Hoffman Princeton Reverb--Help me with flabby bass

[kealonnix]

Hey guys. Just finished building a Hoffman Princeton Reverb. Everything works great and it sounds great for the most part. I built it as a head and plan to run it through a 1x12 cabinet. I primarily play a telecaster and when I get the volume over 6 or so the bass is extremely flabby. The amp is dark, as most princetons are. I was just wondering if I could do a nod to tighten up the bass. I don't want to neuter the bass, just tighten it up. Maybe changing a filter cap in the bass tone circuit?

[shooter]

here's one thread I found searching "flabby bass"
looks like there might be something you can try;

http://el34world.com/Forum/index.php?topic=18008.msg182382#msg182382

I didn't bother looking at the rest from the search.
hope it helps

[HotBluePlates]

... when I get the volume over 6 or so the bass is extremely flabby. The amp is dark, as most princetons are. I was just wondering if I could do a nod to tighten up the bass.

Turning the Bass knob down doesn't do the trick? I found when playing at home & low volume with the '67 Princeton Reverb I used to own, I might crank the Bass up to 7-8. But playing louder or with other instruments (which fill out the bass range), the amp sounded better with the Bass at 3-4. The trick is setting the tone controls with your eyes closed or not looking at the amp, rather than setting them at the same number-settings each time. Note the response of the Bass control changes when you change the Treble control position, so you should tinker both to land on the best sound. That best sound may not arrive at numbers you think they should, because the controls are interactive and don't give the same amount of boost at a given number-setting, all the time.

If simply turning down the bass doesn't work, then trim the overall bass response by changing 1 or more preamp cathode bypass caps from 25uF to 1-5uF (closer to 1-2 uF is probably best). You'll shave bass slightly without gutting the amp's sound. I'd do this 2nd if simply turning down bass doesn't work.

If you still have too much bass, you can swap 0.022uF caps in place of both the 0.1uF and 0.047uF caps in the tone stack. This will change the tone control response and clear up the bass a little, but is much more subtle than changing cathode bypass caps.

Last ditch would be to reduce coupling cap values, starting with the phase inverter caps going into the output tube grids. Reducing those from 0.1uF to 0.047uF or 0.022uF will reduce bass reaching the output tubes.

I strongly recommend trying one step at a time, then listening with the amp at the higher volume where you noticed the bass issue.

[kealonnix]

Hotblueplates, thanks for all of the insight. I called myself playing with the treble and bass pots, but I will go back and experiment more. I will say that a humbucker in the Hi input sounds pretty good and not too flabby. This amp takes humbuckers better than my 67' PR.

I will tinker around with it.

If I wanted to increase headroom do you recommend JJ 6V6s?  I've read that in several places. I have a dr. Z 1x12 cabinet with a Celestion Vintage 30 in it, but I'm not convinced that's the best speaker for what I want. I really do prefer a good amount of headroom. Do you guys have a suggestion on a 12" that won't break the bank?

[kealonnix]

Here is a pic of the amp. This is my 3rd amp and most ambitious. I get obsessed when I start a project, and I think I finished this in 3 nights. I built my others on tdpri and enjoyed it, but I wanted to do this one mostly myself.  Well, I say that but I couldn't have done it without Doug's website. I don't mean to gush, but el34world has to be the most comprehensive website for an amp builder. I've learned more on this website than anywhere else.

I got the amp completed and it powered on, but no sound. I traced the layout with a highlighter 4 times and everything was correct. Determined not to ask for help, I made a signal probe and started probing. Then I realized that I didn't really understand signal flow and couldn't really read a schematic. El34 to the rescue. Found a signal flow layout and read up on how signal flows through tubes. I finally isolated the problem to a capacitor and spent 2 hours checking everything. It all looked correct. So I pulled the board and checked the under board jumpers. Bingo. Soldered it correctly and fired it up. This amp is so much more quiet than the 5e3 and 18 watt I built and I think it's because I followed Doug's recommendations. I think I'm going to rewire both of those.

Sorry to drag on, I'm just excited about this community.

[HotBluePlates]

Beautiful job, and looks like time well spent!!

If I wanted to increase headroom do you recommend JJ 6V6s?

I've never tried JJ 6V6's, so I can't offer an opinion one way or the other.


As for "headroom" my general opinion is the amp does what it does, and clean volume is limited by the output stage. Meaning to get more clean output power (volume) you need a bigger output stage.


That said, you could also look at headroom as accepting the input signal you provide and staying cleaner, further up the volume knob (even though clean power output/volume hasn't increased). This is more a wider range of volume knob adjustment while staying within the same range of clean volume. The reason (as I mentioned in another thread) s in this amp, like most Fender amps, the output tubes distort first.


The easiest way to make the amp cleaner is to swap in 12AT7's or 12AU7's in place of 12AX7's. The amp won't be set up to  ideally operate those tubes, but they will reduce the signal gain and appear to stay cleaner, longer. It's also very simple to reverse this "mod" when you want the stock sound.

[kealonnix]

Hotblueplates, I went to your tweed deluxe reverb build page.  quick question, how did you do the lettering on the control panel?  you said you used "white dry transfer lettering".  Is that something that you printed?  I'm planning on making a faceplate out of aluminum and polishing it to a mirror finish.  my family has a screen printing business and I was going to screen print it, but it's a huge hassle for a single amp.  didn't know if there was an easier way.

[p2pAmps]

Here is a pic of the amp. This is my 3rd amp and most ambitious. I get obsessed when I start a project, and I think I finished this in 3 nights. I built my others on tdpri and enjoyed it, but I wanted to do this one mostly myself.  Well, I say that but I couldn't have done it without Doug's website. I don't mean to gush, but el34world has to be the most comprehensive website for an amp builder. I've learned more on this website than anywhere else.

I got the amp completed and it powered on, but no sound. I traced the layout with a highlighter 4 times and everything was correct. Determined not to ask for help, I made a signal probe and started probing. Then I realized that I didn't really understand signal flow and couldn't really read a schematic. El34 to the rescue. Found a signal flow layout and read up on how signal flows through tubes. I finally isolated the problem to a capacitor and spent 2 hours checking everything. It all looked correct. So I pulled the board and checked the under board jumpers. Bingo. Soldered it correctly and fired it up. This amp is so much more quiet than the 5e3 and 18 watt I built and I think it's because I followed Doug's recommendations. I think I'm going to rewire both of those.

Sorry to drag on, I'm just excited about this community.

Very nice job, these PR are fun to build and sound really good...

[sluckey]

Hotblueplates, I went to your tweed deluxe reverb build page.  quick question, how did you do the lettering on the control panel?  you said you used "white dry transfer lettering".

I think that's my site. Dry transfer lettering is sometimes called 'rub on' lettering. The lettering and numbers come on a sheet of paper. Individual alpha-numeric is a pita when spelling words like volume, etc., but thankfully you can get sheets that have common electronic words. Just put the sheet in place and use a round point stencil (or ball point pen) to rub the letters off the sheet and onto the chassis. Then put the backing sheet over the freshly laid on letters and burnish with the other end of a ball point pen.

http://www.letraset.com/shopcontent.asp?type=info-transfers
http://www.worthpoint.com/worthopedia/datamark-dry-transfer-lettering-audio-244138545

[EL34]

Another very clean build, good job


I plugged my PR into a Marshall 4x12 and there was no flab