Type 45 SETs are NICE!

[David McGown]

I built a Gordon Rankin Bugle Type 45 stereo SET amp probably 12-15 years or so ago, based on an article in Sound Practices (thanks Joe!). Anyway, it has been sitting on a shelf for years, save for it being loaned out to someone for a short time while he did his first build (Decware EL84 SE clone). He said it hummed and when I took it back, I just put it on a shelf. Well, Friday, I got the inclination to drag it out and work on it. First I clean up all the stupid mods I did (mainly bypass capacitor related) and restored it pretty much to the original circuit, with slightly larger cap values in the signal path for extended bandwidth. Well, I plugged in into my system, and was left wondering why I had been wandering in the wilderness all these years. It is only a couple of watts, but man, they are better than anything else I have, and by far. Using AC filament heating, I cannot hear any hum from my 97 db/1W-m bass cabinets on my Basszillas. They have great microdynamics and detail, such that I can listen a at lower level and hear everything. They also sound far more powerful than you would think at higher levels, and even do a respectable job on large orchestral (though they do soften the peaks). FM radio really comes alive and is worth listening to.

My motive for dragging the amp out is I am in the middle of project building a PASS First Watt F4 MOSFET source follower amplifier (zero gain). It is designed to take the output from a low power SET amp and deliver 25W Class A power (current) to speakers. I was thinking about using the Bugle to feed the F4, since it should have enough voltage swing to drive my speakers. Yes, I did do a GM70 SET build less than 2 years ago, but the Bugle is better sounding (within its limits), and with the F4, I should be able to have the drive.

David

[dberning]

Yes, I have built two battery-operated SET amps for clients over the years. One with output transformers and one with the ZOTL. Very nice tubes. As I recall, the power required was 3.8 amps at 12 volts for each stereo amp, making it practical for battery operation. I think the audio output power was just under two watts.

My favorite tube is the 6B4G--just a larger version of the 45 tube. But not so large as to suffer the microphonics that plague the 300B. I have made a number of P-P 6B4G amps for clients and I use one in my own system to drive Merlins plus woofers with a passive crossover.

David

[TubeDriver]

David,

Have you tried the Sovtec versions (6B4G or 6A3)? Do they suffer form the same grid construction issues that Sovtecs 300b have?

Yes, I have built two battery-operated SET amps for clients over the years. One with output transformers and one with the ZOTL. Very nice tubes. As I recall, the power required was 3.8 amps at 12 volts for each stereo amp, making it practical for battery operation. I think the audio output power was just under two watts.

My favorite tube is the 6B4G--just a larger version of the 45 tube. But not so large as to suffer the microphonics that plague the 300B. I have made a number of P-P 6B4G amps for clients and I use one in my own system to drive Merlins plus woofers with a passive crossover.

David

[dberning]

I have a pair of Sovtek 2A3s. The grid is not wound all the way up to properly shield the plate from the heater-so incomplete control at high voltage. Not acceptable for linearity. I assume that if they can't make 2A3s, they cant't make 6B4G either.

David

[Roscoe Primrose]

The biggest problem I've had with 45s is tube life. None of the ones I've used seemed to last more than a few months, and I wasn't pushing them on dissipation either...

Roscoe

[David McGown]

Roscoe,

I am surprised by your comment on tube life of 45 tubes. I was my understanding that these were typically long-life tubes in practice. Did you have any B+ slow turn-on to let the heaters warm up (though they typically get up to temp quickly)? I have 4 pairs of antique 45s (including a very strong sounding a quiet pair of GE STs), plus a set of TJ "mesh plate" 45s, which do not sound as good as the GEs. As far as my amp, I am using AC heating (as I mentioned before), as well as a GZ34 rectifier.

Right now, I am just experimenting. If it is something I want to keep using, I can always pickup more used 45's or spring for pair of EMLs. I first want to see how they end up working with a First Watt F4 clone I am building.

David

[Roscoe Primrose]

I was using 5AR4/GZ34 rectifiers, which heat up more slowly that the 45. AC heated, direct coupled...

http://n4rp.com/images/5842-45.pdf

Roscoe

[Cogito]

First Watt F4 circuit looks like SS adaptation of Bruce Rosenblit's patented OTL design.

[FerdinandII]

That's a joke, right.....?

First Watt F4 circuit looks like SS adaptation of Bruce Rosenblit's patented OTL design.

[Cogito]

Why do you say that?