The Johnson Loredo T25-R repair and mod thread. (guitar amp)

[rock soderstrom]

Hi guys, I bought a small used tube amp for my bedroom "studio".
It's a Johnson Loredo T25-R with supposedly (unrealistic) 25 watts, solid state driven spring reverb and an adjustable FX loop.

These amps are not rare to find used in Germany and are offered between 50-200€.

Mine cost 50€, the speaker had already been replaced by a 10" Jensen P10r, all the tubes too. Now a relatively new set of JJs and EHX tubes is installed. The front cloth was a bit worn , but otherwise the amp is visually in good condition.



My first sound check wasn't that great, there are some things that are annoying. The amp has some fundamental problems.

1. The amp does not sound good, it is strangely quiet. The sound is thin and treble-heavy and somehow choked.

2. there is a strong hum, the thing hisses and sizzles excessively loud and is no fun at short distance when you are sitting directly in front of it. This could be better...

3. For my taste, the reverb sounds weak, the spring is not driven properly.

4. the gain poti has a loose contact, some of the other pots scratch. This is easily solvable.

5. What is the function of C41? How can this work?

So there is a lot to do, let's go.

Here is the schematic for the amplifier:



It's basically an amp similar to the Fender Princeton Reverb, if you ignore the solid state stuff, the master volume and the different power tubes. That's a good cousin, if the amp sounds anywhere near as good after the repairs and mods, I'll have achieved my goal.

My first action was the annoying hum problem. As you can see, 100Hz is dominant, which indicates problems with the filtering in the PSU.



Cheap China amps often also have problems with direct magnetic interference of the power transformer in the output transformer. But that would be 50Hz, which is not the case here. Nevertheless, I changed the position of the output transformer as a test, but there was no improvement.



I could measure almost 1V AC at the anode resistor R3 of the first gain stage, no wonder the amp hums. The PSU is faulty, the resistors are too small. I took my cue from the Princton and increased R51 to 1K and R52/R53 to 18K. At the same time, I increased C29 to 100uf as a test. Now the hum is very reduced, ok for me. Success.

The question remains: what is the function of C41? I have not seen anything like it before.



Since I had already removed the PCB (which is not bad, can be soldered well. The mounting screws are one-way and had to be partially drilled out..), I wanted to take care of the reverb at the same time.

My idea was to push more signal into the op amp driver to achieve more reverb. That's why I increased R10 from 68K to 150K. Seems to deliver more reverb so far, sounds better. What do you think? I will also try another spring. I like it surfy...

The general sound problem was my next stop. There was already information about this on the web, which I used as a guide.

I increased the bass cap C3 in the tone stack from 22nF to 100nF and modified the mid pot as described in the link. Definitely better but the amp still sounds too thin and strangled, something is slowing down the sound.

My next change was to downsize R32 from 330k to 68k to feed the power amp more. This brings a lot, the amp is now louder, but somehow I'm still not completely satisfied.

I'm not sure if this part of the circuit (master level) doesn't suck the right sound out of the amp. I don't know much about such things and have little experience. If in doubt, I'll throw it all out and convert it to a Princeton, although a master level would have advantages in my application.

Abbey pointed out in another thread that this was poorly designed, I don't want to disagree with him. I have also deactivated the NFB at the moment, the change was small. I will reinstall this on a trial basis and remove C23 for it. Let's see what that brings.

Don't get me wrong, in essence the amp sounds much better now than before, absolutely playable, maybe with an optimised master level could get a bit more out of it? Maybe the Jensen P10r is the wrong speaker, I will try a C10q which might fit better.

What would you do, how to do it properly?

The last problem, which is really annoying, is the hissing and sizzling. That's still there and bothers me a bit. It's not super loud but it's noticeable and annoying.

It's not the tubes, I've replaced them all several times. Abbey suspects in the other thread that it also has to do with the circuit at the beginning of the power amp stage.

What do you think?

Edit:

Some impressions

OPT, left Loredo T25, right Fender Blues jr.






PS: This is the reason for the supposed loose contact of the gain pot! This piece of wire grounds the preamp to the housing, which does not work so well. Stupid!

 

[Khron]

C41 - some weird sort of "current limiter", artificial "sag"? Could try increasing its value, or jumpering it out altogether. That is, assuming it's connected in the physical circuit, as it is drawn in the schematic. Standby switch is also missing from the schematic?

With dropping the value of R32, you could consider a larger value master volume pot - the 110k-ish total you have now may well still be loading down the output of V2A.

 

[rock soderstrom]

C41 - some weird sort of "current limiter", artificial "sag"? Could try increasing its value, or jumpering it out altogether. That is, assuming it's connected in the physical circuit, as it is drawn in the schematic.

Strange thing, but is actually installed the way as drawn. I will kick it out.
C41 is a anti pop cap for the standby switch!


Yeah, stand-by switch was forgotten.

 

[Khron]

I now have my doubts. It seems to be across / in parallel with the standby switch.



CT = center tap of output transformer. Just following the PCB traces visible through the board...

 

[rock soderstrom]

You are almost right. C41 is in parallel with the standby switch in this way. This makes decisively more sense, because otherwise nothing would work! C41 is a anti pop cap for the standby switch!

Edit: here the real situation

 

[Khron]

Can you snap a photo of the back of the board, in that area?

The side of the switch that does NOT connect to C29, clearly goes directly to R51, doesn't it???

 

[rock soderstrom]

I have tested it with my DMM. In reality it looks like this. The standby switch (and the parallel cap) is in front of R51 and the OT center tap but after(!) C29.

 

[Khron]

Whew, thanks for confirming i wasn't (THAT) blind or insane then

 

[murrayatuptown]

The problem...well, one of them..with drawing errors is that we have a natural and reasonable tendency to trust them, initially. After all, having a drawing is much better than none...or should be.

I have to remind myself occasionally that a schematic doesn't have component failures 'pre-drawn', when troubleshooting! The drawing doesn't match what's on the bench at those times and I hope I can trust it moving forward.

Murray

 

[abbey road d enfer]

If this schematic is actually faithful to the amp (standby switch excepted), it shows that the designer has not a great command of electronics.
He put a feedback resistor from the OT secondary to the PA input tube (V2a), but included the master volume in the Open Loop, which kinda defeats its action. However, he put a capacitor across the cathode reistor, which completely defeats NFB, which makes it very sensitive to noise.
I would suggest you remove capacitor C23. There would be a loss of gain, which could be compensated by decreasing R32. The taper of teh master volume would also be altered. I don't think it would be a major issue.