Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on November 11, 2010, 08:49:04 am
-
Well, I finished up my revibe, chased down a noise in the tremolo, and thought it was good to go. Finally had a chance to spend some time with it, and realized it's causing some pretty bad volume and tone loss, even if both the trem and reverb are bypassed? I've also noticed the reverb sounds very thin and distant. there seems to plenty of dwell (i can hear lots of splash with the dwell up), but not much coming through in the mix.
I'm assuming I should be only looking at the part of the circuit that the signal passes through, both when off and bypassed, but I'm having a little trouble understanding what's going on when the circuit is bypassed? Could someone possibly explain to my what part of the circuit is eliminated when bypassed? Any help is appreciated and hanks in advance!
-
When you say 'bypassed', exactly what do you mean? Just that you have the trem and reverb footswitches off? Or you have an external A/B switch?
Here's the signal path from input to output. Signal comes in at J1 and gets amplified by V1B. The output of V1B is applied to some tone filters (R42, R43, R44, C29, C31, C32) and split into a high freq band and a low freq band. The highs go thru V4A and the lows go thru V4B. The two bands are recombined in R48 and R49 and then applied to the output jack.
The signal always goes thru V1B and V4 regardless of whether the trem or reverb is on or off. There's no way to kill it. Even if you turn off the trem, the guitar signal must still take this path, although it will not have the trem modulation.
The reverb path is V2A, V2B, V3, and V1A. The reverb signal can be totally killed with the Dwell, footswitch, or Mix control.
-
Sorry, I guess bypassed was the incorrect terminology. You are correct, I'm getting the loss when the reverb and trem are switched off. I will take a look at the areas you mentioned after work. Thanks again sluckey!
-
This is the entire circuit that the signal sees when the reverb and trem are off. The signal path is highlighted. You may even want to pull all tubes except V1 and V4 until you get this part working right.
If it ain't wired correctly, other stuff could cause problems, but this is all that's involved with a correctly wired unit.
-
Thanks a bunch for taking the time to highlight that for me. I really appreciate your effort. I'll let you know what I find.
-
Well, I've checked all of these components, and nothing seems amiss. I know it's not ideal to measure components while they're in the circuit, but one strange thing I noticed is the 470k resistors on pins 1 and 6 of V4 both measure around 260k or so. If I take them out they measure 470k, but in the circuit they measure like they are in parallel? Not sure if this is normal or not, so I'm going to triple check my wiring.
-
but one strange thing I noticed is the 470k resistors on pins 1 and 6 of V4 both measure around 260k or so. If I take them out they measure 470k, but in the circuit they measure like they are in parallel?
You're fine. The calculated measurment is 276K. Let's say you put your probes across R49. But one probe is also touching R48. And the other probe is also touching R47.Also R47 and R48 are connected in series thru R46 for a total of 670K. This 670K is parallel to the 470K (R49) you're wanting to measure. 470K paralleled with 670K is 276K.
-
Ahhh, that makes sense. thanks again. I decided to get my Weber 5g15 reverb up and running so i could compare the two. The Revibe with the dwell and mix on 10 is about the same as the Weber build on 1 or 2, so something is definitely wrong. I am going to go back through every component and wire (again) and try to find my mistake.
-
I finally got around to revisiting my revibe build that is also experiencing signal loss with both reverb and tremelo off. I found this thread while I was searching for anyone else who had this problem. I went over the part of the circuit that Sluckey hi-lited and all of the wiring and components are correct. All of the voltages appear correct and I have gone over the circuit several times. I replaced all of the carbon comp resistors (some had tolerences up to 20%) with metal film and now the unit is very quiet with virtually no hum but am still experiencing some signal loss, distortion, and tone change. With the reverb and tremelo off, I noticed that when the mix control is all the way up, it minimizes some of the signal loss. This doesn't make sense since the mix control is not in hi-lited portion of the circuit. I have swapped all of the tubes without any change. I built a bypass switch to use when playing but have to turn up the guitar a bit to compensate for the signal loss while the unit is on. Both the reverb and tremelo sound great but I am wondering if I am going to have to live with the signal loss. Does any one have any other possible suggestions or is this the best this unit can do?
Thanks,
Dan
-
Does any one have any other possible suggestions or is this the best this unit can do?
Look just below the output jack on the schematic. See that 10K resistor sitting under R23? Change that to 100K to increase the signal level. If 100K gives too much signal, try 47K. Keep trying different values until you get the signal you want. Higher resistance will give more signal. Lower resistance will give less signal.
-
Sluckey, thanks for the tip. I tried various resistors - 100K, 47K, 33K, 20K, and 15K. The 20K seems to have the least impact on the signal. Anything higher, increased the signal quite a bit. Now that the signal loss is solved, I am still getting a bit of distortion and tone change but I will run it this way for a while to see if I can live with it. Hopefully this helps anyone else out there that is having similiar problems with the Revibe.
Again, thanks,
Dan
-
This is good to know. I actually set mine aside for a while out of frustration. I think I'll give this a try while I'm trying to track down why the reverb is less than it should be on mine.
-
Well, I buttoned up my revibe and will start using it when I gig next. I still have a minor amount of hum but I am using a bypass switch so it is not a big issue when I am playing with the unit. When I plug the 3-prong plug in, I get a lot of hum unless I lift the ground with a cheater. Is this safe? I have seen a lot of discussion on this and other forums about this.
Weber uses a complex method of grounding the revibe using insulated jacks, pots, etc and grounding everything though a brass bus and then adds some resistors and caps. They claim that the unit is dead quiet using this method even without lifting the ground. Has anyone tried this and how does it compare to Doug's grounding scheme. All of the other amps I have built using Doug's grounding method are dead quiet. Any thoughts?
Thanks,
Dan
-
As long as the ground (shield) of your interconnect cables are intact then a proper ground should be formed between all interconnected devices simply by plugging in the cables. Only one device needs an earthed ground connection in this configuration otherwise ground loops form (hummm). By lifting the ground from the chassis (sounds like this is what Weber does) you defeat the ground continuity formed by the interconnects and need to have every ground-isolated device grounded separately via 3rd prong for safety. You wouldn't want to both break the ground at the 3rd prong with the "cheater" AND use the weber approach.
Moral: if your interconnects are intact then feel free to simply use the cheater. Layout issues may still give unacceptable levels of noise.
-
I finally got around to changing that resistor. Used a 22k and it seems to be just about right. I still for the life of me can't figure out why the reverb sucks so bad on this build though. Really bums me out.
-
I finally got around to changing that resistor. Used a 22k and it seems to be just about right. I still for the life of me can't figure out why the reverb sucks so bad on this build though. Really bums me out.
Not enough 'verb in the wet mix? Most likely its to do with the gain structure of the amp.
In a stock 6G15, the wet signal (from the single 12AX7 triode recovery stage) is mixed with a dry signal (which is a guitar p'up level signal) that has been through the CF stage (unity gain). So in that case, with the guitar-level dry signal, and the recovery stage's gain on the wet side, there is plenty of wet in the mix. But the "Tore T" unit 'replaces' the CF stage with a conventional inverting stage does it not? (ipso facto, the wet/dry mix will be different, and will need more adjusting. I'm picking that somewhere you would need to lose some of that 'dry' signal). JM2CW
-
I have a Weber Revibe also, never sounded right so I worked on it for a couple weeks then just pulled the tank out and it's in the garage somewhere. Don't think it was meant to work right. It was 400.00 spent on a bad design and I just don't feel like throwing more money and time at it. It's kind of like using a line six pedal it works but with what has happened to your tone it's not worth it.
Tony
-
Yeah, I built the Tore-T version here. It sounds ok, but never exactly what I wanted. I love the classic Fender stand-alone verb, and have built a few with good results. I was hoping the ReVibe would give me the same verb with the addition of some tube tremolo, but I've never been able to get the results I was looking for. I probably should have just built another Fender verb. From a troubleshooting perspective, the revibe may be a bit over my head.