[Too Long in The Wasteland]

I have this Bell 35A (PA AMP) that has two 12ax7 and 12av6 wired
in series from a voltage divider from B+.

The two 12ax7 readings are 12dvc across heaters 4/9 .

The 12av6 heaters have 22vdc across them.

The heaters are only rated for 12v @.15 amp ...
and the resistance between the heaters is 14 ohm.

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 ?




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.

[Phil Allison]

"Too Long in The Wasteland"

I have this Bell 35A (PA AMP) that has two 12ax7 and 12av6 wired
in series from a voltage divider from B+.

The two 12ax7 readings are 12dvc across heaters 4/9 .

The 12av6 heaters have 22vdc across them.


** You real sure about that?

A 12 volt filament would not last long with that voltage.



The heaters are only rated for 12v @.15 amp ...
and the resistance between the heaters is 14 ohm.

ohms law: 22/14 = 1.5 amps .


** That is dumb.

Filament resistance * increases * with temperature - by about 6 times at
normal ( red) running temp.

So a 12 volt @ 0.15 amp filament is 80 ohms when hot and 13 to 14 ohms when
cold.



....... Phil

[John Byrns]

In article ,
Too Long in The Wasteland 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 @.15 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/

[Too Long in The Wasteland]

John Byrns 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.

Exactly !

How many output and rectifier tubes does this amp have?

SS rectifier : + 2 x 6L6 and another 12ax7 PI that uses traditional
6.3 vAC secondary tap.

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.

It was butchered prior to my ownership. A$20 ebait bid that come
in pieces.

The heaters are only rated for 12v @.15 amp ...
and the resistance between the heaters is 14 ohm.

14 Ohms is the cold resistance, it should be about 84 Ohms hot.

This finer detail I didn't know !



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.

I do. It is not an ideal match. One theory I had
there were variant models through production cycle. In particular: the
resistors network scattered around the series heaters are gone !


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,

John Byrns


Thanks !