I got this scheme:
It is basically a vox night train preamp.
But this amp produces way to much distortion for my taste (I like early blues crunch and cleans). This amp gets very little cleans.
I have also tried putting in two 12au7's And I like it better, but I know that they are not in good operating point in that sheme.
How would I need to bias them to work good? What resistor values? And would it make a better PI?
greetings
It is basically a vox night train preamp.
But this amp produces way to much distortion for my taste (I like early blues crunch and cleans). This amp gets very little cleans.
I have also tried putting in two 12au7's And I like it better, but I know that they are not in good operating point in that sheme.
How would I need to bias them to work good? What resistor values? And would it make a better PI?
greetings
Moved thread to instruments and amps where the guitar amp guys are more likely to see it.Well, you don't "bias" preamp tubes - they're pretty much interchangeable. What you need to do is figure out which one is distorting, and reduce the signal going to it.
My guess would be most likely it's V1a or V2b that's distorting first. Does the distortion decrease when you turn down all the tone controls? If so, then the splitter might be the culprit.
Subbing a 12AU7 is an old quick fix, as the lower gain drives the rest of the circuit less, but it's really not an answer. If you (re)design the amp properly, then the higher gain of the 12AX7 becomes clean headroom, which is what you're looking for.
My guess would be most likely it's V1a or V2b that's distorting first. Does the distortion decrease when you turn down all the tone controls? If so, then the splitter might be the culprit.
Subbing a 12AU7 is an old quick fix, as the lower gain drives the rest of the circuit less, but it's really not an answer. If you (re)design the amp properly, then the higher gain of the 12AX7 becomes clean headroom, which is what you're looking for.
Yes, first stage is probably clipping first, because tone controls don't effect clipping so much. In matter of fact they are preety bad, treble knob is doing almost nothing, cleans are little muddy. I am probably putting in fender tone controls
Yes I want more clean headroom. now I turn gain knob like 8'o'clock in the morning and it is already distorting. I don't want that much distortion till 10-11 o'clock
Yes I want more clean headroom. now I turn gain knob like 8'o'clock in the morning and it is already distorting. I don't want that much distortion till 10-11 o'clock
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Well, I assume that you've built it per the schematic and the distortion is just from too much gain causing it to clip. What I always recommend in this situation is to not do anything except remove one or more cathode bypass caps on the 12AX7 stages to cut down the gain until you clean the sound up to where you like it. It's the cheapest solution, easily reversible, doesn't change the bias, doesn't have the expense of buying new tubes.
Leadbellys idea of lifting the cathode bypass cap on the input tube is something you should try. This will help figure out if the first stage is clipping at a given input. I kinda dont think so.
Another expirment would be to bypass the third pre-amp tube. Come off the plate of V1b with a 22 nf cap and go to the junction of the two 1 M resistors on the input to the phase splitter. This can reversed easily. I suspect that the overall gain is too high. (both ideas really easy to do).
Another expirment would be to bypass the third pre-amp tube. Come off the plate of V1b with a 22 nf cap and go to the junction of the two 1 M resistors on the input to the phase splitter. This can reversed easily. I suspect that the overall gain is too high. (both ideas really easy to do).
What if I convert entire second tube into long tail pair Phase inverter? Would I solve something?
I will try your suggestions first.
I will try your suggestions first.
This idea about clipping in the 1st stage is nonsense. If you are clipping in the 1st stage it means you are feeding in a hot signal, not something from a guitar directly, and if that's the case then just turn down the output of whatever is in the signal chain. If you were clipping at the 1st stage the volume knob would have no impact, and you already said turning it down cleans up the sound.
Reduce R3 and R8 to 100k at most. I breadboarded the 12AX7 once and tried getting it in the range of a 12AY7 and used around 50k on the plates with 1.5k on the cathode. I second trying it without the cathode caps, you especially do not need it in the third triode. Your PI is fine, it has a gain of about 1.
You must be joking...
Anyway, changing the anode resistors to 100ΚΩ is the first thing to do, but change the cathode resistors accordingly, to be in the same operating point, Lower them to 820-1KΩ. If you do still have too much gain, change the ECC83 to ECC81 (12AT7). No change of resistors this time, it is close enough.
Not that I am really up on things but I thought the greater the ratio between the anode and cathode resistors the greater the gain. So if you drop down the plate resistor and the cathode values equally you will not be reducing gain.
No, it works like that. Bigger anode resistor=more gain, less current. Smaller= less gain, more current. But , to keep the valve in around the same bias, you have to increase the cathode resistor too.
The cathode resistor is just the feedback mechanism that keeps grid-cathode voltage more or less constant (more - or -less depends on the size of the bypass capacitor, if any). Nothing to do with gain, in a centre biased stage.You can see what happens with anode curves and hypothetical load lines.
Thanks guys! Today I replaced that Vox tone controls with Fender tone controls(improved the tone very much - not anymore muddy), replaced 220k plate resistors with 100k ones and made a bypass for third gain stage with SPDT switch and some caps, I also made a bright switch(like on Twin Reverb). Now I get pristine clean sound like on twin reverb, but without reverb (I also have homemade reverb pedal) on that setting - Rhythm - full gain means just a little crunch now, and if I put it on lead I get classic rock machine!
Here is the beast if you are interested in seeing it.
I just messed the controls positions 😀
Yes, I m cronic DIY-er. I have bunch of homemade effect pedals, 2 tube amps, and homemade telecaster.
Here is the beast if you are interested in seeing it.
I just messed the controls positions 😀
Yes, I m cronic DIY-er. I have bunch of homemade effect pedals, 2 tube amps, and homemade telecaster.
That pretty much is fender type tone circuit, maybe slight differences in values of R & C... I can't remember off hand
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