Another Deluxe Reverb Request - 1981

[bobloblaws]

Hi, I need schematic for a 1981 DR with pull boost on the vibrato channel volume knob. This amp has a blackface panel but it is not the Deluxe Reverb II model. It does not have a master volume. It has jacks on the back panel for "line/recording" and "boost pedal". I've hunted high and low and haven't been able to find it. There is a file floating around called deluxe_reverb_boost_schem.pdf but there is no boost circuit in sight on that drawing. Unfortunately there is no circuit number indicated on the tube chart. I will be eternally grateful if someone knows where this obscure document is hiding.

B.

p.s. I'm interested mostly in the pull boost function, so a schematic for a different amp that has the same circuit would also be helpful.

[shortcircuit]

just eliminating the obvious , have you contacted Fender ?? They have a Deluxe reverb w/boost schematic in the archive, not sure if it's the same pdf you mentioned ,you have to email them to get it.
https://support.fender.com/hc/en-us/...-Copy-Archives

[bobloblaws]

just eliminating the obvious , have you contacted Fender ?? They have a Deluxe reverb w/boost schematic in the archive, not sure if it's the same pdf you mentioned ,you have to email them to get it.
https://support.fender.com/hc/en-us/...-Copy-Archives

Thanks, I'll try that. I'll post it here if I get it.

[g1]

There is a file floating around called deluxe_reverb_boost_schem.pdf but there is no boost circuit in sight on that drawing.

Look at the line coming off the reverb drive transformer. (yes it's an odd spot for boost )

[bobloblaws]

Look at the line coming off the reverb drive transformer. (yes it's an odd spot for boost )

Oops, I feel pretty silly now. But yeah, I was looking in the area of the volume pot.

[galaxiex]

I have a 78-79 Deluxe Reverb Silverface with that pull volume boost.

The circuit matches the schematic above.

FWIW that pull boost is a poorly conceived idea.

Yes... it does give a volume boost... if you like shrill, harsh and all round horrible.

It's well known by owners of these amps with the pull boost... to never pull that knob.

I converted mine to a tone stack lift.
This accomplishes the same thing (volume boost) but in a much more usable way.

I installed a push/pull pot with a DPDT switch.

One of these... https://www.amplifiedparts.com/produ...5-bushing-dpdt

I took the new pot apart and put the 1Meg wafer from the original pot in there, so it's the same as original.

Wired the switch so I can select the stock 6.8k mid resistor on the bottom of the tone stack. (switch pushed in)

Switch pulled out selects a much larger resistor (forgot what value I used) to basically "lift" the tone stack.
I found with the tone stack completely lifted (disconnected from ground) there was too much boost. YMMV

p.s. I'm interested mostly in the pull boost function,

I assume you intend to find out why it sounds so bad? and try to fix it?

[Chuck H]

I'm a fan of the treble cap bump. Fender did this also on some amps. Later Mesa Boogie did it. If I were changing the "boost" switch (which I definitely would) I would make it an extra 500p cap parallel to the existing treble cap when switched in.

JM2C

[galaxiex]

I'm a fan of the treble cap bump. Fender did this also on some amps. Later Mesa Boogie did it. If I were changing the "boost" switch (which I definitely would) I would make it an extra 500p cap parallel to the existing treble cap when switched in.

JM2C

That sure sounds easier than what I did.

With the tone stack lift, the mids get pushed up more.
Mines not foot switchable, so not easy to kick it on during playing.

[bobloblaws]

I have a 78-79 Deluxe Reverb Silverface with that pull volume boost.

The circuit matches the schematic above.

FWIW that pull boost is a poorly conceived idea.

Yes... it does give a volume boost... if you like shrill, harsh and all round horrible.

It's well known by owners of these amps with the pull boost... to never pull that knob.

I converted mine to a tone stack lift.
This accomplishes the same thing (volume boost) but in a much more usable way.

I installed a push/pull pot with a DPDT switch.

One of these... https://www.amplifiedparts.com/produ...5-bushing-dpdt

I took the new pot apart and put the 1Meg wafer from the original pot in there, so it's the same as original.

Wired the switch so I can select the stock 6.8k mid resistor on the bottom of the tone stack. (switch pushed in)

Switch pulled out selects a much larger resistor (forgot what value I used) to basically "lift" the tone stack.
I found with the tone stack completely lifted (disconnected from ground) there was too much boost. YMMV



I assume you intend to find out why it sounds so bad? and try to fix it?

Yes, mine matches as well.

Agree on the sound, unusable to my ears. In my case I want to fix it even if it never gets used, it belongs to a friend. Currently it hisses like crazy with the boost engaged and the noise increases when the volume control is turned up. For that reason I want to think the source is pre-volume control, even though I have to wonder why the hiss wouldn't be apparent in non-boost mode as well. Maybe it is present but barely noticeable. Does this boost circuit have a substantial gain factor?

[bobloblaws]

I'm a fan of the treble cap bump. Fender did this also on some amps. Later Mesa Boogie did it. If I were changing the "boost" switch (which I definitely would) I would make it an extra 500p cap parallel to the existing treble cap when switched in.

JM2C

Hmmm, that might be the way to go, thanks for the tip!

[galaxiex]

Yes, mine matches as well.

Agree on the sound, unusable to my ears. In my case I want to fix it even if it never gets used, it belongs to a friend. Currently it hisses like crazy with the boost engaged and the noise increases when the volume control is turned up. For that reason I want to think the source is pre-volume control, even though I have to wonder why the hiss wouldn't be apparent in non-boost mode as well. Maybe it is present but barely noticeable. Does this boost circuit have a substantial gain factor?

The hiss is there in boost mode because the boost is using V3 reverb driver as an extra gain stage.
The signal from V3 is very "trebley" thanks to that 500pf cap feeding V3 grid.
You don't hear the hiss thru the reverb when the boost is OFF because the tank can't "send" the hiss thru to the reverb recovery stage V4B.

Have a look at the schematic, all they did to add this boost circuit is, add 1 wire with a switch on it.
From the reverb transformer output, to a voltage divider, to the grid of V4A.

The signal feeding V3 reverb driver has most of the lows gone, so it boosts the highs and mixes that into the signal path at V4A.
Hissy and screechy.

I don't know what the gain of V3 is, but I'm sure it can be calculated.

To "fix" this... disconnect that wire from the reverb transformer output, and do "almost anything else" to get a volume increase.

Switch out the NFB.
Do a switchable tone stack lift.
Or as Chuck suggests, add a switchable treble cap.

[bobloblaws]

I'm a fan of the treble cap bump. Fender did this also on some amps. Later Mesa Boogie did it. If I were changing the "boost" switch (which I definitely would) I would make it an extra 500p cap parallel to the existing treble cap when switched in.

JM2C

Do you mean the existing 250p cap at the top of the tone stack? I put a 500p cap in parallel but I didn't hear a difference.

[bobloblaws]

The hiss is there in boost mode because the boost is using V3 reverb driver as an extra gain stage.
The signal from V3 is very "trebley" thanks to that 500pf cap feeding V3 grid.
You don't hear the hiss thru the reverb when the boost is OFF because the tank can't "send" the hiss thru to the reverb recovery stage V4B.

Have a look at the schematic, all they did to add this boost circuit is, add 1 wire with a switch on it.
From the reverb transformer output, to a voltage divider, to the grid of V4A.

The signal feeding V3 reverb driver has most of the lows gone, so it boosts the highs and mixes that into the signal path at V4A.
Hissy and screechy.

I don't know what the gain of V3 is, but I'm sure it can be calculated.

To "fix" this... disconnect that wire from the reverb transformer output, and do "almost anything else" to get a volume increase.

Switch out the NFB.
Do a switchable tone stack lift.
Or as Chuck suggests, add a switchable treble cap.

So I tried the NFB mod and I like it. The only problem is I can't figure out how to implement it with the current open/close switch without it working in reverse. In other words, it's easy to use the switch to switch out the NFB but the volume is boosted with the switch in/open and not boosted with the switch pulled/out/closed, which is the opposite of how a user expects the function to work. Is my only option to replace the switch with DPDT?

[galaxiex]

So I tried the NFB mod and I like it. The only problem is I can't figure out how to implement it with the current open/close switch without it working in reverse. In other words, it's easy to use the switch to switch out the NFB but the volume is boosted with the switch in/open and not boosted with the switch pulled/out/closed, which is the opposite of how a user expects the function to work. Is my only option to replace the switch with DPDT?

Yeah, pretty much need a DPDT switch for that. ( A SPDT would work too)

I tried to puzzle out a way to use the stock switch, and the "opposite" function was all I could get.

[Chuck H]

Do you mean the existing 250p cap at the top of the tone stack? I put a 500p cap in parallel but I didn't hear a difference.

No kidding? I suppose if your treble control is down really low or if you're cranking the amp you may not hear much difference. It boosts the upper mids which is a good way to make clean and slightly distorted tones stand out and it adds gain and roundness to distorted tones that you can "feel" more than hear. Mesa uses a 750p cap and Dan Torres uses a rotary switch with several values up to .01uf. So you could try a bigger value cap. When I used the Torres circuit I likes the .0022uf cap position best for the amp it was in.

But I'm really surprised you didn't hear a difference between a 250p treble cap and a 750p treble cap.?. If you have Duncan TSC you can simulate it and see that it's a pretty big shift.