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floobydust wrote:

One of the reasons I no longer use a tube tester. They make a measurement at a specific target operating point(s). You don't really know what it is, so what are the chances they will be close to your actual circuit operating point? So choosing a close match of tubes that perform the same across the noted parameters (in the actual amplifier(s)) just makes more sense to me. If the pair of tubes are the same manufacturer and internal construction (and unused), then they should age well together and remain fairly matched in long-term operation.  Just for fun, I took the final output stage for the DHT amp and normalized it (i.e., subtracted the DC shift for direct-coupling) and modeled it in the SEampCAD program. I plugged in the details for the output iron as noted. Normalized values result in 1.125K cathode resistor and a 280V plate supply. Values work out to pretty much what you show in measured values.  Transfer curve is quite good considering the lower current. Maxmum power output is about 1.85 watts with pretty low distortion. I can probably create some PDFs for the output specs and curves and email them to you.  Based on the values you are currently using, you can put a 45 triode in the circuit without any change and it will be just as clean but less power... about 1.2 watts. Should be an interesting experience. In many respects the 45 is the purer tube design and many prefer it soncically over the 2A3, "IF" you can deal with 2-3dB less output power.

First, thanks that taking your time for plotting my amp. I think it good and I have no problem with output stage. If would pick fleas on its design then it would be the first stage. It run at juts 10mA and 150V – a VERY conservative operation for this driver. I can not driver it hooter as I would then need a higher voltage on the output tube’s anode. It looks like the driver tube still sound good but I have another concern. At this operation I have only 2.5V on driver grid. At my normal 200V and 20mA I have 4V on grid. The 4V on grid is good as I can swing any input voltage and have an assurance that I will not send the driver into A2. With 2.5V I still far from danger but it is closer then with 4V. So, the 3dB voltage divider at input is kind of extra protective measure to keep the drive state from overload.

Why I said about the déjà vu is because a coupled weeks back I did think about the 45 tubes. I even bought a few for test but they arrived damaged and I sent them back. I need to but more of them and finally try those 45s. You said that might have 2-3dB less output power. I usually do not understand power in decibels I understand gain in decibels, and as I understand I will have both of them less…

Anyhow, my concern about the 45 tube is the same that I have with my YO186/RE604 tubes. All my 4V tubes sound softer then my 2.5V tubes. The 45 tubes with their plate impedance 500R higher then YO186/RE604 I will have even harder loading of those tube against my 6K output transformer. Now, if I find a way to make primary with taps of 6K and 9K then I will be able to drive 2.5V tubes against 6K and 4V against 9K. Since I do not use them at the same time I will be able to keep them permanently connected. Now, the whole beauty of my current arrangement is that I need to do absolutely nothing to switch the amp from YO186 to 2A3. If I stick into the 2.5V socket a tune with 1.7K plate impedance then I need to remaps it to high impedance primary… So, I need to come up with some kind of arrangement that have flexible primaries, also to switch them and do not degrade sound.

Anyhow, I will order some 45s and will see how it goes…

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