Thought it was about time to get a "Tweed Royal mods" thread going, for others like myself who can't leave well enough alone!
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V1 Cathode Resistor and Bypass Capacitor ModsBACKGROUND:It seemed to me that a good place to experiment with the TR circuit was the cathodes of the V1 preamp tube. The two cathodes are tied together and then connected to ground through the parallel combination of a resistor and a bypass capacitor. To save some typing (yeah, I'm lazy), I'll refer to these as CR (the cathode resistor) and BC (the bypass capacitor). Both components affect the gain of this first amplification stage (one half of V1 amplifies the incoming Bright channel signal, and the other half amplifies the Normal channel), though in quite different ways. Other things being equal, the value of CR (shown as R7 on the TR schematic) directly affects the V1 gain--lower resistance gives more, higher resistance gives less. The Tweed Royal instructions recommend using a 2.2K ohm resistor at R7, to moderate the gain and give a bit more headroom to the amp. The original Champ circuit used a 1.5K resistor on one triode of a 12AX7 for it's single channel, while the Deluxe dropped this to 820R, but serving two channels with both sides of a 12AY7 tube. Though a lower gain tube (nominally 45% of the gain of a 12AX7), the Deluxe made up for this by using no negative feedback to increase the gain in its second preamp gain stage.
Rather than attempt to explain the function of the BC myself, I'll just quote directly from
Ampbook's Cathode Bypass Capacitor Calculator webpage:
"The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as "cathode degeneration." This substantially increases gain. When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but as an open circuit for DC, thereby maintaining DC grid bias. We can introduce treble boost by using a lower capacitor value, one that acts as a short circuit for high frequencies but allows negative feedback to attenuate bass. This is often done for the preamp's bright channel. If the additional gain is unwanted, based on the amplifier's overall gain from the input jack to the power amp, the capacitor can be eliminated entirely."The size of the BC determines the frequency where the gain increase effect rolls off. As it happens, the 25uf cap used in the TR is large enough to bypass essentially ALL of the useful frequency range of a guitar, so it's what is referred to as a "total bypass cap". For the TR stock circuit, this cap adds nearly 8 dB of gain across the guitar's frequency spectrum, a very substantial effect.
Besides the effect of these two components, there's one more good reason for modifying the V1 cathodes--it's tough to get in any trouble! There's less than 2 VDC on these cathodes in normal operation, so you really don't need to worry about voltage/wattage ratings on the components. And if you happen to make a bad solder joint, nothing's going to smoke or blow up. My kinda mod!
MODIFICATION PLANFor the V1 cathode modifications to my Tweed Royal, I decided on the following:
- Installation of an on-off-on DPDT mini-toggle switch across the stock 2.2K CR. Across one end-pair of lugs on the switch is soldered a 4.7K 1/4W resistor; across the other end-pair are 2.2K and 3.3K 1/4W resistors in parallel. If you do the parallel resistor math, that works out to three selectable resistance values for the toggle switch: 2.2K, 1.5K and 825R. So you've got the recommended value for the CR per the TR instructions, plus the values used in the vintage 5F1 Champ and 5E3 Deluxe circuits.
- Installation of a second on-off-on DPDT mini-toggle switch across the CR, this one with a 22uf radial electrolytic cap across one side and a 1uf electrolytic across the other. The 22uf cap is essentially equal to the 25uf cap that comes with the kit (which I clipped out of the circuit for this mod), and the middle "off" position takes the bypass cap out of the circuit completely. I chose a 1uf cap for the third value since it shifts the gain roll-off midpoint frequency up around 100 - 200 Hz; the bass and lower mids remain unboosted while the upper mids and highs get the BC gain boost. In effect, this serves to boost the highs relative to the lows, and helps get rid of the flab in the bass that the circuit can tend to have, especially with humbucker guitars. Choosing a smaller cap value here would shift the roll-off frequency higher, resulting in relatively less bass, while a larger value would leave more bass within the boosted range.
Physically, the installation is quite simple. I mounted the two toggle switches through the front panel below the input jacks and first volume pot. From one of the middle lugs of each switch, I ran a wire over to the tube side of the CR and soldered it to the resistor lead. From the other middle lug, I ran a wire up to the sleeve lug of the bottom Normal channel input jack (i.e. ground). For the capacitor toggle switch, just be sure that the caps are mounted on the switch in the same polarity orientation, and that middle lug for the negative side is the one connecting to ground. For the resistor toggle, it doesn't matter which switch lug goes to which connection point. See photos below for a look at the physical installation.
OPERATIONAL IMPACTOnce installed, you have your pick of 9 different combinations between the two 3-position switches. However, they are not all useful. If you plug the various values into the Ampbooks Cathode Bypass Calculator linked above, you'll quickly see that when the cap switch is on the 22uf "total bypass" position, there is no significant difference between the three positions of the resistor toggle. That big bypass cap just swamps the resistor's effect, and the maximum V1 gain is the same in all three cases, about 35.5 dB for a 12AX7 across the guitar's entire frequency range.
With the cap switch in the middle "no bypass" position, the pure effect of the three resistor values becomes clear. Overall gain is reduced--for the 2.2K, 1.5K and 825R resistance setting, max. V1 gains are 27.7, 29.5 and 31.7 dB, respectively. So if you were looking for maximum clean headroom, selecting no bypass cap and the 2.2K CR value would be a logical way to go.
Choosing the 1uf bypass cap gives an intermediate result. In the frequency range where the bypass cap dominates, you end up with the same max. gain as for the 22uf cap setting, about 35.5 dB. At the low end, you see the same gain behavior that the no-bypass setting gave, so the lower the resistor value, the greater the gain spread between the two frequency extremes. Also, the midpoint of the gain transition range moves with the CR value, shifting from about 120 Hz at 2.2K up to about 250 Hz at 820R. So the CR toggle settings with this BC value give a way to take a bit more or less bass off the signal.
A GIF image file showing the gain vs. frequency curves (courtesy of the Ampbooks calculator) for all nine combinations of these two switch settings is posted at
http://duhvoodooman.com/miscimages/musical/TR/12AX7_cap-resistor_combos.gif.
...AND ONE OTHER "MOD"In conjunction with the various CR and BC switch setting combos, you can also change something else, namely the V1 tube itself. Give the original 5E3 V1 tube type, the 12AY7, a try. Another good intermediate choice would be the 5751, whose nominal gain factor (70) falls right about in the middle between the 12AX7 (100) and the 12AY7 (45). These would be especially good choices if you find the overall gain of the Tweed Royal to be a little higher than you'd like. Analogous 9-curve gain vs. frequency GIF files for the 12AY7 and 5751 are posted at these links:
http://duhvoodooman.com/miscimages/musical/TR/12Ay7_cap-resistor_combos.gifhttp://duhvoodooman.com/miscimages/musical/TR/5751_cap-resistor_combos.gifMod installation photos: