Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 18, 2019, 07:10:05 am
-
I have been wondering what makes the relative high difference of the audio power (15W vs. 22W)
of those circuits. Actually the only thing that differs the power amps from each other is the PI.
The plate dissipation is same as the plate voltage.
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf)
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164.pdf)
/Leevi
-
The difference is in the transformers.
-
Yes, nominal output power in Watts for PP = (Plate Voltage)2 / Primary Impedance of OT (which itself is nominal). Deluxe Reverb has 5 more plate volts and "cheats" with the OT Impedance, to declare a nominal 7 extra Watts. The Wattage difference itself is nominal in terms of the volume (SPL) of audio output. I think the difference was meant to emphasize marketing aspects. At the time, Champs & Princetons were considered "student" amps; the Deluxe signified entry into the professional level.
-
The difference is in the transformers.
I have used same transformers (Hammond) in both circuit and power difference can be seen.
5V difference on plate does not affect 7W.
Since the power is measured with a clean sine wave I believe that Princeton will break up earlier which is the reason for lower power rate?
/Leevi
-
Indeed. On a scope you could easily see that distortion is introduced at the princeton's phase inverter, limiting the clean audio output power. But the puny little transformers don't help either. I too have used the same transformers in both circuit, and even then the difference (and character of both amps) remains to a certain extent.
-
BOTH transformers.
--pete
-
In any ab1 amp, the clipping point is the power tube control grids.
Electrode Vdc at full loading may be somewhat lower than at idle.
The saggier HT of the PR, especially at the screen grid node, will tend to result in a lower plate current when that occurs.
-
It's not clear that Fender or any other manufacturer actually measures clean W output; they may simply estimate it per the Power Formula, as stated above, which is fair enough. I think this is why sluckey & I focused on the OT.
Even if W output is actually measured, there's wiggle room in what constitutes a "clean" sine wave output. This too can account for small differences in the stated W output.
Also to double output volume (SPL) requires 10X the W output. So, it takes a 150W amp to be twice as loud as a 15W amp. And this is only a 3dB difference which is barely noticeable. It appears that a claimed 7W difference is insignificant, and within the margin of error.
Accordingly, I submit that many of the comments in this thread are taking the wattage spread too literally.
Furthermore the larger 12" speaker in the DR vs. 10" in the PR moves more air and may also contribute to the louder clean vol. of the DR.
-
Yes, 7W is not much and the difference between 15W and 22W is difficult to hear especially if the output is distorted.
I think the watts are more significant when talking about headroom. 5W and 15W amps sound almost equal loud but
the latter has much more headroom.
Another thing related to output transformers and watts. Has anyone succeeded to break down an OT just by playing too hard?
I have never seen that happen even the OTs have been underpowered.
/Leevi
-
Also to double output volume (SPL) requires 10X the W output. So, it takes a 150W amp to be twice as loud as a 15W amp. And this is only a 3dB difference which is barely noticeable. It appears that a claimed 7W difference is insignificant, and within the margin of error.
[...]
Furthermore the larger 12" speaker in the DR vs. 10" in the PR moves more air and may also contribute to the louder clean vol. of the DR.
SPL and amplifier power may relate approximately as you state under certain conditions, yet there is no direct relation. As a measure of pressure deviation from atmospheric pressure of a sound wave, SPL comes after the speaker, so to speak. If the power (W) indication on a guitar amp had anything to do with SPL, heads would have none, and the same amp with different speaker configurations would have different ones.
It's not clear that Fender or any other manufacturer actually measures clean W output; they may simply estimate it per the Power Formula, as stated above, which is fair enough.
Unless my data are wrong (PR: Vp = 410V; Zpri=8500 ohm), that would put the PR at 20W, similar to the DR. I agree that Fender probably estimated rather than measured the output power, but in that case it would probably be an estimation of clean output power (you know, the 4 on the dial breakup vs 7 on the dial breakup).
-
you know, the 4 on the dial breakup vs 7
Corporate marketing would NEVER put out; "This 30W amp sounds great at 10W" :icon_biggrin:
-
...to double output volume (SPL) requires 10X the W output....
The makers of BIG hi-fi amps like that number "10".
But double the *pressure* is 4X power. If ear were linear that would be "twice as loud".
Ears are not linear, and non-linear different amounts at different SPL.
I would take a middle path: twice as loud is around 6X the power.
-
B (https://www.lifewire.com/loudness-and-amp-power-3135081)lundered into this, which is surprisingly on point to parts of this discussion: https://www.lifewire.com/loudness-and-amp-power-3135081 (https://www.lifewire.com/loudness-and-amp-power-3135081)
It is correct that the loudness level perceived through the human ear varies from measurements taken by instrumentation. It varies by both frequency, and by volume range. At 1000 Hz, the human ear most agrees with measuring instruments. That's an important reason why that frequency is a typical benchmark for measuring audio equipment. (Unfortunately, for guitar most fundamental frequencies are well below that.)
As frequency deviates from 1000 Hz at the same SPL, the human ear does not hear them to be as loud as they "really" are. Also at very high and at very low volume levels, the human ear has difficulty detecting changes in the volume level within that range.
Nevertheless, it is not clear how these truths change the 15W vs. 22W analysis in this thread.
-
Didn't Fender pretty much just list what was on the tube data sheets and the receiving tube manuals for output power for their chosen values? I seriously doubt they went through a whole lot of trouble to even do rudimentary actual measurements.
-
Maybe with the very early tweed amps, but by the time they released the Princeton Reverb they were running the tubes far from anything listed in the datasheets.
By then it was a big company with plenty of resources. Why wouldn't they take a couple of hours to measure output power?
-
Besides the transformers, the stock DR has a bit more gain in the pre-amp, so it gets louder sooner on the dial (so it 'seems' louder).