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Messages - willpirkle

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Thanks! I actually had that batch, but noticed the head version was not in there, so I thought I'd ask to see if there was another source. Mine dates to 1985 according to the serial number. Gotta say that through my Marshall 1960, it sounds great. Even the distortion on this model is reasonable (not awesome, but also not horrific like the 120 twins I've tried over the years) and the preamp/distortion are just so simple in the head version.

I need to fix the reverb on this, which is only barely audible with the control on 10, so will open it up this weekend after verifying it isn't the footswitch jack. Thanks for all your help and input on everything. Guaranteed to learn something new and interesting on each visit here.


Its a Roland JC-120H, not a Marshall  :)

Schematics and Layouts / JC-120H Expanded schematics/service manua
« on: May 27, 2021, 08:22:54 AM »
I’ve scored a JC-120H (head) and I have the ‘84 Schematic as it is called. It is only one page and I’m looking for the rest of it, including disassembly instructions, etc... is there more of the ‘84 schematic than just the one page? Like a more complete service manual?



Dang!!!!!! Thanks man. Wow - appreciate the heads up. Hopefully, they will price it substantially less than the used ones on Reverb ($250-$300), that kinda reminds me of the TS-9 inflation around 1990 — till they reissued it with the JRC op amp. This is killer.

Wish they’d re-release the Heavy Metal pedal. I left mine in a rehearsal warehouse in 1988. That box in front of my Princeton was amazing (still have the silverface). They are stupid expensive on reverb.

Amplifier Discussion / Re: FET Self Biasing Method
« on: May 15, 2021, 08:46:18 AM »
I buy J113’s a hundred at a time from Mouser ($0.13 a piece) and use a FET tester I built (the simple one from runoffgroove) then spend a rainy Saturday sorting them. It takes less time than you’d think (and I live in South Florida so we have many rainy days...). The J113 works great for Danyuk’s circuit.

Amplifier Discussion / Re: Output transformer for Class D Amp?
« on: May 05, 2021, 08:52:18 PM »
Agree with JPB - will likely cause problems and no warmth other than a (potentially) burning ICE board. I’d save that transformer for another project!

Preamps and Effects / Re: Fender STC1 Chorus Pedal
« on: April 23, 2021, 09:05:23 AM »
Here's a freebie chapter from an analog electronics book I was working on back around 1997 or so. I do have the copyright on this (the actual piece of paper from the gov't), but am letting it go here because you guys rock!

This is missing a key ingredient for BBD circuits - the compander (NE571) that limits dynamic range into the BBD and then expands it coming out.

I've been pondering the idea of doing a 3-part Udemy class on analog audio electronics... one of these days.


Schematics and Layouts / Re: Roland JC-40 Schematic
« on: April 12, 2021, 07:44:40 PM »
Thanks! This is great - I didn’t know it was a digital modeling amp.

Schematics and Layouts / Roland JC-40 Schematic
« on: April 11, 2021, 05:32:10 PM »
Anyone got the schematic for the new(er) JC-40 with 2x10” speakers? I am aware of the awesome JC-120 ultimate thread on this forum and also found the JC-50 schematics here as well. But now specifically seeking the JC-40.



Schematics and Layouts / Re: Gallien Krueger 100 MPL
« on: March 08, 2021, 08:06:55 AM »
This is the complete service manual, scanned from my paper version that I got from Bob Gallien back in the 90's -- it has all of the gory details including the digital programming. It is almost 80MB in length and won't fit as an upload, so I added a link from my website below. I have no plan to remove the link, but an admin may want to store it in a more permanent manner on an SSGA server for future use.

This is a digitally controlled analog product; e.g. it uses vactrols for channel switching and other routing. It is also a treasure-trove of interesting circuits (log amp for the compressor, use of the LMC835 to implement the GK active EQ filters (in addition to the graphic EQ), noise reduction ckt, even an R/2R DAC).

The schematic pages were repeated within the documents, so there are two sets of them (didn't have time to go back and remove them).



Amplifier Discussion / Re: Power amplifier capacitor in NFB question
« on: February 28, 2021, 12:01:55 PM »
This is a mixed mode amp (voltage and current feedback). Neglecting the current feedback branch (which changes the power delivered to the speaker), the gain of the amp in this configuration is 1 + (R7/(R6 + ZC4)) where ZC4 is the impedance of C4 at a given frequency. At DC, the impedance of C4 is infinite. This causes the (R7/(R6 + ZC4) a term to become 0.0. Then, the DC gain of the amp is unity. As the frequency increases above 0Hz, the impedance of the cap drops, until it is an effective short. Then, the gain of the amp is 1+(R7/R6). Between DC and the short-circuit frequency, the gain rises from unity to 1 +(R7/R6), forming a low frequency shelving filter.

Rod Elliott has a nice discussion of current feedback (aka “constant current amplifier”) on that site as well. And, this kind of mixed mode feedback is generally limited to guitar and bass amp applications. And, massive unending arguments among audiophiles.

The C4 cap being electrolytic (and polarized) is non-consequential to the gain equation, and does not limit the voltage swing.


Amplifier Discussion / Re: Rocktron Rampage R80 - No Sound problem
« on: February 10, 2021, 05:33:36 PM »
dmeek — now *that* is awesome. I want one!

Amplifier Discussion / Re: Roland JC-120 crossover distortion
« on: February 02, 2021, 09:43:51 AM »
I only have this one:

It looks like yours would be in the next set, but if the power amps are similar to these (which I think will be the case) then the bias is supplied with the Vbe multipliers Q9/Q10 and Q26/Q27 (first schematic, p7) which aren’t adjustable.

Teemuk’s book on SS amps (free, links at this site) is a great resource for explaining some of the more esoteric designs that are guitar/bass amp specific. It also includes a few variations on the Vbe multiplier. If you already know the tube theory, then his book will be easy to get into. Highly recommended.


Amplifier Discussion / Re: Simple 3W amp IC? (for 4ohm speaker)
« on: January 21, 2021, 07:19:54 AM »
I’m using this TDA1517 stereo amp kit in one of my audio electronics classes. I built and tested it over Christmas and it is stable at 4 ohms. For $12 (Jameco, $27 on Amazon) it might be worth a try for your application. The output power will depend on the supply voltage and current capabilities.

- Will

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