Cascode preamp in my DIY guitar amp
Hi,
I've tried to use an ECC82 in Cascode configuration in my guitar amp. Here the schematic: http://www.prowessamplifiers.com/schematics/images/Route66.jpg
...and the cascode configuration (from The vavle Wizard site): The Valve Wizard
So i've only replaced the ef86 with ECC82 (Cascode) mantaining the same tone-stack. I like it too much...it's sound very sweet but when I crank up the volume and/or the treble knob, some audible oscillations starts. I think that pot resistance value are too high and acts like 'Antennas'.
Some one could suggest me a more suitable tone-stack to put in my guitar amp with Cascode configuration?
Thanks!
Hi,
I've tried to use an ECC82 in Cascode configuration in my guitar amp. Here the schematic: http://www.prowessamplifiers.com/schematics/images/Route66.jpg
...and the cascode configuration (from The vavle Wizard site): The Valve Wizard
So i've only replaced the ef86 with ECC82 (Cascode) mantaining the same tone-stack. I like it too much...it's sound very sweet but when I crank up the volume and/or the treble knob, some audible oscillations starts. I think that pot resistance value are too high and acts like 'Antennas'.
Some one could suggest me a more suitable tone-stack to put in my guitar amp with Cascode configuration?
Thanks!
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You have increased the gain. Layout becomes that much more critical. Push the wires around with a pencil. See what makes it worse and better.
> i've only replaced the ef86 with ECC82 (CCS)
Well, actually, that -reduces- gain, from ~~100 to ~~1. Incredibly stable and almost unplayable (too low gain). So you better draw a picture of what you actually did.
> i've only replaced the ef86 with ECC82 (CCS)
Well, actually, that -reduces- gain, from ~~100 to ~~1. Incredibly stable and almost unplayable (too low gain). So you better draw a picture of what you actually did.
No, I don't replace the Ef86 with ECC82 one- I replace the Ef86 with ECC82 in CCS configuration. The output signal of CCS goes into tone-stack.
LOL, did you replace the EF86 or not? Did you add the ECC82 follower after the EF86, or actually REPLACE the EF86 with a zero gain follower? How bout a scetch of the complete mod? BTW that is not a CCS.
Anyways I find a 'potentiometer-stopper' will cure high level squeelings, resistor should be carbon composition type but try with other if you dont have that, like this:
Anyways I find a 'potentiometer-stopper' will cure high level squeelings, resistor should be carbon composition type but try with other if you dont have that, like this:
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well, that gives you no gain and should not give oscillatory problems. If you did not have the problem before when you had the EF86 gain stage, someting is really wrong. Or did you build with the ECC82 CF from the start and never tried the EF86?
Layout is critical, but without gain I dont see how you manage the problems that are typical with high-gain guitar amps. Are you sure it is hooked up that way?
Layout is critical, but without gain I dont see how you manage the problems that are typical with high-gain guitar amps. Are you sure it is hooked up that way?
You're right! I made mess!!! I've used aMu follower like this: http://www.valvewizard.co.uk/Mufollower2.jpg
This kind of circuit tends to oscillate with tone-stack used in my amp.
Sorry to all!
This kind of circuit tends to oscillate with tone-stack used in my amp.
Sorry to all!
That explains it. Try the resistor right in front of the vol-pot. Must be close, preferably soldered right at the input of the pot.
I'm confused. The OP tells us he used a cascode, then draws a cathode follower with active tail, then says he actually used a mu follower. The only thing these have in common is that they all use two triodes wired in series.
Surely. But even this µ-follower's gain won't exceed the lower triode's µ and most probably is far less than the EF86's gain he has replaced. So, why does it oscillate?
I assume he actually built a cascode, calling it CCS, but shows irrelevant drawings to us.
Best rgerads!
I assume he actually built a cascode, calling it CCS, but shows irrelevant drawings to us.
Best rgerads!
Why not show us what you did, instead of showing/telling us something different? Then we might be able to explain what is happening.
You have increased the gain. Layout becomes that much more critical. Push the wires around with a pencil. See what makes it worse and better.
Compared to the original schem with ef86 the gain hasnt changed that much. I bet the layout is a big factor. Picture please. But as I mentioned early, you can usually tame it with a 'pot-stopper'. A mod that takes 5min to do. Have you tried that yet?
Be aware that a 47k grid stopper feeding a cascode will have much less effect on HF attenuation and phase shift than a 47k grid stopper feeding a normal grounded cathode stage. If the wiring layout was sufficiently bad that a 47k stopper was needed then you may now need more like 470k.
In valve guitar amps, I believe the large grid stopper is/ was intended to help the amp cope with unwanted high-frequency interference coming down the guitar cable, by drastically rolling off frequency response above the guitars range, rather than because all guitar amps automatically have abysmal layouts. 😀
-Gnobuddy
Definately. I wonder tho if a pentode with very little input capacitance should have a small cap to gnd, or the gridstop much larger. Vox used the ef86 in triode strap afaik, perhaps after finding it hard to tame in pentode. ?
Btw, that is not the resistor Ive been suggesting, altho Id look into that too if it were my build.
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