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Old 6th March 2010, 01:06 AM   #21
rknize is offline rknize  United States
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Ummm...yeah. Didn't even think about the pin-out, LOL.

For the record, I see about 5.5V on my 5842 filaments. Since this is not intentional, I guess a lower value needs to be calculated by the end user. I'm getting about 6.6V out of the supply in 45 mode, though I imagine it is lower when 300Bs are in there. I wonder if that has anything to do with why I don't care for the 300Bs as much.
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Old 6th March 2010, 01:57 AM   #22
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Quote:
For the record, I see about 5.5V on my 5842 filaments.
You are using an Edcor power transformer? If so that explains it. I have been using Hammonds. Even my Lexan amp which uses a severely underrated Allied 6K56VG (made by Hammond) runs at 6.3 volts with 45's. I have been known to put 300B's in the same amp. This really heats up the little 120 mA transformer, but it is now about 6 years old. I still get 6.1 volts when doing this. I have stuffed 2A3's into this amp but the transformer smells funny and is too hot to touch after 1/2 hour of use. Didn't measure the voltage though.

The Simple P-P on the bench has a 272JX transformer. The "6.3 volt" filament winding measures 7.2 volts. The 600 VCT winding measures 660 volts (both measured under load). I also have a 372HX, but it isn't hooked up right now.
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Old 6th March 2010, 05:03 AM   #23
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Quote:
Originally Posted by tubelab.com View Post
You are using an Edcor power transformer?
Yup...all Edcors here.
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Old 6th March 2010, 01:22 PM   #24
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Different pinout, eh? I guess that explains why the 6c45p's went "poof" after I plugged the 45's in. I was adjusting the bias for one channel, and suddenly there was a flash in both 6c45p's and they both went dark. No big loss since they apparently weren't any good to me anyways, guess I'll hunt down a pair of real 5842s. Hopefully I didn't damage anything else, but I'll start the checkout procedure from the beginning just in case. The 45's and rectifier seem to still be glowing happily away at least, but I'll pull them all out until I'm sure everything's still right.

Last edited by blue lander; 6th March 2010 at 01:25 PM.
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Old 12th March 2010, 07:23 PM   #25
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A new pair of real RCA 5842's came in the mail today, so I'm really to pick up where I left off. I'm going to start the checkout procedure from scratch, of course, to make sure I didn't damage anything. I do have a question about one step, though...

Put a meter from the grid of the output tubes to ground. One meter for each tube. Power up the board. Each meter should read a negative voltage. With ONE HAND BEHIND YOUR BACK carefully adjust one of the bias pots (R12 or R23). The negative voltage for the corresponding output tube should change. Set it to the most negative voltage, and then adjust the bias pot for the other channel. Set them both to the most negative voltage. Power OFF the board. Do NOT touch the board for 5 minutes after the power is disconnected.

What kind of negative voltage should I be seeing at this step? A few volts? A hundred?
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Old 15th March 2010, 02:01 PM   #26
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Okay, two more stupid questions and then I'm done. It's already working fine and making music, but I want to be sure on a couple things before I call it quits.

1: I'm only able to get 165 volts on the plates of the 5842's, but the manual says 175. Is that good enough?

2: I'm a little confused about what the output bias should be. I want to set it to 25ma. My Radio Shack multimeter is reading .025 amps (the range is set to auto). Do I want .025 amps or .25 amps?
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Old 15th March 2010, 02:11 PM   #27
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1) This is fine and pretty normal. The Raytheon 5842s I have vary quite a bit in how they bias in this amp. Just try to get them as high as you can.

2) If you are measuring per Georges instructions, you should set your meter for VOLTS when you place it across the 10 ohm resistor. You want to bias your 45s to 25mA (0.025A), but you are doing this by measuring voltage across the 10 ohm sense resistor. Ohm's Law says V=I*R, so 0.025 * 10 is 0.250V or 250mV.
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Old 15th March 2010, 02:17 PM   #28
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Whoops! Okay, I'll rebias them tonight. Thanks again for all the help. The fact that all my mistakes haven't reduced this amp to a smoldering slag heap says a lot about how well designed it is.
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Old 15th March 2010, 03:37 PM   #29
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Well, biasing them too cold isn't going to hurt them. Should sound a lot better once you have it biased right.
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Old 15th March 2010, 05:11 PM   #30
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It did sound rather... unremarkable when I tried it out with the bias set wrong. What surprised me was that it had more than enough gain to drive my backloaded horn speakers. I only had to turn the preamp up about 1/4th of full volume to get satisfying sound levels.

Actually, that brings me to another dumb question: What's a good value pot to use for the volume knob? 100k logarithmic?
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