I recently decided to have a dabble with a tube amp to see if I can make a reasonable design myself.
As a total tube newbie I started with the veritable 6J1 bile amplifier from China.
I didn't have an AC transformer but I got a smps with hv and heater outputs.
I've prototyped the design, adding gain, tone and volume controls. I changed the tubes to GE5654.
But it sounds... Bad. There is no hum but the distortion sounds crude and rough, more like a bad fuzz pedal.
I'm not totally sure it's being biased right and I'm still reading up on load lines etc
I've captured the schematic and build in case I'm making newbie mistakes!
I know I need to change the caps to polymer - they are on order.
Has anyone built one like this specifically for overdrive distortion?
Cheers
As a total tube newbie I started with the veritable 6J1 bile amplifier from China.
I didn't have an AC transformer but I got a smps with hv and heater outputs.
I've prototyped the design, adding gain, tone and volume controls. I changed the tubes to GE5654.
But it sounds... Bad. There is no hum but the distortion sounds crude and rough, more like a bad fuzz pedal.
I'm not totally sure it's being biased right and I'm still reading up on load lines etc
I've captured the schematic and build in case I'm making newbie mistakes!
I know I need to change the caps to polymer - they are on order.
Has anyone built one like this specifically for overdrive distortion?
Cheers
Attachments
I don’t have 5654 curves in triode, so I can only comment on frequency shaping for a guitar preamp: change C10 and C16 to 1 nF and remove C17.
Here are the 5654 (6AK5) triode curves: https://www.diyaudio.com/community/threads/6ak5-in-triode-plate-resistance.343410/post-5931791
Is easy getting a bunch of gain and blasting a tube to distortion.
Starts to sound basically as described a farty fuzz pedal.
Basic class A gain stage clips heavily on one side.
The typical progression of good guitar distortion is pre filter, post filter.
Far as clipping. Enough to get one side to slightly clip, then it is inverted and ran into another stage.
So you get equal clipping on both sides. Takes 2 tube stages.
The second stage doesnt have much gain it is already provided by first stage.
The inverted signal is clipped again to get both sides clipping.
Either too little or not enough current for each stage. And to much gain.
You want slight clipping and the right current needed to achieve it.
Easier to design proper looking at the waveforms of each stage.
Starts to sound basically as described a farty fuzz pedal.
Basic class A gain stage clips heavily on one side.
The typical progression of good guitar distortion is pre filter, post filter.
Far as clipping. Enough to get one side to slightly clip, then it is inverted and ran into another stage.
So you get equal clipping on both sides. Takes 2 tube stages.
The second stage doesnt have much gain it is already provided by first stage.
The inverted signal is clipped again to get both sides clipping.
Either too little or not enough current for each stage. And to much gain.
You want slight clipping and the right current needed to achieve it.
Easier to design proper looking at the waveforms of each stage.
Not strictly true. Most famous amps clip one side mainly (see cold stage and CF two stages apart), and many unbypassed stages.
That's normal @britincali.
Usually, you never achieve a good preamp distortion tone with only two stages, but rather a fuzz sounding tone... In fact, you need a third gain stage, plus serial resistors of 220 to 470K in the grids. This is the way to offer smooth clipping and sustain.
Marshall, Boogie, Dumble, Matchless (HotBox) all went for this classic, long proven solution, and after much tests, I had to go to this solution myself too... Even on a SF Twin-Reverb with MV, you find 3 stages, and serial resistors.
An exception though : if you plan to use clipping diode in your circuit, like it was the case on the Westbury W-20 The Tube (1978-1982, historically one of the first tube overdrive pedal) and on some Marshall amps (2210, 4210, JCM900 series).
But it's me, OK ? 🙂 😉
T
Marshall used diodes so it did not take so many stages and padding to get = both sides to clip.
The outcome is asymmetrical yes, the art of no fart verses good smooth sustain. Has been well covered by many manufactures.
From Boogie to SLO to 5150 to 6505
Why waste time "proving" how the circuits work and what the pre and post filters do and padding needed for multiple
stages works.
Yet alone recognize the waveforms you would typically see at all those stages.
Might as well resort to wild guessing and magic theories.
The whole thing was well discovered when certain pre amps distorted better than others.
And the original circuits weren't even designed for distortion. They just sounded better with booster pedals.
Eventually the booster pedal was built into the amp with another tube stage.
So yes, One stage for the gain. Then the " magic" circuit = 2 stages 1 side distorts, inverted then followed by another stage less gain
to clip the other side. 3 specific early amps did this to actually drive tone circuits not distort. But sounded rather good boosted.
Hence why copied and modified so much. Mods over time, once you have a good clipping circuit not fart circuit.
Is numerous pre and post filter methods.
Why waste time with this, and bother showing the history of the circuits. Yall got it figured out.
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qwhit dragon :Why waste time with this, and bother showing the history of the circuits. Yall got it figured out.
👍👍👍
Building a guitar preamp without following the circuit of a famous manufacturer is, in my opinion and with all due respect... a waste of time!
Let's look at the Orange tiny terror circuit. Simple but very elegant.
👍👍👍
Building a guitar preamp without following the circuit of a famous manufacturer is, in my opinion and with all due respect... a waste of time!
Let's look at the Orange tiny terror circuit. Simple but very elegant.
The stages are biased at 0.8V, 0.9V, try biasing to 1.5V Increase R26 at least100k, reduce C10/16.