Blowing fuse and hum from an effect loop

bluescaster · Post by **bluescaster** » Sun May 17, 2015 5:18 pm

The loop is after the tonestack (like i draw on the schematic) and i don't understand how it can send DC to the SEND ? There are 250p, and 2x22n caps to stop it !

YouTube · Post by **xtian** » Sun May 17, 2015 6:12 pm

bluescaster wrote:The loop is after the tonestack (like i draw on the schematic) and i don't understand how it can send DC to the SEND ? There are 250p, and 2x22n caps to stop it !

Well, that would indicate a bad cap, right?

Reeltarded · Post by **Reeltarded** » Sun May 17, 2015 7:12 pm

Until any verification of what amp I am talking about I am just wasting the fuck ou of my life on here trying to sift through bullshit.

Tried.

So common, as of late.

Good luck.

martin manning · Post by **martin manning** » Sun May 17, 2015 8:43 pm

Miles, I forgot about that "universal" preamp drawing showing a 1992N with the Hi-Fi tone stack and unused triode. I can't find a gut shot or layout, so it must be a pretty rare bird, no?

Reeltarded · Post by **Reeltarded** » Sun May 17, 2015 9:02 pm

Uncommon because Marshall was hated during those years.

I have gut shots only because of a recent odd discovery by a fellow forumite. I wasn't given any information of what the basis was then had separate conversations with two people.. one was calling the stock thing a mod and the other just hammeresd at the values..

The mod was a guitar amp in a multiple amp rig. I said, "Hey, that would have made a great bass amp....".. and then it was. hehe When I realized half of the mods were stock I laughed at it until I passed out.

back to add pic in a sec.

This is not my amp and I blocked out things because I don't have permission to share. It's nothing special though, we'd all agree.

Not realizing until later that the 100k plate was just hanging on a socket made hell out of decoding this.

bluescaster · Post by **bluescaster** » Sun May 17, 2015 9:10 pm

I'm sorry you don't understand me. When I got the amp, it had the strange tone stack and no cathode follower, like the picture you put here, but the PCB was ready to receive good components like a super lead or a classic super bass. So, I modify all the stages of this amp to have exactly what we have on the schematic that I put at the beginning of this thread.

English is not my main language, sorry.

Reeltarded · Post by **Reeltarded** » Sun May 17, 2015 9:18 pm

I understand that.

You might have a damaged board. These things are a nightmare in lots of ways.

You have 2 para stages and a 2nd stage with a CF then, just like a lead model. Without any pictures we are stcuck here without any clue what might be going on..

I am still not getting the 47k to ground before the send. I dunno what that...

Oh... are you using sheilded wire?

Marshall did on their loops... because it hummed like a UFO without it. That is the most congested place in the amp for stray fields and funny noises of all types. You can change the whole voice of the amp with a grounded spatula hovering over it.

PLEASE DONT HOVER A GROUNDED SPATULA OVER IT heh

martin manning · Post by **martin manning** » Sun May 17, 2015 9:29 pm

bluescaster wrote:All the wiring is like on this schematic.

If that were true, then you wouldn't be having this problem ;^)

bluescaster · Post by **bluescaster** » Wed May 20, 2015 3:41 pm

Hi, i have changed the loop like this:

[IMG:200:136]http://www.image-share.com/upload/2953/235m.gif[/img]

Since, the fuse don't blow and i have less hum.

Thank you for the help.

Regards

bluescaster · Post by **bluescaster** » Wed May 27, 2015 6:07 am

Hi,

I don't know why, but fuse blow again if i don't wait 30s between stand by off and power off. Ceriatone put 3A fuse on the primary of their 100W Marshall. I do it and it seems ok now.

On the other side, i have hum.

[IMG:200:162]http://www.image-share.com/upload/2960/107m.jpg[/img]

I got 0.1V pic to pic, 50 hz on speaker. If i just keep EL34s and PI it's ok. If i add V2 (cathod follower), Hum comes.

There is 180V on V2-b cathod. Perhaps it's too high ?

If i put the cathod follower V2b grid to ground instead of V2a plate, hum disappear.

Can you help me please ? Thank You.

martin manning · Post by **martin manning** » Wed May 27, 2015 6:27 am

With 180V on the cathode, you may have heater-cathode insulation failure on V2b. Try replacing that tube, and consider elevating the heater supply 50-70V.

pdf64 · Post by **pdf64** » Wed May 27, 2015 7:32 am

The schematic only has 2 of the power tubes having grid stoppers; maybe it's oscillating under certain conditions?
Best to fit grid stoppers, mounted at the tube socket terminals.

bluescaster · Post by **bluescaster** » Wed May 27, 2015 7:58 am

I tried several other tubes with no result. I'm gonna consider elevating the heater supply 50-70V.

Thx.

martin manning · Post by **martin manning** » Wed May 27, 2015 10:39 am

Elevating the filament circuit will help prevent heater cathode insulation failure in the future, but if you have tried different tubes and still get the hum it won't fix the problem you have now.

Is the loop tube in or out?

bluescaster · Post by **bluescaster** » Wed May 27, 2015 11:13 am

The loop tube is remove.

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