Lost volume - Vox type build

[sdp1234]

I've been working on converting a previous build to a Vox Topboost type amp.  In order to eliminate a tube I've incorporated a Mosfet CF.  Had problems because the mosfet was installed backwards (oops... not used to these ss devices).  Actually sounded pretty good even with the mosfet backwards.

I was testing it and all of a sudden the volume dropped about in half and became a bit distorted.  All my standard voltage measurements seem to be on track.  I spent an hour or so looking for bad connections. Connected a cd to the input and traced through using a listening amp. 


Swapped pre-amp tubes
Seated and reseated the Ouput tubes (I don't have others to try but the voltages seem correct)
Removed the Master Volume
Removed (bypassed) the tone stack and mosfet CF -
Tried different taps off my OT

Usually voltages point to the problem but my voltages seem good.  Any suggestions where I go from here?

[DummyLoad]

schematic as built, lots of hi-res pics of guts and voltage chart...is the honey that attracts the techno-bears around there parts.

peace.

--DL

[sdp1234]

Makes sense...  I'll try to get a schematic drawn and post everything later today.

[sluckey]

I was testing it and all of a sudden the volume dropped about in half and became a bit distorted.

How were you testing? Just giving it a workout by playing guitar thru it? Or probing guts with a meter or other piece of test equipment?

Did tracing with a listening amp give any clues?, IE, it sounds good up to point "C", but point "D" sounds distorted?

Since the problem occurred "suddenly" I'd have to suspect any solid state device. Can you replace the MOSFET CF? Lot's of other things can fail suddenly also, but that MOSFET has already been stressed.

[sdp1234]

My son was playing guitar and I was adjusting the tone stack pots when the volume dropped.
I bypassed the mosfet cf and tone stack and it still sounds too quiet and a bit fuzzy even.

When tracing sound through the amp I could hear the gain at the output of the first stage
Sounded louder and much less full after the second stage ( I think this is normal due to caps filtering out or not amplifying the lower mid and low freq,)

Attached is the basic schematic with some voltage measurements.

[The_Gaz]

You really need another pair of EL84s to try before you drive yourself nuts taking apart the whole amp. I don't know how long you had it running like that but 340 on the screens is really high for 84's, and it could've just been "their time." Did you check to see if one of the screen resistors opened up? Then again, you did say all the voltages were okay, which makes me think it's the power tubes.

[sdp1234]

I'll be away this weekend but I'll pick up another set of EL84's and I'll make some changes to get the screens down.  Should have an update by Monday afternoon.  Thanks.

[sdp1234]

I finally got new output tubes.  I up the screen resistors from 100 ohm to 2.2 k. I also changed the Mosfet CF with a new one.  Still sounds the same so I added the cf and tone stack back in and took new voltage measurements.

This was a working amp that I changed the preamp to a Vox ac30\express type.  When we were testing it the volume suddenly went down and it became fuzzy and distorted.  It had lots of headroom until this happened.

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12047

[sdp1234]

Seems like it may be a problem with my PI or output stage.  If I inject a radio signal into the PI it starts to get distorted before it is loud.  At very low inputs it sounds good and then as I turn up the radio the volume starts to level off and distort.  I'm figuring that the pi may be going into distortion.


I also tried another speaker cable and a different speaker. 

I may have to take another build apart and try to take the output from the preamp into a known working amp.  Just right now my other 2 amps are buttoned up and in cabs.(it's a pain to get everything apart but it may be necessary.

[terminalgs]

before you take other amps apart,  try bypassing stage#2 in your circuit.  stage#1 is straightforward enough  that i don't think you need to doubt that 220k/1.5k gain stage  (you don't really need to doubt the 2nd stage either).   


you might look at the 1000uf cathode bypass cap on the el84s,  maybe disconnect it just to rule it out  (tho, i think your voltages would be off if it was passing DC).
Is 12v what you had on the cathodes of the el84s when it was working well?

[sdp1234]

I'm out of town for a couple days but I try removing the 1000UF cap.  I think I bypassed the second stage but I'll try it again when I'm home.

[sdp1234]

Took the output from the preamp and ran it directly into the poweramp of a SE build I have sitting around.  It is much louder than the output i'm getting from the amp alone.  While it's not perfect it sounds pretty good.  Looks like my problem is in the PI or power amp.  I did remove the 1000uf cap on the cathode cap with no change.

[sdp1234]

Looks like my PI voltages may have changed.  They are
1-227
2-23.5
3-66.3
6-240
7-25.5
8-66.3

My 2 and 7 voltages seem to be lower than they were before.  I have previous measurements of 45 and 50 for 2 and 7.  Would this cause problems?  If so where should I look for the cause. 
It's probably something stupid but this has caused me more problems than anything I've encountered so far.

[sdp1234]

I'm still unable  to solve the problem.  Changed out all the caps in the PI and power section.  Removed the treble cut and MV from the circuit. Lifted one end of the cathode bypass cap.

Pictures attached.

[sdp1234]

Tried another OT and the volume is back.  I've never had an OT go bad but something went wrong with it.  Still have some fuzzy distortion but I think there are some ground issues and bleed through between stages.  Guess I'll order an OT and start working through the sound issues.