In article <47afad49$0$4077$bbae4d71@[EMAIL PROTECTED]
>,
Too Long in The Wasteland <Lost@[EMAIL PROTECTED]
> wrote:
> I have this Bell 35A (PA AMP) that has two 12ax7 and 12av6 wired
> in series from a voltage divider from B+.
I suspect they are actually in series with the cathode circuit of the
output tubes, with some extra current coming from a resistor to B+,
possibly forming a voltage divider for the screen circuit.
How many output and rectifier tubes does this amp have?
> The two 12ax7 readings are 12dvc across heaters 4/9 .
I would expect about 5 to 6 volts between pins 4 & 9. I would expect
only pins 4 & 5 on the 12AX7s to be used with pin 9 left unconnected on
each tube. This yields a series string with about 30 to 36 volts across
it, and 10 to 12 volts across each heater, pins 4 & 5 on the 12AX7s.
> The 12av6 heaters have 22vdc across them.
It sounds like someone has been messing around with the wiring in there.
> The heaters are only rated for 12v @[EMAIL PROTECTED]
amp ...
> and the resistance between the heaters is 14 ohm.
14 Ohms is the cold resistance, it should be about 84 Ohms hot.
> ohms law: 22/14 = 1.5 amps .
>
> My curiosity is: Is 22vdc normal ? I was expecting 12vdc
> like the other 12axxxx. Is the heater resistance indication
> accurate ?
22 volts is not normal, there is something wrong, I would check the
wiring against the schematic if you have one. It sounds like someone
was in there trying to fix the amp and and messed it up in the process.
> The amp is working fine, but I'm interesting in
> experimenting with a 6SL7/6Sn7 octal preamp and
> I thought this would be an interesting platform for that.
The 6SL7 and especially the 6SN7 are not good choices for this type of
series heater circuit. The 12SL7 will work in place of the 12AX7 and
there is a 24 volt version of the 6SN7 that will probably also work, but
it will probably require some changes in the bias network, as well as
possibly the voltage divider you mentioned.
Regards,
John Byrns
--
Surf my web pages at, http://fmamradios.com/
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