tweed w/ reverb - PI voltage problem

[alange5]

I cloned a tweed Gibson GA-18T using this schematic (GA-16T - same circuit)
http://rudn.nodevice.com/preview/big/365/365453-1.jpg


I have an original to compare the schematic to, and listed voltages are right on the money.


My build incorporates a 1-tube reverb inserted between V1B and V2A
http://el34world.com/Forum/index.php?topic=7957.0 (tubenit's first post)


I upped the 100k resistor between RV1 and RV2 to 220k, and added a .01 coupling cap before V2A's grid.


The amp sounds nice at low volume - more or less just like my original GA-18T, but it breaks up much earlier, and it's significantly louder.  I can get it to break up at "1" on the volume knob with hard strumming - not the case with the original Gibson. 


While troubleshooting, I noticed the V2A cathode voltage is only about 7v - far from the 45v called for in the schematic.  Does this have anything to do with the distortion I'm getting?  I have 218v on V2A's plate, which is in the ballpark according to the schematic. 


In the reverb circuit, I've got 284v on the 1st plate, 3.8v on the first cathode, 165v on the 2nd plate, and .25v on the 2nd cathode.


Anything obvious jump out?  Where should I look to get the V2A cathode voltage up?

[PRR]

You did something to V2A grid circuit.

What?? It isn't shown on these diagrams. Get a crayon and mark-up to show what you did (not what someone else did).

[shooter]

I noticed the V2A cathode voltage is only about 7v - far from the 45v

while the amps off and you're coloring, ohm the 47K and make sure you don't have a 4.7k

[sluckey]

and added a .01 coupling cap before V2A's grid.

You still have the 470K connected to the grid, right?

[alange5]

You did something to V2A grid circuit.

What?? It isn't shown on these diagrams. Get a crayon and mark-up to show what you did (not what someone else did).

Sorry.  I forgot about Crayola's Law.


I inserted the reverb directly after the .005 coupling cap on V1B's plate, and brought it back through a .01 cap into V2A's grid.




Shooter, I'll check out that resistor when I get home from work.




Sluckey, yes, it's still there.

[sluckey]

Shooter, I'll check out that resistor when I get home from work.

While you're at it, measure resistance from V2 pin 8 to ground. Should be ≈48.5K.

[PRR]

The "clone" should have been checked before adding a frill (reverb). If you put 4.7 instead of 47, that's on you.

If it worked "stock clone" and got sick with frill added.....

> inserted the reverb directly after the .005 coupling cap on V1B's plate, and brought it back through a .01 cap into V2A's grid.

This is my interpretation. The purple lines should work (with a reservation). However the DC you are getting on the cathodyne strongly suggest you really connected as in the blue "wrong" line. The DC path through reverb return is pulling-down the cathodyne grid, so far it can't slam the 6V6es.

What I am also wondering: why not connect where I marked "?"? The input to the cathodyne is a BIG signal, more than this reverb needs or can handle well. The level at the Vol pot is more appropriate. Also you do not have a grid at high DC to upset. Also I *think* this is how it was done in the plan you are copying.

[alange5]

why not connect where I marked "?"?

I saw a few 5E3 projects where reverb was inserted before the PI, so I followed suit.


If I end up moving the reverb after the tone control, should I still use coupling before the next grid?


When I get home I'll confirm resistor values and coupling cap placement.

[alange5]

you know what....


Looking at your crayon drawing, PRR, I clearly remember putting the coupling cap in series with the 330k off the reverb pot and attaching it directly to V2A grid, where it met the RV2 end of the mixing resistor.  According to your drawing, that end of the resistor should be on the other end of the coupling cap.  I think that means I took the blue pill.


Excited to finish work and check it out.

[alange5]

I moved RV2 to the correct side of the coupling cap.  I now have about 39v on the cathode.  The amp still breaks up a little earlier than I'd like, but it's not the "something's wrong" distortion I was hearing earlier.


I'll try putting the reverb earlier in the circuit and see what happens.


Thanks for the help.  Every time I feel like a dummy, I learn a little more.

[PRR]

> Looking at your crayon drawing

See? You can wave words all day long. Nobody else is quite sure, and sometimes you aren't either. Scratch a stick in the dirt and everybody is on the same page. Even if it is just you-- sometimes your own scratchings make you say "hmmm."

Always draw it all out, as-built.

[alange5]

this morning I placed the reverb at the output of the volume pot, and now I have a lot of high-frequency buzz/hum when increasing the reverb control.  The hum gets louder as the reverb is increased, and disappears when it's at zero.  Its frequency is constant and is independent of volume and tone controls.  The reverb is present along with the hum.


I've attached an updated schematic showing my new reverb placement.  I experimented with and without a .01 coupling cap at R2, and the hum remains unchanged.  Any ideas?

[alange5]

more info:


The hum disappears when disconnecting the reverb tank.  With the tank connected, I can change the frequency of the hum by moving the reverb cables around to the point of introducing a high pitch squeal when the cables are close to the power tubes.

[shooter]

hum disappears when disconnecting the reverb tank

then you have a grounding issue.  try isolated jacks, shielded cables.

[alange5]

then you have a grounding issue.  try isolated jacks, shielded cables.

I tried both with no change.  I tried new RCA cables, too.  There was no hum when the reverb was before the PI.  All I changed was the insertion point.

[shooter]

the insertion point.

in and out of the TS should be shielded wire, grounded only 1 end.  does the noise change with wire position?

but it's not the "something's wrong" distortion I was hearing earlier.

you might try going back to "here" and seeing if the something wrong is verb or something else.  coming in and out of the TS might take ALOT of work to make it work.

[alange5]

It's hard to tell if the noise changes with wire position...  It changes drastically with the position of the RCA cables to the reverb pan, but only in frequency of the hum/buzz.  The hum volume remains dependent on the level of the reverb pot.  Currently, I have shielded wire grounded at one end on the reverb input and output (R and R2).  The wires running to the RCA in/out jacks are also shielded.  I only added the shielding after the hum started (with no appreciable difference).  When I had the reverb before the PI, I was not using shielded cable, and had no hum.


I could always move it back to the PI, but I'd love to know why it's humming so badly after the tone stack.  I might move it back just to see if the hum follows.

[alange5]

I went ahead and moved the reverb back in front of the PI.  The hum is completely gone.  I'm puzzled as to why it existed after the tone stack. 

[tubenit]

So is this schematic what you currently have?   And if so ............. how does the amp sound?  And how does the reverb sound?

With respect, Tubenit

[alange5]

So is this schematic what you currently have?

Yup, that's where I'm at.  The amp sounds very nice.  Compared to my original tweed Gibson, this build is significantly louder (heavier iron? beefy OT sourced from a vintage PA) and breaks up a bit earlier.  I'm very happy with the reverb.  I'd say the knob on 6 or 7 is comparable to 3 or 4 on a standard Fender reverb.  I'm using a MOD long tank.  With the reverb maxed, I don't know if I can call it "surf" levels... more like "public swimming pool".  Plenty of splash for my needs.


Tubenit, thanks for compiling that reverb thread.  I've referenced it several times.  I'm looking forward to trying a transformerless reverb build a la Ampeg on an upcoming project.