Peavey VK100 6v heaters

[lowell]

Hey my helpful friends. This amp has no 6v heater supply on pin 9 of the preamps... I've removed C119 thinking it was the culprit w/ no change. I cannot for the life of me figure out where the 6v supply is...!!?? This schem is frustrating. Might someone show me the way?

[tedmich]

no heater to just pre tubes? its almost always the heater fuse in PVs...hmm is ground good with the tubes removed? Best to trace the Voltages, ideally with HV (F201) fuse removed

[lowell]

sorry i should've mentioned that the heaters are intermittent, not just dead all around. i think there's a bad cap or resistor getting hot but I cannot see this on the schem anywhere.

[Enzo]

Do the tubes light up? Then they are getting power.

Look at the power supply on page 9, bottom strip. See "HTR" between R212 and R213? That is the DC supply for the tube heaters.

Now go to page 11 with the output stage. Upper left. Note the four 6L6s and the phase splitter wired for 6v are all in series across HTR1. That tells me HTR must be about 30VDC. HTR became HTR1 when it crossed between boards through ribbon P311.

Note the "6V1" there in parallel with V3. That is a tap in the string.
When it passes through P311 it becomes "6V"

Now lower right on the same page. There are V1, V2, the preamp tubes. Wired in parallel themselves, they are also in parallel with V3, since they are3 connectd to 6V.

So the net result is the same .9A of DC heater current passes through the 6L6s and the paralleled small tubes. There is a filter cap between each tube, most shown upper left, but the last one lower right.

if you have intermittent tube heaters, I'd check that ribbon first.

[lowell]

ok I'll check the ribbon and those caps. The last power tube V7? furthest from PI is not lighting up and I swapped tubes w/ no difference. Also the preamp tubes are not lit and that was the original problem.

[Enzo]

Do you follow how the circuit works? They ( cleverly, I thought) reailized that the three small tubes used the same current as one 6L6, so they wired the whole string in series across 30v. SO you should have 30v at the HTR end and each tube drops 6v.

[lowell]

hey no I didn't catch that. that IS clever! In regards to filament hum there are caps b/t each tube so that is also still good design as far as noise goes right?

[Enzo]

I am not sure why they distributed the filter caps like that. Perhaps they found that cathode currents in the 6L6s could modulate the heater supply somehow. So they wouold be a sort of decoupling cap. That is total speculation on my part however.

[lowell]

Should've taken my own advise on this one. EG try stupid things first, new tubes. must've been a bad power tube or PI tube... anywho w/ ALL new tubes the heaters are back. This is a lesson in series wired heaters... try tubes first. thanks enzo.

[Alf]

With this sort of lineup , what happens if one tube has a heater failure ?

The whole amp falls silent , I suppose ?


Alf

[lowell]

yep you got it.. actually only if a power tube heater fails since the pre's are in parallel.