pye mozart hfs 20

[John Caswell]

Hi Jim,
Idly looking at my Mullard data books, there is no way two EL34 are going to draw 400mA possibly 200 mA at the most, so there must be something seriously wrong with your measuring. The negative bias on the valve seems correct and according to Mullard with 250V HT and -12.6v bias the total current drawn would be in the region of 110mA per valve. So I would expect to see about 200mA max current draw.
Further investigation on your part required.

John

[jimangus]

Ok, so a bit of a brief faff about tonight using a 10watt maplins resistor (it certainly isn't up to the job gets a bit hot and smokey after 5 mins or so).


Voltage with no el34 valves in 350v

Voltage with no el34 valves in and 100r resistor 350v (before and after resistor and no temperature gain on resistor)

Voltage with el34's in 295v

Voltage with el34's and resistor, 235v after resistor 265v before.

a bit confused by the last result, i was not expecting the voltage to drop before the resistor.


It looks like i am pulling 0.3amps across the resistor and have measured 0.39amps to the rest of the circuit.

Any how in the dark with this a bit, valves do not overheat when 100r resistor in circuit. Don,t know if they do the throbbing thing as not plugged into anything on the test bench.


Kind regards

jim

[jimangus]

Originally Posted by John Caswell

Hi Jim,
Idly looking at my Mullard data books, there is no way two EL34 are going to draw 400mA possibly 200 mA at the most, so there must be something seriously wrong with your measuring. The negative bias on the valve seems correct and according to Mullard with 250V HT and -12.6v bias the total current drawn would be in the region of 110mA per valve. So I would expect to see about 200mA max current draw.
Further investigation on your part required.

John

Thanks john,

just read this after posting the last message. i bought a new multi-meter today as i did not trust the last one so will have another look at the current draw.

Is there any way of isolating parts of the circuit to get a better idea of what is going on ?


Regards

jim

[pre65]

The voltage drop BEFORE the resistor is most likely because the main transformer is now loaded, and 30v through a 100R is 300ma.

[jimangus]

Originally Posted by pre65

The voltage drop BEFORE the resistor is most likely because the main transformer is now loaded, and 30v through a 100R is 300ma.

Hi pre65,

That is what I thought 0.3amps. The drop before the resitor makes sense as transformer load, ya for that. So I am probably looking for a smaller resistor value to get 265v after the resistor. I may get a second 100r resistor to check if 50r is correct. It will probably looking at the math need to be 75r



Regards

Jim

[pre65]

"Voltage with no el34 valves in and 100r resistor 350v (before and after resistor and no temperature gain on resistor"





That is because with no load you are drawing zero current, and with zero current there is no resistance.


I'm very lazy as far as calculations go, and make great use of this web site for calculations. Most used are Ohms law and resistor wattage.

http://www.electronics2000.co.uk/calc/

[jimangus]

Hi all,

Can I assume the voltage drop across the resistor relates to the current draw of the el34's?

If so one valve fitted at a time and measured 1.7amps brand new pinical valve, 1.5amps old mallard valve, 1.9amps another old mallard valve.


Interestingly with my old dmm I didn't notice the variability in the voltage. It starts of high, then settles to a 100v lower then climbs and fluctuates by about 10v. Any suggestions as to why ?

Kind regards
Jim

[pre65]

Originally Posted by jimangus

Hi all,

Can I assume the voltage drop across the resistor relates to the current draw of the el34's?


Kind regards
Jim

Yes. Ohms law says it must.

[colin.hepburn]

Yes I went through this exact voltage thing as well no voltage will be dead on as not all EL34s test the same that’s why you have to get matched pairs or quads
plus your mains voltage varies up and down at times like +/_10% or so I would say if you know the voltage you require then just get as near
it may help you too post the part of the schematic you are working on

[jimangus]

Originally Posted by colin.hepburn

[size=3][font=Calibri]
it may help you too post the part of the schematic you are working on

Hi Colin,

The schematic is on page 2.


So the resistor on ht stops the valve plates from glowing red put the motorboating is still there, it manifests itself in the measurements by a wondering ht voltage by 10v or so every second.


Any thoughts on this ?

Also wondering if anyone near derby that could have alook for me as getting impatient and probably missing something.

Kind regards

Jim