Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: topbrent on July 08, 2025, 11:04:36 pm
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Looking at the schematics that folks have posted for the ToneKing Imperial, the Rhythm(clean) channel circuit looks like pretty standard fare AB763 Super Reverb tone-stack values .1/.022/100k/250pf, and 100k/100k plate r's.
I noticed that compared to the AB763, the Imperial v1 first stage is missing the typical 25uf cathode bypass cap, and the cathode resistor value is reduced to 820 instead of the usual 1.5k. The second stage is also nearly stock AB763 values, 100k plate and 25uf/1.6k on the cathode.
- What does the un-bypassed cathode resistor, and reduced cathode resistor value do overall in this first stage position in an AB763?
- Is it a method to have overall less gain and a cleaner clean, or some other "feel" based virtues?
The Imperial is a very popular amp, so it is interesting to try to decode what these subtle changes to the traditional AB763 circuit are doing on that Rhythm(clean) channel.
Thanks.
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It would be less gain than the typical 1.5K/25uF combo, but also, it's got an 820 ohm cathode resistor, so I don't really know where it would end up in comparison. Without the bypass cap, there's local NFB that comes into play, as well. Those with significantly more experience may be able to point you towards other circuits for comparison.
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All things being equal a bypass cap boosts the signal 5-6dB in a frequency dependent manner. A sufficiently large capacitor will bypass all useful guitar frequencies.
In this case, compared to a standard 1.5k//22μF network, it will have 4.5-5dB less gain coming out of that stage. This is a bit of a guess (I haven't done the math) as the ToneKing is biased a bit hotter at that stage with the 820Ω cathode resistor and a different set of loading conditions on the plate.
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I have tried the TK Imperial preamp circuit in my 6V6 SE project.
As I understand it, the 820 ohm cathode resistor in stage 1 is considered "warm" biased, whereas the typical 1500 ohm is more "center" biased. As pointed out above, it is unbypassed, so lower gain than Fender.
The Rhythm channel tone stack also deviates slightly from Fender, with a .02uF instead of .047, and 10K resistor instead of 6.8K to set the mids. From the tone stack calculator the .02uF cap gives a little more bass response, and the 10K resistor gives more mids.
In my experience, the Rhythm preamp stays pretty clean all the way up, and the tone is beautiful. I omitted the bright cap on the Rhythm channel volume control. I know some owners of real Tone King Imperials have removed the bright cap.
That triode with the unbypassed 820 ohm cathode resistor (labeled V1B on the schematic) also serves as the first stage of the Lead preamp. I would speculate that the designer chose that configuration to best serve both the Rhythm and Lead channels.
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An unbypassed cathode increases the stage's output impedance, eg from about 40k to about 70k, given typical 12AX7 common cathode circuit values https://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
For an input stage, an unbypassed cathode will tend to degrade the signal to noise ratio, a valve with pretty much zero h-k leakage becomes necessary.
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Thanks for the replies. These help me ask additional questions. There is the mention of local negative feedback when the cathode is un-bypassed. Is there a layman description of what this would do in this position in the circuit? ie, feel, sound, ect.
I highlighted these passages from the Aiken site. If someone wants to expound, I would be interested in learning.
"Rk is the cathode resistor, which is used to develop the cathode bias voltage. The flow of cathode current through this resistor creates a voltage drop across it, resulting in a positive voltage at the cathode of the tube. Since the grid resistor references the control grid to ground potential, this positive cathode voltage creates an effective negative grid voltage with respect to the cathode, providing the bias operating point for the tube. This resistor controls the headroom of the stage (output before clipping) and linearity, or distortion level, of the stage. As the bias point is shifted, the amplifier will clip more on the top or bottom portion of the waveform. If the cathode resistor is unbypassed, this resistor also controls the stage gain to a certain extent."
"Capacitor Ck is used to bypass the cathode resistance to ground for AC signals, which results in a higher gain. Without Ck, there is negative feedback, or degeneration, which reduces the gain of the stage and increases the output impedance. If Ck is not large in comparison to Rk, it will affect the frequency response of the stage, by introducing a "shelving" response, where the stage gain is boosted at higher frequencies compared to lower frequencies. The "breakpoint" of the frequency response is controlled by the value of Ck, in conjunction with the cathode impedance."
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There is the mention of local negative feedback when the cathode is un-bypassed. Is there a layman description of what this would do in this position in the circuit? ie, feel, sound, ect.
The type of 'local NFB' that occurs with an unbypassed cathode is called 'cathode current feedback'. With each increase and decrease in current at the grid, there is a corresponding increase and decrease in current at the cathode - and consequently, an increase and decrease in cathode voltage. This increase and decrease is in synch with the increase and decrease in plate voltage (albeit on a much smaller scale in a typical inverting gain stage). Because of this there is an overall decrease in plate-to-cathode voltage (i.e. compared to what you would get with a fully bypassed cathode). Less plate-to-cathode voltage = less gain.
Conversely, when you fully bypass the cathode with a large enough cathode bypass cap, you essentially hold the cathode voltage constant (because the capacitor stores and releases current to counteract what would otherwise be changes in cathode current). Holding the cathode voltage constant effectively increases the plate-to-cathode voltage swing, thereby increasing the gain.
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Thanks for the reply. It's filling in some gaps that I haven't thought about for a long time. I did some searching around for similar posts and saw this post by
HotBluePlates which had some very pertinent info:
https://www.thegearpage.net/board/index.php?threads/boost-voice-switch-on-hoffman-princeton-reverb.2027261/#post-28211239 (https://www.thegearpage.net/board/index.php?threads/boost-voice-switch-on-hoffman-princeton-reverb.2027261/#post-28211239)
"- An unbypassed cathode resistor for a 12AX7 will cut stage gain by about half for all frequencies. This is what you get with the 1.5kΩ resistor alone, and why it's very clean."
"- The guitar's Low E is ~80Hz, and the A at the 5th fret of the High E string is 440Hz. So there should be a definite bass-cut when going from the 25µF (or 22µF) cap to the 0.47µF cap."
"0.68µF is -3dB at ~301Hz, and full-gain at 3kHz. 1µF is -3dB at ~205Hz, and full-gain at 2kHz. With this value, you should just notice a bit of bass-shave to the guitar's bottom-octave."
"- Bypassing the cathode resistor brings the stage up to full gain for the frequencies at which the cap is "small" compared to the cathode impedance. 25µF has ~777Ω of reactance at 8.2Hz, and 77Ω at 82Hz. Being 1/10th of the cathode impedance at 82Hz is certainly "small" and so the stage has full gain for all guitar frequencies. 0.47µF will have ~777Ω of reactance at ~435Hz, and ~77Ω at ~4.4kHz (the very top end of the guitar/speaker range)."