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Old 18th Jul 2020, 6:36 am   #1
Diabolical Artificer
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Default EZ80/1 wiring.

Been trying to help a bloke via email with hum on a valve guitar amp, after weeks of back and forth I noticed pins 3 & 4 looked connected in a pic, see att , this being cathode (3) and one side of heater (4). as the 80/81 is an indirectly heated rectifier I thought this wrong, however when this connection was cut hum disappeared as did OP of amp. I can only guess there's no HT, trying to remote fault find is tricky as you all know. Anyhoo, he found pics of two other amps wired this way. The schematic shows no such connection.

Yesterday I found a schematic of an amp with the connection, so apart from giving the heaters a reference, but wouldn't that be better wired to gnd? Something just clicked, heater cathode V max will be exceeded .But surely it's going to bung 6v of 50hz ripple on top of your HT, or should i say modulate your HT.

Valve rectification not being my strong point, what gives? Andy.
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Old 18th Jul 2020, 7:39 am   #2
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Default Re: EZ80/1 wiring.

The rectifier has its own heater winding. Thus the rectifier heater and its transformer winding could float anywhere and the heater would be lit and the separate cathode pin would supply HT.

As you said, heater-cathode voltage rating of the cathode insulation is a concern, so the heater-winding loop is made to float at HT by tying one end to the cathode.

There are still only two circuits at work. The heater and its winding loop, and a loop composed of the HT winding, one diode of the rectifier and the reservoir capacitor.

There is only one point of connection into the heater-winding loop so there can be no current in that connection. There is no loop with anything else. One end of the heater is tied to reservoir voltage (cathodes) the other bounces around with 6.3v with respect to reservoir voltage. The 6.3v here is NOT added to the HT on the reservoir because for that to happen, the cathodes pin would need to be wired to one side of the heater and the HT path to the reservoir would need to be taken from the other end of the heater.... or so you'd think?

Actually there are diodes around (the rectifier!) and the 6.3v heater winding will add to the HT winding voltage when one diode is conducting, and will subtract on the other phase when the other diode is conducting. This unbalances the half wave rectifier setup as if the centre tap wasn't perfectly central and the rectifier tries to drive the reservoir with big bump, little bump, big bump..... sequence. So the reservoir instead of having to contend with 100Hz ripple, now gets a 50Hz component. The cancellation of 50Hz component it used to have is spoiled.

But the circuit as shown in the schematic is fine. The cathodes pin goes to the reservoir, and the rest of the rectifier's heater has no other linkage to the HT loop.

The HT loop does have linkage out to other things. The -ve of the reservoir ( == the CT of the HT winding) is linked to chassis, and the positive end of the reservoir (== the cathodes of the rectifier) this is a two-point connection in the HT winding/rectifier/reservoir loop and so it can take current and there is a loop taking it.... the filter inductor/smoother loop. The smoother is grounded at one end and goes off to the HT distribution system at the +ve end. this is a further loop and is connected in two different places.

Network theory takes a complex circuit and breaks it up into loops. It looks for the passage of voltage or current from one loop into the next and to pass current requires there to be an electrical CIRCUIT a loop connection, meaning two different points.

The heater circuit has a one point connection to anything else, so though the loop works and powers the rectifier heater, it has no other influence. It keeps the stress on the cathode/heater insulation down so there is no current from this loop passing out of it anywhere.

You're getting into network theory and topology. Very dry when taught, very easily not paid attention to, and then later you find you need it and that it does have a use. So you sit down with a pad and draw yourself the loops, interconnected where they are and you look at them as that most basic of things an electrical circuit, and you silently thank Mr Kirchoff.

Another trick with a loop, even one embedded in a complex overall schematic is that a trip around it involves a net voltage change of zero. Much like a circular walk in even the most mountainous region means a net height change of zero. Your muscles may ache from all the climbing, but you are back where you started. So if you add up all the voltage differences in a loop paying special regard to the polarities, they all add up to zero.

This is the ground floor and foundations of how all those circuit analysis programs work.

I've spoken of loop currents, but voltages can't be defined without loops. Voltmeters need two probes. All voltages are with respect to somewhere and the voltmeter when connected adds a loop. If the voltmeter itself in very high impedance, there can be negligible current in that loop.

So, sit down and draw yourself some diagrams. Find the loops, find their points of contact. Locate the components whose current flow is shared by two loops. Network theorists talk of 'intersection'. Have a good pot of tea/coffee/beer handy.

*Everybody* nods off or doodles through network theory lectures. Those who find they need it, have to teach themselves, later. There is an exception, the sort of student who, when the lecturer comes in and says 'Good morning', writes it down!

David
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Old 18th Jul 2020, 8:48 am   #3
Leon Crampin
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Default Re: EZ80/1 wiring.

A perfect explanation of the theory. As stated above, the circuit shown is absolutely fine. On a practical note, I would add that the EZ80/81 valves are the one rectifier type for which I have never, ever seen a heater cathode breakdown. So, for the majority of applications in which it shares an earthed heater winding, there's also no problem.

I wish this applied to the infamous 6X5.

Leon.
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Old 18th Jul 2020, 9:05 am   #4
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Default Re: EZ80/1 wiring.

Quote:
Originally Posted by Leon Crampin View Post
I wish this applied to the infamous 6X5.
I think that if we analysed the DNA of the 6X5, we'd find that one of its ancestors was a firecracker.

David
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Old 18th Jul 2020, 4:35 pm   #5
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Default Re: EZ80/1 wiring.

Thanks David, as always well explained.

Andy.
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Old 20th Jul 2020, 7:03 pm   #6
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Default Re: EZ80/1 wiring.

Hi!

It has always been traditional practice with 4V and 5V octal & earlier–base rectifier valves for one side of the heater to be internally connected to the cathode, the original reason for this was the limited number of available base pins, but when the 5V octal–base rectifier was introduced, although the number of pins didn't mandate internal h/k connections in the electrode structure, this connection was retained because it allowed original tooling, etc., designed for 4V rectifiers, e.g., IW4/350, that had indirectly–heated cathodes, to be reused for the new 5V valves with only slight modification for the new heater voltage.

The exception to the rule that 4V and 5V rectifier valves always need their own heater winding concerns the French/Continental B8A Rimlock GZ40/GZ41 series – both of these two 5V rectifiers have indirectly heated cathodes with unconnected heater and cathode, and both these types are rated to withstand the maximum rectified output possible between their heaters and cathodes, allowing running off a 6.3V winding via a series dropper resistor.

The only thing to conclude this point with, of course, that where rectifiers of the 5Z4/GZ34 type are used, the h.t. output current lead should always be wired to the common h/k pin, usually pin 8 on these series of valves!

Chris Williams
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Old 22nd Jul 2020, 6:35 am   #7
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Default Re: EZ80/1 wiring.

Thanks Chris. The bloke's sorted his amp, turns out a new cap he fitted was duff, high ESR, so leaking = more current = more hum.

Andy.
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