Border Patrol SET...hummmmm

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Michael L
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Border Patrol SET...hummmmm

Post by Michael L » Thu Jan 22, 2015 8:28 pm

I am trying out a BP SE300B that someone built from 90% original parts.
The main difference is in the PSU which is in it's own Hammond case
and has very large mains transformer and main choke. The psu must
weigh around 20 kg.
It came with no audio valves so I have fitted them including a pair of EH300b.
I discovered that one op transformer Rp=127ohms and the other 136 ohms.
I set the bias to 70ma. I found that turning the hum pot has little effect
and there is a fair hum whether or not my (active) pre is connected.
I played some music and despite the hum, if the volume is increased
It sounds very capable and in another league to my Leak 20.
Any comments on the op transformer primary resistance difference and
if it might affect anything? I particularly am looking at a way to sort out the hum.
The amp and psu aren't in standard BP chassis but if I decide to keep it I would like to rebuild particularly the amp into something with a wooden plinth.

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Mike H
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Post by Mike H » Thu Jan 22, 2015 9:41 pm

If it matters, you could add a low value resistor to the OPT with the lowest resistance.
 
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rowuk
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Leftovers

Post by rowuk » Sat Jan 24, 2015 9:07 pm

I don't think that a DC measurement of the OPT means anything when they are that close.

Hum with a simple 300B SE amp is normal and there are in my opinion two valid solutions: DC or high frequency AC. For both strategies there are multiple solutions. Andrew here at Audio-Talk has some regulators for DC and I am currently trying some Coleman regs with a separate DC power supply. I have seen two high frequency AC solutions here and both appear to have their followings and merit. All of the filament power solutions can be considered "low cost". There is much division and discussion about what is "better".

I am very happy with the Coleman regs, but will be trying high frequency heating on another amp project in the near future.
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Mike H
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Post by Mike H » Sun Jan 25, 2015 7:03 pm

I believe the original WE amplifier had a centre-tapped heater winding for the 300B.
 
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Michael L
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Post by Michael L » Mon Jan 26, 2015 6:22 pm

After a couple of hours with this amp I knew it was a keeper.
I still have the hum and believe it is down to a fault condition. It's been sitting on a shelf for a long time. Maybe it's a filter cap. When I adjusted the hum bucker pot it seemed to bring on another different hum either side of a central setting. I wonder if I need special HT scope probes to check this out.
Think the B+ is >400V
Don't suppose anyone has a schematic

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Mike H
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Post by Mike H » Tue Jan 27, 2015 1:20 pm

It could be just a wiring issue, pick-up from somewhere. (?)
 
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Michael L
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Post by Michael L » Fri Jan 30, 2015 8:03 pm

Fixed the hum. Just a joint needing re-soldering.
I'm very pleased with my first SE valve amp . I'm using it with Meadowlark
Kestrel Hotrods. Would like to try it with something more sensitive.
I'm wondering also how it would sound if I convert it to low sensitivity
input with no capacitors in it's audio path.

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Mike H
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Post by Mike H » Sat Jan 31, 2015 4:19 pm

A-ha!! Super. :thumbleft:
 
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rowuk
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Re: Border Patrol SET...hummmmm

Post by rowuk » Sun Jun 21, 2015 12:39 pm

Did Avinunca1 make the capless mod? Did anything blow up? I think that caps often get bad press because it is something not to hard to twaek and impossible to objectively compare.
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Michael L
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Re: Border Patrol SET...hummmmm

Post by Michael L » Sun Jun 21, 2015 8:14 pm

Not yet done the mod and I have a pair of Andrew's 300B heater regulators to add too.
I am enjoying the amp hugely meanwhile though

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