Old Trannies

[guitardude57]

I have an old Pioneer SM 500 that is going to be used for a couple of guitar amp builds. The OT's are driven off of 7189's (EL84 heavy duty) nice and beefy. Reported output is 27 WPC.

The PT is a laydown with 1.5A and 6A 6.3V taps, 165V 200mA and 175V 600mA HT taps.....and a few others. So one OT tranny and PT for new build. Have plans for the other OT.

The original design used a 3K5 - 50 watt wire wound resistor behind the strangest diodes I've ever seen for a voltage doubler. A couple of 50uF can caps, a few axial jobbies and a 40/20/20/20 can cap. The reported voltage if I remember is 460V HT.

The tube complement is 3 12AX7's 2 6AN8's and the 4 EL84/7189's

Now what to build? Already have a nice selection of amps.....but nothing in a 6V6. A 6V6 plexi would be cool. I really like nice tone, and some good crunch with the guitar knob. I would really like to build a 2XEL34 amp, but the OT won't work for EL34's as it has an 8K impedance.

Any ideas?

A better voltage doubler to get the volts up would be good. I have several schemos for a Diode bridge and Cap Voltage Doubler. Never built/or used one though..............
I am aware of losing current, raising the voltage............ but we have some to spare with 600mA.

Suggestions/comments welcome. Thanx.

[booj]

cathode biased with 6V6's

[Steve Conner]

I would really like to build a 2XEL34 amp, but the OT won't work for EL34's as it has an 8K impedance.

Just try it anyway. You can connect the speaker to the "wrong" tap if necessary, for instance a 4 ohm speaker on the 8 ohm tap will reflect 4k to the primary.

"Strangest diodes you've ever seen", it wouldn't be those disc-shaped black doofers with the red spot?

[guitardude57]

Thanx for some replies guys,

Yes, the doubler diodes are are large grey discs.........about the size of a pool cue tip. Real thin leads too.

Upon some reading on voltage doubler circuits, I find that a fullwave bridge doubler is pretty reliable. 6 diodes and 3 caps used ala George Hrischenko circuit. Twice the voltage and half the current. With 600mA should be fine. The resultant 120hz ripple is easier to deal with than the 60hz on half-wave that was originally in circuit.

The original had 70-105V on the preamp tubes............so was quiet.
If you approach 150-200V on the preamp tubes, noise becomes an issue pretty easy.

I'm familiar with the 4R into the 8R deal to trick the impedance of the OT.

The several speaker cabs I have are all 16R to use with whatever amp rig that I currently use.
Got some extra EV12L's that need recone jobs, cash flow plain sucks these days..........that will be awhile.
These will go into a H**dTr*ckers style box after being rebuilt. Come to think of it, I do have some single 12 boxes..........but don't remember what the hell is in them.....Hmmmmm

The chassis is large (and steel) so will be fine for experiment purposes.
I just can't see paying the kind of cash for an aluminum pre-punched (or not) chassis. So........keeping my eyes peeled for dead amps for a donor for a more traditional amp/box. The PT and an OT would be too heavy for the cake pan trick.

I'm thinking of a Plexi 1986 (6V6 or EL 34 ?) with the addition of a TMB Fender clean circuit.............sort of 2 amps in one. Allman tone in one, Garcia in the other! Just pick the input jack. Non switchable.
I built a 1986 into my Blues Jr with EL84's and it sounds awesome. Love that circuit.


Questions/comments welcome