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Old 3rd April 2008, 01:49 AM   #1
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Default Bias measurements 6L6

Anyone know how or why i would get two different bias voltage readings on a 6l6 push pull quad?? For example the one pair will will read bias sweep from -59 to -48 and the other end from -59 to -30. I have measured with the tubes removed and they are then equal in measurement sweeping from -59 to -30. I tried different tubes just for S+G's but had the same effect. All of the resistors measure to their respective values. It has (2) new 220K bias supply resistors in parallel with each feeding the (2) 1.5K grid stoppers in parallel. I have attached the schematic. Any help is appreciated. The .047 coupling caps are new and not leaky and the 12AT7 is new as well. I re-tensioned the tube sockets and replaced the 470 ohm screen grid resistors.

-Brian
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File Type: pdf rgt100 - schematic - full.pdf (35.0 KB, 174 views)
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Old 3rd April 2008, 11:49 AM   #2
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anybody? I know this is simply ohms law stuff here but it is mistifying....to give a little more insight, the amp has terrible sounding distortion that kicks in nicely and decays to crap then goes clean as the notes sustain. It sound like the symptoms are directly related to my problems. 2 of the 4 tubes or half of my sine wave is not turning on the power tubes until its driven hard which sounds like cold bias. I actuallly changed the grid stop resistors to a higher value and eliminated the cathode bypass caps in the preamp to eliminate blocking distortion from the list of possible causes.


help!


-B
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Old 3rd April 2008, 12:51 PM   #3
oshifis is offline oshifis  Hungary
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The tubes in each pair must be matched. Try singles first, not pairs until you find the culprit.
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Old 3rd April 2008, 01:27 PM   #4
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I have been using only two tubes one per side. according to my precision912 tube tester the tubes are matched. could this reading be inaccurate from my tester barring calibration?
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Old 3rd April 2008, 01:31 PM   #5
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It doesn't make sense with the circuit that you posted. So the circuit must in fact be different - there's a short or leakage or miswiring.

There's a another problem if you ask me... the grid return resistors are too high per the 6L6 data sheet - 100K max with fixed bias! And with two that should be 50K - but the driver can't DRIVE 50 K! So... you must bias this VERY cold or the tubes will probably run away. My approach would be to add 100 Ohm or more cathode resistors to each 6l6, each bypassed with 100uF or more, giving some cathode bias. These will also provide convenient points for measuring cathode current.
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Old 3rd April 2008, 03:44 PM   #6
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Thank you so much for the responses. This poweramp circuit is ientical to the mesa dual rectifier as all of the values are the same except that the Recto uses a 12AX7. When you say that making the feed resistors 100k and two of them would be seen by the grid of any one 6l6 as 50k??? not sure I follow you there. The at7 certainly can drive more than the higher impedance ax7 correct??
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Old 3rd April 2008, 05:31 PM   #7
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According to the data sheet, the 220K resistors should be 100K for one 6L6, 50K with two in parallel. That's with fixed bias - cathode bias can take higher values. So I'm proposing adding some cathode bias. If you lower the grid resistor value, that will eat up much of the drive from the 12AT7 driver.

You can often get away with exceeding one parameter on a data sheet IF you're not pushing the others. So exceeding plate or screen voltage rating or grid resistor limits may be OK IF you're not running near the dissipation limit. Guitar amps usually must be biased light (by hi-fi standards) to be reliable,
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Old 3rd April 2008, 06:34 PM   #8
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So leave the 220k's and add 100k with 100uF at each cathodes going to chassis ground.....correct? Sorry, electronics tech degree intact but I am new to tubes. They don't teach it anymore. How will adding cathode bias change my problem? what is the lowest impedance an at7 can drive?
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Old 3rd April 2008, 07:12 PM   #9
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Seems maybe something oscillating way in the RF, which is going past unnoticed and is giving you duped figures. It need not be a saturating type of oscillation but simply resident in a small way and knocking the figures. I'm tempted to try without the input tube and see if output stage DC conditions change.

Even if equipment is out of date calibration and is working well, things won't drift so far out ...a bit like an oil change, the engine won't packup and will still run for another 100,000.

richj
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Old 3rd April 2008, 08:22 PM   #10
cerrem is offline cerrem  United States
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For now...leave the 220K resistors in place....I know what the data sheet says...but that is not your problem right now.... the 220K will work fine for now...
First thing you need to do when troubleshooting this type of problem is to DISCONNECT the feedback wire.....
The feedback is an "error correction" signal.... It gets applied to the tail of the phase inverter...this feedback is trying to put the amp into AC balance by pushing the Phase Inverter in opposition to what may be out of balance.... Examine the amp without feedback...you really need a scope to see what the heck is going into and out of the phase inverter as well as out of the output of the amp... For basic guitar amp work...you can just get a cheap used one of ebay.... For sig gen...either use your computer or you can use a CD player with a test tones CD they sell....

Chris
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