DC filament / capacitor
 

Hey Guys,

Just a quick question...please enlighten me for learning purposes...

on the attachment am I seeing correctly that the filament is AC ? I am confused...isn't suppose to be DC with some filter cap's?

Please help as I am getting confused here...

Thank you !

Re: Dc filament / capacitor
 

Hi

The filaments are AC that capacitor arrowed is part of the HT (DC) circuit.

The snip shows the filament supply

Cheers

Mike T

Re: Dc filament / capacitor
 

It's a voltage doubler rectifier creating DC for HT, with the heaters powered directly from the transformer (so yes, running on AC).

These valves are indirectly heated, so are designed to run on AC. Directly heated valves are often restricted to DC only on the filaments (as for battery valve receivers) but can also be AC (eg 2A3 output triode; DY81 rectifier).

Re: Dc filament / capacitor
 

The arrow on your diagram is pointing to a filter capacitor in a voltage doubler circuit. The voltage doubler is in turn supplying 272V DC to the rest of the set.

The filaments (heaters) of the valves (6AL5, 6CG7, 6BC8) and the pilot lamp (PL 1) are being fed from a separate AC supply as normal.

Re: Dc filament / capacitor
 

whooo thanks guys!

So this is the power stage of an Altec schematic where someone advised to put a 4700uf 16V or better capacitor for DC filament supply....that's where my confusion came from looking at the schematic's...

Re: DC filament / capacitor
 

At a glance it does look like there is a connection between the heater supply and the HT supply, but it's just the way the circuit has been drawn.

Re: DC filament / capacitor
 

...thanks guys...in the meantime reading through the modification suggestion, found out where the 4700uf DC filter cap was recommended to go together with a diode...

not sure why or what this will do...i am getting way confused on that :)

attached it....point number 10.

Re: DC filament / capacitor
 

There is a connection, but since it's to chassis/ground/0V for HT- and heater CT it has no effect on the way the circuit works. That's not to say it isn't an unusual circuit- I wonder why a suitable voltage winding wasn't specified on the transformer and a conventional circuit used? It would have been cheaper.

The capacitor and dc heater suggestion sounds incomplete and misleading. Dc heaters in the front end of an amp can reduce hum but there will need to be a suitable rectifier and attention paid to getting the right dc voltage as well as just a capacitor.

The circuit shown here is already good in respect of heater hum since it has the heater ac supply balanced to HT-/ground by the centre tapped winding on the transformer.

Re: DC filament / capacitor
 

This looks like a modification to run some or all of the heaters on DC. It depends on the way the set is physically wired. There may be more than one wire on pin 4 or the commoning point could be elsewhere.

Re: DC filament / capacitor
 

Thanks Guys

yes it is definitely a modification and I was lost on that part at the DC and the diode on pin 4 at 6AL5.

Attached the full modification instruction...maybe you will get some good use to it...someone did write this up in 1991. The rest of mod's are really cool but still not getting the mod at the 6AL5...

have a look at the attachment...great stuff there ! :wave:

Re: DC filament / capacitor
 

following the previous post, the full schematic attached

Re: DC filament / capacitor
 

That will provide a somewhat rough DC heater supply. About 1.2V pk-pk ripple, plus switching transients, and about 7.5V DC so overrunning the heater. The likely outcome is a slightly shorter life for the 6AL5/EB91 and replacement of some 50Hz hum by buzz.

Re: DC filament / capacitor
 

This worries me, first, no indication of which way round to connect the diode and which way round to connect the (polarised) capacitor, the chance is 50:50 that wit will work or blow up; secondly the heaters are pins 3 and 4 of the 6AL5 not 4 and 5, with neither heater pin at chassis potential it is unlikely that pin 5 (a cathode) is commoned with a heater; and last, the heater will be a bit over-run, as others have already pointed out!

Re: DC filament / capacitor
 

Best advice is to forget the so-called 'mod'!!

Re: DC filament / capacitor
 

too right !!! ;)

just wanted to understand the thinking of that person who decided to do this on this old unit...

Re: DC filament / capacitor
 

I sometimes remind people that in order to genuinely improve a circuit you first have to understand it even better than the original designer. Sadly, many 'upgrades' found on the internet come from people whose understanding is significantly less than the original designer. What they lack in knowledge they make up for in misplaced confidence.

Re: DC filament / capacitor
 

Not necessarily- though good understanding may explain perhaps what cost constraints may have been imposed on the original design that made it less than ideal. Unconstrained by these limits, a less competent designer may well "improve" a circuit or at least some aspect of its performance.