Tube guitar amp: tone controls disable when effects loop is engaged???,

[Electarded]

I'm very new to tube amps and brand new to the forum. Looking for second and third opinions regarding the non functioning tone controls when my effects loop is engaged.
I have a Crate USA V3112. Whenever I plug a pedal or my Fender Satellite SFX powered cabinet into the effects loop all the tone controls become disabled. They function fine when any pedal is in front of the amp but the Fender Satellite requires the loop be used to add the CRate amp. Once the amp or any pedals are attached, the Crate loses tone controls???
I had taken it into a Crate repair shop under warranty and all they told me was that it was normal for this amp??? That sounded like an easy way out of any responsibility on their part and it just doesn't make any sense to me at all. That can't be right...can it??? I have checked and double checked the connections, everything is in order and everything works as it should, other than having NO tone controls at all when the effects loop is engaged.
Any knowledge of such things would be greatly appreciated. HELP!
Thank you.

 

[Nigel Goodwin]

Presumably it's designed to be that way, with the internal tone controls bypassed so you can use solely external conditioning in the effects loop. It's not really something you could have by accident though.

Do you have a circuit diagram for the amp?, it would be simple to tell if you do.

However, the block diagram in the instruction manual doesn't show the controls as bypassed.

https://www.electro-tech-online.com/custompdfs/2012/01/V3112_OM.pdf

 

[Nigel Goodwin]

OK, I've done some more googling - here's the circuit.

From that it's obvious that the tone controls aren't disabled when using the effects loop, and can't possibly be - both sets of tone controls come well before the effects loop.

 

[cowboybob]

Just going by the test point data sheet for the signals at TP2, apparently when the "Dirty" channel is selected (by plugging in effects devices and/or a foot switch, or switching channels from "clean" to "dirty", or all, didn't look all that close) the output of V1b goes to null. Since the tone controls are at the front end (grid input) side of V1b, it would appear that the tone controls (as part of V1b's circuit) are electrically isolated from the signal path.

All the various signal enhancement circuits have their output(s) to the driver circuit controlled by the VTL5C3 optocontrollers, which explains the null signal at the output of V1b (and several others as well) on certain conditions

Which means that the block diagram is misleading: the tone control block should be in front of V1B between it and the "channel select" switch, not after V2A & B, according to the schematic.

That being the case, the techs are correct in saying there's nothing that can be done, short of modifying that portion of the circuitry.

Could be wrong, but it sure looks like that's what's happening.

 

[Nigel Goodwin]

Just going by the test point data sheet for the signals at TP2, apparently when the "Dirty" channel is selected (by plugging in effects devices and/or a foot switch, or switching channels from "clean" to "dirty", or all, didn't look all that close) the output of V1b goes to null. Since the tone controls are at the front end (grid input) side of V1b, it would appear that the tone controls (as part of V1b's circuit) are electrically isolated from the signal path.

You appear to have missed the fact that the dirty channel has it's own separate tone controls, on the output of V3.

The whole idea is you can have different settings for dirty and clean, it would be pointless having a footswitch and having to turn round and adjust all the controls every time you changed channels.

 

[cowboybob]

You're right.

I need a bigger monitor.

I printed out the schematics to have a better look.

There's gotta be an explanation in there somewhere.

 

[Nigel Goodwin]

You're right.

I need a bigger monitor.

I printed out the schematics to have a better look.

There's gotta be an explanation in there somewhere.

Assuming the schematic is correct?, then the tone controls DO work when external effects are plugged in.

But we don't know exactly what he was trying to do, or what the actual problem was. Perhaps he was just going in at the effects socket in order to just use the power amplifier?.

 

[alec_t]

If the circuit is as in post #3 then no way can the effects loop disable the tone controls. As Nigel says, the tone controls (clean and dirty channels) come before the effects loop. Is it possible the effects are simply swamping any tonal variation?

Perhaps he was just going in at the effects socket in order to just use the power amplifier?

That would certainly explain it

 

[cowboybob]

If the circuit is as in post #3 then no way can the effects loop disable the tone controls.

The "clean" channel and and its tone controls are disabled when the "dirty" channel is being used, but I failed to see that the "dirty" channel also had tone controls.

But, as Nigel pointed out, and I now agree, there's no obvious reason for the "dirty" tone controls to be disabled.

Electarded, any chance you could provide a detailed description of your system's configuration (what's plugged in where and when, control switch's/adjustment's positions and settings detailed, etc)??

Just speaking for myself, that would sure help a bunch.

 

[alec_t]

The "clean" channel and and its tone controls are disabled when the "dirty" channel is being used

Totally agree. Selection of the clean or dirty channel is by a switch which has nothing to do with the switching which occurs when a jack is plugged in to use the effects loop.
The driver stage is a mixer, so independently of whether the effects loop is used or not there is also a signal path from either the clean or dirty channel (hence subject to tone controls) via the reverb unit to the mixer input.

 

[Nigel Goodwin]

Totally agree. Selection of the clean or dirty channel is by a switch which has nothing to do with the switching which occurs when a jack is plugged in to use the effects loop.
The driver stage is a mixer, so independently of whether the effects loop is used or not there is also a signal path from either the clean or dirty channel (hence subject to tone controls) via the reverb unit to the mixer input.

But if you were only using the power amplifier, then there's no input from the reverb anyway - and isn't if the reverb level is turned down either.

 

[cowboybob]

Unless the front end is being by-passed in some fashion by how the external devices are connected.

They function fine when any pedal is in front of the amp

(My emphasis)

I don't understand this line. The only jack on the front is for the guitar.

I guess my question to Electarded is, "where is the guitar plugged in" while this is happening? Where is everything plugged in??

And this from the owner's manual

...boost switches ... are inoperative when a foot switch is connected.

gives me pause. I went blind looking for a "boost" switch in the schematic. And since it affects the mid range tone attenuation, it caught my eye.

I'm going blind now. "See" you later.

 

[alec_t]

@Nigel
Totally agree again. We don't know where the OP plugged their guitar in.
@cowboybob
You've attributed the OP's words to me

 

[cowboybob]

OOPS.

Fixed it.

CBB

 

[unclejed613]

yes, sounds like the OP either plugged his pedals (with the guitar plugged into the pedals) into the effects loop in, or his effects pretty much overpowered the effect of the tone controls (a fuzz box would negate the tone controls almost completely).

the way i learned it "back in the day" is that you put your most drastic effects (like fuzz) first, then your filters and other subtle effects (wah pedal, flanger, tone controls). putting drastic effects after the tone controls washes out the effects of the tone controls.