Bootstrapping for more gain - Rocket

[rooster]

Mike - Wow, you are right there. And yeah, 'WOW' is about all I can say when I think about the situation in Japan. Now it's a testimony of survival. I hope some lessons have been learned by the world, and I really hope - in spite of the present state of things - that there is/are some serious nuclear genius(es) that surface with a real cleanup solutuion. What a horrible, horrible mess! Eh, sorry to go on.

As to the amp adjustment testing, yeah, I'll be here. Like you, I picked up that book thinking it would really improve my game and amps. For me, it turns out, like I said earlier, to be only a good read. It took me out of my comfort zone a bit mainly, and, even though the three adaptations I tried (from the book) were all reversed, I'm not sorry for the time spent.

Sometimes seeing how one person sees something, whether a story teller or a amp guru, and trying his/her skin on, you learn something. Ha, and sometimes it might just be that it is good to come home to yourself. Er, well, in the case of TW circuits, home to Ken's vision. Ken was a very cool Dude.

[sunnydaze]

OK, so back in Taipei now and had chance to try a few test on the bootstrapping for gain.

Decided to check the effects on a scope and have some pics attached. There are two pics attached - one is measuring a 1000 HZ sine wave at the input and at the cathode of the follower - no bootstrapping, standard Rocket Values

The second measurement taken the same points, but has the 47n bootstrapping cap included in the circuit.

The amps volume is set at about 9:00 - tone and cut controls have no impact as both measurements are before the tone stack

I didn't change the scope settings for either pic.

As you can see with the 47n cap bootstrap connected, the signal from the follower is bigger than without - more gain.

Next, tested with a guitar to hear the difference (used my wife as blind test listener). We could both hear the effect of the bootstap at low volume settings. The audible difference was neglible at higher volumes - this is probably why I didn't really notice much effect the first time I tried.

I didn't try switching the cathode resistor from the standard 56K to 100K. As that would have only increased the load on the CF, which according to merlin can mitigate the effects of bootstrapping (missed that point when I thought about trying the 100K.)

Conclusion - yes, bootstrapping works, but the audible difference decreases as the volume is cranked up.

Thanks
mike

[tubeswell]

The extent to which bootstrapping a CF stage will be effective also depends on what the stage is loaded with, because bootstrapping to the previous stage's plate resistor like that increases the CF stage's output impedance. Merlin mentions an optimal configuration in that chapter IIRC.

As far as turning it up goes, well all amps tend to get to the same stage of mush when they're dimed. Increasing the gain of one part of the amp will have an effect for sure, but only so much effect, without also increasing the HT voltages all around, putting in beefier output tubes and bigger OT, and power supply etc etc.

[sunnydaze]

The extent to which bootstrapping a CF stage will be effective also depends on what the stage is loaded with, because bootstrapping to the previous stage's plate resistor like that increases the CF stage's output impedance. Merlin mentions an optimal configuration in that chapter IIRC.

As far as turning it up goes, well all amps tend to get to the same stage of mush when they're dimed. Increasing the gain of one part of the amp will have an effect for sure, but only so much effect, without also increasing the HT voltages all around, putting in beefier output tubes and bigger OT, and power supply etc etc.

Thanks, Tubeswell. Merlin's book does say that there is an optimal setup for bootstrapping, but that math was pretty lengthy. Instead, he suggested a rule of thumb rule that the value off the split anode resistor on the previous stage (R1 + R2) should be at least equal to RK of the follower and that R1 should be at least equal to 47K. That pretty much fits right in with the standard Rocket values.

It did have an audible effect at lower volumes, but the difference was difficult to hear with the volume at noon (didn't dime the thing). Anyway, it's fun to try new things and always good to keep learning.

Thanks
Mike

[rooster]

Mike - OK, I checked in. Hope you trip was safe, BTW.

Well, so, the verdict from my house is 'eh' on the bootstrapping. I mean, it's not worth implementing, IMO. There is easily more gain to be had by attaching a cap to the second stage before the CF - and this volume is easily heard. Oh well...

But, that said, I appreciate that you posted some pics and shared it. Sometimes, having read the book, I have to think that the author took a lot of info from tube books and just represented it - without the benefit of trying things. And it could be that to him, it's just an exercise in math, and not one in audio for that matter. I guess this is where, even if the person is considered somewhat 'shady, at least someone like G. Weber publishes tried things that tend to deliver. Oh, sure, he's publised some lamo things, too, no doubt.

Anyway, thanks for the testing!

[sunnydaze]

Hi Rooster,
Trip to Japan was fine. There were some pretty strong aftershocks while I was there, but the buildings in Tokyo are built so well, didn’t really worry. I leave a Stat and one of my suits at the hotel I stay in while in Japan. Stat wasn't glowing in the dark or anything when I took her out of the case....

Now, if we have some quakes like that while I’m home here in Taipei, might be different ballgame

Anyway, like you I'm glad I read Merlins' book. I've read all of Gerald Weber's books too. Got started on Aspen Pittman's book, then Webers, then Torres Inside Tube Amps, and a few others. All good reads. It's always good when you keep learning new things. Once you stop learning new tricks, your done.

And then there's this forum - I'm sure I've gotten about as much out of reading this forum as I have from the books.

It's been fun so far.

Thanks
mike

[Structo]

Hi,
Did Taiwan suffer any from the tsunami?

[sunnydaze]

Hi,
Did Taiwan suffer any from the tsunami?

No damage in Taiwan. All of the quakes and Tsunami are really focused on fairly small part of Japan. The area that was hit by the tsumami was devasted. Like the rest of the world, the Taiwanese were very generous in their contributions to relief funds. The video's of the tsunami were such that it was hard to imagine what that must have been like.

mike

[Clyde]

I've tried the CF bootstrapping technique several times but found the tone suffered (to my ears) although gain was definitely increased. YMMV. I think it has possibilities, but tweaking would be needed.

[Ian444]

I think the bootstrapping in Merlin's book was just something he mentioned for experimenters to be aware of. If you need just a bit more gain then here is a method to get it. If you don't, then don't worry about it. I used it in front of a cathodyne with the resultant required unequal plate and cathode resistors to make it work and it was fine. He ran up a test circuit, it wasn't just repeated from some other book. YMMV, different amps, different results.

[sunnydaze]

I didn't really notice any affect on tone, but in my implementation, the increased gain was more audibly noticable at low volume settings. To my ears was a more "full"or sound. Really couldn't discern much difference in gain or tone at higher volume settings.

In anycase, was a worthwhile and fun experiment. I enjoyed Merlin's book, and thought it was a great read - would recommend it to anyone interested in guitar tube amps.

I'm in Seoul, Korea right now, but will return home to Taipei on Friday night, maybe this weekend I'll put the bootstrap on a switch along with the V1b cathode bypass cap I already have on a switch. That way, stock values are only a flip away.

Thanks
mike