5150 Preamp

[erock0138]

Hello, I'm working on a 5150 style tube preamp I made.

It works great, sounds great, exactly how i want it to. Except when you turn up the volume. At low volume it sounds fine, the highs are rich, and classic 5150 sound. When you turn the post volume up, all of the highs disappear, and the mids are so strong. I've triple checked everything, all of my power supply voltages are in spec, as well as grounding, bias, etc.

Any guidance would be much appreciated.

[tedmich]

since the 5150 was NEVER a stand alone preamp I suspect its the interface between the (now stand alone) preamp and whatever power amp you are using. How is the PI set up? I'd look at some stand alone pres, like the Rockmaster, and mimic that PI setup.

[erock0138]

I'm running into the effects loop return of a 5150.

[tedmich]

methinks the 5150 FX loop was never designed to take these levels although I could be wrong, how does a std. preamp-> power amp setup sound?

[erock0138]

It has the similar problems into a standard power amp, but didn't seem as drastic, but was still there. It's almost like the volume being above 3 and it's over compressing all of the highs. I put a scope on it this morning and the output waveform doesn't change with volume increase, but to the ear with a guitar it does.

[tedmich]

I can think of nothing except a (4th) check as your preamp is a completely unique version of a 5150; it highly unlikely anyone has ever encountered this specific problem. Maybe someone knows exactly what might cause this in a 5150 but something HAS to be different. I'd get a bright light and do a sloooow crawl over the preamp with the schematic in hand and highlight each trace and component as its verified.

Or Enzo or some such demigod will post with a single reversed cap that causes just this symptom... good luck!

[Enzo]

Only thing occurs to me is on a real or whole 5150, the louder you play the more the B+ sags. Your B+ voltage may start out the same but won;t be sagging. You can plug this into a real 5150 FX return, and THAT B+ will sag, but the preamp one will not.

I am assuming your project has its own power supply.

[erock0138]

Thank you for the replies.

I have fixed the high frequency issue with the help of a cathode follower, Instead of plate fed tone stack like the 5150
But now I am not having another problem with it being too loud.

It's a "regular" 3 tube High gain preamp, with a cold clipping stage, with a cathode follower feeding a marshall style tone stack then volume out. The C.F. load is a typical 100k, but it increased the output volume dramatically. Looking at some other schematics, there was a resistor placed in series with the 100k to ground, (2.2k, or 6.8k) and the output taken from the top of that resistor meeting the bottom of the 100k. That took care of the super loud output problem, but now am getting a loss good tone like i had before.

I changed that to a 15k, and it sounded the best, and most preserved. Are there any limits that this resistor can be at? The more I increase, the better it sounds.

Any advice would be appreciated. Thanks so much all.

[km6xz]

You have a stage gain problem. How much level can each stage accept and drive to preserve dynamic range? The effects loop problem was driving a relatively low Z return with a high z drive and probably a much different level than it was designed to be driven by. What is the peak to peak output of the preamp? One of the stages was going into heavy compression by being driven way too hard to have much HF content retained. If the resistor network you are talking about is configured like a voltage divider, the bottom the ratio of those resistor values will determine the ratio of the total signal versus output of the divider. I am a little confused by your description, can you post a drawing of your CF and divider? If it is wired as a divider, lowering the lower leg value would drop the output without changing the load on the cathode very much at all so it should sound the same just lower in level. But it depends on how you actually have it wired versus how I assume you have it wired.
When talking electronics versus musician talk, use level measured values instead of "volume at 3", which means nothing. If driven by 10 millivolts, "3" is going to mean something entirely different in terms of sound than driven by 100 millivolts.