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Trouble with a DIY EL34 Tube Amp Kit

elleelle

elleelle

New Member
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


http://thediyexperiment.blogspot.com/


http://3.bp.blogspot.com/_D8VB90EwTZw/TTkv9mRNorI/AAAAAAAAA2M/fNf-gXMJ918/s1600/amp%2B4.JPG
 

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With any point to point circuit it is absolutely imperative that you double check both ends of every wire and component on your build. With no instruction list it is VERY easy to either miss a wire or connection or put a connection where it doesn't belong.

Print out an extra copy of the schematic and then, comparing the schematic to the actual build start identifying each and every lead and component. Make sure that each connection you verify is on the right pin number of the tube socket. Remember that numbering on the socket runs in opposite directions top and bottom. Usually pin numbers are identified on the bottom of the socket. Use those. Mark each connection that you verify on the copy of the schematic and put a small tape tag on the connection in the amp. You are ultimately looking to verify that each of the connections on the schematic matches what you have in the build.

Verifying the wiring is the first place to start.

Good Luck

Shelly_D
 
Assuming all your DC voltages appear correct, can you disconnect the feedback loop to see if the "noise" goes away? Maybe your amplifier is oscillating due to positive feedback (ie, "motorboating".)
 
Lenny-
You actually only have one schematic (3rd diagram); the other two are versions of a 'layout' diagram.
As Shelly_D says, print out a copy of the schematic, and mark with a highlighter every connection, as you double check everything.
Part of the hum problem may be due to the arrangement of the wiring. You really need to keep the AC and power wiring away from the low-level signal stuff as much as possible.

P2P wiring, done well, is a skill. (You have made a good start with the twisted heater wiring...)
Check out some old junked tube hifi chassis if you can find some locally (garbage day? dump? Craigslist Free Stuff?) . A few hours spent studying the underside of well-made factory stuff from the 50s will pay off.
Cheers
John
 
Not the immediate cause of your problem (I think) but you really shouldn't run wires through holes in sheet metal without fitting a plastic/rubber grommet. I notice you have some leads running through a non-grommeted holes. This is a safety issue which you should correct.

While you are double-checking your wiring, some of the solder joints look like they could stand to be re-heated/re-flowed.

John
 
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Thanks so much for the feedback guys. I am going to take some voltage measurements and post them, hopefully that will help.

We did go through all of the points several times, but not with tagging the points as you said. I will double check all of that as well as the solder points and the grommets, as mentioned. Thank you for those tips.

Now, one thing that I feel rather silly about asking..... The ground bus is not grounded to the chassis. Does this need to be? Since the grounds were all joined to it, including the main transformer, we did not see that it needed to be. If this is incorrect, I can see why there would be an amazing about of hum.

Please pardon our amateurness...amateurism....us being noobs.. Your help is very much appreciated.
 
elleelle said:
...we were left with 2 resistors that had no home. One was a 2w 5k and the other was a 220k. ...we did see that there was a 220k resistor present, but on the other 2 it was not there. ...
However, after all is said and done, there is no place for this 5k resistor that we can see.
Click to expand...

The 5K is in your power supply. Do you have one there already? If you do then then check it's value as you've likely got the wrong resistor there.

Edit: the 5K is between the 330uf and the 2nd 47uf caps. (between where it shows B1 and B2) On the schematic it's marked as 5K, on the layout it's marked as 5.1K
 
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thanks chubby. We double checked yesterday and the one in the PS was 5k, but I will triple check tonight.
 
So you received two 5K resistors? Does bill of materials call for one or two? Does the invoice say two were sent? Just things I'd be checking.

I rebuilt the transmission of a sports car once and had more than a few screws left over :dunno: :)
 
haha.. yes, the parts list shows 2 and the bag had 2 in there.. but, for the life of me I cannot seem to find where the 2nd could go. At this point, I am assuming that it was a mistake that it was sent.
 
elleelle said:
..... The ground bus is not grounded to the chassis. Does this need to be? Since the grounds were all joined to it, including the main transformer, we did not see that it needed to be. If this is incorrect, I can see why there would be an amazing about of hum.
Click to expand...
Grounding is often a bit tricky. Too many ground points and you can have ground loops, too few and you can have safety problems.

It is true that the 'signal ground' doesn't always have to be connected to the chassis/safety ground. You do have to think it out, though.

It is a bit tough to see from your pictures, though the pics on your blog are very clear- sometimes wires are hidden behind components or the chassis flange. So I may be stating the obvious here; forgive me.

re: safety ground.

The safety ground (incoming power ground connection) MUST be connected to the metal chassis. Reason: if a 'hot' wire gets connected to the chassis (accident, chafe) then the circuit breaker on your household panel will trip and protect users from shock.
It looks like you have a 3-wire IEC (computer style) power entry? Is the ground connection from the power entry connected to the yellow/green wire that goes to the ground bus? You should establish a good chassis ground connection via bolt/toothed (spur) washer and solder lug(s). One common place for this is on one of the power transformer mounting bolts.

re: signal ground
It looks like your input RCAs are isolated from the chassis with a yellow wire connected from the RCA 'grounds' to the chassis via a nut? (?) You definitely need to get the signal (RCA) ground connected to the ground bus.It may be connected via another wire- this is something you can trace out.....

http://2.bp.blogspot.com/_D8VB90EwTZw/TN4vRnzTVMI/AAAAAAAAArY/kAeNx-U33c8/s1600/IMG_4542.JPG

Hope this helps-
No need to apologize for asking questions- this is a pretty ambitious project for a beginner.

John
 
I hate to seem like I am 'piling on' here, but I think you have your power transformer upside down. That's why you have all those wires going through the chassis. Putting the side with the wires 'down' in the hole is the standard practice. Makes hooking things up a lot easier.

You're going to have to unsolder all the connections anyway, if you fit grommets, so it will be just as fast to flip that baby over.....

http://2.bp.blogspot.com/_D8VB90EwTZw/TNdy36FR8nI/AAAAAAAAApw/-08ZeMBj2Qk/s1600/IMG_4306.JPG

John
 
That was our initial impression, but it turns out that the chassis will only accommodate it in this position.
 
elleelle said:
The ground bus is not grounded to the chassis. Does this need to be? Since the grounds were all joined to it, including the main transformer, we did not see that it needed to be. If this is incorrect, I can see why there would be an amazing about of hum.
Click to expand...

Your schematic shows a chassis ground connection at the input jack -- that's the pitchfork symbol. The other ground symbol is a signal ground. The designer apparently intended these to be joined at the input jack.
 
Why will the power transformer not mount with the wires pointing downwards?

Please elaborate.
 
The layout diagram and the schematic do not match exactly.
One example:
Schematic shows B3 connected to one side of the preamp tube heater.
Layout shows a center-tapped heater winding with the CT grounded.
You don't want to do both of these, I think....
John
 
IMO, step one is to double-check your wiring against the schematic.

Next, get your grounds sorted out.

Then, get out your DMM and measure some voltages.
AC Volts: Heaters (Between Pins 2,7 on EL34, between pins 7,8 on 6N9P, pins 2,8 on 5Z3P)

DC Volts- 500 volt setting, Black lead fastened to chassis ground.
Measure at schematic points B1, B2, B3
Measure at Plate of each EL34 (between pin 3 and ground) and Plates of 6N9P (ie between pin 2 and ground, then between pin 5 and ground).

Measure DC voltage between each cathode and ground, and between each grid and ground.
Write it all down.
Be careful- don't get both hands inside that chassis. Safest method is to get some alligator clips for your test leads - hook them up and then stand back and power up.

Take your time and report back......

Cheers
John
 
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