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Old 1st September 2011, 03:51 PM   #1
jule is offline jule  Slovenia
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Default Want less distortion in guitar amp- 12au7?

I got this scheme:

Click the image to open in full size.

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?

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Old 1st September 2011, 04:11 PM   #2
kevinkr is offline kevinkr  United States
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Moved thread to instruments and amps where the guitar amp guys are more likely to see it.
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Old 1st September 2011, 05:15 PM   #3
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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.
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Old 1st September 2011, 05:23 PM   #4
jule is offline jule  Slovenia
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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

Last edited by jule; 1st September 2011 at 05:25 PM.
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Old 1st September 2011, 05:28 PM   #5
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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.
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Old 1st September 2011, 05:55 PM   #6
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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).
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Old 1st September 2011, 06:00 PM   #7
jule is offline jule  Slovenia
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What if I convert entire second tube into long tail pair Phase inverter? Would I solve something?

I will try your suggestions first.
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Old 1st September 2011, 06:12 PM   #8
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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.
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Old 1st September 2011, 10:42 PM   #9
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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.
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Old 2nd September 2011, 09:04 AM   #10
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Quote:
Originally Posted by Keriwena View Post
Well, you don't "bias" preamp tubes - they're pretty much interchangeable.
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.
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