JCM900 fx loop treble loss

[g1]

I don't know if this will help narrow it down, but you could try with guitar straight into the pedal, and pedal output to the FX return.

[lowell]

I have my Seymour Duncan DejaVu delay in the loop now. I hear zero difference with it on/off. It sounds great. I hear the issue, as I did before when I had this pedal in the loop, if the fx loop level pot is more counter clockwise. But I can adjust that pot to find the happy spot. I have the bright cap removed. I did not replace the 10uf coupling cap or add the parallel tantalum.

So... more evidence that it's his pedal or very possibly one of his loop cables.

Calling him back in with only his pedalboard. And one other head that "doesn't have the issue."

I seem to recall making some cables years ago. They sounded like crap. Turned out the thin film shield had melted when I soldered the plugs on...and caused a minor short or capacitance issue. The high end was GONE. When I cleaned up my work and resoldered it... the cable sounded much better. This sounds like THAT. So I'm gonna swap cables around as well as take some meter readings on em.

[Chuck H]

I have my Seymour Duncan DejaVu delay in the loop now. I hear zero difference with it on/off. It sounds great. I hear the issue, as I did before when I had this pedal in the loop, if the fx loop level pot is more counter clockwise. But I can adjust that pot to find the happy spot. I have the bright cap removed. I did not replace the 10uf coupling cap or add the parallel tantalum.

So... more evidence that it's his pedal or very possibly one of his loop cables.

Calling him back in with only his pedalboard. And one other head that "doesn't have the issue."

I seem to recall making some cables years ago. They sounded like crap. Turned out the thin film shield had melted when I soldered the plugs on...and caused a minor short or capacitance issue. The high end was GONE. When I cleaned up my work and resoldered it... the cable sounded much better. This sounds like THAT. So I'm gonna swap cables around as well as take some meter readings on em.

I think it is as the Dude pointed out. The delay pedals have a "happy place" with respect to input signal level and the circuit and there is detriment to the tone (through the effect, not the amp) outside of that range. I hinted at it in post #5 and Dude mentions it in post #12. I think this is the devilish detail and the amp is doing everything right.

So the only question remaining is whether your guy can run ALL the effects he wants to in the loop with the same level adjustment being ideal for all of them? You will surely need to explain the circumstances and help with this effort.

[Chuck H]

Hoping this catches lowell's attention... I'm interested in how this is working out.

[J M Fahey]

Sorry got late into this, but might add something:

1) checked the loop schematic: it´s ruler flat, nothing boosting or cutting anything.

2) that said, impedances involved are quite low.
Fine when sending, may be a problem when receiving.

3) as Enzo noted, input impedance is very low (as guitar signals go) 13k (pot + 3k3 resistor) in parallel with 47k (Op Amp input is virtual ground so zero ohms) , so can we agree on effective about 10/11k as seen by the pedal?

4) most modern pedals should have no problem driving that, but some vintage ones and the odd modern one might.

Which is clearly the case of the Carbon Copy.

Dunno about a modern pedal being issued with a troublesome output circuit and Factory not correcting it by adding an almost free buffer (1 Op Amp section so 1/2 TL072 or 1/4 TL074 or a humble single transistor) is beyond me, but that seems to be the case.

Although not strictly an "Amp defect" you can correct that by adding a high impedance input buffer at the return loop jack.

Absolutely simplest would comprise a FET (think MPF102 type, currently some J11x N Channel type or similar), 2 resistors and 1 capacitor, so you build it without a PCB.

Here´s the schematic.
Mouser has J112 Fets for 40 cents each, I´d buy 10 and keep the rest for future use.



Carbon Copy is happy driving amp 1M input impedance, this provides the same.

It can be improved, I´d add a 47k in series with FET input, maybe a couple protection Zeners or Leds, maybe 100pF in parallel with input 1M to keep RF out, etc. , but that would require a daughterboard PCB design, probably not practical here.

EDIT: and since Carbon Copy was unable to drive 10k, it will even less drive a bright cap, which by definition presents an even lower impedance load, since it´s bypassing part or all of the 10k pot.

To make things clear: the crappy design Prize lies fully on CC´s shoulders, not Marshall´s.

[lowell]

Guys thanks for the input. He came and grabbed the amp again. I was waiting for him to bring in the OTHER "working " one to run the aforementioned tests and get some clear data. Ah well. If I get the chance I will try again.

[Chuck H]

Couldn't the existing tl072 circuit be modified for a higher input impedance? If what's coming from the send is low z and what's coming from the effects are low z (as it should be) I don't see a disadvantage to having a higher input impedance for the return amp input.?.

[J M Fahey]

It´s not as straightforward.
BIG problem is that damned dual 10k level pot which by itself sets send and receive impedance.
Standard dual pots are same resistance both halves , which unfortunately led to the poor halfway choice of 10k send, haigher than desirable, and 10k receive, way lower than desirable.

A 2k send and 1M return combination would be Heaven ... but a supplier nightmare, so the chose an acceptable value, leaving the buffering problem entirely on the Pedals shoulders.
Statistically a good choice, most pedals work fine there.

Another problem is they chose an inverting Op Amp gain stage, which has relatively low input impedance.

What I designed and suggested above is the absolute cheapest simplest solution, there are better ones of course but have a different price(in $ or in time/ complication which also translates to $ at the end).

Plan B: remove the damn pot which nobody sets properly or at all anyway, leave loop unity gain like God ordered in the 11th Commandment and then we can talk.

IF acceptable, I can suggest some simple changes to current circuits.

[lowell]

HE still SWEARS this wasn't happening before . So it is rather odd. And I confirmed firsthand that it doesn't happen with his other identical amp. Very odd. If he comes back I'll run the tests and if need be I'll add the Fahey mod.

[g1]

If it doesn't do it on the other of the same model, one of them must have a defect. It's not impossible that the one thought to be 'good' is actually defective. I'd suggest your repair rather than modify. I think if you could get them both on your bench it should not be too hard to find the difference.

[lowell]

Agreed. Hope to be able to report back.

[Chuck H]

It wouldn't surprise me to find out that, same model and the known schematic not withstanding, the loop circuit is different in one of the amps.

[J M Fahey]

The big culprit there is the 10k level pot, fed straight from the return jack, nothing you do to the Op Amp circuit will alter that.

Of course the "non tone sucking" amp might have a broken/open/disconnected/solder_cracked pot so input impedance is 47k or 200k something.