Amplifier first start up; Plays tone of signal generator with no speakers! - diyAudio
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Old 28th August 2012, 04:22 AM   #1
mbeards is offline mbeards  United States
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Default Amplifier first start up; Plays tone of signal generator with no speakers!

Hi everyone,

Here's a long one:

So I finished up the first of my PP KT88 mono blocks and put it on the bench today for a first startup. Luckily there was no smoke!

The amp has ammeters on the KT88 cathodes to help estimate the bias on the fly. After running for 20 seconds I noticed that the current on one of the tubes kept growing while the other stayed fixed at zero with full negative grid voltage. I quickly released that the runaway tube's grid connection was made to the wrong pin on the tube socket. ..Whoops

After that quick fix I proceeded to play a signal through the input with my signal generator. To my surprise, with the amp on a dummy load, I could hear the signal generator pitch very clearly coming form somewhere in the amp at any input frequency.

I am wondering if anyone has had any sort of experience like this? I assume its not healthy. I didn't run the amp for to long but I think that the b+ is a bit high although I may not of increased the bias current to an optimal level.

Another note, when I increase the input signal amplitude, the cathode current on the meters moves up away form the bias current setting proportionally. This seems odd to me since the cathode current due to the amplified signal should be AC, not DC and thus the meters should stay at about the same level. Is this assumption correct? I have used multimeters in series with the cathodes for biasing on my guitar amps and notice no difference in the current reading with the volume wide open or off.

Any thoughts or suggestions are appreciated,

Matt
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Old 28th August 2012, 04:59 AM   #2
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The test tones you are hearing are coming from your OPTs .....If an OPT isnt so-called , tight...laminations perfectly secured, bell ends secured.....it can...sing.

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Old 28th August 2012, 05:06 AM   #3
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Quote:
Originally Posted by mbeards View Post
To my surprise, with the amp on a dummy load, I could hear the signal generator pitch very clearly coming form somewhere in the amp at any input frequency.
The sound is almost certainly coming from the output transformer. It will be low in volume, but discernable. It's normal provided it's not loud. Even the best transformers will "sing" a little if you put your ear right on them.

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Another note, when I increase the input signal amplitude, the cathode current on the meters moves up away form the bias current setting proportionally.
Also normal. The positive peaks of the signal cause the output tubes to draw more current. Even though the meters are DC, they will respond as the tube current increases.
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Old 28th August 2012, 05:04 PM   #4
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I think these guys are right on! Your output transformer has a lot in common with a speaker just no cone! sounds like a fun build!
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Old 28th August 2012, 06:11 PM   #5
Bigun is offline Bigun  Canada
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try adding a cone to the OPT - could be interesting !
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Old 28th August 2012, 08:57 PM   #6
DF96 is offline DF96  England
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Push-pull outputs generate second-order distortion. This consists (for a sine wave input) of equal amounts of 2nd harmonic and DC. The 2nd cancels in the OPT. The DC can be seen on a meter.
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Old 29th August 2012, 06:06 AM   #7
mbeards is offline mbeards  United States
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Thanks for the help!

yeah it is defiantly the outputs ringing. I talked with the winder and he said that is typical of a transformer as big as I ordered. (they were custom). He said the ringing is actually is most noticeable with resistive loads. So one of these mono blocks is up and running well.

The amp (well amps as there are two mono-blocks) is a KT88 tube rectified beast with massive output iron. I was getting 80-90 watts out of one of them until the tubes drifted badly off bias. The tube dropped off by 20 ma from the other!!! ....and they were so called matched. I think they were matched to begin with but must have skipped the burn in process.

The second amp isn't working so well. No output unless the input is cranked and with a large input voltage, the output is badly distorted and just a few volts. The power supply seems good and cathode/bias current is good.

Have I mentioned how much I love the cathode current meters??? Have to do some debugging tomorrow....

Matt
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