New build: 5f2-a "Two Stroke" with 6L6

[theprofessor]

I've become interested in a single ended "big bottle" amp and have decided to build the revision of the Dave Hunter "Two Stroke" (in his great Guitar Amp Handbook) that has one power tube. I think I'll call it the "Single Engine." I have been helped so far by looking back at some of @jsnwhite619 's threads and posts on his "big bottle" 5f2-a, in which he used a 5881 (e.g., here and here). I may also do that, but I think I'll try a 6L6GC first, as I've got a couple of vintage singles of those already.

Parts I'm using/ things I'm doing:

- Here's the layout I'm using. I won't have multiple speaker jacks or switchable NFB, but I am going to include the boost and cut switch on one of the inputs.

- Turret board: I've started building my own turret boards out of glass epoxy board and Doug Hoffman's nice turret lugs. This is my first one. It allowed me to design the layout I wanted and then drill the holes and install the lugs. My layout is very similar to the one in the layout I'll post here, with a few changes to allow for keeping two many leads out of one turret and also to fit the size of the parts I am actually using. I've decided to copy Hunter and use a single bus this time. We'll see how that works. I'll probably copy @jsnwhite619 's filtering scheme, though, with a 30μF-22μF-22μF string. My board is 8-1/2" x 3".

- Mojotone 5f2-a chassis.

- Hammond 290AX power transformer. I've used this PT a lot, and I like it. It has a 6.3V heater center tap, which I'll terminate at pin 8 of the 6L6. It also gives me 3A current on the 5V heater winding, which I like, because I like using 5U4 rectifiers. I'm going to do that on this build, and I already have a couple of vintage ones on hand. It makes no sense to me to use a 5Y3 with that much power stage filtering and a single 6L6 tube. In general, I do not like the sound of amps with a 5Y3 -- or at least I prefer a 5U4 (conversely, I also often find a GZ34 to be too "stiff"). I'll use the 325-0-325 high voltage winding. This PT can produce up to 100 mA current.

- Hammond 125ESE output transformer: This thing is a beast! It is heavy, and because it is a universal single-ended OT, it has four secondary windings. That means a lot of wires to go through the chassis punch-out. I had to enlarge that hole with a step bit to fit all of them through. I'm planning now to use an 8 ohm speaker. That will be one of the puzzles I've got to work through: the best impedance ratios for the tube and the speaker. I'll need to review how all that works again.

- cabinet: I'm building my own cabinet out of 3/4" void-free baltic birch ply. Roughly 20-1/2"w x 18-3/4" h x 9" d. Planning on a 1/2" baffle made of the same baltic birch and hoping to get a 12" speaker cutout to fit in there.

- speaker: I'm planning on a vintage Oxford 12T6. I really like this speaker. I've had one in my vintage Deluxe Reverb, and I'm looking forward to hearing how it sounds in this very different amp.

I welcome all feedback! I have much to learn!

 

[theprofessor]

After drawing a layout on graph paper and marking where the turrets would go, I checked the fit of the actual parts I am using and made a few changes. Then I taped it onto the Garolite and started drilling. My board is 8-1/2" x 3".

 

[Peegoo]

@theprofessor You are off to a really good start (NICE work!), and this circuit is a really good one. I recently completed a build using that exact layout, and it is a beast of an amp. Plenty of clean headroom until near the top of the dial and then it really starts to sing. It will rattle the windows through a 2x12. You're going to love it.

A couple of points...

I found that it's substantially quieter by running the heater circuit first, twisting the wires tightly and tucking then into the corner of the chassis. Keep them flat against the metal. Many builders lay all the other conductors on the sockets flat and then "fly" the heater wires in from above, but in the builds I've tried both ways, running the heaters as low as possible makes for quieter operation.

Also important for quietest operation is to index all your non-polarized caps (coupling caps, etc.) with a scope to determine which conductor is the 'outside foil' end. Most capacitors are physically constructed like a roll of toilet paper and the outer layer serves as a shield to help reject interference from other components in the circuit.

I do this by testing with a scope and marking them with a Sharpie. Years ago, cap makers stamped Outside Foil on their products; some makers still stamp a bar or line on one end, but do not trust it. Proof of that is in the pic below; the foil side is different on the two large orange polys.

 

[2L man]

When you have used that hammond PT perhaps you can estimate what is its output? When B+1 estimate is available drawind the loadline is easy.

If B+1 come close to 400VDC I would use 5k OT impedance.

2,5k use its 70mA max bias limit the output power. Edit H125ESE max bias is 80mA

 

[theprofessor]

When you have used that hammond PT perhaps you can estimate what is its output? When B+1 estimate is available drawind the loadline is easy.

If B+1 come close to 400VDC I would use 5k OT impedance.

2,5k use its 70mA max bias limit the output power.

Thank you very much, 2L man! Yes, I believe the 325-0-325 high voltage connection will put ~400VDC on the plates of the power tube. The primary impedance for this OT is listed as a range (2,500 - 10,000R) and only has the brown and blue primary wires, without a center tap with a specified primary impedance. And this is where I need to go back and review output impedances and reflected ratios and so forth. It sounds to me so far like the primary impedance will be determined by the typical impedance load of the tube selected (in this case, 6L6GC; thus: 4200R at ~350 volts) AND the secondary selected (I've posted that chart above). Since I'm using an 8R speaker, it looks as if I should use the yellow secondary lead for a 5,000R primary output impedance.

EDIT: It looks like that same yellow secondary connection would work well for substituting a 5881 into the circuit, since that load should be ca. 4200R at ~350 volts as well. Of course there are other considerations with a 5881 -- the most important probably being a different bias resistor. It seems to me that a 6V6, though, would require another jack at the least: probably the green secondary connection into an 8R speaker = ~10,000R. Plus bias, etc., etc. I don't think I'm going to do that, though. I know what a single-ended Fender-style 6V6 amp sounds like (5f1, 5f2-a), and I'm going for something different here.

 

[2L man]

Loadline might look about this attachment. Here B+1 is 405VDC when over the cathode resistor come 25V bias. I did use 350V screen voltage and making it lower make bias voltage lower as well.

High OT impedance cause 2nd harmonics when operation is not linear which you can see looking green anode sweep both side of the bias point are different lengths. 2nd harmonics sound sweet

 

[theprofessor]

@theprofessor You are off to a really good start (NICE work!), and this circuit is a really good one. I recently completed a build using that exact layout, and it is a beast of an amp. Plenty of clean headroom until near the top of the dial and then it really starts to sing. It will rattle the windows through a 2x12. You're going to love it.

A couple of points...

I found that it's substantially quieter by running the heater circuit first, twisting the wires tightly and tucking then into the corner of the chassis. Keep them flat against the metal. Many builders lay all the other conductors on the sockets flat and then "fly" the heater wires in from above, but in the builds I've tried both ways, running the heaters as low as possible makes for quieter operation.

Also important for quietest operation is to index all your non-polarized caps (coupling caps, etc.) with a scope to determine which conductor is the 'outside foil' end. Most capacitors are physically constructed like a roll of toilet paper and the outer layer serves as a shield to help reject interference from other components in the circuit.

I do this by testing with a scope and marking them with a Sharpie. Years ago, cap makers stamped Outside Foil on their products; some makers still stamp a bar or line on one end, but do not trust it. Proof of that is in the pic below; the foil side is different on the two large orange polys.

Thank you, Peego! I am planning to connect the power transformer heaters and the heater wires for the octal socket at a 2-terminal strip near the pilot light. Then I'll connect one wire from each of those terminals to the terminals of the lamp itself. That will (1) keep all the connections away from the end of the pilot lamp (2) allow me to run the heaters close to the back, and then the floor, of the chassis. I have used the "under the lip" wiring for the heaters in couple of builds, and that has worked well for me. In addition to noise reduction, it allows for much easier access to components than flying the wires over the board.

As for the outer foil: I don't have an oscilloscope, but I do have this little contraption for auditioning for the foil end. I don't like doing it too much, partly because I always have to remind myself which end is which when producing the lowest noise. But also because sometimes I can't hear much of a difference between the ends.

 

[Peegoo]

I don't have an oscilloscope, but I do have this little contraption for auditioning for the foil end. I don't like doing it too much, partly because I always have to remind myself which end is which when producing the lowest noise. But also because sometimes I can't hear much of a difference between the ends.

One thing you can do with that is to clip the cap in, wrap one hand around the AC cord running to the amp, and gently pinch the cap with thumb and finger of the other hand. You will hear a difference that way.

 

[BigDaddy23]

Nice! I built one of these a while back (sold it to a guy who offered me great $ for it after playing it!). Awesome little amp. I found EL34’s to be be best but the versatility to be able to run that, 6V6 or 6L6 whenever was fantastic.

I probably have a build thread on here somewhere but I reckon in the end, I finished with bigger screen and preamp filters from memory….?22uF. Perhaps keep some real estate in that section in case you need bigger components.

 

[theprofessor]

Since I'm on the topic, I went back to this helpful thread on testing for the outer foil end on Mallory 150's. @JD0x0 's post in that thread was what I was trying to find:

Sometimes the caps get flipped on the production line. Outer foil is not always inherently on one end relative to the printing on the cap.


The easiest way is to plug a 1/4'' cable into the amp, and touch one lead to the tip and one lead to the sleeve of the 1/4''
One direction will have notable hum, the other will be significantly quieter. The quieter direction will have outer foil to the 1/4'' cable sleeve. Connect outer foil of cap to the side of the circuit that has less impedance to ground.

 

[jsnwhite619]

Good luck with the build! Looking good so far. I see you changed the filter cap value - 40uf is pretty much set for GZ34 or SS I think, so dropping that is good if you want variety. This last high power 5f2a I built changed my mind about rectifiers on these amps. This was before I got my loop pedal, so the differences could have been due to playing/attack, but at live 110+dB volume, it's splitting hairs on rectifier choice on tone with SE I have finally accepted. BUT, more voltage = more power. Being able to swap from 5y3 to SS and stay safe on your bias will make a difference on final output. 500R cathode will be very cold if you want big tubes only. 300R-350R will probably be closer for what you want. 470R on my last one kept the 5881 at 70-75% but kept 6v6 safe as an option. If big tubes only, the 300 range will be good.

Here is my clip comparing tubes and rectifiers between 6v6/5881 and 5y3/GZ34....not much difference in tone, but output was different. I made a dB chart comparing them on one of those threads. If you need to drop a watt or two in output, it's rectifier swap helps with that. But, overall tone is negligible IMO.

 

[theprofessor]

Good luck with the build! Looking good so far. I see you changed the filter cap value - 40uf is pretty much set for GZ34 or SS I think, so dropping that is good if you want variety. This last high power 5f2a I built changed my mind about rectifiers on these amps. This was before I got my loop pedal, so the differences could have been due to playing/attack, but at live 110+dB volume, it's splitting hairs on rectifier choice on tone with SE I have finally accepted. BUT, more voltage = more power. Being able to swap from 5y3 to SS and stay safe on your bias will make a difference on final output. 500R cathode will be very cold if you want big tubes only. 300R-350R will probably be closer for what you want. 470R on my last one kept the 5881 at 70-75% but kept 6v6 safe as an option. If big tubes only, the 300 range will be good.

Here is my clip comparing tubes and rectifiers between 6v6/5881 and 5y3/GZ34....not much difference in tone, but output was different. I made a dB chart comparing them on one of those threads. If you need to drop a watt or two in output, it's rectifier swap helps with that. But, overall tone is negligible IMO.

Thank you! I went ahead with a 360R bias resistor, given my plan to stay with 6L6 or 5881. I’ll see where I land with that and adjust, if necessary.

 

[theprofessor]

Here’s where I am at the moment. I thought I had a 22uF filter cap for the preamp, but I only had a 16uF and a 10uF. So I went with 16uF there. I am also awaiting another 68k resistor, since I accidentally ordered 68R (dang!). I’m going to make all my connections above board, so I’ll go back and run those tomorrow.

 

[chas.wahl]

Re heater wiring routing:

I found that it's substantially quieter by running the heater circuit first, twisting the wires tightly and tucking then into the corner of the chassis. Keep them flat against the metal. Many builders lay all the other conductors on the sockets flat and then "fly" the heater wires in from above, but in the builds I've tried both ways, running the heaters as low as possible makes for quieter operation.

I have used the "under the lip" wiring for the heaters in couple of builds, and that has worked well for me. In addition to noise reduction, it allows for much easier access to components than flying the wires over the board.

I'm a bit confused -- are both of you talking about running heater wiring down in the bottom long corner of the gopher coffin (down by the circuit board "as low as possible", or "under the lip" of the coffin opening, like in most tweed builds I seem to remember?

Or does it make a difference, so long as the wiring is in one long corner or another?

 

[Peegoo]

Re heater wiring routing:

I'm a bit confused -- are both of you talking about running heater wiring down in the bottom long corner of the gopher coffin (down by the circuit board "as low as possible", or "under the lip" of the coffin opening, like in most tweed builds I seem to remember?

Or does it make a difference, so long as the wiring is in one long corner or another?

Howdy Chas

I've run it down in the corner (the "bottom," looking into the chassis with the open side facing up) as well as under the narrow flange around the top. It may be only a coincidence, but the times I've run it in the bottom makes the amp quieter. It may be due to having more metal on both sides, rather than a narrow flange on one side.

Geno

 

[theprofessor]

Re heater wiring routing:



I'm a bit confused -- are both of you talking about running heater wiring down in the bottom long corner of the gopher coffin (down by the circuit board "as low as possible", or "under the lip" of the coffin opening, like in most tweed builds I seem to remember?

Or does it make a difference, so long as the wiring is in one long corner or another?

I don't know if I'm using the terminology correctly, but I'm planning to run it on the surface that the tube sockets are mounted on, near the back lip where the back panel covers the chassis.

 

[King Fan]

IME chassis style matters. I haven't seen 'down in the corner' much in tweed style (most folks seem to do 'under the lip'). OTOH down-in-the-corner is easy and classic in a tray chassis. And though flying is classic Fender in a tray chassis, in a tweed it has a practical downside exactly as you say, @theprofessor -- it blocks access to the leads and eyelets at the bottom of the board.

 

[theprofessor]

I didn't go back and run those jumper wires on the circuit board yet; instead, I've been prepping the chassis. Here's final fitment. Now to run the wires on the board and start putting things together. And I'm still awaiting on a 68k resistor for the board.

I decided to add another lug on the bottom left-hand corner of the board and to lace it to B+1, since there is so much going on there. In my board design, I am trying to keep turrets from getting cluttered and hoping to keep a design that allows me to swap in other components to test values, if I want, without disturbing the whole nest.

As I mentioned before, that output transformer is quite the beast. When I planned for mounting holes for the circuit board, I forgot about the OT. So I had to go back and drill another mounting hole in another location.

Also: you can see the terminal strip for the 6.3V heater wires. I'll connect the PT heaters there and then also the heaters to the 6L6. I'll run them straight down from that terminal strip to the tube-side of the chassis and then "under the lip" to the tubes.

 

[screefer]

I like the terminal strip idea.
Good planning!

 

[theprofessor]

I like the terminal strip idea.
Good planning!

Thank you! Whatever its value, it was hard-won. I had a problem with a lamp assembly shorting out and taking out power transformers, as I detailed in this thread. That long and expensive saga has left me thinking more about heater wiring than I would otherwise. I decided to use this terminal strip to offset the connection of so many wires in a junction from the lamp themselves. This placement also allows me to get the heater wires to the floor of the chassis faster, rather than "flying" them over the board, which I've grown to dislike.