Replacement OT wiring for Carlsbro 50 Top

[cliffmustard]

Hi all,

I'm replacing an OT in a Carlsbro 50 Top and just wanted to double check the wiring before I go ahead. The OT is a Hammond 1750n (http://www.hammondmfg.com/pdf/EDB1750N.pdf).

Below is a labelled schematic of the amp and the transformer.





I think I have a rough idea but can anyone give me a heads up on colours to numbers?

On separate thread here, some also suggested removing the feedback loop, what kind of difference would this make to the sound?

Many thanks,

Cliff

[oc disorder]

Hi cliffmustard and welcome to the forum.
One side of the transformer numbered the other way made it more confusing.
When you first hook it up and try it don't initially fasten down or cut to length
the two red and blue wires that go to the output tube sockets.
If the amp shows any instability motor boating (sounds like) squealing or howling' - get it a recording contract.... um seriously can get loud.. swap the red and blue wires and then fasten them down permanently.
Murphys law applies if you fasten them down you will have to pull them up and reverse them and if you don't ... you won't !
The reason is negative feedback is used to help counteract any nasty distortion
(cancels out any bumps) and if the wires are reversed at the anodes the negative feedback becomes positive feedback and you can guess the rest...!

Once you get that sorted its a matter of trying the amp without any negative feedback by removing one leg of the 47k(? hard to read) resistor or disconnecting it at the 16 ohm tap.
It should work ok but some amps don't like it and can be unstable.
The amp then may seem to have more gain and a slightly looser feel.
A vox AC 30 for instance doesn't have neg. feedback.
Putting a pot in series is a popular mod so you can reduce it or go back to stock.
Hope it goes well ,never had any complaints from hammond transformer users
in fact usually the opposite !

[52 Bill]

Simply put
1-Violet
2-Orange
3-Yellow
4-Black
5-Blue
6-White
7-Red

And as OC said, the red and blue wires may need to swapped if the amp howls.

[cliffmustard]

Many thanks!

Thanks very much that's really helpful!!

I was just looking for the negative feedback loop and have circled it in the attached file. Is this correct? Looks like 68k, difficult to read though. If I was to add the pot mod would this filter off to ground to remove the negative feedback loop and when open let the signal continue to the resistor as normal?

[Alex R]

Yes that 68K is the nfb resistor. You'll see a wire coming from the speaker jacks that connects to the board on a pin connected to the 68K. Just take it off. Generally this makes these amps sound magnificent.

[52 Bill]

...If I was to add the pot mod would this filter off to ground to remove the negative feedback loop and when open let the signal continue to the resistor as normal?

No, add a high value pot in series with the 68K resistor so that as you increase the resistance of the pot, it reduces the amount of signal being fed back to the driver.

[oc disorder]

I'm unfamiliar with that particular amp.
From what Alex R. said it will sound magnificent with just the neg f/b disconnected.
This would make the pot mod not worth bothering with.
However "beauty is in the eye of the beholder" or magnificent sound is in the ear of the listener,so you could try it to see what it sounds like and get a feel for it at different settings with just a temp hook up without drilling holes etc.
Let us know how the new transformer works out.

[pdf64]

It's got a hardwired presence boost, some Selmar T'n'B had this.
A variable negative feedback control is a useful thing - a 500k or 1M audio pot would do the job.
Less or no NFB seems to work for me with open back speakers, but I've found that I prefer a lot of NFB with closed back cabs.

[cliffmustard]

oscillating feedback....

Hi guys,

Thanks for the advice, I installed the new output transformer with a 500k pot in the negative feedback loop. First time round when i turned the amp on and adjusted the pot I got a very high pitched feedback, so I reversed the power valve wires. Unfortunately now I get a low pitch oscillating sound that becomes very loud with the slightest volume adjustment.

I check the valves are seated properly, they are all new so shouldn't be faulty.

Hoped I'd have it sorted by now but any ideas?

Cheers.

[52 Bill]

Do you have a resistor in series with the pot or just the 500K pot?

[cliffmustard]

I placed the pot in series between the existing 68k resistor and the 16 ohm connection.

[52 Bill]

Only problem I can think of is lead dress.

Where did you mount the pot? Are the leads shielded and kept away from other signal lines?

[cliffmustard]

shielding

The pot is mounted on the front of the amp and the leads are unshielded, I'll try removing the pot for now and checking again with the standard negative feedback loop.

[cliffmustard]

I returned the neg feedback loop back to it's original position and I still get the sound, i think it might be an earth noise. Further troubleshooting to follow...

[Steve Conner]

Try disconnecting the negative feedback as was suggested. It might make the noise stop.

If it does, it probably means the amp is unstable because the new OT has more low-frequency phase shift than the old one. You either need to live without the NFB, or start messing with the sizes of the coupling capacitors in the PI.

Edit: I just noticed you said the volume control affects it. In which case, most likely dried up electrolytics.