bayne
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Post by bayne on Dec 30, 2018 18:49:32 GMT -7
Have a couple questions about a 4/10 combo new to me.
1) There is a “glassy” rattle that comes with certain chords/frequencies. I thought this would be the tubes. So I pulled the guitar cord out, turned the volume to max, and tapped on the tubes with a chopstick. Could not replicate the sound. Used my fist and lightly tapped on top which DID re-create the same sound. Any idea?
2) Bought new tubes recommendations as per Dr.Z through Tubestore. Also have a Bias Rite. How do I adjust the Bias? Don’t see an adjustment screw. Is it located inside the case?
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Post by daddyelmis (Greg) on Dec 30, 2018 19:05:36 GMT -7
The bias pot is on the circuit card. You have to remove the chassis to get to it. It’s not hard to do, and a Bias Rite is the right tool, but if you’ve never done this be very damned careful because there are lethal voltages in there.
Don’t know about the rattle other than suggesting to make sure all the tubes are properly seated. Also, you might try a new or different EF86 because those are tubes that are known to be finicky.
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Post by Ridgeback on Dec 30, 2018 19:11:13 GMT -7
Besides the above, hand tighten all hardware (handle, corner protectors cab back panels, baffle, chassis and speaker mounting screws, etc). Rattles are a bear to find sometimes, especially in combos. Good luck.
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bayne
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Posts: 6
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Post by bayne on Dec 30, 2018 19:11:27 GMT -7
The bias pot is on the circuit card. You have to remove the chassis to get to it. It’s not hard to do, and a Bias Rite is the right tool, but if you’ve never done this be very damned careful because there are lethal voltages in there. Don’t know about the rattle other than suggesting to make sure all the tubes are properly seated. Also, you might try a new or different EF86 because those are tubes that are known to be finicky. I just want to get in, check the bias, consider changing the tubes and get out of the chassis. Hand in the pocket at all times, wood chop stick if required. I’ve been doing a lot of research. Also, the tubes other than the power tubes look a bit more complicated to take out. Any advice?
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bayne
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Post by bayne on Dec 30, 2018 19:15:17 GMT -7
Besides the above, hand tighten all hardware (handle, corner protectors cab back panels, baffle, chassis and speaker mounting screws, etc). Rattles are a bear to find sometimes, especially in combos. Good luck. The amp is amazing. Very happy. Worth the time knowing it as best as I can. Best clean tone I’ve ever heard.
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Post by daddyelmis (Greg) on Dec 31, 2018 5:39:25 GMT -7
Also, the tubes other than the power tubes look a bit more complicated to take out. Any advice? Pretty straightforward- remove the shield (push down and twist) on the 12ax7, and move the retainers on the ef86 - they push off to each side. The power tubes and rectifier have the “shark jaw” retainers and those take two hands (at least for me) so that I don’t bend them. After that it’s just standard wiggle and pull. I’m extra gentle with the ef86.
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bayne
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Post by bayne on Dec 31, 2018 12:31:39 GMT -7
I have the following numbers from the Bias Rite:
Socket 1 (Tube 1 or “A” - 21.1 Amps - 378v Socket 2 (Tube 2 or “B” - 18.7 Amps - 377v
I’m assuming the Plate voltage is fine since they are identical. The lower Amperage on the 2nd Tube is interesting. Could this just be a tube that is on its way out? I would have thought tubes are either on/off.
I have a spare set of tubes that I bought and I’m going to just replace them since I have the glass rattle issue that’s ongoing.
Of note, I replaced the fuse as it had a 3 Amp with the proper 2 Amp (Buss time delay -another thread had Dr Z agree that time delay is correct)
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Post by Christopher on Dec 31, 2018 12:50:43 GMT -7
I believe you mean 21 mA (milliamps) as opposed to 21A. IIRC Dr. Z recommends a bias setting of 23mA per tube which would indicate both your tubes are underbiased. I'd suggest bringing up the current to the 23mA and check your tone with your ears, not your meter. As you have it biased a tad cold you will have a lot more headroom in the amp than if it were closer to the 23mA mark. You may prefer that but the magic in this amp is the breakup.
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Post by Ridgeback on Dec 31, 2018 12:51:48 GMT -7
Those are not drastically mismatched tubes in my experience. Without knowing the wattage rating of your specific tubes, I can't comment on the bias (hot vs cold). If I remember correctly plate v x current = power (e.g 378x.0211 = 8 watts or 57% of the power rating if you have 14 watt 6V6s. That would be a bit on the cold side if my math is correct (but don't take my word for either the math or the formula). It's been a while since I re-biased either of my two adjustable fixed bias amps.
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Post by daddyelmis (Greg) on Dec 31, 2018 13:34:14 GMT -7
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bayne
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Post by bayne on Dec 31, 2018 14:22:36 GMT -7
Thanks for the link. A great write up. How difficult was it to get the chassis out? Looks like take the nuts off the 4 housing screws and it should come right out. I thought if I put a couple shirts on top of the speakers I should be able to do it myself without worrying about the chassis dropping too fast. Devil is always in the details..
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bayne
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Post by bayne on Dec 31, 2018 14:52:50 GMT -7
Replaced all the tubes.
Socket 1 28.8 now (from 21.1) Voltage 376 from 378
Socket 2 29.1 now (from 18.7) Voltage 375 from 377
This is an interesting change! I don’t think I even have to touch the bias now if my math is correct.
Glad I ordered 2 sets at the same time!
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Post by daddyelmis (Greg) on Dec 31, 2018 22:46:08 GMT -7
Taking the chassis out is easy - 4 bolts. I just held the chassis with my hand as I removed the bolts
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