SL+ to BE mods

[CrazyNutz]

So this works by basically adding clipping to the signal (you're using one gate of the transistor right....thought I've seen Zeners used for this) ?

Yes clipping, uses Base and Emitter (basically diode mode). Cameron Atomica uses 20v Zenders, and these MPSA06's switchable.

Here is the sat circuit the BE uses.

 

[djd100]

Nice thread, congrats!

Which BE schematic are you running from, and can you post a link to it.

Thx...

 

[CrunchBunch]

CrazyNutz got me fired up to build my own BE. I've gone a similar route to CrazyNutz with the 220k + 100k/500pf V1B but with the following differences:

- I'm (for now) not running any attenuation in the front end (nothing in R7);
- 0.68uf caps in C6 and C13;
- switchable 2n2 / 22n caps in C3 (BE FAT switch);
- 2k7 in R16 and R17, 820Ohm in R29

This makes for an insanely hot/high gain module with the gain cranked. It is far too 'squeaky' on the top end but will sustain for weeks.

To tame the beast and keep things under control in the top end, I've replaced R10, R15 and R27 with 33k resistors. R10, R15 and R27 are grid stoppers so I'm taking advantage of the Miller capacitance formed by the AX7 to reduce the chance of blocking distortion and also provide a high frequency cut.

This is getting there, but more attenuation is required, so I'm currently experimenting with cap values bridged between R27 and R29 in the 200-300p range.

If I can't get things in control this way, I'll have to start removing coupling caps (C6 or C13) or upping the values of R17 and R29 (like CrazyNutz is doing).

Great fun.

Here's an in-progress pic.

 

[CrazyNutz]

Awesome work, happy to inspire

You should pull that 56k out of r30 and pop in a 100k and try bypassing it with a 500pf


I found 2.7k in r16 a little over the top so I'm back to 4.7 there, however I did do a partial bypass with a 0.68uf in series with a 10k across r16, sounds nice.


Also I've got r11 and r7 at 68k, seems to fatten/darken the tone a little. I think I'm going to put r11 and r7 on a dpdt switch to toggle between 220/100k and 68/68k

 

[JKD]

I vote this most fun thread in a while...you guys recording any clips for reference between changes?

Would love to hear them, even if only a bunch of chords and messing about practice

 

[CrazyNutz]

I vote this most fun thread in a while...you guys recording any clips for reference between changes?

Would love to hear them, even if only a bunch of chords and messing about practice

Thanks

Yes I have a few recorded on my phone, just for the purpose of reference between changes, and yes just messing about so don't judge :lol:

I'll try to post them soon.

 

[CrunchBunch]

...I think I'm going to put r11 and r7 on a dpdt switch to toggle between 220/100k and 68/68k

Nice. This should give you the C45 switch option. Also think about putting a 680pf into C4 for the 220/100k combo.

 

[CrazyNutz]

...I think I'm going to put r11 and r7 on a dpdt switch to toggle between 220/100k and 68/68k

Nice. This should give you the C45 switch option. Also think about putting a 680pf into C4 for the 220/100k combo.

I was actually thinking of trying the real C45 560k/220k (with 680pf) values on the switch. However the dang thing sounds SOOO good right now I'm reluctant to touch it.

I would like to keep this one as a point of reference, and continue modding on a new module.

I know this is a shot in the dark, someone sell me a stock mod non-working is fine or better yet, If you have two I'll BE mod one for you in exchange for the other.

 

[CrazyNutz]

Here's a clip I recorded on my phone:
https://soundcloud.com/user-864489142/voice-160821-1
BTW although I have been known to have a couple beers to many, my wife was talking to her daughter, not me

I'll try to do some classic rock recordings later.

 

[djd100]

Thanks CrazyNutz, the whole BE thing totally went over my head as I never considered making one until your thread popped up.

I'm coming from a Plexi that's already modded so I can blend the bright and normal channels. On my Plexi V1A is the normal channel, and V1B is the bright channel via a dual/concentric gain pot. My custom 50W RM2 has a RM20's pre boost feature sans it's post solid state element (utilizing the host's V1B), so I can duplicate the one-wire mod as well.

a) So far I just added the 220K to V1B's plate resistor, and bypassed the 100K with a 500pf. At this point I still have the stock normal channel to blend in as my FAT component. Think of this as a Friedman modded 4 Hole Marshall, with a stock normal channel and a BE bright channel rather than a full on BE 100.

Result: Increased gain and compression, and perhaps a bit more fizz so next I'll do V2A's Plate Bypass Cap up to 500pf as well and we'll wee what that does to the top end?

It seems to have more gain at 5 or 6 that the stock biasing gave dimed, cool.

Thanks again for bringing this up, and the link to the BE 100 schematic.

 

[CrazyNutz]

Thanks CrazyNutz, the whole BE thing totally went over my head as I never considered making one until your thread popped up.

I'm coming from a Plexi that's already modded so I can blend the bright and normal channels. On my Plexi V1A is the normal channel, and V1B is the bright channel via a dual/concentric gain pot. My custom 50W RM2 has a RM20's pre boost feature sans it's post solid state element (utilizing the host's V1B), so I can duplicate the one-wire mod as well.

a) So far I just added the 220K to V1B's plate resistor, and bypassed the 100K with a 500pf. At this point I still have the stock normal channel to blend in as my FAT component. Think of this as a Friedman modded 4 Hole Marshall, with a stock normal channel and a BE bright channel rather than a full on BE 100.

Result: Increased gain and compression, and perhaps a bit more fizz so next I'll do V2A's Plate Bypass Cap up to 500pf as well and we'll wee what that does to the top end?

It seems to have more gain at 5 or 6 that the stock biasing gave dimed, cool.

Thanks again for bringing this up, and the link to the BE 100 schematic.

Awesome. Man I like how your plexi module is modded, it's like a true plexi with the channels jumped, sweet! Anyways as far as the fizz, you have to up the cap on the tube board, about 250-500pf snubs it out. This can sacrifice some high end, so start with a low value.

 

[djd100]

Thanks.

My thought is that the 500pf plate bypass cap on V2A will cut the highs and fizz a bit, while the 500pf cathode bypass cap on the V2B cathode follower will bring the highs back sans fizz (and with additional second harmonics for smoothness etc), we'll see?

Thanks CrazyNutz, the whole BE thing totally went over my head as I never considered making one until your thread popped up.

I'm coming from a Plexi that's already modded so I can blend the bright and normal channels. On my Plexi V1A is the normal channel, and V1B is the bright channel via a dual/concentric gain pot. My custom 50W RM2 has a RM20's pre boost feature sans it's post solid state element (utilizing the host's V1B), so I can duplicate the one-wire mod as well.

a) So far I just added the 220K to V1B's plate resistor, and bypassed the 100K with a 500pf. At this point I still have the stock normal channel to blend in as my FAT component. Think of this as a Friedman modded 4 Hole Marshall, with a stock normal channel and a BE bright channel rather than a full on BE 100.

Result: Increased gain and compression, and perhaps a bit more fizz so next I'll do V2A's Plate Bypass Cap up to 500pf as well and we'll wee what that does to the top end?

It seems to have more gain at 5 or 6 that the stock biasing gave dimed, cool.

Thanks again for bringing this up, and the link to the BE 100 schematic.

Awesome. Man I like how your plexi module is modded, it's like a true plexi with the channels jumped, sweet! Anyways as far as the fizz, you have to up the cap on the tube board, about 250-500pf snubs it out. This can sacrifice some high end, so start with a low value.

 

[withmittens]

This is such a cool thread and I may be following along. I mean, I certainly know what you mean by replacing Rx with xxvalue or Cx with xxvalue. I can definitely handle that.

I'd like to quietly make a suggestion that once these tests are finished that a chart be made with the final mods so it;s easy to purchase and manage the changes........


maybe I'll give it a go at the end....

Carry on

 

[CrazyNutz]

This is where mine is at ATM


The 220k on v1a (on r24) plate is bypass-able with a toggle.

r11 and r7 is switchable with 68k/68k, 220k/100k combo, this I may change later to the c45 spec.

Changed the gain pot from A500k to A1M, big improvement in clarity.

Most everything is in place for the SAT circuit, the MPSA06 transistors I ordered should be here tomorrow.


On a side note I have a full set of WIMA MKP4 caps coming, I'm going to recap the entire amp (just amp not modules) with them.

 

[CrazyNutz]

This is such a cool thread and I may be following along. I mean, I certainly know what you mean by replacing Rx with xxvalue or Cx with xxvalue. I can definitely handle that.

I'd like to quietly make a suggestion that once these tests are finished that a chart be made with the final mods so it;s easy to purchase and manage the changes........


maybe I'll give it a go at the end....

Carry on

Good suggestion. There are some things/values that for sure seem to be imperative. I'll do a drawing soon that lays these out. Then there are some things that are just up to subjective taste.

 

[CrazyNutz]

Thanks.

My thought is that the 500pf plate bypass cap on V2A will cut the highs and fizz a bit, while the 500pf cathode bypass cap on the V2B cathode follower will bring the highs back sans fizz (and with additional second harmonics for smoothness etc), we'll see?

Yes 500pf plate bypass does indeed cut the fizz. However the 500pf bypassing R30 Is kind of a mid voicing thing I would say, It does not add any fizz, seems like it knocks the upper mids down. It takes some of the HONK out from what I can tell.

 

[CrunchBunch]

MPSA06's arrived today for some saturation switch experimentation...

 

[withmittens]

[/quote]

Good suggestion. There are some things/values that for sure seem to be imperative. I'll do a drawing soon that lays these out. Then there are some things that are just up to subjective taste.[/quote]

that would be awesome of you

 

[djd100]

Interesting regarding the cathode follower's cathode bypass cap. So you think it's taking some mid-range honk, like 600Hz or 800Hz?

I just did the V2A plate bypass up to 500pf, definitely cut the fizz, and it's still pretty bright on mine.

I'll test it again tomorrow with fresh ears before deciding on whether I should do the cathode follower's cathode bypass cap up to 500pf too?

Looking forward to your SAT report, thinking about that too LOL!

TIA...

Thanks.

My thought is that the 500pf plate bypass cap on V2A will cut the highs and fizz a bit, while the 500pf cathode bypass cap on the V2B cathode follower will bring the highs back sans fizz (and with additional second harmonics for smoothness etc), we'll see?

Yes 500pf plate bypass does indeed cut the fizz. However the 500pf bypassing R30 Is kind of a mid voicing thing I would say, It does not add any fizz, seems like it knocks the upper mids down. It takes some of the HONK out from what I can tell.

 

[djd100]

I think it's important to note that Randall had some revisions to their PCB's through out the years, so the various PCB's parts designation numbers are not always the same etc.

Just a FYI...