12AX7... 6V & Series heaters???

[jeff]

Fixing/converting a Masco today, looked at schematic. The heaters for the 12AX7 are wired series on the 6V heater supply instead of parallel. Looked at the amp to see if it was a schematic error, but sure enough the heater wires supplying the other 6V tubes(6SN7, 6SL7, 6V6) are connected to pins 4 & 5. Nothing on pin 9, just one wire to 4 one to 5 from the 6SL7. 6V on a 12AX7 wired for 12V?


Any legit reason to run the heaters at half the voltage?
Should I 'fix' it and rewire the socket parallel?

[tubeswell]

So 6V + 6V = 12V

[kagliostro]

On some old Geloso tape recorder V1 heaters were  deliberately feed with less than the required voltage as to achieve a less noise result

May be they decided to obtain a less gain factor from those tubes using that trick ?

Franco

[Merlin]

Half voltage would be highly unusual. It might help if we had the schematic...

[Latole]

 It might help if we had the schematic...

Right.

[kagliostro]

Is this the schematic you refer to ?

https://el34world.com/charts/Schematics/files/Masco/Masco_mu17.pdf

Franco

[jeff]

https://www.radiomuseum.org/r/mark_simps_d37.html#images
Heater says TO ALL FILAMENTS
Tubes are 6SN7, 6SL7, 6V6 and 12AX7
Under 12AX7 it looks as if the heaters are series with 10ohm resistor.


When I inspect the amp, I don't see the resistor BUT....one heater wire is going to pin 4 one heater wire is going to pin 5, 9 is not connected(as you'd expect for 12V operation).


Can't understand it, if the supply is 12V use 4 and 5.
If the supply is 6V, use 4/5 and 9


But why wire series(tube expecting 12V) with a 6V supply?

[tubeswell]

What VAC do you measure across the heater winding?

[jeff]

I'll fire it up after I mow the lawn, BUT I can physically see that:
1) pin 9 is unused
2) 12AX7's 4 in parallel with 6SL7's 6SN7's 6V6's heater, pin 5 in parallel with 6SL7, 6SN7, 6V6.


So....
Either it's 12V and the 12AX7's correct, all other tubes running 12V instead of 6V(unlikely)
Or it's 6V and all tubes running at 6V but 12AX7 running 6V as well(although it's wired series and wants 12V)


There is a huge dropping resistor(270K) to the 12AX7's B+ cap
Possible they're running 12AX7 at lower than normal voltage and undervolting the 12AX7 for some reason?


Pin 9 is unused
Heaters of 6SN7, 6SL7, 6V6, 12AX7 paralled


???

[kagliostro]

Look to the Geloso g256 and g257 schematics, looking to V1 heater circuit, then read this old thread

https://el34world.com/Forum/index.php?topic=17100.msg170616#msg170616

Franco

[tubeswell]

Pin 9 is unused
Heaters of 6SN7, 6SL7, 6V6, 12AX7


???

The ‘12AX7’ could be a 6N2P

[jeff]

Schem says 12AX7, chassis stamped 12AX7


But check out what Kagliostro posted above... Even weirder! Pin 5 to 9 @6v pin 9 to 4 + 4ohm to lower heater on one triode.
Helps reduce hum I guess.
The 12AX7's amping the tape head play back, so maybe helps reduce sig/noise ratio?


Thanks Kagliostro
Grazie

[tubeswell]

What VAC do you measure across the heater winding?

[jeff]

The WILDEST thing about this amp:
It uses the 6V6 as a power tube ONLY for 'playback' mode.
In 'record' mode, it swaps the O.T. from the 6V6's plate to the 6SL7's plate to use the 6SL7 as the power tube and uses the 6V6 for the erase head.
Wonder if I should keep that feature?
6V6/6SL7 switch.


Wild!

[jeff]

What VAC do you measure across the heater winding?

Checking soon, safety first

[jeff]

7.5V no load heaters
255-0-255 for P.T.

[jeff]

Look to the Geloso g256 and g257 schematics, looking to V1 heater circuit, then read this old thread

https://el34world.com/Forum/index.php?topic=17100.msg170616#msg170616

Franco

Interesting, thanks

[tubeswell]

7.5V no load heaters
255-0-255 for P.T.

Say 6V ish loaded. Inspect the ‘12AX7’ closely. Is there a centre shield? If so, is it connected to Pin 9?

[jeff]

7.5V no load heaters
255-0-255 for P.T.

Say 6V ish loaded. Inspect the ‘12AX7’ closely. Is there a centre shield? If so, is it connected to Pin 9?

Are you thinking that even though the schem says 12AX7, even though the chassis is stamped 12AX7, even though the tube is marked 12AX7, it's not a 12AX7?


What I see as I look inside the tube:
Pins 4 and 5 are bent 90°,
There's a thin wire(filament?) going up, inside the metal plates
There's a thin wire(filament?) coming down, attached to each end of a rod
The middle of that rod is attached to pin 9.


Now, I'm no expert, but, to me, it looks like starting at pin 4 go up into plate, come back down, go to rod connected to pin 9, go up into other plate, come back down, out pin 5.


It looks like 4 connects to 5 through a rod and that rod connects to pin 9.


In other words, pin 9 is a 'tap' in between pin 4 and pin 5, so that if you connected 4 and 5 together, then attached pin 9 to ground and pins 4 and 5 to +6V you'd have 6V across each filament.
OR
If you did not connect pin 4 to pin 5, but rather connected pin 4 to ground and pin 5 to +6V(leaving pin 9 unused) you'd have 3V across each filament.


Does that make sense?

[tubeswell]

The way to verify would be to use your R-meter to check for DC continuity between pins 4, 5 and 9. As to the labelling on the envelope, it usually means what it says. (Having noted that, a lot of strange marketing occurred in the past, especially in the 1980’s LoL.)

[jeff]

7 from 4 to 9
7 from 5 to 9
12 from 4 to 5

[tubeswell]

Is pin 9 on 12AX7 socket grounded? (Can’t access schematic in link)

[jeff]

Nope. Pin 9's never seen solder, clean and as untouched as the day it was born.

[tubeswell]

So, it appears the 12AX7 heater will run starved. Who knows (or cares) why they did that? If it were mine, I’d just rewire that socket to normal 12AX7 6V operation. YMMV

[jeff]

So, it appears the 12AX7 heater will run starved. Who knows (or cares) why they did that? If it were mine, I’d just rewire that socket to normal 12AX7 6V operation. YMMV

Yeah, I'm with you, gonna rewire the amp so that'll get fixed.
It's just so bizarre, wondered about it. That link Kagliostro shared was interesting.


Thanks for the help.

[Merlin]

Oh it's a tape recorder! Well why didn't you say so!  Now it makes more sense, it's not unheard of to run tape and phono pickup input tubes at reduced heater voltage to avoid hum. Admittedly, half voltage is particularly low, but I'm sure not unique.

[kagliostro]

In 'record' mode, it swaps the O.T. from the 6V6's plate to the 6SL7's plate to use the 6SL7 as the power tube and uses the 6V6 for the erase head.
Wonder if I should keep that feature?
6V6/6SL7 switch

Personally I would like to keep the feature and give a try to it, it seems interesting but it depends also from you, I mean from the way you arrange the whole circuit

Franco

[jeff]

Oh it's a tape recorder! Well why didn't you say so!  Now it makes more sense, it's not unheard of to run tape and phono pickup input tubes at reduced heater voltage to avoid hum. Admittedly, half voltage is particularly low, but I'm sure not unique.

I honestly didn't realize that made a difference.
Good to know the 'why', that was puzzling me.
Thanks.

[jeff]

In 'record' mode, it swaps the O.T. from the 6V6's plate to the 6SL7's plate to use the 6SL7 as the power tube and uses the 6V6 for the erase head.
Wonder if I should keep that feature?
6V6/6SL7 switch

Personally I would like to keep the feature and give a try to it, it seems interesting but it depends also from you, I mean from the way you arrange the whole circuit

Franco

On second thought, I think I'll add a second Xfmr and just use the 6SN7 to drive a reverb tank. The switch looks a little flakey.

[kagliostro]

Ok, so, if you use a low impedance input tank you can also arrange a low power speaker exit, but all depends on how the amp circuit is, may be you don't have interest to hear the sound that comes out from only the reverb driver circuit


Franco

[jeff]

Ahhhh...
Interesting. You mean put an 'interupt' jack so the 6SN7 goes to a speaker instead of the reverb tank, also ground signal to 6V6?
Hmmmm.
BTW I started a new post 'Masco Reel to Reel Conversion'. Schematic there.