6j1 supply voltage concerns

[petey twofinger]

boxed up a cheebay 6j1 "tube buffer/preamp" today .

i checked the voltage of the AC/AC adapter , which is rated at 12 volt ac 500 mah .

the meter said 14.37 volt under no load .

now i had read some where a wise man mentioned running this circuit at 15 volts would result in 10-5% tube life .

now i was never that good at math ... tell me , should i be looking for a power supply that meters out closer to 12 v AC for this thing or am i beeing a huge weenie , well wait ... that last part is part and parcell . anyway thanks in advance .

[vigilante397]

Big question: what does the circuit look like?

If the power supply is going straight into the heaters that's a big no, 12V is a terrible idea. 6J1 is supposed to get 6.3V on the heaters, and it's not like 12AX7 where you have a center tap and can run in parallel, this really needs to be close to 6.3V.

I would expect if this is something you bought that there would be a bridge rectifier, a giant cap (or a few), and possibly a regulator. If this is the case then you're fine, it doesn't matter if you feed it 15V, it will be taken care of downstream. If there's a regulator you have nothing to worry about, if it's just a rectifier and some big caps you may want to pull the tube out and measure what voltage it's getting to make sure it's within spec before subjecting your tube to it.

[anotherjim]

I found this. It's a stereo one so both tubes heated in series.
Is it a stereo Petey?

[antonis]

the meter said 14.37 volt under no load .

Under "unknown" circuit load, the meter will say less than 14.37 Volt..
(the more the current comes close to a percentage of the rated output the more the voltage comes close to the rated one..)

You can verify it by connecting some "light" loads (drawing 5mA, 25mA and 50mA say) to your power supply and measure its output voltage..
(resistors of 2k2, 470R/0.5W and 220R/1W should be suitable loads  ..)

[antonis]

What Jim posted in simulator..

https://www.circuitlab.com/editor/#?id=9vnehk

[amptramp]

If the schematic posted by anotherjim is correct, you will probably not get enough filament heat to make the circuit operational.  The diode only allows the voltage to go to the filament for half the cycle and the filament heat requires a full cycle to generate enough cathode emission.  There used to be a number of "instant on" televisions that had the same connection - a diode in series with the series-string filaments to keep them warm enough that applying full voltage by shorting the diode would result in the set turning on within about one second.  But a 50% duty cycle on the filament does not produce usable cathode current.

[anotherjim]

There is a 470uF cap across the heater string. The AC might reach 18v peak so with some of that stored by the cap it could be good enough. I'd be surprised if it gets better than 9v by a DMM across the string though. 12v filament can be ok at 9v.

[amptramp]

Why not just eliminate the diode and capacitor and let the heater run off AC?  Half-wave rectifying the heater supply adds a lot of harmonics into the heater that would be more likely to interfere with the audio than a straight 60 Hz signal.  I have two stereo receivers that use DC for the preamp heaters but one has fullwave rectification and a four-section electrolytic and some resistors for filtering and the other uses the cathode self-bias of the output tubes as the supply.

In a guitar amp and speaker combo, quite often the speaker turnover is above 60 Hz.  My JBL G-730 speakers have a turnover at 70 Hz which is fine for a lead guitar that never goes below 82 Hz.  Even if 60 Hz couples into the signal, there is some rejection of it.  You can't say the same for any of the harmonics.

[anotherjim]

Good point Ron.
If AC, I'd expect it would be better to use discrete twisted pair wires to hook up the filaments?

[amptramp]

Good point Ron.
If AC, I'd expect it would be better to use discrete twisted pair wires to hook up the filaments?

Twisted pairs definitely work to reduce far-field emissions because the magnetic field from one twist is the opposite of the magnetic field from the next twist.