Rebuilding my kit - shielded twisted pair options, and a film cap question

twofires · 770

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Offline twofires

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Hi Bottleheads,

So, my SEX amp has an intermittent popping issue (through headphones and speakers) that is maybe once every hour of use, and is concerning enough that I'm just going to strip the whole thing down and start again. There was a water-soluble flux issue during the build, brought about by my inexperience, that may be causing intermittent shorts, and so I just want to clean the whole thing out, try again, and hope the transformers and chokes are okay.

Anyway, I'm able to account for pretty much every part, except for the shielded twisted pair power wiring w/drain wire. The closest I've found that isn't AUD$400 for a massive reel is this stuff:

https://catalog.belden.com/techdata/EN/8450_techdata.pdf

It's about AUD$95 for a 15m cut shipped from the US, but I can live with that.

The tech data says it's rated to 300V, but as there's around 400V flying around inside the amp, I thought I'd check if this is an appropriate substitute.

Also it seems 1.5uF 630V+ PP film caps are hard to find outside of this general purpose Vishay:

https://au.mouser.com/ProductDetail/Vishay-Roederstein/MKP1839515084HQ?qs=5W%252BGeepUzoc9cdLA%252BylF3g%3D%3D

and a rather chunky and expensive Mundorf M-Cap Supreme:

https://www.soundlabsgroup.com.au/p/Mundorf-MCAP-Supreme-1u5-600V-Capacitor.html

 Is there any leeway with capacitance with that cap? What would raising it to 2uF do to the circuit?

Thanks in advance,
Simon
« Last Edit: September 26, 2024, 02:32:10 AM by twofires »

Simon N


Offline Paul Joppa

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...

Also it seems 1.5uF 630V+ PP film caps are hard to find outside of this general purpose Vishay:

https://au.mouser.com/ProductDetail/Vishay-Roederstein/MKP1839515084HQ?qs=5W%252BGeepUzoc9cdLA%252BylF3g%3D%3D

and a rather chunky and expensive Mundorf M-Cap Supreme:

https://www.soundlabsgroup.com.au/p/Mundorf-MCAP-Supreme-1u5-600V-Capacitor.html

 Is there any leeway with capacitance with that cap? What would raising it to 2uF do to the circuit?

...
There is considerable leeway. The calculated value is 1.25uF, and anything between half that and twice that is acceptable. Larger values give slightly deeper bass with slightly  less power on the deepest bass - neither is likely to be audible in ordinary use.

I'll defer to PB on the wire. You can also ask at replacmentparts <at> bottlehead.com for the cable and capacitor. Mention this thread when you email.

Paul Joppa


Offline Paul Birkeland

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That wire will do the job, the STP in the SEX amp doesn't carry much voltage. 

Water soluble flux can ruin a build, but I have brought kits back to life by running them through a dishwasher cycle without soap. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline twofires

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There is considerable leeway. The calculated value is 1.25uF, and anything between half that and twice that is acceptable. Larger values give slightly deeper bass with slightly  less power on the deepest bass - neither is likely to be audible in ordinary use.

I'll defer to PB on the wire. You can also ask at replacmentparts <at> bottlehead.com for the cable and capacitor. Mention this thread when you email.

Thank you for that, Paul. It seems that the options for caps increase a fair bit if 2uF is acceptable, so I might go with that. I'll keep the replacement parts thing in mind, too. It all comes down to shipping in the end.

That wire will do the job, the STP in the SEX amp doesn't carry much voltage. 

Water soluble flux can ruin a build, but I have brought kits back to life by running them through a dishwasher cycle without soap. 

Excellent. Thanks for that PB. Finding an equivalent wire down here in Oz is pretty much impossible - too small a market for many specialty products I guess.

Re the flux: yeah it was a lesson learned, for sure. The Keystone terminal strips were a bit oxidised and my wimpy no-clean solder didn't have strong enough flux to cut through, so I unwittingly added the water-soluble stuff. It worked in the short term, but then clagged everywhere, hid in places, and trapped forgotten lead cutoffs. The total immediate damage on first power on was one power resistor, a Dayton cap, and an RCA tube. It took out another tube before I got it clean enough to function, but it's not ideal and I'm looking forward to a clean slate. I have better RMA solder these days.

I've seen you mention the dishwasher thing before - how does that work exactly? Are we talking disassembling and desoldering and washing the hardware and passives? Or you just bung the whole thing in minus the wooden base and tubes? I always assumed that would destroy switches and pots and transformers and such.

I'm basically planning to bin everything but the transformers, chokes, tubes, base, top plate and mounting hardware, and replace the rest to rule out gremlins. This still leaves the possibility that I damaged the PT-10 when I (very gently) sanded the chickenwire marks & coating off the sides and repainted. I recall you folks telling someone this may damage the transformer - something about eddy currents? The transformer did get quite hot in operation, and there was a definite hum, but it also seems like these things are normal within certain limits. So maybe it's fine? I dunno if there's a test with a scope or meter or something to confirm.

Anyway, I can try working with the iron I have first - if I have to build it a third time I'm sure I'll have it down pat by then.  :)
« Last Edit: September 26, 2024, 12:50:11 PM by twofires »

Simon N


Offline Paul Birkeland

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Or you just bung the whole thing in minus the wooden base and tubes?
Yes, that will do it, and definitely run a dry cycle. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline twofires

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I opted for the full teardown in the end. That water soluble flux (ChipQuik) was crusted on everywhere, in spite of my best cleaning efforts while the amp was whole. It's a miracle the amp worked at all in that condition. When I was desoldering leads and removing excess solder from the chokes and transformers I could still smell the stuff cooking off - a distinctive black bean stir fry sort of smell.

See the attached for a an example of how the flux caked on to a tag strip, and the horrified look on my now depopulated top plate.

Other than having to bin a lot of components, the transformers and chokes came out okay. The only minor note is that terminal 18 on the PT-10 is a little bit wiggly - I'm wondering if there's a high temp non-corrosive glue I can use to get it rock solid again. Probably not a concern but seeing as I have the thing apart anyway...

Simon N


Offline Paul Birkeland

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You can use a little epoxy to secure the plastic piece of the bobbin that holds that power transformer terminal.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline twofires

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You can use a little epoxy to secure the plastic piece of the bobbin that holds that power transformer terminal.

Thanks for that Paul. Something like JBWeld HighHeat? It says it can withstand up to about 290C. My iron is set to 370C at the tip, but I figure the point of contact should be significantly cooler than that unless I'm dwelling far too long. Unless you recommend epoxy only after soldering? It might be a bit trickier to glue up in situ but not impossible.

Simon N


Offline Paul Birkeland

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I wouldn't even worry about buying high temp products, the heat will only be there for a handful of seconds. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man