| Today's Puzzler [message #8677] |
Wed, 12 January 2005 12:52  |
TubeCraft
Messages: 30
Registered: May 2009
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Baron |
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Noticed the new amp I built is short on bass. I fire up the test tone CD, and sure enough, no reponse at 100hz. (Response at 1khz and 10khz seems pretty normal.)
Using a plain old VOM, I trace good response (test tone at 100hz) to the socket grid connection of the driver tube, a 5687. The 5687 has 170v on the plate (pin1), the cathode is at 7v(pin 3), running at 14.7ma. I measure a 5v RMS signal at the grid pin socket connection (pin 2).
There is little or no signal appearing at the plate (91mv). Swap in several different tubes, 5687, 7044 no change. Any ideas? Rk=475r (bypassed 220uf) Rl=7k5 Grid=100k gain pot
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| Re: Today's Puzzler [message #8680 is a reply to message #8677] |
Thu, 13 January 2005 06:06  |
Damir
Messages: 1005
Registered: May 2009
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Illuminati (2nd Degree) |
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Frequency selective fault, hmm ?! I`d try to disconect input and output connections from this triode and cathode bypass cap, too. Install grid leak (say, 100k from grid to ground) and then you have only Ra, Rk and Rg. Be sure that B+ is OK (no bypass caps to the "wrong" places, etc.).
Then try the 100Hz input direct at the grid pin, and measure the output, direct on the anode pin. You must get the good result, check everything. Check 220uF cathode bypass capacitor, polarity, and solder it, then check again (more amplification). Then check input jack and input pot and install them. Check again. If everything is allright, then check and install output cap, etc.
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| Re: Today's Puzzler - Solved [message #8694 is a reply to message #8677] |
Sat, 15 January 2005 19:22 |
TubeCraft
Messages: 30
Registered: May 2009
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Baron |
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Seems the interstage transformer on the output of the stage (Parafeed connection) wasn't doing it's share loading the tube - or it's shorting. Then I removed the transformer connection - and measured with no load at all - very dumb - I guess my cheap VOM isn't really a "load." I replaced the transformer primary with a cap and a resistor and voila - 2.9v in and 24.9v out. Makes sense - no load - no current flow - no current, no volts. 3 days of beating my head on my workbench.... Thanks for the suggestions guys, the breadboard on the workbench eventually told the tale. It's gonna be a kickin' amp when it's done.
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| Re: Here's the why! [message #8707 is a reply to message #8700] |
Wed, 19 January 2005 10:08 |
TubeCraft
Messages: 30
Registered: May 2009
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Baron |
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The $%#@& lead out diagram packed with the bleedin' transformer was wrong! So if anyone out there's using a Hammond 124E interstage go to the website and check the corrected wiring diagram! The one printed on the carton is incorrect. Transformers don't work well a completely shorted winding.
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