Carlsbro 60 TC power supply

[Ampere]

Hello, I just registered here since I wanted to ask for some help with a situation I have with a 74 Carlsbro Twin Channel Power amp.

Schematic: http://www.chambonino.com/carlsbro/60tc.jpg

I changed the filter caps for new F&Ts. The ones that are 50uF in the schematic were actually 68uF. Two in series without bleed resistors separated by a choke. I replaced these with 100uF/350V and put in 100k 1W bleed resistors across each cap. The one after the 22k dropping resistor was a double 22uF (parallel) and i replaced it with one 47uF/450V. The last one was also double 22uF which i replaced with a double 33uF, since that was the closest i had.

I put it back together after I had double checked everything and started to apply voltage though a variac and a lightbulb current limiter. I got all the way to 240 and after about 20 seconds (with power and SB switches on and no tubes) I see the 1k resistor that is across the choke flash and hear a loud snap. I't didn't smoke or even smell burnt, but It had opened up.

Questions:

1) Why did it blow? I'ts a 1 W carbon comp. And what is it's function? I had turned the power and stanby on before the cap job to make measurements.

2) Why are both the screen grids AND the OT center tap supplied after the choke? Usually I see OTCT voltage taken before the choke.

3) Are the 100 k bleeders ok. I read somewhere that they should be 50/C, so 500 k or 470 k for 100uF.

I'll send pics tomorrow if they might be of use as I get back to it.
Thanks


-Niko

[MarkusBass]

The power dissipated on the resistor was 250*250/100000= 0,625W. It looks like it is to much for 1W carbon comp resistor. I'd say that this should be 220k resistor and most probably 2W. Just try to put two 100k resistors in series.

Mark

[Ampere]

It seems to me that you are referring to the 100 k bleed resistors that i put in. But the one that failed was the 1 K resistor that is in parallel with the choke that was already in there. I don't think there is that much voltage dropped by the choke. Did i misunderstand your post?

100 k/1 W resistors are also used originally in my Bassman 135 across the 220 uF reservoir caps (285 V) across each.
http://www.thevintagesound.com/ffg/s..._135_schem.jpg

Niko

[Ampere]

And I see that I misread the bassman schematic since at first I didn't see that the mains transformer center tap is connected between the two 220 uF caps, and 285 is the cap voltage rating..

[52 Bill]

Welcome to the place.

The only reason I can think of for the 1K resistor to pop is that there was too much current being drawn through it. Check to see that the choke has not gone open and also double check the caps that you installed. There must be some reason for the high current with no tubes installed.

[Enzo]

Agree, with new caps and no tubes, there are limited things to burn that resistor. First among them is one of the caps after the resistor is backwards. or you have created a solder bridge from the B+ to ground.

The reason they took the center tap and screen supplies after the screen is because that is "better." You have smooth B+ after that CLC filter. IN many amps, they take the CT right off the first filter because it is CHEAPER. And the ripple on that first filter tends to cancel out in a push pull output stage.

Of course the question we must ask is this: did the amp work before you did this upgrade? if it did, your work is suspect. if it was blowing fuses or something to start with, then we need to consider the primary of the OT could be short to frame or to secondary, or a short elsewhere along the B+ wiring.

[Ampere]

One of the wires from the choke had its insulation damaged and I'm pretty sure it was the culprit since it's now working with a new resistor in place. Just carelessness on my part. The wire was under the board so it didn't catch my eye.



Thanks for your replies. I'm glad i joined this forum, even though this time I could've just paid more attention. Hope to contribute with something more useful next time

-Niko

[g1]

2) Why are both the screen grids AND the OT center tap supplied after the choke? Usually I see OTCT voltage taken before the choke.

The bulk of the current from the HV winding will go to the power tubes. To run that kind of current through the choke requires a very big expensive choke. The choke in your unit is probably much bigger than what you would normally see in a Fender or Marshall, isn't it?

[Ampere]

Thanks for clearing that out g1. That makes sense, and now I understand what Enzo meant by the other design being cheaper. Yes, the choke is bigger than what you would often see in most amps. It also explains why this amp hangs at an almost 45 degree angle when you pick it up from the handle.

This amp is also really quiet considering hum. Sounds really sweet too. A Marshall type of sound with great cleans to really good tight distortion. The limiter knob works well in dialing just the right amount of gain. This was my first acquaintance with this model (I was repairing it for a customer). Put in new TAD EL34's, it has a Philips 12ax7 (looks like long plate.. something?) in v1 and the other preamp tubes have a code which i didn't chek out yet. they look old too.. It really sings now. I think these should have more recognition among rock players.

[Alex R]

They are lovely amps. They often respond well to negative feedback removal - try lifting the 68k resistor that comes from the 15ohm speaker output.

[Ampere]

Thanks for the advice Alex R. I would try it if was my own, but since the customer didn't ask for any mods, and it sounds really good as it is, I guess I'll leave it stock for now.