over the years commenting that the Princeton Reverb sounded harsh when pushed into overdrive. 

You didn't provide any information about what speaker you are using?  So that could be a huge factor in this.

IF I were experimenting, I'd try these mods to smooth the overdrive:

1)   I'd use a 5751 in V1
2)   I'd reduce the cathode cap on the first 3 gain stages to 5uf
3)   I'd change the coupling caps after the phase invertor from .1 to .047
4)   IF you've used Orange Drops thru out the amp, I might try Doug's Jupiter polyester caps after the phase invertor
      for the .047 coupling caps.

IF those didn't get me where I wanted tone wise, then I might try some 2w 100k plate resistors on the first 3 gain stages also.

With respect, Tubenit

The 5uf bypass caps got me closer. And the change of coupling before power tubes helped also. Still not where I want it. Still a little too much bass with knob dialed all the way down. Im going to try galaxiex suggestion for much lower cap on V1

[TurboGuitarMelton]

The Princeton reveb has a .02uf phase inverter input cap that lets lots of bass into the output stages. If you are looking for less bass/tighter sound you may try changing the value of that cap to something smaller. In the deluxe reverb and other bigger blackface amps its a .001uf cap.

[chriscwill07]

The Princeton reveb has a .02uf phase inverter input cap that lets lots of bass into the output stages. If you are looking for less bass/tighter sound you may try changing the value of that cap to something smaller. In the deluxe reverb and other bigger blackface amps its a .001uf cap.

Actually just did that. And it did help a little. Still not where I want it. At this point the amp is playable and sounds pretty nice with single coils but still cant touch that bass knob. Now I'm trying to pull bass to allow a better response from humbuckers. I'm sure Fender purists would frown at my tone modding, but I know what sound I want with the pedals I use. This is frustrating but all part of the fun when building and modding a circuit. Waiting on the smaller K bypass capacitors to come in and I think this amp will really shine. I also plan on swapping in a 5751 or 12AT7 instead of the 12AX7 PI tube just to see if it will tame a little fizzy note decay. I also changed the resistor value on the V1a cathode from 1.5k to 870ohms. The bias shift seemed to really help out the 5751 when pushed hard with an overdrive.

[tubeswell]

did you try 47k grid stoppers on the 6V6s yet?

[Shack]

LMAO.....I seen this thread and my solution for this problem with my PR build was to change it to a 6v6 Tweed Bassman......maybe I should try this stuff first :P

[chriscwill07]

did you try 47k grid stoppers on the 6V6s yet?

I put 10ks there. Not sure they really need 47k at this point because I’m really just trying to tame the bass.

[chriscwill07]

I got too impatient to wait to buy another 5751 for the phase inverter tube. So I switched the 5751 i put in V1 to the inverter slot and the 12ax7 from there to V1. Weird note decay disappeared but I got more breakup of course. But now more high end fizz. Like what a bad solid state distortion pedal will give you. So I thought to myself how do I give that more headroom? I decided to shift the bias on V1. Changed the V1a cathode resistor from my modded 870 ohm to 10k. I know that is not a typical value for Fender’s 1.5k standard but other amp builders use much higher values in other preamps. Worth a try. The results were great. The tube switch could be a number of different factors but I dig the outcome. So the current list of tone and gain changing modifications that include preventing blocking distortion are:

-5751 sovtek tube in phase inverter slot
-Lower value of cathode bypass caps from 25uf to 5uf for less bass
-Lower from .1 to .047 coupling caps after phase inverter (before 6v6s) for less bass
-.001uf capacitor on phase inverter input like the other Blackfaces for less bass
-raise resistor value from 1.5k to 10k on V1a cathode for a bias shift and more headroom
-10k resistors on grids of the 6v6s to prevent blocking distortion
-470k resistor on phase inverter input for blocking distortion
-adjustable bias pot (10k) with 22k resistor to ground for 6v6 bias adjustment

Pending: .68 cathode bypass caps on 1 or more gain stages depending on how much more bass reduction I want.

This could be overkill on the bass reduction for some people but this isn’t a combo. It’s a head kit that I’m running a 2x10 dr z cab with. Plus it’s closed back. So if you read this and it really speaks to you, don’t change all these at once. Check tone after each step and see if it suits your setup and speaker selection.