DHT Heater using VCCS - PCB or Kit (Possible Group buy)

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Andrew
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#931 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

I suppose the question worth asking is how large, or small, can the virtual cathode resistors be and still provide decent common mode rejection?
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Nick
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#932 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Simple answer is I don't know. The low end is set by the extra current and dissipation in the resistors. The high side is set by the extra resistance being added into the cathode circuit and the degeneration added by the extra resistance.

As I have said, this is what I do with a couple of 22R resistors on a 300b or 2a3, YMMV and connecting to just one end also works fine.

My use of the resistors (and the operation I believe happens to make a hum pot work) is based on the idea of creating a bridge between the centre of the two resistors and the filament. If its balanced then any voltage across the filament will not be seen at the centre tap if its balanced. Won't help with noise via the capacitance between primary and secondary of the heater transformer creating a current through the cathode circuit to ground, but a bypassed cathode will help with that.

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#933 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by IslandPink »

I shall take that saying for use at work, with my younger optical designers !
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#934 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Mike H »

:D :thumbright:
 
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RhythMick
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#935 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

The small boards have the transistors and heatsink mounted directly on them I believe.

Is it ok to consider mounting the transformers remotely and wiring from the transistor legs to the solder pads on the boards?

If the answer is yes, is there a limiting factor on the distance? If the heatsinks were on the side of the case with, say, 100mm wires to the board is that going to cause a problem?

I'm not planning on doing this, just want to be aware of constraints for my layout.
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#936 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

RhythMick wrote: Tue Apr 03, 2018 3:01 pm Is it ok to consider mounting the transformers remotely and wiring from the transistor legs to the solder pads on the boards?
If you needed to do something like that I would be inclined to look at using the 'large' boards, which are designed to mount the active devices on a large heatsink, such as the side of a chassis.
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#937 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Beat me to it, Ray. I would agree, I would rather use the large boards than remotely mount the transistors on the small ones.

Small and large is really a misnomer here, the PCB are physically the same size, the small and large really refers to the board's current capacity.

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#938 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Tue Apr 03, 2018 6:35 pm Beat me to it, Ray. I would agree, I would rather use the large boards than remotely mount the transistors on the small ones.

Small and large is really a misnomer here, the PCB are physically the same size, the small and large really refers to the board's current capacity.

Andrew
Yes that's what I thought I understood. I don't need more than 1.5A (for the 45s) but I'm working through placement options and it may be a convenient option to mount the heatsinks remotely, though not by much. One of several options, I'm just trying to understand which of those options are feasible.

Andrew does it make any difference to the group order which board is supplied? I wouldn't do anything which messed up the order you've put in - I'd rather just dismiss that option than inconvenience anyone.
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#939 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Andrew does it make any difference to the group order which board is supplied?
Nope, no problems either way, and this goes for anyone.

If you take the small boards and then find you'd prefer the large, I'll happily swap as long as you cover the postage and the PCB are returned unused, or visa versa, if you take the large and then swap to the small.

Apart from two extra caps and the heatsinks, the kits contain exactly the same BoM (assuming the same voltage/current ranges).

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#940 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Tue Apr 03, 2018 10:01 pm
Andrew does it make any difference to the group order which board is supplied?
Nope, no problems either way, and this goes for anyone.

If you take the small boards and then find you'd prefer the large, I'll happily swap as long as you cover the postage and the PCB are returned unused, or visa versa, if you take the large and then swap to the small.

Apart from two extra caps and the heatsinks, the kits contain exactly the same BoM (assuming the same voltage/current ranges).

Andrew
OK Thanks Andrew - so they are same size (80 x 50mm) and same BOM apart from 2 extra caps and I assume it doesn't include heatsinks as the large boards expect the transistors to be mounted remotely and wired to the solder pads - do I have that right ?

On that basis I think I'd like to switch to large instead of small. That's assuming it doesn't inconvenience you or anyone else - if it's a pain please let me know, I don't want to be a pain. Not yet, you guys don't know me well enough yet.

Looking forward to Owston. On that note, this weekend I finished my PP "rough-but-safe" prototype build. That was a bit of an eye-opener, absolutely amazing. I really thought I'd cracked it with my 5842/45 SE amps - no slouches with Tribute interstage and OPTs. Put it this way - I'd rather listen to ONE channel of the PP than two of the SE. My brother couldn't believe me, drove up 75 miles to listen. Both spent until 3am last night astonished. I'd like to bring a working pair to Owston.
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#941 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »


OK Thanks Andrew - so they are same size (80 x 50mm) and same BOM apart from 2 extra caps and I assume it doesn't include heatsinks as the large boards expect the transistors to be mounted remotely and wired to the solder pads - do I have that right ?
That's correct.
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#942 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Thanks, is there a restriction on distance from the heatsink to the board?
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#943 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

Andrew wrote: Wed Apr 04, 2018 7:15 pm

OK Thanks Andrew - so they are same size (80 x 50mm) and same BOM apart from 2 extra caps and I assume it doesn't include heatsinks as the large boards expect the transistors to be mounted remotely and wired to the solder pads - do I have that right ?
That's correct.
To be more precise, the 'large' boards are designed so that the active devices mount under the board with the legs of the devices used for the conenctions to the PCB pads; that isn't quite the same as "mounted remotely and wired to the solder pads". The 'large' boards has holes above where the device mounting bolts should be to facilitate clamping them to the heatsink. With that mounting arrangements it will be the length of the device legs that will dictate the max height of the PCB stand-offs.
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#944 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Ok thanks Ray.

So, is it OK to mount the transistors on heatsinks remotely and run wires to the boards? Has anyone done this?

And if the answer is yes is there a restricting factor on the distance?
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#945 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by andrew Ivimey »

I've done this tho not with Andrew's. The modules were variable and I was using 7.5 volts at an amp. The transistors / i.c.s were about six inches away from the modules. No probs.
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