Bench Measuring SS Guitar Amplifier Output Power

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Well-known member
Nov 14, 2018
San Francisco, USA
I'm working on restoring an old mid 1960's solid state VOX guitar amplifier that was provided to me in barely working condition. Very low volume and very noisy - unusable. I've got it cleaned up and working for the most part now with a few minor issues that will need further attention.

I do have a schematic and that also includes some measurement points which I'm working thought to confirm proper working state, but wondering how one goes about measuring the power out (watts) for something like this or is it just checking the indicated voltages are within specifications to confirm proper working state?

I'm familiar with doing this for tube amplifiers - and, I'm sure the calculations are much the same - but, where or can these measurements be taken on a SS device? My solid state experience is limited to building a number SS studio devices, but SS guitar amps are mostly new to me. ATM, the amp sounds good and is fairly loud, but doesn't seem to be 30W loud. Subjective, I understand. I would like to confirm.

Output section uses a ( 2N3054 ) driver transistor and two power transistors ( 2N3055 ). See attached schematic.

Lastly, I've never been quite sure what signal level to inject into a guitar amplifier for bench measurements so the preamp is not overdriven or under driven. For example, when setting up / calibration the studio devices I have built / worked on, 0Db (0.775v) @ 1KHz is what Is usually indicated. My schematic for this indicates 1k as test frequency. Is there a suggested injection level for guitar amplifiers or is that something that needs to be assessed based on the input impedance or other variables?

I've searched the Interwebs for answers in advance of posting here, but wasn't able to find anything that seems directly applicable or that I undertand.

Any help will be appreciated. Thanks in advance.


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Power calculation is the same, and so is the method: set the controls flat/neutral, connect a proper 16 ohm resistive load per the schematic, and measure the full, unclipped RMS output at the speaker jack, tweaking the controls and input signal amplitude as necessary for maximum headroom throughout the amplifier stages.

Cool stuff there, I haven't seen a trafo PI on a SS amp in a loooong time. Looks like a fun old amp to work on!

Do you have an actual recommended quiescent current spec for it? If not, I'd just set the bias trimpots so that both top and bottom of the output waveform are beginning to clip identically. Tweak the driver trimpot, then go back and forth across all the pots a couple of times until you have balance between the output devices and everything is tweaked to symmetrical clipping with max output.
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guitar inputs are generally high impedance inputs generally < 1Vac. full scale.

That amp looks like an old tube amp topology with transistors dropped in place of tubes. 2n3055 are slow but rugged NPN devices.

There may be some merit in replicating transformer output effects, but the transistor saturation will be very un-tubelike

You may have but if not replace the electrolytic caps. It will help the power amp output on that amp. Those Caps are long past useful Life.