A tube guy asks questions about a SS guitar amp

[TimFox]

Power measurement:
Again, this is one of my pet peeves.
During the 1960s, hi-fi manufacturers engaged in what I consider fraudulent practices.
Starting with a possibly defensible practice of "IHF" power, where an external power supply was used to maintain full DC voltage during continuous sine-wave testing (since in musical use the actual power requirement was not continuous due to high peak-to-average demand of real musical programs), or power was measured during a short tone burst to avoid the DC droop.
The fraudulent specifications went further to "+/- 1 dB" ratings, which allowed an actual power to be increased by 1 dB, or an extra 26%, to inflate 50 W to over 60 W.
Even with legitimate measurements, if you measure the true RMS voltage across an 8\$\Omega\$ load and compute V_rms²/R, that is the mean power, not the RMS power.
By elementary mathematics, the RMS power of a sine-wave voltage is an extra 22.5% higher than the mean power.
However, "RMS power" is not a useful value, even though it is mathematically well-defined.

[CaptDon]

I see that perhaps we in the audio production and sound re-enforcement business have been perhaps using the 'RMS' term loosely??? A common value for small amplifiers is 28.2 vrms across an 8 ohm resistive load and we always called that 100 watts RMS. I suppose from your statement we have an amplifier producing 100 'mean' watts. What I do know is that it generates 100 watts of true BTU heat in the resistor and can be verified by temperature rise of one liter of oil surrounding the power resistor. B.T.W., my CBA-1000 amps will do 90vrms across an 8 ohm dummy load all day long producing 1KW of 'mean' power and 70vrms all day long across 4 ohms producing 1.2KW of mean power. Those values are just shy of clipping onset. Of course if you allow for 10Db of 'headroom' for musical peak vs. average power we have only a 100w or a 120w amplifier! I laugh at these 'D.J.' grade amplifiers boasting as much as 5KW of power running from 120vac with a 5 amp line fuse!!! With + and - 80vdc internal power rails and that is before the 'droop'. I calculate a max of about 2.5KW in bridged mono mode at 2 ohms just before the line fuse pops (unless the TO-220 mosfets pop first!!!)

[WimWalther]

So your 125W amplifier only makes 125W when hard clipping is involved? And you are surprised. Welcome to the bigs where people play HARD BALL. That nameplate did not say anything about the conditions where it was capable of 125W output, did it? NO, it didn't.

In fact, it makes 135W or better in a hard clip, so why not just call it "135W" on the placard - working under your reasoning?

Here in the US we have a popular size of "dimensional" lumber called a "2 by 4".

Having grown up and lived my entire life in the US, I'm thoroughly familiar with dimensional lumber. I also happen to own a home built in the early 1870s, so I'm likewise familiar with "full-dimensional" lumber which is sometimes sized by quarter inches.

Again, in another example, I'm aware of the reasoning behind the sudden drop in horsepower ratings of car engines which occurred sometime around 1973.

Anyway.. So guitar amp mfr's play fast & loose with power specs. I get it.

[WimWalther]

Those are my thoughts. My advise is to buy solid state audio equipment: it is far better than tube type equipment ever was. But I doubt you will take it.

This deserves its own reply.

What do you think gives you the right to draw any conclusions about my personal views on this subject?

I come here for friendly & helpful technical discussion, to fill in areas where my own knowledge is sorely lacking. Or at least this is what I try to offer, and what I hope / expect to receive in return. This said, I have no interest in the unsolicited opinions of random would-be mind readers. Nor am I even remotely looking to fight some pointless tube vs. solid-state battle.

This would also be the perfect time to thank ALL of the helpful folks who have so far lent their time & knowledge to this thread. I'm slowly getting my bearings on this SS stuff  - your efforts to that end are much appreciated!

[TimFox]

I see that perhaps we in the audio production and sound re-enforcement business have been perhaps using the 'RMS' term loosely??? A common value for small amplifiers is 28.2 vrms across an 8 ohm resistive load and we always called that 100 watts RMS. I suppose from your statement we have an amplifier producing 100 'mean' watts. What I do know is that it generates 100 watts of true BTU heat in the resistor and can be verified by temperature rise of one liter of oil surrounding the power resistor. B.T.W., my CBA-1000 amps will do 90vrms across an 8 ohm dummy load all day long producing 1KW of 'mean' power and 70vrms all day long across 4 ohms producing 1.2KW of mean power. Those values are just shy of clipping onset. Of course if you allow for 10Db of 'headroom' for musical peak vs. average power we have only a 100w or a 120w amplifier! I laugh at these 'D.J.' grade amplifiers boasting as much as 5KW of power running from 120vac with a 5 amp line fuse!!! With + and - 80vdc internal power rails and that is before the 'droop'. I calculate a max of about 2.5KW in bridged mono mode at 2 ohms just before the line fuse pops (unless the TO-220 mosfets pop first!!!)

Yes:  your example of 28.2 V RMS across 8 ohms is a mean power of 100 W, and would produce the same heat in a water load as 28.2 V DC in an 8 ohm resistor, or 120 V RMS AC into a 100 W incandescent bulb submerged in water.
RMS means "root mean square".  If you square it, you get "mean square" that leads the reader to mean power.

[WimWalther]

Still plugging away here, came across a couple questions..

On the first circuit, with the amp off, I'm measuring 10K from point 6 to ground. With it on, I measure 14.3V to gnd, with a B+ of 15.6V. Neither of these seem right to me. Shouldnt resistance be 147K or possibly 147K || 10K = 9K3? This is part of the "modulator" circuit, which is not working.

What is the point of this second circuit. The Master volume output passes through it, via the LDR in an optocoupler / vactrol. There's a transistor, but it has no (extetnal) control signal..

Is this a timed muting circuit? Like to prevent startup pop or thump? Thanks!

[ELS122]

As the subject states, 90% of my experience is in tubes, mostly hi-end & hi-fi audio, plus radios, test equipment etc. So please bear this in mind as I ask questions about the PA unit from a Polytone Taurus Elite music / guitar amp.

Nameplate rating for this PA is 125W. I bought the unit used, so it's new to me. I did a power test and found that the unit makes 135W / 4R at hard clipping, but only about 80W / 4R with a visibly clean sine. This seems like a problem to me.

The input power rating is 165W. To make 125W out it must be very nearly a class-B design as these figures are very close to the theoretical max of about 76% efficient.

The PSU rests at +/-35Vdc. My first thought is the bias is off.. but I don't know what it should be. I measured ~40mA through the 0R075 emitter resistors R17/R18 , but this figure wanders around. The bias pot R14 is very near full CCW.

Oh, and what are the 10W 0R3 parts R20/R21 used for?

Thoughts & advises?

Guitar amps are typically power rated at 10% THD

Idk why everyone keeps mentioning how the speaker resistance is 3 ohms... speakers are very not-resistors. they can easily have 100x more impedance at higher frequencies.
The amplifier is rated for whatever the speaker rated impedance is. So if it's built for a 4 ohm speaker, power is measured into a 4 ohm (dummy) load, not a speaker. If the speaker messures 3 ohms on a DC ohm meter, it doesn't matter, it's still gonna be rated for whatever the rated speaker impedance is.

With clipping you would get more than 76% efficiency since the output transistors start to saturate, with "100%" clipping you could reach ~90% efficiency.

[TimFox]

There was an old rule of thumb, used for biasing amplifiers, that 5% THD was visible on a simple oscilloscope.
Just like flutes and violins, electric instrument amplifiers make harmonics as part of their application.

[WimWalther]

The more I look at circuit #2, the more certain I am that it's an anti-pop device. On power-up, C46 charges through R68 & 69 gradually increasing the base voltage to Q5 until Vbe reaches about 5.8V. At that point, CR12 zeners and base current begins to flow into Q5, which begins conducting CE current through the opto OP4's LED.

This causes the LDR of OP4 to begin passing signal, which continues until C46 is fully charged and the opto is fully on.

One time constant = about 2.7s, so the whole operation takes just a couple seconds.

Right? Right?? ;-)

[magic]

Seems reasonable.

Regarding #1, it's possible that you are measuring R115 because +V is loaded with enough capacitance or other circuitry to look like a short in series with 10kΩ.

In powered up state, 14.3V is about the expected open circuit voltage at point 6. Remember that R117 is shunted by the BE diode of Q10, so it's 0.7V there.

edit

Shouldnt resistance be 147K or possibly 147K || 10K = 9K3? This is part of the "modulator" circuit, which is not working.

I see your point now. Yes, it should be less than 10k, unless apparent resistance between +V and GND is exactly the missing 700Ω.
At this point obviously continuity of tracks and solder joints near all those resistors should be checked.

[WimWalther]

Right, I need to get the board out and onto the bench to proceed any further. Trouble is.. this thing was not designed to be worked on.

It's like all three preamp boards + modulator board + psu were all wired up on an external jig and then transplanted onto the amp chassis front panel en masse.

Really makes me wonder what the designer intended.

[WimWalther]

The fun never ends with this thing. There is a DIP-16 IC on the board, marked "PT2201" on the schematic. I have signals entering on pins 1 & 4, but nothing leaving it.

And they've scrubbed the markings off! Dandy.

So the NAND gate flip-flop is running, but PT2201 doesn't seem to care. Really wish I knew how this circuit is supposed to work. It's allegedly a Leslie simulator, but I'm not seeing any inductors or cap strings to accomplish phase shifts.

Is one of the op-amps being used to emulate an inductor?

[WimWalther]

Turns out that PT1022 is a TDA2201 BBD (bucket brigade delay) IC. Kind of makes sense, as one of the Leslie speaker's characteristics is a chorus effect.