Fender Hot Rod DeVille & Deluxe Drive/More Drive mod

[nevetslab]

Freq response plots after drive / more drive mod's added

I just finished running the Post-Drive / More Drive mod FREQ Response plots on this Hot Rod Deluxe 112 amp. I've included the Acoustic Measurement plots again, as I had intended to have them all together.





It's interesting that in comparing the Frequency Response plots with all controls set to 5', there's only a 6dB level difference between the Drive and More Drive at the peak of the HF portion of the curves, while there's an 8dB acoustic level difference between them. Also interesting is the nearly identical FR plots of the Drive and More Drive curves, while looking at the acoustic response spectrums, below 250Hz there's considerable difference. Ain't Physics fun??!!!

We also see the mounted open-box resonance of the Eminence speaker @ around 70Hz in the Normal Ch plots. Also interesting how the lower frequency side of the speaker's resonance peak forms a HP Filter response on the Drive and More Drive response. Acoustically, that doesn't really happen. Now granted, running these acoustic measurements in this 12ft x 31 ft room with 12 ft ceilings, hard wall surfaces and the mic at 1 meter from the amp, this isn't precise. Near Field measurements (mic directly at the speaker) may reveal more, as would ground plane measurements. I haven't run any outdoors acoustical measurements in the parking lot, as we're half a block from teh runway at the Burbank Airport. Still, I do have access to large rehearsal studio rooms when they're not occupied. None-the-less, it has provided an intersting look at the Hot Rod Series amp and that over-the-top More Drive boost response, which started the whole thing.

[nevetslab]

604 is a standard 3-digit value in the 1% & tighter tolerance value (6.04, 60.4, 604, 6.04k, 60.4k, etc). I bought KOA-Spear MF resistors from Mouser P/N 660-MF1/4DC6040F. I think heyre about $0.05 ea in qty's over 10 pcs. Shpg always gets ya, though!

[nickb]

I can confirm that there is a 604 ohm precision film resistor available! on ebay

Considering the tubes have a 30% tolerance (or more) and the stock cathode resistor are 5% and the cathode caps at 20% I wouldn't get hung up on using an exact value here.

I'd expect using 560 ohms instead of 604 in a single unloaded 12AY7 stage to give an inaudible gain difference of about 0.06dB. The difference in midband gain between 0 (no resistor) and 604 is just 1.2dB.



Do it to two stages and the numbers double i.e 0.12dB for 560 vs 604.

[bob p]

> Simple matter of jumpering the added resistors.

I was thinking along the lines of demonstrating the difference in the frequency response traces between:

a) Rk
b) Rk || Ck
c) Rk || Ck with the boost limiting resistor

There are lots of textbok plots that show a) and b), but not many that compare the effects of adding the boost limiter as in c). I think that'd be fun from a teaching perspective.

> I wouldn't get hung up on using an exact value here.

when dialing things in by ear I like to use a 1k Pot.

[nickb]

> Simple matter of jumpering the added resistors.

I was thinking along the lines of demonstrating the difference in the frequency response traces between:

a) Rk
b) Rk || Ck
c) Rk || Ck with the boost limiting resistor

There are lots of textbok plots that show a) and b), but not many that compare the effects of adding the boost limiter as in c). I think that'd be fun from a teaching perspective.

> I wouldn't get hung up on using an exact value here.

when dialing things in by ear I like to use a 1k Pot.

Now that sounds like a plan


Ask and you shall receive...

[nevetslab]

FR plots @ V2A & V2B

No problem posting the snippet. I had to look at the circuit anyway, and it was no trouble to capture a snapshot.

While we're on the subject of frequency response curves:

Those are some great plots. From a design standpoint, I think it's interesting to look at the frequency response changes at each individual gain stage. I've seen a lot of plots that show before/after frequency response plots when cathode bypass caps are added, but not that many that demonstrate the effect of boost limiting from the series resistor. By any chance have you snapped a before /after shot of the frequency response curves at V2a and V2b? I think they'd be interesting to look at.

I first connected the Analyzer @ the output of V2A, junction of C2/R18 & R42, with a jumper across the 604 ohm resistor, so I could simply open and close it. Then, did the same at output of V2B, junction of C10 & top of the Master Volume pot. But, the rsults I got don't make any sense, as you'll see:



I first set the Generator's Output Attenuator so there would be plenty of signal on the graph, prior to removing the short across the V2A's 604 ohm resistor. Then, when I set up for V2B, I decreased the gain on the Amber 3501a analyzer to compensate for the added gain of V2B stage, and ran the same plots, now with one and then both 604 ohm resistors in series. Very little difference shown. I re-ran the set-up a couple times, then set the generator at 400Hz to listen, and what I was hearing matched the plotted change. But, I clearly found in the previous tests & plots that I had dropped a good 8dB of output changing from Drive and More Drive. I didn't go back and pull the 12AY7 and put the 12AX7 back in to check.

[nevetslab]

I checked my notes from a few years ago, and found I had played with adding a series resistor on both C8 & C9, when I still had a 12AX7 in V2. So, I re-ran the frequency response plots that I set up for yesterday, taking the output from V2B with a 12AX7 installed. I started with the 604 ohm series resistors shorted out, then removed the short on the C8 resistor, then removed the short from the C9 resistor, finding the level had dropped by 6dB. Back when I was doing this, it was all by ear, using burst pink noise as a source, with the schematic in hand and a decade resistor box with short shielded leads in series with two caps. I had also used a 12AT7 afterwards, but then found some vintage 12AY7's I had from decommissioned test gear, and put that into V2. I liked the sound of the results with the 12AY7 and stuck with that.

So, the last response curve I ran today was pulling the 12AX7 out and installing the 12AY7.

[alpickyuk]

New hot rod deluxe MKIV

I've requested a schematic for the new hot rod from fender without success so far.Does anyone have any access to one or have any idea od any changes that
are worth making? Many thanks alanp

I didn't have to change the JFETS, or diodes in the More Drive circuit. I left the stock values for C8 & C9 as is. I lifted one end of them and added the 604 ohm resistors in series with each cap. You do have to be careful in that operation, as the quality of the PCB foil is very poor...pads lift way too easily. I had played with the voicing caps, but found it wasn't necessary. The other needed change is replacing V2 (middle preamp tube) with a 12AY7, which is a medium gain low noise preamp tube.

I've been meaning to get some photos of the mod, but at present, I don't have any of the hot Rod Deville or Deluxe amps here in the shop. I will be making more of the mod's in our rental inventory soon. Wen I get one back here into the shop, I'll get some acoustic level measurements of the Before and After, as it does tame the amp down, while still having a significant increase in the More Drive mode, but not that er-splitting difference Fender left in place!

[glebert]

I put a 12AT7 in my V1 and a 12AU7 in my V2 on my HRDlx and find it vastly improved. Maybe not as optimal as the mods described here, but really easy. Put an 12AT7 in V3 on my Deville and that helped the clean sound, but haven't messed with the drive channel on that one.

[alpickyuk]

Hi dont suppose you've encountered one of the new Hot Rod 4's yet. I'm itching to find out if all the claims of "shimmering reverb" and "tonally enhanced cleans and drive channel" are correct and more importantly how they've accomplished this. I swapped out my 2 spring reverb tank for a 3 spring medium decay one and think I'll change it back! I find it too much of a rockabilly soundalike for me. Anyone out there seen a schematic. No luck in in the uk so far let alone the highlands of scotland.

[bob p]

"shimmering reverb"? i saw that. it sounded like marketing BS to me.

[Tone Meister]

Reviving this thread because I plan to try this on an early USA version Hot Rod Deluxe.

So, is the recommendation still a 604 ohm in series with C8 and C9 and a 12AY7 in V2? In lieu of a quick response, that's exactly what I plan to do. Also plan to increase the R52 from 180K to 220K at the same time.

[Chuck H]

Reviving this thread because I plan to try this on an early USA version Hot Rod Deluxe.

So, is the recommendation still a 604 ohm in series with C8 and C9 and a 12AY7 in V2? In lieu of a quick response, that's exactly what I plan to do. Also plan to increase the R52 from 180K to 220K at the same time.

Excellent choice of reason to resurrect a thread Looking forward to the report.

[Tone Meister]

Excellent choice of reason to resurrect a thread Looking forward to the report.

Got most of the work done. Was already replacing all e-caps so figured it was a great time to try this mod. Hope to report back tomorrow some time.

[Tone Meister]

OK, I like this mod a lot. It really makes DRIVE and MORE DRIVE much more musical. I tried it with both a 12AX7 and a 12AY7 in V2. Here are my impressions from a guitarist perspective.

First, here is what I did.
All new e-caps, including cathode bypass.
Increased C31 to 100uF
Reduced C1 to 22uF
Added series 604R to the 1uF C8
Reduced C9 to 22uF, added series 604R
Increased R52 to 220K (up from 180K)
Biased to 60mV at TP30

With the 12AX7 in V2 the gain reduction is easy to detect. I can see where many.players would be fine with the 12AX7. Harmonic feedback comes easily and the upper mid frequencies are not as strident. Think Jeff Beck.
Grade = A-

With the 12AY7 in V2 the gain is perfect for my tastes. Harmonic feedback still comes easy, but the DRIVE and MORE DRIVE are much more musical. The tone is searing but in the right way. Think Duane Allman at the Fillmore. Grade = A+