Through trial and error I did stumble upon connecting my 6.3 (8 ohm) speakers to the yellow output. I started out trying to connect my 3.6 (4 ohm) set up to the I think it is green 4 ohm output, but this did not seem to work well. ...
Each case reflected 5kΩ primary impedance, so I'd suspect you're hearing the differences of the individual speakers.
... I think I read somewhere that using more of the OT with the 8 or 16 ohm jack produced a better sound quality ... I haven't tried connecting anything to the 16 ohm output yet. That would use all the iron and windings in the OT and was supposed to produce the best sound quality possible from the OT, if I understood what I read correctly. ...
I've read that before, too. There is a logic to it which may apply if you were building a transformer from scratch and had to evaluate several winding/core schemes to get the same matching of primary to secondary impedance. But
my opinion is it's garbage when considering one tap over another in a guitar amp.
I say that because when a.c. power is applied to the primary,
all of the secondary is energized, not just the tap you're using. I believe the theory of higher-impedance taps sounding better is based on using "more of the core" or having "different parasitic reactances". But once you're playing through a given transformer, all of the core is there all the time, and it's all being used. Likewise, it doesn't matter what secondary tap you use, all of the secondary is physically there and making whatever positive or negative contribution it will make.
The proof for seeing all taps are energized is as simple as playing a tone through the amp. Measure the voltage on the tap you're using (say, the 4Ω tap); Now measure the voltage on the unused 8Ω tap. You will find the 8Ω tap's voltage is √2 times higher than the 4Ω tap, same as the turns ratio is √2 times higher. So the energy is there on the unused taps all the time.
... Soooo, some tubes want to see 5K on their plate, others want 7K, and still others want 10K. If I'm starting to see the picture now, my job is to select the speaker load (4 or 8 ohms) and connect that to the OT's output that when properly calculated most closely matches what the output tube wants to see on it's plate. ...
Close.
The tube doesn't care what impedance is attached to its plate, though there is an impedance given all other conditions in the amp which will yield the most output power. So typically the designer picks an available primary impedance close to that maximum-power impedance.
Let's look at an example. You want to run a pair of 6V6's in push-pull Class A, and have a B+ of 350v. Let's say you know a class A output stage is ideally 50% efficient, so for 2x 12w tubes, you can get a maximum of 12w RMS output power. You idle the tubes at 100% plate dissipation rating, knowing this will maximize voltage and current swings and give the biggest output power. So 12w / 350v = ~34mA. In class A, the tube will have peak current swings of 2x idle current and zero current, so the positive peak swing is 68mA. Power = Current
2 * Resistance, or in this case impedance. But we need to convert the 68mA peak current to RMS for power calculations; 68mA peak * 0.7071 = 48mA RMS. Desired power output is 12w, and our RMS current of one tube will pass through half the primary, so solving for primary impedance we get Power / Current
2 = 12w / 0.048A
2 = 5208Ω. The full primary is double that at ~10.4kΩ, and we'd select the closest typical primary impedance of 10kΩ.
As a check, we could calculate the power across a half-winding dropping the full B+. Power = 350v
2 / 5000Ω = 24.5w peak, or 12.25w RMS.
If you ran through that whole process with a different supply voltage, you'd come up with a different ideal plate load impedance for maximum power in class A. If you were interested in more power output, you'd go to class AB where you'd drop the idle current, drop the plate load impedance (for higher peak current with the same B+), raise the B+ voltage, or all of these. Which is exactly what you see is a Princeton Reverb with 400v+ on the plate, a lower plate load impedance of 8kΩ, a higher bias voltage resulting in a lower idle current typically in the low-20's of mA's.
So plate load impedance is really designed/selected by considering the desired output power, class of operation and power supply capabilities. You check afterwards to verify your selected tube type has characteristics which allow the output stage to work as designed.
Topic: 1st post from new member at Fort Knox KY (Read 13354 times)