Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on October 28, 2011, 10:29:11 am
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Hi. I just signed up mainly to ask a question regarding the PI on an amp i built. It's similar to a JCM 800 single channel amp, but with a it more gain in the preamp. I have been tweaking it to find what i feel is an ideal JCM more or less. The PI is the part i have been going around a bit with. I have tried it with and without NFB, i have tried various changes to the PI values going from the typical JCM values to as high as 1.2k on the cathode and 56k on the tail. What i'm trying to do here is get the pre PI master to me more consistent in tone from low to high. At this point it sounds great till it crank it up past low home playing levels towards stage levels, say the volume you'd be at in a small bar. Somewhere in between those 2 levels it somewhat abruptly loses the smoothness and becomes rather harsh and bright and very different. I was hoping for some thoughts on how i might fight this issue. Thank you.
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The problem is controlling the preamp gain.Lots of preamp gain at lower volumes is almost always trebly and buzzy.
The problem lies with the pre-PI master I would think.
think plexi: lots of gain but not until you crank it up.
I would lower the preamp gain and ditch the pre-PI master for a post PI master.Then you can get power amp and preamp gain and control it with the post PI master volume.
More organic sounding to me.For you the trick would be the balance between preamp and power stage gain.
changing the phase inverter won't really do it.That's my approach on one channel amps.
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Let me clarify this a bit. It actually isn't buzzy or trebly at all during low volume, in fact quite perfect. The issue is as i turn the master up the smoothness and tonal balance and richness abruptly goes away and it becomes harsh and bright somewhere between home volume and small stage volume. I tried a post PI at one point but then it DID become buzzy and thin at low volume.
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TUT 3 talks about putting a 100k resistor in series after the master volumes wiper. I don't know if I'm explaining it correctly;
Let's say you'd take the wiper wire off (middle lug) put a 100k resistor and then wire it from there. Make sense??
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TUT 3 talks about putting a 100k resistor in series after the master volumes wiper. I don't know if I'm explaining it correctly;
Let's say you'd take the wiper wire off (middle lug) put a 100k resistor and then wire it from there. Make sense??
Right. It's called a grid stopper. It sounds like you're experiencing blocking distortion. You've increased the gain of the previous stage(s). Vout of the previous stage exceeds Vin max for the PI and it starts blocking. Can get from schweet to unusable in a 1/4 turn.
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Yes, thats rather what i've thought. But i tried a 100k there and even up to 220k with no luck. Alos tried at the master in lug for the heck of it. But it's not a grid stopper because theres a cap after the master. So i tried it as a grid stopper after the cap right on the pin with no luck there either, tho that was a while back. Does seem like you're right tho. But i don't know how else to fight it. If i lower the preamp gain the tone just goes to sheet. I designed the pre for a good driven tone and lowering it to where i'd need power amp drive to make up for the loss wouldn't work for me anyways since i never turn up that loud. At low volumes the preamp has just the sweetest warmest tone. I wish i could make that happen a bit louder too, but it all goes out the window by the time it's at somewhere a bit below low stage volume.
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post a schematic
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I don't have one, but it's similar to a 2204 but with grid stoppers an all gain a stages a no cold biased stage in the pre.
By the way, i googled the 100k resistor mod on the master's output and someone sad that it s used to keep the tone from changing because t forms a hi pass filter with the tone stack. He sad that it keeps the master from sounding thin at low settings and that without it a pre PI master will sound thin at low settings and get darker as it's turned up. they were discussing jcm 800's too. so what i don't get is, why is mine doing the exact opposite ! Without the resistor it's thick and warm low, and as i turn it up it gets way brighter. Add the 100k to the master's output (center) lug and it then gets very bright at low settings. Seems odd that it would do the exact opposite. But since he said the variation in tone as you turn it up without that resistor is due to the interaction of the master with the tone stack or treble control, then i wonder if somehow isolating the treble from the master would work.
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Can you just draw what you did from the eq to the PI? You're giving a lot of info but to me it seems to contradict itself at times. :dontknow:
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I mentioned this in my other thread, but just look at a 2204 schematic and from the cathode follower through the PI it's the same except of the cathode and tail values on the PI and a 47k slope in the tone stack. But just value differences, no circuit changes other than a switch to lift NFB.