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PostPosted: October 13th, 2017, 2:36 pm 
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Those 11A horns are like bazookas. Western Electric did a demostration in 1927 where they took one of those horns with six 555 drivers, played it in Hoboken and listened to it in Lower Manhattan!

The woofer for that show system was 2x EV 30s and 2x 415s in a slot load format. It was a big "jump factor" installation, that's for sure. Too much for day to day relaxation though.

Dave, man, 25 Watts Class A anything is not going to be fast food DIY. Imagine all the heat sinks and metalwork....bah. Sounds like an ordeal.

I'd imagine if you set those PP 2A3 up as fixed bias, possibly through the IT, they should be punchy as hell. Ya can't know until you try. Maybe install beefy CF drivers after the IT. A few dB of feedback will tighten them up substantially with "modern" woof systems. Anyway there are probably things you can do to make that amp act more like a stout bass amp if that is your goal.

Never heard the Hashimoto OTs but I'll take them if you're just going to throw them away!


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PostPosted: October 13th, 2017, 3:06 pm 
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Joe,

Thanks for the reminder of the Brooks 12A. I remember your article in Sound Practices on this (I actually corresponded with you about it). Should have built that circuit to begin with. Judging from the schematic, since there is a concertina phase splitter before the driver, one could use separate chokes on each driver rather than a center-tapped choke, although the latter would be preferable.

Now you got me thinking....

Oh, forgot to mention, the Basszillas are around 96dB/1W, so probably pushing it for a 2W amp, but getting the power on the bass cabinet should to a long way to making at 45 SET work at higher volumes/dynamics.

David


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PostPosted: October 13th, 2017, 3:35 pm 
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Cogito wrote:
First Watt F4 circuit looks like SS adaptation of Bruce Rosenblit's patented OTL design.

Attachment:
f4.jpg


No Shashi -- not even close. It is a differential amplifier that is pretty typical for SS amplifiers. Actually it has a lot in common with operational amplifiers (aka. Op-amps).

And of course with Nelson Pass designs he uses really obscure transistors, FETS, and MosFets that make his products "sing." Plus he is no stranger to using mil-spec parts.

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PostPosted: October 13th, 2017, 3:45 pm 
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The Peerless A100A uses a split choke CF driver scheme and in the original amp they used the primary of an S-series output trans for the choke, 10k I think. The idea is to get the bias from the DCR of the winding and the AC impedance is free.

I built those amps and that was a nice bass amp. The original circuit has way too much gain and I need to chop it down a stage or so. Only been on my to-do list for 15 years...

i know a couple guys who got Heyboer to redo the 12A choke and I could probably get a part number if you're interested.

The advantage to a PP OT is easy availability and I guess you can get a higher impedance per unit $ in a non-gapped arrangement with balanced DC, but matched chokes would work too.

The 12A was a really good sounding amp but it used some very junky parts, wax caps, offbrand carbon resistors, and so on. I am not even so sure that the output iron was anywhere close to what a Hashimoto would be. I think a well-done DIY version should beat the originals, as is often the case.

I repeatedly mention cathode follower drivers because I have heard them do a lot for dynamics in various triode amps. jc is a big believer in this approach and I have heard the difference when it is there. CFs have a bad name for some reason but they work real good for that. Choke in the cathode, what Gordon Rankin calls "reactor drive," may be a worthwhile elaboration to consider.

In any case, redoing those 2A3s sounds like a lot easier fast food project than building a space heater SS box!


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PostPosted: October 13th, 2017, 8:32 pm 
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J-rob, thanks for the link to the amazing system made up of what some would call trash, junk or museum pieces. Wow


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PostPosted: October 13th, 2017, 10:52 pm 
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At todays prices, "museum pieces" would be appropriate. There's about $75k of rare WE gear in that stack. More than that, probably.

Those 11A trumpet horns are from the pre-electrically recorded sound era. These were part of battery operated PA systems around 1924. 1.5W triode. Better than one would expect but they are literally sound cannons.

The 24A horn in the middle of the stacks is from 1933 or so, movie theater gear. They go for $20k a pair without drivers. We stuck that in to fill out the pattern a bit.

We used the 1928 WE 555 compression drivers on all the horns. The early PA driver was not wideband.

Western Electric theater gear of the 1930s is the best sound I have ever heard, but that mish mosh was an audio show special. Sort of a steampunk sculpture that could hit 115+ dB/W at 1000hz. It sounded pretty cool though.


It is fun to show up with 100 year old tubes and pre-electrical reproduction speakers AND make a lot of really expensive modern gear sound like it is broken! :wave: That year we freaked a lot of people out with low power. Many have no idea what a watt or two can do, especially if certain extreme demands are met.

We do these shows as an educational outreach. Usually we show complete factory WE theater setups, real museum demos, but that year we went nuts.

In the context of this discussion, a 2 watt 45 amp can be great...but there is the question of speakers. Biamp is one option and can be a reasonable one.


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PostPosted: October 14th, 2017, 3:20 pm 
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Joe,

Thanks for getting me thinking about the PP2A3. I think your suggestion of a cathode follower driving the 2A3s is a good one, I can actually retain the input tube setup, probably change it to triode strapped D3A that will have the gain and drive the current lundahl LL1660 SE to PP feeding the grids of the followers. I could use a nice medium-mu dual triode, something beefy, maybe a 6BL7 or 6BX7, 5687, etc., and as you suggest, center-tap choke load the cathode. It would interesting to direct couple the followers to the 2A3s, with a negative supply on the centertap of the choke, but that is a complication that would need working out.

2 W 45 SE sounds pretty good on my speakers right now. Waiting for the chassis to arrive from Italy on Tuesday, all my boards are stuffed and ready for installation, so likely to have the F4 mosfet source follower amp running sometime next week. This will be an interesting experiment.


David


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PostPosted: October 14th, 2017, 3:46 pm 
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Quote:
It would interesting to direct couple the followers to the 2A3s, with a negative supply on the centertap of the choke, but that is a complication that would need working out.


YES! Now that is a driver. You could even bias it up to AB2 and really waffle that 2A3 grid, if you want to get piggish.

Reminds me of the Altec 1420A circuit, which used CF drivers to hammer 6L6s into AB2. Best Altec amps I tried out of many. Still have a pr that I haven't even looked at in 20 years, but that was a wicked bass amp when I used them

http://www.ampslab.com/vintage_altec1420a.htm

A 6SN7 class CF would do the job, but excess has it's own rewards. Consider how the cathode voltages figure into the scheme.

On that note, you could split the bias, part self bias and part fixed bias through the CF. That would give you the benefits of the DC coupled CF driver with additional design flexibility.

I know you like heavy amps, David, so "reactor follower" is a natural progression here! Face it! :lolno:


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PostPosted: October 14th, 2017, 4:49 pm 
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David McGown wrote:
Waiting for the chassis to arrive from Italy on Tuesday, all my boards are stuffed and ready for installation, so likely to have the F4 mosfet source follower amp running sometime next week. This will be an interesting experiment.


David


Remember, the F4 is a follower, so no voltage gain... Only way to get more power out than what the driving amp is capable of is to use a higher impedance tap on the OPT than the impedance of the speakers... What happened to the good old days when OPTs routinely had high impedance taps. Of course, you can also drive it with a line stage that can swing decent voltage....

Roscoe


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PostPosted: October 14th, 2017, 6:44 pm 
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Roscoe,

Point taken, but since the bass cabinet is the same efficiency as the mid/tweet baffle, no problem with coming directly off the 8 ohm tap at 0 db gain. I may rewire my EP outputs on the Bugle (45 SET) for 16 ohms if I just want to use it to drive just the F4, get a bit more gain. unfortunately, the EP output has leads from each winding that you combine in series/parallel to get the impedance, so do not have separate taps. My long term plan is to use a line stage/crossover, or just build a tube voltage stage in the F4 for a hybrid amp. Again, this is just playing around for me and don't know where it will take me.

David


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