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FDP Forum / Fender Amps: Vintage (before 1985) / Why would anyone do something like this?

roadhog96

USA / Sesame St.

Apr 20th, 2017 12:22 PM   Edit   Profile   Print Topic   Search Topic

Opened up a 67 SFDR today and right away found these plate load resistors that are only 24 ohms. Am I seeing things, is this for real, 24 ohms? Red, Yellow, Black, Gold, they are 5% 24 ohm, and look like metal film.

This is the second amp in a week that had wrong resitors used. Maybe they were colored blind?

Just noticed the Cathode Bypass Caps are 22uF 25V instead of the usual 25uF 25V, this is a factory Blackface circuit. Dont think thats going to make a big difference but it's not correct.

The more I probe it looks like there is even more wrong spec resistors being used in place of the correct values.

(This message was last edited by roadhog96 at 11:24 PM, Apr 22nd, 2017)

jazzguy

Philly, B-3 Capital

don't dream it be it
Apr 20th, 2017 02:32 PM   Edit   Profile   Print Topic   Search Topic

who yanked the blue tone caps and put orange drops in there?

roadhog96

USA / Sesame St.

Apr 20th, 2017 02:56 PM   Edit   Profile   Print Topic   Search Topic

Not to worry, I have some good replacements on hand that are going in very shortly. .047 and .1uF 400V blue molded.

Just notice theres a quit a few wires with burnt cloth ends because some nuckelhead didnt take the time to push back the cloth before soldering the wire ends.

(This message was last edited by roadhog96 at 06:22 PM, Apr 20th, 2017)

roadhog96

USA / Sesame St.

Apr 20th, 2017 09:58 PM   Edit   Profile   Print Topic   Search Topic

I still have more to do tomorrow. I was going to replace the phase inverter socket, its been causing problems. I found a heater wire that looked like it was touching pin 8 & 3 that are soldered together on the phase inverter socket. This is the same tube that keeps causing problems every so often. Volume drops off, rumbling noise and static start. Reach in and move the tube and it all stops and goes back to normal till it happens again. Maybe this was causing the problem. I'll have to test it and see if its still acting up, if it is, then it's getting replaced. I've tried everything, re-flowed the solder, cleaned and retention socket, nothing has worked.

Some people will cringe at the thought of using metal film instead of carbon comp. but its all I have left in stock. I havent been able to hear any difference so what the heck.

Plate load resistors replaced and blue molded .1 uF and .047uF 400V caps installed

(This message was last edited by roadhog96 at 11:54 AM, Apr 21st, 2017)

amphead4

Cincinnati, USA

Apr 21st, 2017 12:33 PM   Edit   Profile   Print Topic   Search Topic

Are you saying the amp would actually play with 24 ohm plate load resistors? Sorry, I'm a doubter.

roadhog96

USA / Sesame St.

Apr 21st, 2017 06:45 PM   Edit   Profile   Print Topic   Search Topic

Well this one did even though it doesn't seem possible but it sounded like crap, not what a normal DR should sound like by no means. Dont see why amyone would want to play it that way. None of them actually tested at 24, they were all out of spec on the plus side not that it probably made any difference.

guitarcapo

U.S.A.

Apr 25th, 2017 10:40 AM   Edit   Profile   Print Topic   Search Topic

I'm looking at a schematic and I don't see plate load resistors on the power tubes at all...just 470 ohm screen resistors.

Why don't you think it would play? It's practically open wire. As long as there isn't a huge current going through it frying it out I doubt it's doing anything.

schematic

(This message was last edited by guitarcapo at 12:48 PM, Apr 25th, 2017)

amphead4

Cincinnati, USA

Apr 25th, 2017 11:54 AM   Edit   Profile   Print Topic   Search Topic

I thought the OP was referring to the preamp plate load resistors. If 100k plate load resistors are replaced with 24 ohm resistors, the plate current change with signal would cause very little voltage drop across such a low resistance, resulting in practically no output from the stage.

If we aren't talking about preamp plate load resistors, I apologize for being off topic.

roadhog96

USA / Sesame St.

Apr 25th, 2017 04:35 PM   Edit   Profile   Print Topic   Search Topic

Get this, I unsoldered the heater wires on V7 so I could swap the wires and get them in phase. I removed the 470 ohm resistor to make it easy to get at and look what I found. No Solder on the wire to Pin 1. The wire was just stuck in the hole.

The green heater wire to the right isn't soldered at this time either because I stopped to take a picture.

The green heater wires from the pilot light passed under the two red, and two yellow wires from the power transformer to the rectifier tube. Tranny wires were actually laying on the heater wires and the two black wires from tranny to fuse and switch was lying the top of the heater wires. I did everything over, this should cut down on a lot of the noise this amp was making and it wasn't music type noise.

No Solder on Pin 1

(This message was last edited by roadhog96 at 10:25 PM, Apr 25th, 2017)

FDP Forum / Fender Amps: Vintage (before 1985) / Why would anyone do something like this?




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