Ampeg B-15N rev c Clone Super High Input Sensitivity

[peteheinrich]

I just made my first foray into instrument amps with a bass amp designed after the Ampeg B-15N as described in this schematic. I had a power transformer and some tubes and a bunch of passives left over from previous hi-fi amp projects so I thought I'd see what I could whip up using as many parts in my stash as I could, without being too concerned about exactly replicating the sound of a B-15N. The AF and phase inverter stages are implemented exactly as in the schematic, tone controls and hi/lo switches included, except that I'm using 7F7s instead of 6SL7s. The outputs are 7027As (one of the things I had) instead of 6L6GCs, and due to the limitations of the power transformer they're running at 415v at the plates and 410v at the screens at 100mA for the pair. The OT is a Musical Power Supplies OT33PP unit, 6.6k plate to plate rated for 25w to 40w. Speaker output is taken from the 8ohm tap, feedback from the 16ohm tap.

The problem is that the amp is crazy sensitive. Only 12.7mV RMS drives the output to clipping at ~16v RMS output or 33 watts. The schematic indicates .3v input for 11v output. On an auditory level this means the amp goes from zero volume to distorted well within a quarter turn of the volume pot. At lower volume levels it does sound very nice. The tone controls and hi/lo switches all have their intended effects.

I've checked and re-checked all the component values in the front end and the LTP, the voltages of are all within 5% of those specified in the schematic, and I just can't think of what else to try at this point. As an experiment I even tried decreasing the value of the feedback resistor, and with ~20db of feedback applied (500ohm instead of 10k ohm resistor), the sensitivity becomes reasonable. But that's obviously way more than the original circuit and way more than I want to apply and probably way more than the OT can handle (I have yet to run stability tests). 1.414v RMS at the grid of V3 (the top tube in the LTP) drives the output to clipping.

Does anyone have any suggestions about possible causes of this, or tests I could perform to troubleshoot? It's a lot to ask, but is anyone able to measure how much feedback a stock unit applies, and what the AC output voltages are at the different stages in the front end for a given input voltage?

Thanks very much for any help!

[g1]

You are running at full on volume control for that 12mV sensitivity measurement? They do not specify vol. setting though they do for tone controls and switches.
To me this implies they input .3V and adjust the vol. control for 11V at output.
Getting full volume at quarter vol. setting is not all that unusual, depending on pickup output.

[peteheinrich]

That’s interesting. I assumed the .3v for 11v out was a measurement with the volume at full, but like I said this is my first try at an instrument amp so I’m not familiar with the conventions for specs in those case. I’m taking all my measurements with the volume pot at full and regulating the input with my signal generator.

another thing to note is that the volume pot is designated as “1M LIN” in the schematic, so I am using a linear pot and not an audio taper. With an audio taper I’d be hitting the max output way further up in the travel.

[The Dude]

Do you have a scope? It would be helpful to know which gain stage is clipping first.

[g1]

Your 1.4V at LTPI grid for full output sounds right for the roughly 3VDC at the cathodes.
I know this is a B15, but the only input sensitivity spec. I have is for the V4, which is 11mV for full power.

[Helmholtz]

What LTPI?
Schematic shows a self-balancing paraphase.

[g1]

What LTPI?
Schematic shows a self-balancing paraphase.

Yes, you are correct. I should have said PI but was using his term from post #1 without proper 'quote'.

[peteheinrich]

Yes, sorry, I misspoke when referring to the PI as an LTP.

If the spec is 11mV for full power for the V4 then it's definitely possible there's nothing actually wrong here, and it's just that my reading of the spec on the schematic and my expectations that were mistaken. As I said, considerations of sensitivity aside, the amp does sound quite nice.

And yes I do have a scope, though I'm fairly new to using it and a beginner in electronics. I couldn't produce clipping as I'm used to seeing it at the output stage (with the tops of the waveform flattened), but the visible distortion on both gets ugly (ragged) at right about 35mV RMS input.

[peteheinrich]

To me this implies they input .3V and adjust the vol. control for 11V at output.t.

Looking at this section in the schematic again I see what you mean. They're saying that with .3v in and the volume adjusted to 11v output, the voltages at various spots are as in the table that follows. I was taking the ".3v in 11v out over 8ohm" as the max output power before clipping with the volume at full. It's looking more and more like the amp is behaving just as it's supposed to.