SF Princeton Reverb mods

[ncboy654]

I did some modding many years ago and lost the directions for what I did. One of the things was adding a resistor in series with the cathode capacitor. Also changed plate R to 220K, cathode R to 2.2K and added a 10K between the bypass cap and ground. I've added a 12" speaker and thinking about going back to stock values. I just want to understand what this mod was supposed to do.

Thanks

[tubeswell]

The resistor in series with the cathode bypass cap is a frequency boost limiter. It limits the gain effect of the bypass cap, lowering the amplitude of the frequency gain that the cap otherwise provides. If you take the resistor out, the stage will have more mid-hi gain (depending on what the cap value is). The tyical fender value for a 12AX7 bypass cap is 22-25uF. This tends to amplify all the frequencies that matter. So the effect of putting a resistor is series with 22-25uF cathode bypass cap is merely to reduce the overall gain of the stage. If however, you put a resistor in series with a 0.68uF cathode bypass cap, you will lose some of the highs, and have more mids and bass in the stage. If you keep the resistor in series with (say) a 1uF-2.2uF cathode bypass cap, but introduce a 2nd (say .01uF) bypass cap from ground to cathode, you will get a more scooped sound out of the stage, because in effect you will cut out some mids, but keep bass and highs. Furthermore if you swap that boost limiting resistor for a pot, you get a tone control on the stage, and you can twiddle with the stage's bypass frequency to your heart's content

The 220k plate resistor just provides more gain per se (and more ambient hiss). The typical fender value for a plate resistor for a 12AX7 gain stage is 100k. The load line with 220k is shallower and up to a point this results in a comparatively larger signal at the plate c/f what is at the grid, but it has less harmonic distortion overall than a 100k plate resistor, except when the stage is driven really hard, and then you get a harder more driven tone - By that stage you don't notice the background hiss anymore.

Likewise the typical fender value for a cathode resistor for a typical fender 12AX7 gain stage is 1k5. Raising is to 2k2 provides more negative grid bias, (i.e.; colder biasing - this results in more fuzzy sounding distortion - less bluesy) thus tending to avoid the situation where the signal gets clipped due to forward grid current . Conversely, if you increase the negative grid bias too much, you end up with clipping of the output signal due to cut-off.

[ncboy654]

Thanks, I'm not sure where I got this for a mod, maybe the same place I got the "perfect prince" mod. So what you describe would give more output and reduce distortion, but the gain is being reduced by the resistor in series with the bypass cap, I would guess so it doesn't overdrive the next stage. I want to get more definition, it sounds a but muddy so I plan to experiment with different values.

[tubeswell]

What do you mean by 'more definition'? Cleaner? Less compressed? More bite?

[tubeswell]

On 2nd thoughts if its too muddy, you can try smaller value coupling cap(s) after the plate of each stage. Try one at a time. Fender typically used anything bewteen .1 - .022uF. If you've got .1 then try .047, or .022, or .01 respectively (These need to be rated 400V min).

When you increase the gain the net effect can be more bass, which makes it farty as it gets more distorted (if you don't counteract this by boosting mid-treble/cutting bass).

Or try adding another small value (say 1000pF) bypass cap from cathode to ground - in parallel with your limited cap/series resistor - to boost the higher freqs in the stage

[ncboy654]

Thanks, what I meant by muddy is there's not much highend sparkle in clean and upper harmonics when cranked, more muffled than muddy. When I plug in to a DR or TR (normal, not bright) it sounds like there is a blanket over the PR by comparison. I've switched out speakers and tubes and the PR still sounded muffled, so I'm looking at the circuit.

[tubeswell]

Well its easy enough to solder parts in and out - you just have to heat up your soldering iron. (When I do mods, it can take months of OCD experimentation to arrive at a plateau of non-modding behaviour. )

For a start, try adding an extra .01 to .001 bypass cap at the cathode in parallel with the existing bypass cap/series resistor to lift the hi freqs a bit. (doesn't need to be high voltage in this position and it doesn't need to be electrolytic - even a 50V disc ceramic cap will do for experiment - if you like it you can stick a better quality cap in later) .

If that doesn't ring your bell (once you'e tried it) try changing to a smaller value coupling cap at the plate of that stage

[ncboy654]

ok, I took it completely back to blackface stock values with 1% resistors and Spraig Atoms and 716P. Added 470 ohm to grid screen and used the boost switch for a bright with a 68p. It doesn't sound muffled anymore, good but not great, past 5 was just mediocre tone. I did the stokes mod and it got much more pep, overdrive had more harmonics and grind. Next to try is the PaulC mod, then on to the preamp section if it needs more tweaking.

[ncboy654]

I went back and compared to the DR and TR and there is more low mid mud in the PR, the more it's cranked, it loses definition. I want to get a tighter low end and less of the low mids to open up the sound. I will experiment with the bypass caps to scoop out the low mids. Is there other places to get rid of those frequencies?

[Bruce / Mission Amps]

I went back and compared to the DR and TR and there is more low mid mud in the PR, the more it's cranked, it loses definition. I want to get a tighter low end and less of the low mids to open up the sound. I will experiment with the bypass caps to scoop out the low mids. Is there other places to get rid of those frequencies?

Two totaly different amps and phase inverters... yes use 4.7uF bypass caps.
However, the OT in those Princeton amps are pretty small... that 'is' part of the dubious, magical sound though.

Try a MOJO reissue OT for a tweed Deluxe (#768) ... a little bigger and the same Zed as the smaller Princeton OT.

http://www.mojomusicalsupply.com/ite...008&id=MOJO768

I have had nothing but compliments when doing this mod.

[TD_Madden]

I did something similar with an old Kendrick deluxe OT I had. I'm also using an Avatar Hellatone 30 with a replaced speaker-baffle I ordered from Mojo.

BIG sound.

[ncboy654]

I found a post by Tinman that replaced the 6.8K at the bottom of the tone stack with a pot to adjust the mids. I clipped in a pot and found it will suckout some mids in a nice way. Since I'm not going to drill my chassis for another pot, where can I find a 250K pot with a push-pull switch like the 1M has? That way I can have a fixed mid cut.

[ncboy654]

I going to put the solder iron to rest for a while, the PR is sounding pretty darn good. Here's what I did, V1A cathode cap is 10uf, stokes mod, blueprint PI, 40/20/20/20 filter cap, Allen T022 OT, replaced 1K power resistor with choke. Replaced boost switch with 68p bright cap, replaced treble with switching pot 250p/125p cap, replaced bass with switching pot, 6.8k/1.5k to ground. 10" replaced with 70's Fender Pyle.

Before replacing the OT, there was a lot of midrange mud, I tried to brighten it up with bright switch and mid cut, using the treble and bass switchs but was still pretty honky. The Allen OT really smoothed out the mids and added a nice crunchy compression past 7, the choke was subtle, adding just a little more dynamics.

I also added a progressive power switch and tried a GZ34, but I'm not sure why Bruce recommends against them, added adjustable bias and biased at 24ma with a set of JAN Philips.

Overall, it sounds closer to a DR until after 4, bass is full and tight, then breaks up like a PR. At this point I like it better than my DR, it gets jucier tones and more eq options, just not as loud.

[Bruce / Mission Amps]

I'll have to reread what I said regarding not using a GZ34 in a Princeton.
The only thing I can think of right off the top of my head is:

1.) with a GZ34 in a silver face PR, the B+ will be very high and stock, they do not have an adjustable bias supply.
2.) with too high of B+ comes too high of power tube, DC idle current.
3.) the power tranny is a wimp in these amps and that spells a possible power transformer failure with high idle current plus a bigger OT that can pull more power from the power tubes.