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Post subject: Bassman 10 Volume Drop
Posted: Mon Jan 06, 2014 12:41 pm
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I bought a very clean Bassman 10 and replaced the filter caps and the bypass caps. I still notice some hum when the amp is powered on. After 15-20 minutes there is a volume drop. Is this likely one or more tubes that need to be replaced or possibly the output transformer? The tubes appear to be original and are RCA. In this amp which tubes need re-biasing when replaced? Many thanks.


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Post subject: Re: Bassman 10 Volume Drop
Posted: Mon Jan 06, 2014 1:35 pm
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
1- Did you know how to replace those caps , they are polarized ?

2- Did it work fine before you work on it ? not volume drop ?
a) No volume drop before, check your work .


Drop of volume ;

3- Voltage reading first , did B+ voltage drop with the volume ? Check B+ on filter caps
a) voltage good , see 4
b) voltages wrong , very low see 4

4- Remove output tubes , check voltages
a) voltages OK check output tubes
b) voltages too low , isolate B+ from filter caps from the amps section

I stop here for now , good luck

5- Check bias on your RCA tube, if it is good , tubes might be good .

Transformer bad ? Could be many many thing .

Give answer to my questions and I will try to help you.

By the way check the fuse , many second hand amp have wrong fuse and this can damage the amp

Schematic ;

http://www.el34world.com/charts/Schemat ... MAN_10.pdf


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Post subject: Re: Bassman 10 Volume Drop
Posted: Mon Jan 06, 2014 10:37 pm
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Did you replace the power-rail resistors under the doghouse when you re-capped it?

Likely they're at least as old as the electrolytics you removed and often just as tired.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bassman 10 Volume Drop
Posted: Thu Jan 09, 2014 9:33 pm
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Thanks. Yes I knew the caps were polarized and was careful to orient them correctly when I soldered in the replacements. Yes there was a volume drop before the cap job along with a persistent hum. I will need to check the B+ voltage drop. Thanks for the suggestion. And yes I did check the resistors. They all seemed to be within range but as Retroverbial mentioned they appear to be original. Should they be replaced even if they test in range? If so, replace with wire wound? Many thanks stratele52 and Retroverbial.


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Post subject: Re: Bassman 10 Volume Drop
Posted: Thu Jan 09, 2014 11:27 pm
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Wire-wound will be fine if space permits. Otherwise, use metal-film types. Do not over-spec them.

Is this amp an ultra-linear Bassman 10 (with solid-state rectifier) or the original version with a 5U4GB rectifier tube?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bassman 10 Volume Drop
Posted: Fri Jan 10, 2014 11:22 am
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It is the Bassman 10 Combo at 50 watts. There are only 5 tubes, 2-6L6GC power tubes, 2-7025 pre-amp tubes and 1-12AT7 phase inverter. From what I read elsewhere the ultra linear 70 watt models were built later.


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Post subject: Re: Bassman 10 Volume Drop
Posted: Fri Jan 10, 2014 12:40 pm
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Have you checked your B+ at pin 3 of both power tubes?

You should be seeing something in the neighborhood of 430 VDC.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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