Parallel Series Heaters 6l6 to 12ax7

[Zephedone]

I dug an old Conn organ amp chassis out of my garage the other day . Thinking about building an all octal Fenderish circuit like a Pro . This is how the heaters are wired on the organ chassis as far as I can tell . The PT fed two 6l6gc tubes and two 12ax7 tubes plus about 40 or more 12au7 tubes . Plenty of filament power I would think . This is just a guess as to the description . The 6l6 pair is paralleled with the ground going through the cathode resister or "above ground" ??? and the 12ax7 pair is in a 12.6 volt series configuration rather than the usual 6.3 series configuration . I have a hand drawn schematic that I think shows how the filaments were wired . I was hoping to get a few more experienced eyes on the schematic and possibly some discussion so as to better understand what is going on here .

[Stevem]

I know the amp you have and you will have tube filament blowning heater voltage without those un-neeed tubes to load that down, your going to need to get a sperate PT ( like a Hammond 166m6 )just for your filament needs to use that PT!

[martin manning]

The 6V 6L6 filaments are wired in series so they can be powered by the 12V winding. Grounding through the cathode resistor is done so that the whole heater circuit is elevated and centered at the 6L6 cathode voltage, to reduce hum. To Steve's point, what voltage do you measure across the outer leads of the heater winding with no load on it?

[Zephedone]

I measure 13.4 volts unloaded on the heater tap martin manning . Stevem , I suspected there might be a problem without all the extra 12au7 tubes in the heater circuit and so I asked the question . I have not built a DC heater circuit as of yet . Would this be a possibility with this heater setup ?
I know they make a 12 volt variant of the 6v6 ie 12v6 . Is there a 12 volt variant of the 6l6 or even el34 tube ?

[martin manning]

As specified by the manufacturers, heater voltage can be within a range of nominal +/- 10%. At 13.4V you are at +6.3%, so you are okay.

[Zephedone]

I seem to remember having two of these chassis . I think I left the other one as it was wired and replaced only the original pre amp circuit with a deluxe style pre . If my memory is correct , and if indeed it was the same chassis, it seemed to work well . Can't remember what happened to it though . Probably sold it or traded it for more unneeded music stuff . The filament circuit is still intact other than the two 12ax7s . I think I'll wire up the filament circuit as per the schematic and see what happens . I have a box of weak tubes that I can use for testing purposes . Thanks for the replies . I'll post some pics and voltage readings once finished .

[Zephedone]

So I wired up the other noval sockets . Installed two 6l6 and two 12au7 tubes . Theses are my readings on the filaments . I get 13.4 vac on each 12au7 tube . The next tube in line (6l6) is reading 6.0 volts and the last tube (6l6) is reading 7.2 volts . I measured 121.3 volts coming in on the primary side of the PT . I am letting it sit as I write this . I'll go and check voltages again and report if there is any change .

[martin manning]

Is the center tap connected between the two 6L6 filaments?

[Zephedone]

No it is not . I grounded it to the chassis and forgot that I removed the K resister when stripping the chassis . Good call . I'm out to do that right now .

[Zephedone]

That fixed it . I am now measuring 6.6 vac on both of the 6l6 filaments and still 13.4 on the 12au7 filaments . Nice ! Thanks Mr . Manning . It's still running and the tubes seem fine . Of course this is nothing but the heater circuit hooked up . What are the chances of that voltage coming down ? And I am still within the +/- 10% . Is this much of an issue ? Should I do some more troubleshooting or testing ?

[martin manning]

I don't think it will come down from there, but it'll be fine where it is.

[Structo]

Strange since 12AU7's draws about 10x the current a 12AX7 does.