Some mods I have made to my Hot Rod DeVille
Warming up the Amp, and Improving Gain Quality:
I recently decided that I wasn't totally happy with my Hot Rod DeVille. There seems to be some room for improvement in the areas of warmth, dynamics and gain. So I decided to "hot rod" my Hot Rod DeVille. I got the idea from Steve Ahola's Blue Guitar site. You can find there some mods for the Blues DeVille. Here is an excerpt from the mod that interested me:
Date/Time: 8/29/99 3:32 AM
Subject: Re: Modding Blues DeVille to a Hotrod Deville
"I have done this mod to several Blues DeVilles for pro musicians who play the circuit night after night in Omaha and Sioux City. It is simple and will result in increased gain and dynamic response. Here goes...
Change C1 from 25uF electrolytic to 1uF/50V electrolytic. Add a 0.68uF/50V electrolytic in parallel with R10. Change R11 slope resistor from a 100K to 47K 1/4 watt. Change C6 from 0.1uF to .022uF/630v. Change C25 and C26 from 0.1uF to .033uF/630v. Now here is the super secret! Remove C22 from the presence circuit and replace it with a straight wire. You now have a type of resonance control instead of a presence control. As you turn up the presence the gain increases and the tone becomes fuller and looser (spongy)."
Bruce from Mission Amps had this commentary to add:
"I think shorting the cap (C22) changes how much negative FB is sent to ground at ALL frequencies instead of just to the phase inverter. That sounds like a form of variable FB and some variable PI gain control. The presence control is doing the same thing too, but since the FB is held from ground with the pot's resistive value, there is more NFB applied to the PI. With a cap to ground through the wiper acting as a shunt around the pot's resistance, it acts as an RC circuit and thus, frequency selective, too. With no cap and the wiper connected to the grounded end, it acts just like a variable resistor instead of a pot. The long tail pair PI with a presence control is held above ground by the 2.5K to 5K of resistance, and this mod now bypasses that too. That means a little more gain through the PI/Driver when the resistance is very low and at the same time less negative FB voltage to the PI. A bigger value cap will do a similar thing and you still get a little presence out of the presence control."
After reading this, I thought that a similar mod to my Hot Rod DeVille might make it sound a little warmer while also increasing and improving the quality of the gain on the drive channels. So I compared the Hot Rod DeVille schematic to the Blues DeVille schematic, and here is what I came up with:
1. Change C1 from 47uF electrolytic to 10uF/50V electrolytic
2. Change C23 from 1.5nF to .022uF/400V cap of same type
3. Change C7 from 250pF ceramic to a 330pF/1KV ceramic cap
4. Change R12 from 130K to 82K 1/2 watt resistor
5. Change C26 and C27 from 0.1uF to .033uF/630V cap of same type
6. Remove C29 and replace it with a wire jumper
Replacing C1 changes the size of the bypass cap on pin 3 of V1A. Replacing C7 changes the value of the treble cap. Changing C23 will result in a little fuller and stronger sound. Changing the size of R12 affects the tone control section, allowing it to be a little more dynamic. Replacing C26 and C27 changes the size of the caps on the control grids of the power tubes, resulting in a little more headroom and power tube distortion when driven hard. Removing C29 does something similar to what is described above in the Blues DeVille mod, but I couldn't get the pot to be silent as it was before. With the cap removed, it makes kind of a rumbling scratchy noise when the Presence control is turned. So I put the cap back in and paralleled it with a wire jumper. So now it seemed to be a combination of presence and gain boost control. I finally decided to install a switch to be able to change the presence control to a gain boost, and vice versa. But I do not switch it while the amp is on, or I will get a loud pop. I just like having the option... The only drawback is that when the Presence control is turned up past half, the amp starts to have a bit of "white noise", but with it up to around half, you get a nice gain boost.
Having performed this mod to my Hot Rod DeVille, I can say that it most definitely has improved the amp's dynamic response. It sounds much more lively now. And the drive channel has a fuller sounding distortion. This turned the amp from being just okay to sounding really good. One thing to note: While the actual mod is easy to do, it is a lot of work get the PCB out of the chassis to do the mod itself. You have to basically pull out all of the PCB's to do it. But if that doesn't intimidate you, then go for it!
There always seem to be one slight problem, though.... I had a problem after the mod, which was an occasional loud pop when switching from the drive channel to the normal channel. It seems that was because I also changed the value of R45 (47K at the Master Volume control) to 150K, and that it was messin' with the voltages that the channel switching relay saw, resulting in the pop. I tried adding a bypass cap in parallel with R45, but I still had the pop. So I decided to put the 47K resistor back in. This also helps keeps the impedences matched on all the channels, too. The power tube bias setting has an affect on the pop. At factory setting it seems fine, but I like to run mine at 70mA. But I never change from the drive channel to the normal channel in the middle of a tune, so it's not a big deal to me. The amp will start to feedback quicker now because suppressor caps C26 & C27 are now 2/3 smaller. I changed these caps to .05uF/630V, and I get better results with that.
Touchy Master Volume Control:
finally figured out a way to make the Master Volume control a
little less sensitive. It goes from nothing at "0" to
roaring at "2". The schematics don't clearly state what
type of pot is used for the master, but it appears that Fender
uses a linear pot there, instead of an audio pot. In theory, one
should be able to simply replace the stock pot with an audio
taper type to make the control more user-friendly. The hard part
is finding the exact same pot assembly. Looks like a custom job
for Fender. I just decided to make one myself, using the existing
one as a guide.
Here is how I did it...
I put some Celestion speakers in my amp on a hunch that it would make for a good combination, which it did. Here is what I did...