A few years ago I had a RocPro 700 that was having this exact issue. At first I thought it was the speaker. Went through everything but couldn't figure out why it was misbehaving. After installing the 47pf or something close it finally sounded like an amp again. So yes this seems like a bandaid repair but it certainly solved the issue.

Try the guitar into the Return Jack on the rear of the amp.
A dirty connection on that jack will kill the signal going to the power amp.
It's a Normally Closed jack. Any dirt will open it.

It was pointed out to me recently that even when within the power limits, it is usually better to have larger surface area for power resistors.  When they are mounted on the circuit board, spreading the heat across a larger circuit area helps prevent overheating of the board and solder joints.  Especially with components getting smaller and smaller for the same power rating.
So sometimes when you see resistors with power ratings that seem way larger than necessary, it is because of heat issues.
Standing them off the board also helps

Case in point: that Crate GX-20M amp, in a recent thread:

https://www.ssguitar.com/index.php?topic=5743.0

0.1 current sensing resistor, only 3W rating. That would still be enough even with a 4 ohm speaker and more powerful amp. You need 5.5A through 0.1Ω to get 3W. That current is well over 100W through 4Ω.

Anyhoo, that Crate schematic has some interesting features (in this regard, but also others).

The voltage feedback has fairly hefty treble bypass resistor of over 500 pF.  So that by itself would give the amp a rounded high end. Then in the current feedback though, there is a low pass filter. That will reduce the high frequency feedback, boosting highs.

I'm going to simulate that exact network to see what that thing is doing in terms of frequency response against the typical speaker model.

Almost certainly, they tuned the parameters to that thing's 10" speaker and cabinet.

Prompted by the thread, I went to Youtube to hear this thing in action and it sounds good!

In the top of the 1980s, Crate got a boost from an Yngwie Malmsteen endorsement, haha. He wouldn't have done that for something that is complete *s!!t*; it would have to be at most slightly *s!!t*. Around that time, they had lots of them in guitar stores all over so I remember playing around with them.

The schematic is impressively clean and professional. Everything is crisply readable, and there are useful notes to remove guesswork: like that unless otherwise noted, all resistors are 5% and 1/4W. They give a table of all test points, showing the expected approximate waveform shape, and voltage range. Amazing! Pretty much all you need to troubleshoot that thing and return to factory spec. And though the draft job is from 1992, it's exported to crisply scalable vector graphics shoved into a PDF. Kudos!

I've made one of them close to 40 years ago. The recommended wattage was 100 Watts.
It is still the same Osram bulb, but although I don´t need it often, I should start to find a spare bulb.

Yes the bulbs with a HV halogen lamp inside will work, but we won´t get them with 100 Watts methinks. At the time they were meant to replace high wattage bulbs by lower wattage, probably a 60W halogen for a 100W filament due to the higher efficiency. I am fairly sure I never saw a 100W bulb version. Except maybe one for the floodlights, those sticks.

A pity I have thrown away my last dead halogen floodlight.

Quote from: g1 on May 04, 2026, 11:41:55 AMThe VA issue is exactly why amp power is measured into non-inductive (resistive) loads.

Never thought about that, but it makes sense of course.

If I was there I would scope the output. I would want to observe where I have signal and where it stops. It probably involves a bad connection. Someone or something removed the positive lead connection on the speaker. Trace the signal there.

The VA issue is exactly why amp power is measured into non-inductive (resistive) loads.
Measuring power into resistive loads takes reactance out of the equation so we can compare apples to apples as far as raw power capability.
Aside from that, with inductive loads, output impedance (damping) becomes a factor.  As does frequency response and transient response.  These factors are why amps sound different in terms of loudness even though they may have the same power specification into non-inductive load.

So I actually tried this exact thing illustrated in my above example: reduce the current sensing resistor to 0.1 Ω and make the other compensating adjustments. I was not looking to explore this exact aspect, but simulation has a way of taking you places.

I have to say, wow! While it has about the same amount of "scoop" as predicted by the bode plot, the sound quality is noticeably better. The top end is smoother and can take more presence out of the preamp. But not in a way that presence is lacking, such that you have to add it. If you know what I mean.
Probably the excessive negative feedback from the larger resistor was causing a problem for the amp, leading to distortion that trashed the high end? Wild-assed guess. Plus there is the fact that the input had to be 7 dB louder to achieve the same level as now.

There is a lesson here: for current feedback, use a small value current sensing resistor; you don't need more than around 0.1 Ω.

That also keeps the wattage down. For current sensing resistors, the dissipated heat is proportional to R. If you cut R by four, you cut the wattage by four.

BTW looks like I registered here on April 20, 2011. Just passed my 15th birSSday.　🎂🎉🥂

When I joined, Posh Spice and Sporty Spice were still 30 something, right?

The one for me that never grows old: LTSpice!