2ch dual rectifier transformer hum

[benjy]

Hey SSO.

3 days ago I replaced the power tubes on my *older* 100watt Dual Rec head.
I was going for the "half power mode". After some research, I put 2 6l6 power tubes in the right locations, and 1 rectifier tube in. I plugged an 8 ohm cab into the 4 ohm out of the head.

It worked great and sounded great for 3 days, but today it made a humming sound coming from the output transformer(i think). It still powers on and sounds good, however, It hums and starts to smell after a minute or so. I also tried it in silicone diode mode, and tried with other power tubes.

Have I done something to blow the output transformer?

Do they blow after time?

With the pic provided, can you guys confirm that I'm looking at the output transformer?

Thanks for any help you can give

 

[Shask]

That is the power transformer... Not the output transformer...

 

[loktide]

did you bias the amp after changing tubes?

does the amp still hum without a guitar plugged in?

does the hum get louder with increasing master volume?

 

[Edika]

Humming and worse the smell are not good signs for electronic components. This means something is overheating. Best if you take it to a tech and not play it in the meanwhile.

 

[benjy]

That is the power transformer... Not the output transformer...

thats valuable information, thanks.

did you bias the amp after changing tubes?

I switched it from el34 to 6l6. it had el34's before. all mesa branded tubes.

does the amp still hum without a guitar plugged in?

Yes it does. Even on standby it hums...

does the hum get louder with increasing master volume?

As I increase the master volume, The hum starts coming through the speakers, so yes.

 

[benjy]

Humming and worse the smell are not good signs for electronic components. This means something is overheating. Best if you take it to a tech and not play it in the meanwhile.

Will do, Thanks

 

[Seanthesheep]

Kind of obvious and condesending but did you make sure the bias seitch is set to 6L6?

 

[loktide]

I switched it from el34 to 6l6. it had el34's before. all mesa branded tubes.

Even on standby it hums...

but did you adjust the bias of the amp or not?

it sounds like you just pulled the EL34s out and two 6L6s in.

6L6 draw a lot less power than EL34s. that means that if you didn't adjust the bias, the tubes are running WAY too hot now, and might even overload the power transformer.

additionally, since you're only running two 6L6, the load is even greater on these.

could you post a pic of the powertubes after 5 min of the amp being on? not standby, but on/on

if the plates are glowing, it means that your tubes are biased WAY too hot. this might damage the power transformer.


here's how glowing plates look like:

if you see that, then take your amp to a tech to properly bias it. you'll likely need new tubes. hopefully not a new power transformer as well

 

[benjy]

but did you adjust the bias of the amp or not?

it sounds like you just pulled the EL34s out and two 6L6s in.

6L6 draw a lot less power than EL34s. that means that if you didn't adjust the bias, the tubes are running WAY too hot now, and might even overload the power transformer.

additionally, since you're only running two 6L6, the load is even greater on these.

could you post a pic of the powertubes after 5 min of the amp being on? not standby, but on/on

if the plates are glowing, it means that your tubes are biased WAY too hot. this might damage the power transformer.


here's how glowing plates look like:

if you see that, then take your amp to a tech to properly bias it. you'll likely need new tubes. hopefully not a new power transformer as well

I switched the bias selector switch to 6l6. as far as i know, thats all i had to do to bias it... Maybe it needed further adjustment for running it with only 2 power tubes.

I took it to a tech today, so i cant answer the plate glowing thing. But Im sure that was it. They were glowing pretty good when I tried it on/on last.

Now i just have to hope it's a $100 repair vs a $500 repair.

Thanks for the information

 

[loktide]

I switched the bias selector switch to 6l6. as far as i know, thats all i had to do to bias it... Maybe it needed further adjustment for running it with only 2 power tubes.

I took it to a tech today, so i cant answer the plate glowing thing. But Im sure that was it. They were glowing pretty good when I tried it on/on last.

Now i just have to hope it's a $100 repair vs a $500 repair.

Thanks for the information

you have to manually bias the amp EVERY time you change tubes. even with the same brand. the 6L6/EL34 just adjust the bias range. otherwise the bias pot wouldn't reach to go from 6L6 to EL34.

just biasing should cost you less than 50$

 

[eaeolian]

^ It's a Recto - there IS no bias pot. You should be buying tubes in Mesa's spec range - many vendors can help you with that.

 

[Dead Undead]

EDIT: Nevermind, missed something OP had said already.

 

[loktide]

^ It's a Recto - there IS no bias pot. You should be buying tubes in Mesa's spec range - many vendors can help you with that.

wow, i've never owned one. but it's the first time i hear this except for the 5150. i wonder if it's also set to be in the cold bias range like the 5150

 

[benjy]

just a little update to this.

the power transformer has been replaced
$450 after taxes

thanks for all the help everyone
il be buying brand new amps from now on

 

[Deliverowned]

"I put 2 6l6 power tubes in the right locations"

Though you had to pull out the 2 outer or 2 inner tubes but not 2 tubes on the right or left sides. Not sure on this tho.

"il be buying brand new amps from now on "

I would better blow out a transformer on a used mesa than a brand new one but thats just me. And people should really need to educate themselves on how amplifier wroks before trying out something as "risky" as running 2 power tubes instead of 4.

 

[Thrashman]

^ He did pull the two outer tubes and flick the bias switch. Obviously there was something else going on with the transformer, should be fixed now that it's replaced though.

 

[Deliverowned]

^ He did pull the two outer tubes and flick the bias switch. Obviously there was something else going on with the transformer, should be fixed now that it's replaced though.

I always miss the obvious.