Best way to add heater current?

[pullshocks]

Years ago I bought a nice Canadian PT from Angela.  Supposedly it was made for a SE guitar amp with one 6V6 and one 12AX7 but there was no actual heater current spec given.  The company, Audio Transformer AFAIK is long gone.

I used it for Champ and AX84 P1X circuits for a number of years but too many interesting preamp circuits have been coming up that I want to try.  So now I want to add a 2nd preamp stage and tube effects loop. 

I am assuming the additional .6 amp heater current draw might be questionable for the PT.  I found a cute little 6.3v .6amp filament transformer at Seattle's last remaining electronics shop.

Question 1:  Do you think I can get away with mounting it inside the chassis?  Would it be better to relocate the choke inside the chassis and mount the filament transformer externally?

Question 2:  Should I wire the 2 transformers up in parallel, or run separate heater circuits for the 2 transformers, e.g. power amp and effects loop on one, and 2 preamp stages on the other?  Neither one has a center tap

Question 3:  Currently, the heaters are referenced to the cathode of the 6V6. If separate heater circuits are the way to go,  should both heater circuits be referenced there?

[quayhog]

Have you tried hooking up another 12AX7 heater on the existing transformer?  My hunch is there is enough reserve heater current for one more tube.  Hook one up using clip leads and measure the voltages and compare the voltage drop with it in circuit and out of circuit.  If there is no significant change you'll be alright without hooking up an additional transformer.

I'd do this before I'd hassle with hooking up an additional transformer.

[Geezer]

Have you tried hooking up another 12AX7 heater on the existing transformer?  My hunch is there is enough reserve heater current for one more tube.  Hook one up using clip leads and measure the voltages and compare the voltage drop with it in circuit and out of circuit.  If there is no significant change you'll be alright without hooking up an additional transformer.

I'd do this before I'd hassle with hooking up an additional transformer.

What he said........

[pullshocks]

Thanks guys.  I do want the ability to add *2* preamp tubes, one for the overdrive stage and one for the tube driven effects loop.  It would be great if I didn't have to add the filament transformer, but I don't want to toast the PT, either.

So, can I do the voltage drop test with just the heaters connected, or do I need to have the amp all the way wired up?  Currently,the chassis is completely stripped out but I could temporarily wire up the heaters.

I would like to resolve the question before I rewire the amp.  Or I suppose I could ditch the choke if it turns out I do need the extra transformer

[PRR]

See if Radio Shack still stocks a 10 ohm 10 Watt resistor (or get it HERE). Get two. Put in parallel for 5 ohms. Hang on your 6.3V heater winding. 6.3V/5= 1.26A, essentially your 0.3A+0.6A+0.45A demand. Measure voltage with and without the 5 ohm resistor.

If the loaded voltage is over 6.3V, the unloaded voltage no more than 7V, it will probably be fine even when you add the plate loads.

Even if the fully-loaded voltage drops to 6.0V, the tubes will work fine. The total added heat from 0.6A added load and 0.3V drop is 0.2 Watts, a totally trivial amount on a 40 Watt PT. Maybe on paper you reduce PT statistical life from 10,000 hours to 9,500 hours.

[pullshocks]

Thanks PRR.  It read 6.95 with no load, 6.65 with the 5 ohm resistance in there. 

So if I understand you correctly,  I can probably add the tubes w/out frying the PT?