Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 15, 2014, 08:10:05 am

Title: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: kagliostro on January 15, 2014, 08:10:05 am

A Geloso G 261-A was converted to guitar amp

Resistors and electrolytic cap where changed with new one an the schematic around V1 was changed (see schematic)

If the amp is connected to a guitar speaker all is fine, but when tested with an HiFi speaker high frequency noise comes

What to do, connect the expected guitar speaker and don't worry

or there is something to do as to avoid the high frequency noise

may be the substitution of the grid stoppers of the final tubes and/or what else ?

Thanks

K

Title: Re: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: jjasilli on January 15, 2014, 08:54:37 am

If used just for guitar, the guitar speaker itself attenuates hi's -- as a practical matter, there's no problem for guitar use.

For hi-fi there is a problem.  Maybe worse than apparent: hi's maybe be generated beyond 20,000 Hz, which we can't hear.  But such excessive signal-noise can burn out a hi-fi tweeter over time.

To diagnose I suggest a scope or at least a listening amp to determine the source of the hiss.  A scope will be necessary to detect hi's -- for guys our age maybe above 10,000 Hz.  

The solution is probably a bad tube; or global or local NFB.  The schematic already shows a post-PI suppression cap; and seems also to have a weird global NFB circuit; but the latter is not clear to me.  If these circuits are functioning, then in theory there should be no hiss.  So it needs to be tracked down & isolated.

Title: Re: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: kagliostro on January 15, 2014, 09:19:08 am

The use will be only as guitar amp with a guitar speaker

but if you say it is better to investigate, it will be done

Thanks

K

Title: Re: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: jjasilli on January 15, 2014, 01:17:26 pm

Investigation is optional UNLESS the amp will be used with hi-fi speakers.  The coil of a guitar speaker is robust enough to absorb excess hi-frequency energy without harm. 

Title: Re: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: PRR on January 15, 2014, 09:38:00 pm

.

Title: Re: Noise in Geloso G 261-A converted to guitar use (if using HiFi Speakers)
Post by: kagliostro on January 16, 2014, 01:28:31 am

Many Thanks PRR

it will be done

K