JM Two Rock build - Almost there!

[beasleybodyshop]

Almost done with my build. Aside from a few squirrely bits, just waiting for the head cab from amplified nation (Looks amazing Taylor!)

Now that she actually fires up, i have an odd issue:

When i put a full set of tubes, I get very weak output. When I pull two and run it in 50 watts, I get sound just fine. I swapped in different sets of tubes and it doesn't matter what sockets have tubes in them - if i have one pair, i get sound. if i run the full set of 4, i get really weak, farty output; when I flip off the standby switch It gets loud for a moment and then settles into that wimpy sounding output. What could be causing this? Oscillation?

[xtian]

First thing I'd check is heater voltage with all four power tubes installed.

[beasleybodyshop]

Im getting proper heater voltage in both 50w and 100w configurations. Plates and screens are around 451v in 50W, 447v in 100W. ive double checked wiring and solder connections on all the sockets.

[Aaron]

Maybe the bias?

[beasleybodyshop]

Maybe the bias?

Bias is stable and working on all tubes. My thought is this might be oscillation - but if that is occurring wouldn't I see it on a scope? I've scoped the output in both 50w and 100w. It shows almost no output waveform with no signs of oscillation in 100w. My next guess is to inject a signal straight into the PI and bypass the pre, see if it changes.

That wire connecting the effects loop to the master volume does have to make a mighty long run to the front of the amp. Has anyone who has built one of these run into issues with this connection?

[xtian]

Stumper! What about the OT? When you've got low/bad output, have you tried alternate secondary taps?

[sunnydaze]

From the pics, it looks like there is some kind of stand-off attached to the chassis at the junction of the two 470 ohm screen resistors. The one for the power tubes closest to the fuse holder kind of looks like metal in the pictures.

Hard for me to see whether these are made of conductive material, and if so, are isolated from the chassis. I doubt if this is your problem - any kind of short here would probably cause a complete meltdown or at the least blow a fuse. Still kind of curious as to whether I'm really seeing a stand-off in that spot, and if so, what its made of.

Anxious to hear some clips from this build.

Thanks & Best regards,
Mike

[M Fowler]

Is that standby circuit wired correctly?

[sunnydaze]

I see you posted the issue on the Tube Guitar Amp builders group on Facebook. Looks like Nik of Ceriatone spotted the issue. Apparently, the power tube pairing on the grids was inconsistent with the pairings for the OT wiring. Is that correct?

Still curious about the material used for the stand-offs at the 470 ohm screen resistor junction. And, especially would love to hear.

Congratulations on resolving your issue.

best regards,
mike

[M Fowler]

Mike I'm sure Dustin is using insulated turrets like what I use.

[sunnydaze]

Thanks Mark. Haven't seen those before.

Would love to hear some clips of your JM sig, too Mark.

Best regards,
Mike

[M Fowler]

http://www.partsconnexion.com/solder_po ... denyo.html

http://www.newark.com/wearnes-cambion/5 ... dp/05C9717

I got mine years ago from another vendor but don't seem to have them anymore. http://angela.com/terminalpostsforcircuitbuilding.aspx

[beasleybodyshop]

Yes my connections from my PI to the tubes was backwards. Apparently a push pull amp won't work if you hook up one side of the PI to each side of the OT

I am using some isolated turrets that I bought from people here on TAG - I love to use them for prototyping and flying components across the chassis.

Also, they are good to use as turrets for turret boards. Just drill your holes and screw them onto the board using 4-40 screws. Mess up a turret location? Drill a new hole and screw 'er in!

[sunnydaze]

Glad you got it straightened out. Great looking build.

Those insulated stand-offs are a bit pricey but nice. Can certainly see how they come in handy.

Hoping to hear a clip soon!

Best regards,
Mike

[amplifiednation]

Looks like you figure it out...each side of 2 tubes in parallels should have the same grid connection to the PI. If you have the pair wired to both sides of the PI, phase cancellation will kill the output.