Yet Another Two Stroke build

[jer_vic]

Hi everyone. I'm starting this to (hopefully) document and (mostly) ask stupid questions about building my first amp.

Having bought Dave Hunters Tube Amp Handbook (2nd edition), I've decide on his updated Two Stroke design based on the 5F2A. It looks simple enough, has some interesting changes to the basic 5F2A design, and there are kits available that will get you most of the way there. To that end, I've ordered a 5F2A kit from Weber, sans resistors and jacks, which I'm sourcing, along with a couple of capacitors, from DigiKey.

I didn't think the output transformer that ships with that kit was appropriate, so I asked them to swap it for this model:

https://www.tedweber.com/w022905m/

It claims to be 6-15 watts, with a 7000 ohm primary and 2 secondaries - 4 and 8 ohm.

But now I'm second guessing myself. Dave states that the OT should have a primary between 4000 and 6000 ohms.

So:
1) Is that Weber really a single ended transformer?
2) Does it matter that the primary is out of the 4K-6K specified?

I looked at the tube charts here:
http://tdsl.duncanamps.com/tubesearch.php
and am even more confused.

Why? Because the 2 stroke is supposed to accept a number of different power tubes, and I'd like to roll a few different ones through to see what happens.

But according to the tube charts, the output impedance of the various tubes I'm interested in (6L6, 6V6, 6K6, EL34 and KT88) vary substantially. So I don't see
how one OT, whether it's primary is 5K or 7K ohms, can satisfy all of those different tubes.

Is it just the case that either primary value is ok, and it's just that some of those tubes will be "happier" than others? I realize this is something I will probably learn the answer to along my journey in building the amp, so I don't REALLY need this question answered right now. Mainly, I need to know if I have an appropriate OT or not....

My order with Weber is going to take a few weeks, because of the cabinet build, so I have time to swap OTs (again) if I need to.

Thanks in advance.


(I should mention, Weber has been great so far. Good communication (from TA Weber himself) and they're swapping out the OT and throwing in a few carbon comp resistors at no charge.)

[johnnyreece]

Did one of these myself several years back. Did you know they actually make this as a kit (they call it a Maggie): https://www.tedweber.com/maggie-c-kt/

It comes with a very beefy upgraded OT, as well as a 8"/10" speaker combo. Looks like they finally updated the layout to get the screens on the right tap. I've always had good luck with Weber. I live reasonably close, so I usually go pick my stuff up when it's ready, and they're always great. Good luck on your build!

[jer_vic]

Did one of these myself several years back. Did you know they actually make this as a kit (they call it a Maggie): https://www.tedweber.com/maggie-c-kt/

It comes with a very beefy upgraded OT, as well as a 8"/10" speaker combo. Looks like they finally updated the layout to get the screens on the right tap. I've always had good luck with Weber. I live reasonably close, so I usually go pick my stuff up when it's ready, and they're always great. Good luck on your build!

Thanks for your reply. I noticed (after I ordered, of course) the Maggie kit on the Weber site. I took a look at it and it's BOM, and it's a bit confusing. I'm basing my amp on the updated 2 stroke design, the major changes being it's now a single 12" speaker, and a single power tube, so not a dual-single ended amp anymore. I think there were some other smaller changes (tweaking resistor/capacitor values?) as well. Looking at the Weber kit, it's still 2 speakers, but it comes with a single tube.

Yeah, I see that it comes with a beefy OT - the Weber WSE25, which would be a good match at 25w, 5000ohm primary and 2,4,8 and 16 ohm secondaries. They also have a WSE15, which would be an even closer match, being identical to the WSE25, but 15 watts.

So why don't I get one of those? Because they are respectively $95 and $91, whereas the W022905M is ~$32. I have no issues paying more if the W022905M won't work, but if it will do an admirable job, why spend the extra money?

[maxkracht]

But according to the tube charts, the output impedance of the various tubes I'm interested in (6L6, 6V6, 6K6, EL34 and KT88) vary substantially. So I don't see
how one OT, whether it's primary is 5K or 7K ohms, can satisfy all of those different tubes.

Is it just the case that either primary value is ok, and it's just that some of those tubes will be "happier" than others?

Yes, some tubes will be "happier" than others. Whether this sounds good or bad is up to you. The recommended load changes with plate voltage and 7k and 8 ohm tap is 3k5 with a 4 ohm load. So there are some variables you can play with.

[jer_vic]

But according to the tube charts, the output impedance of the various tubes I'm interested in (6L6, 6V6, 6K6, EL34 and KT88) vary substantially. So I don't see
how one OT, whether it's primary is 5K or 7K ohms, can satisfy all of those different tubes.

Is it just the case that either primary value is ok, and it's just that some of those tubes will be "happier" than others?

Yes, some tubes will be "happier" than others. Whether this sounds good or bad is up to you. The recommended load changes with plate voltage and 7k and 8 ohm tap is 3k5 with a 4 ohm load. So there are some variables you can play with.

Ah, thank you. So when they say 7000K primary/4,8 ohm secondary, that's not actually correct, as it's 7000K to 8ohms, and 3500K to 4ohms.... That explains some of the things in Dave's book that weren't clear, like why you might want to plug an 8ohm speaker into the 4ohm secondary when you run a 6V6 tube in the amp....

[maxkracht]

It is a ratio, not a fixed value.

[Mark]

]The Two Stroke seems a strange amp to me.

The tone control on these amps is nice but it does mean the amp loses signal so it doesn’t have the same break up as a Champ. I don’t know why the second 1.5K cathode resistor wasn’t bypassed by a 5uF to 22uF cap for extra gain and a 47 ohm tail resistor put between the 1.5K resistor and ground to accommodate the negative feedback. (Refer to the Champion 600 ri circuit diagram.)

https://www.thetubestore.com/lib/thetub ... ematic.pdf

The power transformer is going to have to be pretty beefy to accommodate the extra current from the 6L6, I tried using the stock transformer years ago and it was hotter than the glass on the 6V6. The power transformer has to have extra capacity. I used a custom wound output transformer wound to 5K. It worked brilliantly and the amp was putting out about 10 watts.

I don’t know if many people would use the bright dark switch which changes the cathode cap on the first stage as Champs and Princeton amps are pretty bright to begin with.

[johnnyreece]

So why don't I get one of those? Because they are respectively $95 and $91, whereas the W022905M is ~$32. I have no issues paying more if the W022905M won't work, but if it will do an admirable job, why spend the extra money?

Yes, I agree that subbing one of those in is a large difference in cost. I only meant to say, the Maggie kit is a pretty good upgrade for not much more money. The OT you selected, I believe, should work. It's just that it's really small...I'm guessing it's going to saturate pretty quickly (as tweed Champs/Princetons do). I don't know what you're trying to get out of this amp, so it's hard to advise on the OT.

Short answer, though: Yes, that OT will work.

[Phil_S]

Ah, thank you. So when they say 7000K primary/4,8 ohm secondary, that's not actually correct, as it's 7000K to 8ohms, and 3500K to 4ohms.... That explains some of the things in Dave's book that weren't clear, like why you might want to plug an 8ohm speaker into the 4ohm secondary when you run a 6V6 tube in the amp....

This is not clear,

If you plug an 8Ω load into a 7K transformer that is derived from a fixed turns ratio. If you plug in a 4Ω load to the SAME secondary winding or tap, then it is 3.5K to 4Ω. If the output transformer has two secondary taps, one 8Ω and one 4Ω then the primary is 7K for both of them. The turns ratio is constant.

Specifically, in the case of two secondary taps, 7K:8Ω calls for a turns ratio of about 30:1 and 7K:4Ω calls for a turns ratio of about 42:1. The formula for getting the turns ratio is the square of root the impedance ratio. IOW for 7K:8Ω the impedance ratio is 7000:8 or 875:1. sqrt (875) = 29.58 (or about 30.)

This only tells part of the story about the transformer. Many factors are considered including the actual number of turns, the size of the core, and the size of the laminations. Much is published about transformer design. To me, the material is rather dense.

[Mark]

I probably would look for something like this Hammond output transformer.

https://www.hammfg.com/part/1628SEA

If you disagree that’s fine, but I would like to know why.

It is touted as a Hi Fi transformer, but I don’t like Class A amp with rubbish bass response generally. From my experience this transformer should kick out 10 watts with a 6L6.

[jer_vic]

This is not clear,
If you plug an 8Ω load into a 7K transformer that is derived from a fixed turns ratio. If you plug in a 4Ω load to the SAME secondary winding or tap, then it is 3.5K to 4Ω. If the output transformer has two secondary taps, one 8Ω and one 4Ω then the primary is 7K for both of them. The turns ratio is constant.....

Thank you for clarifying! I'm very new to all this, and was clearly talking out of my backside....

I probably would look for something like this Hammond output transformer.
https://www.hammfg.com/part/1628SEA
If you disagree that’s fine, but I would like to know why.

Thanks. I guess I would say that DH recommends the Hammond 125ESE as a possible choice. But I don't have enough knowledge about the subject to say why to not use the 1628SEA. Also, to add in a "better" Weber OT (WSE15 or WSE25) is easier and cheaper at this point (an additional $60, and no additional shipping).

Yes, I agree that subbing one of those in is a large difference in cost. I only meant to say, the Maggie kit is a pretty good upgrade for not much more money. The OT you selected, I believe, should work. It's just that it's really small...I'm guessing it's going to saturate pretty quickly (as tweed Champs/Princetons do). I don't know what you're trying to get out of this amp, so it's hard to advise on the OT.

Short answer, though: Yes, that OT will work.

Thank you! I didn't mean to imply that the cost was prohibitive, just whether the upgrade would be worth it. After all, ~$60 more on a ~$800 project is not a big deal if it means a better amp. From what you and others have said, I think I will swap to either the WSE15 or WSE25. I sent an email to Weber to get their input.

[johnnyreece]

Well, the upgraded OT will give you better bass response, for sure. In a little Champ or Princeton, the little 8" speaker can't necessarily handle more bass. Since you're going with a 12", though, I'd think a better OT would be worth it. In the old tweed Champ, that's kinda part of the charm: The compression of the OT into the little speaker on the verge of exploding. The Two-Stroke is a little more refined (not a crazy amount more, mind you).

[jer_vic]

Decided on the WSE15, after discussions with Weber. We were both concerned about the physical size of the WSE25. Got my tools all assembled, the resistors and capacitors that I swapped out are here, now just waiting for the amp kit to arrive!

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