JMP 50W Lead/Bass Build

[bbaug14]

Working on a 50W with a Lead/Bass switch. Sourced most parts from Valvestorm with a few odds and ends at other places for personal taste. Marstran PT, MM oversized OT, etc. Its slow going due to a wife and two little girls, but it's coming along.

Made a little more progress last night. I'm short a few resistors, but I'll pick those up tomorrow. Once the back panel gets here, I should be able to wrap this thing up. Probably another month I'm guessing and then I'll be back in the 4-hole club.

[danman]

Nice work on the filaments! They are the most time consuming part for me!

[bbaug14]

Made a little more progress on my 4-holer project. Finished up the main board and wired in the Lead/Bass switch. I did it this way to make it easier to install when the main board goes in the chassis. The pots also arrived, so I installed those and routed some of the grounds. The back panel came in yesterday, so I'm hoping to do the fuses and IEC today when the kids take a nap. With any luck, I'll wrap this thing up in the next couple of weeks. (NOTE: Not sure why it's only showing part of my pictures, but if you click on them and select open image, you can see the whole thing.)

[bbaug14]

Finished the amp up today and I have some issues that I'm not sure what the cause is. I powered up with no tubes and all my voltages were about right, except for my heater to ground which is sitting at 2.8VAC. Across the heaters I was getting 6.1VAC and I read here that this isn't all that uncommon, so I figured this was alright. Installed some tubes, powered the amp up, and listened for hiss, and everything was great (although, there was more hiss on Volume 1 than on Volume 2). Installed power tubes, powered up, set the bias (my tubes appear to be unmatched as one was reading about 5mA lower than the other), and power down. When I switched the mains I got a loud pop that decayed like it was losing power. I powered up again and moved the bass pot and got a loud buzz/hum. Tried the same process but moved the treble pot and got the same result. Weird. Nothing plugged in and the Volumes are set to zero. Every time I power the mains down I get the same high pitched pop that decays like an echo. Any ideas????

[neikeel]

I do not see a PI filter cap wire up.

I see mains 50+50, I see screens and pre a mp but no PI?

Where do you choke wires go?

[bbaug14]

I do not see a PI filter cap wire up.

I see mains 50+50, I see screens and pre a mp but no PI?

Where do you choke wires go?

Sorry. In order to separate the PI and Screens caps, I installed a 3rd PI cap can on the top of the chassis. You can't see it, because it's under the board. I have the ground going to ground 3 and the output one side of the 8.2k (Yellow Wire) going to the PI filter cap.

The choke wires are going to the Screens cap can and the Heater Fuse.

One thing I've read is that you can have issues with the MM OT if the wires going to the power tubes are flipped. I thought I had them right, but if someone can verify I'm correct or tell me if my symptoms match this being wrong, that would be great.

FYI - IF YOU RIGHT CLICK ON THE IMAGES AND SELECT "VIEW IMAGE" YOU'LL SEE THE WHOLE PICTURE. I'm not sure why it's cropping them when in the forum. You can see the choke connections when you do this.

[bbaug14]

One thing I just realized is I have the OT impedance switch wired wrong. Basically, the MM OT I have is different than the other one I had in the past and I didn't note it. I will fix the wiring to make sure the switch is wired correctly. I suspect this is not the source of my issue though. I guess I really don't know though.

[bbaug14]

One thing I just realized is I have the OT impedance switch wired wrong. Basically, the MM OT I have is different than the other one I had in the past and I didn't note it. I will fix the wiring to make sure the switch is wired correctly. I suspect this is not the source of my issue though. I guess I really don't know though.

I was correct in my assumption. The amp still pops like crazy with a decay when the mains is turned off. No idea what's causing this issue.

[bbaug14]

Is it possible the primaries are swapped? The MM OT has it

Brown
Red
Blue

I have Blue to V5 and Brown to V4.

Someone also suggested maybe a bad main switch is arcing at shut down, but that wouldn't explain the loud squeal/hum when just turned on.

[bbaug14]

Is it possible the primaries are swapped? The MM OT has it

Brown
Red
Blue

I have Blue to V5 and Brown to V4.

Someone also suggested maybe a bad main switch is arcing at shut down, but that wouldn't explain the loud squeal/hum when just turned on.

Okay, I think I have this figured out. I measured the bias voltage, no tubes, at Pin 5 of V4 and V5 and got around +50VDC. This is wrong. So, I had the OT Primaries swapped. Rather than try to change the primaries (which would really suck since they were cut to size already), I just swapped the 5.6K grid resistor lines between the two tubes. This should have the same exact net affect as switching the primaries in a push/pull power amp such as this.

I do not think this will make any difference on the loud popping noise when shutting off the mains though, but we'll see. Probably won't get to more testing until tomorrow (kids are sleeping). If anyone has any other suggestions or sees anything wrong with my wiring, please let me know.

[danman]

Did you say that you had a reading of +50 volts on pin 5. The power tubes should be redplating if you are not seeing a negative voltage there.

[bbaug14]

Did you say that you had a reading of +50 volts on pin 5. The power tubes should be redplating if you are not seeing a negative voltage there.

I knocked it down a bit before putting in tubes, but I suspect they were redplating. I'm hoping now it'll be OK though. We'll see tomorrow.

[bbaug14]

I tested today after swapping the 5.6k connections to the tubes and all my issues are gone. All voltages are correct the amp is functioning properly. Very happy.

[neikeel]

Interesting as your symptoms of reversed primaries are pretty atypical (maybe your grid stoppers lessened the racket?

Usually you get a loud banshee like scream through the speakers and motonboating when you turn off, oh well, live and learn!

Glad it is fixed (why did you separate the PI and screen caps, BTW, first I have seen of this and do not understand the logic?

[bbaug14]

I wanted the screens and PI separate for two reasons, one being such that I can easily try different values in each location, but more importantly, so I could separate the grounds for proper Larry Grounding. It worked too, because I used 32uF for the PI and this thing is dead quite.