Well hello to another Ham…
Yet, another......Tubelab = KB4LRE, novice call, extra ticket, ham since the mid 80's, although with a few powerful exceptions all of my DIY ham stuff has been solid state, primarily because most of my 41 year career at Motorola was spent as a transmitter design engineer. Free silicon, GaAs, GaN, or SiC beats buying tubes! Truth be told, I have blown up more sand than glass in my life.
I went to the Dayton ham vention about four years ago
I have been to the Dayton hamfest almost every year for the past 15 years, and sporadically before that. What has been the largest purchase every year.....tubes, of course.
I should be there again this year unless $hit happens....yes it can happen....just search YouTube for Dayton hamfest 2011 to see the smelly lake of brownness that erupted in the swap meet lot.
it seems that it’s rated for up to 15,000 Hz, do we know what the Roloff is after you hit 15,000 Hz?
No OPT is perfect. All of them have unwanted capacitance and inductance from the method of winding and types of insulation used. These unwanted effects combine to form a notch in the frequency response curve somewhere in the high frequency region. The depth, width and center frequency of this notch depends on the OPT and the surrounding circuitry.
In a high quality OPT this notch is far above the audio frequency range, 30 to 50 KHz, so that the response is flat to beyond 20KHz.
In a budget OPT this notch can be pretty close to the upper end of the audio range, say 22 KHz. This can cause a gentle rolloff starting in the 12 KHz or so range with -3dB occurring at 15 KHz. The response will continue to drop to maybe -15 dB or more at the point of the resonant notch, 22KHz in this example. The response will then rebound somewhat above this notch, but there will be considerable phase shift causing possible instability in designs that use a lot of feedback.
The previous paragraph just discussed the response of the OPT itself when tested in a fixture designed by the manufacturer, which we know nothing about. Its actual response in your circuit could be better, or worse than what's stated in the manufacturer's data. Why?
We have an unwanted resonant circuit formed by the OPT's leakage inductance, and it's stray capacitance. It is electrically in parallel with the primary and "sucks out" a chunk of signal. Every "resonant circuit," even ones we don't want, have a "Q" factor. We try to maximize Q in most RF circuits to favor a single frequency, but here we would like to reduce the Q as much as possible to reduce the depth of this notch.
We can kill the Q of a parallel resonant circuit by putting a resistor across it, but putting a low value resistor across the primary of an OPT will suck up much of the wanted signal too....but,
The tube that we have driving this OPT has an internal plate resistance. For a big fat triode this plate resistance can be pretty low, 500 ohms or less for a 300B DHT...our budget OPT will like this and it's notch will shrink and move UP in frequency getting somewhat out of our way. Drive that same OPT with a pentode that does not run feedback and it's plate resistance may be 25,000 ohms. Our unwanted notch will have nothing to stop it from stealing all the HF from our audio. Clearly a low plate resistance is desired here.
There is a similar loss of response at the low end of the frequency spectrum. Every OPT has a primary wound of wire wrapped around an iron core. This forms an inductor, and many OPT's will specify this primary inductance. It's directly related to the amount of iron and copper used in the transformer, so SIZE MATTERS. Yes, bigger is better, and bass response of an OPT can be measured in pounds (or kg). A single ended OPT requires a core gap, killing some of our needed inductance, so a SE OPT must weigh about 3 to 4 times as much as a push pull OPT of the same (true) power rating. Again some lack of primary inductance can be compensated for by lowering the driving impedance by using a tube that has a low plate resistance.
We can use a triode, the bigger the better because is has an inherent low plate resistance. We can lower it further by applying some feedback. We can get more power with a pentode, but a LOT of feedback must be applied to compensate for it's high plate resistance. We can also take the pentode and wire it as a triode and get almost as low of an impedance as a triode.
So, pick the biggest triodes in your collection, or the biggest pentodes, and triode connect them.
What do you use for an OPT? Well that depends on your tube collection. You have some 6L6 types. They like a load in the 5K range for SE. Do you have any 6CA7, EL34, 6550, KT88 or similar tubes? They also like a 5K ohm load and a B+ in the 375 to 450 volt range.
Have a bunch of TV sweep tubes? They prefer a lower B+ and a lower load impedance. Many can be triode wired, but the B+ should be 300 volts or less in this case, needing a load in the 3K ohm range.
