80s Mod Board in the works

JTyson":2c3xeziy said:
I think its a cool idea, given that it delivers the intended tones. I know a player or two who would be able to install this, but would not try to install this same mod within the original pcb. Its all whats familiar to you and how far beyond that you are willing to venture ;)
Plus, it looks like it would be easily reversible.
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It's all reversible except the two 1/4" holes required for the switches. There are hole plugs that can be used to clean up the rear panel if the switches are ever removed. This might be a concern if the amp is a classic, but otherwise it cleans up well.
 
JMMP":1xskisny said:
Color me interested, especially if it works in a Peavey Windsor! I need to go compare schematics to check that.
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It should probably work. I can provide tips for optimizing things for this amp, but I won't be able to test the mod board in every circuit. The Windsor has a plate-driven tone stack with a coupling cap between the 4th gain stage and the tone stack. That should be no problem (I've installed clipping circuits with plate-driven tone stacks before (although not this particular clipping circuit).

Feel free to shoot me a PM/email to discuss.
 
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I populated the first Mod Board with components and loaded it into my Egnater. She's a GO!
I'll build all of the first ten this week. One will stay in the Egnater, which leaves nine from the first batch. I have a list of potentially interested folks going, and I'll create a classifieds thread here once all of the boards have been built and tested.

Oa0LRz3.jpg
 
Metlking":3hrc4r3w said:
Is your Egnater a seminar head? I built one a few years back and would like to get more out of it!
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Yes it is (although I didn't go to the seminar; got the same amp as a kit).

The mod board would help expand the tonal options of this amp and get saturated tones at lower volumes...
 
Update:
I've built the first batch of (ten) mod boards and have started to work on the illustrations that'll go along with the simple installation instructions. I'm hoping everything will be ready to ship in 7-10 days, so I'll start to get in touch with folks who have expressed interest here.

The installation instructions will focus mainly on 2204 clones that have eyelets or turrets. I won't be able to fully support installation in all amps, but I'll add some installation notes about the Marshall JCM800 and Egnater Seminar Amps. I am guessing that on at least some amps (e.g., Jet City JCA20H), the PCB would have to be partially or fully removed in order to solder on the underside of the PCB. The Mod Board can be installed in Egnater Seminar Amps without removing the PCB (with a simple modification of one component). I think the Mod Board could be installed in a Marshall JCM800 without removing or modifying the PCB; the same might be true of the Jet City JCA20HV.
 
mxr2000":1vtqmmcd said:
Can this Mod can be Installed in a Marshall JCM800 2205 or 2210 amp with good results?
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The 2205 and 2210 have an entirely different topology and already has diode clipping. It would not be designed for your amp
 
Hey, I just wanted to add a little review of the Mod Board to this thread.
I finally got it installed on a Marshall 1987xl that's being converted to a 2204-type circuit, and it came out pretty nice.
First of all, the board itself is very high quality, with through plated holes. This was important for me because I had to remove the switches and mount the board on a stand-off to squeeze it into the amp. Not a lot of panel space on this build. Also, all the components used are good quality as well.
Anyway, the clipper section functions perfectly and sounds great. The volume drops a bit, but that's normal for this type of circuit. I like the choice of 20volt zeners, so it's not too compressed.
The structure section functions nicely as well, with nice choices for the values, but it adds a TON of gain, and this may a bit of an issue for some amps. I ended up having to do a couple of tweaks to the circuit, but it sounds great now. What I ended up doing is removing the standard "2204" 470pf / 470k peaker, the volume pot and the bright cap that is located between the first and second gain stage. I replaced this with a 68k / 68k voltage divider and a 33k on the grid of the second gain stage. I then re-installed the gain pot after the second stage .022uf coupling cap, and before the 470k / 470k voltage divider with the 470pf cap. This got everything under control and there is still a crapload of gain available. I did NOT install a bright cap on the gain pot.
So all in all I'm pretty happy with the purchase and the results.
I think V2a did a nice job on these boards and I'm going to order another one for another project.
 

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Thanks for the review, Adam. There is indeed a crap-load of gain available. It takes the 2204 into Friedman Dirty Shirley territory. I've added a few tips on taming the gain in the new installation document. In brief:

- the amp wiring and mod board wiring should be nice and neat. The "lead dress" is very important.
- grounding is also very important.
- it is useful to limit bass by swapping out the stock 22nF coupling cap (after stage one) with a lower-value cap (2n2F). Add a back-panel switch to go from 2n2F to 22nF.
- An anode bypass cap on the first stage can help prevent oscillations. 100-200pF should do it. There are other places to add low-pass filters, but these are often easy to add (at least with a turret board). For PCBs, you might try adding a small-value (pf) cap from anode to cathode, right on the tube socket (I think Laney did that at some point).

Cheers!
John
 
Looks really simple to install without having to drill... theres a ton of gain in a dirty Shirley though?
 
lespaul6":2hvzr7ir said:
Looks really simple to install without having to drill... theres a ton of gain in a dirty Shirley though?
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Yes, a ton of gain.
Also the current Egnater Seminar amps (boutikits) have similar JCM+ topology, with the gain of the second stage maxed out. The Mod Board actually reduces the gain in such amps (same as the Friedman S switch).

Regarding installation of the board, you need to drill two holes for the two switches.
 
lespaul6":3qtbfjkx said:
Looks really simple to install without having to drill... theres a ton of gain in a dirty Shirley though?
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Just to be clear, the board comes with the switches attached to the PCB, and you would need to layout and drill the chassis for the 2 switches at that spacing. I couldn't find a spot on a panel anywhere where I could get the correct hole spacing, so I removed the switches from the board, drilled the chassis where I had room for the switches, and mounted the board on a stand-off inside the amp.
If you have room to panel mount the board/switch assembly, then yes, the install is easy.

No idea about a Dirty Shirley, but the structure section of the mod board, installed in a 2204 style circuit, adds a ton of gain @ V1b by dropping the un-bypassed 10k cathode resistor to 2.7k or 2.7k bypassed with a .680uf cap, depending on the position of the switch. Very cool, but the amp will most likely oscillate if you don't do a few other tweeks.

The Mod Board actually reduces the gain in such amps (same as the Friedman S switch).
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Yes, the Friedman amps before the "S" switch update had 2.7k bypassed with a .680uf cap as the standard value on the second gain stage, so in that case you are in fact reducing the gain there. Those type of amps have a slightly "modified" 2204 circuit ( extra voltage divider, gain pot placement etc.. ) that makes them stable with 2.7k / .680uf on the second stage.
 
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