Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Armageddon T Thunderbird on October 17, 2020, 05:41:16 pm
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I have a question regarding what can I expect for the output voltage on a transformer that is rated for more amperage than what I need, I know the circuit itself will only draw the current it requires but I read the voltage rating of the transformer is when the it is drawing maximum amperage rating and using a circuit with a lower amperage draw than the rated max amperage rating will cause the output voltage to raise. I’m using a Antek 200v toroidal transformer and the HT winding is rated at .13A and I’m just using it to power two 12ax7’s by using a full wave bridge rectifier and assuming I did my calculations right via the formula Hammond provides on its rectifier guide I don’t think those two triodes will draw anywhere close to 80ma so from what I read I can expect the transformer to give out more voltage since the transformer is seeing a small fraction of its max amperage rating. Is there a way to calculate what voltage the transformer will output when it isn’t fully loaded? Am I better off using a transformer with a lower amperage rating for the HT? This is a Marshall style preamp project. I know I will probably need to use some dropper resistors but I have no idea what voltage to expect coming out of the bridge rectifier. Thanks in advance! If it matters I am aiming for 250v at the plate resistors.
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I'm thinking you're fine
2 pre tubes, maybe, maxed~~ 20mA at the peaks
so you drop the volts with resistors. calc the wattage
steal a couple PS Taps from a fender or marshall and temp it together, dial in the R's to taste
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> what voltage to expect coming out of the bridge rectifier.
So you actually have the AS-05T200 - 50VA 200V Transformer:
https://www.antekinc.com/as-05t200-50va-200v-transformer/
The data you need is on the "Download specs" PDF:
https://www.antekinc.com/content/AS-05T200.pdf
Input 120VAC 60Hz
207V 0.0A 0W
179V 0.24A 43W
163V 0.38A 62W
Your two Marshall/Fendery 12AX7 will suck 4mA at most, 0.004A. This is MUCH-MUCH closer to 0.0A than to 0.24A, the next data-point. So expect 207VA. A sharp pencil suggests 206.55V, which is pointless math.
207V rms * 1.414 makes 290V of DC. I do not see a gross problem running a Marshall preamp on 290V. But the buzz will be awful without a 2+ stage R-C filter. 3-stage is better, 4-stage is often seen going from raw wall power to a guitar preamp stage.
You "can afford" 290V-250V or 40V of drop. 40V of drop at 4mA of current suggests 10k total.
From too much time wasted with schematics, I would try three 3k resistors and four 22uFd caps.
Looks like 25uV ripple at B+, so 13uV at plate, so a quarter uV referenced to grid, or far below the tube's self-noise. We could probably squeeze a little cost out, but pennies, not worth it.
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Would it make any difference if you use a 40uf cap at the first stage filter?
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Thank you PRR for the help. Good thing that part of the datasheet wasn’t a snake!
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Would it make any difference if you use a 40uf cap at the first stage filter?
Yes.
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Hammond make a tiny one for stand alone reverb units that is good for upto 3 x 12AX7 at about 320v