Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 19, 2015, 01:01:13 pm

Title: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: kagliostro on March 19, 2015, 01:01:13 pm

The amp in question is the old Gem Venus 20 of this tread

http://el34world.com/Forum/index.php?topic=15511.msg149538#msg149538 (http://el34world.com/Forum/index.php?topic=15511.msg149538#msg149538)

(http://i.imgur.com/vbHGK8p.jpg)

My friend had some problems and only now he is trying to ultimate the restoration of the amp

The electrolytic cap are new and also the power tubes (but the problem appears also with the old tubes)

the problem is this:

when the amplifier is turned on, for a small time (10 seconds), all seems to be OK

after this small time B+ has a sensible drop (from 365v to 315v --- B+ without tubes inserted is 372v)

and appears a noise that remember the rumble of the coffe on the Moka (  :icon_biggrin: )

listen from 1:20 till end

Fuoriuscita del Caffè da Moka (http://www.youtube.com/watch?v=yR3z3MNFDac#ws)

any idea ?

Thanks

K

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: eleventeen on March 19, 2015, 01:48:01 pm

If they are old, I would have to suspect the plate resistors on the 12AX7.

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: kagliostro on March 19, 2015, 03:24:15 pm

Thanks


Franco

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: AZJimC on March 20, 2015, 04:38:55 am

With B+ dropping like that, I wonder if the power tube cathode resistor or cap is causing the bias to drop on the cathodes? Just a thought

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: kagliostro on March 20, 2015, 06:29:32 am

Thanks

Franco

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: sluckey on March 20, 2015, 07:48:11 am

It's a coffee pot. It's supposed to sound like that!    :laugh:

Title: Re: Gem Venus 20 - Odd B+ and coffee ready problem
Post by: kagliostro on March 20, 2015, 01:03:53 pm

 :l2: :l2:

Franco