Ampeg V4.com

forum

You are not logged in.

#1 2012-10-15 18:36:44

bingdylan
New member
Registered: 2012-10-15
Posts: 8

B 15s drivin' me nuts

Ok, low output, distorted sound. Voltages are really wierd from v3 and above. Had a 12AU7 instead of a 12DW7, and was sorta functioning. I guess I really don't quite get the V2 style power supply. V1 and V2 are biasing up, but thats about it. Any ideas?

Offline

 

#2 2012-10-16 08:24:32

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

Do you get sound if you put signal through the ext amp jack?

Can you be more specific about these weird voltages?

What is it about the v2 power supply that trips you up?

Offline

 

#3 2012-10-16 15:12:22

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

V3 (with 12au7) p1 440v p2 26v p3 278v p6 107v p7 0v p8 5v...
What wigs me about the supply is that it seems to reference to ground through D5... External amp jack seems to function, thought not a tight sound...

Offline

 

#4 2012-10-16 18:17:14

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

And what did you check so far?

My first thought would be the tube,  but if not check r37,  followed by c20 and r36.

Offline

 

#5 2012-10-16 20:04:12

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

Im leaning tword leaky coupling caps. Tried tubes, but nothing really changes...

Offline

 

#6 2012-10-16 20:45:01

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

Here is what I don't understand...   Pin 3 and pin 8 should have the exact same reading.   But according to what you wrote... They do not.   
With the amp off does pin3 measure a short to pin 8?

-steve

Offline

 

#7 2012-10-16 21:19:14

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

V3 not v4...

Offline

 

#8 2012-10-16 23:24:47

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

Sorry,  saw 12au7 and ran with it.   You've tried it with the 12dw7 and it still sounded bad?
If you don't have the 12dw7 on hand you could try a 12ax7 instead.   Big thing is that the second stage is biased for the high-mu triode equivalent to a 12ax7,  and the first is just a cathode follower,  I think it would sound better to usea12ax7 in that position.

It's that second stage tat looks wonky.

Offline

 

#9 2012-10-29 20:01:38

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

Checked all those resistors and replaced C20, no different. Whole thing sounds like a fuzzy 5 watt amp...

Offline

 

#10 2012-10-29 20:08:35

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

and the speaker is good?   
do you have an Oscilloscope?

Offline

 

#11 2012-10-29 23:45:24

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

unhooked the out put transformer... 138 ohms on each side of the primary, 1.3 ohms on the speaker side...

Offline

 

#12 2012-10-29 23:46:30

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

@ 8 ohms...

Offline

 

#13 2012-11-03 16:34:44

bingdylan
New member
Registered: 2012-10-15
Posts: 8

Re: B 15s drivin' me nuts

I thought it should read about 4k...

Offline

 

#14 2012-11-03 19:09:25

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: B 15s drivin' me nuts

your output transformer measurements are not odd.  I usually expect the secondary to measure below 1 ohm,  but some volt meters measure low resistances poorly,  especially if their batteries are low. 
and since its not too far off i didn't say anything.

the primary measurements look right on.

if you had a scope you could take a look at the output of the power tubes going into the OT,  and compare it to the output of the amp.   
if you have a signal generator you could unhook the output transformer from the circuit and drive the secondary with a 1k signal,  measure it the voltage on the secondary,  and then compare it to the voltage on the primary from center tap to either of the other wires.
the ratio (Vprimary/Vsecondary)^2  should be the same as the impedance ratio.   which is perhaps the 4k that you are thinking of.   as in that amp it should be somewhere around 4000/8 or 500/1

so in an ideal world if you drive the secondary with a 1volt signal you should measure about 22.36 on the primary.
if you do that and it doesn't pan out then your OT is bad,  otherwise, you're running up the wrong tree.

Offline

 

Board footer

Powered by PunBB
© Copyright 2002–2005 Rickard Andersson