Bassman 5b6

[rp]

I still think deviating from a proven layout (and that was recently successfully built by a member) will only lead to more confusion. Every mod you make could add a bad solder joint, wrong value part etc, happens to me sometimes, if you compound things then you are really in the weeds. So again, I would make it picture perfect stock, and then shotgun each stage, it's a very simple amp so parts and time won't be anything too consuming. You are a better man than me as you you have a scope and know how to use it, can't you isolate the stage, then shotgun it? At this point I was going to suggest replacing the NOS Ruskies (maybe yours were made right after the 10:30am union vodka break) but saw you did that to 3 of 4 - but it's always the last one you know!

You don't need to clean the sockets, all that tube pulling did that already, you need to tension them. Be gentle.

What grid resistors did you use on the output tubes that you heard so clearly? 470 ohm? I ask 'cause I was going to put them on my 5C8 as I didn't think I'd hear 470 ohm.

[M Fowler]

Must be something else wrong not the amp, the speakers or guitar or home AC electrical system.

The circuit works great without power tube grids, screens,or any other modification.

Mark

[rp]

The circuit works great without power tube grids, screens,or any other modification. Mark

All those old tweeds work well, Leo built up from success to success. If those old tweeds sucked or had been troublesome there would have been no browns, blondes, or blacks.

[Matt D]

Thanks guys. RP, the screen grid resistors were 430. I heard the difference immediately. I tried a bunch of different guitars and different speakers. I haven't yet tried a different electrical outlet, though I've never had any problems with the one in my practice room. I'll try that today. The scope seems to indicate that the problem is in the output section, but there really isn't anything to troubleshoot. I've tried new tubes and adjusting the lead dress. Perhaps the topology of the amp is off in some fatal way due to the larger chassis? The cap can isn't measuring any leaks. I am using some more unusual guitars: Rickenbacker Bakelite, Fender steel w/ trapezoid pickups, and a Clinesmith with a Bigsby-style pickup. Maybe this week I'll start over from scratch with the Fender layout.

[Matt D]

Took it to a different outlet. Same problem. Clipped in a 20Hy choke between the first two filter caps. No effect. Spent a bunch of time with the Telecaster playing through it. Same problem above the 12th fret. When playing three notes at a time with the tone control at 9:00 it sounds like someone hit a gong. Down in first position the amp sounds gorgeous, simply stunning. Up the neck all kinds of garbage appears.

I took two sine wave generators, tuned one to a G# and one to a high E. Then ran each signal into one input of the amp. When I did that I did not hear a third note. Hmmmm.

Replaced the bypass cap on the preamp tube with a 33uf cap. Not the problem.

[Matt D]

Replaced the last Russian coupling cap. No luck.

[Milkmansound]

Did you double check PI resistor values?

[Matt D]

Hi Tim, I'm going to try replacing those this afternoon. One is something like 267k and one is 272k not off by too much but maybe enough to make it unbalanced?

Here is something I read about the paraphase PI on Merlin B's site:

"The two triodes could also share a common cathode resistor. No bypass capacitor would be requied since the two valves amplify out of phase, and this will help to compensate for any imbalances."

How important in this design is it that one side of the PI grounds above the 27r resistor and one side goes directly to ground? I realize this is really deviating from the proven design. Just curious.

[rp]

Here's something I tracked down, I remembered coming across it when researching my 5C8 build.

The 5D8 arrangement is easier to build, the value of the quoted "6.8K" in the 5C8 is a lot more sensitive to inbalance and can sometimes need careful tweaking. http://music-electronics-forum.com/t15681/

I didn't have any trouble but maybe you are unlucky w/ your 6SL7s?

[Matt D]

Thanks RP, I was really hoping that would solve my oscillation/ghosting problem. I took out the 6.8k resistor and went through a bunch of different values with my decade box. Changes there definitely changed the sound of the amp but nothing helped with the low ghost notes.

Last night I took the amp over to a friends house and used leads and alligator clips to clip in a potted output transformer to make sure the topography of the amp isn't causing problems. With the different OT clipped in the problem persisted.

This afternoon I replaced the PI resistors with perfectly matched carbon films. Didn't help.

[Cliff Schecht]

Matt, have you verified that all of the screws and nuts/bolts in the amp are firmly tightened. I've had this cause problems in a few builds in the past, especially if the loose screw/nut is for a pot or solder lug that makes some sort of chassis ground connection.

[Matt D]

Thanks Cliff, Everything is nice and tight, but I just double checked. This is a real head scratcher. Thanks to everyone for sticking with me this far and for all the ideas. The low frequency ghost notes happen at all volumes, not just when the amp is turned up. I'm thinking about converting the PI to the long tail design just to see if the problem persists then. Anyone have any suggestions for values? I know Aiken has a 6sl7 LTPI example on his site. I'll study that and see what I can come up with.

[rp]

I think you just won this week's "Stump The Chumps" your TAG t-shirt and mug will be in the mail.

[Matt D]

The only thing left is to build another one on a different chassis using the Fender layout and see if it does the same thing. I have and old Magnavox PT in the basement salvage pile that should have about the right voltages. I'll have to see what I can come up with for an OT.

[Blackburn]

I too am having an issue with my 5b6 (re)build in the form of the loud putt putt putt when the volume is at 11 o clock or so and I give the strings a hard hit. I go to stby and back on to stop it but I'm at a loss.

Suggestions?