This article first appeared in the April'12 issue of MIX Magazine

Part-3: Fender Hot Rod Deluxe (HRD) Mods...
2012 by Eddie Ciletti

This is Part-3
Go to Part-1
Go To Part-2

This is the third in a series of simple tweaks and signal flow 'exercises' for the Fender Hot Rod Deluxe (HRD). 

My inspiration comes from questions and challenges - from students, readers, customers and broken gear - anyone and anything that pushes me has the potential to translate into a light-bulb moment. 

I have always tried to help people do more with what they have (and can afford). When one student brought the HRD in for a session, my own needs as a recording engineer intersected with theirs and we set about the process of finding out what could make this affordable amp more complimentary to their style. I hope this series has inspired Engineers, Guitarists and DIY'ers to coax an unruly guitar amp into one that can tolerate the scrutiny of being recorded.

It should be noted that there are specialists who know and perform all of the common instrument amplifier fixes, mods and upgrades on a daily basis. 

I am not one of those people. I have been tinkering for years, know what works for me and am thrilled when my 'independent exploration' yields similar conclusions to those with more experience in that niche market. That said, a freshly tinned tip-o-the-iron to my friends John Frondelli and Blackie Pagano of the Pro Audio Service Technicians Group. (I'm sure there are non-Italians in this group, so don't be intimidated - at least you know the food will be good!)

Worth repeating is the standard 'hazardous voltage' warning: Power Down, unplug and discharge the power supply (explained in part-1 and part-2).

Part-1 (February'12) involved one cap change and one cap removal to improve the range of the existing tone stack. In Part-2 (March'12), a real Master Volume Control (MVC) was inserted between the Preamp and the Power Amp. (The stock MVC only affected the Lead / Drive channel.) In addition, a negative feedback control was added to yield a more tweed-like tone.

Part-3: You are Here...
Tracing signal flow through a vacuum tube guitar amp is fairly straightforward. Because tubes are so big, 'real estate' contributes almost as much to the simple circuitry as any of the other 'design parameters. Simple designs have a better chance of being more 'musical,' which is code for the type of distortion often described as 'warmth' when the circuit is pushed toward saturation. 

When the Transistor and then the Integrated Circuit (IC) introduced 'miniaturization,' the resulting increase in component count and circuit complexity inversely reduced distortion - the character and range of expression (think 'soft-knee' or 'bloom') that we associate with vacuum tubes took a back seat to features and versatility. Many modern guitar amps suffer from being either too clean or too dirty with no 'happy accident' in between. Which brings us back to...

Tu Be or not Tu Be?
With the exception of the reverb tank circuit, the Hot Rod Deluxe has an all vacuum tube signal path, unless 'something' is plugged into the Power Amp Input Jack (J4). Two dual Operational Amplifiers (OpAmps) - ICs labelled U1 and U2 - do three jobs: buffer the vacuum tube preamp (to J3), drive and amplify the Spring Reverb and, when necessary, incorporate the new 'external' signal from J4 into the reverb driver IC and the power amp. 

The sleight-of-hand required to transparently pull all of this off made the factory schematic look like a traffic jam, the modified version (Figure-1) makes signal flow easier to trace. For example, note that J4 serves not only as the Effects Loop Return and the Power amp Input, but it's also a multi-pole switch.

Figure-1: The low voltage, IC opamp section of the Hot Rod Deluxe


At the top-left, the wiper of Relay K2A (pin-4) selects between Rhythm (Clean, pin-6) and Lead (Drive, pin-8) before being split into three parts: 

  • A voltage divider (R28 and R29) creates a 25dB pad that makes the tube signal compatible with opamp U1A - the Preamp Out / EFX send amplifier. 
  • The unpadded signal feeds both the reverb spring drive amplifier (U2A) and the Power Amp Input. 
The ratio of the Voltage Divider is calculated by dividing R28 by the sum of R28 and R29: 2k7? / 47k? + 2k7? = 0.05432. The formula for converting this ratio into its more familiar dB form is: dB = 20 times the 'logarithm' of this ratio, which is ?1.265, times 20 = -25dB. 

Q-1: Where does distortion come from?

A-1: For all modern opamp based circuits (interfaces, mixers, etc.) the audio signal is linear-clean (typically well below .01% distortion) from a whisper until the point where clipping becomes visible (about 0.5% distortion), beyond which an overdriven sine wave becomes a symmetrically distorted square-wave, a.k.a. your gear as fuzz box. Speaking of which, 'fuzzy' square waves are rich in odd-order harmonics (predominantly 3rd, with progressively lesser amounts of 5th, 7th, etc.). 

Opamps consist of a few dozen transistors that have the potential to provide much more gain than is typically required. Negative feedback is employed to control the amount of gain and in the process, reduce distortion and improve bandwidth (frequency response).

By contrast, the typical for individual tube and transistor gain stages can be much higher at nominal (0.1% is not uncommon at nominal). From 6dB below maximum output, asymmetrical sine wave distortion gradually becomes visible to the eye - think of it as 'parallel processing,' where one-half of the wave is clean while the other half is 'dirty,' generating predominantly even-order harmonics, A.K.A. Octaves. Percentages can easily fall between1% and even 5% without being harsh or edgy and still be described as 'musical,' 'euphonic' and, dare I say it, 'warm.' 

When I do ask fellow techs for advice, it's mostly to get tube recommendations, and by this I mean which tubes to avoid in the event of a bad run. Sad to say that as much as you'd like to choose tubes purely for their tone, Noise (hiss), Microphonics (mechanical sensitivity) and Reliably (build quality) come first. To meet the goal, I often buy a minimum of ten preamp tubes and then sort through the batch to find the better ones. I will buy an extra pair of matched output tubes just to hedge my bets, which is my advice to you as well - that and to encourage you to experiment. The alternative is to pay premium prices for pre-tested tubes, but only if they come with a good warranty / return policy.

I buy tubes and related parts from the following sites.

Several years ago I tested three dozen dual triodes of the 12A?7 family - 12AU7 (ECC-82), 12AY7 (6072) and 12AX7 (ECC-83). (See Figure-2.) The test was done using a combination of a Hickock TV-7 tube tester, a modified preamp section of a Gibson GA-6 guitar amp and an NTI Minilyzer (distortion and spectrum analyzer). I wish I had access to the rig that Larry Janus (of Tube Equipment Corporation) created. Probably a good excuse to take my tubes to Florida for a visit...

The preamp mods included a rectified filament supply and a cathode capacitor switch. Capacitor IN circuit (Note-2 & Note-6) increases gain and cap OUT (Note-1 & Note-5) trades gain (a la degenerative feedback) for a more linear dynamic response, reduced distortion and extended high frequency response. The capacitor value is what we changed in Part-1 to make the first stage amplifier act as a high pass filter.

Each of these tube types has a different Amplification Factor (mu), the 12AX7 being highest and yielding 6dB more gain with the cathode cap switched in. The other tubes showed less of an increase as per their lower 'mu.' Increased gain from the lack of feedback is about four times more for the 12AX7, double for the 12AY7 and about 1.5 times for the 12AU7.

Vacuum Tube Evaluation: 12A?7 Dual Triodes 

Triode 1

Dist LO 

@ +16.7dBu


Triode 1

Dist HI 

@ +23dBu

High Gain (mu=100)
Telefunken / 12AX7
28.28 dB
.185 %
34.33 dB
.913 %
EHX / 12AX7
28.69 dB
.246 %
35.07 dB
1.096 %
Mullard / 12AX7
28.66 dB
.320 %
35.00 dB
1.33 %
GT/ 12AX7M-1
28.37 dB
.278 %
34.45 dB
1.343 %
RCA / 12AX7
28.47 dB
.420 %
34.64 dB
1.690 %
GE / 12AX7
28.50 dB
.417 %
37.41 dB
1.739 %
JJ / ECC83 
28.67 dB
.547 %
35.05 dB
2.317 %


EHX / 12AY7-1
25.59 dB
.204 %
29.61 dB
.480 %
GE / 12AY7
25.49 dB
.236 %
29.49 dB
.554 %


EHX / 12AU7-1
20.9 dB
.362 %
23.09 dB
.549 %
RCA / 12AU7
21.22 dB
.401 %
23.51 dB
.633 %

Figure-2: Sample results from Dual-Triode tests.


It has been interesting to review these tube tests, now eight years old. I also reviewed an earlier article about the Altec 1566a mic preamp. I have worked on - and learned - so much since I first began exploring. The design process is a series of trade-offs, of balancing goals, features and flexibility. This is why we like those obscure amps from the late fifties and early sixties. They do one great thing and force us to play to that strength.