Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: separateness on January 17, 2024, 07:41:43 pm
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Gentlemen, I have an academic question.
I would like to build a Princeton Reverb-ish type amp so I can play electric guitar from the safety of my couch. In looking into my options on various fora, I have seen that it is frequently the case that people upsize the transformers. I would, I believe, like to do this also because I tend to prefer cleaner tones. The reported effects of folk putting a Deluxe Reverb output transformer into a Princeton Reverb circuit is that it is a) louder b) cleaner c) low end gets tighter.
This last one I can easily understand, I think, on account of bigger iron->more inductance->etc.
B made sense at first until I plotted the load lines with every one's favorite load line plotter. I have attached the two loadlines, as best as I understand how to plot them. 8.5kΩ tranny is the stock Princeton, 6.6kΩ is the Deluxe 'upgrade' tranny. To me it looks like the 6.6kΩ is much more perpendicular to and has much less travel in reference to cutoff. This does not say to me 'this is a cleaner loadline' (effect b above), nor does it say to me 'this is a loadline more apt and capable of swinging' (effect a, noted above).
Can any one in the goodness of their heart show this simple fool what he is missing here? While they are at it, can any one tell me any good reason (besides "The designer designed it that way") that these power amps are operating at that cramped as heck looking area on the right side of the graph when there is all that beautiful real-estate around the bend in the Max-power curve? Preferences for high voltage instead of high current operation maybe?
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That's a lot of thinking for a couch guitar. :icon_biggrin:
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No kidding. I wish I could just shut off my brain, purchase the standard parts and get building but no, everything must be coupled with an endless cycle of interrogation, doubt, research, bewilderment repeat ad nauseum.
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You start with an assertion about "upsize the iron", then seemlessly slide off the couch into comparing two impedances.
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... To me it looks like the 6.6kΩ ... has much less travel in reference to cutoff. ... can any one tell me any good reason ... that these power amps are operating at that cramped as heck looking area on the right side of the graph ...
The plotter is showing you the loadline of, let's say, the "Push Tube."
Where the "Push Tube" is cramped & about to shut off, the "Pull Tube" is conducting & getting ready to pass double-current when the "Push Tube" shuts off.
The Princeton Reverb and Deluxe Reverb are push-pull Class AB amps: each side of the power section shuts off for a time if you push the power section hard enough.
Check out "Composite Loadlines" in RDH4 (https://el34world.com/charts/Schematics/files/Books/Radiotron_4th_edition.pdf) (pages 575, 583).
... The reported effects of folk putting a Deluxe Reverb output transformer into a Princeton Reverb circuit is that it is a) louder b) cleaner c) low end gets tighter.
... 8.5kΩ tranny is the stock Princeton, 6.6kΩ is the Deluxe 'upgrade' tranny. ... the 6.6kΩ ... does not say to me 'this is a cleaner loadline' (effect b above), nor does it say to me 'this is a loadline more apt and capable of swinging' (effect a, noted above). ...
The shut-off side of the loadline doesn't matter, because the other output use is doing the work (established above). So let's look for how far the other side of the loadline goes.
There is a vertical line for "50v" on the 6V6 plate of each set of curves, and it is near-enough to the maximum plate current achieved when the 6V6 is fully driven.
8.5kΩ loadline seems to reach "50v" at 168mA of (peak) plate current.
6.6kΩ loadline seems to reach "50v" near 220mA of (peak) plate current.
Plate Voltage in each case seems to be around 410v (the red dot on the loadline).
Plate Voltage Change is 410v - 50v = 360v
We can use the info above to estimate RMS Power Output (RDH4 page 584):
RMS Power = (Peak Current x Plate-Volts-Change) / 2
8.5kΩ --> (0.168 A x 360v) / 2 = 30 watts -----> Seems optimistic for 6V6
6.6kΩ --> (0.22 A x 360v) / 2 = ~40 watts -----> Seems optimistic for 6V6
I think the loadline calculator estimates peak plate current will reach higher, and/or the tube will pull its plate voltage lower, than what real tubes exhibit.
15w from 8.5kΩ: 119mA through 8.5kΩ/4 ---> (0.119A2 x 2125Ω) / 2 = 15w
22w from 6.6kΩ: 163mA through 6.6kΩ/4 ---> (0.163A2 x 1650Ω) / 2 = 22w
While we might doubt the specifics of the calculator's output, it does show that the lower-impedance OT enables higher Peak Plate Current, and so higher Power Output for the same supply volts. There's "Louder" sorted.
Most people don't have a power-meter in their ears, so they turn up an amp to "some loudness in the room," and notice the amp seems less distorted than with the smaller, higher-impedance OT. There's "Cleaner" sorted.
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Brilliant explanation! Thank you very much for taking the time. I was embarrassingly missing the entire other half of the graph necessary to see what was going on at all. I may now purchase a DR output transformer with a placid mind.
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with a placid mind.
I don't recommend working on live equipment in this state of mind :icon_biggrin: