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Old 30th Dec 2019, 3:08 pm   #21
PortugalTV
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Location: Serzedo, Portugal.
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Default Re: Need help with Ferranti 146e radio

ahhh more headache from this radio...i think it's possesed by evil
how hard can it be do diagnose a simple 6v6 se output stage ?

now i found the reason for the 6v6 to run hot...the tone pot is wide open, it looks like it acts as a grid leak, i was getting positive voltages developed at the grid even with the coupling cap disconnected
shunting the pot with a 100k resistor mitigated the problem, with pot shunted and the 9v wall wart otput transformer, the cathode voltage is now 13.5v wich is 54ma. changing the resistor value doesnt affect the cathode current that much, tried a 33k and a 270k. i will open the pot and see if i can fix it, meanwhile any ideas are appreciated. on the scheamtic the pot is labeled as 100ohms wich i thinkis impossible because it would shunt all the audio to gorund...tried it anyway and i had almost no volume

the audio transformer (9v adapter) has a dc resistance of 180ohms on the primary and that lowered a little bit the plate voltage
right now the 6v6 operating conditions with the new audio transformer are these:
plate 280v
g2 295v
grid 0v (with resistor in place)
2 to 3v positive without resistor and potentiometer damaged
cathode 13.5v
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Old 30th Dec 2019, 6:19 pm   #22
Herald1360
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Default Re: Need help with Ferranti 146e radio

If the coupling cap is good, the actual value of the grid leak won't make much difference as long as it's less than about 1 megohm.


You say you have 250ohm cathode R- the service info suggests it should be 270ohm and it wouldn't hurt to go a bit higher still to reduce the stress on the 6V6. The primary resistance of the ouitput transformer will have very little effect on anode current since beam tetrodes / pentodes are pretty good constant current devices- look at the flat top to the Ia vs Va curves in the datasheet. https://frank.pocnet.net/sheets/093/6/6V6GT.pdf


Sounds like you're nearly there......
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Old 30th Dec 2019, 9:36 pm   #23
PortugalTV
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Default Re: Need help with Ferranti 146e radio

Quote:
Originally Posted by Herald1360 View Post
If the coupling cap is good, the actual value of the grid leak won't make much difference as long as it's less than about 1 megohm.


You say you have 250ohm cathode R- the service info suggests it should be 270ohm and it wouldn't hurt to go a bit higher still to reduce the stress on the 6V6. The primary resistance of the ouitput transformer will have very little effect on anode current since beam tetrodes / pentodes are pretty good constant current devices- look at the flat top to the Ia vs Va curves in the datasheet. https://frank.pocnet.net/sheets/093/6/6V6GT.pdf


Sounds like you're nearly there......

tried a 330r resistor and cathode voltage has now uped to 15.5 wich equals 47ma..i think that might be it. i have plenty of volume and the tube is cooler. will see now what's up with the pot .

i have acid dipped and repainted the old speaker basket and i have a good used 8ohm voice coil and a cone waiting to be fitted, will put pictures of that
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Old 31st Dec 2019, 9:28 am   #24
Silicon
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Default Re: Need help with Ferranti 146e radio

The HT seems to be higher than I would expect.

What is the voltage tapping on the primary of the mains transformer?

Another important thing to check is the heater voltage.
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Old 31st Dec 2019, 12:23 pm   #25
PortugalTV
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Default Re: Need help with Ferranti 146e radio

Quote:
Originally Posted by Silicon View Post
The HT seems to be higher than I would expect.

What is the voltage tapping on the primary of the mains transformer?

Another important thing to check is the heater voltage.
hi
thanks for the tip
i am on the 230v tapping, wall mains today reads 231.5v and its preety much always around that overhere.
heaters are 6.4v under load so we are preety close
rectifier is the original ferranti r52 so i will dare to say that this radio had little use, it was just poorly stored all these years.
i still have a 250v tapping to select if needed.......


here is the speaker basket, i acid dipped it and scrubbed it until it was all shiny metal..primed it and painted it...still looks like crap because it's so pitted but its protected now
Click image for larger version

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here is the new cone i found from a woofer i had in the junk
Click image for larger version

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Click image for larger version

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here the cone with a 8ohm voice coil that fits the field coil's "shaft" diameter perfectly

the condition of the old potentiometer cover, i went ahead and replaced both volume and tone pots, the tone pot is also an on off switch
Click image for larger version

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had to make shaft extenders for the "new" pots
Click image for larger version

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Last edited by PortugalTV; 31st Dec 2019 at 12:52 pm.
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