Help Me Diagnose an Issue

[blown240]

Tonight I spend some time with a jumper wire trying to figure out exactly whats wrong with my Bassman build.  It has a terrible, super loud sound, and no real signal comes thru unless all the controls are at max, and it still sounds like garbage.  I think I have found the problem, but I wanted to get other opinions.

In the following schematic there are 4 jumpers that I tried.  They are color coded and numbered.  3 of them took care of the problem, 1 didnt.

A.  BROWN - problem went away when I jumped from the volume pot sweeper to the .0005 cap in the power stage.
B.  Red - problem went away when I jumped from the treble sweeper to the .0005 cap in the power stage.
C.  Green -  problem went away when I jumped from the volume sweeper to the treble sweeper.
D.  Blue - Problem DIDNT CHANGE when I jumped from the treble sweeper to the treble ground side.



This leads me to think that the problem is in the treble pot ground.  This makes sense to me, and seems to go along with the problem being less when all the pots are maxed.

But thats where my knowledge stops, so how can I verify and what do you guys think?

THANKS!!!

[Willabe]

Well, I think the way you have the treble. control wired up your not only changing the treble, but it's acting like a volume control also. It's changing the grid return R from 1M to 1M250. I don't see how that's a good thing.        Maybe have to add a separate 250k grid return to ground would be the fix?

You have 3 gain stages and a CF before the PI and the 3'rd gain stage is the jacked up treble control with at least 1M for a GR R. A 250k would be a more common value there. The larger the GR R you use there, the more gain you get. (to a point)  
 
B and C take out a gain stage, A takes out two.

Can you describe the super loud sound better?


             Brad        

 

[blown240]

The best I can describe it is a pissed off robot sound mixed with the sound of a speaker plugged straight into the wall, its LOUD.

I know thats a terrible description, but its the best I can describe it.  


I purposely bypassed gain stages to try to narrow the problem down.  It threw me for a curve when I couldn't narrow it down to a specific gain stage.  The way its wired is exactly as the fender schematic, the only difference is I used a 500k pot instead of a 250.

[Willabe]

Most of the guys here know more than me, but I'm curious what Fender amp is that? I don't remember seeing that treble control before.

I could be wrong.       


           Brad       

[blown240]

Its a Fender Bassman 6G6b.  Here is the original Schematic:

http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_6G6B.pdf

[Willabe]

Thanks. I don't think I ever looked at that before.

Some think the Fender engineers used the bassmans as their platform to try different ideas, with different levels of success. I have no idea if this model/channel is thought to be a good one.   

Is there a reason you happened to pick this one?


                Brad        

[blown240]

From what I understand its the classic bassman.  Guys like Brian Setzer love em.   I built the bass channel because its often modded and I play bass.  But I want it to work in the stock form before I do anything to it.

[Willabe]

I knew Setzer has used a brown face bassman for a long time, forgot the model #. But I doubt he's plugin in to the bass channel and he's not playing a bass.  

And I guess this is the one that guys love to hot-rod the bass channel on because it has 4 triodes to work with.

Well anyway, I'm sure you'll get it to work and sound for you the way you want it to. I've been following your threads and you have a lot of stick to it in you.  


               Brad  

[blown240]

Thanks.  I have a lot of fun with this stuff.

[tubenit]

Looking at what you tried with jumpering ........... I would be suspicious of a bad tube in that position. OR a bad socket.

Brown bypassed two of the triodes. Works
Green bypassed V2a triode. Works
Red bypassed V2b triode. Works.

Blue doesn't bypass any triodes. Doesn't work.

I don't understand why one triode at a time would work BUT I'd still try replacing the tube if you haven't already.

With respect, Tubenit

[blown240]

I swapped tubes around to see if the problem "moved" with a tube.   It doesnt, which makes me think its either a bad cap, resistor, pot, or ground.

If I hook up the NFB it get way worse.  But that may be a separate issue, since I don't see how the NFB would effect the preamp side of the circuit.

[John]

Based on one of my screw-ups, also check the solder joints and correctness of connections at the arrows.

I'm not saying why I'm suggesting that though. 

[Geezer]

Has the amp ever worked correctly? If yes (and this is a recently arisen problem, then disregard this idea)

When you jumper across "B" OR "C", you are eliminating a gain stage, or a phase reversal, and thus it "fixes" the problem. "A" simply lowered the gain enough to where the oscillation stopped.

Try unhooking one end of the 56k NFB resistor....better?

If yes, then swap the OT primary leads around.....you have POSITIVE feedback. (and re-attach the NFB resistor)

g

[sluckey]

This thread really needs to be in the thread about your radio project. People are not realizing this thread is about the same amp.

EDIT... Here's the link...

http://www.el34world.com/Forum/index.php?topic=12299.0

[Geezer]

This thread really needs to be in the thread about your radio project. People are not realizing this thread is about the same amp.

Oh....I guess I should go read that one. I haven't had much forum time lately & i've not seen any on that build. 

[Colas LeGrippa]

uhhhhhhhh, did you try swapping output tranny blue and brown leads to the tubes ?

[blown240]

Sorry, I guess I should have posted this in the same thread.  It just seemed that at 4 pages long that thread was a little overwhelming.  I figured if I started a new thread it would be fresher.

I swapped pin 3 on the power tubes last week and it only helped a little.  Tonight after work I may bypass the resistor and cap on the treble pot to see if that helps.   That should basically make it a volume pot, I think.

[terminalgs]

have a look at the 20/600 filter cap.   make sure its well grounded and well connected to the preamp B+ rail.  Also make sure its on the correct side of that 56k/1W resistor.   That cap sends  any AC (instrument) signal to ground that makes its way to that B+ rail,  you get plenty of it on that rail from the plate of the CF.   also make sure that .002 cap (parallel with 100K plate R on 12ax7 section #4) is indeed a .002... you might actually lift a leg of it to eliminate it as well.

IIRC, in the other thread you triple-checked all your grounds, right?

[Ritchie200]

What kind of condition is the .1 cap in?  Is it somehow bypassed?  Silly question, did you check and double check all your wiring?

Good luck!
Jim

[blown240]

The cap should be new, and the wiring has been checked numerous times, but you never know.

The grounds are good, I will double check the filter caps.

Thanks for the replies!

[jojokeo]

That "Treble" pot idea is whacky! WTF? If it was me I'd put it back where it belongs in the stack and make that thing a gain control where it would actually have a lot more usefullness. But, IYA

Still waiting on the OT wiring response...??? If the amp is built to the schem and there are no component failures, there shouldn't be issues.

[DummyLoad]

Still waiting on the OT wiring response...???

i believe that in the original thread, he did swap OT plate leads. we have made near full circle again.

ship it to texas, zip 78712. i'll rebuild it for 100.00 + any needed parts - u pay shipping both ways. it'll take me about a week.

peace.

--DL

[blown240]

My buddy will bench it for $30 and after that I only have to pay for parts.  But I REALLY want to try to fix it myself.

I have a couple more things to check once I get done with work.  Probably will be tomorrow.

[blown240]

  also make sure that .002 cap (parallel with 100K plate R on 12ax7 section #4) is indeed a .002... you might actually lift a leg of it to eliminate it as well.

I lifted the leg of the .002 and it sounds great now with the treble all the way down, but if I turn the treble up, it squeels.  Bass is much better now too.   The cap I had was a .0022 cap, is that close enough to a .002?

[blown240]

OK, So I put in a new.002 cap and now the amp sounds pretty good unless I hit the strings hard, then it gets all distorted and sounds like garbage.  But we definatly have made some progress!!!

[blown240]

So why would it sound good when played lightly, but sound like crap when you hit the strings hard?

[Ritchie200]

Double check your .1 coupling cap to make sure it is the correct value and is reading properly.

Jim

[terminalgs]

I lifted the leg of the .002 and it sounds great now with the treble all the way down, but if I turn the treble up, it squeels.  Bass is much better now too.   The cap I had was a .0022 cap, is that close enough to a .002?

.0022 is close enough to .002.   I think the problem is not so much a matter of an incorrect value, but when connected, the .002 isn't functioning as it should.   disconnect that 20/600  filter cap and see if it behaves exactly the same.

[blown240]

I swapped in a new .1 coupling cap:  No Change

I disconnected the #4 filter cap.  Now I dont have a 0 volume, and it gets sounding bad easier.

I swapped in different 12AX7 tubes in the preamp and PI.  Nothing changed.

I swapped in new power tubes, and it got alot better.

Then I swapped in a 12AU7 into the PI and its better still.  Now I really have to get on the strings before it sounds like crap.  And the volume isnt as touchy. 

I may try 12AU7 for the preamp as well, and I need to see if only one of my old power tubes is bad, or both.

[blown240]

Its funny how tubes that test fine can cause so many problems.  After much testing, I ended up with the following tubes:

Groove Tubes 12AX7-R for both preamp tubes
Raytheon 12AU7 for the PI
Raytheon 6L6GC for the Power tubes
Tung-Sol 5U4GB for the rectifier

The amp actually sounds pretty good.  And for my first real build I am pretty happy.  It still gets out of hand sometimes, but I can tweak the controls to remedy that.  I cant really play it that loud, but for a living room amp it doesnt need to be loud.

So now I just need to finish the enclosure and button up a couple loose ends.

Thanks so much for everybodys help.  I REALLY appreciate it!!