Peavey tnt130. Issues or not?

[catalin gramada]

Hello. I bought a second tnt130 to be used as sound reinforcement. Both amps will be used just for its power stage inputs. From begining I noticed a slight diference in between: one is a little bit darker. So I did a kind of test using one speaker as reference. It seems to be equally loud but one have more airy, open sound. Can I suspect the coupling caps? Both are made in same year, so no manufacture difference. One more thing...touching with finger tip the inputs of power amps one of it seems to have more sensitivity, it is more noisy than the other, but when plug the guitar in it sound the same as loud. I have just an analog voltmeter at mine so I checked the outputs just strumming the guitar cords.No serious investigation around, but why so input sensitivity difference as time the outputs are almost equal, please?

[Enzo]

Are the speakers identical? Peavey made those with a stamped basket Eminence, with a cast basket Scorpion, and with a cast basket Black Widow. If the two amps have different speaker models in them, that would surely explain it.

[catalin gramada]

Thanks for You kind reply Enzo. I thought as same, so I used just one speaker for test for both amps. But yes the speakers are same.

[catalin gramada]

One of it is loud buzzy when I touch the power amp input, the other is just a bit. I was surprised to found the outputs are the same, as far as my test could be relevant... I used a chop stick to found mechanical issues but didn.t found nothing suspect

[g1]

The low gain input jack is a switching type. Make sure that the switch in the jack is working properly.

[catalin gramada]

Thanks. The story is happen at power stage inputs where preamp stages were disconnected when plugged in, and they are.

[g1]

Oops. Sorry I missed that.
Right after the power amp in is non-polar coupling cap C3, which would be a suspect. Also, muting circuit Q12.

[Enzo]

There is only one 8-leg IC on the power amp, pull it. MAke a difference?

[catalin gramada]

Thanks. I pulled out those IC you're talking about and not found any diference. Also checked with finger tip probe the signal path after those input bipolar cap ,as g1 said, and did not found any difference.
After some empirical investigation found the power transistors are different in my amps. One have 67376 C the other 67376 D. That means different amplification factor, please?
I will do an effort this week end to buy a soldering iron to inject signal from my phone and measure the voltages along the signal path...

[g1]

You can also remove Q12 to test, the amp will still work without it. Q12 just prevents thump when amp is turned on.

[Enzo]

67376 and other Peavey transistors were never matched for gain. Besides, those are current amps not voltage amps at that point in the circuit.

[catalin gramada]

I put everything in back and playing just with power amps. One of it definitely rolls treble off way more like the other. Can be done a investigation without proper tools, please?

[J M Fahey]

YouŽll have to measure, so it confirms or denies what you get by ear.

If both amps work the same as intended, say putting out some 22V RMS (undistorted) into 4 ohm at 440 or 1kHz, then they are not "damaged" at all.
Even 20V RMS (real 100W RMS into 4 ohm) would be fine.

IF one of them has noticeable treble loss, then it might put out significantly less than the other, say 15V RMS or less, at, say, 10kHz.

That would not mean the darker one is "bad", there are a couple capacitors in the circuit meant to stop RF interference, typically very low value, from 10pF (C9, C6, C10) to 100pF C7 , 39pF C14 or 470pF C21 .
*All* of them are HF killers but typically beyond 20kHz so inaudible effect.

*Sometimes* a shotgunner replaces caps for no reason, or pulls them "for measurement" and puts back the wrong value.
Say you put similar looking .01uF ceramic in any of those places, ceramic size is NOT a reliable indicator, FWIW I have exact same size 100pF and 47nF ceramics, and you KILL HF response for good if you mix them wrong ... yet amp "still works fine".

So if you wish you may check those caps I mentioned for misplacement or wrong value.

Such "Tech" created problems may drive you crazy, because "amp works".

[catalin gramada]

Thanks JMF. First I thought one power amp have low input sensitivity than the other but now realise some treble are missing so my impression regards lacks of sensitivity can be in fact explained by some missing highs from musical programm. I have to find a solution to inject some measurable signal and to measure along the path.

[J M Fahey]

Yes, but not even along the path.

SS power amps are just "large Op Amps" and always are engulfed inside HUGE NFB loops, so what happens "inside" tells you nothing.

You just inject a continuous tone at the input, youŽll need about 1V RMS so earphone outputs are not enough here, you probably must go through at least an external preamp or single gain stage , it may even be TNT own preamp but use same to drive both power amps, so you are certain they get the same, and measure output.

In a nutshell:
inject 440 Hz into one preamp, all tone controls flat, rise volume until you get , say, 20V RMS at speaker out, with or without load but do same on both amps.
Turn amp OFF, lift right side of C3 away from R19, run a wire from C3 now free end to R19 on the other amp, where you previously lifted its own C3 right leg.

Turn both amps on , you will be using the originalpreamp driving "the other" power amp,and repeat teat and measurement, of course you did NOT touch any control or vay signal level so second amp "should" have the same output, around 20V RMS.
10% and maybe even 20% variation can be possible, because of component tolerance, anything beyond that is a red flag.

Then repeat exact same test but with a 10 kHz signal.
IF HF response is compromised you "should" at least find a 3dB difference (30% down) or more, 6dB would be 50% down in the weakest one.

Even if multimeter has poor HF response, we are looking for *comparative* measurements here so results are still valid.

If you want more HF level precision and find your meter behaving very poorly, I can suggest an HF probe, but weŽll cross that bridge when/if needed.

Ok, do your Homework

Pick the 30 second ones and set player to Loop or Repeat 1 so you have continuous tone (with a small glitch every 30 seconds when it restarts).