Bootstrapping for more gain - Rocket

[sunnydaze]

Wondering if anyone else has tried this trick from Merlin's first book - bootstrapping for more gain. It's also shown on his website here at the bottom of the page:

http://www.freewebs.com/valvewizard1/dccf.htm

I did this to my Rocket and really couldn't tell much difference - put it on a switch and tested for awhile. Have to admitt was underwhelmed. Am I missing something about how this should be applied? I used the same values as the pic on the website.

I ended up using the switch to switch in or out a cathode bypass cap on the normally unbypassed V1b. Makes a lot more audible difference than bootstrapping.

Anyone one else tried the bootstrapping technique in Merlin's book in a Rocket?

Link fixed now - had a period on the end before. Thanks KT66
Thanks
Mike

[sunnydaze]

Deleted

[guitarsnguns04]

rj implemented something similar in his high gain rocket..you might check that...also you could lower the cathode resistor in merlins example for more gain...or lower what you have now if your looking for more bite..adjust bypass caps to taste also

[Ian444]

I've used it in a different cct, did the trick for me, as far as I can remember. You did have the cap wired to the cathode of the follower?

[sunnydaze]

rj implemented something similar in his high gain rocket..you might check that...also you could lower the cathode resistor in merlins example for more gain...or lower what you have now if your looking for more bite..adjust bypass caps to taste also

Thanks for the comment. Let me clarify my first post. I didn't actually use all of the values in Merlins example - only used the split anode resitors and 47n cap to cathode of the follower - my other values are the Standard Rocket values. For example, I didn't change the cathode resistor on V1b to 820r as per Merlin's example (left it at standard 1.5k). Also, my the resistor on my follower is 56K rather than the 100k in Merlin's example. I thought about lowering the cathode resistor value in the follower, but it's already pretty low at the standard 56K. According to Merlin's book, lowering that value tends to compress the top side of the wave form. I guess I was thinking the compressed wave form would make the amp sound more compressed, not really what I was going for - That said, might play with the CKR on the follower and see how it sounds.

Thanks

[sunnydaze]

I've used it in a different cct, did the trick for me, as far as I can remember. You did have the cap wired to the cathode of the follower?

Yes, the 47n cap is from the junction of the split anode resistors of V1B to the tubeside of the cathode resitor of the cathode follower.

I might try different cap values and/or changing the resistor value of the cathode follower.

Thanks
mike

[RJ Guitars]

Sunny,

I've done a few different things to get more gain out of the Rocket. The amp already has a lot of gain although one of it's real attributes is fantastic clean tones. When you push it it does crunch very nicely though.

I have worked through the various components in the original circuit and was able to dial in a lot of creamy sustain on one of the Octal Rockets that I built. I haven't really taken that exercise the other way but would expect you could make it more clean if you wanted to go that way.

Look on page 6 of this thread and you'll see that I used several trimmer pots and my ears to dial in what I wanted. https://tubeamparchive.com/viewtopic.php?t=7300 There is an audio clip on page 6 that demos the great sustain.

If you want some gain and a little fun, you can also try the "Rocket Booster". It's a cool project and won't cost you much if you already have a Rocket to experiment with. https://tubeamparchive.com/viewtopic.php?t=11401


Good luck and let us know what you learn,

rj

[sunnydaze]

Thanks RJ.

Are the post you have referenced in your note the ones you wanted me to check? I see an audio clip but there is no page six (only two pages), both references are to topic 11401.

I love the cleans on the Rocket, too, so didn't want to do anything to drastic. I thought the bootstrapping for gain trick from Merlin's book would be an easy switchable implementation. Just kind of underwhelmed.

On the other hand putting a switchable cap on the V2b cathode was easy and works and sounds pretty good. Just a quick flick of the switch and its back to stock values.

I might try playing around with the cap values, or cathode follower resistor value to see if the bootstrapping improves.

I think I'll save using the unused triode for a next build . Don't really want to move this Rocket to far from standard values.

Thanks
Mike

[RJ Guitars]

Thanks RJ.

Are the post you have referenced in your note the ones you wanted me to check? I see an audio clip but there is no page six (only two pages), both references are to topic 11401....
Thanks
Mike

Oops! I cut and pasted the same thing twice... should have been this one - https://tubeamparchive.com/viewtopic.php?t=7300

rj

[Richie]

Some of the schems show a 50pf treble cap. If you use that value the amp will stay pretty clean.
Some schems show a 500pf treble cap. The amp will have much more growl using the 500pf.
And as was posted the bypass cap which you could play with values of the resistor and cap to fine tune what you like.

[rooster]

sunnydaze - Yeah, I have not tried this mod from Merlin but I have tried a few others. 'Underwhelmed' is a good word. He signs in here but not often lately, he may respond to your post, who knows?

I think his book is a good read, yes, but the best part of it may be that some ideas presented in it make you take a fresh look at the things you already know and trust. Well, at least for me.

And on that subject, I think you just learned that the 100K cathode tail on a 12AX7 produces as much gain as the tube stage is capable of - at least as far as human ears are capable of recognizing. So, yeah, Merlin uses the word 'gain' throughout his book, but I think you just learned that his definition of gain is not yours or mine.

[sunnydaze]

RJ, Richie & Rooster, (that's a lot of R's..)

Thanks for the replies.

Richie, I haven't been fooling around with the treble cap, but might play with that some time. The cap I was talking about was the "bootstrapping for gain" cap that runs between the split plate resistors on V1b and the tube side of the cathode resistor of the follower.

Rooster>I've enjoyed Merlin's book along with many others. I definitely appreciate they way he covered pre-amp theory including the load sheets and how to do those type calcs.

I may not have time to fool with anything before my next trip. I'll be away for a few weeks starting Sunday.

I'll resurrect this thread when I get back and have a chance to make some changes.

Thanks
Mike

[sunnydaze]

Ok....

Before giving up on the "Bootstrapping for gain" mod I need to implement as actually intended.

I just found a piece in Merlin's book I must have missed the first time around. Giving credit to Merlin Blencowe, page 133 of his "Desiging Tube Preamps for Guitar and Bass" . Here he talks about losing the additioanal gain if you don't pay attention to the CF's RK values with respect to the previous stage's split anode value. He provides some math, and goes on to give a rule of thumb saying 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.

I was trying this with the standard Rocket CF RK value of 56K. Maybe the CF RK needs to be closer to the 100K in his example for the to work.

I won't have time to test this for several weeks - on my way to Tokyo for a few weeks.

Will provide an update when I get a chance to test.

thanks
mike

[rooster]

Mike - Ha! Or not, me thinks. Just switching to the 100K - without the bootstrap thing - you'll get more gain. (And more distortion, if that's your goal, though most here are not seeking that with this amp this way - permanently - because the clean headroom suffers.) Which would beg the question: How could you tell whether you increased the gain overall if you didn't start with the 100K cathode resistor? And here again, provided you started from scratch with 100K, and then added the BS idea, I am betting that the gain difference is minimal if even audible to the ear. But yes, by all means, if you do this experiment correctly, starting with the 100K and then adding the BS idea, I would like to hear your report. Too, as you read the book, he talks about the compression from the cathode follower circuit in general? Which begs another question: Will this 'potential' gain - which, IMO, is prolly occuring simply because of the new 100K resistor - alter the compression/feel of the stock circuit to the point that it, well, it ain't no Rocket anymore?

BTW, enjoy Japan as best as you can, there's a lot of suffering there right now, and for years to come it seems. If you're going there on a care giving mission, good on you then, Brother!

[sunnydaze]

Mike - Ha! Or not, me thinks. Just switching to the 100K - without the bootstrap thing - you'll get more gain. (And more distortion, if that's your goal, though most here are not seeking that with this amp this way - permanently - because the clean headroom suffers.) Which would beg the question: How could you tell whether you increased the gain overall if you didn't start with the 100K cathode resistor? And here again, provided you started from scratch with 100K, and then added the BS idea, I am betting that the gain difference is minimal if even audible to the ear. But yes, by all means, if you do this experiment correctly, starting with the 100K and then adding the BS idea, I would like to hear your report. Too, as you read the book, he talks about the compression from the cathode follower circuit in general? Which begs another question: Will this 'potential' gain - which, IMO, is prolly occuring simply because of the new 100K resistor - alter the compression/feel of the stock circuit to the point that it, well, it ain't no Rocket anymore?

BTW, enjoy Japan as best as you can, there's a lot of suffering there right now, and for years to come it seems. If you're going there on a care giving mission, good on you then, Brother!

Rooster, valid points, I'll make sure I do "controlled" test and check with the 100K resistor in CF first. It will be awhile before I get chance to mess around around with it - I don't get back home to Taipei until the 20th (unless my schedule changes). Starting to look like I might need to spend a few days in Hong Kong.

Japan is a business trip - I live in Taipei, Taiwan but spend a good deal of my time in Tokyo. Our largest operation is in Japan, and fortunately, all of our staff and sales people have been accounted for and are safe. We do have lot a sale people and customers who live in the severely impacted areas. All of staff, world wide have made generous contributions to a relief fund to help out out sales persons and customers.

Will let you know how the test goes, when I get I chance to fool around with the amp. Not really interested in changing the amp from Rocket to something else permanently.... Switches could be nice though.

thanks
mike