Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on August 20, 2010, 10:38:47 pm

Title: Valve Junior Layout Error
Post by: plexi50 on August 20, 2010, 10:38:47 pm

Do you see any errors in this layout file? If so what and where are they?

Title: Re: Valve Junior Layout Error
Post by: PRR on August 21, 2010, 10:03:15 pm

IEC jack pinning is unclear to my eye.

Title: Re: Valve Junior Layout Error
Post by: RicharD on August 21, 2010, 10:58:12 pm

Seems like R2 should be a 33k, not a 68k.  They "omitted" cathode bypass caps for R8 & R9 but didn't say why.  No tone control...cool!  The flow of the board looks fine as does the grounding scheme.  Not that it's "wrong", but I'm not a fan of separate jacks for the different speaker impedances.  I prefer a single shorting jack with a selector switch.

>IEC jack pinning is unclear to my eye.

Ditto.  Where is the fuse and power switch?  Where is the pilot light? 

Title: Re: Valve Junior Layout Error
Post by: sluckey on August 21, 2010, 11:04:44 pm

Maybe the fuse is in the IEC and the pilot is in the double pole switch?

Title: Re: Valve Junior Layout Error
Post by: plexi50 on August 22, 2010, 08:19:49 am

The fuse is in the IEC and the pilot is in the double pole switch. I almost burned up a second PT when i looked at the rectifier diodes. I lifted there ends and tested them. They were reading good. I replaced them anyway seeing as they are a dime a dozen. I wound up wiring the PS caps neg to chassis ground and the rectifier diodes the old Fender way with each HT lead to it's own diode

Amp fired right up with no light bulb current limiter dimming this time
Tested all preamp and power tube socket voltages before installing power tube
Working fine now / What was the issue?

Title: Re: Valve Junior Layout Error
Post by: sluckey on August 22, 2010, 08:29:39 am

Quote

I wound up wiring the PS caps neg to chassis ground and the rectifier diodes the old Fender way with each HT lead to it's own diode

That statment suggests there is a center tap for the HT winding. If so, AND you had the CT connected to chassis with the bridge rectifier shown in the drawing, that's the reason you burned up the PT.

Title: Re: Valve Junior Layout Error
Post by: plexi50 on August 22, 2010, 09:10:06 am

Yes the second PT had a CT. The original PT had no CT lead that i recall. The shell is all that is left. Im using the old PT as a paper weight for projects

Title: Re: Valve Junior Layout Error
Post by: sluckey on August 22, 2010, 09:39:38 am

Quote

Yes the second PT had a CT. The original PT had no CT lead that i recall.

That explains why the second PT began smoking. Gotta be careful when replacing a PT. Just because it has a center tap doesn't mean it's gotta be connected to chassis. Only the shadow knows why the original failed.

Title: Re: Valve Junior Layout Error
Post by: plexi50 on August 22, 2010, 11:22:15 am

Thanks. The Shadow is unavailable. All worked out. I will remember this one

Title: Re: Valve Junior Layout Error
Post by: J Rindt on August 22, 2010, 03:52:30 pm

What is the advantage of that .1uf poly-film cap in parallel with C8?
Is there some advantage of a small, non-elytic cap?
Thank You