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tube preamp potentiometer recommendation

I want to add a potentiometer for volume control to the tube pre amp in the schematic. What should the value of the potentiometer be? Will I have a problem?


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I want to add a potentiometer for volume control to the tube pre amp in the schematic. What should the value of the potentiometer be? Will I have a problem?
Although they're not drawn like what you see in most schematics, it looks like there are two pots already, labelled W1 and W2, both 500k. I don't think I've ever seen pots with values that high before in a preamp or amp, though.

I'm no expert, but that circuit looks overly complicated to me. The 6N2 is the Russian / Chinese equivalent of a 12AX7 which has a mu of 100 but the pinout is different and their heaters are 6.3v only. So the preamp will likely have much more gain than necessary.

So, I do think you'll have a problem but I doubt it will be related to the value of the pot.

What amp will you be using this with? What is the input impedance of the amp? How much signal is needed to drive it to full power? You really need to start by answering those basic questions and then choose a circuit that fulfills the needs of the amp.

Most all modern amps don't need any gain at all to be driven to full power from a line level source. Sometimes an active gain stage can improve the sound but, IMO, using a design that has a huge amount of gain when none is actually needed doesn't make much sense.

Some of this - pot values and placement options - is discussed in the thread linked above. You'll need to read through the thread, though, not just the title.

In terms of gain, the design you posted seems to use two mu 100 voltage amplification sections while the one in the thread linked uses one mu 20 amplification stage and a cathode follower stage which adds no gain at all.
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fazildiken and FlaCharlie,

Those 2 pots are the Tone Controls.

Volume Control:
Put a 50k pot from the input connector signal to ground, and the pot wiper to the grid (or better yet to a 1k grid stopper resistor and the other end of the grid stopper to the grid).

Tone Controls:
My recommendation is to eliminate tone controls.
This is a Hi Fi Stereo, Right?
Tone Controls are best applied to electric guitars, Right?
Just my opinions (I have not used tone controls for Decades).

Suppose the playback of the recordings you listen to sounds like it needs a tone control, then . . .
Check the model of your speakers;
Check the adjustment of any controls on the speakers;
Check the reflective nature, or the dead nature of your room;
Check the placement of your speakers in your room;
Check the orientation of your speakers . . . pointing directly to your ears, or not;
Check the recordings that you listen to . . . the sound mix and tone settings of some recordings can sound bad to you (Lots of them ARE bad).
Have your ears tested.
Sorry, I am running out of more ideas of what to check.

Have Fun!
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I would install the pot at the input, right after the 0.22 cap. The viper connects to the input grid but keep the
grid resistor(500k)

Pot should be of the "log" type as it corresponds better to our perception of volume. Value of pot is not
critical, the sources ability to drive is limiting here, bit 100k could be a starting value.
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Please note that normal sources will have no trouble driving a few kOhm and this device definitely will have issues with that. So it will be worse in that aspect compared to the sources driving it possibly leading to low roll off. Output capacitor is too small for modern amplifiers. It lacks GND reference resistors at the outputs (and the novelty called muting) so it is dangerous/unpleasant to both users that change cables and connected amplifiers certainly if these are solid state. I always chuckle when I write "solid state" :) In its current state it will be an effect generator like many ChiFi tube devices. Volume control should be at the inputs before C5 and today 10 kOhm log is one of the better choices unless one has tubed sources.

First ask yourself if you really need that tone control circuit and the inserted drawbacks before taking action. I have observed many DIYers building and inserting stuff they normally would never use and tube preamps are a number 1 in that field. Needing volume control and source selection does not automatically mean one needs a preamp.... If one likes to be busy one can solve the issues that follow like the few I started with in this post. Devices like these also introduce the need for matching/marrying devices as they are designed non standardized with regards to impedances, ridiculous high GAIN! and signal levels. The famous "my volume is already too loud at 9 o'clock" possibly will occur just like noise, low roll off and the other stuff you would never experience when connecting modern sources to a standardized integrated amplifier.
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I believe that circuit is a Chinese PCB for a Baxendall tone control, and it has been featured herein the past. The usual misconception is about input and output impedances since, as mentioned above, it needs another stage to drive a lower impedance output.
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Position choice depends A LOT on what signal levels or signal input/load we are talking about.

If signal driving it is 200mV or lower, say it´s coming from an input RCA connector selector in a Preamp, choosing from Aux - Tape - Radio - CD - etc. it is better to put volume control at output, since it will not only control Volume but also attenuate hum-hiss-buzz down to zero when lowered.
100k Log would be a proper choice there.

It will NOT clip since it has Gain (when set flat) around 40X.
Given those plates as shown can swing up to some 60-70V RMS before clipping, input can handle up to 1.5V RMS, an ample margin/headroom.

Now if we come, say, from a DAC , say some 2000mV RMS possible, we ned a volume pot at the input, on penalty of unadjustable saturation (unless we control volume from DAC itself, which would turn preamp volume control redundant/useless)
For general purpose use (suppose you come from an unbuffered Tube preamp output) a high impedance would be better, say a 500k pot (even 1 M) , always Log of course.

High impedance will not cut highs, since shielded cable after it would be internal, 20-30 cm at most.

Problem being that even setting volume to zero will NOT stop internal hum buzz hiss.

If you come from a high level low impedance source, say a DAC, you can use a lower value, down to 10k.

But in that case, 40X gain would be excessive, WHAT cn you drive that needs such a high signal?

It would be better to replace first stage with a unity gain follower/buffer which would also improve tone control functioning.

My beef with these random "tube module" offerings is that they are just chopped parts from real/complete Tube preamps, made by "entrepreneurs" who want to make a quick buck or two, bought by consumers who just want to add "something Tube" to their 100% digital/SS audio chain, both buyer and seller having no clue, just based on perceived anything with orange glow inside a bottle is "better" :rolleyes::rolleyes::rolleyes:
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Confused, the schematic shows the valves used as a "6V3" which is a damping diode NOT a triode.

Re the OP's question, use what you have and see if it sounds ok, anything from 50k up should do but also depends on the source.

After experimenting and having tried out numerous valve tone controls I gave up and instead knocked up a Baxandall using 5532 opamps into a low gain common cathode gain stage direct coupled to a cathode follower using something like a 6CG7 or 6SN7. Opamps make better tone controls, you don't need so many de-coupling caps as used in a valve tone control for one thing & opamps are less noisy. It does mean an extra power supply though, however that's not really an issue if you use a little 15v 0v 15v tfmr.
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Guys, thank you all for your answers.
you said circuit is a Chinese PCB for a Baxendall tone control. I am using a media player as the audio source.

seller's comments about the product:

6n2 Baxanda Ll Vacuum Tube Tone Board with Treble and Bass Adjustment, Low Distortion and Low Noise for Diy Amplifier Audio


  • Some components on the circuit board are second-hand disassembled products, which have been tested to be normal. Due to different assembly batches of components on the finished product board, the brand, model and specification parameters of some components will be slightly different. If the component parameters on the finished board are different from the schematic diagram, the component parameters assembled on the board shall prevail! Because the batches of boards are different, the colors on the boards are different, but the use is the same, so the colors of boards are not selected.

Product introduction:
1: The most reasonable finished board of Baxandall type 6N2 electronic tube tone circuit, the Baxandall type electronic tube tone control circuit designed by British PJ. Baxandall.

2: It has been published and applied as early as the mono period. It uses the frequency negative feedback to work with a low distortion tone circuit. Although it is a negative feedback tone control circuit, it is not β Loop to complete all tone control functions. It changes the frequency characteristics of the coupling part with V1, and together with the changes given by the negative feedback, it improves or attenuates the low frequency and high frequency.

3: The characteristic of this circuit is that the control range is very wide, and when two potentiometers are placed in the middle position, a flat response curve can be obtained. During control, the shape of the treble frequency response curve is almost unchanged, but it is translated along the frequency axis. Although the bass frequency response curve is not constant, its change is smaller than that of most continuously adjustable circuits. The Baxandall type tone control circuit is the best sounding tone circuit. Today, professional stage audio equipment is also widely used.

4: Because of its unique working mode, the electronic tube device, as a tone circuit, has the following advantages compared with discrete components and operational amplifier tones: high input impedance, large dynamic range, low thermal noise, good transient response, low distortion, good air sense, simple circuit and many other advantages.

5: Double sided epoxy plate, imported metal film or domestic Hongpao resistor, Dewima capacitor frequency divider, 500K dual potentiometer high and low pitch adjustment, German EPCOS 22 μ 450V filter or other imported brand capacitors, Dewima capacitor coupling, high-frequency ceramic tube base. High voltage 250V DC power supply, filament 6.3V DC power supply. Plate thickness: 2mm, plate size: 115X115