Converting Filaments AC/DC

[J M Fahey]

If the owner complains than now the amp lost its Mojo, staple some dried fish to cabinet walls, so tube radiated heat vaporizes it and Mojo vapour oozes through ventilation slits, Mojoing the whole rehearsal.

Today I repaired one of my own PA amps, which sits in a Restaurant: head and 1 cabinet full of dead cockroaches and that weird empty capsule they leave behind.

I sprayed both cabinets and the head guts with some Raid (tm) and the owner even dared complain at the smell.
Oh well, the nerve/chutzpah some of these customers have.

Even worse: he even invited me to dinner there, for free of course, any day I want.
Ugh !!!!!!

[lowell]

Hijackers!!!!

[Leo_Gnardo]

Hijackers!!!!

Jolly pirates are we! ARRRGGGHHH! Back to regularly scheduled activities now... sans roaches and phô juice.

[lowell]

Pho juice haha. I love me some pho. War...what is it good Pho?

[Justin Thomas]

head and 1 cabinet full of dead cockroaches and that weird empty capsule they leave behind.

You got lucky. That coulda been an extra 100 roaches in about a week... eggs.

Justin

[Gregg]

In TWEED mode I get 11.47V DC on the first tube. It means that the regulator is not working.

Did you use an LDO voltage regulator?

Without those resistors heaters of other tubes are not balanced. One side of the heater winding is grounded with a diode. Juan said that the resistors are not needed but you find them in almost every amp. So someone considered them to be important.

Generally speaking those resistors are there to eliminate hum. When this type of circuit grounded it does the same. In some amps one end of the filament winding is grounded so "balancing" filaments is not an issue here.
Most of the amps have those resistors but they don't have that circuit. It can work with them in place but concerning the main reason they are used - eliminating hum, they are not necessary.

[nickb]

<snip> It can work with them in place but concerning the main reason they are used - eliminating hum, they are not necessary.

If the resistors are in place with this voltage doubler, and I'll assume only one ground is hooked up else that would be bad, then this, as Juan already said, unbalances the circuit. What this means is that you end up with a common mode 3.15Vrms AC signal that the DC heater rides on. Considering that the objective of the balancing resistors is to eliminate the common mode voltage as far as possible, then you can see that adding the resistors defeats the objective of having a quiet heater supply.

Of course, the other AC heater supply, if used, are now unbalanced and so is not so suitable for sensitive stages e.g input, reverb recovery.

[MarkusBass]

Did you use an LDO voltage regulator?

No, the regulator is L7812 from ST Electronics and I didn't change it. This is 1.5A regulator and in this case it provides only 150 mA@12V (plus current for the relays). It doesn't have a heatsink (again there is no space to use it). After few minutes of work it gets quite warm - 100 degrees Celsius. This is another interesting "feature" of this amp.
I did some measurements and here they are:
with 230V mains power supply and 220V primary winding used I get:
in FULL POWER mode: 489V, 365V and 362V voltages in the power supply.
in TWEED POWER mode: 406V, 307V and 305V in the power supply. So they switch voltages by 80-60V.
The voltage regulator:
FULL input: 16.2V, output: 12,41V
TWEED input: 12,66V, output 11,4V. You see that the regulator does not work properly in this mode. I think that LDO also wouldn't work here.
Filaments V2-V9:
FULL: 6,6VAC, (a little to high)
TWEED: 5,56VAC (a little to low)
Filaments V1:
FULL: 12,33VDC,
TWEED: 11,3VDC

Mark

[Gregg]

According to the service schematic I have the export power transformer #561137 has a 230V tap (brown/red) and all kind of other taps. Please check if you have the same part number PT in your amp.
Concerning the doubler I'll repeat myself: your best shot at it is Schottky diodes, 10000uf caps (these caps' value affects voltage directly) and an LDO. Some LDOs need only half a volt difference to run properly. If necessary use a heatsink but since the voltage regulator is grounded you can also run wires from the PCB and bolt it to the chassis. On the other side if it survived so long it doesn't seem to be a problem however personally I wouldn't feel comfortable knowing there is something in my amp that runs that hot.

Of course, the other AC heater supply, if used, are now unbalanced and so is not so suitable for sensitive stages e.g input, reverb recovery.

According to my experience this is not an issue but you can always use your ears to make sure it's OK.