Output transformer center tap

a fine line between stupid and clever

Output transformer center tap

Postby Frank2 » Sun Jun 23, 2013 10:52 am

Hi guys, i have made a diy guitar amplifier 6 month ago. It was going well with my active pickup guitar but has some squeel with passive pickup. I decide to replace half of the wire with shielded wire. Doing so, i did place thing differently like moving the filter caps. Anyway, i did an error that i think is very bad. I forgot to plug the center tap of the primary of the output transformer and tried the amp. Absolutely no sound has come out, i have induced guitar signal and turn the volume to more than the 1/4. I've found my error but the amp is now having an insane hum and almost no signal. I mean, i can hear guitar but it is very low compared to the hum. I have double check every connection, nothing seem broken visually. It act differently when i turn the volume (preamp section) if it is on solid state or tube rectifier. Also, it is very affected by that preamp volume and is very loud even at all volume to 0. I have resistance in the primary of the output transformer. I wonder if it could be the power tube? The amp is a badcat clone base on ceriatone model "stray cat"
here is a layout(sorry i don't have the electrical schematic)
http://www.ceriatone.com/images/layoutPic/matchlessLayout/StrayCat30Ceriatone.jpg

What do you think?
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby Geek » Sun Jun 23, 2013 4:00 pm

No CT = no anode B+. You likely melted the EL34 screen grids :'(
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
User avatar
Geek
KT88
 
Posts: 3585
Joined: Sun Oct 21, 2007 3:01 am
Location: Chilliwack, British Columbia

Re: Output transformer center tap

Postby Frank2 » Mon Jun 24, 2013 10:23 am

aaoooowwww! that is what i feared... two el34 from tubedepot.. Thank you for your help!

Frank
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby Frank2 » Fri Jul 05, 2013 9:29 pm

New matched El34 set, same problem...
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby Geek » Sat Jul 06, 2013 1:49 am

Do you have continuity on that tranformer? (???) What's the DC resistance from CT to anode leads?
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
User avatar
Geek
KT88
 
Posts: 3585
Joined: Sun Oct 21, 2007 3:01 am
Location: Chilliwack, British Columbia

Re: Output transformer center tap

Postby Frank2 » Tue Jul 16, 2013 9:15 pm

hi, sorry for the delay, i was out of here.

I have 107.8 ohm on one side and 93.3 ohm on the other
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby Frank2 » Wed Jul 17, 2013 9:18 am

hi, i have made test this morning and i narrow down the problem, i think. I did flip every switch, play with every button exept one... the 30/15 watt switch. it was on 15, when i turn it on 30, everything run perfectly. I have no more time this morning, but i am so happy that it seem to be close to a resolution :)
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby mesherm » Wed Jul 17, 2013 1:34 pm

Check the 1K 10 watt resistors at the EL34 sockets. The 15/30 switch changes the El34 operation from 15 watt triode to 30 watt pentode.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
User avatar
mesherm
KT88
 
Posts: 1231
Joined: Fri Aug 27, 2004 10:33 pm
Location: Alvin Texas

Re: Output transformer center tap

Postby Frank2 » Wed Jul 17, 2013 8:41 pm

i read .986 K and 1K, but is it possible that they are okay with the ohmeter but when it is on amp power they does not give the good resistance?
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby mesherm » Thu Jul 18, 2013 9:44 pm

verify that the amp operates with the switch in the 30 watt position but not in the 15 watt position.
Check the voltages on EL34 socket pins 4 both with the switch in both positions.
Is the toggle switch possibly wired backwards. A standard double pole double throw switch connects the center contacts to the set of contacts OPPOSITE the toggle position. If the toggle is up then the lower contacts are engaged and when toggle is down the upper contacts are engaged.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
User avatar
mesherm
KT88
 
Posts: 1231
Joined: Fri Aug 27, 2004 10:33 pm
Location: Alvin Texas

Re: Output transformer center tap

Postby Frank2 » Fri Jul 19, 2013 9:28 pm

When the switch connect S1 to S4 and S5, it work perfectly, i presume this is 30 watt
when the switch connect S4 to S2 and S5 to S3, it does not work. i presume this is 15 watt
Am i right?
The switch seam to work fine, it is not shorted and do not offer resistance

I have 364V on pin 4 of both El34 in 15 and 30 watt
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby mesherm » Fri Jul 19, 2013 11:17 pm

When the switch connect S1 to S4 and S5, it work perfectly, i presume this is 30 watt
when the switch connect S4 to S2 and S5 to S3, it does not work. i presume this is 15 watt
Am i right?
The switch seam to work fine, it is not shorted and do not offer resistance


When the switch connects s1 to s4 and s5 that connect the screen grids to a section of the power supply through the 1K 10 watt resistors which should put it in pentode operation.
When the switch connects s2 and s3 to s4 and s5 that switches the screen grids to the plates through the 1k 10 watt resistors which should put it in triode operation.
It works in pentode mode but not in triode and you get the same voltage on the screen grids regardless of switch position.
OK..I am officially stumped. I cant see how that can be. The 1k screen resistors must be ok because it works in pentode but when the screens are connected to the plate it doesn't work.
What voltage to you get on EL34 pin 3 (plate) when the switch is in each position?
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
User avatar
mesherm
KT88
 
Posts: 1231
Joined: Fri Aug 27, 2004 10:33 pm
Location: Alvin Texas

Re: Output transformer center tap

Postby Frank2 » Sat Jul 20, 2013 7:18 am

On pin 3, when the switch is on triode, voltage is 370V
When the switch is on pentode, voltage is 376v
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby Frank2 » Sat Jul 20, 2013 7:42 am

Sorry, i take that voltage on DC measurement
on AC its 38V for all, volume bellow 1, no guitar pluged in
Frank2
 
Posts: 17
Joined: Mon Jul 16, 2012 9:25 am

Re: Output transformer center tap

Postby mesherm » Sat Jul 20, 2013 1:50 pm

The DC range was the correct way to measure the plate voltage.
Frankly I am stumped as to why it works in pentode but not in triode. Does switching from SS to Tube rectification make any difference?
Check the resistors on the 450 volt 30 mf filter caps.
When you first forgot to connect the OT center tap was that the first time the amp had been powered up?
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
User avatar
mesherm
KT88
 
Posts: 1231
Joined: Fri Aug 27, 2004 10:33 pm
Location: Alvin Texas

Next

Return to guitar amps

Who is online

Users browsing this forum: No registered users and 2 guests