300b lovers

@alexberger 

If I were you I'd be concerned about the current for a driver tube that must pass through the core of the interstage transformer. That current is DC and causes the core to be magnetized. That in turn leads to saturation which increases distortion.

To reduce distortion, a saw cut is made in the core of the transformer. This dramatically reduces inductance at low frequencies and such frequencies will result in a poor load for the tube because the load line becomes elliptical. 

If i were you I'd be looking for less current rather than more. All you really need is the ability to saturate the power tube. If you can do that with a 6SN7 you'll be in good shape. 

BTW this is one reason I really don't think interstage transformers are a good idea for SETs- its why I prefer to direct couple. 

BTW this elliptical load line thing is a problem for most SET's output transformers (excepting parafeed circuits). For this reason, if you're using an SET you really want to limit bass frequencies getting into it as they cause the amp to make a lot of distortion, and are also hard on the (sometimes expensive) power tube as the load impedance literally vanishes at lower frequencies due to the lack of inductance in the output transformer. 

If you want the most out of almost any SET, preventing bass signals getting into it is one of the major ways to do so. 

Hi Alex! Unfortunately, I think you are limited by the maximum DC current the interstage will accept. As Ralph mentions above, SET transformers, whether interstage or output, have air-gaps in the core so they can tolerate the DC offsets. This has the side effect of requiring much larger cores, since air-gapping the core reduces inductance several times. OK, so the IT grows in size. Unfortunately, there is another downside as the core gets larger ... stray capacitance between windings also gets larger, which is a fancy way of saying less bandwidth.

So the effective cost, assuming clean-sheet transformer design, is decreased bandwidth on both ends of the spectrum. But transformer designers are clever and now have access to computer modeling that wasn’t available in previous decades, so clever interleaving schemes, and analysis of flux density in different parts of the core, lets them "do the impossible" compared to 20 or 30 years ago. So I would suggest looking at Cinemag or Monolith if you want an exotic custom design, but I warn you that requiring 60 mA as well as wide bandwidth will push the transformer to the limits of what can be done. And it will not be cheap ... custom work never is.

Don and I are using both Cinemag and Monolith, and they are custom designed for us. They are not off-the-shelf parts, and we collaborated with the transformer designers over several iterations before we got to where we are now.

You are limited in scope as long as you are buying off-the-shelf transformers, whether IT or output types.

However, it isn't bad as it sounds. Even a massive tube like the KT88 works just fine at 36 mA. Conveniently enough, all of these family of beam-tetrode tubes have the same octal socket and the same pin wiring. So just twiddle the cathode resistor to run the tube at 36 mA, and off you go. And if it's a little guy like a 6V6, just run it around 25~30 mA, and compare your favorite types. Frankly, the choice of cathode bypass cap may be more audible than the tube you choose.

If my vague memory serves, a neighbor of mine, Thom Mackris of Galibier Design, has been building an amp similar to yours. The lineup is: 1/2 6SN7 RC-coupled to a triode-connected 6V6 driver, with SE IT coupling to the 300B grid. Isolated B+ supplies for the input+driver and output section.

I haven’t spoken to him in a couple of years, so it might be different now, but before the pandemic hit, it sounded pretty good, a lot better than most commercial SET amps I’d heard at shows. But that was a while ago, and it could be quite different now. But the overall concept was solid ... basically an updated Herb Reichert Silver 300B amplifier.

If an 8-watt SET is your thing, what you’re already doing is as good as any. The rest comes down to cap tuning and whichever wire you build it with.

Hi @lynn_olson ,

I think the Hashimoto a-305 is an extraordinary transformer. The measured frequency response of this transformer (with 6f6) in my amplifier is 6Hz to 90KHz -3dB. The difference in sound between RC coupling and this IT was huge, it is a completely different amplifier.

http://www.tube-amps.net/images/Hashimoto_Specs/A-305.jpg

I also can recommend 5881 over 6f6. With 5881 the sound is bigger, more air around instruments and better separation. 6f6 gives more chamber and lush sound.

Hi @atmasphere ,

I understand your point. I will not implement it in my current amplifier, but I can try this idea in the next project.

I have another question, how do you balance a number of tubes in parallel on your OTL amplifier without fixed bias. Is it necessary to select all these tubes by parameter or the amplifier circuit balances all parallel tubes automatically? 

The measured frequency response of this transformer (with 6f6) in my amplifier is 6Hz to 90KHz -3dB.

@alexberger while that looks excellent, I suspect these measurements at the link are not at full power. If you want the amp to perform, full power bandwidth is important. Did you measure your amp at full power?

how do you balance a number of tubes in parallel on your OTL amplifier without fixed bias. Is it necessary to select all these tubes by parameter or the amplifier circuit balances all parallel tubes automatically? 

Actually our OTLs are fixed bias. A manual DC Offset control is provided to balance the two output tube banks using the front panel meter.

We've never practiced power tube selection.

You can get a bit lower distortion by doing so; the distortion created by mismatch tends to be lower ordered harmonics. 

Most of the distortion in our design depends on how well matched the internal sections are in the input tube, since the input Voltage amplifier comprises all the gain of the amplifier. Usually that defines the measured distortion; apparently the output section is very low distortion on its own. 

This approach allows for much lower distortion than is possible with an SET. For example our MA-2 (which is zero feedback) can make 220 Watts (230 at clipping) with distortion at full power around 0.5% (with excellent tube matching); up to 1.8 to 2% if the tubes are not matched at all. Most SETs cannot make that sort of power; if the MA-2 is running at any level an SET can make the distortion is several orders of magnitude lower and its mostly 2nd and 3rd harmonic. Lower distortion is directly associated with greater transparency and detail and this is easily heard. 

Unlike an SET or push-pull tube amp, the distortion of our OTLs is unaffected by frequency since the amp has a direct-coupled output. Its full power down to 1 Hz.