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I'd shoot off an email to Mesa Boogie support while waiting for responses here. They did bother to forward one of my earlier requests to the technicians, and I got a good answer from them. They might be able to confirm that your output is too weak. Since you seem very competent with tube amps...

Hi J.J, I agree with your calculation in the non-clipping case, but in the "almost square wave" I think the calculation should be a bit different. In both cases, one can use Vrms in the power formula P = Vrms^2/R, but in the square wave case, Vrms is the same as Vpeak, i.e. for a square wave...

Strange, I have always just had a red light on my Express 5:25. It does not change when I flip the standby switch. The only yellow light on mine is the Class A 5W indicated LED. Torquil

Let me wrap this thread up. I already have a simple footswitchable passive volume control unit that I can put in the FX loop. So I'll use that instead of modding the amp. Instead I would like to modify the solo boost function in my AX84 amp. Compared to the Express, that amp is a pleasure to...

Yeah, there are several possible reasons for that. First of all, keeping all the knobs at 9 o'clock won't give the same frequency response as keeping them all at 3 o'clock. Also, the tone stack might be located between a different pair of gain stages than the gain control (I'm not sure). In...

Don't be afraid of turning those knobs in all directions to find what is best. This depends on playing style, guitar, tubes, cabinet, speaker and room, and other instruments in the mix. Torquil

Conclusions: I now know how to make a switchable solo bost. For both channels, the audio signal goes through the 475kOhm resistor close to the master volumes. So for me this is a good position to put a solo boost "global master" volume. Of course, one could have two independent master volumes...

I have bypassed the contour and loop. It works fine apart from a lack of volume. So there is some attenuation going on somewhere. I think the master volume pots are only 100kOhm and that may contribute, and/or it is in front of the phase splitter. I'm hoping it is mostly the last one, so that I...

Ok, I think I know where the "bypass return" signal should return to the circuit, before the phase splitter. It's at the rectangular plate coupling cap of V3B, called C31 and it is next to the two 10uF vertical capacitors near V3. The bypassed signal should return to the circuit here, on the...

Woah, wait a minute.... I meant 41cm. The thing is, my measuring stick starts at 40cm because it is broken. I forgot to subtract 40 on the result... Of course it is not 81cm, that's nearly a whole meter (ie. almost a yard). :lol: Torquil

That wasn't a picture of my own 5:25 (although it looks the same), but just a picture I found on the web. In my 5:25, the long circuit board is 81cm. I don't know what the difference will be between the 5:50 and 5:25, though. I hadn't even heard of Tracy McGrady before. Basketball isn't...

Wow, I don't like PCB! Not only does it make it more difficult to modify, but it's also difficult to understand the circuit topology... Per chance, does anyone have a schematic of the preamp? Torquil

Hi! I'm thinking about making a solo boost function on my Express 5:25, and and FX loop switch. So I would like some opinions on my thoughts about this. Obviously the solo boost should be remotely switchable (I will use a 12V relay), but I'll settle for a simple on/off switch on the FX loop for...

How about this picture of the 5:25: Torquil

Yes.

I found Lonestar schematics, but not with a class A switch on them. Maybe that is a newer version. Measured DC resistanses from plate to OT centre tap: 30W mode: Tube L: 222 Ohm Tube R: 213 Ohm 5W mode: Tube L: 2.9kOhm Tube R: 213Ohm This must be measured in while power is on, since the 5W...

Very interesting results. So one tube doesn't heat up as the amp is taken out of standby, in 5W mode. On the other hand, my measurements indicate that plate and screen voltages are turned on for this tube. I found the cathode current to be around 40ma for the actuve tube (in my case). After...

The tube that actually amplifies the signal is cathode biased through the 120ohm resistor located on the circuit board that has the speaker outputs. The other tube which balances (at least partially) the current through the OT, is still fixed bias with the cathode at 0V and the grid at around...

I wouldn't place a bet that it is possible to prove that humans cannot create something more advanced than their own brains. Already the human race have made intellectual achievements that are out of reach of any one person's intellect. The entire structure of mathematics and physics is more...

I think I made an error on the plate voltage of the left tube on the last measurement. It is 280V, not 360V. Also I didn't measure the cathode voltage accurately enough: 5W mode: Tube L Tube R Cathode: 4.8 0 Control Grid: 0 -11 Screen Grid...