Isn't the Princeton Reverb PT undersized?

[Victor Denance]

The current capacity of the Champ / VChamp / Princeton / PR / Tweed Deluxe power transformer's HV tap is 70mA.
(IIRC the code for this PT is 125P1B).

Now in the case of a Princeton Reverb w/ 2 6V6s and 4 12AX7s, the max current draw to the HV tap is:

say we have an average 240V on each 12AX7 plate and 380V on each 6V6 plate.
The max plate dissipation is 2.2W for a 12AX7 and 14W for a 6V6.
Therefore we have:
- 2.2W / 250V = 9mA for each 12AX7
Total for 4 12AX7s is 4*9= 36mA
- 14W / 380V = 37mA for each 6V6
Total for 2 6V6s is 2*36= 74mA
Total dissipation for all the tubes is in the 36+74= 110mA ballpark.
Now let's say the circuit is designed to work the tubes at 70% of max plate dissipation.
The total plate dissipation (for all the tubes) would be 110*0.7= 77mA.

Since the PT HV tap feeds 70mA at max, it looks like the PT is undersized (tubes plate dissipation exceeds what the PT can feed).

Now, the questions:

What do you experts think regarding the dimmensionning of that PT?
How comes all those PRs have withstood the test of time w/ such a tiny PT?
Is it safe to run a Princeton Reverb full tilt for hours long gigs w/ the original PT?
Is that tiny PT responsible for the saggy tone (especially when cranking a Tweed Deluxe and hitting a big power chord on the low notes)?

Thanks!!!

Victor

[Enzo]

Hi Victor, welcome to the group.

I'd say your premise is flawed. That 12AX7 may have a max dissipation spec of 2.2w, but that doesn;t mean we operate them anywhere near that. 9ma would be an awful lot of current through a 12AX7. I usually expect more like 1-1.5ma.

Look through some schematics for the expected cathode voltage, and look at the resistor there. The typical 1.5k resistor will usually have something along the likes of a volt or so there. 1.5k resistor and 1ma current means about 1.5v there. SO there is only 6ma maybe instead of 36. That reduction of 30ma alone brings us down to 80ma, not far from your 70.

The preamp tubes don't run anywhere near their max dissipation, and you want to spec the 6V6s at 70%? 70% of your 74ma is about 52ma, so there is another 20ma savings, which brings us down to 60ma, well within the 70ma ratings target.

[Bruce / Mission Amps]

And, adding to Enzo's reply, the BF Vibro Champ and the Princeton, Princeton Reverb amps are one of the very few Fender amps that I have had to replace power trannys in with any obvious failure pattern... well obvious to me over the last 26 years of messing with guitar amps.
This could be due to the many knuckleheads and wankers sticking 6L6s in these amps too!

Regardless of part numbers stamped on bell covers, the tweed Deluxe PTs I have reversed engineered were more likely 90 to maybe 100ma by today's standards.

[Alby]

So what changes if any would show in the amp if you stuck in a 120 or 150ma PT with the same supply voltages?

[R.G.]

Let's put it a different way.

The ONLY things which determines a power transformer's power ratings are
(a) does the voltage sag out of the necessary range when loaded
and
(b) does the transformer get too hot?

As a rough test, if a commercial transformer is cool enough that you can keep your index fingertip on it for 15 seconds or so, it's not too hot. If you won't hold your fingertip there, it's above the 130F or so that is the too-hot-to-touch temperature for most people, and it's likely that the insides will be too hot for the lowest grade of insulation materials.

However, there do exist insulation materials which will let transformers operate stably and safely at temps so hot they'll raise blisters on your skin. But that's the exception.

If the voltage sags too much, it's not good for electrical operations reasons. If the shell is too hot, it's at risk of failure from insulation breakdown. Otherwise, it works.

[bazookajoe]

I did not check your calculations. I hope i am allowed to copy this lines from the tonelizard site:

Part of the real 'secret' to great vintage guitar amplifier tone is the 'underdesigned' power supply. When you crank up the smaller powered amplifier to play in the modern gigging environment, the power supply 'sags', and you get the wonderful power amp distortion thrown in for good measure. There are sonic trade-offs, however. The audible result of having a high power supply impedance is a 'mushy' bass response at high volume levels and a peak compression. However, if you grew up with classic Rock 'n' Roll albums, you heard underdesigned power supplies with a high impedance.
To read more do a google search for "tonelizard" and go to "mods and odds"

Although i thought that this would apply to small 1940s and 50s amps not to a Blackface amp known for higher voltages and more punchy attack.

[drspencer]

Originally Posted by R.G.

Let's put it a different way.

The ONLY things which determines a power transformer's power ratings are
(a) does the voltage sag out of the necessary range when loaded
and
(b) does the transformer get too hot?

As a rough test, if a commercial transformer is cool enough that you can keep your index fingertip on it for 15 seconds or so, it's not too hot. If you won't hold your fingertip there, it's above the 130F or so that is the too-hot-to-touch temperature for most people, and it's likely that the insides will be too hot for the lowest grade of insulation materials.

However, there do exist insulation materials which will let transformers operate stably and safely at temps so hot they'll raise blisters on your skin. But that's the exception.

If the voltage sags too much, it's not good for electrical operations reasons. If the shell is too hot, it's at risk of failure from insulation breakdown. Otherwise, it works.

I have a healthy 66 BF Princeton Reverb that gets ridiculously hot. Not only can't I touch the PT for more than a second, the faceplate even gets unusually warm to the touch.

Any suggestions?

Thanks

[Enzo]

It is 40 years old. It still works.

[Luthin You]

Hey Enzo, I have a BFPR and the output clips one side more than the other, I have tried new tubes, any ideas?Len

[Bruce / Mission Amps]

Originally Posted by Luthin You

Hey Enzo, I have a BFPR and the output clips one side more than the other, I have tried new tubes, any ideas?Len

That is very typical with any amp using a cathodyne driver.
Because if it's very nature, the phase inverter is being driven into an unbalanced looking condition.
One side is clipping before the other and the power tube(s) is following that distorted signal. It is not the power tubes.

Hook the amp up to a dummy load and clip your scope probe on the dummy load... turn it up until you see that distortion.
Note the volume setting.
Now use your scope on the grids of the power tubes and watch the scope as you turn it up louder and louder... you'll see a clipped display on the scope is coming from the signal driving the power tubes.
Note the volume setting?
The power tube is probably not distorting the clean signal at all... as that point it isn't clean in the first place.
The scope display, as viewed across a dummy load, should look nearly the same but smaller and with much more current of course. Yes the negative feedback loop alters this but I think you'll see the point of all this text... ha ha