5f6a diode/start up Q's

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es347
Junior tube assistant
Posts: 9
(1/24/04 1:19 am)
 5f6a diode/start up Q's
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 Hello, this is my first scratch build, just want to get few things crystal clear. With the IN4007 diode is the end with the band on it the cathode? Does that mean that the end with the band is going to connect to the board and the other end is connected to my red-blue bias tap? Also, When I do my first start up, correct me if I'm wrong, I'm going to put a 100 watt lamp in series. Turn on with no tubes, if the light does'nt burn brightly or even at all ,thats good. If I start to add tubes While still using the light as a safety, should i connect a speaker? If I hook up an 8 ohm speaker, just to test the amp, not playing loudly, is that O.K.? Its a 2 ohm output tran. Thanks for support, Mike.
 
Tiny Daddy
I will work on all amps
Posts: 451
(1/24/04 12:14 pm)
 Re: 5f6a diode/start up Q's
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 The diode cathode end is marked with a band. The banded end (cathode) connects to the bias tap so you will get negative voltage out.

You are mostly correct about the lamp, just try it and you will see what happens.

Always connect a speaker. 8 ohms may not be OK if something is wrong causing a power oscillation, for instance if the feedback is backwards. On a new build there is a 50-50 chance of it happening so assume it will and load your amp with 2 ohms.
 
es347
Junior tube assistant
Posts: 9
(1/24/04 3:39 pm)
 Re: 5f6a diode/start up Q's
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 Thanks for the feedback. I guess the thing that confuses me is that on the schematic it shows the selenium rectifier with the positive end going to the bias tap, and the negative end going to the board. Am I correct in thinking the cathode is negative? Just trying to learn how to fish, if you know what I mean.  
 
GroundhogKen
Forum Moderator
Posts: 2229
(1/24/04 5:23 pm)
 Re: 5f6a diode/start up Q's
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 This drawing:

www.ampwares.com/ffg/sche..._schem.gif

shows the diode repesentation correctly, BUT, the + and - symbols are reversed. The arrow points from positive to negative and the line represents the cathode ( banded ) end of the diode.

It sounds like you're on the right track and your suspicions about the drawing are correct.

Also, in the bias circuit make certain all the positive ends of the electrolytic caps connect to ground. It's easy to get that messed up.

Good luck,
Ken


 
jakamp59
Junior tube assistant
Posts: 2
(1/25/04 6:24 am)
 5f6a diode/start up Q's
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 If you don't want to use a speaker at startup, you can use a speaker dummy. I found this DIY scheme. It's in Dutch, but it speaks for itself i guess. It's disigned for 8 ohms. Between the brackets are the values for a 4 ohm version. The 2 ohm version isn't hard to guess :-)
If you want i can translate it for you.

www.circuitsonline.net/circuits/view/90
 
es347
Junior tube assistant
Posts: 9
(1/25/04 12:22 pm)
 Re: 5f6a diode/start up Q's
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 Thanks for the link. I dont mind using speakers at start up, Its just that I dont have a cabinet yet to mount them in. I may make a temp board to mount them in untill I get the cab. Thanks again.
 
es347
Junior tube assistant
Posts: 9
(2/4/04 7:52 pm)
 Re: 5f6a diode/start up Q's
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 Will different preamp tubes give different plate voltages? I put a ax7 in v1 and was getting 260 volts on plates. put 12at7 in and got aprox 180 volts. The 12at7 also caused all my other voltages in the amp to drop closer to what the schem calls for. Other than not being able to properly bias the amp(waiting on parts to install adjustable pot) the amp seems to work fine. Right now theres -56 bias volts, this only lets about 13ma thru the tubes. I ordered a 12ay7 from Doug, Am I correct in assuming that will bring the plate voltages down even closer to what there supposed to be. thanks for help.
 
 
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