Hoffman AB763 Single channel Princeton?

[proaudioguy]

What are the implications of using the AA1164 Transformers and chassis with the Hoffman AB763 circuit?  Would be adding 1 TUBE next to the input jacks.  T1 would be on the FRONT side of the chassis and the rest of the tubes would be on the back side of the chassis.  The turret board would be in the middle.  This would facilitate getting everything I want into the package I want.  I already built an AA1164 with a 12” EV speaker.  This would be essentially the same cabinet (although I may make it deeper to recess the knobs and to allow for more space for the acoustic properties of the box.  The chassis would be PR and the transformers would be PR but the circuit would be DR.  This would affect the voltages available I suppose.  Is the output transformer smaller?  How would that affect things?  I am totally OK with a DR sound at lower volumes.  I want the LTPPI, the Bias Vari trem, the big fat reverb, 1 channel.  Tube rectifier, Compression sag, early breakup.  Preamp and power amp distortion.  I like to be on the edge between clean at barely touching the strings and dirty when digging in the pick, without much change in volume.  I want to be able to crank it up for a lot of crunch without being too crazy loud.  Would this work?  What components in the circuit would have to change?  Has anyone done this?  This has been in my head for a long long time but I never get around to working on it.

[tubeswell]

The 125A1A (DR) is 6k6 Pr 20W


The 125A10B (PR) is 8k 15W (12-18W), so smaller size = a bit earlier saturation, and shallower load line = a bit more g2 dissipation (and therefore tending to lower Vg2 - so less gain) under big signal conditions, which it will hit 'sooner', given the DR pre-amp - especially on the vibrato channel - (and including LTP inverter) has more gain (seeing as how you seem to be wanting 5 preamp tubes, which indicates you want to keep the DR vibrato channel, although you haven't supplied a schematic of your vision, so I'm making guesses). This tendency to lose gain would be offset slightly by the B-load line tending to kick in sooner (depending on output tube bias) given the bigger pre-amp driving signal (although the likelihood of cooler output tube biasing to accomodate the bias-vary trem may counteract the transition point between A and B load-lines)


That's my guess. YMMV.

[jordan86]

You may need to upgrade both transformers. The stock Princeton PT is a little meager, even for a Princeton. Adding another tube will be really pushing it I think.

Check this site: https://thesubjectmatter.com/calcptcurrent.html

Other sharper minds may be able to speak with more authority.


If you want a drop in upgrade power tran, an Allen TP25 is great. But if you upgrade the PT you may run the risk of overpowering your OT. So it may need to be a package deal.

[jordan86]

The Hammond 290AX might work. Has 100ma. Much cheaper than the Allen. You’d be a little short still on the heater current. You need about 2.5A+. It’s only 2.25A

[mresistor]

think I would use an Allen TP25H

[proaudioguy]

(seeing as how you seem to be wanting 5 preamp tubes, which indicates you want to keep the DR vibrato channel, although you haven't supplied a schematic of your vision, so I'm making guesses)
That's my guess. YMMV.

You are a good guesser.  Thinking Hoffman 1 channel AB763 in a princeton chassis with princeton iron.  This saves space because bigger transformers take up more space, and I like earlier breakup and sag.  I have a twin (if I can get the gremlins out----Other thread) so I have gobs of loud clean.  I would like to use an AB switch and go to this for dirty.  Single EV 12”. I already built a stock princeton with a single EV 12” so I know it will fit with the stock transformers.  That was a gift for my step father so its out of the equation, otherwise I would have it open.


I want the deluxe reverb sound in the princeton package and if I get a little earlier breakup and sag, I’m all about that too.


Will those “upgrade” transformers FIT without cutting into the chassis?  Do they stick out further?  These are questions I need to get answers to before contemplating alternative transformers.  Thanks for the part numbers!

[tubeswell]

I want the deluxe reverb sound in the princeton package and if I get a little earlier breakup and sag, I’m all about that too.

What about a PR with a DR PT and DR OT and a 12" speaker?

[glass54]

Do it proaudioguy 
I did a 1Ch in a Princeton Aluminium chassis, with custom toroid (had a few in my kitty) and a DR output Tx.
I wanted the switches at the front and an IEC mains socket and I used a Fender 12" speaker (had one in a box for a long time),
It has a SS rectifier, little sag and very clean tones.
Used the Hoffman turret board, works very well. The switch at rear in gutshot was replaced with a Switchcraft 12A socket to switch Rev and Trem. (Didn't have one in stock when photo taken).
Trust this helps.
Kind regards
Mirek

[mresistor]

Will those “upgrade” transformers FIT without cutting into the chassis?  Do they stick out further? 

The Allen TP25H is a drop in upgrade for a princeton chassis but it's about an inch taller.  It's a very serious upgrade. I have one in my Hoffman PR. http://www.allenamps.com/trans.html
"310V-0-310V @ 150ma, 6.3V at 4.5A, 5V at 2A. 50V bias tap, center-tapped heater winding"
Capable of using 5881, EL34 and 6L6 as well as 6V6.

The Hammond is also a drop in.

[daveneary]

The Hammond 290AX might work. Has 100ma. Much cheaper than the Allen. You’d be a little short still on the heater current. You need about 2.5A+. It’s only 2.25A

I have used a 290cax from Hammond. Sounds good and no noticeable difference from a full deluxe spec.

[daveneary]

What are the implications of using the AA1164 Transformers and chassis with the Hoffman AB763 circuit?  Would be adding 1 TUBE next to the input jacks.  T1 would be on the FRONT side of the chassis and the rest of the tubes would be on the back side of the chassis.  The turret board would be in the middle.  This would facilitate getting everything I want into the package I want.  I already built an AA1164 with a 12” EV speaker.  This would be essentially the same cabinet (although I may make it deeper to recess the knobs and to allow for more space for the acoustic properties of the box.  The chassis would be PR and the transformers would be PR but the circuit would be DR.  This would affect the voltages available I suppose.  Is the output transformer smaller?  How would that affect things?  I am totally OK with a DR sound at lower volumes.  I want the LTPPI, the Bias Vari trem, the big fat reverb, 1 channel.  Tube rectifier, Compression sag, early breakup.  Preamp and power amp distortion.  I like to be on the edge between clean at barely touching the strings and dirty when digging in the pick, without much change in volume.  I want to be able to crank it up for a lot of crunch without being too crazy loud.  Would this work?  What components in the circuit would have to change?  Has anyone done this?  This has been in my head for a long long time but I never get around to working on it.

I have built a single channel deluxe  reverb in a princeton chassis. 290cax xformer and deluxe output. 5 preamp  valves with the extra one slightly forward of the others between the 3rd and 4th tube. Works very well. Alternative is to go with 4 tubes... Dropping the tremolo.

[glass54]

Another alternative is 4 off 12++7 and an IRF840 Mosfet for tremolo. All audio is in tube circuits (to be pure  ) and a SS control.
See attached sch. Not sure where you might get a better resolution drawing but Sluckey may be able to help (sorry to dob you in Sluckey)
If I was doing mine again I would definitely have used an IRF840 and save a little heat and space.
Just for the record my Toroid had secondaries of: 295VAC @250mA and 6.3V@4A, so I could use 5881s if I wanted.
Kind regards
Mirek

.....and the link to sch below: https://el34world.com/Forum/index.php?topic=26928.msg295794#msg295794

[jordan86]

290ax and 290cax are both drop in replacements. No cutting. Same with the Allen that was posted. But Allen will be taller.

These two are nearly identical. cax has more current on the HV secondary. But less current on the filaments and rectifier secondaries.

https://www.hammfg.com/files/parts/pdf/290CAX.pdf

https://www.hammfg.com/files/parts/pdf/290AX.pdf

[proaudioguy]

Good to know there are drop in replacements available.  What happens if you just use the stock princeton transformers though?  Is it the same only it distorts at a lower volume (les clean headroom) or is there more to it than that?  Is the PR transformer able to supply the heater current needed?  I am thinking that is the most important question to answer.

[daveneary]

Suggest but don't know. If you go for the 4 x preamp tubes. Heater should be adequate ish but main supply will sag. Would be interested.

[jordan86]

My guess is that none of them truly supply the needed heater current, except the Allen unit. Bigger money though.

From what I have read, I don’t think a stock PT stands a chance with 5 preamp tubes. Sounds like someone has done this successfully with the 290cax, so that’s something. Good price too. I think you will still get plenty of sag with those Hammonds.

[proaudioguy]

Yea I may just do a version without the Tremolo and see how it sounds.  Gotta have the reverb though.  So at that point it would be using the princeton iron but have a long tail phase inverter.

[proaudioguy]

OK so as I’ve.been thinking about this more and more, I’m far more interested in the tone and the package and I could give up the tremolo.


Has anyone done a DR with only 1 channel, no tremolo, and using PR iron?  Would only have to add a choke and swap out some components which would make this a whole lot more economically feasible.  I could cover the cap can hole pretty easily too.


V1 Input and tone stack
V2 Reverb driver
V3 Reverb recovery and makeup gain
V4 LTPI
V5 V6 6v6gt
V6 rectifier


Looking for that compression.  The opposite of the twin reverb I already have.


I already have a DRII which I am working on, but although the clean sounds great with a few minor mods, it is still incredibly loud and there isn’t much compression even cranked which I assume has to do with the SS rectifier.  Also its almost as big as the Twin and its heavy.  I definitely want to get it where its a really solid studio quiet amp, but eventually, I’m thinking I’m going to be down to just 1 amp and I want to build the one I’m going to use the rest of my life.  Maybe I’ll keep one more for a backup, or maybe I’ll just build 2 of the same once I get it where I want it.  I have enough kids and grand kids also just getting into music that I should be able to pass on the rest of the amps.

[sluckey]

I would just use Hoffman's single channel AB763 board and omit the trem section. The board could even be shortened about two inches.