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Old 24th February 2006, 04:32 PM   #11
anatech is offline anatech  Canada
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Hi Victoria,
Tube amplifier equipment should always have a load when powered up. This can be cheaper speakers or dummy load resistors.

Do a visual inspection first. If you see a white deposit around the terminals of an electrolytic cap, replace it before applying power. Part way up in powering up, measure the voltage on both sides of the smaller coupling caps. If they are passing DC, stop and replace them.

Like burnedfingers suggested, replacement of coupling caps should be done anyway. Better now than later (before they "get you").

-Chris
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Old 24th February 2006, 10:38 PM   #12
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i do suspect i would prefer the 1000A
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Old 26th February 2006, 08:31 PM   #13
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Default Re:Old Tube Equipment Turnon

Am I correct in assuming that the " coupling capacitors", for the 500C, are the four .047 's, 2 per side, which connect a pin on each of the 7591's and the 12AX7's? If so they look original and OK. All are W. German and have a black band on the ends closest to the 7591's. Is that the + end?.
In fact, the whole under side looks good. The only mark on the under panel is a little oil stain under a brown jelly bean .oo47 cap just behind the ON-OFF/Volume switch.
My plan is, with speakers connected, to bring it up slowly with the mode selector switch in Aux with nothing connected to it. If the amp finally goes a couple of hours at 110/118 (house voltage) I will back it down, enable the FM tuner, and slowly bring the FM/Amp combo up. I will have a 100W light bulk hooked into this test setup.
The 500C has options for 4/8/16 ohm speakers . My speakers are ESS AMT 1B's which are 6 ohm. I assume I should hook them to the 8 ohm tap. Is that correct? Thanks again, Vic
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Old 26th February 2006, 08:55 PM   #14
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Default Re: Re:Old Tube Equipment Turnon

by Victoria

The 500C has options for 4/8/16 ohm speakers . My speakers are ESS AMT 1B's which are 6 ohm. I assume I should hook them to the 8 ohm tap. Is that correct? Thanks again, Vic
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might try the 4 ohm to see how the amp settles

then try both

could you post a pic of the underside?
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Old 26th February 2006, 08:56 PM   #15
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Quote:
My speakers are ESS AMT 1B's which are 6 ohm. I assume I should hook them to the 8 ohm tap. Is that correct?
Experimenting with both the 4 and 8 Ohm taps is in order to determine which way sounds best. FWIW, I think the 4 Ohm taps are indicated, as that arrangement maximizes damping factor and minimizes distortion. If your speakers have significant dips in their impedance curve, using the 4 Ohm taps is (IMO) clear cut. Net power O/P will be lower using the 4 Ohm taps.

Quote:
My plan is, with speakers connected, to bring it up slowly with the mode selector switch in Aux with nothing connected to it.
Use shorting RCA plugs inserted into the Aux RCA jacks.
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Old 26th February 2006, 09:27 PM   #16
anatech is offline anatech  Canada
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Hi Victoria,
The coupling caps can look to be in perfect condition and still be leaky. This would then cost you much, much more.

So if I understand you, you will use a variac to power up your unit? Somewhere over 1/2 voltage (leave it there) your will start to get some B+. At that point look at your bias supply and coupling caps. If it's cathode biased (can't remember off hand) the grid voltages should be mV within ground. If there is a neg. supply, the grids should be with mV of that set voltage. This is very important. Continue to check these voltages as you bring the unit up.

If you see any plates turning red, shut down. Same if you hear a big hum from a speaker.

-Chris
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Old 26th February 2006, 10:58 PM   #17
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Where valve amplifiers are concerned, always assume that all loudspeakers are 4 Ohm. It's not a danger thing - it'll just sound better.

Look for hemispherical bulges or cracks in the black rubber base of electrolytic capacitors. If either are present, replace the part without question.
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Old 26th February 2006, 11:50 PM   #18
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Spoken like a true triode user!

For those of us who sometimes fall prey to the siren song of those extra two grids, the following chart (from Crowhurst, "Understanding Hifi Circuits") may prove useful.
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Old 27th February 2006, 12:14 AM   #19
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Yes, triodes presently rule.

An interesting chart, but is it useful?

It quotes efficiency (which is arguably useful) but doesn't quote the power level at which the distortion measurements were made. Suppose that the distortion for one measurement was 0.1% and for another 0.25%; you'd assume that the 0.1% configuration was superior. But what if the 0.25% configuration produced 30W, and the 0.1% configuration produced only 10W? Then, (if we make the sweeping assumption that distortion is proportional to level), the 0.25% configuration would produce a little less distortion at 10W than the 0.1% configuration...

We really need graphs (and lots of them)...
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Old 27th February 2006, 12:19 AM   #20
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The relevant columns were the load variation tolerance. Pentode users are people, too.

I would not complain a bit if some contemporary author took your suggestion and did a more comprehensive survey, but to be fair, this is just a summary chart following quite a bit of explanatory text.
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