6G15/5G15 Reverb - trashy thin distortion

[theunrulychef]

The burnt CC resistor gave me all sorts of weird noises til I sorted it. I didn't quite realise how bad it was until I was de-soldering it - it was so badly compromised, it virtually crumbled when I came to yank it out.

Hmm, maybe this isn't it then. I haven't been able to get much of anything on the board to so much as make a peep, though maybe I'm not hearing it because it's going into the reverb pan & not directly into a speaker like I'm used to. After messing around last night, I was thinking about seeing if I don't have another dwell pot to try. If I don't, I should at least disconnect the leads & wire in a resistor so I can take it out of the equation.

[tubeswell]

Hmm, maybe this isn't it then. ..., I should at least disconnect the leads & wire in a resistor so I can take it out of the equation.

Well the final clue for me was that the resistance measured a little 'too' low (or was it 'too' high?) once I measured it in the circuit, anyway it wasn't right (at least, allowing for the fact that it was a CC, it wasn't as 'right' as it should've been), and that was because if had drifted too far because I had cooked it. You expect some variation with CCs, but if you cook them, then you can't rely rely on how they'll perform with voltage going through them when they get warmed up.

The problem (which I remember now, was intermittent staticky noise on my 5E3 when I first built it) went away when I replaced that V1 plate resistor (it helped that the 5E3 has two channels for V1, and this noise was only happening in one of them, and the coupling cap in that channel had tested good for blocking DC when I measured the lifted grid end). You might not have exactly the same symptoms, but it could be a similar cause - if you can't find anything else.

[Diablo]

Some more info. I checked the Revibe schematic from Jeff Gehring and discovered he has changed the 100K plate resistor on V2-pin6. He changed it to a 69K + 33K, so it acts as a voltage divider for the signal going into the 6K6 power tube. Hmm....So, I think he also discovered the fuzztone, and concluded it is the 6K6 that is being driven too hard into saturation. I think I may try his mod.

[theunrulychef]

Just had time to do 1 thing tonight so I tried that voltage divider going into the .002 cap. It cut the attack fuzziness down by about half or so with the Les Paul. Seems like the Jazzmaster is coming through almost completely clean. That was a good find, Diablo. Thanks! This thing is 80% there now.

I'll have to check out those other plate resistors too. Once my scope arrives, hopefully that will help me get this thing finished for good!

Thanks again,
Jay

[Diablo]

Glad the voltage divider worked out for you. I just finished adding it myself, and I think it's fine. I exchanged some e-mail with Jeff Gehring, and he said the voltage divider was added to tame the overdrive on the 6K6. He also suggested playing around with the resistor values to get it just right. For me, I'm sticking with 66K and 33K.

[theunrulychef]

So I finally checked this thing out with the scope (using the built in function generator as a test signal). I looks like I'm not getting any clipping distortion to speak of with normal input levels, but I've got a funky looking trace coming out of the 6k6 (nice innocent looking sine going in). Doesn't look like crossover distortion from the examples I could see online - I wasn't sure if you even can have crossover distortion with a SE output like this. I wonder if it could be some sort of parasitic oscillation or something. As I back off my input signal, it morphs back into a regular sine. It also is frequency dependent, notching up more the lower I go with the frequency. Either way, it's sounding good tonight. The Gehring voltage divider trick does pretty well to tame most of the fuzziness.

I did do one thing though that really brought this unit to life. I was messing around trying to see if I couldn't get the wet signal closer to the dry signal volume-wise, and I bridged the 250pf cap coming off the wiper with a 500pf - boom! I had reeeevvvvveeeerrrbb like I haven't heard out of this thing before. I can still dial it back for "tasteful", but now I can get it soaking wet & cavernous, which is what I really wanted the external reverb for anyways. I ended up putting a 1000pf in there.

Thanks again for all your help guys.
-Jay

[Diablo]

So I finally checked this thing out with the scope (using the built in function generator as a test signal). I looks like I'm not getting any clipping distortion to speak of with normal input levels, but I've got a funky looking trace coming out of the 6k6 (nice innocent looking sine going in). Doesn't look like crossover distortion from the examples I could see online - I wasn't sure if you even can have crossover distortion with a SE output like this. I wonder if it could be some sort of parasitic oscillation or something. As I back off my input signal, it morphs back into a regular sine. It also is frequency dependent, notching up more the lower I go with the frequency. Either way, it's sounding good tonight. The Gehring voltage divider trick does pretty well to tame most of the fuzziness.

I did do one thing though that really brought this unit to life. I was messing around trying to see if I couldn't get the wet signal closer to the dry signal volume-wise, and I bridged the 250pf cap coming off the wiper with a 500pf - boom! I had reeeevvvvveeeerrrbb like I haven't heard out of this thing before. I can still dial it back for "tasteful", but now I can get it soaking wet & cavernous, which is what I really wanted the external reverb for anyways. I ended up putting a 1000pf in there.

Thanks again for all your help guys.
-Jay

The distortion you see on the scope is exactly what my ears were telling me before I installed the voltage divider. It was fuzzing up the low notes on the guitar, not the high three strings. I'm no expert, but I just think this shows that the signal strength is too high - too much voltage swing for the grid of the 6K6 tube, so the plate current doesn't follow the grid signal accurately.

[twangOmatic]

This is a great forum, Im having the same problem with my 5g15. Ill have to try that voltage divider trick.