AR88D PSU question.

[TubeType]

Just to say that I agree with Al in his earlier post insofar that I would have expected a lower value discharge resistor if they were fitted with quick discharge/shock hazzard in mind, I would say 100k or a lot less maybe. Clearly they are supposed to be there though which the main thing. I have since discovered that the manual I was reading from had AR88 (R's not fitted) written on the front and not AR88D (R's fitted)I must try harder.

Lawrence.

Hello Lawrence,
Normally I would agree with the 100k Ohm scenario, but in this case it may be pushing the 5Y3 current too hard.
Kind regards,

[pwdrive]

Hello Terry, yes having had a conversation with some one earlier I realize that is the case, good call.

Lawrence.

[Skywave]

The 5Y3 rectifier is rated for a mean current of 125 mA. An additional bleeder of 100k-ohms is O.K.

With the receiver in "REC. MOD.", I measured the +HT2 voltage at 235-v; current at approx. 60 mA; 200-mV p-p ripple.
The +HT1 rail was at 260-v.

I connected a 100k-ohm, 12-watts, wire-wound resistor from +HT1 to chassis. The +HT2 line fell to 220-v; current at approx. 60 mA; 200-mV p-p ripple.
The +HT1 rail fell to 245-v

The new 100k-ohm bleeder resistor draws approx. 2 mA and dissipates about 0.5-watt.

Bleeder performance. When the receiver was switched of at the a.c. supply, the voltage at the cathode of the rect. valve fell to 100-v in less than 5 seconds. Prior to the fitment of that 100k-ohm resistor, the time was approx. 25 seconds.

Al.

[pwdrive]

Mmm...

Lawrence.

[Mark R]

Lawrence does that mean you are ruminating?

cheers Mark

[pwdrive]

I don't know wot ruminating means but my Mmm usually means interesting, it's interesting because I have already said that i would expect a lower value for such resistors, that was then followed by a confirmation from me that I was probably wrong because others had given valid reasons as to why they should be a higher value, now I am wondering again since Al's latest tests, I am guessing though that the manufacturers chose the correct values for the correct function so it's back to square one for me but my thoughts have been answered.

Lawrence.

[Mark R]

Ruminating Lawrence is to think.

cheers Mark

[pwdrive]

Thanks Mark, that fits the bill as when stuff is interesting i tend to think.

Lawrence.

[Skywave]

Let's just run through the basic factors that determine the ohmic value of a bleeder resistor in a valve PSU such as this.

Its primary function is to provide a discharge path for the filter capacitors when the a.c. supply is switched off. It also provides a minimum load on the PSU, so that in the event of the load becoming disconnected (whilst the rectifier is delivering volts to the filter), the output voltage does not soar to a possibly dangerous value. Depending on the L and C values, it also provides a minimum value of load resistance that will optimise the filter's performance. Of these three factors, in most PSUs of the type we have here, it is the capacitor discharge function that is usually the most predominant and important. Ideally, we want a low enough ohmic value for that resistor so that the filter capacitors are discharged as fast as possible. On the other hand, the lower that value, the more current 'goes to waste' from the rectifier - and more heat is dissipated in that resistor. A given rectifier valve has a maximum value of mean current, so that sets one limiting factor. Then there is the consequent increase in the PSU transformer temperature and the rectifier valve to consider. And finally, the greater the bleed current, the greater the ripple on the HT rail and the lower the d.c. voltage of that rail becomes. So a compromise has to be found. An appropriate value can be calculated, but I usually find that trying a few values within a limited range, (that range based on experience), is usually faster and a lot easier. And in fact, I did try 47k-ohms: it produced a current demand on the rectifier that was well within its capabilities - but I considered that the ripple was excessive, the HT drop excessive and the discharge time too short in relation to those factors. So I traded discharge time against HT drop and settled on 100k-ohms.
Since wire-wound resistors are much more reliable in maintaining their nominal ohmic value than carbon composition and other types, I always use a wire-wound R for this task - and make sure that it is mechanically secure where its lead-out wires join the rest of the circuitry. A bleeder resistor is a component that you fit and forget! And I always use a resistor with a very generous wattage rating - purely to maximise reliability. The very last thing you need in a HV PSU is an open-circuit bleeder - a false sense of security - which could be fatal. :omg:

Al.

[Skywave]

To ruminate, (verb intransitive): cows ruminate. A cow has more than one stomach. When it eats the grass, it simply swallows it. Later on, and usually when the cow is lying down, it recovers that undigested grass, chews it and passes it to its other stomach. This process of re-digesting is called ruminating - commonly called 'chewing the cud' - the cud being the mass of grass and digestive fluid brought up from the cow's first stomach. Cows usually 'chew the cud' when they are lying down and then take on a distinct "I'm pre-occupied and I'm slowly thinking about what I'm doing" look on their faces. Hence the phrase, applied to humans, when someone appears to be in deep thought, trying to make a decision - "he's chewing the cud".

And that little zoological missive is about as far removed from vintage radios as I think it is possible to go! I hope it helped.

Al.