How to voice a Hoffman Princeton 5f2a for keys?

[newguitarsmell]

Any thoughts guys? What would best accommodate the range/tone?

[PRR]

Link?? (Like we remember every amp ever proposed...)

Anyway.... keyboards should normally be reproduced "clean". This is a one-6V6 amp, maybe 6 Watts. You need an efficient speaker in a small venue to be happy this way.

The 5F2a is not a high-color amp. It is probably too sensitive for e-keyboards, omit the first cathode cap, maybe decrease the NFB resistor from the output. The 1Meg Volume pot loses some highs, change to 250K. May want a larger coupling cap here. This will reduce tone-knob action, but keyboard does not need heavy tone control. You may like a larger cap under the 6V6. Speaker will be critical: a small guitar speaker will make all keyboards sound like small guitar.

[newguitarsmell]

Nice! What speaker do you recommend?

[newguitarsmell]

PRR - what amp do you think would be best revoiced for keys?
I love my 5f2a with the guitar - want something that isn't super loud at the get go. Any interesting links you can send or refer me to?

[jjasilli]

Keys??? Full range organ / piano; or just 1 or 2 mid octaves?


Guitar amps need only be voiced for 80 > about 1600 Hz fundamental notes.  Guitar speakers tend to drop-off sharply @ about 4500 - 6000 Hz.  Do you want 20 - 20,000 Hz?  This makes a difference re frequency range of the OT; 2 or more speakers in the cab with crossovers.


You need to state your design goals more clearly.

[Ritchie200]

To reference one of the greatest (in my humble opinion!) Hammond sounds in history I give you Deep Purple Made in Japan and the three albums before that. Jon was running the Hammond into a Marshall Major into several full range Marshall PA cabinets. I believe they were 15” with horns. The Major is a very clean amp although he was pushing the front end to death.

Maybe build a head and pick your speakers to taste.

So basically what PRR and jjasilli said!


Jim

[newguitarsmell]

I would like a full range amp - 88 keys optimum. The farther I go down this inquiry, the more fascinating it becomes. What does a crossover do? I love the Deep Purple reference.

The signal coming from the keys is significantly hotter than from a regular guitar.

If I run my keyboard into a guitar amp, what will happen? Distortion? I don't want to try now....

Also - starting to figure out why there are no decent tube amps for keys. I used an Ampex flip top bass amp long ago - but there was no sparkle... I currently use a PA or my Bose tower - decent but nothing like tubes.

[newguitarsmell]

Revamp box?
Passive DI backwards?

How to lower the keys level to guitar level so I don't fry stuff...

[jjasilli]

This can be done.  The path is to work backwards.  I'm thinking a vintage style Altec cab with a 12 - 18" driver, hi efficiency.  This is needed to get deep bass with few Watts.  Maybe use a coaxial driver for midrange to save space.  Also a piezzo or horn tweeter.  Build passive crossovers as needed, or go crazy and use active crossovers to preserve power.


Use a hi-fi grade SE OT for full range frequency output.


You can use an outboard buffer to match keyboard output to input impedance and signal voltage > tube amp input, e.g.

[newguitarsmell]

I have an altec Santana 4 speaker set - not too portable...

[newguitarsmell]

Recommendations on SE OT? I'm currently using a Mercury 5w..

[DummyLoad]

if you have a thin wallet.


http://www.classictone.net/40-18031.pdf


if your wallet is much thicker + chassis mods:


https://www.electra-print.com/singleended.php


--pete

 

[newguitarsmell]

Pete
I love that avatar. What a monkey!

What do you recommend from the list? 15w?

[jjasilli]

The Santana's seem to have mixed review's.  I think they should be OK, especially since you already have them.  For this purpose their efficiency is less than ideal since they want a min of 12W input:  https://www.google.com/search?q=altec+879a+santana+specs&tbm=isch&source=iu&ictx=1&fir=WWs8q9J9GMKufM%253A%252CTx_fJ1HvU5-TSM%252C_&usg=AI4_-kRHtU2JlstUru7pjTGha8FAGyC8MQ&sa=X&ved=2ahUKEwjr7saym6rfAhXQct8KHfAhDWcQ9QEwDXoECAQQCA#imgrc=WWs8q9J9GMKufM:


If you parallel them down to 4 Ohms and drive them both with an 8 Ohm OT tap, you're just about there, power-wise. But, the lowest A on the modern piano is about 27.5 Hz; the Santana's only go down to 40Hz. 



"not too portable..."  The Laws of physics are not on your side.  Hoffman's Iron Law was discovered in the 1960's:  http://sites.psu.edu/speakerdesign/2013/01/24/hoffmans-iron-laws-of-speaker-building/



"Hoffman’s Iron Laws of Speaker Building: 1) Bass Extension 2) Efficiency  3) Small Enclosure.  The law is that you can only have two of these three attributes in speaker building.  Ideally we would want all three, we want speakers with good bass, can play really loud, and are small.  Unfortunately we cannot have all these."


If you insist on a 5W amp and deep bass, then you need an enormous speaker enclosure the size of a Voice of the Theater (Altec VOTT).  If you want a smaller enclosure and deep bass, then you will have lower efficiency and need a hi power amp.

[newguitarsmell]

HA HA I just looked at those speakers! Not coffee shop friendly! Aha ha ha

Ok - what size amp would accommodate the Santana or smaller - 12” speaker - which I have?

I like the Hoffman amps a lot - and I can upgrade PT and OT so the spectrum is great.
As for the lowest notes on the piano... overrated. I’ll use my 76 key

[jjasilli]

To make 12W: a 6L6 would make about 10W, close.  KT88 should hit 12W.  PP 6V6, 12 - 22W. 


It's not just the size of the driver (speaker), but its frequency range, efficiency (SPL rating), and the size of the enclosure it needs to produce deep bass.  Assuming you have the right driver (speaker), as you decrease the size of the cab enclosure, the amp power needed to drive bass goes up exponentially.


Another solution is, say, a vintage Bogen MX-60A PA amp > PA (full range) enclosure.  But this amp is Heavy! And so is the PA Speaker cab.  Hence, SS.


It's your choice where to compromise.  It's all Faustian choices.  Deep bass and decent volume mean heavy weight to lug around, and lots of floor space.  You've already moved from 20Hz to 76Hz.  You no longer need a hi-fi grade OT!

[Ritchie200]

If your keyboard(s) have a line out you are not going to burn anything up.  Plug it in.  You will probably be surprised at the glorious noise your Princeton will make when you tip that volume pedal.  Play around with it.  Twiddle some knobs.  What do you like and not like?  This will guide you on your build.

Jim

[silverfox]

I know on a tube amp forum this is heresy but: If you want a clean sound, why not run it into a mixer and power amp or powered speakers?
silverfox.

[Ritchie200]

GASP!!!!!!

 

Because then he would not sound like this:

Jim

[PRR]

> I love my 5f2a with the guitar

If you have a 5F2a in hand, you can make most of my changes in a few minutes, see how it sounds. Leave the Vol pot 1Meg. Snip one end of first cathode cap. Tack 22K across the 22K coming from the speaker. Does it play? Loud and clean enough for the dives you play? The Vol-pot change will pass more of the synth's supersonic squeegles (no effect on piano). The fat cap under 6V6 may support deeper bass lines more solidly.

A next step up from a classic 5F2a rig is a BIG speaker. The usual 5F2a has a speaker like a ukulele; pushing 6 Watts at it still only makes a small guitar. Not a Piano, Organ, or Rick Wakeman. Yes, it may be fine for tinkling behind a mild singer; OTOH most voices sit better accompanied with a phatt bottom and clear sparkly ornaments--- a large hi-fi speaker, not a table radio.

[newguitarsmell]

A long time ago I requested some voicing changes for a simple single ended amp to use with my keyboard.
PRR sent a schematic and today I finally got to the place where I could build it. . I used a fancy OT made in Sweden - don’t know if that helped - but I wanted to report that it sounds FANTASTIC!!
Thank you everyone. It’s just what I wanted.

[Colas LeGrippa]

IMHO I think a keyboard needs a hifi output tranny otherwise it will sound too mushy. the best would be a 6V6 PP for better headroom than a se, ideally with a 1650N Hammond transfo in UL configuration. Then you d have to double the load to reflect 8k on the power tubes or use a 8k OT with UL taps.
Colas