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PostPosted: 22 Mar 2017, 08:34 
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Joined: 25 May 2016, 16:30
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Thanks for the input. 2 amps were made by one person in Belgium, the other two by myself in the UK. So the chances of an identical mistake would seem remote. We've been through components, wiring, solder joints etc etc to no avail.

In the repair side of my business I've just got a 30 year old Rose preamp in whose main smoothing capacitor behaves similarly to the one you mention. Fine for 2 or 3 hours then it gives up and the hum starts.......

I'm soon to start on the next pair so it will be interesting to see how they perform!
David


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PostPosted: 22 Mar 2017, 12:52 
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When I'm faced with situations like this in the shop, I sometimes have to do a "shotgun repair" - replace everything and hope you hit it. Better to lose money than a customer!

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PostPosted: 22 Mar 2017, 14:26 
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I believe you really do need a third party to examine the amps - someone who has never seen them before and has no notions or thoughts, preconceived or otherwise.

I also strongly suggest you look for some commonality of parts across all four units, especially those in the relevant circuit(s). That they are in different countries separated by water is not relevant to this last issue, as the originally sourced parts/lot(s) could be defective or counterfeit.


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PostPosted: 22 Mar 2017, 14:28 
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Geek wrote:
When I'm faced with situations like this in the shop, I sometimes have to do a "shotgun repair" - replace everything and hope you hit it. Better to lose money than a customer!


There is always that. Funny thing is that it works.

I keep spare boards for amps I favor that are ready to go. When such a device arrives, it gets 100% re-boarded. Only then will I start to troubleshoot the 'bad' boards.


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PostPosted: 22 Mar 2017, 16:50 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Keep the cards and letters coming....they are still misbehaving. Lots of good input for anyone trouble shooting a difficult piece of gear.

Good listening
Bruce

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PostPosted: 22 Mar 2017, 23:07 
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Location: Canada
Quote:
four transformer sets came from the same Edcor production batch.

Try a different output transformer. That seems to be the biggest commonality.

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DTS Audio Electronics
www.dtsaudioelectronics.com


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PostPosted: 23 Mar 2017, 03:44 
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Here's a few more clues. The redness is always either on the metal fold adjacent to Pins 2 and 3, or adjacent to Pins 6 and 7. In other words, it's adjacent to the screen grids. The redness is more common by Pins 6 and 7 than by 2/3, and in the 'best' tubes is normally by 6/7 and not 2/3.

Some folk might miss the redness on the 'best' tubes: it's that slight. You need a blacked out room and /or nighttime to see.

I'm used to over-dissipation in 300bs which manifests itself as patch which is not in a consistent place on the anode. Also, you reduce dissipation slightly and it goes. With these KT150s, on the worst tubes, dissipation has to be reduced to 40 watts before it goes.

The Oddwatt circuit is pretty straightforward and I've replaced all the coupling caps. If the voltages are correct under load then the resistors and power supply are behaving themselves.

If it were the output transformers, surely there would be some measurable artefact? The latest ones from a different batch are identical in DC resistance.

No stone will remain unturned until this problem is cracked so do keep the suggestions rolling!


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PostPosted: 23 Mar 2017, 06:13 
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Not to fall into the trap of "impedance" and "resistance" might be another avenue to explore.

>> The latest ones from a different batch are identical in DC resistance. <<

Just for giggles and if you have a meter than can do this, do an ACV reading on the B+ power-supply and let us know what you see.


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PostPosted: 23 Mar 2017, 09:08 
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Hi, An AC voltage test on the B+ line could be interesting, but with the amps as quiet as they are the B+ has to be pretty clean. If not there would be significant hum. I will try to measure one of the amps I have here to see what is normal.

A question for the folks in Europe and UK....where do you get your tubes and who (name ) is the supplier.

Good listening
Bruce

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PostPosted: 23 Mar 2017, 10:07 
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I use three sources principally: Hot Rox (UK), an Banzai and Tube Amp Doctor (TAD) in Germany. TAD are European Distributors for Tung Sol. All the faulty 120s and 150s s far have come from Banzai and HOt Rox. TAD are more expensive. All three are normally pretty good on refunds /replacements.


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