Help with Hybrid Design

[SerpentRuss]

Here is my start on the pre-amp of this build.  I decided to squeeze it onto a little project board to try and keep things small and neat.

[phatt]

OK thanks for the pics and explanation I now see how it has been done, i.e, the second primary is used inductively. (I kept thinking of Centre tapped primary )
But still it's my understanding that most modern Mains Tx's are wound with extra isolation between Pri/Sec.
So are you aware there is normally extra insulation between Pri/Sec windings?

I do know that most IE type Tx's now use split bobbins so there is even better isolation than the old overlay winding system.

I'm not up to speed on the physical structure of Torids but I'm assuming something similar would be in place. If so that would put both primaries on the same stack with far less insulation which makes it a potential hazard if something ever shorts in the primary stack.
Tiss a fair bet that design concept would never pass most Electrical regulations that I'm aware of. 

OK it will work but I still see it as a potential hazard and I would never build such a circuit based on that Tx trick.
When I build stuff I always think of the wotiffs?
What if your best mate uses this and he dies,,, how would you feel?

I have several muso mates who often use gear that I have built and I feel much better knowing that I have used as much safety margins as possible to ensure no one gets zapped.
Layout and wiring of Mains is often overlooked even in big name gear, often I find mains wires zip tied to secondary wires. OK it may never fail but if those wires did melt you have mains on the circuit,, of which you are directly connected to when holding an electric guitar or microphone. I choose to remove that possibility by keeping those mains wires well away from the working circuit wires. One less gotcha in the equation and it only takes a minute to layout a couple of wires to remove that danger.
In this case I can only assume the idea has inadvertently put the mains/secondaries dangerously close together.
Likely the person who designed it never realized the danger he created.
Phil.

[SerpentRuss]

I understand your concerns.  They are valid.  One thing to consider here is that shocks from 120-volt mains are very rarely life-threatening.  In a perfectly coupled system, there would be a maximum of 120-volts AC (RMS) induced on the secondary.  A shorted system could never produce more than 120 volts and in many cases would cause the 1 amp fuse to blow since the AC supply ground and the amplifier's ground will be tied together.  This is not the case on many tube amplifier designs.  I'm interested to see if it will be noisy.

[Loudthud]

It should be noted that when you hijack one of the primary windings to make high Voltage, you should de-rate the VA of the transformer by half.

[SerpentRuss]

Well, my project is currently a bust.   I finished soldering everything together (without breadboarding, LOL)  and it didn't work.  When I was trying to diagnose the issue, I shorted out one of the LND150s.  I have spares, but I'm going to start over and build the circuit between terminals so I can follow the signal path

I tested by using my phone with a guitar tuner app.  It was supplying 196 hz at 45 mV which I scoped before I connected it.  When the amp was turned on, it produced a 6k tone whether the input was connected or not.  The volume knob seemed to work making the squeal louder and softer.

The heater Voltage was dead on 6.3 volts AC with the tube up to temp.  B+ was a scant 123 Volts DC.  I could not find my test signal anywhere in the circuit, starting at the input jack.  It was being sucked away by some soldering or component mistake.   DC power was clean and steady, but as stated, lower than I expected.  I didn't troubleshoot long before I let the smoke out so I didn't learn much other than the output was quiet as a tomb after I killed the MOSFET. 

[SerpentRuss]

I got back to this project and got it working on a breadboard.  I'm transferring that design to terminal strips and when I get that working I'll post some audio samples.  I'm not much of a guitarist, but I should be able to give some examples of the tone.   Initially, it was pretty noisy, but I'm currently rebuilding it using a shielded heater run and shielded input wiring.  The tone is never quite clean, and the amp has a very interesting distortion at full bore.  It is never really loud, so it would only work as a practice amp.

Here is a photo of the amp near its final configuration, re-mounted in the phenolic board "chassis"  I re-worked the tube mount using a piece of aluminum to remove flammable material from the build.

[SerpentRuss]

Well, I "finished" my first junkyard amp project.  I decided to make this a small combo with an external speaker jack that will bypass the internal speaker.  I goofed on the dimensions of the cabinet cross pieces.  Where there is a triple thickness of wood on the sides, I only planned for a double thickness.  My idea was that a three-inch tall strip attached to each side would support the top board of the cabinet and the middle board of the cabinet which served as both the shelf for the amp innards and the top of the sealed speaker enclosure.  The bottom, front, and back of the "chassis" are pieces of phenolic board salvaged from computer floor tiles.  This was salvaged from floor purchased new around 2006, so it probably doesn't contain asbestos but I wore a respirator when I cut it, just in case.

The speaker is an Oxford 6x9 speaker from an old Chrysler complete with a high-tech whizzer cone.   I know these trunk speakers pretty much operated in an infinite baffle environment, but I tried the amp both ways and I liked the sound better with the back sealed.

All in all, I have about 45 dollars in parts.  The PT was $10, the OT was $3 and the tube was less than $2.  Speaker was junk, the floor tiles were junk, wood was lying around the shop.  The biggest expenses were the filter caps and the grill cloth.

Modifications to the original circuit included a couple of high-frequency shunt capacitors to prevent oscillation in the SS preamp, two cathode bypass capacitors (one in all the time, the second put in circuit by the right-hand switch),  a bright capacitor around the volume control, and a tone control pot between the 1st and 2nd stages.  I played around with values for all of these but ultimately the differences in tone are all very subtle, the volume pot has more impact on the "tone" of the amp.  Turning down the amp to where it plays cleanly makes it really quiet but the tone is warm and round.  About half-way up, things get interesting, and running full-tilt there are bizarre things happening to the low frequencies that may or may not be musical, depending on your personal taste.

I truly hope to have sound samples soon.