Hot Rod Deville (USA) - what does this mod do?

[Tone Meister]

Opened this original USA model HR Deville to replace a couple failed filter caps and ran across this mod. Amp sounds great on the clean "channel" but the drive "channel" sounds somewhat tame as compared to my memory of how the stock amp sounds.

[EDIT TO ADD] Went back and played the amp again and the drive channel is not "tame" as originally described; however, it does seem like the amp is a tad bass heavy, particularly in the drive channel. Are these mods correlated with an increased bass response perhaps? I'm inclined to leave this work in place but would like to see what you guys think first.

Annotated picture and schematic attached.

[Tone Meister]

Here's what I think is happening. What effect would this have on the circuit?

[52 Bill]

[ATTACH=CONFIG]39494[/ATTACH]

Here's what I think is happening. What effect would this have on the circuit?

The added 47uF cathode bypass cap to V1B will drive the tone controls harder and the bright switch has been moved to the cathode of V2A.

Edit: Yes, the bass response will be increased with the addition of the 47uF cap.

[Tone Meister]

I'm thinking the amp is a bit bass heavy, particularly in the Drive/More Drive mode. Bill would you recommend putting this back stock, leaving it be, or reducing the size of that bypass cap?

And did I annotate the schematic correctly?

[52 Bill]

You can try smaller values for the bypass cap, that will reduce the low frequency boost.

For your drawing, the green jumper and the 0.068uF cap need to be disconnected from the cathode of V1B.

[Tone Meister]

Thanks Bill. Already caught the drawing error and fixed the schem. Will re-post for sake of accuracy.

[Tone Meister]

OK, I play tested the amp before giving it back to the owner and in case anyone is interested, here are my thoughts on the above mod.

The amp has a pronounced increase in LF to low-mid frequency response as opposed to a stock HR Deville. Also, I wasn't able to dial in the crispy Fender tones we associate with this amp. The bright switch isn't nearly as effective as with the stock configuration, but it is audible when engaged, just not as effective as the stock arrangement, IMO. Overall, the amp sounds much "bigger" and I can see where some players (jazz) may love the result of moving the bright circuit and adding that large bypass cap to V1B.

For my tastes, I would leave the stock bright switch configuration without question. I can see the value of bypassing R10 for some players, but if I did so I would certainly try a much smaller cap (10 - 16uF) as a starting point.

Simply put, the above mod adds too much gain across the board and too far upstream in the preamp for my personal tastes. Of course, as always, YMMV

[Enzo]

Did you play both single coils and humbuckers into it? It might tame some overly bright thing the owner has.

[Tone Meister]

Did you play both single coils and humbuckers into it? It might tame some overly bright thing the owner has.

Yes I did, and overall the circuit seemed to heavily favor single coils over humbuckers. The owner uses single coils exclusively, Stratocaster and Rickenbacker, and those guitars really shine on the amp.

But again, for my tastes, I'd not move the bright circuit downstream nor would use such a large bypass capacitor. Other players may find those mods favorable, but it's just a tad too "gainy" for me.

[Pedro Vecino]

My opinion is that this adaptation is original. I've seen it more times in older HR Deluxe and probably Deville. Resistor type is identical to the others and also electrolytic capacitor. Furthermore it makes no sense extract the plate for so little. That was done when it was built and it can be checked by examining the internal welds.

[Tone Meister]

Pedro,
I don't believe this work to be original to the amp. This is an original USA Hot Rod Deville from the 1990's and that capacitor is not there on the original amp or the schematic. Look closer at the picture and you will see where Wire 4 was removed and a jumper wire added from the bottom of the 68nF cap to R17. The value of R10 is the same as stock, yes, but it is now bypassed with the 47 uF capacitor. You can see where the 1.5K resistor was wrapped around the leads of the 47 uF cap.

I did check the solder pads of those components and they are definitely not factory welds.

A higher resolution picture is attached.

[Pedro Vecino]

Yes. Itīs different from the schematic. I usually keep pics of the work, but not in this case. I have searched the net and I have not found anything except this video.
Inclined electrolytic with paralleled resistor and same wire associated with the cathode bright capacitor.
Iīve seen many times in older H.R. Deluxe.


https://www.youtube.com/watch?v=sD_s5fCmalA

[Pedro Vecino]

More info. See pic in post 5.

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