PI pushed too hard issues

[isaac]

Hey guys, I have built an amp that has a deluxe reverb normal channel single channel (and PI) with some mods in the preamp, and then the back-end of a 18w Marshall. Here's the schematic I drew up, no power section sorry... It's just a 18w marshall, (or similarly an AC15) oh well. But the preamp is creating too much gain and is driving the PI into nasty distortion, not just overdrive, and then of course, it also has lots of gain, but oh well. Actually right now though, it doesn't have PI push issues because of a voltage divider, but it shouldn't have that problem at all, and I want to figure it out. So here it is, please try to figure out how to not need the voltage divider.

I can NEVER get the upload image to work on this website... so here's the link.

https://lh3.googleusercontent.com/-x...o/gt18+pre.PNG

Notes: No NFB, obviously modded a little bit, the B+ voltage where the plate resistors get voltage is ~280VDC, I can get that lower obviously, with a bigger dropping resistor.

But yea, this amp has a problem with pushing the PI too hard, and I not only hate that problem, but it has a bit more Gain on tap than it should. Also, that TS lift boost I have there, creates a totally different tone, which is basically a drive channel, so when the tonestack gets lifted, and the extra tone control turns on, I have basically a separate drive channel, and a pretty good boost (DB-wise).

I'm hoping to return the one plate resistor back to 100k, and the cathode pairs to the exact DR (more like a TR because of Mid control... but oh well...) specs, soon.

Any Help is appreciated.
-Isaac

[loudthud]

Why the bigger than normal cathode resistors on the 12AX7 ? Have you tried moving them down to the normal range? Post some plate and cathode voltage readings.

[isaac]

Hey, yea, I don't know why I did that, but I did... I'm going to restore the plate and cathode resistors to normal values next chance I get. I think I referenced that briefly. Should I change the cathode caps while I'm at it?

Hey, and I have posted numerous times since joining here, and the about 10 of you guys who I find your names on a zillion of the posts, have been great. and I thank you for that. I just also in the last 5 minutes posted again about a tweed amp I built, so maybe you guys might help me there, also.

[loudthud]

The cathode caps will change the overall tonal balance. I would see if just changing the resistors moves the amp in the right direction and if so, tweek the caps to see if it can be improved more. You'll probably want to tweek all the caps in the amp once you fix the ugly distortion.

[isaac]

Yea, I'm hoping the resistors will get me there. If that helps, maybe then the tonestack lift boost won't be overdrive as much, at least.

[Chuck H]

Since you didn't include anything past the PI I don't know if your using a post PI master. If not then you may not be hearing ugly PI distortion. You may be hearing the notorious EL84 buzz. EL84's don't like to be shoved around at the same signal levels as 6V6's.

Are you using a post PI master?

Have you scoped the signal from the PI to determine that this is where your ugly distortion is coming from?

[soundmasterg]

I assume the divider that you are talking about is the 1M and 220k before the phase inverter?

You can adjust the phase inverter to accept a larger input without distorting by changing the 470 ohm resistor in the cathode of the PI to a larger value...perhaps 680 ohm might be a good starting point to see if you are going in the right direction. Another thing to keep in mind is that EL84's as mentioned by Chuck don't like to see as large of an input signal as 6V6's since the EL84 has more gain, so you should adjust the phase inverter to output less voltage gain when working with EL84's.

Greg

[pdf64]

Yes, a good way to reduce the output voltage from the phase splitter is to reduce its VB+.
To keep the same time constants, a suitable zener can be used on that node.
Too much output voltage p-p swing from the phase splitter will cause blocking distortion at the power tube grids, which can get ugly in large doses.
EL84 are very susceptible to this, as noted.
If the VB+ is reduced significantly, eg 20% or more, then the phase splitter bias may need attention to retain the same stage gain / operating point (it may not be needed).
Pete

[isaac]

I hadn't thought of it being the el84's. No PPIMV it's just plain out there. What would be the best way to get less output from the phase inverter so as to not get nasty distortion, (assuming it is the el84's.) I have tried an 820 ohm resistor on the PI before, I couldn't hear much difference. But will definitely try this stuff. Once I get my preamp changed a little bit so as to not be too gainy, maybe my problem will be smaller, then I can deal with it a little better. I'll increase my dropping resistor at the last filter cap stage so the voltages are lowered here. I just converted to a SS rect. a little while ago, so that might not have helped at all either... Since I got probably a 40V jump. I actually right now have on the plates of my 84's, about 410VDC, pretty high, I don't worry though, since you are supposed to be able to do up to 450V, as long as your screen voltage is under the plates a bit. Although, I'm not sure of tone at those levels... It still sounds pretty good though.

[Chuck H]

There are a number of ways to get the voltage back down. 18W type amps typically run a hot bias, the high voltage complicates problems with crossover distortion and grid loading.

You said the power supply was a typical 18W type. But now you are telling us that you have a diode rectifier. Is there anything else we should know? A schematic of exactly what is in your amp now would be best.

I would dare call myself proficient at eliminating grid loading and crossover distortion problems with EL84's. I can help with this. But it will require effort and diligence on your end.

[Enzo]

I am a little foggy at the moment, I just woke up, but how have you determined the power tubes are where your distortion is occurring? I'd get out a scope and look at the signal path. For example, the Peavey Classic 30, very popular amp, on the clean channel, it starts to get a little ratty after maybe 4 or 5 on the volume. What is happening there is the phase inverter stage is clipping. The power tubes could handle a lot more signal, but the PI can;t send more without clipping. I'd hate to see you changing power tubes and dropping voltage to them and all this stuff if your PI was clipping.

[Chuck H]

Enzo is right. The only way to KNOW is to scope the amp. I'm speculating the problem based on the available information and my familiarity with the circuits in question. IMHE (provided the circuit is as shown and working correctly) the power tubes in this amp are clipping significantly before the PI. But I wouldn't make any such assumptions if the amp were on my bench. I would scope it. Sans scope, which seems likely, we need to proceed with the available information or the amp needs to be turned over to a qualified tech that DOES have a scope and is willing to complete the design for an acceptable fee... Which seems unlikely.

[isaac]

I need to scope it, the reason I started to assume it could be power tubes is because my design in teh preamp and PI is SO similar to a deluxe reverb normal channel, that it would seem that that much should work, but I need to get to work on it, as I haven't had time as of late... Maybe monday I'll be able too, tonight I can't get to it, tomorrow definitely no, and Monday is the first day that I'd have the chance, but yea, I will change some components around, and see what I get, and yea, My power supply circuit is just if you take a 1974x marshall and replace the EZ81 with two diodes, that's the only difference there. I'll scope it first chance I can also, as I do have a scope, so once I restore some components back to normal values, I'll scope it out. Thanks for your help, I'll see what I get ASAP, hopefully monday...