Kat 6550 build (yes really!)

[Infinitely Baffled]

Hello, All. I wonder if someone can put my mind at ease about this. I am in the middle of building an old Kat 6550 kit that I have had for ages, and I am not too knowledgeable about the principles involved. You probably know the design employs 4 x 6550 valves and I am at the stage where I am wiring up the heater wires between the tag boards and the four valve bases. The instructions say to connect tag CL15 to pin 7 on each of the four valves bases, and connect tag CL18 to pin 2 on each valve base. That's clear enough. Because the two wires going to each valve base are to be twisted together I carefully used a continuity meter to ensure that I connected the correct wire of each pair from the correct tag to the correct pin. No problem, but once finished I did some testing with the continuity meter to ensure I had got it right. I found that all four pins 7 connected back to tag 15 (correct), and all four pins 2 had continuity back to tag 18 (also correct) BUT I was also getting continuity signals between the wrong pins and tags and also between pin 7 and pin 2 on each valve base. In other words, despite my efforts to observe the right tag to pin connections, it turns out that everything has turned out to be commoned up somehow. Looking back at the tag boards, whilst the soldering is not the highest standard, it is perfectly OK and I'm fairly confident that I haven't inadvertantly linked up tags 15 and 18. So I'm left wondering how the two separate circuits have come to be joined together. The only possible explanation I can think of is that the two heavy grey wires that run from the tag board back to the mains input transformer somehow join the two circuits. One of the grey wires is soldered to tag 16 (which is connected via a jumper wire to CL15) and the other to tag 17 (again, connected via a jumper wire to CL18). Now, I have no idea what goes on inside a mains transformer, but is it possible that the two grey wires and the transformer are somehow bridging tags 15 and 18, and with that the pin 2 heater wiring and pin 7 heater wiring? And if they are, is that correct, or is it going to end in tears when I finally switch on? Whew, that's a lot of information, and thanks for sticking with it to the end. What do you think - has something gone wrong here? Cheers and thanks. Infinitely Baffled.

[Phil Y]

I am not familiar with that amp but are you sure you are not just measuring the resistance of the transformer heater winding ?
They are low voltage and high current, so have only a small number of turns of thick wire.

Phil.

[bob orbell]

Hi, as Phil say's, if you have connected to the transformer 6.3 sec., you will read as you are doing, the 6550 heaters work on AC, so it matters not that they are all connected to exactly the same pin, it is, if my memory serves me correctly, what you have read refers to V1-V2 the rectifier valves, don't get this wiring wrong. BOB

[Richard]

Yes as Phil and Bob say, your continuity meter will buzz with a connection less than about 200 ohms. The 2 grey and 1 yellow heater wires are all connected to the heater winding inside the transformer with only a few ohms from the windings separating them. You might find this by switching to low range ohms but otherwise just check the connections physically

[Infinitely Baffled]

Thanks to you all. Your memory is indeed correct, Bob. The instructions contain a specific caution against getting the wires crossed in relation to valves 1 and 2, the rectifiers. That caution is not repeated in the section dealing with the heater wiring for the 6550s, but I simply assumed that the same applied.

Interestingly, given the cautious instruction regarding V1 and V2 I took especial care to ensure that V1 pin 8 went to V2 pin 8 and V1 pin 2 went to V2 pin 2. I am confident that I got this right - it is quite easy to check visually. Nevertheless, putting my continuity meter onto the respective pins of the two valve bases, I am getting a continuity signal not only pin2/pin2 and pin8/pin8, but also between pin2/pin8 and pin8/pin2 both across the two valve bases and even across the pins on the same base. This threw me a bit, since I don't understand why the big deal about not crossing the wires if they are all commoned-up at some point anyway.

I think perhaps the answer is that I do not really understand what I am measuring with my multimeter. I should probably have taken up collecting beer mats instead. Many thanks to you all anyway. Regards. Gary.

[Richard]

Hi Gary,

Your meter uses DC to measure the resistance of the connection between those heater wires. It will buzz with less than approx 200 ohms so cannot tell you where abouts the wires are connected along an AC transformer winding which has very low resistance.

The power for the heaters coming off the winding is AC. This means it "pulses" (rises and falls in voltage).

As it pulses the heater gets hotter and then cools again - very minutely - but enough to modulate the valve and cause a little hum.

If both valves in a push-pull pair have the heaters wired the same way around, "in phase", they will "pulse" at the same time and much of the resulting hum is then cancelled in their output transformer.

So the reason for wiring the heaters in that particular way is to lower heater hum. To do that we need to know the AC phase which cannot be measured easily by a DC meter and so we physically mark the wires

[Shane]

A transformer is effectively just two long bits of wire wrapped around a lump of iron. The symbol on the circuit diagram is a pretty good representation. Bearing that in mind, you can see that since cl15 and cl18 are connected to opposite ends of one of those wires (the secondary winding), your circuit tester sees a short circuit, because it works by passing a DC current between the probes. At DC the transformer windings act just like bits of wire. It's only with AC that the clever stuff starts to happen.

[Infinitely Baffled]

This is really helpful, fellows. Thanks very much, especially for taking the time to explain the very elementary things. Best regards. Gary.

[davebms]

hello all , would recomend that you do the FAQ - Common Problems mod
to the rectifier valves

dave