Brownface Potentiometers
Moderators: pompeiisneaks, Colossal
Re: Brownface Potentiometers
V7 pin 3 - 446v
pin 4 - 451v
pin 5 - -49.3v
pin 8 - 25.2mv
V8 pin 3 - 446.3v
pin 4 - 453v
pin 5 - -49.6v
pin 8 - 26.2mv
I also have 1k5 grid stoppers on pin 5 of the power tubes.
I also swapped the output transformer leads to no avail.
pin 4 - 451v
pin 5 - -49.3v
pin 8 - 25.2mv
V8 pin 3 - 446.3v
pin 4 - 453v
pin 5 - -49.6v
pin 8 - 26.2mv
I also have 1k5 grid stoppers on pin 5 of the power tubes.
I also swapped the output transformer leads to no avail.
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Brownface Potentiometers
With only 25 mA (assuming you have grounded the cathodes through 1 ohm resistors) your bias is very cold, only 11W. It should be more like 18W, Vpin3 x Vpin8.
Re: Brownface Potentiometers
Sorry Martin have got a pair of 6p3s-e tubes in not 6l6 i just use them for starting an amp up.
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Brownface Potentiometers
I see. The 5881 on the schematic are 23W tubes anyway, so I should have suggested 14W dissipation. 6p3s-e are listed a 20W, so things look reasonable.
Re: Brownface Potentiometers
Changed tubes to JJ 6L6 and rebiased signal on pin 5 with grid stopper is fine but pin 3 is horrible loud and distorted and sounds out of tune is this looking like the output transformer?
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Brownface Potentiometers
Could be. Do you have another one to sub in?
Re: Brownface Potentiometers
As it happens yes i do same transformer from same supplier
https://modulusamplification.com/f-styl ... 3295-p.asp
Will have to wait start my shift pattern tomorrow night.
https://modulusamplification.com/f-styl ... 3295-p.asp
Will have to wait start my shift pattern tomorrow night.
Re: Brownface Potentiometers
Just tested the output transformer primaries resistance to the b+ winding 183 ohms on V7 and 88.2 ohms on V8 then tried both primaries to ground both read well over 1m and it just carried on going up, then tried 8 ohm tap to primaries 1m and still going. 8 ohm tap to ground 2.3ohms.
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Brownface Potentiometers
That seems to indicate shorted primary turns. The resistances should be much closer than that.
Re: Brownface Potentiometers
Thats what i thought too ,brand new transformer too
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Brownface Potentiometers
How does the other one measure?
Re: Brownface Potentiometers
more or less exactly the same
- martin manning
- Posts: 13209
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
1 others liked this
Re: Brownface Potentiometers
It would be pretty strange to have two defective ones. You could try the "super secret transformer test" if you have a neon bulb: https://www.premierguitar.com/the-super ... mer-tester
Some discussion on that here: https://ampgarage.com/forum/viewtopic.php?f=6&t=35771
Some discussion on that here: https://ampgarage.com/forum/viewtopic.php?f=6&t=35771
Re: Brownface Potentiometers
Thanks Martin will have a look when I get home from work tomorrow
Re: Brownface Potentiometers
Just tried the resistance between the blue( 183) and brown (88) and got 97 ohms, I wonder if the colours are the wrong way round. What's easiest way to find the correct b+ wire?