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oldradio99
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Post subject: Marshall Amp Posted: Mar Wed 02, 2022 6:53 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 3268
Location: North Port, Florida
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Have a friend who plays guitar using Marshall tube amps.
Has has noticed he has had to increase the volume from 3 to 5 to get the same volume.
He suspects the tubes are getting weaker as he is playing more than ever before.
Marshall states you have to adjust the bias when replacing the tubes (EL84 and 12AX7).
Is it really necessary to adjust the bias?
Thanks
_________________ Art's Antique Radio Collection - A work in progress
http://www.myantiqueradio.com
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dberman51
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Post subject: Re: Marshall Amp Posted: Mar Wed 02, 2022 7:10 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 5077
Location: Boston, MA
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EL84 yes, 12AX7 no. And I suspect it’s his ears that are getting weaker, not the tubes!
-David
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Tom Bavis
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Post subject: Re: Marshall Amp Posted: Mar Thu 03, 2022 12:02 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 5928
Location: Macedon NY 14502
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Bias has little effect on gain (what he's talking about). Weak 12AX7s might drop in gain though. Most EL84 amps don't have bias adjustments. But EL84s vary a lot... if you get some hot (high current) ones, you might have to change a cathode resistor. Would need a model number to be more precise.
_________________ My web page: https://bit.ly/2rxq4qx
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oldradio99
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Post subject: Re: Marshall Amp Posted: Mar Thu 03, 2022 2:25 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 3268
Location: North Port, Florida
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Tom Bavis
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Post subject: Re: Marshall Amp Posted: Mar Thu 03, 2022 3:22 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 5928
Location: Macedon NY 14502
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From what I can gather, the DSL20 uses EL34s, not EL84s, and has cathode bias (no adjustment). Loss of gain would be caused by the 12AX7s. But if sounds good, just turn it up!
_________________ My web page: https://bit.ly/2rxq4qx
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Barry H Bennett
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Post subject: Re: Marshall Amp Posted: Mar Thu 03, 2022 10:27 am |
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Joined: Nov Wed 30, 2016 6:35 pm Posts: 13339
Location: Sunbury, Ohio 43074
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Bias is going to have an effect on the heat, and distortion, of the output tubes. The secret password back in the day was "distortion or destruction" ... when talking about bias As others have noted, bias on the output tubes won't change the volume of the amp for any given setting of the volume control. To answer the question "do you have to adjust bias" ; pretty much yes, if you want to maximize the performance of the amp. Adjustment may be via a control, or by changing a resistor, also as noted by others. Will it work with any new tube plugged into the outputs? Yes, but it won't be optimum, might reduce the life of the tubes, in extreme cases could cause red plating, and/or if not properly biased, you might notice it distorting earlier or later ... which could be good or bad depending on your musical tastes  I assume it's push-pull, in which case matching the output tubes would also maximize the performance.
_________________ Preserving the hist. of electronics, one boat anchor at a time! :) https://www.bbtvtestequipment.com
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tube42
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Post subject: Re: Marshall Amp Posted: Mar Sun 27, 2022 3:09 am |
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Joined: Jan Tue 23, 2007 9:23 pm Posts: 3038
Location: Chicago, IL. 60609
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By any chance is this a multi channel amp?
If so the 12AX7s can be swapped around to confirm the issue is coming from there.
_________________ -Carlos
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reeves03
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Post subject: Re: Marshall Amp Posted: Mar Sun 27, 2022 3:50 am |
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Joined: Jun Sat 18, 2011 2:36 am Posts: 2831
Location: Pace, FL 32571 USA
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Just wait till he sees the price of 12ax7's now $30 a piece.
_________________ Thanks, Jason N1EBC
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ampstamp88
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Post subject: Re: Marshall Amp Posted: Apr Thu 07, 2022 3:28 am |
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Joined: Apr Sat 04, 2015 5:15 am Posts: 2236
Location: Willits, CA.
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If the amp had been sitting awhile not being used before he started "playing it more than ever before", it could be the filter capacitor block needs renewal, too? Not necessarily the tubes...
_________________ "How come we never watch Television?" - sonuva TV repairman in the fifties, Mac Rebennack.
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oldradio99
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Post subject: Re: Marshall Amp Posted: Apr Thu 07, 2022 2:20 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 3268
Location: North Port, Florida
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ampstamp88 wrote: If the amp had been sitting awhile not being used before he started "playing it more than ever before", it could be the filter capacitor block needs renewal, too? Not necessarily the tubes... He uses it 2-3 times a week
_________________ Art's Antique Radio Collection - A work in progress
http://www.myantiqueradio.com
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Tube Radio
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Post subject: Re: Marshall Amp Posted: Apr Tue 12, 2022 2:11 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 22401
Location: Warner Robins, GA
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If the amp uses carbon comp resistors and paper capacitors I'd check those first after verifying the tubes are good.
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Lou deGonzague
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Post subject: Re: Marshall Amp Posted: Apr Wed 13, 2022 9:58 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 10185
Location: Latham NY 12110
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Output tubes could be getting weak, they are probably Russian or China tubes.
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oldradio99
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Post subject: Re: Marshall Amp Posted: Apr Wed 13, 2022 10:07 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 3268
Location: North Port, Florida
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Lou deGonzague wrote: Output tubes could be getting weak, they are probably Russian or China tubes. nope USA RCA
_________________ Art's Antique Radio Collection - A work in progress
http://www.myantiqueradio.com
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reeves03
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Post subject: Re: Marshall Amp Posted: Apr Wed 13, 2022 10:27 pm |
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Joined: Jun Sat 18, 2011 2:36 am Posts: 2831
Location: Pace, FL 32571 USA
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Well then it's time to go about this logically. Get a schematic and start with the power supply an monitor the HV an LV rails while playing a 1k sine at the input and a dummy load on the output.
_________________ Thanks, Jason N1EBC
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Tbirdkid
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Post subject: Re: Marshall Amp Posted: Apr Thu 14, 2022 1:49 pm |
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Joined: Aug Sun 23, 2015 5:01 pm Posts: 1428
Location: Deptford NJ
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Is he saying the sound is weaker at the same settings after always using the same guitar? Obvious question, but needs to be asked...as different pickups have different output voltages. Changing string gauges can also affect output too, He could have a weak or bad cathode bypass cap on one of the PREAMP tubes. That would truly affect the sound.
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Tube Radio
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Post subject: Re: Marshall Amp Posted: Apr Thu 14, 2022 2:53 pm |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 22401
Location: Warner Robins, GA
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Tbirdkid wrote: Is he saying the sound is weaker at the same settings after always using the same guitar? Obvious question, but needs to be asked...as different pickups have different output voltages. Changing string gauges can also affect output too, He could have a weak or bad cathode bypass cap on one of the PREAMP tubes. That would truly affect the sound. Or if the guitar has a volume knob on it maybe he bumped it and didn't realize it. Often simple things such as that get overlooked when trying to troubleshoot a problem as one thinks in their head I know the knobs and switches are set right so they don't verify they are set right. Has happened to me before at work. Spend several hours troubleshooting something only to find a switch in the wrong position.
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westcoastjohn
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Post subject: Re: Marshall Amp Posted: Apr Wed 27, 2022 12:55 am |
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Joined: Dec Sat 24, 2011 8:17 pm Posts: 10689
Location: Qualicum Beach, Canada
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He plays guitar thru a Marshall amp with EL-34's at half volume 2-3 times per week, and he notices his ears are getting weak. This was already mentioned, it sounds less loud but is still pretty darn loud. Many guitar players want a way too big amp to practice in a small room. Anyway, because of the way these amps are designed, it is best to measure cathode bias current (thus calculating plate current) after retubing, and that is what he's asking for. Install matched 1 ohm or low ohm resistors on the EL34 cathodes and measure the voltage across them. Leave the resistors in there and the cathode current can be checked any time. If tubes appear to be mismatched, try swapping them, one socket is usually hotter from slight differences in the output transformer windings. Edit: Googled it this AM and found this - an attenuation switch he might have forgotten about. They say it is on the back. Here also is a picture of a schematic, might be legible: https://www.tdpri.com/threads/believe-i ... g.1065931/I gather from this thread that the attenuation switch reduces plate voltage.
_________________ Read the War Diaries: A Radio Amateur in Kyiv [:l>I
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stuben
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Post subject: Re: Marshall Amp Posted: Apr Sat 30, 2022 1:18 pm |
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Joined: Oct Thu 23, 2014 4:15 pm Posts: 354
Location: Old Saybrook Ct. 06475
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As others have correctly noted this is a cathode biased amp that uses 3 12AX7’s and a pair of EL 34’s all tubes are cathode biased and there is no adjustment provided or needed according to Marshall.
This amp also has a printed circuit board making modifications difficult and unadvisable. Should it be modified it will void any warranty and most likely make the amp harder so sell if and when that happens.
Sounds to me like the owner of the amp is not the least bit interested in checking the bias of the power tubes. The short answer to his question about checking the bias is NO it’s unnecessary.
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COM-NAV-ECM
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Post subject: Re: Marshall Amp Posted: May Tue 10, 2022 2:55 pm |
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Joined: Dec Sun 25, 2016 4:53 am Posts: 628
Location: Santa Barbara, California
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Check the DC resistance of the speaker to eliminate a change in the voice coil, which will impact output volume. An 8 ohm voice coil should be around 6 ohms DCR, 16 ohm around 12 ohms. I bought a well used JBL 15" K series that sounded OK but seemed to have a weak output. Its DCR measured 40 ohms. It was really smoked but hung in there somehow.
Just a thought.
_________________ 1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C
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Barry H Bennett
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Post subject: Re: Marshall Amp Posted: May Mon 30, 2022 10:15 pm |
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Joined: Nov Wed 30, 2016 6:35 pm Posts: 13339
Location: Sunbury, Ohio 43074
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Well, this is really all guesswork unless you can measure the output of the amp for a given input. More or less what the factory or any good pro audio shop does.
If that’s not possible, you cannot really know if the amp is up to spec or not
_________________ Preserving the hist. of electronics, one boat anchor at a time! :) https://www.bbtvtestequipment.com
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