Tweeter Amp for OB or Horns

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IslandPink
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#1 Tweeter Amp for OB or Horns

Post by IslandPink »

I’ve been putting off this job for ages. To get any of my speakers systems working properly in stereo, I need two more channels of amplification. I was thinking of adding output devices to the 300B breadboard and using the main ( shunt ) power supply, but it gets a bit marginal for current very quickly , and I don’t want to compromise the main amp too much. So, I’ve decided to try & make a separate tweeter amp with its own power supply. I have a standard wood casework with alu top-plate ( a Bluebell audio one ? ) that Ant sold me. That should be enough real-estate to do a few options.
Since the ribbons are nearly 100db/w , and any compression driver if also used with the tweeters would be well over 100db/w, the power requirements are not great – anything over 1.5 WPC will be fine. The lowest frequency of interest at the moment for the OB’s will be about 2000Hz , if I was to use this with the 288 horn too, that would come down to about 500Hz ( with some margin below that) .

I have a pair of special Bud Purvine outputs with litz-wire secondaries that can do 150Hz upwards – they are 3k into 4/8/16R nominally. If I play around with the secondaries I can get more like 6k>8R if required. These will handle 2A3 type current and power – and a bit more.

The aims of this at present are a bit of a mixed-bag. My prime concern is to get something done in a reasonable amount of time – so it can’t be too complex. It needs to fit on/in the chassis ( I will measure this tonight ) . Needs to be quiet, although 100Hz hum is not going to be particularly audible if it’s being used at 2kHz upwards.

I’ll put down a few specific combinations I’ve thought about, in next posting
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#2 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

1. So, I started off thinking a SPUD ( one-tuber ) amp with 6C45Pi might be enough ( or 6E5P ) . This seems OK until I factor-in the need to use the transformer with the four-ohm tap , to simulate a 6k anode load rather than 3k ( 3k is a bit too low for a valve with ~1.5k anode resistance ) . Then I can work out that the overall circuit voltage gain is about 1.5x , which even for 100db/w ribbons seems a bit marginal. The amp I’m using at the moment is 4P1L pentode, no bypass cap on cathode ( Spice says 22x gain ) , into 300B loaded with 3k . Total circuit gain with 19.4x step-down at the end is therefore about 4x . The volume control for the tweeter is around a quarter or a third of the way round ( will check tonight ) . So, the 6C45Pi idea might just work, but will have no margin for playing anywhere other than my front room.

2. Cheap & cheerful option with 6EM7 or 13EM7 which have a voltage amp triode and output triode in one envelope. Advantage of these, limited space required, single heater for each driver/output , also the anode R of the output section is about 750R which allows the transformer to be run as 3k>8R as normal. Plenty of gain. This valve very popular with Paul and others a few years back. Pete Millett has a 13EM7 circuit on his website. With more B+ and more faff in setting-up, can be wired for direct-coupling between the two stages.

3. Two-stage with DHT outputs. Choice of 45’s, 4P1L’s or 71A’s to hand . More work/space as needs a pair of Lehane fil supplies for the outputs. Ideally would have DHP or DHT drivers, but I think the chassis would not take another pair of fil supplies and it’s more work/time. Hence I’m wondering about something simple like a good 6SN7 or 12SN7 , could load each side with 55H chokes which I also have ( Bud P ) which would not work full-range on a 6SN7 , but are already 100k load at 300Hz , so fine for upper mids & tweeters. Would take up a bit of space, but on top of plate. Could use C3m pentode drivers, or some E83F’s which are a bit smaller. Wouldn’t need the chokes in that case, but would need G2 stabilisation, but resistor-divider and cap would be OK for those if not doing lower-mids and bass.

Power supply : think it will have to be a conventional passive supply , not much space for shunt-reg, although I might be able to spare enough space for a pair of OB3’s and 20mA to clean-up the small signal swings. Not sure if I need valve rectifier. My fave these days is two SS diodes eg. BYV26E’s plus two damper diodes, however if a SS-only bridge can be made and be quiet in the HF ( Nick ? ) then I might go for that. Not sure of voltage yet, circuit is going to be anywhere from 180 to 250V B+, my prefernce is for a non-CT secondary and a full bridge, these days. May be on the cadge for a transformer. I have some BG WKZ 220+220u @ 350V caps stashed :D
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#3 Re: Tweeter Amp for OB or Horns

Post by andrew Ivimey »

This is all good home territory stuff and always worth a work out/ re-visit. I'll follow your wanderings.
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#4 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Feel free to lob in any ideas .
I did a bit of calculator/spreadsheet work on the 6EM7 earlier. I wanted to find out the min drive current for the output section of the valve. Similar calc to what I've done with 300B , 2A3 and GM70 , derived from rules of thumb James used. On this basis , for 2W output via a 3k output transformer, I reckon 3 to 4ma required in the driver ( takes slew-limiting out to 60kHz ) . In other words the top end will be clean and extended, which is essential in this application. So, it's a bit harder to driver than a 45 , they only need about 2ma maybe 3ma. The input capacitances are a bit higher and the plate is likely a bit bigger, anode load 3k instead of 5k also relevant.
So... well, the datasheet 'typical' or 'suggested' is only 1.5ma ( hence my interest ) but looking at the section 1 specs, it can be run up to 1.5W anode dissipation. So, instead of 250V on the anode and 1.5ma, seems like 200V and 3ma would be better , and get you out of the 'mud' on the operating curves.
All good, but then what would the B+ be ?
The output section looks like it'd need only 150 to 200V , but if the driver section needs nearly 200V , then it would have to be choke or CCS loaded. Choke not very practical for this.
Hmm, better check Pete Millett's scheme again.
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#5 Re: Tweeter Amp for OB or Horns

Post by Nick »

Yep, 150v is a good point for the larger triode, its why the direct coupled method works, stand the cathode at about 150v with a 300v B+, and directly couple from the smaller triode.
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#6 Re: Tweeter Amp for OB or Horns

Post by andrew Ivimey »

Please sir, quick check - we are here talking about 6em7?
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#7 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Yes . I know Paul used to direct-couple them.
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#8 Re: Tweeter Amp for OB or Horns

Post by andrew Ivimey »

That's right I tried it. I did lots of versions using 6Dn7, direct coupling, parafeed, choke loaded ccs and so on. That all began with what became the Bottlehead SEX amp. Brilliant stuff that I never improved on really.but learned a lot on the way. I still have two working versions. I never got on with Paul's 6em7. DtB for example did.
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#9 Re: Tweeter Amp for OB or Horns

Post by pre65 »

My KT66 SET amp has a 6DN7 (or is it 6EM7 ?) driver and the cathode follower has a choke in it.

I'm not sure where the circuit is but I can find it if necessary. It was a Mike H design, clever fellow that he is. :)
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#10 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Ah, sounds like a good combo. Different application but I can see the logic.
6EM7 or equivalents are good and free up some space, but I'm thinking if this needs doing then it needs doing properly, and for the high treble, well, it probably ought to be DHT. In every IDH triode there's a free ceramic capacitor hanging off the cathode ( formed by the filament surrounded by its insulation ) . I can't help feeling it might affect the top octave, even if everything else sounds nice.
I've just been double-checking the 71A and much as they are lovely things and I'd like to use them, they are only 0.8W output max, and need 5k load , and only x3 gain. Not really enough of what I need, I am sceptical that valves much below 1.5W can drive cables properly ... just a hunch..
https://frank.pocnet.net/sheets/127/7/71A.pdf

So, I'm heading towards 45's. I've just been upstairs to have a look at my bits ( before I went in the bath ) :D and there should be space for two 150Hz+ Bud transformers, two 45's, and two 55H chokes to load a 6SN7 ( or 12SN7 , or 7N7 or 14N7 ) , with some room left for a few power supply items. I looked at the similar unit ( chassis ) I already had for two channels of 6B4G to feed the horns, and sadly there's too much of the layout, valve holes etc that's wrong for this amp, would be more trouble to bodge it into shape than start again with the other , new, chassis.
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#11 Re: Tweeter Amp for OB or Horns

Post by andrew Ivimey »

45s are lovely.
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#12 Re: Tweeter Amp for OB or Horns

Post by Cressy Snr »

Mark,
You need to get this lot done before the next Owston. :D
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#13 Re: Tweeter Amp for OB or Horns

Post by simon »

Yes, from what you have I think 45s should be good. (The Bugle circuit works surprisingly well and is simple enough.) I reckon all the stuff you mentioned should be good though. :-)

What would be interesting is 10Y/801A, but you don't have the right OPTs :-(.
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#14 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Cressy Snr wrote: Tue May 22, 2018 10:06 pm Mark,
You need to get this lot done before the next Owston. :D
That's 2019, right ?
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#15 Re: Tweeter Amp for OB or Horns

Post by Cressy Snr »

IslandPink wrote: Tue May 22, 2018 10:29 pm
Cressy Snr wrote: Tue May 22, 2018 10:06 pm Mark,
You need to get this lot done before the next Owston. :D
That's 2019, right ?
Yes :D
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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