Recent content by Matador

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...

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...

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...

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...

Can take these on a case-by-case basis: 1) LDR1 - shorts across a 10M, and even hundreds of ohms is small compared to the follow-on resistance of the treble pot which is 250k. 2) LDR2 and LDR3 engage the lead channel - LDR2 is in series with 680K, and LDR3 terminates into a 680K grid resistor...

I have used the NSL-32 from Advanced Photonix (previously Luna Optoelectronics) successfully in the circuit with stock values. The SR2 variant has a maximum on resistance of 40 ohms, however all of the samples I tested were below 10 ohms (@15mA forward current). The maximum off resistance is...

Stunning work! From what I have gleaned between schematics and looking at exactly one ++ mod in person, the mod is: 1) A 3.3M + 20pF cap (in series) are added parallel to the existing 3.3M + 20pF cap (that are in parallel) that bridges across the lead channel gain stages. The tonal change by...

More progress! Got the boards stuffed (minus a handful of components I'm waiting to arrive from being backordered). Now that I can mount all of the PCB's in place I can get an idea of how to route the wiring. But everything is in place, and everything fits properly. I substituted LED's in...

If signal follows audio 0V, then loops shouldn't be possible. This is the ideal arrangement, with only two connections to the chassis: one, at the safety ground to the power cord, and another, to bridge analog 0V with chassis right at the input jack.

Here's all the PCB's installed in the amp chassis:

Finally the preamp board. This has a bit different layout, however those familiar with the original RP11A will feel at home where things are placed. Obviously all of the channel switching stuff is missing. No direct socket soldering just like the power amp board. I had to take some poetic...

Next is EQ PCB. Not so many changes here. Switched transistors to TO-92 ECB layouts, using MPSA06 and MPSA56 NPN and PNP transistors, rated at 80V. Beta is similar to original MPSA70 / MPSA20 transistors. Axial caps were switched to radials. The dominant-pole compensation capacitors were...

Next is the power amp PCB. Since I had the original PWR-7C board de-populated I could follow close(r) the stock component positions, except with (of course!) my own improvements: NO DIRECT SOCKET SOLDERING TO THE BOARD. This is the most common failure mode on these amps, which is either a) a...

I finally made a bit more progress. Someone offered to purchase the amp from me provided I could restore it as original as possible. So my tact has changed somewhat, given I still want to fold in everything I've learned repairing these amps for the past 20 years. Here is the new PSU board...

Is it true that you only get the noise when you are in lead mode? I inferred that was the situation based on your question but I realize you never said so explicitly...