5F6A: Squeal

[carrejans]

Hi,
I almost finished my second tube amp. It's a 5F6A bassman. I bought parts from Ceriatone, and also based my build upon his layout:
http://www.ceriatone.com/images/layo...one_100508.jpg

I doublechecked all connections, so I put it on. But only for 1 second, because there is an enormous loud squal making me almost deaf. (I'm not exaturating)
I also saw some blue glow in my power tubes.
My volume controls are at zero.

I found this page on R.G. Keen's website: Squeals
But I'm not sure if we're talking about the same kind of squeal?

So, I should first change the preamp tubes. Mine are brand new JJ's for the ECC83 and 6L6 and Sovtek for the rectifier and EH for the 12AY7. Aren't these tested at the factory?

Maybe you guys know what the exact problem can be?

Also, here are some pictures of my build (if it is useful, I will try to make some more decent pictures):
http://members.lycos.nl/carrejans/amp2/amp001.jpg
http://members.lycos.nl/carrejans/amp2/amp002.jpg
http://members.lycos.nl/carrejans/amp2/amp003.jpg
http://members.lycos.nl/carrejans/amp2/amp004.jpg
http://members.lycos.nl/carrejans/amp2/amp005.jpg
http://members.lycos.nl/carrejans/amp2/amp006.jpg
http://members.lycos.nl/carrejans/amp2/amp007.jpg

Some remarks of my build. I soldered the 1 Ohm resistors of the power tubes to a screw of there socket. I also soldered the ground of the speaker jack to the same screw (not to my main star ground point)

I don't have the "E" connection at my power transformer, like on the Ceriatone layout.

I doublechecked all my wiring, but it looks all the same as at the ceriatone layout. Maybe there is a mistake in that layout?

There is a 5E7 labelling at the output transformer. Are they the same?

Another question: how should I use the bias pot? First at zero?



I hope we will find the solution.
Thank you...

[txstrat]

Wow, great and tidy layout.
Have you tried to interchange the plate wires of the output transformer? This can lead to the squeal you're talling about.
It might help when you take readings of all the voltages with the tubes in and out.
And BTW welcome to the forum.

[carrejans]

Wow, great and tidy layout.
Have you tried to interchange the plate wires of the output transformer? This can lead to the squeal you're talling about.
It might help when you take readings of all the voltages with the tubes in and out.
And BTW welcome to the forum.

Thank you for your answer.
I interchanged the plate wires of the output transformer.
Result: no squael; but also no sound.

Can someone explain why the squael is gone? What's the theory behind it?
(at the Ceriatone layout, they are connected the other way around)

Also, what's wrong now?
I absolutely hear no sound at all. But when I rotate the "presence" knob, I hear a crackling sound. (don't know if this has to do anything with the problem, though)

[Chuck H]

Here's a possibility...

The squeal was there because the plate wires were opposite what they should have been. You reversed the plate leads and the squeal is gone. That means the PI and power amp are operating (probably). So the fact that you have no sound now is most likely related to the preamp. Check connections, wiring, make sure everything is in place, etc. for the preamp. Do a signal trace to see if the signal drops out anywhere behind the PI.

Chuck

[carrejans]

The problem is solved!
There was indeed a problem in the preamp section.
One of the heaters of one of the triodes of one 12AX7 was not connected properly.
Now the pilot light doesn't work anymore. Strange, didn't hold my solder iron that long against the heater wires. (and the light is not that close to the solder joint of the 12AX7, that I had to redo)
EDIT: Sometimes it seems to work. Bad connection probably.

But I still have some questions.

- I still don't know why interchanging the primary connections of the output transformer was necessary. (different than at the Ceriatone layout)

- The power tubes have a blue glow insinde them.(they are also much hotter than the rest; but this is probably normal) This isn't good, I assume? Is this because the bias isn't good? (there is a bias pot in the ceriatone layout; not in the original I think)

- I want to add a fx-loop. Should I just search the internet/this forum (in other words: is this pretty standard for every kind of amp), or is this very specific for this amp? Is it just placing the effect between the preamp and power amp?

Thank you guys, for your help.
Nice forum by the way.

[Alex R]

- if the plates are connected the wrong way around it turns your negative feedback loop into a positive feedback loop, hence the howl.

Blue glow is normal.

Effects loops work ok if you just slot them in before the PI, after the last coupling cap so there is no DC there of course - simple break-circuit loops are generally at a point where there is high impedance (like mostly everywhere in a tube amp except after a cathode follower), and zero gain. The reason some loops are more complex than that, with little amps before and after ('buffers'), is to make sure impedances match and gain isn't lost. With most pedals it is no problem - maybe if you have long long leads to the pedalboard it might sap the signal a bit.

[carrejans]

Would this work good as fx loop?
http://www.ceriatone.com/images/mods/FX-Loop.jpg


Layout of amp used: http://www.ceriatone.com/images/layo...one_100508.jpg


Thanks...