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Old 19th July 2013, 06:18 PM   #11
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The problem is not entirely confined to the one channel, however, one channel is significantly worse than the other. I will point out that both channels of the amp were operated for some amount of time without a speaker attached.

I noticed that some other forum members talking about replacement output transformers here: link

The "Edcor XPP10-8-10K" seem like a reasonable price. I can order them right now, but I just want to have some kind of reasonable assurance that the replacement transformers will fix the problem.-

You mentioned that there could be also be some oscillation on the lines. Is there any way I can test for this without an oscilloscope?
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Old 19th July 2013, 07:31 PM   #12
rmyauck is offline rmyauck  Canada
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A nice step up would be GXPP series because of the extended frequency response and I would go for the 15W versions for its extended headroom for a few $ more. You get a much bigger trans. with nice covers. You should really notice another step up again.

Also try a different source or two as sometimes there can be issues with them. OK that was mentioned.

Maybe a bad cap or short or even backwards cap (check polarity) or diode in the PS as it's affecting both channels and you mentioned PS trans. buzz.

Last edited by rmyauck; 19th July 2013 at 07:39 PM.
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Old 19th July 2013, 07:38 PM   #13
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+1 Would really like to compare my Hammonds with the Edcors

Quote:
The "Edcor XPP10-8-10K" seem like a reasonable price. I can order them right now, but I just want to have some kind of reasonable assurance that the replacement transformers will fix the problem.-
Don't think we've proved beyond doubt that the OT's have been damaged. Have to say it's not looking good

Swapping might still settle the issue if "the significantly worse" sounding channel travels with the OT. If it doesn't then the fault lies elsewhere.

A lot can be done without an oscilloscope. Remember this is a new build so the most likely fault is a component(s) in the wrong place, or back to front or missing. A dry joint ( or short ) may be present. Make sure OT primary and secondary wires are correct. ( This usually causes an unmistakable wail of a noise so isn't that likely.)

Check R5 (and R11) is correct as this sets the amount of negative feedback.

If all seems fine then we need to check voltage levels with respect to earth at various points within the amp. Each channel should have similar voltages at similar points. Different voltages should help to narrow down where the fault is. Valves should be removed and no music played while testing.

Caution - High voltages can kill, have you read our sticky thread on safety.. - ( never too late )
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Old 19th July 2013, 08:04 PM   #14
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As Rmyauck says-

Quote:
Maybe a bad cap or short or even backwards cap (check polarity) or diode in the PS as it's affecting both channels and you mentioned PS trans. buzz.
You only need to worry about getting C8 and C9 in backwards. It doesn't matter which way round the rest of the caps go in as they are non polarised. C8 and C9 help to filter out the mains " buzz " passing through the mains transformer so are important.

The bridge rectifier can only be inserted correctly IIRC
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Old 20th July 2013, 02:02 AM   #15
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Okay, so after switching the output transformers the problem channel moved from the left to the right. I think that's good enough evidence for me. I'm going to go ahead and buy a couple Edcor GXPP 10-8-10.

For the sake of completeness though:
-I checked the electrolytic caps and they're both oriented correctly.
-I checked and tested all of the resistors. They're all performing as expected.
-I checked the orientation of the bridge rectifier. It's good too.

P.S.
The wires on the GXPP output transformers seem to be a little different than the ones that I have now. Could someone please tell me how to properly hook them up to the amp? Thanks.

Last edited by bananajamslam; 20th July 2013 at 02:08 AM. Reason: added a question
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Old 20th July 2013, 10:29 AM   #16
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Looks like the Edcor GXPP 10-8-10 has ultra linear taps. ( white/blue and white/brown wires) These should be insulated and left unattached for the moment. In the future you might like to try the ultra linear mod. Or you could build a new amp using these taps.


GXPP-8.png

Not sure if the colour of your OT wires are the same as my stock OT (SA0734). The centre tap (Ct) of my primary is red and would be the same on the Edcor. My P1 is brown and my P2 is blue. According to my reading of the above Edcor diagram you would use the blue wire for P1 and the brown wire for P2. Getting this wrong would result in positive feedback instead of negative feedback coming off the 8ohm secondary. The amp would go into oscillation and you would quickly hear this "wailing" sound I've talked about. (You'll want to turn the amp off right away ) The phase problem is easily solved by either reversing P1 and P2 of the primary or reversing the secondaries, whichever is easiest.

Perhaps others who have experience of this Edcor transformer could confirm my advice.
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Old 20th July 2013, 01:33 PM   #17
rmyauck is offline rmyauck  Canada
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A second reason to maybe go for the 15W version besides the better sonics it should give you is that down the road the tube used in your amp may be hard to find so you may want to use in production tubes like the 6BQ5/EL84 which will give you up to 17W/CH depending how it's set up or if the PCB's go bad you can build a new amp.
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Old 20th July 2013, 03:04 PM   #18
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Okay, so Red goes to Red, Blue goes to Brown, and Brown goes to Blue? I looked in the manual that came with the amp and it seems to confirm that.

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Originally Posted by rmyauck View Post
A second reason to maybe go for the 15W version besides the better sonics it should give you is that down the road the tube used in your amp may be hard to find so you may want to use in production tubes like the 6BQ5/EL84 which will give you up to 17W/CH depending how it's set up or if the PCB's go bad you can build a new amp.
I did notice that the 10GV8 tubes that came with the amp are rather difficult to find replacements for. After I hook-up the new output transformers, what would be involved in swapping the 10GV8 for 6BQ5? Would they be drop-in replacements? Or would I have to change resistor/capacitor values and/or do some rewiring?
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Old 20th July 2013, 03:30 PM   #19
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The 10GV8'S contain a triode and a pentode in one tube so no can do. The heater supply is 11.4Vac in your amp whereas the 6BQ5 ( EL84 ) uses 6.3V and is usually teamed up with an ECC83 triode to drive it. Better to start from scratch with your new transformers.

There are many designs on this forum you could consider building in the future so Rmyauck's comments make sense if you get bitten by the bug.
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Old 20th July 2013, 04:06 PM   #20
rmyauck is offline rmyauck  Canada
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If you can swing the $ in the long run for great sonics and power handling capability for a future amp get the Dynaco Z-565 clones (used on the ST-35/SCA amps) say from Triode Electronics the first to really make them readily available. Dynakit parts came later. Magnequest is a noted high end winder making exact copies with a top finished look. Heyboer also makes a clone I believe. The Magnequest one is the only one built with the 8 & 16 ohm secondary taps.

In the future if you want to stick with PCB'ss for ease of building etc. then you can use tubelab's simple EL84 PP PCB board (he sells it with bard components also) or a Dynaco clone PCB from diytube or an original ST-35 or SCA from different vendors. The EFB mod from Dave Gillespie is well worth adding and he has info for diy and PCB's on the Tronola Projects site.

Last edited by rmyauck; 20th July 2013 at 04:17 PM.
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