No sound after new OT installed

[pbman1953]

HI all,


I'm installing a new Mercury Magnetics  OT in my Fender Studio bass head. I wanted to have 4 & 8 ohm capability. After turning on there was not a huge hum , as if I connected backwards but a level hum and no sound.


 I think I may the reason but 'll explain.


I'm using the Output / recording line jack hole for the 8 ohm jack. The original hole will be for the 4 ohm jack.


Taking out the recording line jack there are 2 wires. A ground and a line to the original positive speaker connection. I'm suspecting that the ground wire needs grounding and the other wiree needs connection. Before the install I cut them off and shrink tubed.


If that wire needs a home , and now I have both the 4 & 8 terminations, does that wire need connection to both positive terminals?

[Latole]

Schematic;

[pbman1953]

Yes, that the close up. But I removed the wires and resistor off that jack to use it for the 8 ohm connection. Am I getting no sound because the wires that were there are not connected?


The OT has a 4 and 8 ohm output wire. I sent each positive wire to its own jack


The left over wires were a ground and I guess a negative feedback wire?

[Latole]

Show wiring diagram of your wires connections and OT wiring and color
Wich MM  OT ?

[pbman1953]

FST-OM-4-8


7 total wires

Pri side
Red
Brown and Brn/wh
Blue and Blue Wh

Secondary side
Blk- ground
Green- 4 Ohm
Yellow- 8 ohm

[Latole]

It is your wiring ?

It must be like this;

[Latole]

Can't find any information about the primary, no sketch , nothing in the package ?

[pbman1953]

Exactly what i did.


2 jacks on $ (green) , another 8 (yellow)


In the past, I was told if you get a hum, the  OT (Brown and blue combination wires) were backwards. but would there still be sound?


If if they were really  would there be sound or no sound and only hum? Maybe compatibility wise I need to reverse them?


Off the OT I was matching the Fender colors- Browns to the Browns and Blues to the Blues

[pbman1953]

It's too  simple

[SEL49]

Maybe you have a jumper between sleeve and switch lugs on one or both of the speaker jacks. If so, remove the jumper(s).

[pbman1953]

No Jumpers, individual wires for 4 and 8 ohm.


Individual jacks

[pdf64]

What impedance is the cab you're using?
Use that OT wire, insulate the other tap, and retest for sound.

[pbman1953]

I must not be explaining clear enough


If you look at the MM sheet they sent me , there's a common, 4 (green) and 8(yellow) wires

The common are jumped to both jacks. Each positive wire goes to its own jack.


I'm only trying the 8 ohm to an 8 ohm speaker


The sad thing i that I did the exact same thing 3 years ago to a Fender Super Twin and it was the same exact output circuit. I can't remember what I did with the feedback wire.

[Latole]

If secondary wirings are ok , look at the primary wiring

[pbman1953]

If secondary wirings are ok , look at the primary wiring

In a  previous post, I matched the Fender wire position.


If were reversed would I get sound?

[pbman1953]

If secondary wirings are ok , look at the primary wiring

I decided to reverse the primary wiring per tube and the hum went away. I have a whisper of sound.


This must be something with the 2 wires that were on the output recording jack. One was a ground and other went back to the 820 ohm 1w resistor.


I'm guessing that one needs a proper home

[Latole]

Plug both output jacks as show in my sketch for test.

It is a easy way to check secondary, forget resistors

[pbman1953]

You see, 2 jacks. grounds are jumped between the ground lugs.


Yellow is the 8


Green is the 4

[bmccowan]

I hesitate to jump in, but I don't think anyone asked: did the amp work fine before you replaced the OT?
I see a clipped wire? Is that in play at all?
What did you do with the feedback wire?

[pbman1953]

I hesitate to jump in, but I don't think anyone asked: did the amp work fine before you replaced the OT?
I see a clipped wire? Is that in play at all?
What did you do with the feedback wire?

Thanks for chiming. The amp was fine before. The clip wires are only markers. I'll take them out after final wire cutting. I left the wires from the OT long.


The feedback wire needs a placement, where?

[Willabe]

The feedback wire needs a placement, where?

In the Fender schematic, where did Fender tap the -FB wire from the OT secondary?

[pbman1953]

It connects to both the recording output jack and the positive of the speaker jack




The schematic is listed earlier in this feed

[Willabe]

It connects to both the recording output jack and the positive of the speaker jack

No, I asked you where did they tap the -FB from? Where did Fender TAKE the -FB from? 

I didn't ask where it went to.

[Willabe]

I'm trying to help you figure it out for yourself. To see it for yourself.

The Fender schematic shows the OT secondary 8 ohm tap/green wire has a wire connected to it, tapped off of it, that goes straight up and then makes a left turn and goes to a 1w R, which is the -FB R, I can't make out it's value, then after that R it goes on to the LTPI.

So, the -FB wire is taken from the OT secondary 8 ohm tap.

The feedback wire needs a placement, where?

That's where the -FB wire goes, to the OT secondary 8 ohm tap. The OT 8 ohm secondary tap, green wire in Fender schematic, is the source of the -FB wires acv.

It connects to both the recording output jack and the positive of the speaker jack

Don't worry about the recording output jack, you took that out with the 2 R's, 3K3/270 ohm, that were connected to it. And you know what to do with the 8 ohm speaker jack's positive wire from the OT secondary.

[Latole]

Forget  all resistors , keep the 2 jacks wiring at minimum for test as show on my sketch.

You don't need those resistors to see if amp is working.
If amp do not work you issue is in the OT primary

Answer 5 and 16

[pbman1953]

The feedback wire needs a placement, where?

In the Fender schematic, where did Fender tap the -FB wire from the OT secondary?

On the schematic, the -FB wire , towards the top right of the schematic, comes from a 820 ohm 1w resistor. That 820 value is heard to read on paper.  Its connection goes to the 3.35k resistor which is connected to the positive of the output/recording jack. I've removed the output/recording jack to install a second jack to have both 4 & 8 ohm capability.


There's also a ground wire that was on that jack which I grounded. The -FB wire is not connected at this time. Th stock original connection was to the speaker out +


This is driving me crazy because I did this same change on my Super Twin with great results. At this point its either the new OT and I my rewire the old one just prove that, but it's hard to imagine the new OT being bad out of the box

[Willabe]

On the schematic, the -FB wire , towards the top right of the schematic, comes from a 820 ohm 1w resistor. That 820 value is heard to read on paper.

Yes, that's the -FB wire. So hook up that -FB wire with the new OT's yellow wire/8 ohm tap, at the 8 ohm speaker jack. 

But, you've got it backwards. It doesn't come from the 820 ohm R. 

The -FB acv is taken from the OT's secondary and 'fed back' to the LTPI.

The -FB wire is not connected at this time. The stock original connection was to the speaker out +

Yes.

So why any confusion, hook it back up to the 8 ohm speaker output jack +.

[pbman1953]

Morning,
After some thought, I remember off the jack was a ground that providing ground to 2- 33k resistors and a wire that was also connected fed back in to cap can. This must of been the ground for the storage caps. Please correct me if not


The amp works. I suppose I'd need to rebias.


the -FB is still not connected. Could I connect this to the positive terms of both the 4 and 8 ohm positive term? They will never be used together.

[Willabe]

The amp works. I suppose I'd need to rebias.

When did the amp start working? You said the amp has no sound.

the -FB is still not connected. Could I connect this to the positive terms of both the 4 and 8 ohm positive term? They will never be used together.

No.

That would short the OT secondaries together.

Only connect that -FB wire to the 8 ohm tap.

The 4 ohm tap has a different output acv than the 8 ohm tap. Fender figured the -FB from the 8 ohm's acv output.

[Latole]

The amp works. I suppose I'd need to rebias.


the -FB is still not connected. Could I connect this to the positive terms of both the 4 and 8 ohm positive term? They will never be used together.

Bias : always check.
-FB ; at only one output, anyone. The output where you plug the speaker have nothing to do with -FB

[pbman1953]

The amp works. I suppose I'd need to rebias.

When did the amp start working? You said the amp has no sound.

Yes, check bias.

It started working as soon as I reconnected a ground I spoke of in #27. Did you see that?

No, I missed that. Glad it's working.

the -FB is still not connected. Could I connect this to the positive terms of both the 4 and 8 ohm positive term? They will never be used together.

No.

That would short the OT secondaries together.

Only connect that -FB wire to the 8 ohm tap.

Ok, will do that

Edit; Fixed quotes, Willabe

[pdf64]

... off the jack was a ground that providing ground to 2- 33k resistors and a wire that was also connected fed back in to cap can. This must of been the ground for the storage caps. Please correct me if not
...

Photos / identifying them on the schematic is needed for anyone to  be able to do that.

[pbman1953]

On the re-connection and biasing, I have a hum. but, if I turn up the Output control just past 2, the hum is gone. Any thoughts on that?


Plus, the bias is a bit high . The tubes are running at 22-23 watt per tube for a 6l6. I'd like it lower.


This amp has  the Fender Output tube matching (balance pot) and thanks to the forum this amp has a bias pot, too.


A 10k pot with a 15k to ground, but the pot is pinned so I need less. What part needs the adjustment to lower the bias?

[Latole]

I never bias more than 60% often 50% power .Never never at 70% in class AB

6L6  or 6L6GC. ?

Hum; one tube issue ?

[pbman1953]

I never bias more than 60% often 50% power .Never never at 70% in class AB

6L6  or 6L6GC. ?

Hum; one tube issue ?

Actually, Philips 7581a

[pdf64]

...
This amp has  the Fender Output tube matching (balance pot) and thanks to the forum this amp has a bias pot, too.


A 10k pot with a 15k to ground, but the pot is pinned so I need less. What part needs the adjustment to lower the bias?

In the absence of the actual circuit you've got there, what can anyone do other than guess?
My guess is that the 15k needs increasing.

[Willabe]

Actually, Philips 7581a

Real NOS Philips 7581A's in a bass amp? 

[Willabe]

Plus, the bias is a bit high . The tubes are running at 22-23 watt per tube for a 6l6. I'd like it lower.

Further down you say their really 7581A's, those are 35w tubes and your already only running them at 22w/23w? And you want to run them lower than that?

A 10k pot with a 15k to ground, but the pot is pinned so I need less. What part needs the adjustment to lower the bias?

Like pdf64 said, go to a larger tail R on the bias 10K pot. You can also go to a 15K pot.

But your already running those tubes pretty low.

[Willabe]

After some thought, I remember off the jack was a ground that providing ground to 2- 33k resistors and a wire that was also connected fed back in to cap can. This must of been the ground for the storage caps. Please correct me if not

In the Fender schematic it shows 2 x 39K R's across those 2 x 220uF stacked B+ filter caps. Those are to force the same dcv across both of those filter caps. And those 2 caps/39K R's are the grounded together. Look at the schematic.

[Willabe]

... off the jack was a ground that providing ground to 2- 33k resistors and a wire that was also connected fed back in to cap can. This must of been the ground for the storage caps. Please correct me if not
...

Photos / identifying them on the schematic is needed for anyone to  be able to do that.

There 39K R's across the 1st 2 x 220uF filter caps, between the SS rectifier and the stand by switch in the schematic.

See above, reply 38.

[pbman1953]

Plus, the bias is a bit high . The tubes are running at 22-23 watt per tube for a 6l6. I'd like it lower.

Further down you say their really 7581A's, those are 35w tubes and your already only running them at 22w/23w? And you want to run them lower than that?

A 10k pot with a 15k to ground, but the pot is pinned so I need less. What part needs the adjustment to lower the bias?

Like pdf64 said, go to a larger tail R on the bias 10K pot. You can also go to a 15K pot.

But your already running those tubes pretty low.

Ya, forgot about the extra 5 watts in the equations. May not need to mess with the pot. What are your thoughts about the hum when the output control is all the way down but gone turned up a hair

[Willabe]

What are your thoughts about the hum when the output control is all the way down but gone turned up a hair.

 

Just don't play the amp lower than 2?

[pbman1953]

🤣, cool, I thought there would be something to check




Thanks to everyone!

[pbman1953]

I edged the tubes up to 23 watts and its whisper quiet`