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.

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.

Quote from: tonyharker on January 21, 2021, 02:21:35 PM
12a?7 valves have a 12v6 heater between pins 4 & 5 and a centre tap on pin 9. for 6.3 v common pins 5 & 6 and feed supply between these and pin 9.  AC or DC  doesn't matter.

Thank you very much for answering that question.

I'm curious about a design like this and I have possibly a very stupid question.  If you're running at 12 volts and this is your schematic, what are you doing to attenuate the filament voltage?  It seems like you should have a resistor in that path.  Also, the filament is happy to run on AC and does so in traditional Tube Amp designs.  Is this a normal setup for a "starved plate" design?

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. 

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.

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.

We agree that is drawn incorrectly, and I've already stated it's not an isolation transformer, but it does work.  The primary windings and secondary windings are wound around the same toroid core so they are definitely "coupled" inductively.   In most applications, this transformer would have both primary and secondary winding sets paired in parallel.  That said all the winding sets are independent.  I've tested. 

In this design execution, the secondary windings are paralleled since the filament of the valve has the highest current draw.  I estimate that the TOTAL current on the 2nd primary (used as a secondary) winding is about 20 mA.

Here is a picture of one of this guy's builds, kind of sloppy point-to-point wiring, but you can clearly see that one black and red set of primaries are connected to power and the other set are connected to a diode bridge in the upper left-hand part of the photo.  The blue and green 7 volt AC secondaries are twisted together and are wired to the tube socket and dropping resistors. 

I appreciate the other design, but I've already purchased the parts for this amp and started, so I will definitely attempt to complete it.  I will be done with this soon and I will post photos and sound files if it works.

Quote from: phatt on December 28, 2020, 08:14:57 PM

Also the way in which the power Tx is drawn up in that schematic seems strange.

I agree, I would not have drawn it this way.  I would have placed the 117 VAC label under the AC source symbol and I would have put the 7 VAC symbol on the there side of the bottom transformer symbol.  Essentially those two series windings on the left side represent one pair of the 120-volt primary leads.

Here is the web page for the transformer.   Antek makes some inexpensive and interesting toroidal transformers.  I will probably use one of their tub amp models for a Rob Robinette "Deluxe Micro" project.

https://www.antekinc.com/an-0107-10va-7v-transformer/

https://robrobinette.com/Deluxe_Micro.htm

This is my first amp build, but I have about 40 years of experience working with electricity all the way up to medium voltage circuits like 4160-volt motors.   I know how to work safely.

You're right, it's not an isolation transformer, but it's being used as one.   This design has a very low current draw because it is a sub-1-watt amp.  The guy who builds these has used this exact transformer in this way on several small amp builds. One primary set of leads on the transformer gets driven by the mains inducing about 120 volts on the other set.  The 7-volt secondary is dropped to 6.3 volts to run the filament on the tube.  The 6CL6 power amp doesn't require a lot of DC voltage.  I think this supply will put out about 135 volts DC.  The OT is wired using the 1/2 watt tap.

Rather than screw with this design, since it's my first project, I've decided to build this as drawn as a small combo amp using a switched output jack that will allow me to use another cabinet if I want to experiment.  I will include some build photos as I go.   Wish my luck.

That's interesting.  I really didn't want to build a pedal.   I'm wondering if there will be enough power in the 7 volt AC side to make a 9 volt supply.  I imagine the load is very small for this.

Phil, I'd like to see it, if you don't mind posting it.

As it happens this seller has an amp up for sale made into a computer power supply.   Interesting build and he included a schematic for this one.  I wonder if he's on this forum.  And no, he's not me.


https://www.ebay.com/itm/Custom-Low-Watt-Tube-Push-Pull-Guitar-Amplifier-with-Tone-and-Headphone-Output/133599983433?hash=item1f1b2df749:g:hbkAAOSwsbFfzwds

Hey guys, new here.  I've been lurking for a few weeks and enjoying looking back through old posts hoping to gather some knowledge.   I started a project a couple of months ago and I have a couple of questions.

There is a seller on eBay that occasionally sells small tube guitar amplifiers housed in small, unique enclosures.   I have been outbid on the couple that I've tried to buy.  I look on eBay infrequently and he seems to offer amps on a rare basis.  I contacted him directly and asked him if he would sell me a kit and he was kind enough to reply with a schematic, but he wasn't interested in selling me a box of parts or supporting his design.  I get the feeling that he makes each amp a bit differently.

The power supply for the amp I'm trying to build features an inexpensive 7-volt Antec Toroid transformer with the 120-volt coils split, making it a small isolation transformer.  The 7-volt side is used for the tube filament.  The output transformer is a cheap 70-volt distribution transformer. The output is supposed to be in a 1/2 watt range.  I have sourced all the parts and have started working.  I think I have about 30 dollars in the parts.

If I was going to insert a tone stack in this circuit in the solid-state pre-amp, where would it go?  I'm thinking it would insert prior to the gate of the third LND150.  Also, the quality of the schematic jpeg is pretty bad.  I've assumed the gaps in some values are missing decimal points.  For example, this would make the cap in series with the volume pot 4.7N.  Does that seem right?