Weird Heater Supply Problem - And other solutions!

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neikeel
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Re: Weird Heater Supply Problem - And other solutions!

Post by neikeel » Thu Oct 26, 2017 3:15 am

end to end 30ohms is probably right but not a particularly useful metric! Approx 15 ohms between primary each end and centre tap.
Better way is use a 100vac (typically I use a bias tap) across the primary and measure what you get on the secondary. The ratio is what is important (1.7k for 16 ohm, 3.4k for 8 ohm and 6.6k for 4ohm tap) you can use any small transformer to do this but 100 is easy to work with.

Could just be quality issue!
Neil

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Thu Oct 26, 2017 3:30 am

Can you upload a new pic of the presence control please? What did you go with in the end? Old or new style?

Are your input jacks definitely wired up ok?

It could be the OT. I would have thought with a cheap OT you'd get less bass not less treble! Unfortunately I haven't experimented much with different OTs so I can't really say.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Thu Oct 26, 2017 11:18 am

neikeel wrote:
Thu Oct 26, 2017 3:15 am
end to end 30ohms is probably right but not a particularly useful metric! Approx 15 ohms between primary each end and centre tap.
Better way is use a 100vac (typically I use a bias tap) across the primary and measure what you get on the secondary. The ratio is what is important (1.7k for 16 ohm, 3.4k for 8 ohm and 6.6k for 4ohm tap) you can use any small transformer to do this but 100 is easy to work with.

Could just be quality issue!
Sounds like I could just wire it straight to the wall at 120vac too. I will do that and report back.

I will also snap a pic of the presence wiring. I have a 25k pot, .1 cap to ground and a 4.7k resistor from the right lug to ground. And then the nfb wire to where the 4.7k resistor attaches to the pot.

All the controls are responsive on the front panel. The depth mod pot is removed from the circuit at the moment just to eliminate any confusion. It just sounds like the power section is being boxed in. I would have figured the .0022 cap across the pi coupler would have for sure forced only bright stuff through the power amp.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Thu Oct 26, 2017 2:50 pm

Alright. Here are some pics of the input jack wiring and the presence pot.

ImageIMG_20171026_075202050 by Paul Abbott, on Flickr

I swapped the treble peaking 470K/470pf cap for a 68K to see what it would do. I like the gain it introduced.
ImageIMG_20171026_075211539 by Paul Abbott, on Flickr

ImageIMG_20171026_075220653 by Paul Abbott, on Flickr

And I have this old Traynor YBA-1 Output Transformer I could swap in and pull two of the tubes and see what gives.
ImageIMG_20171026_083034089 by Paul Abbott, on Flickr

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Thu Oct 26, 2017 7:15 pm

Hang on I can see what's going on now. You've got a .0047uF cap across your PI plates. That is way too big. You only need 47pF. .0047uF = 4700pF.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Thu Oct 26, 2017 7:19 pm

Littlewyan wrote:
Thu Oct 26, 2017 7:15 pm
Hang on I can see what's going on now. You've got a .0047uF cap across your PI plates. That is way too big. You only need 47pF. .0047uF = 4700pF.
Well sheit!

:peace:

I will rewire everything up and see if I have a 47pf cap laying around. Any specific voltage rating?

https://www.ampbooks.com/mobile/amplifi ... alculator/

Holy crap. I just input those value differences and no wonder I am passing so much low end through.
Last edited by marshallnoise on Thu Oct 26, 2017 7:25 pm, edited 1 time in total.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Thu Oct 26, 2017 7:23 pm

Your HT is 525VDC if I remember rightly so go with 630V to be safe. The PI Plates could go up to 525VDC if you turn the amp on with no valves for testing.

1kV Ceramics are real cheap on ebay if you haven't got any :)

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Thu Oct 26, 2017 7:41 pm

Littlewyan wrote:
Thu Oct 26, 2017 7:23 pm
Your HT is 525VDC if I remember rightly so go with 630V to be safe. The PI Plates could go up to 525VDC if you turn the amp on with no valves for testing.

1kV Ceramics are real cheap on ebay if you haven't got any :)
*heads off to ebay right now*

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Re: Weird Heater Supply Problem - And other solutions!

Post by neikeel » Fri Oct 27, 2017 3:19 am

I agree a cheap OT is thin and shrill

You do not actually need that cap there for testing on the PI plates, quite a few amps don't use them.
Neil

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Fri Oct 27, 2017 11:55 am

neikeel wrote:
Fri Oct 27, 2017 3:19 am
I agree a cheap OT is thin and shrill

You do not actually need that cap there for testing on the PI plates, quite a few amps don't use them.
Ok, I will pull the cap and place a jumper across or leave it off all together? Seems like just pulling it would be the trick.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Fri Oct 27, 2017 2:18 pm

DON'T PUT A JUMPER THERE! You'll short the plates! Just leave it off for now :)

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Fri Oct 27, 2017 2:21 pm

Littlewyan wrote:
Fri Oct 27, 2017 2:18 pm
DON'T PUT A JUMPER THERE! You'll short the plates! Just leave it off for now :)
Hence why I waited. Ha!

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Sat Oct 28, 2017 12:17 am

I had an old YBA-1 board laying around with a 47pf cap on it. Bam!

Last thing I am chasing is a 120hz buzz.

https://youtu.be/UqNuddH2iLw

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Sat Oct 28, 2017 4:53 am

We were brilliant? You're too kind! :D

Do you get the buzz with nothing plugged into the input? Have you got a headcab for it? If so has it got a shield in for the bottom?

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Sat Oct 28, 2017 1:16 pm

Well, mentioning that you guys are brilliant does imply I am not so brilliant. Ha!

The buzz is there no matter what, but it does get louder with a cable plugged in. I haven't made a case for the bottom of the amp yet. I am thinking the diodes in the full wave bridge need some filtering. But that's just a stab in the dark at this point.

Freaking wow this thing is loud and has tons of low end. As a baseline for mods, this thing is going to be killer!

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