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Post subject: Fender Bassman 10 speaker fart question
Posted: Wed Sep 25, 2013 10:49 am
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Hello..I just got a 1978 fender bassman 10 ..and i am wondering why the speakers vibrate so hard? low notes cause some really bad farting that kinda scares me that the speaker will blow up? they vibrate really hard..at about 7 on master and ch volume (regular channel). using with a fender strat..also hooked up to a 4x10 with newer celestion speakers and still does it? makes me think maybe it is the capacitors,tubes, or biasing? it has the bias balance control and tube hum control? any tips or help would be great thanks!!! J


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Wed Sep 25, 2013 1:33 pm
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Check the output tube biasing. And the power supply caps. If these seem ok, you prolly have a bad coupling cap in one of the stages.

A photo of the circuit may show a modification that has effected the amp's performance.

Do you have a separate set of known good speakers, to test the amp?

Good luck!


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Wed Sep 25, 2013 2:34 pm
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i tested with a different 4x10 with newer celestion 10s and it still did it?


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Wed Sep 25, 2013 4:25 pm
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I'd opine that the chassis is tired and in need of some servicing, as Steve (above) has indicated. And if you replace the power tubes I'd *strongly* recommend the Tung-Sol re-issue STR's. Your Bassman 10 is the ultra-linear version, which pushes substantially higher plate voltages than the earlier version of the amp. Lesser-grade 6L6's will prematurely melt down as those used in the UL platforms are seeing nearly 500VDC at the plates.

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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 2:29 pm
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This must be the problem? what do you think? thanks J

http://s1302.photobucket.com/user/jimleach/slideshow/fender%2078%20bassman%20ten

reading from right to left in photos 2-3-4:
2x 220 mfd/285 VDC
3x 20 mfd/500vdc
1 x 70 mfd/100vdc
1x 80 mfd?/vdc75 (white one on the end)


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 2:53 pm
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are these the original specs? (from a cap kit on ebay)

2 x 100uf/350v F&T electrolytic capacitors

3 x 22uf/500v F&T electrolytic capacitors

2 x 100uf/100v F&T bias capacitors

3 x 25uf/50v Sprague Atom electrolytic capacitors


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 3:34 pm
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Compare them yourself......

http://web.archive.org/web/200805160806 ... _schem.pdf

I wouldn't feel safe with anything less than a 600VDC rating for the filter caps.

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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 8:56 pm
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I cant locate them on the schematic ..i am new at repairing amps and the picture is blurry to me..could you tell me what you are seeing so i can go buy the correct ones? that would be really cool! thanks J:-


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 10:25 pm
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A novice should not be cutting his eye teeth on something as complex as a dual-channel Fender tube amp.

Do yourself a favor and spare yourself a whole lot of grief -- take the amp to a competent professional. I've had to broom-and-dustpan hundreds of messes made by the foolhardy who thought they'd save a penny but ended up paying me hundreds of pounds in clean-up costs.

JMOOC......

Arjay

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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sat Sep 28, 2013 11:27 pm
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jimitrader, those paper Mallories appear to leaking. I would not trust this amp until you replace them. Really, I'd have all electrolytic caps in the power supply replaced. Peace of mind.

Fender does not show the layout of the main filter caps in most of their drawings. You need to decipher them from the schematic. For these amps, I use the 80mfd/450VDC F&T caps in the first position. To get a total filtering of 40mfd at 900VDC. And replace the 220k-ohm load leveling resistors with good 2-watt metal oxide. This first section takes the brunt of power ons and offs.

Just how much electrical circuit fixing have you done? Much tube circuitry? These amps can push nearly 500VDC within the guts. Enough to fry you silly. Faster than you can drop a screwdriver. Just a warning.


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sun Sep 29, 2013 9:21 am
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2 x 100uf/350v F&T electrolytic capacitors

3 x 22uf/500v F&T electrolytic capacitors

2 x 100uf/100v F&T bias capacitors

3 x 25uf/50v Sprague Atom electrolytic capacitors

are these values wrong?


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Sun Sep 29, 2013 12:39 pm
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jimitrader, use two 80mfd/450VDC caps in first filter stage (instead of 100mfd/350VDC). The higher voltage rating will withstand power-ons-&-off and standby action better. The higher the voltage rating the better the cap tolerates current surges and heat. In amps with GZ34 tube rectifiers, you must keep this stage to no more than 50mfd total capacitance, 40mfd is better. Two 80mfd/450VDC in series = 40mfd/900VDC total capacitance. That's why I buy a bunch of 80/450's. They will work will solid-state and tube rectified Fender amps.

You might as well build the best PS rail you can, first time around. Then, it'll give you years of quiet, steady service.

WATCH THE POLARITY! If you've never done a cap job before, take pictures or drawings of layout BEFORE you start unsoldering things. Be as neat as possible. Do good soldering. A well-done PS goes a long way for making a reliable amp.

Now, your "farting" issue may still exist. But, at least the PS will be good (it needs R&R anyways). The other caps in the kit are fine. The 25mfd/50VDC are bypass caps for the 12AX7 and 12AT7 tubes.


Super Reverb:

Image




Twin Reverb:

Image


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Mon Sep 30, 2013 10:34 pm
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BMW2002Ti wrote:
In amps with GZ34 tube rectifiers, you must keep this stage to no more than 50mfd total capacitance, 40mfd is better.


This amp is rectified via diode stack, Steve.

Arjay

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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Tue Oct 01, 2013 12:01 am
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Arjay, even more the reason to use the 450VDC rated caps in the first stage. SS diodes tend to load the PS quicker during cold starts. The total voltage rating of 900VDC coupled with healthy 220k-ohm load leveling resistors makes that first stage rugged. The total mfd of 40mfd seems adequate for filtering good UF5408 diodes.

Jerry's Dual Showman has a nice black background. Very little hiss even with volume at "10." I feel that a lot of the proper filtering is being done at the first stage. Just my opinion, of course YMMV.

:D :mrgreen: :D


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Post subject: Re: Fender Bassman 10 speaker fart question
Posted: Tue Oct 01, 2013 12:08 am
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+1

I just though you might not recollect the ultra-linear Bassman 10's power-supply architecture.

Arjay

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