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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 21st Oct 2020, 11:32 pm   #61
DMcMahon
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Default Re: Brenell Purchases

Have now powered up and done some quick measurements, monitoring the HT voltage and current while slowly bringing up the mains voltage from Variac.

Only got to 150 volts mains with the HT current already up to 45 mA (highest I have seen it) and HT voltage 165, then the EL84 Output pentode started flashing/arcing internally, together with loud horrible noises from the speaker. Mains ripple also seemed higher than normal at around 400mV p-p.

Switched off and removed the valve and repeated. At 200 volts mains, HT voltage was 356 volts and HT current 3 mA and ripple 30mV, I think these figures are very similar to previous values seen with no EL84 fitted. I did not take the mains voltage up to 240 because the HT voltage was already at 356 volts and power supply capacitors only rated at 350 volts.

Not sure yet if the EL 84 has developed an internal breakdown problem or something to do with the external connections to the valve.

David
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Old Yesterday, 11:32 pm   #62
DMcMahon
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Default Re: Brenell Purchases

Thoroughly checked out all the circuitry around the EL 84 but nothing untoward found.

Repeated tests as above and now the problem has gone away. Able to go up to 240 volts mains voltage without excess HT current or the EL 84 arcing, with typical good results as seen before.

Results at 240 volts (Playback mode) :-

Variac AC current 0.16A
DC HT voltage +332V (schematic value = 285V)
DC HT current 40mA
EL 84 Anode voltage +325V (schematic value = 270V)
EL 84 Screen grid voltage +290V (schematic value = 250V)
EL 84 Cathode voltage +9.6V (schematic value = 8.5V)
HT mains ripple 240mV p-p

Still have the very original issue of the HT voltage being too high. The value of 332 V above is 47V higher than stated value on schematic and no obvious reason found. The AC voltage across the EZ 80 full wave rectifying valve anodes is 615V which is 45V higher than stated value on schematic.

I know the values shown on the schematic are approximate but my values do seem just too high to me.
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Old Today, 12:11 am   #63
Ted Kendall
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Default Re: Brenell Purchases

Quote:
Originally Posted by DMcMahon View Post
Still have the very original issue of the HT voltage being too high. The value of 332 V above is 47V higher than stated value on schematic and no obvious reason found. The AC voltage across the EZ 80 full wave rectifying valve anodes is 615V which is 45V higher than stated value on schematic.

I know the values shown on the schematic are approximate but my values do seem just too high to me.
Contemporary Grundig manuals give voltages as read by an Avo 7, and it is likely that Brenell specify something similar. A modern meter is going to give a higher reading as it will load the circuit less.
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Old Today, 12:25 am   #64
DMcMahon
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Default Re: Brenell Purchases

Thank you Ted, a very good point.

The Brenell documentation states Avo 8 for the Mk. 5 Series 2 and Avo 7 for the Mk. 5, I know from a few comparisons on other similar machines, surprisingly the difference between Avo and high impedance DVM is not that great but will recheck the Brenell using Avo.

David

Last edited by DMcMahon; Today at 12:29 am. Reason: Update
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Old Today, 11:36 am   #65
Uncle Bulgaria
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Default Re: Brenell Purchases

Saw the e-mail about post #62 and arrived to find Ted had said just what was on my mind. I discovered the excessive voltage and circuit loading effect thanks to Lawrence while doing my Ferrograph...
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