New Sluckey Style AB763 Lite Princeton Size! Choke question and PI question

[ToneJunkie]

OK I have a champ sized chassis that I'm going to fit a AB763 Lite into (just normal channel of a AB763).  Do I need a choke?  Also if I do is the only reason the Deluxe Reverb has a choke and a Princeton does not is because of the PI type?  What I have on hand is a Princeton sized PT (330V @ 75ma) and a Deluxe sized OT but I don't have a choke on hand.  Here is some pictures of the cab that I finished over the summer...


Oh and I want to say that I make amps maybe once a year and I find this forum to be such a blessing for a tinkerer like myself.  Thanks in advance for all the past help guys!

[sluckey]

The AB763 needs a choke. The Princeton Reverb does not have a choke because it's a cheap amp.

[ToneJunkie]

Dang it. Ok I got to order one last part a choke :-)  LOL... I will order it from Doug he always ships quick.  Its my week off of school (I'm a high school teacher) and I want to bust out this amp over break!  Thanks Sluckey

[sluckey]

The AB763 needs a choke. The Princeton Reverb does not have a choke because it's a cheap amp.

I guess I should clarify a bit. No amp really "needs" a choke to operate. They will operate quite well without one. But any amp will benefit from having a choke because the B+ will be smoother. Cheaper amps often don't use a choke simply because of the $$$ factor.

When I say the AB763 needs a choke my reason is based solely on the fact that all original AB763 amps were designed with a choke.

The type of PI in an amp is not a factor in the decision to choke or not.

[bnwitt]

Just be aware that Doug is on holiday and isn't shipping again until January 3rd.  The Fender 5E3 didn't come with a choke but it benefits greatly by the addition of one as in Doug's version as it tightens up the bottom end frequencies.  I'm building an AA1164 Hoffman Princeton reverb at the moment and I'm adding a choke to it as well.

[dude]

Is a choke more like a filter cap than a dropping resistor? I noticed that the dropped voltage through a choke is very minimal, a few volts whereas even a low value dropping resistor drops more....?


What does a choke actually do in theory. I know it smoothes the bass freq., replaces a filter at times, doesn't lower voltage very much and is used in a lot of higher quality Fender amps.


Speaking of a 5E3, which stock is known for "farting out" without the cap changes, using a choke would it replace the 5K dropping resistor or simply add it before that resistor?


thanks,
al   

[sluckey]

A parallel filter cap opposes (resists) a change in voltage across the cap. You could say it tends to smooth out any ripples (changes) in voltage.

A series choke opposes (resists) a change in current flowing through the choke. You could say it tends to smooth out any ripples (changes) in current.

When caps and a choke are combined in a pi network they work together to produce a pretty smooth dc voltage at the output of the network without actually dropping the voltage significantly. (You can get a pretty smooth dc voltage by just using caps and a big resistor. But the big resistor also means a big voltage drop. This is wasteful unless your goal was to get a much smaller voltage in the first place.

The reason that a choke has a relatively smaller dc voltage drop than a big resistor is because the dcr of the choke is much smaller than the typical dropping resistor.

5E3... yes, replace the 5K with a choke. And add a 470Ω screen resistor for each tube. Probably gonna loose some of the charm of a 5E3.

[dude]

Thanks Sluckey for the info.


Just curious, when you say you might lose the mojo using a choke in a 5E3, I'm trying to understand what effect a choke would have inplace of that 5K. Only thing I can think is the voltage across the rail would be higher. A 5K in say a PT 300-0-300, would lower the voltage approx. 15v... or so, true? Would that 15v really change the tone, audiablely ...?  I guess it's subjective to one's ears...? Answering my own question, ha.


I got a kit from Mission amps years ago, at that time Bruce was using a Mojo 340-0-340 PT, kind of high for a tweed Deluxe (guess he got a good deal on quantity) but he included a 15v 15 watt Zener to go between PT and ground to lower the B+. The B+ was about 350v with the Zener, I assume about right for a 5E3.


Bottom line: Seems to me there are several ways to skin a rabbit in modding, fixing amps, etc. Simple things as changing a speaker can also be done by changing a coupling cap, etc... So many ways to solve a problem, things can get confusing to which is the right way, maybe there is no right way and it's all subjective...? With experience I'm just finding several different ways to solve the same issue.


Am I out to lunch here?     

[sluckey]

maybe there is no right way and it's all subjective...?

I totally agree. And even that is subject to change over time.

[bnwitt]

Well after having built some 30 plus 5E3 amps with both the original Fender circuit and the Hoffman circuit which has a choke I would say the only "charm" ones loses with the choke addition is farty bass and an inadequate power supply.  The real charm of the 5E3 IMHO is the interactive channel controls and the tone one gets when jumping the two channels together.  With a choke the result of the channel jumping is phenomenal.  Without a choke....meh.  I personally think the Hoffman 5E3 is the finest Tele/Strat tone machine every designed.  Every time I thought I was done building them and built a "last one" for myself, I'd let someone else play it and bam! it would be gone.  I now have one that no one is allowed to play but me.

[ToneJunkie]

Thanks guys I will wait till Doug gets back in to order my transformer and various other things.  Thanks for all the great information!  I ended up finishing a build that I started over a year ago!