Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 27, 2011, 11:26:59 am

Title: MusicMan problem
Post by: Leevi on June 27, 2011, 11:26:59 am

I started to repair a Music Man 1650rd amp. I noticed that the first of the power tubes
glows red (or orange). I measured the bias voltages (pin 5) that are on level 30v.
Tubes have been measured and they are OK. Any idea what could be the reason.
/Leevi

Title: Re: MusicMan problem
Post by: tubenit on June 27, 2011, 11:57:50 am

I used to own one of those. Really a very LOUD amp, IMO.

Have you measured the current (ma)?

With respect, Tubenit

Title: Re: MusicMan problem
Post by: phsyconoodler on June 27, 2011, 12:07:52 pm

Quote:"Tubes have been measured and they are OK."

Try another set.Your tube tester cannot duplicate the voltages in that amp(725v) and a tube tester is not infallible.
   

Title: Re: MusicMan problem
Post by: Leevi on June 27, 2011, 12:38:05 pm

Quote

Try another set.Your tube tester cannot duplicate the voltages in that amp(725v) and a tube tester is not infallible.

I switched the tubes with each other and the problem is still in the same socket position so I still believe the tubes are
OK. But if I measure the resistance from the cathode (pin 8) to the ground the first one shows about 5k and the
second one 77k. I think the first transistor is probably burnt.
/Leevi

Title: Re: MusicMan problem
Post by: Leevi on June 27, 2011, 01:49:33 pm

I'm quite sure that one of the transistors (JE1692) is burnt.
I got another problem where to find these transistors that are probably produced by MusicMan.
Does any one have experience in replacing types?
/Leevi

Title: Re: MusicMan problem
Post by: phsyconoodler on June 27, 2011, 02:50:44 pm

Here's a quote from another website on musicman woes:

"I've now tried tried both the 2N6292 and 2N6488. They both sound fine and work fine. However, the 2N6292 do not seem to be as consistant from one to another as the 2N6488 from one to another in terms of hFE (and probably other things, but that's all I can measure), which will cause a bias im-balance. I discovered this after installing a pair of Central Semi 2N6292 from Mouser and finding that my bias differed by 15mV! Whereas the 2N6488 from RCA and ST all measured very closely to each other. I eventually "matched" a pair of the Central 2N6292, and that's what I have in now. Although I probably should have kept in the RCA 2N6488, but I was too curious to let it be."

Title: Re: MusicMan problem
Post by: Leevi on June 28, 2011, 08:50:09 am

Yes, I succeeded to order some 2N6488s. I have to change both cathodes after I have matched the transistors.
Thanks
Leevi

Title: Re: MusicMan problem
Post by: Leevi on July 04, 2011, 10:03:09 am

I fixed the fault and want to confirm that 2N6488 works fine in this if someone has similar problems.
The amp is very loud as Tubenit stated in his post.
Thanks for the help
/Leevi