what limits the clean headroom of my 64 princeton reverb?

[pula58]

I have a '64 princeton reverb. I have had the plate voltage on the phase inverter increased (the "stokes" mod). Also, the previous owner had put in a 12" speakewr baffle. I have a 12" Cannabix Rex speaker.

When I turn up the amp past 3.5 it starts to get broken-up. Nice sound but I would like a little bit more clean headroom.

What limits the headroom of my amp? And, by headroom, what I meanis the volume at whoch the sound remains fairly clean. So, I don't want a V1 tube with less gain (since it won't be as loud). What I am after is more clean volume.

Of course, I could get another amp but I would rather not as I love the light weight grab and go aspect of this amp.

1) How much does the output transfer limit the clean volume the amp is capable of?
2) Is the Phase inverter still limiting the available clean volume range?
3) Is the power supply transformer limiting the available clean volume range?

Thnanks!

Paul

[Blue Strat]

The phase inverter is the primary limiting factor. After that, the output transformer and the power supply.

I converted my SFPR to a virtual Deluxe Reverb (one channel, no tremolo) by converting the PI to a long tailed pair and installing an Allen Ampification upgrade OT. Also plopped the chassis into an Allen upgrade cab with a 12" speaker. The amp draws a lot of smiles at blues jams until I plug in......

[pula58]

Mike,
The PI stilll limits things even with the stokes mod?

Thanks

P

[Blue Strat]

Mike,
The PI stilll limits things even with the stokes mod?

Thanks

P
Yes, the Stokes mod is a "patch" at best.

[Wakarusa]

3.5 on the volume seems a bit early for breakup. What are you driving the amp with? The reason I ask is that if you've got hot pickups or some kind of boost in your pedal chain it's not hard to push the preamp harder than you might want to (so the breakup may be happening before you ever get to the PI). This could also be an indication of a weak tube (preamp or power) or a drifted/failing plate load resistor in the preamp greatly limiting headroom in that stage.

[roknroll]

Try using 12AT7 in the first preamp tube position. If that doesn't do it try 12AY7. You will probably find you have to turn the volume up a little more but the amp will stay cleaner in general.

[Blue Strat]

Try using 12AT7 in the first preamp tube position. If that doesn't do it try 12AY7. You will probably find you have to turn the volume up a little more but the amp will stay cleaner in general.

This MAY keep the preamp cleaner but doesn't increase overall headroom in the amp. You'll end up cranking the volume control higher to achieve the same volume level and you won't get any more clean power from the amp.

[phsyconoodler]

The PI in the princton does not add as much gain as a long tailed pair,so I am thinking you may have the amp either biased too hot or you have a speaker that's not efficient enough.Check your bias and try a high-efficiency speaker like one of weber's high-powered 10" models.
Case in point: a tweed deluxe breaks up really early,but if you use a speaker like a vintage 30,it has tons of headroom and is louder to boot.

[Blue Strat]

The PI in the princton does not add as much gain as a long tailed pair,so I am thinking you may have the amp either biased too hot or you have a speaker that's not efficient enough.Check your bias and try a high-efficiency speaker like one of weber's high-powered 10" models.
Case in point: a tweed deluxe breaks up really early,but if you use a speaker like a vintage 30,it has tons of headroom and is louder to boot.

It's not really a matter of gain, it's a matter of headroom. The stock Princeton PI's don't have much.

[phsyconoodler]

Lots of factors involved in the 'headroom' discussion.The 12 whopping watts of the princeton may be the real problem here.But i do have good success with high-efficiency speakers.

[Blue Strat]

Lots of factors involved in the 'headroom' discussion.The 12 whopping watts of the princeton may be the real problem here.But i do have good success with high-efficiency speakers.

The PI is the big bottleneck in the Princeton. See my post above about my Princeton conversion.

[jbrew73]

how do amps like orange and some ampeg models have good headroom using the same p.i. as the princeton?

[Blue Strat]

how do amps like orange and some ampeg models have good headroom using the same p.i. as the princeton?

What amps? Got schematics?

[jbrew73]

http://schematicheaven.com/ampegamps/b25b.pdf

http://schematicheaven.com/newamps/orange_mkii1972.pdf

http://schematicheaven.com/newamps/sunn_sorado.pdf

[Blue Strat]

http://schematicheaven.com/ampegamps/b25b.pdf

http://schematicheaven.com/newamps/orange_mkii1972.pdf

http://schematicheaven.com/newamps/sunn_sorado.pdf

25% higher plate voltages, beefier power supplies, higher wattage output tubes, bigger iron. The Ampeg uses a 7199 for a PI which is much different than a 12A*7.

[jbrew73]

so would increasing the power supply and output tranny increase the princetons headroom?

[Blue Strat]

so would increasing the power supply and output tranny increase the princetons headroom?

I'm sure it would, but since you can increase the headroom just by changing the phase inverter (you do lose the tremolo though) I'm not sure why you'd want to do this. The PI and OT upgrades made a BIG difference.

[Wakarusa]

A split load (i.e. cathodyne) phase inverter always has a gain of less than one. That is, the output signal is always less then the input. Half of this thing is a cathode follower, eh?

Headroom is a question of how large an input signal you can apply before the output signal distortion exceeds some desired value. Keep in mind that "distortion" is any difference between the input and output wave forms. For our purposes we'll say that you've run out of headroom any time the input signal is (a) positive enough to cause the PI to clip or (b) negative enough to drive the PI into cutoff.

So in the stock Princeton Reverb with around 240VDC fed to the PI it will handle an input voltage of around 30 volts before it clips. Depending on the tube, the phase of the moon, and your particular karma the actual input voltage may be lower. It's also worth pointing out that this PI setup will clip asymmetrically with the inverting output (the plate) clipping before the non-inverting output (cathode).

If you move the plate connection for the PI upstream one resistor in the power supply voltage divider (and if you believe the published voltage on the schematic) then you're feeding the same circuit with 320VDC. Now the PI will take an input voltage of around 50-55 volts before it clips -- which means more theoretical headroom. However, you'll never get 50V on the outputs of this PI because the 6V6 grids will go positive long before that.

Also, unless I've forgotten to carry the one in some really important underlying equation, a LTP should yield less headroom in this amplifier. Unlike the split-load, an LTP has gain greater than one. Using the same component values as a Deluxe Reverb, a 12AT7, and fed with 240VDC the LTP setup should have a gain somewhere around 17. I'd imagine the big difference is one of balance -- how and when the different PI topologies start to distort relative to when the power tubes (and anything else in the preamp) starts to distort.

None of this makes any commentary on how the different setups sound -- if you like the LTP setup, or the different mods then you're in a great place. I'm just saying that from a "how it works" viewpoint, some of the comments in the thread so far don't make a lot of sense to me.

[Blue Strat]

All I can tell you is my experience in converting to a LTP and the increase in headroom is VERY notable.

[Wakarusa]

All I can tell you is my experience in converting to a LTP and the increase in headroom is VERY notable.

In the conversion did you move the NFB loop from the second gain stage to the second input of the LTP PI?

[Blue Strat]

In the conversion did you move the NFB loop from the second gain stage to the second input of the LTP PI?

I replicated the DR circuit.

[Wakarusa]

I replicated the DR circuit.

It's late so I'm a bit too scatter brained to do the math right now but it seems that the much larger NFB signal in the DR circuit more than offsets the increased gain in the LTP. So, and shame on me for not suggesting this earlier, another way to increase headroom in the PR is simply to change the NFB voltage divider. Replace the 2K7 with a lower value and keep the tremolo :)

[strumminsix]

The C Rex is not a very efficient speaker. IMO, that could be part of the problem.

[Blue Strat]

It's late so I'm a bit too scatter brained to do the math right now but it seems that the much larger NFB signal in the DR circuit more than offsets the increased gain in the LTP. So, and shame on me for not suggesting this earlier, another way to increase headroom in the PR is simply to change the NFB voltage divider. Replace the 2K7 with a lower value and keep the tremolo :)


Interesting. So, you don't think that the lower plate voltage "rail" in the split load PI limits the peaks more than the LTP? In the LTP you effectively have ~350V for both halves (peak to peak) while in the Stokes Modded SL PI you have about 300 or less for both halves.


<Edit>
LOL! Oops, I was looking at the DR schematic when quoting the peak to peak PI voltage limit for the LTP. I guess that would change in a PR with a LTP ;)

Not sure why, but my modified PR has virtually the same headroom as a DR with the LTP and upgraded OT. The PR with Stokes mod and upgraded OT didn't come close. Maybe I'll scope it someday, but I've got way too much more pressing stuff to do.

[harryjmic]

Blue Strat what is a long tailed pair 12ax7? Is it a Sovtek LPS? List some other LTP tubes, Thanks

[Blue Strat]

Blue Strat what is a long tailed pair 12ax7? Is it a Sovtek LPS? List some other LTP tubes, Thanks

It's not a tube, it's a phase inverter circuit. Google "long tailed pair".