Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on May 28, 2013, 08:24:48 pm
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http://s111.photobucket.com/user/zzmoore/library/scope (http://s111.photobucket.com/user/zzmoore/library/scope)
I built an amp.
Basically a Fender BF circuit with a LTP phase inverter and a bias vary trem from an Ampeg M12.
There is NO audible output. I have looked for every stupid mistake I can think of, and can find no reason for the lack of sound. Even if there was straight line DC at the Out Jack, there would still be SOME kind of sound wouldn't there.?
So I took my first shot at a scope. Should this output be audible.?
I am not too good with the pics. Picture Number 1 is the last pic on Bottom Right. The last pic in the series (number 6) is the first pic on the top left. I have them exactly in reverse order is what I am trying to say.....
What the 6 pics show is...:
1. V1a grid
2. After tone stack at plate of V1b
3. Phase Inverter plates
4. Both power tube grids
5. Both power tube plates
6. The out put jack itself
So where do I look for this lack of sound.?
Thank You
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There is NO audible output.
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6. The out put jack itself
So where do I look for this lack of sound.?
Scale of your scope traces (especially the speaker jack)?
It would appear there is a.c. at the speaker jack. So if there is no sound at the speaker (and the scope trace is showing volts and not showing millivolts), then look at the speaker plug, wiring and speaker itself.
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Yeah, I tested the speaker cab and cable with another amp and they work fine.
Amplitude at the jack:
5V/D at 3 divisions would be 15 volts. The probe is x10. So that would be 150V peak to peak at the jack.....right.?
At the V1 grid the amplitude was 300mV peak to peak.
I kind of thought that even my distorted signal should be making Some Kind of sound. The amp is totally quiet...like there is no speaker plugged in. Can I have the jack wired wrong, but still have an AC wave on it like that.?
Thank You
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Amplitude at the jack:
5V/D at 3 divisions would be 15 volts. The probe is x10. So that would be 150V peak to peak at the jack.....right.?
Only if the probe's switch is in the x10 position. You should have an idea of what's "normal" to interpret the traces (hint: 150v pk-pk is not normal).
Let's figure it's 15v pk-pk. 15v / 2 = 7.5v peak. Normally, we'd be looking at a sine wave and figure RMS voltage, but the waveform is closer to square wave. For a pure square wave, RMS voltage equals the peak voltage, so we have 7.5v RMS. Yes, we're rounding up because the wave isn't a perfect square wave.
I'm guessing an 8Ω speaker: 7.5v2/8Ω = 7w output
If you're getting a.c. at the speaker jack, it's getting all the way through the amp. As long as the contacts are good on the jack, the speaker's plug is good, the speaker wire is not broken and the speaker functions, you should be hearing plenty of output.
Keep looking around the area of that speaker jack, and don't assume anything is making good contact or is free of cold solder joints. Especially any ground connection at the jack.
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5V/D at 3 divisions would be 15 volts. The probe is x10. So that would be 150V peak to peak at the jack.....right.?
You're talking about a speaker jack? I don't believe those numbers. 150Vpp is 75Vp is 53VRMS. 53v into an 8Ω speaker would make 350 watts!
Can I have the jack wired wrong, but still have an AC wave on it like that.?
Anything is possible. If it's a 3 lugs switching jack you could have the OT secondary lead on the switch lug. If so, when you connect a phone plug, the OT will not be connected to the plug. This is assuming you are using a 1/4" phone jack.
Show us some close up pics, especially the speaker jack.
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Yeah, I cannot believe those power numbers either. Not from a 2x6V6 amp... :w2:
I had output at the speaker Jack, but not at the other end of the speaker cable (at the cab).
Oh Boy.! I had a fine solder bridge under the jack that I did not see. That was the problem. My Fault.!
I have sound, but it now looks like a sawtooth.
Have not even tried it with a guitar yet. Will try that, see how it sounds, and maybe take new pictures of the scope.
Thanks for all the advice so far guys.
I WILL be back soon
Thank You
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Oh Boy.! I had a fine solder bridge under the jack that I did not see. That was the problem. My Fault.!
Happens to me too!
I have sound, but it now looks like a sawtooth.
Have not even tried it with a guitar yet. Will try that, see how it sounds ...
Yeah, listen with your ears, not eyes. That's because you can easily overdrive the amp and change a sine to a square without the audible feedback. Further, a sine wave applied at the input jack will look like a very strange waveform if you probe at the tone stack itself.
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OK -
As this is my first time using a scope...do the pics seem "normal" until I get to the output.?
Once again, my pics are in reverse order. Sorry about that. Still need to figure out how to operate Photo-bucket.
So looking at the "last" pic first we have.....
1. V1a grid 300mV
2. V1b grid after tone stack 60mV (is this pretty much the normal "loss" we see from a tone stack.?)
3. V1b plate 220mV
4. Phase inverter 46V
5. Phase inverter 42V
6. Both power tubes look like this 190V
7. This is at the speaker jack. 30 Volts peak to peak. The scope looks like there are 4-5 traces, pretty much all in phase, but with some differing amplitudes.
With the amp volume real low, you can gently strum a chord and it sounds OK. But with ANY increase in the Volume Pot, or any aggression in the pick attack, the sound is quite distorted.
I have built quite a few amps, and I have never had this type of a problem before. In the past there has always been a mistake I made that caused more normal problems .....like low voltage, snapping a fuse, feedback, real uneven bias, etc etc etc. Stuff that you could search for.
This amp seems like it has prettty normal voltage everywhere, but I will checkk all those and tell you what I have.
I am not sure if I have a pair of 6K6 that I know are working properly. So I am not sure if I can try another pair of power tubes.
Do the scope pictures suggest anything to you guys.?
Thank You
http://s111.photobucket.com/user/zzmoore/library/scope%202 (http://s111.photobucket.com/user/zzmoore/library/scope%202)