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Old 21st January 2011, 04:37 PM   #1
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Default Trouble with a DIY EL34 Tube Amp Kit

Hey all! A buddy of mine and I took to building a small stereo amp from a kit that we got on eBay. It was quite the process, but we finally finished it today. The kit did not come with any instructions and only came with a set of 3 schematics and a parts list.

When we finished the amp and powered it up, all of the tubes lit up and there were no sparks/flames or smoke. However, I could hear some cyclical noise coming from the chassis. When connecting it to a source and output, it sounds as if it is in a feedback loop along with a nasty hum. Only if I turn the pre way up can I hear any signal. The volume does not affect the output level of the noise and it comes out of each of the 4 channels. If connected to the right ground, the right channel is louder than the left and vice versa.

When we were finished, we were left with 2 resistors that had no home. One was a 2w 5k and the other was a 220k. The parts list show that they belong with the kit, but we could not find where on the schematics they belong. After checking on the power stage schematic, we did see that there was a 220k resistor present, but on the other 2 it was not there. Because we were not 100% we felt it was safest to add it.

However, after all is said and done, there is no place for this 5k resistor that we can see.

I have attached all 3 of the schematics and a photo of the underside of the amp. Also, there is a larger version of the build photo at the link. We have been tracking it on a blog if anyone is looking for more information.

Any help whatsoever would be amazing. We have not gone ahead and prodded any further and will sit tight until we know what to try.

Thanks again for reading all of this and I hope to hear from some of you!!

Lenny


The DIY Experiment (Attenuated Attempts)


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Old 21st January 2011, 04:49 PM   #2
SY is offline SY  United States
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Looks like the 5k goes into the power supply, between the first two electrolytic caps.

CAREFULLY measure voltages at the power supply output and at each of the tube pins and make a voltage map on the schematic. That should clarify things and help us figure out where the problem is.
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Old 21st January 2011, 04:56 PM   #3
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Thanks SY! Regarding the 5k, we did put one between the 2 caps, but there was one left over when we were finished. Also, we have the 220k over the 47uf on the 5z3p as shown on the first schematic. Does that seem to make sense as it does not show up on the other 2 schematics, we were not sure if it belonged there.
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Old 21st January 2011, 05:19 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Hi,

Your first post looks closest to correct. As SY says you need to check the voltages with a multi-meter on B1,B2,B3 and tell us what you get to Gnd. The 220K is probably a Discharge resistor to stop the B+ staying UP after power off. Don't switch it on with out the speaker connected or a load resistor!

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M. Gregg
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Old 21st January 2011, 05:23 PM   #5
Bone is offline Bone  United Kingdom
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This amplifier is already being discussed on this thread <EL34B 6N9P 5Z3P amplifier kit - any thoughts on schematic? > Seems not to be too popular.
Tony
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Old 21st January 2011, 05:27 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Hi,

Don't forget to check the B+ is dead before you touch anything with your hands!

Also where is B3 connected to? Looks like it should lift the heaters not connect to cathode.
What voltage have you got at pin 3 of the 6N9P? Also when you measure this you should hear through the speaker a "noise" as you connect the meter. Pin 3 to Gnd DC voltage!

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M. Gregg
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Old 21st January 2011, 05:59 PM   #7
M Gregg is offline M Gregg  United Kingdom
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If you want the next step give us voltages on pin 6 of the 6N9P.
And voltage on Pin 8 of each EL34.

We need these readings for each channel.

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M. Gregg
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Old 21st January 2011, 06:04 PM   #8
SY is offline SY  United States
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That 220k resistor does nearly nothing- the schematic shows a 47k bleeder, which would swamp the effect.
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Old 21st January 2011, 06:13 PM   #9
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by SY View Post
That 220k resistor does nearly nothing- the schematic shows a 47k bleeder, which would swamp the effect.

I thought there was a 120K and a 5K in series with the 47K to the 47uF after the rectifier so would we have a parallel value?


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Old 21st January 2011, 06:14 PM   #10
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Thanks for the input! We are reading all of this and trying to keep up. We are in over our heads, but are trying hard to get up to speed. So, I will power it up, with a load on it and measure the voltage at the 6n9p and at pin8 of the 34s. Is there anything else that would be helpful? Would any other photos help and any other precautions in getting these measurements?
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