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Old 10th July 2006, 08:13 AM   #1
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Default New to forum! Or: A few newbie questions

Hi all,

Let me first say that I've been lurking here for a little while, and I find that you guys are probably the most knowledgeable, unbiased folks around when it comes to tube amp advice. I'm something of a neophyte to the area, and I'm in the process of building my first tube power amplifier. I'm not completely electronics-dumb (recent EE graduate), but I *am* tube-dumb. As a result, I've got questions all over the place.

This amp that I'm tinkering with is the power amplifier out of an old organ, as far as I can tell. It had 12ax7s driving el84s in push-pull with a 5u4gb rectifier tube. Since getting the amp, I've basically gutted one channel and disconnected the other from the power electronics completely. I figure it's probably best to work on one channel at a time.

Now, I removed all the tubes, and did some scope/meter measurements on the transformer with it plugged in; the HV windings have 350/350 on them, plus some LV windings for heaters.

WIth the rectifier tube in, the open-circuit voltage on the big silver multicap is in the neighborhood of 450vdc. Am I mistaken in thinking that this is a tad high to be B+ for an el84 push-pull? I figure I could use a resistive divider to pull the voltage down to ~350, but I'm not sure where to start in calculating how much voltage a resistor is going to drop when there's a bunch of tubes in parallel with it.

Last, any advice on a simple "first-project" el84 push-pull that sounds decent? The only tubes I've really got in my possession to tinker about with are EL84s, 12ax7s, some 12at7s, and 6au6s; I'm not sure at all if that's enough.
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Old 10th July 2006, 08:21 AM   #2
arnoldc is offline arnoldc  Philippines
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open circuit = no load

your B+ will shoot up.
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Old 10th July 2006, 11:16 AM   #3
SY is offline SY  United States
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First, what Arnold said; you need to stick a dummy load on the supply to see what the loaded-down voltage is. Assume that the idle current will be something like 80mA per channel, then do a rough Ohm's Law to see the size of the power resistor for the dummy load.

Second, you have to account for the bias voltage if this is a cathode-biased circuit. That could knock another 10-15V off the effective plate voltage.

Third, you have to realize that the max plate voltage rating of EL84 was, in many classic amps, honored in the breach. For example, the very popular Dynaco SCA35 ran the plates at 30-40V over the max rating, yet got good tube life. The secret was keeping the dissipation down, especially the screen dissipation.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 10th July 2006, 02:30 PM   #4
poobah is offline poobah  United States
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SY is being modest; as usual. Check out the thread: "red light district". There you shall find an amp using 2 EL84's and an ECC81 (12AT7). This amp features a unique cathode biasing scheme that should leave you twitterpated.

Remember, the inductors in tube PSU's will have significant resistance... another voltage drop for you.

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Old 10th July 2006, 05:29 PM   #5
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Assuming I've done this right, I futzed around with Duncan's PSU simulation software for a bit and found that a dummy load of 3.3k sets the current draw right around 120mA. This gives me a loaded supply voltage with a fair bit of ripple around 400VDC.

Of course, I've probably chosen exactly the wrong assumption here. I don't have many resistors in the junk box that are likely to be able to handle dumping that much power directly into; 400*.12 = 48 watts. Think two ~6.6k 25w resistors oughta handle it without burning down my house (and with it, me)?

Assuming the 400VDC figure is correct (feel free to thwack me with your nerf bats in the likely event that it isn't), where do I head from here?

(also, the Red Light District seems intriguing. I'd like very much to not deal with a tube-based power supply just yet, but it's what I've got )
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Old 10th July 2006, 05:38 PM   #6
poobah is offline poobah  United States
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The tube based supply should concern you that much... and it is in hand.

Does your supply have an inductor... most organ amps did.

If so, be sure to measure it's resistance. You can guess at it's inductance value knowing resistance and physicle size by comparing to inductors on Hammond's website.
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Old 10th July 2006, 06:51 PM   #7
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Welcome to the forum, friend!

make sure you replace the old capacitors in this amp, because if you don't on a bad day they can turn into small explosive devices and spray costic fluid everywhere
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Old 10th July 2006, 07:19 PM   #8
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There don't appear to be any chokes or inductors in the power supply besides what is in the power trafo; that is, no seperate device. Unless the 5v transformer secondary for the rectifier heater counts, I don't think there is one at all.

Since the provenance of this amplifier is somewhat of a mystery to me, if there was an inductor, it could very likely have been scalped for some other project before I got my grubby little hands on it.

Also, thanks for the advice, alex. I've got replacements for just about everything; there are only a few wax-coated caps left in there, plus the big multicap that I was told to leave alone unless something went terribly, horribly wrong.
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Old 10th July 2006, 07:22 PM   #9
poobah is offline poobah  United States
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The advice re the multicap was perhaps misguided. Lytics are lytics... they dry out, and, they eat themselves. I just had to replace them in my Hammond Organ and Leslie Speaker.
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Old 10th July 2006, 08:18 PM   #10
nickds1 is offline nickds1  England
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Smile It's OT, I know...

Originally posted by poobah
The Poobah
"The birds kept calling his name, thought Caw" -Jack Handy
"Caw" said the crow. "Balls", said Milligan.

From Puckoon, by the late, great, Spike Milligan (who is buried opposite my brother's house).

(sorry for the OT)
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