Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TurboGuitarMelton on April 02, 2019, 05:32:06 pm
-
Hi,
I built ANOTHER champ... Vibro champ this time. I love the tremolo but am experiencing a few issues and would like to hear what you guys think could be going on.
1) There is a "fizz" at low volume settings that accompanies notes. Its almost like it sounds like there is a second signal that is putting out a distorted version of the guitar sound with the clean/normal signal. Its very quiet and oddly only happens when I play a note softly. It seems to not happen when I play louder notes. It seems to be related to the intensity pot. When I turn it I get a scratchy sound from 1-5 and this issue gets more noticeable. I had to use 50k instead of the stock 25k here aswell.
2) With the footswitch plugged in and the tremolo turned off (via switch) and intensity turned up all the way there is a significant amount of volume reduction happening. The volume jumps back up when I hit the footswitch and turn the trem back on.
Few important notes about the build
*no NFB
*40uf for first cap
*50k intensity pot instead of 25k
*non-original layout ( I used terminal strips)
*Power transformer is not very beefy (235-0-235 @40ma) gets warm but not hot at all.
6v6 is biased @~35ma
B+ is ~300V so all voltages are about 50v lower than BF champ
-
"fizz"
scratchy sound
those are both typical of something not well connected, pin, solder, bolt....
fizz has also been "linked" with sketchy plate R's, typically old skool carbon type.
http://el34world.com/Forum/index.php?action=post;topic=24552.0;last_msg=264363
looking at this version
intensity turned up
it appears to me the intensity pot interacts with the cathode :dontknow:
-
I had the same problem with my VibroChamp. NFB fixed it. Note the B schematic has both global NFB and local NFB at the power tube.
-
NFB might help but would not explain all problems, I believe. (That being said: it would almost always be a better idea to start from a known good design and modify from there.)
If a search for bad connections or cold solder joints doesn't deliver, it would be good if you could try to isolate your problem(s). For example:
- temporarily break the connection between the intensity pot and V1b and remove V2. You now have a non-vibro champ. Any of your problems persist? Any difference with V2 inserted?
- temporarily break the connection between the lfo cathode follower and the intensity pot (also a non-vibro champ but with the intensity pot in circuit). Any problems now when you work the intensity pot?
-
2) With the footswitch plugged in and the tremolo turned off (via switch) and intensity turned up all the way there is a significant amount of volume reduction happening. The volume jumps back up when I hit the footswitch and turn the trem back on.
The voltage increases too high on 2. amplification stage cathode and causes gain drop. Try to decrease the 1.5K resistor or change the 68K resistor connected to the pot.
/Leevi
-
Frankly, i wouldn't mod it any further. Fix it first. A stock vc is not supposed to exhibit the behaviour you described. Get it working first, than tweak to taste. Any other approach is likely to induce (more) problems unless you know very well what you're doing.
-
*50k intensity pot instead of 25k
Use a 25K pot even if you can't find a reverse audio taper. Should take care of the volume reduction issue. Might even take care of the fizzes.
-
Finally Back!
Found that screen voltage on 6v6 was higher than the plate voltage. Fixed that by changing 1st power resistor to a 4.7k. and I upped the bias to 39ma @ 285V
added the little nfb cap to the 6v6 and added normal NFB resistor
Fizz is gone!
Footswitch volume issue:
I changed the 50k pot to 25k. No change...
Changed the 68k resistor on intensity pot to 100k. Slight change here but the trem got weaker.
:dontknow:
Whats going on with this thing? I swear I've read other people saying that vibro champs just do this. I've never seen a footswitch being sold WITH the old vibro champs... is it possible they all do this but nobody knows it because nobody has/uses the footswitch?
Ive noticed that I have 10V on the intensity pot @4-6 when its at 0 I have 0V when its on 10 I have about 2V. Odd....or is that normal?
***With V2 (trem tube) disconnected it operates as expected. Plain old champ. The trem pots do not chnaged the volume level at all
-
N.B.: higher screen voltage at idle is normal. Per Ohms Law. Screens draw near 0 current at idle, so near 0 screen voltage is dropped. Plates draw current at idle, so they cause a voltage drop.
Glad you solved the hiss issue.