Bassman 5b6

[Matt D]

How about adding grid resistors and/or screen stoppers to the 6l6's? Is there any reason to think these could be helpful in quelling low frequency parasitics?

[M Fowler]

Matt, very puzzling that every thing tried hasn't resolved the issue.

How about shielded wire from wiper of volume pot with 820R grid blocker soldered directly to the tube pin.

Mark

[rp]

You're not trying to quell distortion or a parasitic, you shouldn't have any. This stuff happens to me because I'm a sophomore and stray from known good layouts, but you have a building flaw somewhere.

Bad E caps motorboat (putt putt putt) hum real bad, or squeal loud as hell. I've never had a bad new E cap, but old worn out caps make for an amp that's dark, soft, and lifeless, but I don't know all the failure modes. When ever I've come across a bad coupling cap I've caught it from scratchiness on the vol or tone pots. I've never had a bad new cap either.

I've had some distortion problems and after days of aggravation found that both times it was a socket. AES had some $2 plastic octals that after 4-5 pulls lost contact, after tightening the problem was cured, lesson: use good sockets. On an other amp I did some mods and got a mild distortion I couldn't find for days, till I noticed one output tube was sitting 1mm higher. I had checked the sockets with a flashlight but had missed it, a tiny bit of solder had wicked up. I only found when I stuck a needle in the pin holes. Needless to say if you poke around the pins drain the E caps. Just an other stab in the dark for you.

[Matt D]

Thanks guys for sticking with me through all of this. I'm puzzled as well. My only departures from the Tino Z layout originally were adding grid stoppers to the 6sc7, using a larger chassis, and using oversized iron. Maybe the larger chassis is less forgiving of the grounding scheme for some reason. I'll try the shielded wire from the volume pot wiper with grid stopper this afternoon. It also occured to me to replace the 6sc7 with a loctal 7f7. Might be less noisy and easier to find a good tube. Most of my 6sc7s are used from the local junk/surplus store and maybe all of them are no good.

[M Fowler]

Twice now in both of my 5b6 builds I've had to replace V1 6SC7 one due to oscillation and second tube lost voltage to plates.

I have more on order but the best tube so far is a black metal NOS tube.

Mark

[Matt D]

What were the oscillations like?

[M Fowler]

When turning the amp volume up the oscillations would kick in you know the usual annoying high pitch crap.

[Matt D]

This morning I have replaced the 6sc7 with a 7f7, tried a 820r grid stopper on the PI input, added screen grid resistors to the to the 6l6's (not good sounding) changed the ground location of everything that I have moved back to Zottola spec, reflowed every ground connection, and cleaned out the tube sockets with gobs of contact cleaner. None of it made a bit of difference for the positive. I'm going to try my alternate ground bus/preamp cap setup.

[Matt D]

Redid the preamp grounds and filtering. That made the amp quieter but didn't help the strange distortion. Moved the power tube cathode resistor off of the tube socket and onto a terminal strip so I could have more space to work with the lead dress around the power tubes. Nope. Changed out the tone control shunt capacitor. No dice.

Is it possible that I have crossover distortion? I'm going to try 1.5k grid stoppers on the power tubes.

[Matt D]

1k5 grid stoppers might have changed the sound a little. It seems like the ghost notes might be in a bit of a higher register. When playing two notes I definitely hear a 3rd, distinct ghost note. For instance, when I play G#on the 3rd string, 11th fret and and E on the 1st string, 12th fret simultaneously on the lap steel, I hear a B pitched a minor 3rd above the G# (king of a nice effect). When straighten the bar for A and E on the 12th fret, I hear A-flat pitched right below the A (not so nice). Is that consistent with blocking distortion effects?

[Matt D]

Increased the grid stoppers to 10k. No effect on the ghosting/distortion. Bad cap can?

[xtian]

Drive-by comment here: Try clipping in additional smoothing cap to see if this affects ghosting at all.

[Matt D]

Thanks xtian. I've any suggestions as to what resistor to add to the dropping string? I'm going to change the tube socket at the PI and change out the cap can. Maybe add a choke?

[Matt D]

Replaced the 6sl7 tube socket. Not the problem.

[Matt D]

Here are some pictures of where I'm at with this thing. Almost ready to throw in the towel and turn it into something else.