SE versus PP

[benito_red]

hello to everybody,
I recently discovered this interesting forum, and I would like to ask a question about what differences there are between a SingleEnded and a PushPull amp. Of course the first difference is the amount of the power, but beside that, what difference about tone?
Let's talk for istance about two different Fender: the AA764 (Champ BF) and the AA964 (Princeton BF), the second seems to be the PP version of the first. Only one difference in the tone stack: the 15k resistor in the Champ vs the 6,8k in the Princeton.
If they would be connected to the same speaker/cabinet, how would they sound?
Thanks a lot
benito

[MWJB]

Firstly there are other relevant differences...you're not really comparing like with like.

You have noted already that the Princeton has a 6.8K mid resistor value, whereas the Champ has 15K...these values could be swapped, or both amps could be fitted with a 25K mid pot to allow the mids to be adjusted to taste. Champ probably has 15K to give it a more "forward" sound, given the limitations of speaker & design.

The Princeton, as well as being PP, is also fixed bias - this gives a tighter sound, better headroom & perhaps slightly less interesting harmonic content than cathode bias (Champ). Cathode bias tends to have a more emphasised envelope of attack & decay, runs the tubes at less voltage (sounds browner, less power) and has richer harmonic content. Again, either amp could be converted to fixed or cathode bias, or fitted with a DPDT switch to allow operation in either mode.

Being PP, The Princeton requires an additional tube, the phase inverter, to split the signal between the 2 power tubes. This is less relevant to the Princeton (non-reverb) as this is "missing" a tone stack recovery stage and is low on gain for a Fender. The Princeton Reverb subsequently has much more gain (even discounting anything added by the reverb).

If all things were equal (mid resistor, method of bias, preamp stages) the SE amp would sound crunchier & fuzzier when overdriven. The PP amp would be cleaner smoother and make around 3 times the power.

Champ through a 10" will still sound like a champ (assuming output impedances are matched in all cases), just with a little more headroom & better projection (assuming a reasonably similar style speaker). Princeton through the champ's 8" would probably not be a good idea in the long run due to the power difference, but a typical 8" guitar speaker would cut headroom/fidelity, but may sound warmer.

[imaradiostar]

I think the phase inverter plays a part too- the Princeton in stock configuration gets asymmetrical when driven hard. This can have a big effect on the tone though the negative feedback takes care of a lot of it.

jamie

[benito_red]

SE versus PP

hello MWJB and Imaradiostar,
thanks a lot for your answers.
MWJB wrote:
"... If all things were equal (mid resistor, method of bias, preamp stages) the SE amp would sound crunchier & fuzzier when overdriven. The PP amp would be cleaner smoother and make around 3 times the power. ..."

but what about the clean sound? I suppose that perhaps even below the clipping the SE performs more trebly and thinner, however it should retain his Fender "imprinting".
What do you think about?
bye benito

[MWJB]

The SE won't necessarily sound more trebly & thinner...in fact if I was recording, I'd go for the champ every time, as the tighter, cleaner PP amp will normally sound thinner when recorded. Yes, the Champ will still sound like a Fender.

Other than a very controlled environment, you're not likely to play a Champ "below clipping". So the Princeton will still hold up for small club gigs with a sympathetic band behind it, the champ won't unless mic'd up.

That's the heart of the matter...people choose amps for the volume levels they have to work with. Small practice amps are made to be cheap, so SE makes sense to keep parts count low & final product cheap. If you want more than 10W you go PP.

[Old Tele man]

Champ is SE Class-A (lowest distortion, predominately even harmonics); has air-gap OT to control core saturation; B+ about 350Vdc; 1 x 6V6GT = about 2-3W power output.

Princeton is PP Class-AB1 (higher power, predominately odd harmonics); has split primary OT to cancel power tube induced even harmonics; B+ about 420Vdc; 2 x 6V6GT = about 10-12W power output.

[MWJB]

"Champ is SE Class-A (lowest distortion, predominately even harmonics); has air-gap OT to control core saturation; 1 x 6V6GT = about 3W power output" True enough, but very few players will play a Champ at RMS rated output...so a crunchy 6W is more typical.

Princetons can push 20W under normal gigging circumstances.

[Bruce / Mission Amps]

Champ is SE Class-A (lowest distortion, predominately even harmonics); has air-gap OT to control core saturation; B+ about 350Vdc; 1 x 6V6GT = about 2-3W power output. ...

Thank you for that.. it drives me crazy when sometime keeps calling these cheap class A Champs with a postage stamp sized OT a 5 or 6 watt amp.
I've never had or seen one that could make more then 2 or 3 "clean/linear" watts into a dummy load at any reasonable low freq and drive level.
Many come in here and can't even muster 1.5 to 1.75 clean watts to be honest.

[HTH]

I've got into SE amps in a big way over the past year or so - one day I'd love to build a SE amp in the 30w range so I could have a SE amp loud enough to gig.

It'd be one beast of an amp in terms of weight, but I bet the tone would be sweet.

[MWJB]

Many of us have/had the same fantasy...in reality though you'd end up with an amp that weighed as much as a Super Reverb, maybe a Twin, but would only keep up with a PV C30! For the cost & effort a PP amp always seems to win out as the sensible option, there are things you can do to increase even order harmonics, to give a more SE vibe, or even try running one KT88/90 in a 2x6L6 PP amp & run hotter plate current (not so hot as to burn the OT of course & you still won't get 30 RMS). You won't have the air gapped SE OT if you try this, but even a 30W SE guitar amp isn't going to run clean at typical gig volume.

[mooreamps]

A couple years ago I built a couple 36 watt SE amps using a KT88 and an EL34 running a plate voltages switchable between 300 volts and 600 volts. I though it was pretty loud enough...

[Old Tele man]

A couple years ago I built a couple 36 watt SE amps using a KT88 and an EL34 running a plate voltages switchable between 300 volts and 600 volts. I though it was pretty loud enough...

...in one of my engineering textbooks is a statement I've always remembered: '...only 15W is needed for the average size room...' said of Hi-Fi stereo system. Of course, the text didn't clarify whether that was TOTAL power or per channel, ha,ha!

[HTH]

A couple years ago I built a couple 36 watt SE amps using a KT88 and an EL34 running a plate voltages switchable between 300 volts and 600 volts. I though it was pretty loud enough...

nice one, glad that someone else is as crazy as me in even considering such an amp

I'm envisaging a 4x EL34 SE head which will hopefully put out around 40w-50w with the plates around 400v.

Am I right in thinking that each EL34 will pull around 80mA at that kind of voltage and I'd therefore need a PT with a HT winding capable of handling at leas 320mA. Of course the OT would also need to be able to handle at least 320mA too.

Trouble is, where do I get a 300-0-300 PT with a 320mA (or more) HT winding and also an OT to match ??? - am I in the territory of having to parallel the transformers up?

[mooreamps]

Well, at 400 volts you don't want the plate dissipation on an EL-34 too much more than about 12 watts. Therefore, you will want to keep the plate current in the 30 mil range.


-g

[MWJB]

50-60mA would be more typical for a EL-34 at 400vdc on the plate, IF cathode biased. Fixed bias, anything up to 50mA might work? Really though, for the power you are thinking about, more plate volts would be a good idea (adjust current accordingly), as would fixed bias.

Parallelling 4 tubes sounds quite ambitious?