Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cboysen on December 03, 2021, 02:47:05 pm
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Hey fellow tinkering tube nerds!
Lets talk about the E-series tone stack for a moment. You know, the mysterious tone stack found on the tweed amps like the Tweed LP Twin, Super and Pro (5e4-8). Here is a schematic from LTspice, including the Treble and Bass plots. Top 1M-A is treble, bottom is Bass, where the plot shows what each knob turned down simultanously will do. Sorry for the odd numbering of components.
(https://i.imgur.com/PPdGKhl.png)
The bass knob lowers the overall volume, favoring more of the bass. When nearing full clockwise rotation, C13 will start to scoop out the mids from 600Hz to 1kHz depending how far you turn the dial.
The Treble knob is tuned to a cut-off way below 100hz due to C11 at 10nF, and will sort of act as a normal volume knob. C11 also scoops out the high-bass around 100hz, so when turning the knob clockwise, an increase in both treble and bass will occur. As a result, at full rotation, the frequency response is flat until 600hz, where it starts to rise as a 1st order high shelf. If you want the treble knob, to only affect treble frequencies, derease C11 to something like 1000pF to 250pF. C9 gives a slight high-mid boost if increased, but mostly serves to isolate the bass and treble side.
Resistor R20, 220k to ground, is a leftover from the local NFB loop from cathode to grid from previous cathode follower stage. It forms a high-pass filter with C10, cutting below 20hz at 220k. You could lower this for a fixed bass-reduction, or just remove it, in case you don't use the NFB.
Resistor R18, is neceesary to provide a high enough RC cut-off point, for the bass knob to be usable. As a side effect, the bass knob also lowers the overall volume since R18 forms a voltage divider with the bass knob.
Resistor R43, 220k provides isolation between the treble and bass knob. It also slightly eases the signal path through the treble side, giving more highs than bass.
I have recently built an E-series type amp; Something like 5e8 preamp, including reverb, a 12ax7 for the cathode follower, and a 5e3 output section. I wanted to remove the bass/treble interaction, and have them only cut their respective areas. So, I have come up with two variations that does just that, without losing too much of the E series vibe, imo.
First variation looks like this:
(https://i.imgur.com/X3lbjX1.png)
At the bass side, R20 is removed, and a 10nF capacitor is added in parallel with C10 and R18. R18 in increased to 470k, to accomodate pin 1 of the volume pot, being tied between C10 and R18.
When the bass knob is turned counter-clokwise (down), the signal path will favor C13 (10nF), as R18 (470k) is paralleled with a high resistance from the volume pot. As the Bass knob is turned clockwise (up), the resistance from the volume knob is decreased, until C13 and C10 are shorted at one end, providing a full bass response.
Since we lose the volume drop from the orignal circuit when the dials are at noon, a -6dB voltage divider is provided between R43 and R44, to avoid overloading the preamp. In this variation, the treble pin 2 (output) connects directly to grid of the next stage. This is fine, if bass isn't reduced prior to the tone stack. In my amp though, I have a 1uF cathode cap at the first triode of the cathode follower. In that case, you may want to tie the treble pot pin 2 between a 220k resistor and the voltage divider, like this:
(https://i.imgur.com/Y3IvjL9.png)
Hopefully this will be interesting to some.
Kind regards, Christian.
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Good work. I do like your new approach.
I am curious. Your stock E is not SPICE plotting like Duncan's TSC. At a glance your setup seems equivalent; I offer it for the keen-eye readers to see if they have been set-up different (I don't want to say 'right' because clearly Leo was fumbling here).
One difference noted with a red *. 100k or 220k? I have not gone back to Fender docs; I doubt it can be a huge difference. (Yes, my tattered copy of TSC is using a font that overflows the spaces. "20K" is in fact entered as "220K", etc.)
Your plot covers ~~22dB and TSC took it down to -40dB, so I made some yellow numbers and lines to get similar reference.
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Lets talk about the E-series tone stack for a moment. ...
The Treble knob ... will sort of act as a normal volume knob. ...
I wanted to remove the bass/treble interaction, and have them only cut their respective areas. ...
Guys that play 5E_ series amps & don't tinker perceive the tone controls to shape how much drive the preamp sends to the power section. Intuitively, they leverage the feedback to clean up the sound (or not) at their desired loudness.
So there are some players who find utility in the "defective nature" of the 5E_ series tone circuit.
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Good work. I do like your new approach.
I am curious. Your stock E is not SPICE plotting like Duncan's TSC. At a glance your setup seems equivalent; I offer it for the keen-eye readers to see if they have been set-up different (I don't want to say 'right' because clearly Leo was fumbling here).
One difference noted with a red *. 100k or 220k? I have not gone back to Fender docs; I doubt it can be a huge difference. (Yes, my tattered copy of TSC is using a font that overflows the spaces. "20K" is in fact entered as "220K", etc.)
Your plot covers ~~22dB and TSC took it down to -40dB, so I made some yellow numbers and lines to get similar reference.
The plot dB ranges, are determined by the iterative nature of Ltspice .step param functions, where I only deal with 5% to 95% range, thus any really crazy settings won't show up on the plots. About the 100k or 220k resistor, I noticed this as well. The 5e4, 5e5 and 5e7 iI believe, all uses 100k, where as the 5e8 (Tweed Twin LP) uses 220k. Increasing the value, drops the amount of signal passing through the bass circuit. Together with R43, it also determines how much bass is cut, relative to the pot level. If you play with duncans tone stack, shorting R18 (duncans R3), will result in no over all volume drop, but also the bass knob does practically nothing above 10'oclock.
Lets talk about the E-series tone stack for a moment. ...
The Treble knob ... will sort of act as a normal volume knob. ...
I wanted to remove the bass/treble interaction, and have them only cut their respective areas. ...
Guys that play 5E_ series amps & don't tinker perceive the tone controls to shape how much drive the preamp sends to the power section. Intuitively, they leverage the feedback to clean up the sound (or not) at their desired loudness.
So there are some players who find utility in the "defective nature" of the 5E_ series tone circuit.
I completetely understand, and respect that way of perception, but after playing the amp like that, I found it too much of a hassle getting a good cosistent tone. For example, if I want to cut the bass, I HAVE to accept quite a volume loss too, giving me more preamp gain. Not necessarily something I want. Same goes for the treble knob. In that case, I might as well just have a single volume knob after the cathode follower. Also, since the reverb in my amp is tapped just before the tone stack, I would also get more or less dry signal passing through.
Kind regards
Christian