Jose v1, Bootstraped Cathode Follower, and CF Boost switch

[mightymike]

Am I double dipping on the Cathode Follower? Can I do both of these CF mods at the same time? The Bootstraping the CF boosts the gain of the previous gain stage (v2a) to what you would get out of both sides of a 12ax7 according to Valve Wizard (if I'm undestanding him right), the switch that adds a 100k/.001uf in parallel to the 100k v2b rk reduces the value of v2b rk to under 50k, and is supposed to drive the Tone stack harder (like a Plate driven tone stack), but with the CF voice..

The bootstrap circuit is from Valve Wizard, and the CF boost switch idea was posted by a member years ago. I have yet to try either of these CF mods. I would like to make the Bootstrap .047 cap switchable too. I guess a spst switch between the .047uf bootstrap cap and v2b grid would work.This way I could try both, just one, or none to see if I even like it.

If all three of these mods can be used like this, I think that the gain would be close to the level of a 4 stager (maybe more), but more stable with only 3 pre amp tubes because the boostrap will make v2a have the gain of an entire tube, and with less blocking distortion and fizzies because no CK on v2a. Again according to Valve Wizard.

Here's a link that describes the Bootstraped Cf mod in detail. Scroll to the bottom of the page http://www.valvewizard.co.uk/dccf.html" onclick="window.open(this.href);return false;

I based my diagram off his diagram and applied it to a Marshall.

[harddriver]

Hey Mike,

How are you, the bootstrapping thing looks kind of cool. I don't have much experience with it. I did build a 4 gain stager that is two channels that switches via an A/B box. If you look at the SLO schematic that is the exact layout that I used but substituted different values and options.

If you completely isolate your two separate input jacks and ground them correctly you essentially end up with a 2203 3 gain stage and the extra fourth gain stage that you can toggle back and forth. There is no clicking when switching but the A/B box must be of good quality because it is grounding the unused channel. One jack goes to the fourth tube input grid and the other jack goes to the other input grid on the 2203 side along with the output from the fourth tube, and you get to retain the Cathode follower just like the SLO. It has worked very well and this amp has been working since 2008 and the owner and I have been very pleased with the tone. He has a hot 2203 channel and my take on the Aldrich Cameron mod, depth mod, adjustable feedback, sat switch, dual gain controls, and a footswitchable gain boost, it's quite the monster. I actually prefer having the CF IMHO.

Another thing you may want to do is DC elevate the heaters that keeps the noise down and actually helps protect the tubes. Cameron does this on all his amps that he mods and builds. I can send you a schematic for that if you want. I have an old thread with pics from 2008 when I built the amp and I think the pics are still up if you can find the thread here.

[harddriver]

Heres some pics of the board layout of my amp in this thread.

http://forum.metroamp.com/viewtopic.php ... 54#p290966" onclick="window.open(this.href);return false;

The amp has been through a million evolutions and tweaking since but the basic layout remains.

[mightymike]

Hey man, I can't wait to check out your build later today. I've done similiar with a 3 stager to one jack , with a 4th tube to another with an a/b box. The 3 stager was the same v1 my drawing above, but with a .002 after the 2nd stage, 69 tone stack, 25uf on v2a, and .1 output, sat switch, ppimv, reasonance, and half power.. The 4th tube was just like v1 (Jose spec) with the 330 plate.

On the x3 thread in the EVH room I'm working off that design and using the b side of the extra tube for a clean channel, and I'm working on a foot switched setup with relays and like a 5 or 7 pin din/midi switching. 3 voice from a foot switch.. Plexi, hot 2203, and 4th tube.

I totally agree on plate driven tone stacks. I'm a Cf guy.

With this drawing I wanted to try something different. To get a higher level of gain, with less stages, and more purity for lack of a better word. I'm thinking of changing the mixer resisters and cap too. For the same frequency but with a lower resister and higher cap. I did that once before and it had a cool compression thing going.

There's so many places to get gain and v2 gets over looked.

If I change the mixers, I will be triple dipping. I don't know how much that poor tube will take.

[harddriver]

I hear you on just using 3 gain stages, you can get alot of gain and not need to DC reference the heaters and the amp is still relatively quiet with judicious use of shielded cable on the input grids between stages and all the inputs. The 4 th gain stage running into a 3 with the CF can get a little noisy without the DC referencing.

I have a really cool gainboost setup you might be interested in using that works fabulous, is footswitchable with any latching type footswitch and can be tailored to your desired shelving frequency you can use in addition to the bootstrapping idea. I can send you the schematic if you don't already have one.

I would recommend keeping the 2.7K on your cathodes to keep the Jose jangle and dynamics, the amp kind of loses that if you gain out your stages with 820 ohms which is pretty much full bore maxed out gainstagewise. Even running V2 at 1K helps as well. I have also utilized a lot from the Bogner blue channel on a three gain stager with a plexi switch that turned out pretty ripping where I used a three way rotary on the second gain stage cathode for gain and voicing selections.

I would go with the three stages if your looking to get closer to the Jose Atomica type amp and gain, which I think you are. With that schematic and some clever gain boosts the amp would be a great VH monster all the way and then some, I used the standard JCM800 master since instead of the Jose so I could get a 2203 channel and an Aldrich type channel.

I have a few layouts I can send you if you want to help you brainstorm what you might want. Let me know how the bootstrapping works it would be a cool addition to a bag of tricks.

I actually can't remember my exact Photobucket logon and password since I posted those pics from 2008-2010 but the links are still up.

Good luck on the build.

[mightymike]

Yes definitely send me your layouts and links. I want to check them all out.

[joey]

Am I double dipping on the Cathode Follower? Can I do both of these CF mods at the same time?

Yes to both

The Bootstraping the CF boosts the gain of the previous gain stage (v2a) to what you would get out of both sides of a 12ax7 according to Valve Wizard (if I'm undestanding him right),

No. You will only realize the full potential of the previous gain stage and it will not be an equivalent of two cascaded gainstages, especially ones that are conceptually voiced, and designed for clipping. IMHO this circuit is best used as an input stage for smaller signals, (it's not a new circuit it has been around for quite a while and is very popular in SS design for Low signal input stages) it will tend to mush out in a place where clipping is involved.

the switch that adds a 100k/.001uf in parallel to the 100k v2b rk reduces the value of v2b rk to under 50k, and is supposed to drive the Tone stack harder (like a Plate driven tone stack), but with the CF voice..

It doesnt drive the Tonestack harder, it loads itself down causing a more compressed sound, and making the ts harder to drive, as it's an additional load.

[mightymike]

Well shit. Looks like I was way off in my undertanding on both accounts

Is the 100k in parralell pushing more current to the tone stack or doing anything that would get more gain? I thought in parralell would reduce the load to 50k. Trying to understand the benefit of this mod if any.

[luiscristiano43]

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