Need voltage troubleshooting help

[kwijabo]

i built a stang ray clone this weekend. turned it on and it sounded like mush when i dug in. also, considerably louder on the low E and A strings. after a little troubleshooting, i found the issue, but i'm not sure of the root cause yet! maybe someone can help.

- the output of the PT is right on, 292Vacrms.
- without any other tubes in, the output of the rectifier is 330Vdc. this is low.
- with the other tubes in, output of the rectifier is a paltry 280Vdc! so low - should be up around 350+. consequently all the other B+'s are low.

unfortunately i don't have another 5AR4 laying around so i can't just switch it to see so i'm checking in before i order another. can it happen where the rec tube doesn't completely break down but just fails to the point where it still "works" just not at it its spec'd output? or would you suggest something else in the build is dragging this down? obviously the rec. output is tied right to the output transformer, so can i disconnect the OT and try taking voltage measurements then?

[M Fowler]

Do you have a SS plug to use rather than the 5AR4?

Mark

[kwijabo]

thanks for the idea mark. it won't be as simple as a plug or anything but i can take the couple minutes to mod it for some 1N4007's - i have two extra of those parts. will try when i get home tonight.

[tubeswell]

Before you totally bag the 5AR4, have you got a bleeder resistor in which is the wrong value or something (which is dragging down the B+)?

[M Fowler]

Hard to tell what is going on in the amp without more info or pictures.

[kwijabo]

ok got home and did some more troubleshooting. can post a scan of the schematic tomorrow during the day, but for the meantime...

it was not the rec. tube - i tried SS and got the same low voltages with the tubes in.

When all tubes are in, the preamp (a single EF86) is:
B+: 285
Plate: 12.061 (Rp = 250k)
Screengrid = 149V (Rsg = 2M)
Cathode = 3.8V (Rk = 2.2k)

so i lowered Rp to 100k and Rsg to 1M and get a plate @ 4.6V!! B+ and cathode remained constant.

is this starting to sound like a bad EF86?

[tubeswell]

So you're sure about the value of the plate, screen, (and cathode) resistor(s) huh?

But if they are correct then it could be a shorted tube.

Or a partially shorted filter cap.

Or a wrong value inter-stage attenuator/voltage divider

Or a wrong value bleeder resistor

(i.e.: something somewhere is causing excessive current to pass through the B+ rail methinks.)

[Phil_S]

Agree with Tubeswell, this is likely a short of some kind.

In addition to his list, under a magnifying glass, look for a whisker that is touching the chassis or other ground point. Probe both ends of each connection in an attempt to isolate where voltage is OK and where it is not. For example, do you have 285 at the B+ node and at the entry point to Ra (whether 250K or 100K).

Take a voltage reading across Ra, one probe on each side of the R and use clips, not hand held. This is the voltage drop. Make sure it is consistent with what you probe on each side. IOW, if you see 285v on one side and 4v on the other, you should see a reading of 281 across that R. This will confirm or refute your readings.

[tubeswell]

You could try lifting the +ve end of each filter cap (one at a time) to see whether this makes a difference to the (unloaded) B+. If one of them is partially shorted, the B+ should shoot up to more 'normal' levels when you take it out of the circuit.

[kwijabo]

ok i'm embarrassed to post this and sorry to have wasted your time.

turns out three things were the issue.
1. started off removing the supply resistors 1x1. it was the pre-amp stage. bam. for whatever reason i didn't tie it to the right cap and never noticed it. but i wouldn't come here and waste your times without troubleshooting myself! i did triple check the build, just not well enough. probably becuase it was like 3am and i was tired.

2. during troubleshooting, until i found out what was wrong with 1. above, i was changing resistors. i had incorrectly swapped the screen grid and plate resistors which were 100k and 1M, so i had a 1M plate and 100k screen grid.

after way too much trouble shooting, my first amp is up and playable. still going to make a few minor value adjustments but its not bad.

thanks again everyone.

[Cygnus X1]

Great news!

Don't worry about it, it's a learning experience for all of us.

Cheers!

[Phil_S]

BTDT. Writing it down for someone else really forces you to think about it in another way. Triple checking or not, you had made an error. I can't speak for others, but I don't think folks mind just holding up the mirror so you can have a good look. It is very difficult sometimes to be objective about your own work.