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

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

Post by RhythMick »

Nick wrote: Wed Mar 21, 2018 10:33 am Andrew will say for certain, but there is no reason why you cant feed the boards DC if you want to do the initial rectification remotely. But consider you are connecting the cathode of your valves to a long wire going back to the second box just to have two boxes. TBH, dont underestimate the problems that two boxes can cause.
Thanks Nick. I'll post up a circuit for comment when the time is closer. Lots of prototyping and listening to do before then.
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#887 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

It's entirely possible to do as you suggest, but I'm with Nick on this one, don't underestimate the issues two boxes bring, by all means post a circuit.

Ask Ray on here what he did, his 300B has several boxes.

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

Post by simon »

Agree that they're not without complications but I've used multiple boxes, well breadboards, for years and managed fine - it certainly helps with weight and portability. But I'd want to have the regs (HV and LV) in the same box as the audio with raw DC in the PS box.

But the downside is the umbilical, especially at higher voltages; connectors can be expensive. I found decoupling caps in the audio box work for me.
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#889 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by ed »

DTB is not at school today so I'm the fat thicko at the back today...

please sir, what are the issues/problems with two boxes?
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RhythMick
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#890 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Wed Mar 21, 2018 7:09 pm It's entirely possible to do as you suggest, but I'm with Nick on this one, don't underestimate the issues two boxes bring, by all means post a circuit.

Ask Ray on here what he did, his 300B has several boxes.

Andrew
Thanks. I assume that the requirement is 4v headroom for regulation whether the supply is AC or DC. I also assume that the heat dissipation isn't reduced, although the caps on the board will see less ripple voltage obviously.

For me the main advantage would be to avoid the AC hum in the umbilical by having the pre reg in the power box, countered by the complication of more wires in the umbilical.

The working prototype will be monoblocks, but if I go with a separate power box I'll try simply passing AC mains through. If that doesn't hum, no problem to solve.
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#891 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by davebms »

hello all,, IF im not to late, can I have 2 small kits if possible with 63 mm heat sinks
sorry to be late
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#892 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

For me the main advantage would be to avoid the AC hum in the umbilical by having the pre reg in the power box, countered by the complication of more wires in the umbilical.
The cathode of the valve, is the point where any signal on the grid is referenced, so any signal on the cathode is the same as one on the grid. By making the supply (even if its only the pre reg in the remote box, the cathode circuit covers a lot of area. Remember the heater supply is floating, so without a ground reference, any signal picked up in the remote box or the connecting cable will be directly coupled to the cathode. If the regulators and transformers are in the power amp box, there is no problem with AC in the umbilical. Its only a problem if you mix AC and DC, and the transformer isolates the heater from the umbilical and second box.

But that’s just what I would do.
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#893 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

For info, my 300B OTL amp is a three box arrangement. I have two mono amplifiers and one power supply 'box'.

The power supply box just houses the mains transformers (and a delay circuit for the B+ supply). I feed AC via umbilicals to each power amp.

Each power amp contains four of Andrew's DHT filament supplies each fed from it's own transformer winding (incidentally, these supplies are the main source of heat in the amps, they do get warm and IIRC I have 63mm tall heatsinks) as well as a DC supply for the 12AU7 and, of course, the B+ supply (SS rectifier, CLCLC arrangement).

The umbilicals are just twisted cable pairs enclosed in some braided sleeving; maybe I could have included a screen too but as I don't have a noise problem so I don't lose any sleep over it. When I demo'd the amps at the last Owston meet there was no hum/noise despite the amps being on a table with something of a nest of various cables and they sounded rather nice through Steve's speakers.

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

Post by RhythMick »

Ray P wrote: Wed Mar 21, 2018 10:41 pm For info, my 300B OTL amp is a three box arrangement. I have two mono amplifiers and one power supply 'box'.

The power supply box just houses the mains transformers (and a delay circuit for the B+ supply). I feed AC via umbilicals to each power amp.

Each power amp contains four of Andrew's DHT filament supplies each fed from it's own transformer winding (incidentally, these supplies are the main source of heat in the amps, they do get warm and IIRC I have 63mm tall heatsinks) as well as a DC supply for the 12AU7 and, of course, the B+ supply (SS rectifier, CLCLC arrangement).

The umbilicals are just twisted cable pairs enclosed in some braided sleeving; maybe I could have included a screen too but as I don't have a noise problem so I don't lose any sleep over it. When I demo'd the amps at the last Owston meet there was no hum/noise despite the amps being on a table with something of a nest of various cables and they sounded rather nice through Steve's speakers.

Ray
Thanks Nick and Ray both, plus Andrew. I think that makes my mind up.
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#895 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

If I were to build the 300B OTLs again, which I am actually contemplating, I would go for a two box arrangement, just two mono amplifiers with toroidal transformers located in/on the chassis but with careful attention to layout/placement. I would use the 'large' versions of Andrew's DHT filament supplies mounted on large external heatsinks (using a chassis such as the modushop dissipante), saving chassis space and easing heat management.

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

Post by RhythMick »

Ray P wrote: Thu Mar 22, 2018 6:28 am If I were to build the 300B OTLs again, which I am actually contemplating, I would go for a two box arrangement, just two mono amplifiers with toroidal transformers located in/on the chassis but with careful attention to layout/placement. I would use the 'large' versions of Andrew's DHT filament supplies mounted on large external heatsinks (using a chassis such as the modushop dissipante), saving chassis space and easing heat management.

Ray
How many valves in your OTL amps Ray? I take it you use 1 DC board, plus 1 transformer, per valve?
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#897 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

RhythMick wrote: Thu Mar 22, 2018 11:37 am How many valves in your OTL amps Ray? I take it you use 1 DC board, plus 1 transformer, per valve?
Each channel has four 300B valves. Each 300B has its own DHT filament supply (Andrew's 'small' boards). Each DHT filament supply has its own secondary winding. I use one transformer with four secondaries. (9V @ 3A IIRC).
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#898 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

Thanks Ray, I thought your perspectives would be useful.

I think Nick summed it up.

Finally, keep the PCB as close to the valve's cathode as possible, this is good practice from a noise perspective, also the sense resistor resides on the PCB so any resistive losses in the wiring from the heater PCB to the cathode will not be accounted for.

In practice, this results in a very slightly lower voltage at the cathode, often just a few 10's of mV, well within tolerance, you may see this phenomena on my bench photos. Losses depend upon the type of hook up wire, its resistance, its length, the amount of current required to drive the voltage on the cathode and how carefully you select the voltage set Rx/Ry resistors.

Keep the the PCB close, the wires short and use the appropriate hook up wire for the current being passed.
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#899 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

Andrew wrote: Fri Mar 23, 2018 8:31 am Thanks Ray, I thought your perspectives would be useful.

I think Nick summed it up.

Finally, keep the PCB as close to the valve's cathode as possible, this is good practice from a noise perspective, also the sense resistor resides on the PCB so any resistive losses in the wiring from the heater PCB to the cathode will not be accounted for.

In practice, this results in a very slightly lower voltage at the cathode, often just a few 10's of mV, well within tolerance, you may see this phenomena on my bench photos. Losses depend upon the type of hook up wire, its resistance, its length, the amount of current required to drive the voltage on the cathode and how carefully you select the voltage set Rx/Ry resistors.

Keep the the PCB close, the wires short and use the appropriate hook up wire for the current being passed.
Thanks Andrew. I've been working my way through the Hammond Transformer lists to find a close match to the requirements. Looking forward to getting going with them, but no rush as my working prototype pair (with AC heating) isn't done yet.
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#900 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

I'd like to share my working on transformer selection, in the hope any errors can get pointed out before I get too far. Taking as example the 45 valve pair.

Vf = 2.5v
If = 1.5A
Required V supply to DC board = 2.5v + 4v = 6.5v
TX rms required = 6.5v / 1.41 = 4.6vrms minimum

Hammond 162H10 (PCB Mount type) has 2 secondary windings each 5v rated at 2A (TX VA rating is 20 VA total).
The DC Board will dissipate (5*1.41 - 2.5) * 1.5 = 6.8W

Using 63mm heatsink with thermal resistivity of 4.5 degC/W and adding 1 for mica tab and paste. Not sure if the mica tab is required with the small boards ?

Junction Temperature = Ambient + DISS * Total Thermal Resistivity
Jt = 50 + 6.8 * 5.5 = 88 degC

The heatsink bit is complicated because as I understand there are 2 transistors on the same heatsink. My poor little head breaks at that point, but I figure that I just need to worry about the TOTAL dissipation and the fact that it's split over 2 transistors will help not make it worse.
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