Tremolo Problems

[dude]

Super Reverb, 1971 AA1069, been black faced but tremolo is not AB763.
Speed works ok, but intensity only comes on faintly on 8, at 10 it's on but not deep.
Checked all resistors, changed all e-caps (25/25) in trem circuit, wiring in circuit all good, changed a few different 12Ax7s, no differences, roach blinks, footswitch works.


Searched here, elsewhere, stuck.


Schematic attached
 

[sluckey]

Get a new roach.

[dude]

Thanks, sluckey  I ordered one.


Since I know you'll read this, post.
I put all the right caps and rail resistors in the Super Reverb, added two 1 watt 1 ohm R's off 6L6 cathodes for biasing with meter.
The question is the schematic says, -52 bias also posted on the schematic 465 plate voltage. When I look up the mA chart in Weber's Bias calculator for 6L6GC, says 45mA (70%). My plate voltage is about the same and entered, so when I set the mA across the 1 ohm R to 45mA, the bias voltage reads -43.


Pay no attention to the that - bias voltage difference that's on the schematic and set the mA at 43...?   


I'm assuming that each set of 6L6s are different, some require a higher neg bias than others? Go with the mA reading across the 1 ohm and bias for that set of tubes according to the plate voltage (maybe minus the screens which is usually only a couple of volts difference than the plates)?

[sluckey]

The -52v shown on the original schematic is a conservative bias setting that's easy to adjust and doesn't require using a slide rule. And nobody was obsessing about 70% max plate dissipation in the '60s.

But today, you better use the 1Ω resistors and set the bias to exactly 70%, or you risk a spanking by the bias police! 

[dude]

OK, I get it, just getting too anal in my old age, ha. My entire family is engineers of some kind, I'm not, but think like one "without the education". Guys like me probably drive you guys crazy,  .  Just can't help it, I'm into everything, jack of all trades master of none, well, except cabinetmaking. I was hooked when I picked up the guitar at 16, then "cursed" when I started digging into the guts of an amp, trying to figure out how it all worked. My dream is to have a good tech, in person teach me the ropes. I have a decent knowledge of amps but far from understanding how it all goes together. I read several books but when I don't quite understand what I'm reading have no one to ask. I taught many kids how to use woodworking tools, machines, designing, you name it and they all had questions I answered. All I have is this forum and my dog, he may know more than me. 


Thanks for getting back and don't send the bias police,   



 

[sluckey]

There's nothing quite like having a knowledgeable tech in the shop with you. But really, you're doing very well. Keep at it and ask when you don't understand something.

I don't need to send the bias police. They see everything!   

[ginger]

Dude... it's really simple . The Global Moderators are pretty , well Global... spoon feeding  , is out of the question... learn to ice fish

[ginger]

I think what S.Luckey says is correct... he just has this amp/fishing thing

[EL34]

I think what S.Luckey says is correct... he just has this amp/fishing thing

Please tone down your post
I am getting reports from members about them

[dude]

I've been playing this SR pretty hard before I let it go, friends kid's first major small tour. Amp sounded great, put a new roach in, same problem, just need intensity on 9 or 10 but sounds good. I think the 50k RA pot might be shot, as nothing till 9 or 10, I'll replace it later.


But a new problem popped up, after a hard workout, I checked voltages again and the plate on one 6L6 was 30 volts higher than the other, wasn't before I drove it hard for an hour. I put set of Sovtek 5881's I had laying around, plate voltage came back to normal on both 5881s. I figured the tube just went, have no idea how old the JJ 6L6s were...?


I replace the screen grids, 3 watt 470  ohms (originals were 1 watt), all voltage down the rail match the schematic but about 10 volts higher, figured the higher wall current is the cause.


Anything, I should check out that might have caused the higher voltage on one tube beside just going...? The plates are now even at 469 volts each, B+ is 475v, I set the bias at 40mA.   

[shooter]

was 30 volts higher than the other,

I always go for the cathode 1ohmers next, the higher volts one should be lower current
next I swap tubes n see if it stays or follows.

[dude]

Thanks Shooter, don't know why I saw 30 volts  higher on one plate, maybe I'm losing it . You're right trying other tubes some were way out of a match and the current draw is different too. The one ohms are on the money in resistance. I put the 6L6 JJ back to see if the higher voltage followed the tube but now they're the same, played amp for a hard drive, still the same bias and plate voltage. Guess, I read the voltage wrong, didn't use clips just the probes, probably didn't have a good contact.


Delivered the amp to the kid, he was stoked, that was my pay and well worth my time for a friend. Here's a before and after pic of the doghouse with correct caps, thanks all for giving me suppliers that were fast delivery and inexpensive.


I muse a 1K5 first rail R rather than a 1K to compensate for the higher wall current.


Thanks for all the help on this one.

[Willabe]

Good job and a very nice thing to do for the young man and your friend/your friends son.