LANEY supergroup 100 question

[andresound]

I have finally got around to refurbing this 69 SG 100. I see that the schematic shows the CT connected BEFORE the choke. I did mark cables before dismantling everything, and I am sure the CT was connected AFTER the choke. Which is correct?
Does the choke drop a bit of voltage? If so, that might be a good thing considering the HT in this amp (600v).
I also replaced the 470 screens with 1k (7w) to be easier on tubes. Is this OK?
Your input would be appreciated.

[martin manning]

That's the usual setup where "HT1" goes direct to the OT CT and the choke, and the screens are fed from the other side of the choke. There are examples where the power supply is "choke input," but I doubt an amp of this vintage would use that architecture.

[andresound]

Thanks Martin... As usual, you are always the first on the scene to help. You are a good man. Don't you work?

Regarding the 2nd question. Is the use of 1k instead of 470 OK? (When I got the amp, it had 10k on the screens!) And is there a voltage drop accross a choke. Excuse the idiot questions.

[Roe]

please measure the dc resistance of the choke. it makes a big difference whether it is 110ohms or 690ohms

[andresound]

Can I measure with it in circuit? (It is the original Partridge choke)

[Phil_S]

Voltage drop across the choke depends on the values of the various components, but there should be some drop. Chokes are different from resistors, but you can make a decent guess from measuring DCR from one end to the other. Remember, while it looks like a screen choke, the rest of the B+ ladder is also loading the choke, too. You might model it in Duncan's PSU.

I don't believe you'll see a meaningful difference between 470Ω and 1KΩ on the screens. Either should do the job.

[andresound]

Voltage drop across the choke depends on the values of the various components, but there should be some drop. Chokes are different from resistors, but you can make a decent guess from measuring DCR from one end to the other. Remember, while it looks like a screen choke, the rest of the B+ ladder is also loading the choke, too. You might model it in Duncan's PSU.

I don't believe you'll see a meaningful difference between 470Ω and 1KΩ on the screens. Either should do the job.

The choke measured 14.6 ohms in circuit and 15.7 out of circuit

[Roe]

seriously? Then you need big screen resistors if the tubes are to last. I guess at least 2.7k

EDIT: 15ohms doesn't sound right with a 20h choke suitable for 4xel34 (160mA?)

[andresound]

I'm not technical, but it does sound low (or wrong)!! That's why I lifted one leg to check for "out of circuit" DCR. I am using a Fluke DM, so measurement should be accurate. I also have tester a tech friend sold to me, which measures all kinds of components, and I got the exact same reading.

Maybe guys can chime in.

[martin manning]

With resistance measuring that low I wonder if it is shorted? Does your component tester measure inductance? You won't get an accurate read at low current, but it'd be another clue.

Don't you work?

Yes, but this is play!

[Phil_S]

With resistance measuring that low I wonder if it is shorted?

Yeah, I'm thinking this one should be at least 50Ω and possibly a few hundred Ω.

I'm no expert on winding transformers, but 15Ω doesn't seem like there is enough wire inside.

[andresound]

Thanks guys. This is sad news. I will have to get it rewound. Can anyone tell me what the specs should be. I looked on the Patridge/MM site and the they don't give a DCR. Only 20 or 30H

[andresound]

Here it is. Sorry about orientation

[Roe]

A RS style plexi choke might be suitable here. the high dc resistance will save the screen grids

[martin manning]

Another option is Hammond 194E AC30 replacement. It has the same general form (2-bolt U-frame with 3.56" c-c mounting), and 30H, 100 mA, and 500Ω DCR. http://www.mouser.com/ProductDetail/Ham ... Hi8Q%3D%3D