Blowing fuse and hum from an effect loop

[bluescaster]

Hi,

I'm working on a 1979 Marshall SuperBass 100W for a friend. I change all the old caps and clean sockets. My friend says that since somebody add loop effect in this amp, there is hum and Mains fuse (2A) blows when you turn the amp off.

If, I pull off the loop tube and bypass the loop (Treble whiper straight to 22n PI), all the problems disappear.

For hum: All the grounds in the loop goes to the send jack and then to the ground star of the preamp. It's seems ok for me ?

For fuse blow: I don't know ?

this is the schematic:

[IMG:200:136]http://www.image-share.com/upload/2949/130m.gif[/img]

Can you help me please ? Thank you.

[Stevem]

A mains fuse popping when you turn the amp off is strange to say the least , have you seen that happen while you have been going through the amp?
If you have had it pop that fuse on you then if you pull out the HT fuse and power off the amp does the fuse then still blow?

By the way does the amp have the spare heater current to power the effects tube, is the heater voltage still above 6.1 volts?

[JMFahey]

It "shouldn't" blow although it *might* be a marginally rated fuse and the turn off "pop" blows it.\\Remember it *must* be a slow/delayed/"time"/slow_blow fuse, not a standard one.

Standard fuses have a plain piece of wire inside, slow blow add mass so it takes longer, the filament may be spiral wound, have 1 or more solder "blobs", have a small strip of lead and a spring, etc.

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[img]http:292:355]http://www.lakesidepinballparts.com/fuse.jpg[/img]


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[img:281:175]http://i1238.photobucket.com/albums/ff4 ... 1/Fuse.jpg[/img]

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[img:500:605]http://www.mojotone.com/Fast-Acting-Fus ... -image.jpg[/img]

Check the label besides the fuse holder, I'm quite sure it specs a "T"-something fuse (say: T2A) .



That said, that's not "just an added loop", he also added an extra gain stage+cathode follower, , that will be an extremely "hot" Plexi type amp and hum is to be expected, since he raised gain around 20X or more.

As an example, Soldano does add a loop recovery gain stage, but previously pads down normal signal 20X , so it evens out, that was not done here, don't know if on ourpose or for lack of experience.

[bluescaster]

Hi, thank you to help me,

I think i don't use slow fuse, so i'm gonna to try one.

if you look at the schematic, there is a 47k at input of the loop, and you can see with duncan calculator that this add a 15db loss. Perhaps it's not enough, so i will replace the 47k with a 25K pot .

I tried to pull off the cathode follower : Hum is always here but fuse doesn't blow ! Strange.

[Reeltarded]

Do you have a schematic of the added circuit?

This amp is supposed to be plate drive with an atypical tonestack (bax/james)..

I believe you are working from an incorrect scheme to the model. The next couple years are different.

I think you are in the middle of the tone circuit or something.. there.. even a sketch would help.

[Reeltarded]

I was thinking... you could have a bad jack in the loop..

Have you tried terminating the treble out to the send jack and terminating the return jack straight to the PI? Simple loop, no buffer no recovery.

It's gonna be hairy.. but if that works we'll have another clue. Amp should operate as stock with a cable from send to return..

Did that work without hum?

After you try it with the tube bypassed and verify that works reboot the amp with the tube in teh socket but still unconnected.

Does that hum?

[bluescaster]

This amp had a strange tone stack and no cathode follower. I though someone modify it so i change wiring to have what you can see on my schematic. Do you know where i can get the original schematic ? Which one is best for guitar please ?

i'm gonna try your idea for the loop. Thank you.

[martin manning]

From Dr. Tube pages: http://www.drtube.com/en/library/schema ... ll-schemas

Scroll down to:

1992 JMP Super Bass, 100W head
This amp was in production from 1967 up to 1981.
GIFAmp schematic with 3x ECC83 & 4x EL34, 70-13-11 (Unicord, 1970).
GIFMk2 amp schematic with 3x ECC83 & 4x 6550, 70-6-11 (Unicord, 1970).

[Reeltarded]

I had no idea this model existed until a few weeks ago myself.

Don't be confused by the 100k on the socket, they used that as a tie point so it pretends there is a CF, but there isn't. I wonder if somehow that worked to get DC to the effects send from using the wrong schematic.

It will take 3 huge coffees to draw that conclusion.

A picture would really help.

[bluescaster]

what picture do you need, i draw the loop at the bottom of the schematic in my forst post.

[Reeltarded]

I want a picture of your tone controls from inside just to verify which amp I am looking at, because I think we are seeing a scheme for the previous model and I am very confused about where this loop is and if it has anything blocking DC on the send side and what the 47k is added to ground from the signal path is supplsed to be doing.. be right back.

[bluescaster]

All the wiring is like on this schematic.

[Colossal]

Here is the preamp schematic for the later 1986 and 1992 Super Bass models. Note the James tone stack, not the earlier 1970 Unicord standard Marshall, cathode follower driven tone stack.

[Reeltarded]

There is a 100k tied on the v2 socket.. that is not a CF. it's a plate for the B side of that tube. the pin is a tie point only.

We must verify which amp you are actually working on.

If it's the later model that is should be, you are pulling down the signal with that 47k and sending DC to the send jack on the loop... as far as I can tell.

One picture would make this almost easy.

[Reeltarded]

Look, plate driven. No CF.