Posting the schematic of my tube preamp:
Sounds ok but when breaks up, the overdrive is too thick.
Swapped the cathode bypass caps from 22uf to 2.2uf but can't notice any difference.
Swapped the cathode resistors from 820 to 2k4 (should be towards less gain and cold/ asymmetrical clipping).
Could it be the MOSFET follower that alters the signal spectrum in a way?
F.ex. input capacitance, how it AC loads the valve, changing the harmonic distortion (f.ex. odd vs even order harmonics)
I put it so that it buffers the valve output, thus preserving all the top end and harmonics..
I want the overdriven sound not that thick and with more chime on the top end.
There is a 220k audio pot between both stages (the gain).
I think of treble bleed cap (some more top end) but it won't affect full gain setting and the general sound signature.
Sounds ok but when breaks up, the overdrive is too thick.
Swapped the cathode bypass caps from 22uf to 2.2uf but can't notice any difference.
Swapped the cathode resistors from 820 to 2k4 (should be towards less gain and cold/ asymmetrical clipping).
Could it be the MOSFET follower that alters the signal spectrum in a way?
F.ex. input capacitance, how it AC loads the valve, changing the harmonic distortion (f.ex. odd vs even order harmonics)
I put it so that it buffers the valve output, thus preserving all the top end and harmonics..
I want the overdriven sound not that thick and with more chime on the top end.
There is a 220k audio pot between both stages (the gain).
I think of treble bleed cap (some more top end) but it won't affect full gain setting and the general sound signature.
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Hi @emosms ! Good attempt, indeed... But possibly too "Linear / Hi-Fi / Audiophile" for suitable guitar tone, by giving too much gain with a hard breakup ?
Assuming you don't want to change your design, here's what I would test, if doable, to see if there's an improvement :
C1=22µF ----> 10 to 22nF instead.
V1 RG=10K ----> 100K instead.
OUT 1 ----> insert a 470K in series.
V2-C1=0nF ----> 4.7nF instead.
This should help to reduce excess of saturation by damping, transforming it in compression and sustain.
Plus it will reduce some muddy / farty clipping tone in the bass.
Of course, you may have to tweak those values to achieve as best the expected tone ! But you already have an idea...
Another tip : back a little bit the volume pot of your guitar, it may help to smooth the ovedriven tone too...
It's difficult with tubes to achieve that singing overdiven tone with only two gain stages, and yes I'm not sure that the additional FETs are very fine for a good overdiven tone... The old Boogies and Dumbles only have plate resistor cascaded gain stages (usually 3 or 4), with serial attenuation in-between each, in order to obtain a softer clipping with sustain.
T
Assuming you don't want to change your design, here's what I would test, if doable, to see if there's an improvement :
C1=22µF ----> 10 to 22nF instead.
V1 RG=10K ----> 100K instead.
OUT 1 ----> insert a 470K in series.
V2-C1=0nF ----> 4.7nF instead.
This should help to reduce excess of saturation by damping, transforming it in compression and sustain.
Plus it will reduce some muddy / farty clipping tone in the bass.
Of course, you may have to tweak those values to achieve as best the expected tone ! But you already have an idea...
Another tip : back a little bit the volume pot of your guitar, it may help to smooth the ovedriven tone too...
It's difficult with tubes to achieve that singing overdiven tone with only two gain stages, and yes I'm not sure that the additional FETs are very fine for a good overdiven tone... The old Boogies and Dumbles only have plate resistor cascaded gain stages (usually 3 or 4), with serial attenuation in-between each, in order to obtain a softer clipping with sustain.
T
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I disagree with changing V1RJ from 10k to 100k, will only add more noise. Also a grid stopper on the second triode may be in order.
Changing/ setting input caps would form a high pass filter with the 1M to ground.
It would still have a pretty low cutoff.
7 to 33 hz (22-4.7 nf).
Especially C1 - would it make any difference?
OUT1 - 470k in series with 220k gain pot - a voltage divider that would reduce the max gain to 1/3?
Would it be any different than turning the gain knob to 1/3?
It would still have a pretty low cutoff.
7 to 33 hz (22-4.7 nf).
Especially C1 - would it make any difference?
OUT1 - 470k in series with 220k gain pot - a voltage divider that would reduce the max gain to 1/3?
Would it be any different than turning the gain knob to 1/3?
I'm not seeing a lot of gain in that circuit, not something that easily explains hard clipping. What is the B+ (or HV+)? If it is low you are just saturating.
Well, have a look to the schematic of a little Danelectro amp, the DM10, from 1964 which offers a great vintage smooth overdriven tone : the coupling capacitors are small with 3nF.
T
Changed cathode caps to 0.68 uF
Input cap 22 nf.
Second stage input cap 6n8 and added 10k grid stopper.
Now the sound is not that thick, but there is still unpleasant dirt when it breaks up (like cracklings).
Especially when playing chords.
I plan to bypass the first MOSFET follower and feed the gain pot via smth. like 470k resistor in series.
Could it be the MOSFET in this setting adding odd order harmonics or some kind of dirt?
Just assume the 'dirt' and 'cracklings' are odd order harmonics, not sure.
Also the diodes? I could not measure over 18v AC after any of the stages.
The opposite zener diodes are protection for the following opamp stages (tone control and other stuff).
Input cap 22 nf.
Second stage input cap 6n8 and added 10k grid stopper.
Now the sound is not that thick, but there is still unpleasant dirt when it breaks up (like cracklings).
Especially when playing chords.
I plan to bypass the first MOSFET follower and feed the gain pot via smth. like 470k resistor in series.
Could it be the MOSFET in this setting adding odd order harmonics or some kind of dirt?
Just assume the 'dirt' and 'cracklings' are odd order harmonics, not sure.
Also the diodes? I could not measure over 18v AC after any of the stages.
The opposite zener diodes are protection for the following opamp stages (tone control and other stuff).
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Next stage is opamp EQ which have to be adjusted.
F.ex. there probably won't be any need to roll off the bass. Setting should go smth. like 0 to + 12 db.
So I will regain the bottom end but keep it out of the valve clipping, to get my base sound.
Like tubelectron said, the smaller the coupling cap, the less 'farty' sound from bass (I think these are the unpleasant 'cracklings' at the signal decay).
Now I go with the vox top boost preamp and went down with the coupling cap to 470-220 pf - pretty much there 🙂.
I guess the phase inverter and power tubes would add saturation, gain and handle more gracefully the remaining dirt from the preamp, but that much for a hybrid amp.
HT+ is low, ~220v loaded, I can lift it up to ~250v
F.ex. there probably won't be any need to roll off the bass. Setting should go smth. like 0 to + 12 db.
So I will regain the bottom end but keep it out of the valve clipping, to get my base sound.
Like tubelectron said, the smaller the coupling cap, the less 'farty' sound from bass (I think these are the unpleasant 'cracklings' at the signal decay).
Now I go with the vox top boost preamp and went down with the coupling cap to 470-220 pf - pretty much there 🙂.
I guess the phase inverter and power tubes would add saturation, gain and handle more gracefully the remaining dirt from the preamp, but that much for a hybrid amp.
HT+ is low, ~220v loaded, I can lift it up to ~250v
I think smth is not right.
It is not completely quiet when the gain pot is turned all the way down.
The pot is grounded at the same ground where the second triode input is 🙁
It is not completely quiet when the gain pot is turned all the way down.
The pot is grounded at the same ground where the second triode input is 🙁
Got it, the filament trimpot to ground - found a minimum hum setting 🙂
Leaving it now as it is (a reminder to look up):
1. 22nf input cap, 10k/ 1M grid, R820 || 0.1uf cathode, 100k plate.
Gain. 220k pot, 470pf bleed cap
2. 330pf input, 10k/ 1M grid, 2k4 || 22uF cathode, 47k plate.
PS is +~265v loaded, can't really get a difference with the lower voltage winding.
The coupling cap is not before the gain pot, but does it really matter..
The MOSFET followers are as in the first schematic, they have output caps.
Leaving it now as it is (a reminder to look up):
1. 22nf input cap, 10k/ 1M grid, R820 || 0.1uf cathode, 100k plate.
Gain. 220k pot, 470pf bleed cap
2. 330pf input, 10k/ 1M grid, 2k4 || 22uF cathode, 47k plate.
PS is +~265v loaded, can't really get a difference with the lower voltage winding.
The coupling cap is not before the gain pot, but does it really matter..
The MOSFET followers are as in the first schematic, they have output caps.
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This means that something in the circuit turns into a blunt clipping over the overdrive. Yes, I agree : I would suspect the FET, diodes and Zeners to be the responsible... Cracklings are typical of Solid-State circuits that are not really designed to be used in overdrive / distortion mode...
Below here is the tone of my VOX AC3 Top Boost :
The guitar is a Fender Stratocaster, played by an pro friend. Volume on the amp is maxed, my pal Dood controling the volume and overdrive with the pot of his Strat. Recorded in a living room with a ROLAND R-05 digital portable recorder.
Here's a view of the amp - a defunct 1968 VOX Escort solid-state, that I rebuilt in a Tube Amp :
And here's the schematic - very simple and very classic : no FETS, no diodes...
I may be wrong, of course, but again, I would suspect the solid-state section of your preamp circuit to be not very well convenient for the use you plan and tone you expect... 😕
T
Now cracklings are more or less gone. How can I set the fet properly?
Taken from 'mossfet follies', I did some calcs years ago and have to reload it in ram/ brain, cause it is completely GONE..
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
The point is to use mossfet follower whenever u need a low Z output instead of valve cathode follower.
Not using it in gain stage and it should not alter the sound.
I can eliminate diodes and output electrolytes, but I am struggling with other module from the amp and wonder if it worths the time/ struggle?
Also change how the mosfet is set, but changing towards what?
Lower/ higher current, whatever else ?
Taken from 'mossfet follies', I did some calcs years ago and have to reload it in ram/ brain, cause it is completely GONE..
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
The point is to use mossfet follower whenever u need a low Z output instead of valve cathode follower.
Not using it in gain stage and it should not alter the sound.
I can eliminate diodes and output electrolytes, but I am struggling with other module from the amp and wonder if it worths the time/ struggle?
Also change how the mosfet is set, but changing towards what?
Lower/ higher current, whatever else ?
What are the capabilities of a PC based scope, could I monitor the nature of the harmonic content and distortions (spectral analysator?) to tune the preamp?
I just have an onboard audio and no special probes/ interface, but the audio frequencies are low
I just have an onboard audio and no special probes/ interface, but the audio frequencies are low
I am not quite content with the result.
Has anyone tried DC coupling as from the Merlin Blencowe book
The gain is set by controlling the cathode bypass capacitor.
Seems it needs kind of a reverse log pot to get it right (a lot of gain in top 10% rotation)
So a pot cold be made to react like reverse log, the problem is at top position the cap would still be 'attenuated' by (R pot || R tap).
If R tap is small enough, it would be almost full gain, but then the ratio R pot/ R tap would be probably more than 5.
I will try to put that in simulator
Has anyone tried DC coupling as from the Merlin Blencowe book
The gain is set by controlling the cathode bypass capacitor.
Seems it needs kind of a reverse log pot to get it right (a lot of gain in top 10% rotation)
So a pot cold be made to react like reverse log, the problem is at top position the cap would still be 'attenuated' by (R pot || R tap).
If R tap is small enough, it would be almost full gain, but then the ratio R pot/ R tap would be probably more than 5.
I will try to put that in simulator
Seems it responds quite linear with a 10k pot (on simulator)
And it is the same high shelf I get with a small value coupling cap (AC coupling) that I need to get rid of the farting lows.
But there are still some speckles of crackling noise.
As far as I understand, Blencow says the coupling caps themselves (AC coupling) cause some unpleasant distortion.
(or I misunderstand)
The point is I get more or less the same frequency responce (shelving), but I hope for nicer tone, without the unpleasant stuff.
And it is the same high shelf I get with a small value coupling cap (AC coupling) that I need to get rid of the farting lows.
But there are still some speckles of crackling noise.
As far as I understand, Blencow says the coupling caps themselves (AC coupling) cause some unpleasant distortion.
(or I misunderstand)
The point is I get more or less the same frequency responce (shelving), but I hope for nicer tone, without the unpleasant stuff.
- the first graph is the second stage, full gain cathode bypass
- second graph is the total gain through stages 1 & 2
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