JTM 45 fixed!

[Reeltarded]

-sigh- It is making terrible sounds with signal as if a cathode was loose, like a metal man talking.

So I checked the bias at the 1Rs and they are both reading the same now. Unfortunately they read nearly ZERO. I have lifted the board looking for trouble but none can be seen so far. I am suspicious of this trimmer.. I really am.

If it isn't fun at least it keeps me off the streets.

[Firestorm]

Zero??! Bad cathode ground? What's the B+ at the plates? What's the B-?

[Reeltarded]

I have a long day tomorrow and a pile of chores so alas I bid the amp farewell for the next 36 hours and then back to it.

It sure sounds awful!

Mbrrrbrebrrbrbrerbrrbrbrrbrrbrr.. and that is the high E. it is in the power section. What can break that can't be seen in a low impact jolt? Damned. It was working last PM again. S'ok.

Thanks for all the guidance, everyone. Salute!

[Reeltarded]

Ok. Rebuilt output sockets. Checked ever ground to every other ground. Looks good. Installed new bias pot (much better action!) heated the amp up and started checking.

No bias reading on either output socket @ 1Rs.

I looked at the tubes. They looked kind of dark. I plugged in and it sounded like a poor joint on an early cathode.. but no output hiss. It only made sound if I thrashed the guitar. Like an octavia with a dead battery even..

SO.. I started turning the bias pot toward the positive. I got a .1 then a .2 then a .3 and when I arrived at a .4 reading at the 1R, I played.

The output is in cutoff when the amp tells the meter it has a .4 mV condition at the bias reading point.

This board had some obvious mistakes when I bought it built as a JTM45 RI replacement. I think I should remove the board, rebuild the bias section, and run all new bus wiring if for no other reason than to set my mind at ease, that is if you don't have some great idea to talk me out of it.



I have much patience. All the voltages are just as described earlier. Even with the GAIN at 0, it passes some signal, enough to do the cutoff sounding output with the output at around 3. Increasing the gain helps to push enough signal so it just sounds like it is in powerstage cutoff for 2x as long. Readings make no sense.

Dunno. I read everything about everything and I am still unsure. Should I suspect a problem with this brand new output transformer? I don't know. You tell me.

[martin manning]

Filament voltage is good? Before you rip it apart, check the voltages in the bias circuit starting at the PT (at the front panel side of the 150k, AC volts there should be the same as HT secondary), then after the 150k, after the diode (DC), after the 15k series resistor, after the 56k, after the trimmer.

[Colossal]

Filament voltage is good? Before you rip it apart, check the voltages in the bias circuit starting at the PT (at the front panel side of the 150k, AC volts there should be the same as HT secondary), then after the 150k, after the diode (DC), after the 15k series resistor, after the 56k, after the trimmer.

Martin,

I spoke to Miles about this last night and we talked about the same thing; going through every point in the bias circuit, testing AC and DC voltages to verify the assumption that the bias circuit is working as expected. He's getting raw AC off the secondary, and voltages check out from the entrance through the bias circuit and terminate in a range at the pot of -33 to -55V. So with the tubes pulled, the amp can provide the appropriate negative bias voltage and range. It seems that once the power tubes go in, the grids start to go positive, it spirals towards runaway, eventually taking out the B+ fuse.

Dave

[cbass]

So are both tubes "running away" ?or just one?

[Structo]

Known good tubes?

Check bias splitter resistors (220K?)

grid resistor values?

Bad PI caps?

[Reeltarded]

I went methodically back through it.

There is no real runaway in all conditions. The tubes change out of cutoff by adjusting the trim. It doesn't blow HT constantly, only twice ever, so far. Brand new Sozos in the outs. I stripped the 1Rs and retightend the grounds before disassembly.

I removed the board and decided to wire in the screen feed 1987 style because I am more familiar with that plan. The BBRI circuit has caused more than one person a lot of headaches. The only thing I am unsure of is having less filter sections because I am not good with the power side of things, yet.

The 1987 has 6 sections, the 45 more commonly has 4 in this configuration. I am using 50uF at the mains instead of 32. I am using 32 instead of 16 at the front end. I just need to get that figured out and reinstall the board after swapping screen and preamp bus wiring sides of the dropping string. (When I get back from spanking a client's pooputer)

http://home.comcast.net/~mamp17/index.html

^^^^ Steve Miller's excellent layouts to show the differences I am talking about. 1987 vs BBRI if anyone cares to see what I mean.

Only one thing different than the alternate screen layout. I will have an 8.2 back on the bottom of the dropper. NO TREM IN THIS AMP just a dead socket waiting for an arrangment.



Thanks, yo.

[Reeltarded]

So are both tubes "running away" ?or just one?

Everything looks generally dark. There is no evidence of a runaway condition other than it is what you'd expect. Tubes work as expected in Petey, the mule 50wttr. Petey freaking blows like Coltrane on a freight run... Wooooowooo!

[cbass]

Try dropping it again see if that helps.

[Reeltarded]

Like I hat'n already, DUH!! It just dug a new hole in my floor.. shucks.

lol grape minds tink a link.

[Structo]

Does your amp look like this?

[Reeltarded]

If you want it to, sweetie! $20 will make you hollah!

It's almost back inna piece, just preamp dressing and back to voltage fun-fun.

[Reeltarded]

It looks just like this.