Where can I get ??

[David W]

A 50W 1 Ohm Rheostat to gauge to heater resistance required for my Poor Man's 660?
Apparently 1 ohm is a guestimate on the resistanse required for the stereo version so I guess this is what I need to test the resistance required.

I pretty much have everything now accept the Torriod which hopfully is only a coupple of weeks away. PCB's are stuffed and Case has arrived today,.

[Milo]

You can get slider or rheostat types at RS

[David W]

Yeh checked the website and 10 Ohm was the lowet value they had at 50W. Unless it goes down that low.

$148 yoww. That's the most expencive part of the build.

[Milo]

There's a 1 ohm slider rheostat, 100 watts, PN 235-7207, $53, but also a 55 watt dial rheostat, 10 ohms, same PN 235-7027, $124.00, so it looks like there's a catalogue error. They're both on page 279 of the catalogue, so if you ring tech support and say you've found an error, they'll acknowledge it, and you can specify which of the two you want.
Hope we're talking about the same thing

[David W]

Thanks mate.

[no-fi]

why don't you start with around 2R, and then parallel resistors alongside it till you get the heater voltage you want??

I expect some parallel combination from a kit of 3 x 2R2 and 3 x 3R9 will get you pretty close.... bonus is you don't need as much power dissipation/part that way! 5W resistors are 0.32 ea. at jaycar.

(I have almost everything, except a few transistors and some diodes for the PSU... oh... and meters....)

[rob]

ditto to no-fi

spending $50 for a one shot to select the final resistor value seems nuts and why 50 Watt anyway. If you are dissapating 50watts in one ohm, then you have 50amps flowing through the resistor. Are you building a tube limiter or an arc welder?

look up the data sheets for the tubes involved, see what heater current they each draw, add this up for all tubes. Thus you have the current draw on the heater supply. Measure the unloaded heater supply voltage, find the difference between that and the target voltage under load ( 6.3V ? ). Apply ohms law R = V / I and bob's yer uncle, you have the value of R that you need, plus or minus some fudging factors.

or just do as no-fi suggests.....

btw was a schematic for this beast ever released for the project? Or, are all the kit builders just winging it in the dark?

Rob

[no-fi]

I'm not 100% familiar with tubes (well, I'm not even really 1% familiar - never done anything with tubes before) that's why I suggested the suck it and see method.. :-) but yeah.. apparently heater current draw is 6A for 8 tubes (I find this a bit dubious) and heater voltage is meant to be 6V... (which is 36W.. just to heat the tubes?????) anyway - I expected tube heater to be more like a light bulb in terms of resistance vs temperature...


schematic was released. in paper format, shipped with PCBs.
they asked us not to post the schematic online.
from what I can tell, nobody has done that so far.

[Thirteen]

Buy a $10.00 toaster from Woolworths, pull out the elements and short across sections of them with an alligator clip until you get your desired resistance.

[rob]

what are the tubes?, cause it is easy to look up the heater draw? 6A seems a fair bit, but could be right. Even so, if you need a dropping resistor in the heater supply, then 6A flowing down a 1R resistor will cause 36W of heat dissipation in the resitor, so a 50W resistor would be recommended. However, this situation will also be dropping 6V, so your raw heater supply would need to be up around 12V. If i had 12V available for a target 6.3V supply i'd be putting in a regulator circuit. Then it will be hum free, constant regardless of the number of tubes installed ( good for servicing ) and can be easily designed for a soft start up. But maybe you have something like 8V available and a resistor is more the go. I'd still be doing the maths, at least just for the mental exercise.

R

[David W]

Thanks for all the input.

A rheostat 50w is what has been recomended by many doing this project who have far greater knowlage than me.
Rob you definitly not the first to bring up using a regulated circuit but I guess this is what were stuck with.

I agree even $50 for this exersise seems a waste but considering the overall cost of the project I'll just ditch my plans to get the cool reto knobs and use ones I already have. That way I'm still in budget.

[no-fi]

david... the toaster element with an alligator clip IS a rheostat! just DIY one.
:-)

I don't think there's a hell of a lot of people actually thinking for themselves on this particular DIY build....
and I don't really trust a lot of the specified currents either. I guess I'll find out when I finally have the last bits here, and the time to put it together......

As for the the supply for heater voltage - that is dependent on what transformer you use.
mine has a 9VAC winding, but I'm using the cheapo edcor E-core option.

[Sorr]

From the valve data sheets the heater current of the stereo version will be approx 5.2A.
You will get slight variations between valves.

Ross

[Sorr]

There will be varations in the full load voltage, depending on the voltage and current ratings and the regulation of the transformer used.
Lets say we get 8V under load and we want around 6.3V.
That gives us a required voltage drop of 1.7V.
If the heaters draw 5.2A then we need an R of 0.3269 ohms
It will dissipate 8.84 watts.
You could use a 0.33 ohm 50W resistor from RS ($8.14) part # 160-887 and bolted to the chassis with a tad of heat sink grease.
In parallel with a 6.8 ohm is 0.3147, with 12 is 0.3211, 15 is 0.3228, 18 is 0.3240, 22 is 0.3251, 27 is 0.3260, 30 is 0.3264, 33 ohms is 0.3267 and 39 is 0.3272 ohms.
Using a W21 Series 2.5W axial from RS ($2.00, but you have to buy 5 pcs)

If the current drawn was 5.5A then we would need 0.3090 and a 5.1 ohm resistor would give 0.3099 ohms

If the input was 9V ( at this point I would be looking at a low drop out 5V regulator with C (ground) raised via two diodes and bypassed with a 10 -27 mfd cap, the valves will still be happy at 6.0V heaters) and current is 5.2A then we would need a 0.5192 ohm resistor.
In this instance the next value is (50W) is 5.6 ohms the the parallel R would be 62 ohms (5.1360 total).

There is a small problem and that is the mains voltage is rarely 240V.
So when you do this, measure the mains at different times of the day, because this will effect the voltage out of the transformer..
It is normally a small variation, but try and keep the heater volts between 6.0 and 6.5 VDC.

I hope this will help the DIYer in selecting the correct heater voltage as an alternative to using the 50 W rheostat

Ross

[Sorr]

I forgot to mention that these resistors have a +/- 5% tolerance, so there will be a small difference between different resistors of the same marked value.

[rick]

ok ross
that kinda response gets you in on the trm endorsed tech list !

Ryan active rosses "TRM endorsed 5 tube rating"

and to all you other non tech guys that have to earn your tubes the hard way
well you gotta let the techs have some kinda easy win .. right
we need these guys !

[David W]

Hi Ross

Thanks for such a detailed response.

So a .33R /50W resistor with a 33R/ 2.5W in series seems ideal? (In a perfect world)

Given that:
1) We have 8V and wanting to get between 6.0 and 6.5V (is this likley to to vary much)
2) Power is 240V ( I have the US torroidal so will have a stepdown transformer) How will this effect things??
3) Heaters draw 5.2A

But as you say things very. How important is this?
Over time do valves change there current draw?, Using the compressor in a different studio may have a different voltage variation?
If this was a commercial product would the manufacturer just use the above measurements and be happy with that or is it absolutely critical to get this 100%.

What are the effects if someone was to just use a .33R /50W resistor with a 33R/ 2.5W. based on these calculations.

Will it go bang, valves not last as long, poor performance ??

[Ryan Collings]

ok ross
that kinda response gets you in on the trm endorsed tech list !

Ryan active rosses "TRM endorsed 5 tube rating"

and to all you other non tech guys that have to earn your tubes the hard way
well you gotta let the techs have some kinda easy win .. right
we need these guys !

Done

[Sorr]

David,

Does the Tx have a 9V @ 6A winding?

[David W]

David,

Does the Tx have a 9V @ 6A winding?

Pri: 0-115V x 2 50/60Hz
Sec 1) 250V @ 0.2A
Sec 2) 9V @ 6A
Sec 3) 15-0-15V @ 1A
Low noise Design
Finish: Polyester tape , approx 8" stranded leads
Resin filled center with M6 mounting hole, pad
Resin is suggested for this design due to the mix of wire gauges involved, and in case bolts get over tightened when installing.
Size: 4.3" x 2.0" approx
Weight: 4.5# approx


So I guess this applies

"If the input was 9V ( at this point I would be looking at a low drop out 5V regulator with C (ground) raised via two diodes and bypassed with a 10 -27 mfd cap, the valves will still be happy at 6.0V heaters) and current is 5.2A then we would need a 0.5192 ohm resistor.
In this instance the next value is (50W) is 5.6 ohms the the parallel R would be 62 ohms (5.1360 total). "

[Sorr]

The 9V is the AC voltage, the rectified and filtered DC voltage will be higher.
Because you will be using a step down Tx it gets a bit more complicated.
Just because it says 240V -115V does not mean it will always give you 115V.
It depends on the regulation of the Tx and its load.

If we assume it will give you 115 under load, then the DC out will probably be 10V.
In practice it could be anywhere btwn 9V & 11V.

Anyway lets take an example, 10V - 6.3V = 3.7V
3.7V/5.2A = 0.7115 ohms.
So we need a 0.82 or 1 ohm resistor as the next value down is 0.68.
Using 1 ohm the R across it will be, 2 is 0.6666, 2.2 is 0.6875,2.7 is 0.7297
If we used 2.7 then it would drop 3.794V and 10V-3.7944V = 6.205V.
Which is OK.
But how much will the 2.7 dissipate?
5.332Watts and our 1 ohm will dissipate around 14W
Hmmm, its a bit harder to find a 10W 2.7 ohm resistor.

Lets try 0.82, in parallel with 5.6 gives 0.7152 and will drop 3.719V and the 5.6 will dissipate about 2.47 W.
We could use a W22 series 9W axial.
The 0.82 will dissipate 16.86W
However from RS we can only get 0.82 ohms in a 25W HS25 series ($6.33)
At 25 C without a heatsink its power rating is 12.5W
With a 4.2C/W heatsink its power rating is 25W.
If you bolt it to the chassis with heatsink grease, you could get away with it.


If you are a bit sus about that, then the alternative is to series or parallel the 25W or 50W resistors and then work out the final parallel trim resistor.

If I was doing it I would start with a 0.82 25W resistor and see how far out I was.

[David W]

OK

0.82R/ 25W = RS 107-3841 *1=$3.59
Heatsink = RS 490-7214 *5=$21.90
H/S Compound = RS 490-7214 *1=$9.49

And parallel resistor to be worked out. Will that need to be 9W??
Sounds like a plan.

BTW: When testing this will the Power Supply board need to be connected to the channel boards or can it be done in isolation.



Thanks Ross.

[Sorr]

You can buy just one heatsink from Jaycar.
If you decide to use a heatsink, use H/S compound between the resistor and the H/S and between the H/S and case.

You need to try it first with the 0.82R and see what the volts are to detirmine which direction to go.

And yes you need to do this under load, with the PSU board connected to the channel boards and all the valves in thier sockets.

[Barney Loveland]

If anyone needs transformers for this project or any others I can highly recommend Voller Transformers in Highett, Vic. No website but a good guy and good prices.

[no-fi]

The 9V is the AC voltage, the rectified and filtered DC voltage will be higher.
Because you will be using a step down Tx it gets a bit more complicated.
Just because it says 240V -115V does not mean it will always give you 115V.
It depends on the regulation of the Tx and its load.

Of course, you also have variations in mains voltage, depending on where you are on the grid, and maybe also the time of day it is.

240V is not always 240V.... I seem to remember mains voltage at an outlet can be +/- 20% of nominal voltage, and still be "in spec" (ie you get down to 192VAC or up to 288VAC on your power points, you can't complain to energy australia, and have it fixed) so even with everything after the power point designed perfectly, your transformer output voltages can also be up or down 20% depending on your mains supply..
:-)

[JulienG]

240V is not always 240V.... I seem to remember mains voltage at an outlet can be +/- 20% of nominal voltage, and still be "in spec" (ie you get down to 192VAC or up to 288VAC on your power points, you can't complain to energy australia, and have it fixed) so even with everything after the power point designed perfectly, your transformer output voltages can also be up or down 20% depending on your mains supply..
:-)

Australia's nominal voltage is *NOT* 240V, it's 230 (+10% / -5%). Now in practice inner city areas still see as much as 252V, and in rural areas below 200 happens.

[no-fi]

Australia's nominal voltage is *NOT* 240V, it's 230 (+10% / -5%). Now in practice inner city areas still see as much as 252V, and in rural areas below 200 happens.

Where'd you get that little nugget of knowledge?

I remember there was mumbling about moving to 230V a while ago, but I was pretty sure it hadn't happened.

According to this, which appears to be the current network standards for my electricity supplier, 240V still stands as nominal.
http://www.energy.com.au/internet/pdfs/ENOS-June06.pdf

also, they're claiming the "objective" is 216-264. which is... gee... +/-10% before they will adjust the network. and then only if they're informed of the problem.... so if there is an undervoltage or overvoltage issue, will it really be hard-limited at 10%, if no customers realise or have the ability to test it?


As for your story about where this variation might be expected to happen, well..... I've measured AC voltage down towards 200V at work..... and we're not in the centre of sydney by any means, but we're not rural either. Plain old boring suburb. (though to be fair, our factory unit is near a bakery, we're on the end of a branch, and they pull a fair bit of power.)

Also in my experience, there's inner city areas where voltage is way down too... what it really depends on is the distribution network, the rest of the load on the substation that supplies you, and your building's place in the area's network. not some general city/country thing.

[David W]

So the obvious next question is >>>>>

Power Conditioners.

Is it benificial to keep the power going into your gear stable at 240V.
And what devices actualy do this? A standard UPS only cut's in when the power drops out so what yould someone look for if they wanted power stabalised at 240V?

[TimS]

So the obvious next question is >>>>>

Power Conditioners.

Is it benificial to keep the power going into your gear stable at $240.
And what devices actualy do this? A standard UPS only cut's in when the power drops out so what yould someone look for if they wanted power stabalised at 240V?

I wish I could get AC power conditioning for $240 ;-)
I have a UPS at home but its more for power failures giving you time to backup and shutdown (if needed too)

[David W]

So the obvious next question is >>>>>

Power Conditioners.

Is it benificial to keep the power going into your gear stable at 240V.
And what devices actualy do this? A standard UPS only cut's in when the power drops out so what yould someone look for if they wanted power stabalised at 240V?

Corrected!