BAJA Mars Hall 1959 plexi amp overdrive pedal 9v 090318

HEAD · Post by **HEAD** » 17 Apr 2018, 18:12

Hi,

is their a sigificant change in treble content with the bright switch activated? 'Cause in my build I don't hear any change. Wonder if that's normal or if I have a gremlin somewhere (did check everything around the bright section visually so far).

Cheers
Helge

Frank_NH · Post by **Frank_NH** » 18 Apr 2018, 00:11

I'm in the middle of building mine, but the 1nF cap is a treble bleed and is bypassed when the gain is maxed. You should hear it when you turn down the gain.

HEAD · Post by **HEAD** » 18 Apr 2018, 14:08

Thank you! Could have come up with that explanation on my own but ... sigh. Yeah, the effect becomes obvious when you turn down the gain pot - something I barely did, because its fairly clean before full gain. To me the pedal could need a little bit more gain to squeeze out some distortion. It works well with booster in front. Really breaks up like the real amp, which also isn't too distorted at full bore.
Any mod ideas? Upping some of the resistor values in the feedback loop uf UA1 before the gain control or better before the clipping diodes?

Cheers and thanks!
Helge

Frank_NH · Post by **Frank_NH** » 18 Apr 2018, 15:43

Hmmm. I'll have to finish my build to know for sure, but most online demos of the Marshall Plexi/1959 amps display a very crunchy classic rock overdriven sound when maxed out. It would seem to me that the diodes in the latter stages of this effect should give you that crunch.

HEAD · Post by **HEAD** » 18 Apr 2018, 17:42

Gave this a try today. First starting to experiment with the gain stage around IC2b putting in a 25k pot in series with the 5k6 resistor. This didn't give me - even maxed out - a lot of distortion increase. It shifted slightly the frequency response, too. Funnily I had the impression it alters the sound a little bit like you were fiddling with the bias of a real tube amp. Anyway... times had come to take some more drastic measure: I put a 50k pot in series with the 2k7 resistor in the f.loop of IC1a. This did the trick so far. WIth half way up this gave me a lot of distortion.I measured ~23kOhm in total for the sweet spot. So a 22kOhm resistor would do the trick here. If you're facing the same issue you can try this mod for this otherwise nice sound emulation.

Cheers
Helge

Frank_NH · Post by **Frank_NH** » 18 Apr 2018, 18:17

Thanks for report. I was thinking you could, alternatively, make IC2a a gain stage, substituting a 22K resistor for the 2.7K in the feedback. As it is, it has a gain of only 2x - upping the feedback resistor would give you 10x.

Looking at the original Marshall 1959 preamp schematic (link below), it appears that the stage after the volume control consists of a tube gain stage with a 820 ohm cathode resistor and 680 nF bypass cap. To get similar frequency response for IC2a, you could keep the 2.7K resistor in the feedback loop, and put a 220 nF cap in parallel the 2.7K resistor to vbias. That would kick up the gain above 250 Hz, perhaps giving you some added crunch. I may try that with my build as it would be easy to add to my vero layout.

Marshall 1959 schematic:
https://drtube.com/schematics/marshall/1959u.gif

HEAD · Post by **HEAD** » 21 Apr 2018, 08:35

Hi,

to just give you a feedback on the idea of increasing the gain with the 220n cap: It works. Somehow. But it leads to oscilalltion with higher presence settings and doesn't sound too pleasing in general. So far my "solution" sounds best if you want to increase the gain.

Cheers
Helge

Frank_NH · Post by **Frank_NH** » 23 Apr 2018, 13:30

Hi Helge,

Thanks for the feedback. I realize now that I goofed in my recommendation, and that you should really use a cap and resistor in series, with the resistor sized based on the gain you want and the cap sized to give you the desired frequency response. However, there are some schematics that have just a single 1K cathode resistor and cap in the second tube stage for the normal channel. For example:

https://robrobinette.com/How_the_Marsha ... Super_Lead

I'm going to build my version stock for now and increase the feedback resistor in IC1a if I think it needs more crunch.

Here's an interesting discussion on the breakup in a real Marshall 1959 amp:

https://www.thegearpage.net/board/index ... p.1656622/

HEAD · Post by **HEAD** » 23 Apr 2018, 16:27

No problem. Always glad for advices and an open discussion of how to improve/alter designs. And this was a really quick try - so no real time has been wasted. Of the baja amp designs so far I tried (BEO, 5E3, VH4, SLO II, AFD II), 1959) only the SLO mkII and AFD mkII had a somehow similar same amount of gain compared to the real amps. With an added tone control these two designs sound really, really good. Love both of them but decided to only box up the SLO since it had a little more ooomph and was more useable/flexible on the gain pot (from light crunch to mean hi gain).

Frank_NH · Post by **Frank_NH** » 24 Apr 2018, 13:21

Last night, for fun, I created a simulation of the Bajaman 1959 amp sim in LTSpice and discovered two things that may be of interest:

(1) The output level is on the low side with the master volume maxed, and you can increase the output level by substituting red LEDs for the 1N4148 diode pair before the last op amp buffer stage. I will try this out as I generally like LED clipping.
(2) If you want to get more distortion at lower gain pot levels, you can play around with the 2.7K feedback resistor that sends the signal from the output IC3a to the negative input of IC2b in the power stage part of the sim. Increasing this resistance will increase the stage gain and cause more clipping/distortion. You could put in a 10K trimmer pot in series with the 2.7K resistor to adjust this to taste.

HEAD · Post by **HEAD** » 11 May 2018, 07:58

Hi Frank,

did you finish your build? Really would like to hear of your experiences with this circuit.

Cheers
Helge

Frank_NH · Post by **Frank_NH** » 11 May 2018, 14:25

Hi Helge,

Unfortunately, this has been put on the back burner for now as my basement workshop space is being renovated and I have a bunch of other activities on my plate. Probably early June before I get back to it.

Frank

jhergonz · Post by **jhergonz** » 15 May 2018, 02:38

please make a emulator of PRS archon .... PLEEEEEEEEEASE!

bajaman · Post by **bajaman** » 15 May 2018, 06:25

jhergonz wrote:please make a emulator of PRS archon .... PLEEEEEEEEEASE!

Please post schematic and i will take a look

bajaman

temol · Post by **temol** » 15 May 2018, 08:16

How about Sunn that you have announced before ?

T.

DiegoJorge · Post by **DiegoJorge** » 14 Nov 2022, 17:49

Hi everyone! It's one of my first posts here.
Thanks Bajaman for this proyects!
Here is my attemp of Layout, that includes Bassman & 59 Plexi on the same board. It's designed for 1590B.

TheGunn · Post by **TheGunn** » 07 Sep 2023, 15:55

In the schematic there is 1u cap after buffer but layout shows 100n. Does it make any difference? Also is there possibility to add a 39# mod?

TheGunn · Post by **TheGunn** » 16 Sep 2023, 18:11

I started to mod this circuit to emulate bright channel. Also there is not enough gain comparing to real amp. I tried changing IC1A feedback resistor to 27k and gain amount was about right but it didn't clean up so well. So I changed gain of first two stages to get roughly same gain as with this mod but I kept 12:7 gain ratio between first and second stage as in real amp. Are my modifications correct? Also can anybody help with emulating real amp 470p bypass cap and 470K mixer resistor?

TheGunn · Post by **TheGunn** » 16 Sep 2023, 18:19

By the way anybody thought about adding gain stage emulating bright channel with separate gain control in parallel with IC1A and switch enabling selection between bright/normal/parallel and then using IC2A as summing amplifier? This way you could get possibility to mix channels like real amp does.

BAJA Mars Hall 1959 plexi amp overdrive pedal 9v 090318 [documentation]