Tweeter Amp for OB or Horns

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IslandPink
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#76 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Good questions ! Making me think ( ouch )
ps. I'm Mark
Wolfgang wrote: Mon Jun 18, 2018 4:04 pm IslandPink,
I was wondering what could have been the reasons for some of the choices you have made in this circuit. For example the ECC40 gets a CCS but the 45 doesn’t. I would have expected it to be equally important there (constant current sink).
Well, I'm not really loading the ECC40 with a CCS. I actually have an aversion to solid-state in or near the signal path , I find a lot of the ideas don't work or sound quite right to my ears - however I slowly accept these things where it's done well and I have proven it myself. For instance I have not found a CCS I fully like when used as an anode load. I have not tried a CCS in a cathode, but then I'm not sure how well that would work in SE. I can see the benefits in PP, but have heard some poor implementations that did not interest me.
So, going back to the ECC40, it's loaded by the choke really - the CCS and VR tubes form a stabilised supply with shunt-regulation. I'm hoping the CCS and the choke will not interact in a detrimental way. It's rather like the method used by Lynn Olson ( +Gary Pimm ) for the driver stage in the Amity & Aurora amps, although they are PP.
Wolfgang wrote: Mon Jun 18, 2018 4:04 pm How do you calculate the AC load line with a CCS, a choke , and a resistor? Do you use a Spice simulation? The linearity (and benefits) of the CCS depends after all on the impedance to which it is connected but if you want to adjust this as RC filter how do you make all these factors fit?
I really should do a Spice sim now before it's too late. I just haven't got the valve models downloaded or working yet ( I had it before, but lost all the set-up in a computer crash ) . But explanation as above - I believe the CCS and VR tubes form a little 'wall' from the main supply and then the ECC40 interacts with the choke load. Note the choke value is too low for good full-range action - this is where I have concerns that something odd might happen when feeding this amp some bass - which it will get. After the coupling cap, the output stage operates only in the upper mids or HF.
But anyway, at 500Hz upwards the choke will look like a CCS in terms of load line - so a horizontal line across the ECC40 curves.
As far as the 45 goes there aren't too many choices on load line etc if you want to get decent power out of them - ie. 1.5W or more. I've not spent a lot of time playing around with load lines over the years, it's usually a trade-off between 2nd harmonic and 3rd-harmonic at the end of the day - both have their pros/cons depending on the music you play - and I play a pretty mixed range of music all the time.
Wolfgang wrote: Mon Jun 18, 2018 4:04 pm When you design a circuit like this do you start from a specific "sound" you want to achieve which would be the reason for setting the operating point of the tubes (considering the harmonic/distortions)and adjust the rest around this or do you simply define the output gain and work from there backwards?
It's built up from previous work & amps plus what other people have built that sounded good. I know 45's can sound very special, clean and pure, if not over-loaded by speakers - hence prime choice for a low-power HF amp. The transformers are specially built for HF. The ECC40 , I first heard in a lovely 3-stage 45 amp . I've since learned that it has low levels of higher harmonic distortions ( 3rd and above ) compared to other valves, and I used it in a 3-stage 300B amp for a while. I want to hear it again , and use with a 45 , to see how good the top-end sounds. Other than this, the decision was based on getting just enough gain, plus using items I had around.
But mostly I just work from the datasheet charts to get the basics set up. I believe the choice of valve determines the 4th/5th etc harmonics.
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#77 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Ray P wrote: Mon Jun 18, 2018 7:32 am Just another thought. In the past I have mounted similar boards vertically by hanging them from the chassis top plate; I carefully tapped threads into the holes in top of the heatsinks, seemed secure and I used some heat transfer compound so some of the heat would be dissipated into the chassis plate, though I had vent holes too.
I've been thinking more today about this issue, Ray . Also had some email with Nick.
I feel I've clarified my ideas a bit.

It seems to me the situation on convection and dissipation level is only clear if the heatsink is out in fresh air.
Once you get the heatsink under a chassis top-plate it gets messy.

if you have the plate un-vented, it's not clear to me the convection from a vertical heatsink will be much good because it's blocked by the top-plate.
Also the PCB is quite a significant area - if you have the heatsinks bolted to the top-plate by the ends, the PCB will be blocking some of the convection from below. ( My amp will either be open or have some coarse mesh on the underside btw. )
It seems to me that having the heasink laid-out horizontally will actually give more free movement of air in this case.

If you now cut some holes in the top-plate for convection ... well, I'm not sure it changes anything. You still have the PCB blocking some air-flow from below if you have the heatsink vertically.
Does this make sense ?

I do agree though that the heatsinks being bolted to the top-plate does have advantages in conduction - that's the main plus I can see.

I will try to run the 45 off the supply on the 'bench' and see how hot things get. I want to get the voltage drop from the feed to the minimum 'safe' level to keep dissipation down. The side-mounted boards with horizontal heatsinks is the easiest for the fitting-out operation, that's my concern. Also I am trying to keep the top-plate as free from screw-heads as possible, for aesthetics. ( However if I can put the heatsink screws under the 55H chokes, then this might be nice - hidden, and with a big lump of metal to absorb heat ! )
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#78 Re: Tweeter Amp for OB or Horns

Post by Ray P »

Hi Mark, no problem with anything you've mentioned and it'll probably work just fine. I was just making the point that for optimal performance the 'sinks should be vertical with a route for air to flow along the fins, which is why I always try to have holes in the PCB under the 'sink and vent holes above.

When I 'hung' some similar boards from the chassis top plate I had a row of vent slots either side of the heatsink.

On my Transcendent 300B SE-OTLs the DHT filament supplies get pretty warm; the bottom of the chassis is largely open and there are a lot of vent slots above the supplies and I suspect that without them to allow some heat out of the chassis the service life would be shortened.

Good luck with the project...
Sorry, I couldn't resist!
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#79 Re: Tweeter Amp for OB or Horns

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So did you wire-up some circuit-boards yourself for the fil supplies rather than using Andrew's boards ? Otherwise there's no unused area for making holes to improve circulation.
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#80 Re: Tweeter Amp for OB or Horns

Post by Ray P »

No Mark, holes in the PCB isn't an option with Andrew's 'small' DHT filament supplies so I just did the best I could with the vent slots. If I was starting the project today, knowing what I've learned, I would probably opt for the 'large' versions of the boards mounted on an external heatsink...
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#81 Re: Tweeter Amp for OB or Horns

Post by Wolfgang »

Thanks for the detailed answers, Marc. Some time back I was considering to use a “tube only” active x-over for my Klipsch La Scalas but I couldn’t find any satisfying (simple) practical solution. So I am still driving them with 3 fullrange OTLs and a Behringer Ultradrive Pro which is not such a bad thing as long as the source is already digital.

Your approach to put the filter in the amp itself is interesting but I still don't understand something. You wrote: “at 500Hz upwards the choke will look like a CCS in terms of load line - so a horizontal line across the ECC40 curves.” But it will still be parallel to the grid leak resistor (no horizontal AC load line possible no matter how stable the choke impedance will be from 500Hz upwards) and frequencies below 500Hz would still affect the filter function. No matter how little this fluctuations might be wouldn't they have quite some audible effect as part of an active x-over? How do you want to get a stable cut off frequency with RC if the choke shows varying impedance at different frequencies? Wouldn’t the cut-off point and the phase shift of the filter move audibly with the music signal with the cutoff point still in the audible frequency range?
Did I miss something?
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#82 Re: Tweeter Amp for OB or Horns

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Wolfgang wrote: Thu Jun 21, 2018 7:40 pm .. not such a bad thing as long as the source is already digital.
Yes, unfortunately source will be analogue at least half of the time. I'm not very impressed by the digital crossovers I've heard so far, Nick has a long-term ambition to do something better, in 24-bit regime, till then I prefer in-amp analogue slopes.

Wolfgang wrote: Thu Jun 21, 2018 7:40 pm You wrote: “at 500Hz upwards the choke will look like a CCS in terms of load line - so a horizontal line across the ECC40 curves.” But it will still be parallel to the grid leak resistor (no horizontal AC load line possible no matter how stable the choke impedance will be from 500Hz upwards) and frequencies below 500Hz would still affect the filter function. No matter how little this fluctuations might be wouldn't they have quite some audible effect as part of an active x-over? How do you want to get a stable cut off frequency with RC if the choke shows varying impedance at different frequencies? Wouldn’t the cut-off point and the phase shift of the filter move audibly with the music signal with the cutoff point still in the audible frequency range?
Did I miss something?
No , I missed the grid-leak resistor ( done that mistake before ! ) .
I was intending to keep the grid leak at around 100k for the filter ( & very small coupling cap ) so the load will stabilise at ~100k at high frequencies. The choke will be 138k at 400Hz , and at a more likely frequency ( if horns driven ) 700Hz it will be 240k . That seems an acceptable compromise. At the low drive levels I think I can tolerate small changes in th eload line on the ECC40.

However I'm glad you've pointed out the issue with the choke again, I am now more concerned about what happens to high-order distortion products of bass frequencies. The load line will be very steep and non-ideal below 100Hz and that could throw extra harmonics through the circuit where there is 'mixed' musical input.
I think I will have to get the Spice simulation working. I'm off work for a few days now and will try & get the sim going.
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#83 Re: Tweeter Amp for OB or Horns

Post by izzy wizzy »

I realise you're using stuff you already have. To get you going first, you could even go resistive load for the ECC40. Not ideal as you see things today. Or you could put a full range choke in there to avoid "issues" if you have one of course.

Then once it's going, sub in the planned choke. I like doing things like this; build somewhat "sub optimal" and then put the "experimental" stuff in and if it works, call it an upgrade. That way, you stick with a known path to get it going before going off piste.

cheers,

Stephen
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#84 Re: Tweeter Amp for OB or Horns

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Yes, I'm flexible here up to a point. I will continue doing stuff on the amp that's 'definite' and work around the driver stage, I think. A simple resistor-loaded anode is an option, though I'd still like to have the CCS & VR tubes in there for smoothing and low noise - though I will need towards 300V for the resistor and ECC40 to get the valve into the good part of the curves. Then there's still the pentode option , probably E83F for this.
It looks like I might need a sub-board and cip-leads for the driver stage to try out two options.
I'll keep moving on other bits like setting up the filament supply voltage and the B+ , while I sort out the Spice model and mull over things.

I was listening to the 4P1L>300B amp the other night with some ECM label stuff, with Sitar and little bells, and I thought - any tweeter amp is going to have to go some to beat this ! This was with the ribbon tweeter - just amazing transparency. Oh and also the elliptical re-tip on the 103R . Magic .
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#85 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Just wondering about how to get a couple of holes made in the top-plate for the UX4 socket. I need to get the (45) valve to go down onto the socket, ideally, so it needs to go through the top-plate. Unfortunately my biggest Q-Max cutter is about 27.5mm and that's not enough . Might have to go to my local machinist to do it nicely.

Just occurs to me I have some hole-saws in a box somewhere. I wonder if they can handle 2mm or 2.5mm aluminium - might try.
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#86 Re: Tweeter Amp for OB or Horns

Post by Ali Tait »

Might do, though they tend to wander in aluminium.
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#87 Re: Tweeter Amp for OB or Horns

Post by Ant »

It will leave a nasty finish to the plate, id change drill it then file it out, shouldn't take long with aluminium
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#88 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

What's 'change drill' ? I could imagine drilling a load of little holes around then cutting through and filing it circular, but I can't see that being very neat ( in my hands ) , at least no better than the hole saw. The slight advantage of this plate is it's a black one which I could spray paint over at the end to cover any errors.
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#89 Re: Tweeter Amp for OB or Horns

Post by simon »

37mm dia hole punch I think is what I've used for UX4s (I should check). I wouldn't mess about, just buy one. Treat yourself to a thrust race if you're feeling flush.

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#90 Re: Tweeter Amp for OB or Horns

Post by IslandPink »

Yes, you're right. Valve base is 34.6 on the 45 I have here.
Maybe I thought the punches only went up to 35mm or so.
Plus I have some birthday money :D , I'm sure my mum will approve of this purchase.
Good exercise for the forearms as well.
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