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Jan Oelbrandt

RD65: looking to treble-cut the REVERB output

I want to cut the amount of treble on my reverb output.
There's nothing wrong with it: it works as you would expect.
But I play pedal steel through my RD65, and when I turn up the amount of reverb, there's just too much going on in the high register. I'm looking for a fixed cut-off of the frequencies to get a nice, warm sounding sound.
I've looked at the schematic, but I'm in no way an expert. I can solder and I have basic understanding of how things work electronically.
I think by altering some values of capacitors and/or resistors in the circuit, the job can be done. But which part(s) and what values?
Thanks for all feedback.

Attached here, the part of the schematic I'm talking about

Images: 
mgriffin155

Treble Cut

This looks like the early GD1 version of the RD65. I've never worked on this vintage but I'll go out on a limb and make a suggestion. R22 4.7K and C15 .015uF form a small tone booster circuit for higher frequencies going through IC-2. Try lifting the end of R22 where it joins R20 & R21 47K feedback resistors. In theory, this should flatten the frequency response of IC-2. Take a look at the Bright Switch components around IC-1 (R7 100K and C3 .0033uF) and you'll see that it is a similar treble boost configuration. The switch just activates and deactivates that circuit. For safety sake, make sure the amp is off and the high voltage power supply is fully discharged before you stick your paws in there. Getting zapped with 700+ VDC is not a pleasant experience. Good Luck. -mgriffin

Jan Oelbrandt

thanks Mike. It's try that.

thanks Mike. It's try that.

Jan Oelbrandt

After opening the amp I

After opening the amp I discovered it is a GD-2 board, not a GD-1. So the R22 was actually R36 in the schematic. I lifted that and indeed, the reverb sounds a lot warmer and is now usable on my steel guitar sound. And I connected the lift to the "bright" switch on the front, working nicely. So I still have the option on choosing the reverb with or without treble boost. Of course, I no longer have the bright switch on the input stage, but that doesn't bother me at all. Thanks Mike!

mgriffin155

I'm happy it worked out. I

I'm happy it worked out. I rarely ever use the Bright switch. Maybe I'll try this on my RD65. -mgriffin

Jan Oelbrandt

I will do a recording soon

I will do a recording soon where you can hear the difference.
There is an other problem though with my reverb sound: a hum. There's one of those O.C. electronics type 271 in my amp. I suspect the hum has something to do with the grounding. Would you happen to know if both outer contacts of the RCA plugs should be grounded, or only one of them?

mgriffin155

Reverb Hum

Hi Jan, Both (all) outer contacts should be grounded. Have you tried disconnecting the foot switch cable to see if the hum goes away? By default, the reverb output is active with foot switch un-plugged. A bad ground/shield in the foot switch cable can introduce hum. Verify that all shielded cables have good continuity from end to end, ie center to center and outer to outer. Same as you would do for a guitar cable. Others have had this problem using replacement foot switch with un-grounded shield. BTW: The 4.7K resistor you lifted is R56. See the GD2A schematic which is less fuzzy than the GD2. Good Luck. -mgriffin

Jan Oelbrandt

Thanks. As you can hear in

Thanks. As you can hear in this video (the moment I turn the knob to 10) the hum is becoming louder. And yes indeed it is gone with cables detached. I'll check all grounding next week or so.
Demo of the mod, using pedal steel and a Strat, reverb at 4 and at 10, bright switch on/off
https://www.youtube.com/watch?v=B_6JSD0BmM0

mgriffin155

Cool

Hi Jan, Sounds less "Fenderesq" with the mod. I like it. Just curious: What kind of speakers are in the cabinet? -mgriffin

Jan Oelbrandt

That is an open back cabinet

That is an open back cabinet with a single 12" Jensen Blackbird 100W speaker

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