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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: Tube tastingPosted by floobydust on: 1/27/2009

Okay, I think I understand.... and thanks for the link on the YO-186... if I can ever locate some of these I'll be sure to grab some (hmmm, a trip to Russia perhaps). As tubes get very hot internally, there were some procedures for trying to eliminate the possibility of "out gasing" from the tube elements, if they were exposed to internal heat levels beyond their ratings. Transmitting tubes were a good example as many of these had cherry red to orange anodes for CCS operation. Even the high powered VR pass tubes (6336 and 6528) had anode materials and processes to ensure "out gasing" would not be a problem when deployed in high-current applications. It's not very clear how much of this exists today with some of the new manufactured tubes.

I would agree that sticking with a 45 or 2A3 seems more promising as they are still readily available and can operate at lower voltages and a lower load impedance. I won't say anything bad about the 10Y... it was essentially a replacement for the 10 as noted in the RCA Receiving Tube manual as far back the 1940's. The 10Y also was listed in the Transmitting Tube manual.... you may well do better to find some 801 tubes as these can also be used in audio amps and can yield around 4 watts with a 7K load. In many cases, they can be a drop-in for the 10Y.

In any case, I agree about not liking taps on the OPTs... but, not all OPTs are created equal either. While I certainly prefer to use the full windings, in the rare case where it's not possible, I will terminate the unused section. This is akin to not letting it "flap in the breeze". Case in point, the Hashimoto H507-S OPT has a 7K primary with a 5K tap. I use this in my 45 SET with the 5K tap. I also load the unused section (between the 5K and 7K points) with a calculated load... I have my own reasons/logic on how the value is calculated. The end result is very good... within 1dB to 45KHz at full power and the square wave response is excellent.

Increasing the load impedance on the 2A3 to the 5K range will drop your useful power to about 3 watts and will also drop the distortion a bit. It may, depending on your OPT characteristics and driver loading, increase your effective damping factor as well. I would probably argue that despite your limited bandwidth application, that you ensure a reasonable power bandwidth on the amplifier. This will yield good phase characterstics at the response range you are looking at and provide a crisp response to transients. Having an extended power bandwidth is rarely a bad thing, unless it's responding to frequencies far beyond what is required.

Regards, KM

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