Fender Champ AA764 I think '73

[SAG Electronics]

G1: I ran a signal from my pc speakers to the Amp speaker to make sure that even though the specs tested correct, it would actually make noise. You know, cone not detached,etc.

Dave H:Cathode resistor tests 508 ohm, good.

No, all my probes are at x1. With no signal going, pin 3 and pin 5 on 6v6 have negligable voltage difference, with signal of 1000Hz 460 mv pk-pk, volume knob at 4 only 4x, volume knob at 10 only 1.2x.

Helmholtz: Changed cathode cap, probe 10:1 signal 1000Hz 460mv pk-pk. Not a pretty sine wave at volume 4, pointy at the positive peaks. Max 5.52V Minimum -8.87V lightly undulating.
Sound still outputs as tinny and far away.

Is there any chance that using the headphone out on my p.c. speakers could be part of the problem?

[Helmholtz]

Well. the 6V6 may have been loose. Now the voltages are much better. 296-306 and 14-18. That's a relief. I also found that pin 5 on the 6V6 is not showing 205VDC or anything really. It is coming out of the 12Ax7 at 206VDC, but not getting into 5 on the 6V6....

You still may have an intermittent socket contact problem resulting. Clean and re-tension contacts.

[Helmholtz]

G1: I ran a signal from my pc speakers to the Amp speaker to make sure that even though the specs tested correct, it would actually make noise. You know, cone not detached,etc.

A somewhat damaged speaker can sound decent at low volume but get farty and thin at higher output.
But it seems you primarily have an amp problem in the power stage.

No, all my probes are at x1. With no signal going, pin 3 and pin 5 on 6v6 have negligable voltage difference, with signal of 1000Hz 460 mv pk-pk, volume knob at 4 only 4x, volume knob at 10 only 1.2x.

If I understand correctly, it seems to confirm that the 6V6 is not amplifying as it should. Please re-measure plate and cathode DC voltages.

Helmholtz: Changed cathode cap, probe 10:1 signal 1000Hz 460mv pk-pk.

Does that mean 0.46Vp-p at the plate with 10:1 probe? So actually 4.6Vp-p at the plate?

Max 5.52V Minimum -8.87V lightly undulating.

Don't understand. There's no negative voltage in this amp. Where did you measure?
Please be more precise and try to structure yout text.

Is there any chance that using the headphone out on my p.c. speakers could be part of the problem?

Hardly, but if you suspect your input signal, just scope it.


Did you ever change the 6V6?

[SAG Electronics]

O.K. let me try to clarify.

The measured input signal from my p.c. speaker headphone output was 460mv pk-pk. Measured on my oscilloscope.
The measured signal at the plate was peak 5.52V and trough -8.87V also measured on my oscilloscope with the probe set at 10:1.
The input signal was a nice 1000Hz clean sine wave, the output at the plate was not nearly so clean, with sharp peaks at around 80 degrees sharply dropping by 90 degrees then smooth through the rest of the wave.
Cathode measures 20VDC, Plate measures 365VDC according to my DMM.

It does seem to be amplifying and the speaker seemed undamaged when I ran a music signal to it using the same method that I am currently running the clean 1000Hz signal. upon trying the 1000Hz signal directly through the speaker it is louder and clearer that when it is run through the amp and turned up.

I hope this clears things up for you. It still does not for me.

[Helmholtz]

The measured signal at the plate was peak 5.52V and trough -8.87V also measured on my oscilloscope with the probe set at 10:1.

Still confused by the minus (-) sign.
Now you say peak voltage, which would be half of peak to peak. Please always specify the full unit with the numbers: VDC, Vp, Vpp, or Vrms.

If its actually peak to peak voltage with a 10:1 probe, it would actually mean between 55Vpp and 89Vpp, because of the 10:1 attenuation in your probe.
Why is it not constant? What was the grid signal level when you measured this?

Best repeat grid and plate measurements with same 1:1 probe setting.

As Dave H said, we expect the plate signal to be 20 to 30 times larger than the grid signal.

[g1]

A picture of the waveform on the scope would be very helpful. And tell what the settings are.

[SAG Electronics]

Alrighty, RE tested with o-scope and DMM.
Input signal 1000Hz 960mVpp
Plate voltage DMM: 342VDC signal o-scope: 99.2Vpp Pin3
Grid screen DMM: 360VDC
Grid Voltage DMM: 26VAC signal 0-scope: 68.4Vpp Pin5
Cathode DMM: 31.5VDC

All o-scope probes at 1x

Not seeing the 20-30 times. IS this a bad 6V6gt?

[Dave H]

Still confused by the minus (-) sign.

I think he must be measuring the 6V6 plate with the scope set to AC coupling and the positive and negative peaks are not equal because the waveform is distorted.

[Helmholtz]

I think he must be measuring the 6V6 plate with the scope set to AC coupling and the positive and negative peaks are not equal because the waveform is distorted.

I'd think peak-to-peak can't be negative by definition.

[Helmholtz]

Alrighty, RE tested with o-scope and DMM.
Input signal 1000Hz 960mVpp
Plate voltage DMM: 342VDC signal o-scope: 99.2Vpp Pin3
Grid screen DMM: 360VDC
Grid Voltage DMM: 26VAC signal 0-scope: 68.4Vpp Pin5
Cathode DMM: 31.5VDC

All o-scope probes at 1x

Not seeing the 20-30 times. IS this a bad 6V6gt?

Ok, the plate to grid gain of around 30 only holds within the linear range of the stage. As soon as clipping occurs, voltage gain reduces.
The grid signal of 68.4Vpp is far more than necessary to drive the stage into clipping.
Anyway the plate signal should be around 600Vpp at full output (because of that high expected voltage I originally recommended a 10:1 probe).

As you already tested the OT, it definitely makes sense to try a new 6V6.

Why is the cathode voltage higher than before? Did you measure with signal? DC voltages are typically measured at idle, i.e. without signal.

[Dave H]

I'd think peak-to-peak can't be negative by definition.

I wasn't thinking of peak-to-peak. Post #82 appeared as I was typing. I was referring to post #79

[Helmholtz]

I wasn't thinking of peak-to-peak. Post #82 appeared as I was typing. I was referring to post #79

Sorry, I misinterpreted.

The OP probably didn't mean "minus" but rather "up to", though I don't understand why the voltage wasn't constant.

[SAG Electronics]

right, sorry guys, I had the settings n my oscilloscope wrong up there, I thought it was peak to peak but that channel for whatever reason was set to minimum and maximum Highs were at 5.blah volts and lows at -8.blah volts. I corrected that in the last set of readings. I do not think it is the 6V6gt unless it has been always the 6v6gt doing the wrong after I cleaned up the short issue.

Sorry for the continued confusion. I am still a neophyte with no one in the real world to talk to regarding these issues. I am doing my best and striving to improve.

[SAG Electronics]

Yes, all channels are set to ac coupling. all probes to 1x and I have set the scope to measure all 4 channels to peak-to-peak as I had intended.