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!
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!
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