peavey conversion

[Andy Le Blanc]

Interesting build, may be, I have and old peavey heritage in the the shop.
The owner killed it, loose speaker clips caused a catastrophic failure.
Tubes gone, opt gone, its a real mess... even into tube driving power transistors.
But... the power supply is still good.

So the discussion came up to covert the rig into a simple all tube rig.

It looks do-able... maybe an 80w twin opt... the only issue seems to be getting heater voltage for a couple 12 ax7 in a basic tube pre.

There is the +15/-15 dc supply for the existing pre, the question is whether there is enough ma. to utilize it for a couple tubes?

[ampdoc1]

If it's the Heritage VTX, I see a 6V AC power supply protected with an 8A fuse. That should be enough power for most any combination of tubes.

a'doc1

[Andy Le Blanc]

Oh ya... I was bit hesitant not having the specs for the PT, using the DC source on the existing board is attractive.

The rig is built for 4 EL34... 6 A .... going with 6l6 .... 3.6 A a bit lower
Most other popular power pentodes will work too, I know I can..
But I'm eyeing that DC supply for the heaters, it actually simplifies the build.

[JazzGuitarGimp]

Most 15V bipolar audio circuits, whether disctrete transistors or opamps, draw very little current (unless the power amp is also powered by the same rails). Are the 15V rails fused? If so, the fuse rating might give you at least an approximate idea of what to expect from the supply.

[Andy Le Blanc]

More noodling,... the supply with the existing load puts out just under 31 v.
I can creatively wire the heaters of 3 tubes for 31.5, and the current draw drops for series.
Probably get down to less than 75ma draw.
The fuses are 1A, beginning to think I might be in the clear.
But its that turn on draw just as the filaments heat that worries me

Backing up thru the supply, its 55v, I can series 4 tubes and only draw
37ma, with the extra v to light the string.

[beasleybodyshop]

I have just recently finished converting over my old Peavey Deuce II to a Marshall JMP 2X12 Combo. Granted I pretty much stripped everything out of the old chassis and started from scratch, but they seem to be similar ideas - hybrid pre/tube power section. Plenty of space to build pretty much anything you want in there - the sky is the limit!

[Andy Le Blanc]

Alright... I have 3 ways to heat pre-amp tubes for this project.

1)... The 6.3 V heater string that's already in whats left of the rig.

I'm leery to do this, the string is grounded to the chassis on one leg and
I'd have to frig with it to make her go, parts, parts board, standoff's, time to
re-dress the circuit etc.

2)... The existing DC supply that's already in whats left of the rig.

This is more do-able and would give me DC for the pre as I was thinking originally,
but it suffers all the foibles of #1, and it occurred to me that
I still have to bias the P-O-S peavey carcass before I'm done with it and it is a 55v winding off the PT....
jeez louise wtf.

3)... CHEAP, SIMPLE & EASY

I can run the heaters series off the cathodes of the power tubes.

This is the easiest solution to all the issues

a... no wasted time or parts, one resistor and half a dozen solder joins, that's it.
And, no time spent farting around with the remaining circuit.

b... it meets all the needs of project, it gives me dc for the pre amp and bias for the power side

c... safe simple reliable... and it sounds good. The client just needs a big, clean, simple old fashioned rig.

[DonMoose]

The heater current for a 12A*7 wired for "12.6V" heaters is gonna be 150mA. Any number of them in series is still gonna be 150mA.

The 31V source for 3*12.6 is 82%, which is juuust within spec for the heater in that mode.

Not at all sure how running heaters from the output tubes' cathodes would work - is there an example you can point to?

Hope this helps!

[Andy Le Blanc]

notice what happens to the current when you go from 6.3 v to 12.6 V

It goes from .3A to .15A... you run a series string, it increases the voltage
and decreases the current requirement, you use it as a big resistor.

this is very old news, theres some wards valco supro gretch things that utilized it.
not to mention television... before transistors... old school consumer electronics.
if you heater string in volts adds up to 110 your in business.

This case I need to bias 4 6l6 ... conservatively 85 w plate dissipation

170ma @500v ... 75ma going to keeping the heaters lit, I'm gonna have to put a resistor in the string
to give some control over the bias point, and raise the bias V over the 25.2 v of the heaters.

[Andy Le Blanc]

guts... not much left but plenty O-room

The dash boards front and back are big stickers, the only real difficulty so far is getting the old stickum off the metal
so I can re-stickum for a new sticker of my own