I am building a little single 12AX7 tube preamp for bass guitar , and the output is way too hot so I am going to put a voltage divider attenuator between the two triodes. I have been looking at a schematic for a Gallien Krueger amp that uses several 12ax7 in the preamp and then uses a voltage divider 1M / 100k to reduce a 22v pp output to 1.9v pp. So I think it's ok to do something like this , but then I'm thinking of other amps that use bypass caps on volume controls to retain otherwise lost treble. Now I realise that a bass guitar signal doesn't contain much high frequency , but then there are harmonics and so forth that do give a bass guitar an "airier" sound , and I don't want it to sound boomy. So , depending on how much the voltage is reduced , how does one know what high frequencies and how much of those higher frequencies will get attenuated, over and above the attenuation of the other frequencies ? Or is it more of a psychoacoustic thing? Or is the loss simply negligible as far as a bass guitar sound is concerned , even when cutting the output by 90% as the Krueger does?
Why would you amplify the signal so much, only to burn it in a resistor? If you built it yourself, maybe change the load/bias to reduce the gain instead?
In place of wasting the amplification, you can either use negative feedback that enhances the quality of an amplifier, or use a lower gain tube like 12AU7 or 6BQ7 or ECC88 for saying some options, or both: use lower gain plus NFB.
An attenuator reduces all frequencies equally (except well above the audible range).
You won't need a bypass capacitor unless you want to change the tonal balance and
also attenuate the signal at the same time.
Or use a lower mu tube and an appropriate operating point, so there won't be excessive gain in the first place.
You won't need a bypass capacitor unless you want to change the tonal balance and
also attenuate the signal at the same time.
Or use a lower mu tube and an appropriate operating point, so there won't be excessive gain in the first place.
The caps you mention are there for "loudness" compensation. It is assumed that the sound level is high when the pot is set high and low as you turn it down, so to maintain a pleasant tone, the bass and upper highs are boosted at the lower pot position, to compensate for the Fletcher-Munson curve.
https://www.first.edu/blog/fletcher-munson-curves-explained/
https://www.first.edu/blog/fletcher-munson-curves-explained/
By ear.
A cascade of no-NFB stages with attenuators in between is an old-old (WWII) technique for pulse-shaping. See the MIT RADAR books. These taught a NFB cascade for other purposes (mostly bandpass).
Guys, this is a bass guitar amp, not a hifi amp. Normal rules do not apply.
You can calculate the roll-off by knowing the divider resistance and the following Miller capacitance. That doesn't necessarily tell you how it will sound of course, it's just a clue. Ultimately you have to cut and try.
The amount of cut doesn't correlate directly with loss of treble, it all depends on the component values used. Counter intuitiviely, you get the most loss of highs with a 1:1 divider. A 10% divider, or a 90% divider, will both leave you with more highs than a 50% divider!
Having copied a GK amp, the circuit will have two classic gain stages 100k on the plate and 1k5||22u on the cathode, an eq stage between the two gain stages, a B+ around 320V, and a voltage divider to adapt the signal to the sensitivity of a class D power amp.
I suggest you a source driver DC coupled to the second tube gain stage to have a low output impedance. I did it for a very similar bass amp I built 20 years ago.
Shall this thread be moved to the instrument amplification section?
I suggest you a source driver DC coupled to the second tube gain stage to have a low output impedance. I did it for a very similar bass amp I built 20 years ago.
Shall this thread be moved to the instrument amplification section?
So not sure any of these replies answer the OP's question.
That pot bypass cap is called a high pass, and it's because in many guitar amps, the response is skewed to have less bass and more highs, especially at lower volumes. To match the actual instrument and to what we want to hear at any given volume, or in a mix with drums, bass, and whatever else is in the band.
I am guitar player and amp builder, not a bass player. So I don't know if bass amps have this. Hearing it might be in a G&K, I can say those do sound punchy and maybe a little more treble happy than a normal bass amp. So maybe they do.
If adding, I'd at least make it switchable, so can have it on or can turn off. As far as a voltage divider to match output levels. Harmless, and you should, as long as the resistance values of your divider match to what the output circuit is expecting to see. If those are way off, as any output load being way off, will significantly affect tone and performance.
That pot bypass cap is called a high pass, and it's because in many guitar amps, the response is skewed to have less bass and more highs, especially at lower volumes. To match the actual instrument and to what we want to hear at any given volume, or in a mix with drums, bass, and whatever else is in the band.
I am guitar player and amp builder, not a bass player. So I don't know if bass amps have this. Hearing it might be in a G&K, I can say those do sound punchy and maybe a little more treble happy than a normal bass amp. So maybe they do.
If adding, I'd at least make it switchable, so can have it on or can turn off. As far as a voltage divider to match output levels. Harmless, and you should, as long as the resistance values of your divider match to what the output circuit is expecting to see. If those are way off, as any output load being way off, will significantly affect tone and performance.