Fender Hot Rod DeVille & Deluxe Drive/More Drive mod

[nevetslab]

After listening to the complaints from many users of the Fender Hot Rod Series amps, where the level difference between Drive and More Drive is just way over the top, boosting the level probably 15dB or more, I set one up on the bench, replaced V2 (12AX7) with a 12AY7 Medium gain low noise tube, and added 604 ohm resistors in series with C8 & C9, being the cathode resistor bypass cap around R17 & R21 cathode resistors (V2 tube circuit). The More Drive mode uses JFET switches to short out the 100k resistors in series with those bypass caps. The initial mod was done empirically and, after finding all who have played thru the mod'd amps loving it, I've been adding the easy mod to them as they come in for service.

I'll get real acoustic level difference numbers the next time one comes back into the shop. Just never got to measuring the difference. There's still a good 6-8dB increase in level between Drive and More Drive, without it ripping your head off as the stock amps do. I thought I had some photos of the mod, but was mistaken. I'll add that on a later post, with the other details.

[alpickyuk]

Hi , I was thinking of modding my hot rod drive circuitry along the lines you suggested in your post.Can you confirm the value of 604 ohms for C8 and C9 and is it worthwhile changing out the diode for the 10meg resistor change on the switching jfets as mine is an earlier one .I didnt find the fromel mods startling but didnt actually buy the kit and instead gleaned info online about changes including those from others users. I use KT66's in the output and as a jazzer find the sound to my liking for warmth.Any other mods worth doing.Manythanks alanp

[nevetslab]

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!

[Chuck H]

I've used those amps at "jam nights" and such a few times. I remember doing Voodoo Chile and using the boost switch. Perfect for THAT! If you just want a bump, way too much. And I love the sound of 12ay7's plugged into "typical" 12ax7 sockets (1.5k cathode/bypassed, 100k plate). They sound very throaty and "tubey" for lack of a more technical analysis. So your mod would clean up the normal mode a tad with the lower gain tube, but then bump it back up with the partial cathode bypass circuit. NICE. Then a reasonable boost to follow. All with the added benefit of the 12ay7 tonal contribution. I'm in Nice mod.

[alpickyuk]

Thanks. I guess it doesn't matter which end of the cap you lift? I've worked on worse pcbs ( mine for instance!) and try to suss whether desoldering braid or solder sucker is best just by feel. No problems so far. I'll try that mod and fancy doing a global mater volume by adding a pot on the rear chassis just where the preamp signal comes in. all the best alanp

[Chuck H]

I've used braid and a sucker bulb on mediocre PCBs. I vote solder sucker for less damage. If your iron is the right temperature you're in and out faster. That usually means less stress on the pads or traces.

JM2C YMMV

[Pedro Vecino]

To many I have simply removed the first capacitor in "more drive" mode. I do not see the interest in the focus produced by 1uF as a cathode bypass in that circuit added to the full gain of the second (22uF).

[nevetslab]

Frequency response plots & acoustical response spectrums

As luck would have it, a Fender Hot Rod Deluxe popped up with a microphonic tube and intermittent input jacks today, so I took the opportunity first, replace the micophonic tube, then run a series of Frequency Response plots and Acoustical measurements in it’s Stock configuration, then pulled it apart, cure it’s ailments, make the Drive/More Drive mod to it (installing 604 ohm resistors in series with C8 & C9).

I did get acoustical measurements After the changes, but ran out of time today to complete the modified Frequency Response plots, which I’ll complete tomorrow. I ran the FR plots driving it’s stock Eminence 12” speaker, and began by setting all of the controls to ‘5’ across the panel. Then, ran plots in the Normal Ch mode, Drive Ch Mode and finally More Drive Ch Mode, setting the Bass, Mid and Treble controls to ‘1’, ‘5’ & ‘12’, with the Presence Control set at ‘5’ on all the plots. Never did engage the Bright switch.





As you can see and hear, the level change between Drive and More Drive is about a 15dB change....way over the top for most folks.

in the next post, I'll add in the acoustical response spectrums AFTER installing the 12AY7 tube into V2 and the pair of 604 ohm resistors in series with C8 & C9.

[nevetslab]

Freq response plots & acoustical response spectrums after modification

This post only has the acoustical response measurements at the moment, having run out of time this afternoon to complete the Frequency Response plots similar to my first four plots in the previous thread. I'll finish those up in the morning.

It's been very well received by all who have heard the modified Hot Rod series (Deluxe 112, DeVille 212 & DeVille 410). And, not so noisy as it always has been in the More Drive mode.



In this series of plots, I kept the panel controls all at '5', and only changed from Normal Ch, Drive Ch and More Drive Ch. On the Normal Ch spectrum, I set the Max SPL to 100dB, which reading the second line of the header, shows 95.2dB SPL Wideband. The Drive and More Drive spectrums were run with the Max SPL set to 110dB, showing Drive level at 101.5dB SPL Wideband and More Drive level at 108.2dB SPL Wideband, an increase of around 8dB instead of nearly twice that in the Stock mode. We'll see the change in the frequency response that results with the 604 ohms in sereis with C8 & C9, as it does change it some. The Measurements were run with a 10 sec Integration time to arrive at a stable reading (Pink Noise is constantly fluctuating, it being in this case, true Random Noise from the Bruel & Kjaer 1027 Sine/Random Generator.

[Tone Meister]

I'm going to ask the inevitable question: how did you settle on the 604 ohm resistor, and in your estimation what effect would a R with higher or lower resistance have on that part of the circuit?

Also, have you considered Pedro's comments regarding removing the C8 1uF cathode bypass cap altogether?

I dabble with these circuits a bit and am always looking for tweaks that will suit individual players. I'm always grateful for the wealth of the knowledge shared by the folks here, and this info is something I will document for future use.

[bob p]

for those of you who don't have the schem handy:

[nevetslab]

When I was working with the circuits, I had found a reasonable gain increase (between Drive and More Drive) happening between 560 ohms and 620 ohms. 560 seemed a bit too high in level, 620 a bit too low in level. Put a pair of 1.2k's in parallel, and decided that sounded like enough level increase over the Drive mode. I do recall changing the 1uF cap early on (break freq @ 264Hz) to 470nF (561Hz) and 2.2uF (120Hz), but decided to leave it where it was. I had also played with C3 & R8, which in Normal mode gives a break frequency of 637Hz, but decided to just keep it simple.....inserting two resistors and changing V2 to a medium-mu 12AY7 kept basic sound of the amp, and took that huge edge off from the More Drive mode.

I had initially installed a 12AT7 in V2, prior to adding series resistors in with C8 & C9. It wasn't enough of a change, and it still didn't address the large gain increase of the More Drive function. The 12AY7 with the two resistors just sounded really nice, and was a simple change, so I've left it at that for the time being.

Thanks, Bob....I meant to add that section of the schematic to clarify all this.

Now, b ack to running the new FR curves, to be posted early afternoon PST.

[bob p]

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.

[nevetslab]

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 haven't looked at those stages with this plotting set-up thus far. Now would be the time to do it, as I still have the amp set up and open. Simple matter of jumpering the added resistors. Shouldn't be any difference with teh 12AY7 installed . I did just look to see if there was any difference with and without the rear panel in place, as I had that off on all the plots, having clipped across the wiring side of the speaker jack. You'll see that minor difference on the Normal Ch mode where I added the Bright Switch.

I'll first post the plots I just ran, then look at V2A & V2B at the plate circuit

[alpickyuk]

604 ohms resistor

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