Original JTM45 oscillation??

dobbhill · Post by **dobbhill** » Thu Mar 24, 2011 9:31 pm

I've got an original JTM45 w/tremelo (like Bluesbreaker) that has what I believe is an oscillation problem. It manifests itself when the amp starts overdriving as a buzzy extra note riding on top of the original note or tone. It sounds great clean. Is there a specific area to target on these old amps' layout?
D

pdf64 · Post by **pdf64** » Thu Mar 24, 2011 10:05 pm

Have you got grid stoppers on the output tubes? Pete.

Phil_S · Post by **Phil_S** » Thu Mar 24, 2011 10:08 pm

I think you should try substituting known good tubes one at a time. I had that problem. It turned out to be a bad power tube. It was at a particular frequency that I got a double tone and it was very annoying. I don't think that amps are likely to develop an oscillation problem late in life.

Other things to look for are parts that vibrate because they just don't stay where the assembler put them when the cab vibrates. I'll for caps especially.

ToneMerc · Post by **ToneMerc** » Thu Mar 24, 2011 10:11 pm

That issue is called "ghosting" and it's not uncommon even with new builds.

How original do you want to keep it?

Here's what I would do.

1. Considering it's age, I would replace the power supply electrolytics first.
2. Increase the filtering on the screens.
3. Install a larger value choke. I'll assume it has a 352-114, which is 3H. A 7H would be good median value.

TM

Reeltarded · Post by **Reeltarded** » Fri Mar 25, 2011 9:21 am

Your amp needs a cap job. You should change the values of the filtering, as stated above me.

People who use much gain notice it at some point. It's loud as Hell, makes me insane.

dobbhill · Post by **dobbhill** » Fri Mar 25, 2011 2:38 pm

I have replaced the power supply caps with original value JJ caps. The tone I'm whining about is a gritty, buzzy anoying sound riding on top of original note, not really a double tone as I understand Ghosting to sound like.
I would like to keep it original as possible. It has a GZ34 (Mullard) rectifier and I have swaped tubes with known good/tested until I wanted to throw something. I really don't know how long the amp has had this problem, as I haven't used it for distorted tones in quite a while.
I did try 5.6k grid stoppers on the power tube; didn't help.
Thanks much for all the replies. I need to record it and take some photos for you.
D

Firestorm · Post by **Firestorm** » Fri Mar 25, 2011 3:10 pm

Take a look at Ken Fisher's tips for Marshall lead dress. http://ampgarage.com/forum/files/twreck_pages.zip

Pages 18-19 (original pages 36-37) is the pertinent section. In particular, check the grounds, especially ones made to the back of the pots. You may need clean up the chassis behind the pots to make sure the connection is solid.

When you take a look inside, could you try to determine where the choke is electrically connected? The old Marshall schematics show a 32uF cap, then a 20H choke, THEN the OT CT. That's not where it would be if Ken Bran was copying the 5F6A circuit verbatim as everyone says he did. Just curious.

dobbhill · Post by **dobbhill** » Fri Mar 25, 2011 3:46 pm

Firestorm:
Thanks! I'll do some lead dress homework this weekend. Mine is about '67 vintage and the choke is after the OT before the screens.
D

paulster · Post by **paulster** » Fri Mar 25, 2011 6:23 pm

http://forum.metroamp.com/viewtopic.php?f=4&t=19696

It's a long read but Raindall Aiken suggests a way to identify if this is what you're suffering and a way to resolve it with a simple zener clamp that could always be removed to put the amp back to stock.

I wouldn't increase the filtering too much on a '45 or you can lose the organic feel of the amp, but you certainly want to at least be running new electrolytics so they will be at specified value rather than dried out ones that will be well below.

Cliff Schecht · Post by **Cliff Schecht** » Fri Mar 25, 2011 7:03 pm

Paul I thought that problem only occurred on cathode-biased amps..

paulster · Post by **paulster** » Fri Mar 25, 2011 7:38 pm

No, it occurs on fixed bias amps as well.

I've been too busy to go through the whole thread to pull out the salient points this morning but it's in there and as usual Randall Aiken delivers the goods!

pdf64 · Post by **pdf64** » Sun Mar 27, 2011 8:59 pm

How are the screen supply resistors arranged on your JTM45?
Like a regular plexi, 470 or 1k per screen grid, fed from the choke output node?
Or like the Iss7 of the bluesbreaker RI, with a 1k from the choke output, then splits into 2 470ohm resistors to the screen grids
http://www.marstran.com/1987T.gif
Thanks - Pete.

dobbhill · Post by **dobbhill** » Sun Mar 27, 2011 9:34 pm

Mine has a 1k? Off the choke, then splits into 1k for each tube. They have been replaced.....maybe I should use the original values?? It is a JTM 45 MKIV with tremelo in a head box.
D

pdf64 · Post by **pdf64** » Wed Mar 30, 2011 9:20 pm

So, for the screen supply, that's three 1k resistors in total?
Thanks for the info - Pete.

Mikka · Post by **Mikka** » Wed Mar 30, 2011 10:18 pm

Did you try new valves ?

The Amp Garage

Original JTM45 oscillation??

Original JTM45 oscillation??

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