JTM 45 fixed!

[Reeltarded]

Haven't biased yet. Motorboating.

Pulled v1. Put put put put

Pulled v2. Put put put put

Pulled PI. No put put put put.

Layout is here: http://home.comcast.net/~mamp17/index.html

It's the BB layout and is that exact layout from the bias section of the board all the way back to the tone stack EXCEPT the 10k dropping string has an 8.2k at the bottom.

The choke is connected AT that end of the string. (The alt)

Grounds confirmed all around. Passes no signal but I think the jack I installed is bad. Physically doesn't like a plug. 470R in PI, checked not 470k as I have seen before

I have never had a boat. I'm going fishing.

[Reeltarded]

Reversed OT primary pair. Hoooooooooowwwwwwll..

What else besides filters and reversed OT does this?

[xtian]

Lead dress? Have you gone a-chopsticking?

[Reeltarded]

Yeah. My dress is way better than the Marshalls I have here. heh..

I am just glad it howls puts and doesn't eat fuses for the moment. Damn, this is what makes them ugly..

(shakes head)

[Reeltarded]

Power on ok. Stby flipped to run about 3 seconds then the put put.

Hmmm.. headache. Yay.

[Firestorm]

Mark your leads for phase. Move the in-phase ones around. Marshall and Fender got lucky. We usually don't.

[statorvane]

Found something on Metroamp forum that recommends installing grid resistors when using modern KT66 tubes. Supposed to help stop oscillation. It doesn't look like the BB layout shows these resistors.

Recommended value for the grid resistors is between 820 ohm and 5.6kOhm.

Here's a link: http://metroamp.com/wiki/index.php/Outp ... _resistors

That should help with any squealing, but motorboating, I'm not sure.

There's also this which indicates there may be a NFB problem, and how to determine and correct: http://forum.metroamp.com/viewtopic.php?f=15&t=32266

Hope this helps. BTW what transformers are you using. I am really tempted to build a 45 instead of an Express.

[martin manning]

Grid stoppers are definitely more about stopping HF oscillation. I think you have it isolated to the power amp, with FB going from power tubes to the PI. Any effect with NFB connected or disconnected? Motorboating is often power supply related, so maybe the decoupling from B+ to the screen node to the PI node isn't quite right. Are all your filters known to be good? And, what exactly does your power supply string look like? Metro JTM 45 (built to one of several variations of schematic published by Marshall) is:

Standby Sw, 32u, OT CT -> Choke -> 32u, Screen R's via 1k R ->8k2 -> 16u, PI Plate loads ->10k -> 16u, CF Plate, Preamp Plate Loads.

BTW, I can't believe there is any problem with adding 1 ohm between power tube cathodes and ground (described in the linked Metro Forum post).

[ToneMerc]

Grid stoppers are definitely more about stopping HF oscillation. I think you have it isolated to the power amp, with FB going from power tubes to the PI. Motorboating is often power supply related, so maybe the decoupling from B+ to the screen node to the PI node isn't quite right. And, what exactly does your power supply string look like? Original is:

Standby Sw, 50u, OT CT -> Choke -> 50u, Screens ->10k -> 50u, PI Plate loads ->10k -> 50u, CF Plate, Preamp Plate Loads.

I had started to reply, but Martin summed it up.

TM

[cbass]

Have you tried unhooking NFB

[Roe]

Have you tried unhooking NFB

+ 1

[Structo]

RG Keen has some great troubleshooting articles.
http://www.geofex.com/ampdbug/ampdebug.htm

Most likely a power supply problem, specifically the B+ nodes are not decoupling properly and a cap or caps are going high impedance.

So check out the caps and make sure you have the proper dropping resistors and amount of capacitance on each node.

[JazzGuitarGimp]

Try doubling the reservoir cap value.

[Reeltarded]

It has 5.6k grid resistors. Yes, I discoed the NFB and it ran for 30 seconds without put put then ate the HT fuse BUT the OT Pri. is correct.

I couldn't find a schematic match for my early reissue. This is a Metro board that was built when I bought it. I found two big errors (1 was bus wire dropped in NFB exit instead of top of 10k dropper) before I started.

I used SDM's layout archive, and Metro JTM 45 instructions to figure out the power parts. They obviously don't match the original ways of doing things because of switches wiring and wiring layout from PT to rectifier to board. I can easily see this being a power supply issue.

Thanks guys. I have a couple of hours to do here on other stuff before I jump back in, but I think checking values on other board components and mapping out a workable power supply in in my near future. I will check back in here asap with what I do. I am excited that it works at all. put put put put put put..

You're all great, and fun too.

[martin manning]

Deleted. Damn cell phone likes to post things multiple times.

Whew! I was having some serious Deja vu there!