I think things are becoming unclear so I'll take things inline for clarity.
Andrew wrote:Question, is a TX loading different to a choke loading?
no, the same rules apply.
How will the TX reflecting whatever it sees on its 'secondary' different for a triode to a pentode.
Simply put a pentode given its high source impedance forces you to size the primary inductance to match the reflected load. Swapping that out for a triode allows much less inductance to be used to get the same LF response which in turn allows you to design for much better HF response. Assume a 5K reflected load and a triode with a 1K Rp and a pentode with a 100K Rp. In order to be -1dB @ 10hz for the triode you would need 26hy's of inductance. For the same reflected load and a pentode you would need 151hy's
Look at the plots below where I increase the stepdown ratio of the transformer. The loss in bass is due to the need for more inductance as the reflected load goes up. If you triode wire the D3A you will not see the bass roll off with increased stepdown since the Low Rp of the source will dominate.
I'll ask you guys a question about what I just said. Have any of you ever seen the argument made that a transformer for a pentode should be designed differently than one for a triode? I have been saying this to deaf ears for some time now and I'm trying to figure out why nobody seems to "get it" Who Knows i may be the one who doesn't "get it" but in the event that this concept makes it into some commercial spiel, please "have my back" so to speak. (sorry i'm in a ranting mood... now back to your regular program)
I'm not questioning that a choke loaded pentode is out, a constant current source loading a constant current source is the problem.
an active device (ccs) and a pentode (high resistance) in series are not a problem. The CCS will easily dominate. A choke (or transformer primary) if unloaded attempts to be an ideal CCS so the two are similar. In order for your pentode to behave as desired you need a fixed defined load which neither a choke or CCS can provide. Put a resistor in parallel with the choke or the CCS and you suddenly have the load the pentode needs to see.
However, a transformer loaded is going to reflect what's on secondary. I was sure Dave had a sim of this working? Looking at this another way, don't we use tx-coupled pentodes on output stages. OK, I don't but amps like this do exist.
EXACTLY... If transformers and pentodes did not mix, how could we ever have a PP pentode amp?
dave