I measured my amp's power output - it doesn't seem right.

[Enzo]

What is clipping? And by that I mean what in the amp is doing the clipping. Just because you see clips on the waveform doesn;t mean they are coming from the power amp. For example, in the little Peavey Classic 30, the phase inverter clips at anything over about 4 on the clean volume. The power amp is capable of much more amplification clean beyond that point.

I don't want to stir up the whole debate over measuring power output at clean only. After all, if you kill one side of the signal so the thing NEVER plays clean, does that mean it puts out zero power, even when at deafening levels? But Just watching a scope doesn't answer that. If your tone stack stage is clipping, then it isn;t fair to the power amp to say the amp only makes 3 watts because the thing "is clipping" over than amount.

Since all your readings are in peak to peak, I have to assume these are scope screen readings, not AC voltmeter readings.

Just for science, crank the thing up to where it seems as loud as it is going to get to your ears. Forget whether it is clipping or not for now. Connect an AC voltmeter across the speaker, and what do you get. Make a power calculation from that figure and the speaker nominal impedance. What power do you come up with that way?

I understand the idea of clean onset of clipping measurements for hifi amps, and why that might be the only "fair" means of comparisons between amps. But this is a guitar amp, not a hifi, and in my opinion, power ratings for guitar amps are not about fair hifi power readings, they are basically a general idea of how loud the thing will be. If amplifier B sounds about 6db louder than amplifier A, most players are not concerned that amplifier B is really 20 watts less than amplifier A when measured "fairly."

[Sonicboom]

You are correct, I am using a scope to measure the wave forms. The amp does get louder beyond the point of clipping. I'll check the output with an AC meter as you suggest. For grins, I put a 12AU in V1 of the normal channel. This obviously affected the gain and vol. It stayed fairly clean up to 10 on the vol. with the lower gain tube.

Enzo, in your time on the bench, have you ever come across an OT that seemed weak in that it lacked punch or saturated easily? I thought you mentioned in another thread that they are pretty binary - worky, no worky. Just curious.

[MWJB]

There are schematics for many of the Fender circuits at Fender.com, with AC voltages, set your amp as per the schem for the one that is closest to yours...what do you get?

Note that typical procedure is to set controls at 50% rotation (checkl specifics for tremolo controls etc.) then tweak the input AC voltage for max clean voltage at the speaker. Often that input AC is considerably less than the 100mA that seems to be the rule of thumb.

An OT with some shorted windings will still make sound but kill output & fidelity...it's always a possibility, but one that I would investigate after trying Enzo's voltage tests described earlier.

[Enzo]

Sure, a shorted turn can ruin performance but allow it to continue. The work/not work thing is more situational. It depends upon what the problem is one is trying to solve.

[Sonicboom]

I checked the power using an ACVM. Results as follows:

Vibroverb
8 VAC
8 ohms nominal
8 watts

Super Reverb
9 VAC
2 ohms nominal
40 watts

I should have 18-20 VAC on the VV. A thought on testing the OT is to cross-wire the VV plates to the SR OT. Before I get that drastic, I'll put it on the bench again and record all of the DC voltages. Wish I just had a spare OT laying around to eliminate it as a possibility. A bad OT is my greatest fear.

[Chuck H]

Just for the heck of it try turning the amp up to where it begins to clip and then scope the PI waveform to see if the clipping is visually similar. It was mentioned earlier that in order to isolate he problem we first need to know if the clipping is in an earlier stage than the power tubes.

If I'm trying to pump up a tire that needs 35PSI of pressure but for some reason I can't get more than 12PSI into it I don't know if it's the fault of the tire or the pump until I find a problem with one, the other or both. If some stage prior to the OT is clipping then the power tubes never get the chance to produce full power because they're amplifying a clipped, and therefore limited drive signal.

If the PI is clipping about the same as the output your OT may be fine. So then try scoping the stage feeding the PI. Is it clipping about the same? If yes the the PI is probably fine. Isolation of the problem is paramount.

[Sonicboom]

Thanks for the input. I completely agree. I'll be checking the DC levels and chasing signals later this evening.

[MWJB]

"Wish I just had a spare OT laying around to eliminate it as a possibility. A bad OT is my greatest fear." It's everyone's greatest fear (well that and nuclear holocaust) that's why when a fuse blows novices look skywards and proclaim "that'll be my OT gone then?". But there is an order to trouble shooting, first thing is to check the dc voltages. If you had a spare OT, you stil wouldn't fit it until you had checked voltages. If you had a spare OT and you did have to change it, then it's only 5 wires & 2 or 4 screws...in a go/no go situation it's not exactly a demanding job.

[Sonicboom]

Actually, I fear a blown OT more than a nuclear holocaust. In the later, you're dead, so who cares; in the former, you're around, but your amp is still dead.

I haven't chased an AC signal yet, but I have checked all the voltages. They're borderline within tolerance in the PI bias section. I've got 100v (vs. 118v) on the cathode and 98v (vs 116v) at the junction of the bias resistor network. All other voltages are within a couple of volts of spec (~5%).

[R.G.]

What's funny about the irrational fear of a blown transformer is that:
- they're easy to check out; it takes a schematic, a filament transformer, a multimeter, a neon bulb and a battery to check one for all but the rarest of situations if it has ever worked before
- they are possibly the second-most electrically reliable parts in an amp
- people in general resist doing anything to protect them from damage

[Sonicboom]

Update: I checked the 1M, 22K and 470R resistors that make up the PI bias network. The 470R was 610R, so I replaced it. Cathode voltage is now 106v - somewhat improved and within 10%. Some of the resistors seem to be borderline within 10% spec. For example, 820R resistor on the output ckt. measures about 912R. Not sure how much that matters - none seem to be more than 12%.

I have signal bleeding through to the output on the Effects chl. when the vol. is all the way off, so I suspect that I have a leaky coupling cap somewhere. Is there a best way to locate it w/o unsoldering one side of every one?

As for signal amplitude, the signal from the PI to the power section is about 200 volts p-p. On the plates of the power tubes, it is about 250, maxing at 300. At the speaker, it's 20v.

[Chuck H]

I have signal bleeding through to the output on the Effects chl. when the vol. is all the way off, so I suspect that I have a leaky coupling cap somewhere. Is there a best way to locate it w/o unsoldering one side of every one?

With the volume all the way down there should be no bleed even with a leaky cap, BUT, I've seen bad preamp tube exhibit this behavior and I have no idea why. Perhaps a leaky cap could cause the same problem.

But your still checking component values instead of isolating the problem. The first and easiest step in solving tube amp issues is to substitute known good tubes. How old are your power tubes? And if that suspected leaky cap problem IS actually a bad preamp tube a tube swap will correct this too. If a tube swap doesn't do it, the next step is to isolate the problem.

Ever see 'that guy', car on the side of the road with the hood up and he's staring at the engine with a hunted look on his face? Eventually he's tuging random vacuum lines and wires, wiggling molex connectors and trying to look through the dust and grease for some kind of leak. HE hasn't isolated the problem and probably isn't going to fix his his car. He will waste a lot of time before handing the job over or taking advice on where to look though.

The voltages in that amp are +/- 20%. Components 10%. There were also times when a part value that was just wrong was put into many BF amps. Short of randomly locating a grossly bad component you would save time if you isolate the problem. The next step in doing that is to check ALL voltages and trace the signal through the amp with the scope.

I don't know what to look for as to signal voltage on the power tube plates without measuring a known working example first. How that number is affected by OT impedance/inductance, etc. It sure sounds like your PI is doing it's job. How old are the power tubes? And what is their bias condition?

[Sonicboom]

Interesting comment about the preamp tube. I'll check into that. I'll replace all of the tubes with ones from my Super Reverb, which is working fine. FWIW, the power tubes in the VV are new Winged Cs, biased at around 70% and checked a few weeks ago. I'll recheck the bias current and post it.

How is it that I'm not isolating anything? I've encountered situations where there have been multiple bugs, so I've learned to correct problems as I go. Part of my methodology involves taking the time to check for similar issues with other components in the affected section, especially on an old amp. Is that wrong?

As for characterizing the problem, I know that both channels are affected, so one would suspect the PI, supply or output sections. The next step is to verify all quiescent DC voltages in the various sections. All DC voltages are within spec (after I replaced a resistor in the PI bias network that had changed value, throwing the PI DC voltages out of spec). This would indicate a functioning power supply (supply caps are new).

Power tubes are new and biased correctly. I have a healthy input signal from the PI driving the output section. However, issues I have are signal bleed through on one channel and clipping in the output that may or may not be a symptom. I'll know more after I compare it to the same signal in my Super Reverb.

[MarkusBass]

How is it that I'm not isolating anything?

Chuck mentioned previously a simple test; scoping the stage feeding the PI (whether clipping is there or not). It would help if you do it.

Just for the heck of it try turning the amp up to where it begins to clip and then scope the PI waveform to see if the clipping is visually similar. It was mentioned earlier that in order to isolate he problem we first need to know if the clipping is in an earlier stage than the power tubes.
If I'm trying to pump up a tire that needs 35PSI of pressure but for some reason I can't get more than 12PSI into it I don't know if it's the fault of the tire or the pump until I find a problem with one, the other or both. If some stage prior to the OT is clipping then the power tubes never get the chance to produce full power because they're amplifying a clipped, and therefore limited drive signal.
If the PI is clipping about the same as the output your OT may be fine. So then try scoping the stage feeding the PI. Is it clipping about the same? If yes the the PI is probably fine. Isolation of the problem is paramount.

Some clipping may be due to the feedback loop but maybe you could try scoping the signal without the output valves? Is clipping stil there with the same level of the input signal?

BTW, I had exactly the same problem with old resistors changing their values. I had 580 Ohms instead of 470 Ohms. Over the years they change their values.

Mark

[Chuck H]

Well, we've been trying to make it NOT the OT. But if it's the OT it's the OT. One thing did pop into my mind. Voltages on those amps are usually too high. After changing the PI resistor values to spec your voltages there are still a little low. With modern wall volts it should be high. So... I'm wondering if this may not be a power supply issue. If the power supply can't provide the current the amp will never reach max output. It could be that in use the voltages are sagging lower than they should. There's also the signal bleed issue. That could be a preamp tube or leaky cap but it could also be a problem with the power supply decoupling function.

Also, as noted by R.G., it's a pretty easy thing to just test the OT that's in the amp. OT's, while doing many complex things in the circuit and to the tone and being complex to design, are really simple in construction. If the impedance ratios work out and there are no shorts then you can be pretty sure the OT is good and can move forward with some peace of mind.