45 SET

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simon
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#1 45 SET

Post by simon »

I have a hankering to build another valve amp. It's nearly 9 years ago that I started building the last amp, the GM70 jobbie, and I realise I've forgotten a great deal... In truth the copper GM70 is by far the best amp I've built, but it's a bit power hungry for these times. And whilst regulated 100W filaments are toasty in winter it's a little warm right now for such madness.

I reckon a nice 45 SET should run to about 60W, a light bulb, and is I think the second best amp I've built - the Bugle. I used 5751 as driver in the SRPP and it really rocked, way more than I ever expected from 2W. It's intended to be direct coupled to the Fostex 208e Sigmas above 400Hz, so a 45 should be more than enough in my play room/office.

So I could do a "proper" build of a Bugle, but I'm leaning towards a pentode driver, albeit as a triode. I'd be interested in the panel's thoughts.

I'd like to fully drive the 45 from +/-1V rather than 2VRMS, so I'm looking for a gain of around 50 from the driver. The obvious candidate is a D3a, which is closer to 70, similar to the 5751. Other options are 12GN7, which I get 39 from in the GM70 amp IIRC, or possibly C3g which looks to have a similar 40ish gain looking at the triode curves. 70 might be better but I could live with 40 with a CCS.

I have a couple of spare D3a somewhere I'm sure, though they appear to be on holiday somewhere at the moment. I have a couple of spare 12GN7 too but they're out on loan. I think I have 6 or 7 C3g.

I suspect they'd all do a good job, but just wondered if anyone had any views?
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IslandPink
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#2 Re: 45 SET

Post by IslandPink »

Well James always said you can drive a 45 with a piece of wet string !
Myself, i'd use the pentode, but keep it as a pentode :)
My tweeter amp uses gold-pin E83F's .
Basically by the comment above, a driver running about 3ma current should be enough to put slew limiting well outside the audio band and hence keep the top end clean and sweet.

Who was the guy from years ago who built a 45 amp and demoed it at Eggborough ?
We used it on a couple of the early OB speakers were building at that time.
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#3 Re: 45 SET

Post by simon »

Jono?
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Nick
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#4 Re: 45 SET

Post by Nick »

I remember building one of these many years ago from a kit.

https://exclusive-audio.jp/en/products/a08s45

It was very nice, used a 717a as the driver
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#5 Re: 45 SET

Post by simon »

Love the Japlish :-)
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#6 Re: 45 SET

Post by steve s »

Jonos was a nice amp as I remember Simon and a good choice to build.
But my understanding is you need a little more inductance than the 2a3/ 300b with the 45 to get the low frequencies
As I recall, that amp was rolled off at the bottom end, but it highlighted the mids beautifully
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#7 Re: 45 SET

Post by simon »

It was a good amp Steve, one of the half-dozen most memorable I've heard I suppose. Ultimate bass extension isn't that high a priority in this case as there's a 400Hz high pass filter in front of it.
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#8 Re: 45 SET

Post by simon »

One of the consequences of not building an amp in nearly 10 years is not buying valves. Wow, prices sure have risen. In the region of 10 times what I paid a decade and a bit ago.
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#9 Re: 45 SET

Post by IslandPink »

simon wrote: Thu Aug 11, 2022 9:44 pm It was a good amp Steve, one of the half-dozen most memorable I've heard I suppose. Ultimate bass extension isn't that high a priority in this case as there's a 400Hz high pass filter in front of it.
In that case I would definitely talk to Dave Slagle about dedicated output transformers. the ones i used on my 45 tweeter amp were built by Bud Purvine to work from 150Hz up , and have significant extra tone and detail in the treble.
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#10 Re: 45 SET

Post by simon »

That's a good point. I intend to use what I have to hand though - Bud wound me some 4k4 OPTs for 300B which should make a decent start.

Looking through my valves earlier I came across some 6AM4s which I couldn't remember buying for the life of me, had to look them up. Then found some posts by James recommending them. Gain of 85, but a bit low on current at 10mA. Wet string did you say? :-). Could perhaps use a MOSFET source follower to drive the 45 grid if needed.
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#11 Re: 45 SET

Post by IslandPink »

I said above that 3ma is enough !
No need for the Mosfet. Might need to keep RF under control though, should be do-able.
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#12 Re: 45 SET

Post by simon »

I know. 3mA just seems so weedy in the world of pentode drivers though. It's easy enough to look for solutions when none are required. I was probably influenced by some of the reading I did that suggested it wasn't ideal as a pre or driver.
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#13 Re: 45 SET

Post by Nick »

Yep, I think you need to think about delicacy with a 45 not brute strength. The 717a I mentioned that worked well, ran at 7.5ma
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#14 Re: 45 SET

Post by simon »

It's funny, but over the years when I think about a "nice" 45 build I always come back to the 5751 SRPP driver and the Bugle, it just worked so well.

In this situation I perhaps don't need as much welly, so delicacy is probably a very good thing.
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#15 Re: 45 SET

Post by simon »

HT power supply - passive or shunt reg? The Bugle uses a small Hammond 30H choke and 220uF cap per channel. But I have some Salas simplistic shunt reg boards I've never used which I'd be curious to try. The answer of course is to try both and see what I think but just wondered if there were any thoughts on pros and cons?
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