Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big chief on January 28, 2025, 11:31:54 am

Title: Supro 1650 rt
Post by: Big chief on January 28, 2025, 11:31:54 am

Power tubes red plating changed tubes did not help
Can’t find schematic on internet
Any help would be appreciated
Have to travel to my home where I work in my little shop due to flood at my house

Please help

Title: Re: Supro 1650 rt
Post by: shooter on January 28, 2025, 11:42:29 am

self biased or fixed bias?

Title: Re: Supro 1650 rt
Post by: Big chief on January 28, 2025, 11:51:17 am

Fixed bias as there is no bias pot that I can find

Title: Re: Supro 1650 rt
Post by: shooter on January 28, 2025, 12:40:27 pm

fixed bias doesn't NEED a pot
IS THERE a cathode resistor on the PA tubes, if so what value

Title: Re: Supro 1650 rt
Post by: Big chief on January 28, 2025, 01:05:14 pm

I’ll go over tomorrow and take readings with tubes out and post this thing has a pcb
Thank you

Title: Re: Supro 1650 rt
Post by: shooter on January 28, 2025, 01:42:03 pm

post a pic of the PCB

Title: Re: Supro 1650 rt
Post by: Big chief on January 29, 2025, 10:29:40 am

Voltage on power tubes
V2 p1 0     V3 p1 0
     P2 8.         P2.8
     P3 0.         P3 0
     P4 512.     P4 508
     P5 0          P5 504
     P7 3.7.      P7 .5
     P8 .4.        P8 .5
Can’t get photo to post.  Hope I did not get junk

Title: Re: Supro 1650 rt
Post by: Big chief on January 29, 2025, 10:30:56 am

Pin 5-54

Title: Re: Supro 1650 rt
Post by: Big chief on January 29, 2025, 10:35:38 am

Photo

Title: Re: Supro 1650 rt
Post by: Willabe on January 29, 2025, 01:20:51 pm

Quote from: Big chief on January 29, 2025, 10:29:40 am

Voltage on power tubes
V2 p1 0     V3 p1 0
     P2 8.         P2.8
     P3 0.         P3 0
     P4 512.     P4 508
     P5 0          P5 504
     P7 3.7.      P7 .5
     P8 .4.        P8 .5

Somethings wrong. Power tubes are 5881/6L6GB?

If so, pin 4 is the screen grid, that looks pretty good, then pin 3 is the plate, should be the same or a little higher than the screen grid dcv.

Pin 5 is the input/control grid, should be the -bias dcv, maybe ~ -45? 

Title: Re: Supro 1650 rt
Post by: Big chief on January 30, 2025, 09:29:37 am

Rectifier tube cold 5u4
Pin 4 122
Pin 6 128


Pin 2 .05
Pin 8 .05
Have no idea from here

Title: Re: Supro 1650 rt
Post by: shooter on January 30, 2025, 09:31:45 am

with no schematic, PCB built, strip it and build something new, otherwise we're just blindfold duck hunting

Title: Re: Supro 1650 rt
Post by: Big chief on January 30, 2025, 09:34:58 am

Thanks

Title: Re: Supro 1650 rt
Post by: Willabe on January 30, 2025, 10:10:24 am

Quote from: Big chief on January 30, 2025, 09:29:37 am

Rectifier tube cold 5u4
Pin 4 122
Pin 6 128


Pin 2 .05
Pin 8 .05
Have no idea from here

With all tubes out, measure for 5acv across rectifier tube pins 2 and 8. That's the acv for the rectifier tubes heater. If you have 5acv, then try different rectifier tube, that tubes heater went out.

If no 5acv then make sure there's not a bad/broken 5acv heater wire, or a cold solder joint, might be a push on spade luge.  Or lastly the PT's gone bad. 

And pin's 4 and 6 look low, but you do have some acv from the PT's secondaries B+ high voltage wind.

And please put acv or dcv when you post voltages.

Title: Re: Supro 1650 rt
Post by: Big chief on January 30, 2025, 02:21:04 pm

Thanks for the info I’ll go over tomorrow and measure and post

Title: Re: Supro 1650 rt
Post by: Willabe on January 30, 2025, 04:00:04 pm

Pins 2 and 8 should have both a 5acv and the B+ dcv.

For the 5acv for the heaters measure across pins 2 and 8. For the hi B+ dcv, measure for dcv from pin 8 to ground. 

Title: Re: Supro 1650 rt
Post by: Big chief on January 31, 2025, 07:49:20 am

Pin 2-8 ac 0.6
Pin 8 to ground 522 dc
Pin 4 127 ac
Pin 6 131 ac

Title: Re: Supro 1650 rt
Post by: Willabe on February 03, 2025, 02:33:03 pm

I was out of town for the weekend. Somebody should have replied on this.

The 0.6acv has to be wrong because your getting 522dcv at pin 8.

Pin 4 with 127acv and pin 6 with 131acv seem low since you getting 522dcv at pin 8.

131 x 1.3 = 170.

Somethings wrong with these readings/voltages.

Title: Re: Supro 1650 rt
Post by: Willabe on February 03, 2025, 02:40:00 pm

And;

Quote from: Willabe on January 29, 2025, 01:20:51 pm

Quote from: Big chief on January 29, 2025, 10:29:40 am

Voltage on power tubes
V2 p1 0     V3 p1 0
     P2 8.         P2.8
     P3 0.         P3 0
     P4 512.     P4 508
     P5 0          P5 504
     P7 3.7.      P7 .5
     P8 .4.        P8 .5

Somethings wrong. Power tubes are 5881/6L6GB?

If so, pin 4 is the screen grid, that looks pretty good, then pin 3 is the plate, should be the same or a little higher than the screen grid dcv.

Pin 5 is the input/control grid, should be the -bias dcv, maybe ~ -45?

Those #'s show no plate dcv. And the V3 pin 5 502dcv has to be wrong.

Measure for negative dcv at pins 5 on both tubes. That is the -bias dcv.

And put a new battery in your meter. See if the readings change.