6AS7 Push Pull

What people are working on at the moment
simon
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#226

Post by simon »

Lee S wrote:What rectifier? We are using AC aren't we? :?
simon wrote:Simple CRC. 5V Hammond filament tx into bridge into 4700uF - 0.1R - 4700uF.
My gut instinct is also to put one in each wire, though I can't see why it would make any difference either. Of the few amps I've built the AC filament voltages have been spot on, this is the first time it's been out. I'm going to Maplins shortly, shame they don't do anything suitable. If the 0.1Rs don't fettle it, it's looking like a special order to Farnell in the New Year.
richardcooper2k
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#227

Post by richardcooper2k »

i have done the same to drop my ht as my mains is a bit high and the transformers are so 'stiff' (v doesn't drop much as expected under load)

do people think there a sonic downside to this with dht amps ?
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Nick
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#228

Post by Nick »

do people think there a sonic downside to this with dht amps ?
Even just giving my view on this is a complex thing, normally I would say increasing the supply resistance would be a bad thing, except, having a higher resistance, may reduce the Q of the supply and prevent overshoot. It may also work out better, as the higher resistance may lead to a less strict bass response, which giving a particular amp/speaker/room/ear combination may be a good or a bad thing.

So I think for me the answer is "depends"
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simon
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#229

Post by simon »

I think this is going to be trial and error.

The additional two nominally 0.1R resistors dropped the voltage from 5.48V to 5.22V. Paralleling a pair of nominally 0.47Rs instead gave 5.07V. I guess the values of the resistors are so small the tolerances of the resistors will be important. A little job for the new year I think.
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Paul Barker
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#230

Post by Paul Barker »

the thing is Simon as you reduce the voltage to the filaments they pull less current so the predicted value estimated from the current drawn originally is always out. It's exactly the same when calculating cathode bias resistor. It's like chasing a fighter plane, it shifts parameters as you go after it.

That's why it's best to adjust everything empirically.
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#231

Post by simon »

Hadn't thought about it like that Paul, makes perfect sense now you say it.

I've managed to bodge it with various resistors I have - paralleling some, series others - and I've got 5.04V and 5.07V which will do me for now. What's interesting is that laid back sound that the amps had developed which I really didn't like has disappeared now I've got the filament voltage up from 4.4V to 5V. I think 5.5V might have been best of all, but I'm trying not to think about that. Burning up KRs prematurely would make them very expensive indeed...

I feel a lot happier about my humble efforts now I know I'm trying to outwit fighter pilots. :)
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Paul Barker
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#232

Post by Paul Barker »

And so you should. Actually making something is a major achievement.

I have been outside in the cold and the rain making some Open back boxes for my newly arrived Supravox woofers. I decided to keep them seperate and make fullrange driver baffles to sit on top. That way I can try the lowther pm7a's against the aer mk1's and against anything else I care to try.

When I've got speakers sorted I'll have to find a new hobby. (imagine four rolling about on the floor emoticons and insert here).
simon
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#233

Post by simon »

Where did you get the Supravox from Paul? Is there anywhere in the UK, or did you have to go abroad?
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Paul Barker
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#234

Post by Paul Barker »

Yes it's the person who Thorsten went into business with. You do need to specify that they are for open baffle.

"info@realhi-fi.com

Dear Mr Barker,

Thank you for your interest in Supravox drivers.

A pair of 285GMF would cost £345.00 all inclusive (postage and VAT).

Regards,

Matthew Jameson
Real hi-fi"
simon
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#235

Post by simon »

Hmm, they're an expensive option but ultimately may give superior results. I'm weighing up options at the moment, and the Supravox are contenders, depends what I can find. Good luck integrating them.
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Paul Barker
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#236

Post by Paul Barker »

simon wrote: Good luck integrating them.
Shucks you been in my listening room?
simon
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#237

Post by simon »

:lol:
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#238

Post by Cressy Snr »

Hi everyone.

Had Mum & Dad round for tea yesterday evening. The amp was burbling away in the background playing some jazz. Suddenly Mum piped up,
"ooh! hasn't it got a lovely tone Stephen"

Oh dear :cry:

Steve.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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Nick
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#239

Post by Nick »

I would take that as the higherst praise :-)
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Dave the bass
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#240

Post by Dave the bass »

SteveTheShadow wrote:Hi everyone.

Had Mum & Dad round for tea yesterday evening. The amp was burbling away in the background playing some jazz. Suddenly Mum piped up,
"ooh! hasn't it got a lovely tone Stephen"

Oh dear :cry:

Steve.
I think you're miss-reading what she said Steve, secretly she's a hard-core be-bop jazz buff and commenting on the Selmer Series 3 Sax tone :wink: She probably let being uber-cool slip out and was hoping you wouldn't notice... :)

DTB
"The fat bourgeois and his doppelganger"
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