Diagnosis for a second hand 50/50 power amp

[rsanz]

Hello:

This is my first post here. First of all i want to thank "crane" and his post (http://forum.grailtone.com/viewtopic.php?f=5&t=32130) I think it's a great and usable mod i was looking for and opened my eyes to what i'm about to tell you.

I have just bought a second hand 50/50 power amp. Even though I know electronics i never opened the amp until a few days ago. The first thing i saw was that it didn't have any mesa boogie tube... i said okay i will have to add a bias potenciomenter for the 4 SOVTEK 6L6WXT+ . Another thing i was thinking about is "are this ENGL preamp tubes good?". But the thing that scared me was an output transformer different from the other.

Yesterday i read the post from Crane and i saw his fifth picture where can be seen 2 identical output transformers ( which should be the original ones) placed parallel to the front. In my amp i see one of the output transformer ( the left one) that is far from looking like crane's. The other one looks like it's Mesa Boogie but has a label saying " MESA BOOGIE, 562051, 40-16639, 166-0511, MADE IN USA". This one is placed in the corner in a different way from Crane's fifth picture.

Furthermore there is one more transformer missing in my amp (the one that's closer to the output jack's in metallic in Crane's picture). I must say that my amp is a European (or is working for 220v at least).

So i have a bunch of questions about this:
1. Are any of these output transformers right for the amp?
2. I guess the Mesa Boogie one could be...but what about the other? Could this be a good supply part? How can i Know?
3. Why is this amp missing the transformer close to output jacks?
4. What do you think about the power tubes?
5. What do you think about the preamp tubes?

I know about electronics not to much about transformers or tubes (it's my first tube amp). I have access to scopes and multimeters. i haven't played to much with this amp and i haven't played loud enough yet to hear a difference in sound in each of the channels to tell the difference in the output transformers. Some screws for the transformers have been sealed with red wax, even though i don't think it went to an officcial repair service.

Thanks for all in advance.

 

[crane]

Thanx for your compliments !

You're right, mine is a 220v version.

It seems that one of the trannies has been replaced.
4 capacitors have been replaced too in your amp (the 4 black labelled "ruby")

Don't know about the preamp tubes.
The bias mod is easy to do if you're carefull, the hard part is to build the adapter from an old tube.

Does it work well ? no hum, cracks ?

 

[rsanz]

thanks for replying.

Mine is 220v too, so i don't know why your amp has 1 transformer more than mine.
Your output transformers have the same label as my mesa boogie transformer?
Good to know about the four capacitors replaced.
I haven't played enough yet to say if there's hum or cracks.

Any other comments are welcome.

thanks.

 

[rsanz]

Crane or any other, i need your help:

I have done the mod for rebiasing the 50/50 (http://forum.grailtone.com/viewtopic.php?f=5&t=32130 ).
I was trying to see the difference in the output waveform in both output channels measuring with a scope but as i was touching cables inside the amp powered on and looking the scope's screen i noticed something strange in the output. When the output of one of the transformers was close to the cables coming from the input towards the front pots there was a feedback in form of a high frequency depending of the volume of the signal. The higher the volume the wider the feedback over the 1khz test tone. When you have lower volume there's no problem but when you start turning the volume up the second half of the positive cycle shows a higher frequency overlapped and it gets wider covering the hole sine period as you go to higher volumes.

I talked to a friend how know much more about electronics than me and he talked to me about the importance of the shielded transformers. As my amp has one rare unshielded transformer i rewired part of the amp trying to take the output and the input of both transformers far from each other and also far from the bunch of cables coming from the input connectors. I have tried to reproduce Crane's amp in terms of where the wires come from one place to other but then i saw another strange thing: my Mesa output transformer has one of it's output unconnected (the yellow one),so i don't know if this should be the 4 ohm conector that someone forgot to connect or if it's a transformer with an 16 ohm output that obviously should be unconnected in my amp. I also saw that it's not in its place because the unshielded tranny is bigger so that's why the last "technician" that put his hand on the amp decided to do another hole and put the mesa tranny in diagonal. I wonder also if your trannies have their shield connected to ground mine doesn't because it's not screwed in the place where the ground is.

So basically i have these questions:

1. what are the colours of your output transformers and where are they connected?
2. are your shield output tranformers connected to ground?
3. do you think it's a real problem an unshielded tranny?
4. if i try to change the unshielded transformer do you think i should get a mesa or can i get some other to save some bucks?, where?, how much do you think it should be ( i live in Spain)? any web site for buying trannies?

Next week i would like to take a look at the output waveforms in scopes and bandwidth measures using fft analyzer i would like to post the results here.

Thanks again.

 

[Old BF Shred]

i don't know why your amp has 1 transformer more than mine.

It's not a transformer; it's a filter choke used to help maintain higher current output of power supply. It gives the amp a little more "punch". This is a mod available from Mesa...call them and they will give you the details.
It is easy to install, BTW, as I have done 2 of these myself.

As far as noise, my 50/50 is far from totally quiet, but can't comment on what's normal for your amp as, I can't hear it from here.

Re-routing wires can help, and it is possible the "alien" transformer is generating some of the noise (60hz hum, anyway).
Don't expect it to ever be as quiet as a HiFi amp....don't think it's in it's DNA.

Good luck with that, however!

 

[Old BF Shred]

PS....my bad...the output transformer generally won't give you 60hz hum (unless the power supply is not filtering out enough of it.... The filter choke mod could help here). The power transformer definitely can.

Noise usually is introduced early in the signal path.
I'd look there.....

Good luck