A few 2 Stroke Questions

[Chester]

First of all, I’d like to say that this was the first amp I’ve ever built. More than that, it’s the first time I ever soldered anything; it was a bit of an adventure for me. After finishing the build, I took the amp to a tech. He immediately found a bunch of problems. I made a mistake with the primary of my OT which was causing oscillation and he found a bunch of problems with the layout in the book. In any case, the amp works and sounds great! The main change that I made was that my Power and Output Transformers are both made by Mercury Magnetics. It’s one of the most versatile amps I’ve ever used. Now onto what might be stupid questions; I’ll never learn if I don’t ask.

I know that it is possible to use multiple tube combinations. If I am going to, let’s say, use one power tube (e.g. one 6L6), which octal socket would I put it in? Will this be next to the rectifiers socket or the socket farthest to the right? Does it matter? Also, am I unplugging one of the speakers, depending on the change that I am making?

All of the parts that seem like they should be extremely basic that are really throwing me. In the past, all I’ve done is plugged in and played.

[Robin]

Congratulations on the amp. You built the original, 2 power tube version so I’m wondering if you got the updated construction info that DH put out about the Two Stroke design.

Does the MM OT have the same specs as the Hammond 125ESE transformer that is spec-ed in the book? If so, you can use the table on page 188 to help you select the correct secondary winding to use with each power tube / speaker load combination. If not, you can use a turns radio formula to calculate the correct load for each secondary lead depending on the primary input impedance. The OT can stand some degree of impedance mismatch so don’t get too worried if the secondary impedance does not exactly match the load. Fender gets away with 100% mismatch without issue. You can put the output tube in either socket, your amp is a duel-single-ended design which means that the tubes are on and working all the time (unlike a push-pull design) so it does not make a difference which socket you use. Don’t use two different tubes at the same time though. In fact, the only 2 tube configuration you should use is 2-6V6s. My favorite is one 6L6.

What “problems” did your tech find with the layout? Which MM transformers did you use?

[Chester]

Thanks for the helpful answers. As far as the MM PT and OT here’s what I used:
PT – Tone Clone FTPP-5
OT – Axiom SE-9K-OS
Both of these were correct for this amp. I called up MM and talked to them about the project. They were able to lead me in the right direction. I accidentally purchased the SE-9K and did not realized at the time that it was wrong. I only had two wires coming from the secondary; one common and one 4 ohm. Due to the fact that I’ve spent about two years saving up and collecting the parts, I did not realize this problem for, well, two years. MM were really cool. They sent me the upgrade; all I had to do was pay shipping both ways.

As far as the issues that the amp tech encountered, a lot of it has to do with grounding. While I’ve emailed Dave Hunter and seen some updates, most of them seem to apply to the newer version of the Two-Stroke. This man’s been designing and building tube amps for 20 years and made some changes.

One thing he really did not understand was the shielded wire; he really does not believe that there is a need for it. I did not realize that this has to be grounded, so that was one thing he took care of. If you look at the layout, there’s a wire going from the 16uF/500V capacitor going to ground. He completely removed that and attached a wire to the negative lead of the 40uF/500V cap and took that to ground. Also, he thought that it wasn’t a good idea to have the terminal strip attached to one of the screws on the PT. He drilled a hole in the chassis and stuck a star washer beneath that. The ring terminal is now on V3 instead of V2 and pin 1 is grounded there; pin 1 was grounded to the terminal strip, in the first place.

Don’t get me wrong, I made plenty of mistakes; I’m not trying to knock the book at all. For all I know, these changes have already been made and I’m just getting to the show kind of late. These just happen to be the changes that I noticed that he made that I can remember. He spoke quickly and I got a little lost. What I do know is that the amp does not hum to the degree I’ve heard on most Youtube videos from others who have made this. The tech told me that there is still a hum; it’s so minor that I’m not really worried. In fact, I expect a tube amp to make some noise.

I did say that it sounds great and it does. The only place that I’ve heard this was when the tech plugged it into a speaker that he had and used some dumpy tubes for testing the thing. Even with that, it sounded better than most amps that are available. I’m going to give Tone Tubbys a try for the speakers. So far, the one I’ve purchased is the San Rafael Alnico; it’s supposed to be a good replacement for Champ and Princeton speakers. Thanks again!

[Robin]

Thanks for the details. The revised chassis layout for the 2 Stroke that is floating around the internet shows the circuit with one power tube and clears up the vagueness of the shielded cable connection at pin 2 of V1.
The Weber Maggie kit is the same amp, you can see it at the Weber site,
it addresses the “boost” switch pop, if your tech did not already fix it.

Regarding the grounding issues, it’s true that single ended designs can be noisy but with proper lead dress and grounding the 2 Stroke can be quiet (both of mine are wired pretty much like the original layout and are VERY quiet). Unless there were other issues not mentioned, it looks like your tech’s grounding changes were personal preference. The 8,16 and 40uF caps all tie to the same ground. Using one of the transformer mounting lugs as a ground point works too.

The shielded cable cable is there to reduce noise and interference, you are correct that it is possible to not included it (Fender never did) but it’s still a good idea. Using a shielded cable with one end (only) grounded, can really help quiet a circuit in some cases.

Good luck with your amp.

[Author]

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