Princeton on Steroids

[jbefumo]

Just finished this build and after playing with it for several days am really happy with the results.  Actually the only resemblance to a 5F2 are the chassis, control layout, and single-ended topology, but I tried to stay true to the spirit, but with 20W, it definitely has the oomph to hang with a small combo/gig.  Still working on the cabinet, but will post pics and sound clips shortly.  Tried it with a bunch of different speakers, including a 15" ceramic Rola, a pair of alnico 12" Webers, and a 12" alnico Rola, but the one that it really seems to like is a Chinese made ceramic Celestion, so that's what's going in the box.  Attachments show what's inside.

[uki]

Wow this is one interesting amp,     does it stay clean at max volume due to the kt120 ? Why use so powerful tube for a single ended ?
Got any sound samples ? How fat it sounds ?

[jbefumo]

Actually, it stays clean and round up to about 60-60%, then breaks up smoothly, but retains note articulation.


As to why ... because I don't know what I'm doing?  ;^)


Actually, I've been messing with this idea for several  years now -- I originally thought I could do it with a KT-88, but single-ended puts some serious stress on the output tube, and I didn't want to build anything that was going to flame out at a crucial moment.  I started looking at the KT120, and lo and behold, the datasheet actually had a 20W single-ended configuration as a "typical application," so I felt fairly confident that I was on the right track.


The tricky part, at least for me, was in keeping the grid steady while applying adequate B+ to the plate.  400 on the plate, and 225 on the plate suggested a pretty hefty resistor, and as the current draw of the grid changes with the plate current (if I understand my theory correctly), the voltage is going to start fluctuating.  Then I stumbled upon that nifty little high-voltage, low-current regulator, and it seems to be working just as it should.


Am thinking for my next project of a single-ended version of a 40-45W Super Reverb ... talking with Mercury Magnetics about the iron, and they suggested a few changes to my calculations (REALLY helpful folks), but don't think the idea is totally wacko.  The OT will be the size/weight of a Marshall 100, which I can live with.


As to why -- that's another question -- just ... because?  I've always loved the tone of those little single-ended amps, and I KNOW the rational approach is to just stick a mike in front of one and run it through the PA, but that would be too easy ...


To boldly go where no geek has gone before ...

Wow this is one interesting amp,     does it stay clean at max volume due to the kt120 ? Why use so powerful tube for a single ended ?
Got any sound samples ? How fat it sounds ?

[jbefumo]

Wow this is one interesting amp,     does it stay clean at max volume due to the kt120 ? Why use so powerful tube for a single ended ?
Got any sound samples ? How fat it sounds ?

Will be recording some demos as soon as it's back together -- building the cabinet at the moment (always my downfall!).


As to breaking up vs. staying clean -- as most audiophiles know, single-ended amps with big tubes are capable of remaining distortion free, if that's what's desired.  I wanted clean to be available, but I didn't want it to sound like plugging into your stereo receiver, ya know?  the paralleled input triodes (copped the idea from Matchless and Geezer on this forum) give maximum clean headroom up front, but can overdrive that second preamp stage easily, so you have that preamp overdrive on tap.  My goal was to have that cathode follower provide enough drive to push the power tube.  To tell the truth, I initially planned to use a 12DW7 there, with the 12AU7 half in the cathode follower slot, with, say, a 10k load resistor, but decided to stick with the more straightforward approach, and experiment with the 12DW7 later, as it's own experiment, once I had everything else where I wanted it.


Joe

[jbefumo]

At one point I had a 75-Ohm cathode resistor under the KT120, but that put my plate dissipation right at the max -- what can I say, I'm a whimp, so I backed it off a bit. Still, the sucker is LOUD, and can do pretty loud AND clean.  Bypassing the tone stack really ups the grind, as one would expect, but that's all in the preamp, of course.


I have a Db meter, and will quantify all this when I record the demo.


Joe

Wow this is one interesting amp,     does it stay clean at max volume due to the kt120 ? Why use so powerful tube for a single ended ?
Got any sound samples ? How fat it sounds ?

[tdvt]

Looks cool!

I will look forward to hearing it.

Your design is rooted in the amp I just built & further than I might go on the next one, but down the same path.


TD

[jbefumo]

Great minds, as they say, think alike ;^)

Looks cool!

I will look forward to hearing it.

Your design is rooted in the amp I just built & further than I might go on the next one, but down the same path.


TD

[jbefumo]

Next in my queue (under way, actually) is one of those cool Tweed OD Specials from this forum, but . . . with a 100W 2XKT88 back end ...


Stay tuned!

Great minds, as they say, think alike ;^)

Looks cool!

I will look forward to hearing it.

Your design is rooted in the amp I just built & further than I might go on the next one, but down the same path.


TD

[uki]

I see your point, and it is a great idea, instead of just copying an existing amp, why not take the idea beyond ?!! I would do the same if my knowledge were big enough!! I'm just a learner so far, I've built one amp, just one  , with considerable success, but to create something new, that is the thing !! Oh by the way, it would be really groovy if you could post some pictures !!

Next in my queue (under way, actually) is one of those cool Tweed OD Specials from this forum, but . . . with a 100W 2XKT88 back end ...

WOW I like the idea !!

You will like to see this video(playing starts at 1:15) 3 different Tweeds, IMO the 1st one the guy play is the best, huge tone. Check out, I'm not sure but I think that one is the 5C4, or something close to it.

[jbefumo]

Will post some pics shortly.  Just finishing up the cabinet as we speak.  Expecting grill cloth tomorrow. 


We're ALL just learners! When we stop learning, we stop living.


Seems to me, that just building clones of existing designs would be like ... a JOB!  We can stand on the shoulders of giants without necessarily walking in their footprints, if that makes any kind of sense.


Wow -- some impressive picking on that vid!  Guy reminds me a bit of Albert Lee. 


I'll have to re-watch that vid when I'm more awake, and less under the influence of shellac fumes ;^)


Funny thing about instruments -- sometimes the ones you think you like least turn out to be your favorites at some other point in time.


Case in point -- I had this way funky 1961 Melody Maker 3/4 a while back.  The single-coil sounded really weak to me.  Didn't want to mess up a vintage guitar, so I had someone make me up an entire new pickguard, with a new set of electronics, and experimented with a succession of single-coil, and pseudo-single-coils -- hot rails, cool rails, stacked humbuckers.  Later, when I was getting ready to sell it, I put the original parts back on, and found that I REALLY liked it the best of all.  It had a personality all its own, and with a bit of amp tweaking to suit, it made some wonderfully unique tones.  Just goes to show -- everyone SAYS they want their "own sound," but when push comes to shove, what they really want is to sound like Hendrix, or Clapton, or Leslie West, or Van Halen, or .... 


but it's late, and I'm rambling...

I see your point, and it is a great idea, instead of just copying an existing amp, why not take the idea beyond ?!! I would do the same if my knowledge were big enough!! I'm just a learner so far, I've built one amp, just one  , with considerable success, but to create something new, that is the thing !! Oh by the way, it would be really groovy if you could post some pictures !!

Next in my queue (under way, actually) is one of those cool Tweed OD Specials from this forum, but . . . with a 100W 2XKT88 back end ...

WOW I like the idea !!

You will like to see http://www.youtube.com/watch?v=m4xbAcNx-WI]this video(playing starts at 1:15) 3 different Tweeds, IMO the 1st one the guy play is the best, huge tone. Check out, I'm not sure but I think that one is the 5C4, or something close to it.

[uki]

... Didn't want to mess up a vintage guitar, so I had someone make me up an entire new pickguard, with a new set of electronics, and experimented with a succession of single-coil, and pseudo-single-coils -- hot rails, cool rails, stacked humbuckers.  Later, when I was getting ready to sell it, I put the original parts back on, and found that I REALLY liked it the best of all.  It had a personality all its own, and with a bit of amp tweaking to suit, it made some wonderfully unique tones. 

Sometimes the noise of single coils does bother, and people look after those pseudo-single-coils, and then the tone changes way out of the expectancy, have you tried a dummy coil ? I've found this, got this on my guitar and it does the trick w/o mess up with the tone of my single coil !

[jbefumo]

Personally, I LOVE P90s, but wound up replacing the ones in my Tele Special with mini-humbuckers because there was no way I could record near a computer unless the switch was in the middle (hum-canceling) position.  Since then, though, I've come to adore those minis, which I never before even gave a thought.

... Didn't want to mess up a vintage guitar, so I had someone make me up an entire new pickguard, with a new set of electronics, and experimented with a succession of single-coil, and pseudo-single-coils -- hot rails, cool rails, stacked humbuckers.  Later, when I was getting ready to sell it, I put the original parts back on, and found that I REALLY liked it the best of all.  It had a personality all its own, and with a bit of amp tweaking to suit, it made some wonderfully unique tones. 

Sometimes the noise of single coils does bother, and people look after those pseudo-single-coils, and then the tone changes way out of the expectancy, have you tried a dummy coil ? I've found this, got this on my guitar and it does the trick w/o mess up with the tone of my single coil !

[jbefumo]

The aforementioned Tele: http://www.befumo.com/music/instruments/tele.html

Personally, I LOVE P90s, but wound up replacing the ones in my Tele Special with mini-humbuckers because there was no way I could record near a computer unless the switch was in the middle (hum-canceling) position.  Since then, though, I've come to adore those minis, which I never before even gave a thought.

... Didn't want to mess up a vintage guitar, so I had someone make me up an entire new pickguard, with a new set of electronics, and experimented with a succession of single-coil, and pseudo-single-coils -- hot rails, cool rails, stacked humbuckers.  Later, when I was getting ready to sell it, I put the original parts back on, and found that I REALLY liked it the best of all.  It had a personality all its own, and with a bit of amp tweaking to suit, it made some wonderfully unique tones. 

Sometimes the noise of single coils does bother, and people look after those pseudo-single-coils, and then the tone changes way out of the expectancy, have you tried a dummy coil ? I've found this, got this on my guitar and it does the trick w/o mess up with the tone of my single coil !

[uki]

I was browsing your page and went across a video, and I was pleased with the content ! 

We are  so what ?!!   

Ok just to not stray way too much here a pic of my "beast".

[jbefumo]

OK, poor audio quality, but here's a first look/listen:

[uki]

Yeah that is a good tone amp !!! Way to go !!!

Here my beast sound: https://goo.gl/3Yxk8e

[jbefumo]

Nice tone and playing!  I have to start spending more time with a guitar in my hands and less with the soldering iron (I say as I just ordered another board from Doug Hoffman ;^)

Yeah that is a good tone amp !!! Way to go !!!

Here my beast sound: https://goo.gl/3Yxk8e

[jbefumo]

Been talking about several topics on multiple threads -- DC Filament supplies, regulated screen supplies, bias supplies, etc.  Consolidating what I think I have gleaned from many responses, I came up with this admittedly rather unnerving idea.  Seems like it should work, in theory, but would like to solicit any dire predictions before I risk my transformer ....


(I'm actually already using a Carling 3PST switch for the standby, with one set of poles unused, because it was the only thing I could find that was actually rated for the voltages being controlled.)