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Old 16th October 2011, 05:51 AM   #1
cancon is offline cancon  Canada
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Default 1st build; Adding load to PT secondary

Dear reader:

The power section in my home-made design is overpowering the circuit I've built; single-ended, 1ea 6V6 & 12AU7, 5k-8w OT.

The PT secondary is rated at 250vac @ 42mA and 285vac @ 0mA (exhibiting a linear voltage change between 0-42mA).

In circuit I measure 275vac before the bridge rectifier and 375vdc after the filter section (150uf/350 & 20uf/350). My circuit isn't putting a dent in the load on this transformer and I need an operating voltage of 250vac.

Please advise!
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Old 16th October 2011, 06:04 AM   #2
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PSUDII

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Old 16th October 2011, 02:15 PM   #3
DF96 is offline DF96  England
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How much current is the 6V6 taking?
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Old 16th October 2011, 04:41 PM   #4
cancon is offline cancon  Canada
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I'll measure when I have a chance soon.
Right now the amp is not working very properly...there is arcing in the 6v6, strange oscillation noises, no signal at all - even when the effect is bypassed...(half-true bypass...input tip is shunted to output tip, but input tip is never actually disconnected from the screen of the 12AX7. Signal does bypass properly if the unit is off however), the input tip is actually hot as well, gave me a bit of a bite.

I'll try to get that measurement soon. thank you!
I'll check out that link too, looks promising Aardvark!
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Old 16th October 2011, 06:37 PM   #5
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you have a bigger problem than B+ voltage.

A schematic and measured voltages (tubes out) would be a good start.
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Old 16th October 2011, 10:28 PM   #6
cancon is offline cancon  Canada
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Please please please excuse my amateur drawing skills. First schematic i've ever drawn, did this up with tinycad. Unless otherwise noted all cap values are uf/VDC & resistor values are ohm/1/4watt

Much appreciated!

Excuse the original PT 205 rating, that was what I was copying. In reality it was more like 225 because of the 115V tap and my 120-122V actual supply. My revised target (based on current stock and prices) was 250V, but there is also a 125V tap. I figured that if I hit 42mA I would achieve between 240-250 which would be ideal.

Can you tell me exactly which measurements you'd like?
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Last edited by cancon; 16th October 2011 at 10:32 PM.
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Old 16th October 2011, 10:43 PM   #7
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very odd front end - do you mind describing what is meant to be going on from the input jack to the second section of the ax7? Don't worry about voltages at this point.
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Old 16th October 2011, 10:47 PM   #8
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sorry - kinda got it now...
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Old 16th October 2011, 10:56 PM   #9
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100K in series with the power transformer primary?
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Old 16th October 2011, 11:54 PM   #10
cancon is offline cancon  Canada
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All I can say...this is not my brainchild...I copied it from an existing design. I think it is very similar to a fender champ. This is meant to be a tube head "preamp"...feeding an unbalanced tube driven Leslie speaker (which does not have a built-in pre), hence the 10ohm/5w resistor across the OT.

I feel that there are 2 problems...but I can only pinpoint one - I'm supplying the circuit with too much juice. I want the transformer to deliver under 250v so I need a full load, possibly by adding beefy resistors before the bridge?

Anyhow....let me know if your diagnosis requires any present voltages.
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