Mark IIB to IIC+ Conversion

[Matador]

I did a quick SPICE simulation of just the bass control. Assume the LDR is 1 ohm, the maximum boost comes out at 17dB.

Increasing the LDR resistance until I saw a 3dB drop in the boost (e.g., 14dB) resulting in roughly 500 ohms: this makes perfect sense, as when the LDR resistance equals 470ohms, that should be a 3dB gain reduction, all other things being equal.

So if you run the EQ sliders at their maximum, and had an LDR near 500 ohms, you might be able to hear a difference depending on which EQ's were maxed. If you don't run them at max, you might need to raise the fader a bit to get it to match the stock circuit.

(The grey line is 68K, or the EQ 'disabled').

 

[Matador]

The other interesting thing in the simulation is that with modern, medium-voltage, high bandwidth transistors, the op-amp (might) be significantly under-compensated. The stock bandwidth with 2N4403 transistors extended out beyond 1 MHz with the stock 10pF dominate pole compensation capacitor. Increasing this up to 100pF drops the high frequency turn point down to about 200kHz which might stabilize the circuit if repaired/replicated with modern transistors.

In addition, the Q of the filter(s) fall as a function of the pot (slider) travel, with Q narrowing as boost/cut is increased.

I *really* want to try swapping the EQ board with an op-amp gyrator based EQ to see if I can get it close, as the inductors are such a PITA to source.

 

[quad48uF]

It appears another tough part to source are the eq inductors for the iic+?

 

[Jay Omega]

It appears another tough part to source are the eq inductors for the iic+?

Not the case, Boogie sells them.

 

[quad48uF]

Very good

 

[Jay Omega]

The other interesting thing in the simulation is that with modern, medium-voltage, high bandwidth transistors, the op-amp (might) be significantly under-compensated. The stock bandwidth with 2N4403 transistors extended out beyond 1 MHz with the stock 10pF dominate pole compensation capacitor. Increasing this up to 100pF drops the high frequency turn point down to about 200kHz which might stabilize the circuit if repaired/replicated with modern transistors.

In addition, the Q of the filter(s) fall as a function of the pot (slider) travel, with Q narrowing as boost/cut is increased.

I *really* want to try swapping the EQ board with an op-amp gyrator based EQ to see if I can get it close, as the inductors are such a PITA to source.

I've had good luck with op-amp based gyrators in this circuit, they simulated well and sounded fine to my ears. I abandoned gyrators when I found I can buy the inductors right from Boogie's online store.

This dude's C+ amp uses gyrators for the EQ:

 

[Matador]

Yes, it looks like Boogie still sells them: although it is $100 for a set ($15 each, plus $19 in shipping)!

Jay, it looks like you used an original output transformer. Did you give any thought as to which replacement output transformers might work in this circuit if you wanted to build a completely new one?

I've found a few transformer shops in Shenzeng, China, that will wind transformers to custom specs. I ordered a few 60W ones with around 5K primary impedance, with 4, 8, and 16 ohm taps, that work well with the stock Mesa power amp circuit (with ~425V on the plate). My Simul-class output transformer measures about 3K across the outer (non simul-class) windings which seems consistent.

 

[Jay Omega]

Yes, it looks like Boogie still sells them: although it is $100 for a set ($15 each, plus $19 in shipping)!

Jay, it looks like you used an original output transformer. Did you give any thought as to which replacement output transformers might work in this circuit if you wanted to build a completely new one?

I've found a few transformer shops in Shenzeng, China, that will wind transformers to custom specs. I ordered a few 60W ones with around 5K primary impedance, with 4, 8, and 16 ohm taps, that work well with the stock Mesa power amp circuit (with ~425V on the plate). My Simul-class output transformer measures about 3K across the outer (non simul-class) windings which seems consistent.

The inductors are like $4 a piece from Boogie's factory store, where are you looking? Usually I buy other stuff as well when I order so ends up being free shipping.

I've put a lot of thought into output transformers! The OT I used in the 60W build was a Magnetic Components ClassicTone (now defunct) Bassman replacement with 4/8/16 taps. I wouldn't go the custom route unless it saves you cash, I'd recommend a Bassman replacement for 60W and Twin for 100W. For simul just buy from Boogie, they sell those as well.

I'm working towards a four power tube chassis and PCB kit based on the IIC+ with some features ported over from the Mark IV. Chassis is designed to allow five or six available output transformers that I believe should all work great, I'm going to do a shootout to see which sounds the closest to the OT in my IIB.

 

[Matador]

The inductors are like $4 a piece from Boogie's factory store, where are you looking?

You are completely correct: I was looking on the "Tube Amp Doctor" site in the EU, which was charing 13 euros per inductor: I do see that Mesa sells direct for $4 each. My misunderstanding!

https://store.mesaboogie.com/products/inductor-toroidal-1h.html
The custom 60W OPT's I use come out to about $50 each, accounting for sea-shipping, and are fantastic. However the only comparison I can make is against a simul-class 100W, which I almost always run in half power mode, hence looking for a two-tube solution and a smaller/lighter OPT since the 100W MArk 2C+ is such a beast to lug from place to place.

 

[okgb]

Epic thread! Almost makes me want to convert my Mk III , thanks for all the info & observations.

 

[Less02]

Wow! I just bought an early XHRG markiic (from the date and serial number) but unfortunately it has the RP-9C board. Having mylself clone a MKIVa 21 years ago, your project is inspiring me!

here some picture of my build, not as precise as your. At the time, I had in hand only some fraction of the schematic. And I had never seen any gut shot from the real mkiv…. I built my own vactrol using existing LDR using neon. I simply replaced the neon by a LED. Unfortunately, when I drew the component in PCad, I mirrored two of the pins… this has forced me to install all the LDR on the secondary side…. No time to redo the PCB, I was still at school at the time…