Fender 75 effects loop

[pdf64]

But at least it stops the loop driver circuit clipping?
Try a trimmer instead of the fixed resistor, increase the level until clipping is noticed, then back it off a bit.

[Rocket Roll]

Yes it does, but the signal is more than 6dB lower and I'm not really sure how usable is the amp in live settings with that kind of limitation. Besides, Klein-ulator should be able to overdrive the PI when Recovery pot is set high, and with that input configuration it cannot do that.

So I've returned my Klein-ulator to regular schematic and got this:

(Strat -> Fender 75 head, low power, lead channel, treble and bass pulled -> Celestion G12M Greenback 1x12'' cabinet. The amp is "tired", it waits for new tubes and new caps.)

At least in my room, it works more or less as it should. Now, I don't know what will happen on stage/gig levels, but the sound is useable, and if you keep the Send and Recovery pots low, you don't really notice any added gain, just what you should expect from the Klein-ulator, as I understand it - more compression, darker tone.

(Nota bene, I'm not the person to notice any unwanted gain. I just try to work with what I have. All gain sounds more or les ***tty to me, it's just a matter of finding something you can work with.)

Maybe someone could test Klein-ulator with Fender 75 so that we have more documented experiences. Maybe even a test rig is in order, so that it could be measured how much gain it adds. But so far, it works, nothing explodes...

[Rocket Roll]

Here's Klein-ulator and Fender 75 on clean channel, it sheds some light on how much distortion does Klein-ulator add:

For the whole time, volume on clean channel is on 10. Because the master is on 3, there's no overdrive when you play it like that, even with humbuckers. As you can see, Recovery pot does add some very, very slight drive - and that's enough - but you must really push it. Bear in mind, I used 250K C-taper pots because that's what I had.

You cannot really "fry" the effects with Send pot. Maybe that's because that Boss CE-2 clone has Burr Brown opamps and that Behringer delay has good headroom. I've read in the Klein-ulator manual that you must be careful with the master volume if you use your Dumble with it, but I don't know whether the same applies for the Fender 75. On Fender 75, master volume comes after the send/return loop. (I'm really not all that knowledgeable in these matters.)

YMMV, as I said earlier, this Fender 75 head has original preamp tubes and original caps, it buzzes a lot (hopefully it's because of electrolytics), and it also has export PT and I'm running it on 240, so that the B+ voltages do not shoot through the roof.

[Rocket Roll]

Update on the overdriving the Klein-ulator: I had some time to do more testing, and there's definitely added drive in Fender 75 "lead" mode. And that drive is there, even if I return it to pdf64's recommended values - 100K in series with input, 4K7 to ground. Even with Klein-ulator turned down. You have basically two options - drive or whisper quiet.

In "clean" mode, there's volume boost but no added drive.

So, it has to be something to do with gain stages, V2B and V4B, but what exactly?

[pdf64]

It would need to go on the bench with a sig gen, and scope along the signal path to see what was going on.
Most likely it's just that the signal level there is much higher in the lead mode.

Regarding being careful with the master volume, yes, the loop here is before the master volume, so the loop gets slammed with a rather high signal level.
Hence my suggestion with the potential divider on the input. It should avoid overdriving the cathode follower 'send' stage.

[Rocket Roll]

Thanks for the help, pdf64. I do have a scope and signal generator and all other neccessary stuff like 8 ohm dummy load, I just don't quite know what to measure.

(I told my mom that I wanted to go to electrotechnical school, but she insisted I go to high school. Some thirty years later, I know all there is to know about Shakespeare and still very little about really important stuff, like cathode followers.)

A couple of thoughts, before opening the amp (I'm waiting for new caps):
- Parallel loop? Adding 100K between tip and ring of effects send/return jack? (See one of the earlier posts, where shorting "clears up" the amp, probably because of the impedance.)
- Replacing V4 with ECC82? It has the lowest mu. Because V4A -> effects loop -> V4B, it could reduce gain?

[pdf64]

Rather than attempting to 'polish a turd', why not just move the insert point for the loop to after the master volume?
It would solve all these issues, allowing your loop box to work properly.
There wouldn't be any obvious mod to the amp, I suspect most players would never notice.

[Rocket Roll]

At this point I would agree that the most efficient solution is to move the effects loop.

However, let me leave here the final update on Klein-ulator/Fender 75 situation:

If you build a Klein-ulator and place 58K in series with input, and change the first resistor to ground on the layout up there (lowest leftest part of PCB) to 4K7, you will hit the balance as far as this circuit is concerned.

You will lose some volume (maybe 3dB) and you will have a slight pleasant drive, and all the pots should be turned up to the max, but it will work and be very usable.

By the calculator here, you should, at that point, have 24dB attenuation. (It's a very rough calculation.) Maybe that can be useful for some other projects.

Thanks again, pdf64, for advice and suggestions!