tightening up BF Princeton reverb ?

[Bluefinger]

Hi,

I have built me a BF Princeton Reverb clone. It's built mostly like the original with the excecption of a 12" speaker and a Deluxe Reverb OT.

I really love the sound of it and the compact dimensions but the main problem is that the low end is farting out with a Les Paul from around 3 on the volume knob.

Is there an effective way to tighten up the low end? A little more headroom would not hurt either. I thought about converting it to 6L6 power tubes by replacing the OT with maybe a vibrolux tranny.
Bigger filter caps would be the other thing. The princeton has this 4-in-1 FIlter cap but I have not found it with different values yet. Is there a replacement 4-in-1 cap available that tightens up the amp that fits the original mounting or would I have to convert it to a cap board? How high can I go with the filter caps on a princeton to stay in the safe zone?

thanks a lot!!!!

[Jazz P Bass]

On the first preamp tube cathode resistor there is usually a 25 uf capacitor across it.
If that is what you have installed, try a lower value cap.
I have gone all the way down to 4.7uf on certain Princetons.

[Chuck H]

Also, get a bassier and efficient speaker. The main reason BF type amps fart is because they don't have a lot of bottom end capability by design and they are typically fairly shallow combo cabs with bright speakers and this makes players tend to turn up the bass knob. Since there is very little attenuation of bass in the early preamp stages the amps EQ isn't balanced well for clipping when the bass knob is turned up. Simply put, if the bottom end is flabby or farty, you are pushing the amp beyond it's ability to reproduce bass and the primary limiting factors are the shallow cabinet and the speaker. Changing to a bassier, more efficient speaker will help you get the bass level in the EQ your after without the need to turn up the bass knob.

A little revoicing can help too, as Jazz suggested. I usually change the second stage bypass cap to 1uf or 2.2uf. Your other notes in the OP are also good. Bigger main filter and OT. But do this first...

Turn the bass knob down until the amp doesn't fart. If there isn't enough bottom end balance in the EQ a bassier and more efficient speaker will get that for you without changing the overall feel of the amp.

[Bluefinger]

Sorry, I should have mentioned ... The Speaker is not the problem. I took the time and put every 12" speaker I own in that amp last weekend. A Jensen C12N, Jensen P12Q, Eminence Cannabis Rex, Weber 12A150A, Weber 12F150 and finally a 50Watts version of a Weber 12A150 which turned out to be a MAJOR improvement over the C12N that was in the amp previously. The Cannabis wan't bad either but the Weber is fantastic.
I am pretty sure that all the remaining farting is happening inside the amp, not the speaker.

I will try the suggested cap mods.

thanks a lot!

[Bluefinger]

ok, I dont have 1, 2.2 or 4.7 caps but a couple of 8µF ones. I will try those on both sides of V1 (stage 1 and 2) to see if that puts me in the right direction.

thanks!

[ron vogel]

I'd still play with speakers to be honest. I tried more than a dozen I had laying around before one worked with that amp. Although my circuit is a teeny bit different it's finicky on speakers because they require a colder bias to deal with the bias wiggle trem. What is the B+ like at the output tubes?

[Chuck H]

ok, I dont have 1, 2.2 or 4.7 caps but a couple of 8µF ones. I will try those on both sides of V1 (stage 1 and 2) to see if that puts me in the right direction.

thanks!

8uf won't get you there. The knee frequency for that value bypass cap is still in the fart zone. You can wire two 8uf caps in series for a value of 4uf though.

I'm still thinking you won't get the fart out. If your bass knob (and bass level preferences) have the amp farting, cutting bass in the preamp will only have you wanting to turn up the bass knob more or compromising on your original vision. It may be time to try a bigger OT and 6L6's. A better solution might be to make the amp a head and play it through a 2x12 closed cab.

[Bluefinger]

Thanks ... yes maybe I'm expecting too much from such a small amp. I own enough big stage amps so I don't want to turn it into another one. I like the small package with trem and reverb. It's a great session and studio amp.

if I turn down the bass to an amount where the farting ends it sounds too thin for my taste. I am not the twangiest guy on earth when it comes to guitar tones. I guess the 6L6 route sounds most promising

[Bluefinger]

I'll have to check the B+ but I guess we can rule out the speaker. I had it in a 1x12 cab before I put it into the princeton and it handled a bassman head there without any farting so I guess the little Princeton shouldn't be a problem ...

[ron vogel]

Well, food for thought; I set mine up for 6L6, 6V6, and 6K6....I actually preferred the tone of the 6K6's. Between all three there was a volume difference for sure, but not a lot. The 6K6's had a better bass response than the 6V6. My B+ is too high to run 6K6's w/o popping them at full tilt for anything longer than an hour of playing time (I'm seeing about 380-390V)...fortunately I have a lot of 6K6's!

[Richard]

When I reworked my Princeton I replaced the stock 20/20/20/20 cap can with a 40/20/20/20
Can Cap, Multi-section, 40/20/20/20uF 525VDC, CE Manufacturing | Antique Electronic Supply
They have other values as well. I also revoiced the tone stack to move the frequencies higher (the mid dip in a stock tone stack is somewhere around 300Hz mine is around 800Hz). I raised voltages in the preamp and phase inverter, installed caps to bleed the treble from the NFB loop and installed 6L6's (and raised bias voltage) and ran through a 2x12 cabinet. The amp has a lot more headroom and sounds excellent (to me).

Revoicing the tone stack seems to be the most effective change. Sometimes cutting the mids can make the bass sound bigger by the contrast of less mids the bass stands out, sometimes raising the lower mids seems to help, 100Hz can't get any louder so adding 200Hz can enhance the low end content (the theory I was working with on my amp.

Increasing the voltages as also a big help. I think it is the "Stokes Mod" that applies to the phase inverter and this might be as effective as tone stack mods.

I changed back to 6V6's and don't think that the 6L6's made that big of a difference. I don't think that the split load PI drives the output tubes that hard to where the extra headroom makes a big difference.

I would suggest the order of mods to be: Speaker/cab; Stokes mod to PI; revoicing tone stack or playing with bypass caps, coupling cap,etc.; 6L6 output tubes; output transformer.

By the way, I never changed the output transformer (although it may make a big difference) I believe you just need to run a 4 Ohm load instead of 8