Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JT on June 09, 2011, 11:18:52 am

Title: fuseable resistor
Post by: JT on June 09, 2011, 11:18:52 am

I'm working on a Knox amplifier with a schematic attached to it identifying it as a DR-55, I have found that this schematic has nothing to do with the circuit actually in the cabinet.  I have been studying the schematics available on this website and have finally determined that this amp is a Kent DR-19-A

http://www.el34world.com/charts/Schematics/files/kent/kent_unknown.pdf (http://www.el34world.com/charts/Schematics/files/kent/kent_unknown.pdf)

which has the tremolo section and tube configuration on the amp I have.  I have a few questions if anyone can help.  First there is a 47 ohm 3 watt fuseable resistor just before the rectifier, I'm not familiar with this term.  What kind of resistor is this?  I'm guessing it is a wirewound resistor, but want to make sure I install the right component.  Second, in looking at the schematic it appears to me that the output transformer is hooked from its secondaries straight to the speaker wires, but the amp I have has one end of the secondary soldered to the chassis and the other end soldered to an isolated lug, the speaker wires are not hooked up.  Final question, there are two filter caps that I plan to replace, one is 60 uf 160 volt and the other 40 uf at 160 volt.  I would like to find a two in one style to put inside the can with wire leads, but the closest I can find is a 47/47 at 160 or a 47/80 at 160.  Any suggestions would be greatly appreciated.

Thanks.

Title: Re: fuseable resistor
Post by: John on June 09, 2011, 11:31:49 am

http://www.welwyn-tt.co.uk/pdf/application_notes/EMC2_AN_A.pdf (http://www.welwyn-tt.co.uk/pdf/application_notes/EMC2_AN_A.pdf)

Dunno if that will help or not, but that explains the fusible resistor thing.

Title: Re: fuseable resistor
Post by: sluckey on June 09, 2011, 12:31:18 pm

Don't worry about the speaker connection. Either way is fine for that amp.

I'd use the 47/80 cap.

You may consider an isolation transformer to protect you from tingles caused by the line voltage present on the chassis.

Title: Re: fuseable resistor
Post by: kagliostro on June 09, 2011, 01:59:59 pm

+ 1 for Sluckey

Kagliostro

Title: Re: fuseable resistor
Post by: JT on June 09, 2011, 09:44:53 pm

Thanks for all the advice,

after searching for a fusible resistor of that value it looks like they're hard to find.  I'm thinking a 1 amp fuse in series with a 47 ohm 3w resistor would essentially do the same thing.  I don't totally understand how a slow blow fuse works, but instinct tells me that would be the way to go.  Any thoughts on the subject? 



Isolation transformers are good things, I'll be careful.


Thanks

Title: Re: fuseable resistor
Post by: kagliostro on June 10, 2011, 07:08:43 am

Slow blow fuses are used when at startup the circuit has a high inrush current

if you use a fast blow fuse it easily blow at startup also if there are no problems in the circuit

Kagliostro