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BYPASSING A GAIN STAGE - QUERY
#1
Hi Ultimate Tonesters,

I have had to shelve amp work for most of this year, due to other pressing issues, but I am now getting back into it.
The TUT Forum is looking a bit quiet; where are you all?

I have a query regarding bypassing a Gain Stage, which is a bit foreign to how I would arrange a Drive stage, so this is not something I have tried before.

The attached circuit shows a 4 stage preamp that uses 3 for Clean and adds the 2nd stage (V2B) for Drive.
The cct. is shown in Drive mode and when switched to Clean, Relay 1B enables a full bypass of the 2nd stage. 

In Clean mode; would you advise grounding the output from V2B, as shown with Relay 2, to remove any possible signal or noise from V2B entering the grid of V1B; or will the Clean Bypass be sufficient to ensure uncorrupted Clean sound?
Modifying the PCB traces and installing wiring to enable Relay 2 to ground V2B output is not a fun job but, if it is advisable, I will do it rather than have the clean sound compromised.
The owner loves the Clean sound and doesn't want to lose it.

Thanks for any advice,
Noel

   
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#2
Hi Noel

You could simply use a single pole to select the output of stage-1 or stage-2 to feed int stage-3. It is best not to have the output of stage-2 tied to the input of stage-3 all the time if stage-2 is supposed to be optional.

As a refinement of course, when the output of stage-1 is selected, it would in fact be the output of the Clean volume control that is selected wired as a standard level control.

An alternative is to use all shunt switching and re-arrange the attenuations so that the shunt effect contributes a part of it. The Clean volume would be fed by a resistance from the stage-1 coupling cap in parallel with the feed to the Drive control via its resistor. The output of the Clean control would feed to stage-3's grid via a series resistance as would the output of stage-2. The shunt points would be the input or output of the Clean control, the input to the Drive control and the output of stage-2. A DPDT would be needed

Have fun
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#3
Thanks for the input Kevin.

Unfortunately I am tied into using what is available in this amp to do the switching.
It is another of those horrible flexi Fender PCBs and I am stuck with the 2 relays fitted and all those suspect traces. 
I am hoping that I can achieve a good result by using what is available, with a little bit of modification.

I have been told by another Tech, who had done this job to a few of these amps, that he just utilised the Bypass via Relay 1B - shorting the top of the Volume pot to the grid of V1B - and it worked fine for him. I was just not sure that having the output from V2B also connected to the grid, was not going to inject some noise, despite the whole stage 2 being shorted out via the Bypass.

Relay 2 is a DPDT and one pole's function is switching ground to the Master Volume in Drive mode, with the other contact having no connection. The contact, shown with NC in my drawing, is the one that is actually connected to the Master Volume. 
This leaves the possibility of using that unused contact, when the relay switches to Clean, to place ground on to the juction of the 100k & 1M resistors. This will ground the V2B output via the 100k and the 1M will be grounded and then be in parallel with the 510k, as part of Rg of V1B.
What is your opinion of this approach?

Cheers, Noel
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#4
Well, I just had a 'Light Bulb Moment' - thanks for flicking the switch Kevin.

I read your post again and looked at the schematic and the answer was there all along.
Relay 1B has contact #11 unused, which provides the means to select the output of Stage 1 or Stage 2.
#9 is already connected to Stage 1 and #13 is the input of Stage 3. 
I will connect the output of Stage 2 to #11; which will be a bit difficult, but it is quite doable and that should solve that.

The attached, revised drawing shows cct.

Thanks again, 
Noel

   
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#5
Hi Noel

That will work better.

I would still be inclined to slash some traces unless gaining access to them is seriously difficult. I would change the connection of RLY1A so the wiper ties to ground, one contact to the top of the Drive control. Then wire the Clean volume in the usual way and connect its wiper to the pin-9 of RLY1b.

Although you say the relay is shown in the Drive position, is that the default? I would expect it to default to the clean sound. In any case, drawing the default position (nonenergised) allows one to refer to the contacts as C, NC and NO.
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#6
Hi Kevin,

The relays switch; with one on while the other is off.
Therefore: Relay 1 switches ON for Clean and Relay 2 switches OFF for clean.
Fender shows the schematic in Drive mode because that is how the voltages are to be measured.
"All voltages measured with respect to ground - Footswitch connected. Drive Ch. Sel."

I normally wire my amps your way. It is just a bit of a chore, with these Fender PCBs.
I will have a look at the degree of difficulty of cutting traces and wiring how you have suggested. 
If it is too difficult to warrant the time involved, will the changes in my Post #4 suffice without detrimental effects?

I have shown the new changes to the cct. in RED.

Thanks again for your advice.

   
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#7
Hi Noel

Yes, the post-4 circuit will work fine. The only concern as you know is whether the signal from stage-2 will be large enough to bleed through, which is why it is desirable to mute either its input or output ( input-muting is preferred).

Have fun
Reply
#8
Hi Kevin,

You are right, of course and I don't want to take the chance on any bleed through.
I will persevere with muting the Drive Input also.
The revised wiring and switching as per Post #6 is the best option then.

Thank you,
Noel
Reply
#9
Hi Kevin,

I hope you don't mind me imposing on your expertise again.

I have now done most of the work to this Blues Deluxe and due to the difficulty in wiring the existing relays, to mute the Drive stage, I am considering adding another relay.
In Clean mode, this will mute both the input and output of the Drive stage, which will prevent any High Gain signal being created and mute any noise from the Drive stage output.
I am concerned that without the mutes, I will reassemble it all, only to find that there is some distorted signal riding on the clean sound and then it will have to be disassembled to rectify the problem. I have done so much work on this amp now that I think it is unwise, not to finish it off properly. I think prevention is better than a cure.

With the components, for the extra gain stage, mounted on an aux. board, connecting to the relevant grounding points will be much easier.
If I can add another relay to the existing circuit, the (foot & panel) switching will activate the added relay as well.

The relays fitted by Fender are OMRON 24V DPDT and I have some in stock, which I can use for this purpose. (I also have other DPDT relays: 6V, 12V and 48V.)
My only concern is whether the +-16V power supply will handle another relay. 
There will then be two Relays on and one off, in one mode - and one on and two off in the other.
My knowledge of SS electronics is not the best and I would appreciate your advice on the best method of wiring in another relay and whether this is feasible.

The +- 16V supply is known to draw enough current to overheat the dropping resistors R85 & R86 to the point of cooking the PCB (which has happened to this amp).
My solution to this problem, on other amps, has been to remove these 5W resistors from the PCB and mount new aluminium body 15W resistors to an external heat sink; which I am doing to this amp also.

I have attached the Blues Deluxe Schematic.

Thanks again, for your help.
Noel

PS: I also need to have a talk to you about buying some Power Scaling Kits, now that I'm back into the amp work. Should I contact you via the forum, or the LP phone or email contacts?


.pdf   Blues_Deluxe REV G.pdf (Size: 343.81 KB / Downloads: 4)
Reply
#10
Hi NGW

The actual supply fix is to replace the shunt regulated aux supply with voltage regulators. So, remove R85,86 and CR22,23 and replace them with 7815 and 7915. Bolt these to the chassis but insulate their backs and mounting bolts OR find fully epoxy encapsulated versions and bolt them directly.

The switching should work at +/-15V instead of +/-16V, but if you are concerned, add a diode in series with the adjust pin of each regulator.

As far as whether the 4560s will handle the current for the extra relay is unknown without knowing the complete part number off the chip. A search at Digikey shows 4560 opamps with maximum output currents of 25,30,50mA.

For the PS:
Power Scale kit ordering should be done via email or the London Power store.
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#11
Star 
Thanks for the info and advice, Kevin.

After thinking about the options and inspecting the PCB again; I decided to cut some traces and make the existing relays do the job.
A bit time consuming, but less work than adding a relay and having to worry about loading the switching supply.
It has worked out fine, after some strategic wiring.

I wired the Clean Volume in normal mode, between ground and R6 and wired the wiper to Pin 9 of Relay 1B, Pin 11 to RA5/RA6.
I connected Pin 4 of Relay 1A to the wiper of the Drive pot, grounding it in Clean mode.
I swapped connections on Pins 9 & 13 of Relay 2B and connected Pin 11 to Pin 11 of Relay 1B, which grounds the junction of the Stage 2 output voltage divider, when in Clean mode.
So I now have Stage 3 selecting Stage 1 or Stage 2 output and mutes on the Input and Output of Drive Stage 2 when in Clean mode.

This should now take care of the switching and eliminate any Drive bleed-through. 
I can reassemble tomorrow and see how it sounds.

Thanks for your patience and help.

Noel
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