6D22S conversion for KAT6550

[bob orbell]

Question for John/Richard or any one that can help. My chums KAT6550 has one 5U4G that is intermittent, resulting in hum from time to time, I am looking to may be covert to the above and have studded the FAQ. One thing puzzles me, why does the 6D22S have the HT connected to the heater circuit ? unlike the 5U4G, it has individual pins so surely the HT need not make contact with the heaters. Help please. Bob

[bikerhifinut]

Originally Posted by bob orbell

Question for John/Richard or any one that can help. My chums KAT6550 has one 5U4G that is intermittent, resulting in hum from time to time, I am looking to may be covert to the above and have studded the FAQ. One thing puzzles me, why does the 6D22S have the HT connected to the heater circuit ? unlike the 5U4G, it has individual pins so surely the HT need not make contact with the heaters. Help please. Bob

This is a standard practice Bob.
Study any of the classic valve circuits and you'll see what I mean.
certain Rectifier Valves were specifically made with good heater to cathode insulation so that you could use one heater winding for All valves in a circuit.
I only really Know of valves like the EZ81 and EZ80 usually found in old radios and domestic Audio likerecord players etc.

So, usually a Quality item would use a separate heater winding, in the case of valves like the 5U4 it's 5V as is the GZ34 and other indirectly heated Cathode rectifiers.
We have an additional issue here, in that higher powered amplifiers will operate on high HT voltages of 400 plus Volts and more. This puts a real strain on the heater/cathode insulation, so the way to eliminate this problem is to ensure there is no potential difference between Heater and cathode. Hence we tie one side of the heater to the cathode.
To add to this, even with an EZ81 and given that there's a separate heater winding to enable it, I'd tie the heater to the cathode. Be nice to your valves.

That's kind of it in a nutshell Bob.
Anyway what's wrong with just getting a new 5U4? I'm not keen on em but its not a rational dislike assuming you aren't stressing it with too much capacitance directly off the cathode. Complicated issue here as things likethe internal resistance of the HT windings and any resistors in the rectifier circuit will alter the way a rectifier "sees" the reservoir capacitor.

regards
Andy

[bikerhifinut]

Had a quick look in my old Mullard Data book.
Can anybody tell me why you can't substitute a pair of GZ34 in the KAT6550?
1 x GZ34 is well capable of firing a pair of KT88 in a cathode biased configuration.
You get the advantage of a couple of amps less heater current and it will take up to 60uF as first Capacitor.
There is the issue of a lower Vf which means the HT will be a fair bit higher. A series resistor will take care of that assuming you can get a good high wattage one and you could even add an extra RC filter as a tweak?
Just an Idle musing.

A.

[bob orbell]

Thanks' for the reply Andy, quite a simple fix, we got two new JJ valves from ebay, £50.00, not bad considering nothing has been done to it for 22 years. Bob

[John Caswell]

Hi Andy
Absolutely no reason at all except GZ34 are expensive because the audiophools have given some of them magical properties.
Mullard usually quote a minimum anode resistance, so measure one half of the tx then add a series R from tx to the GZ34 anodes to make up this minimum. It also helps to even out the current to each anode. I use 25 Watt metalclad resistors bolted to the chassis in my amps for exactly this purpose, except I use 6D22s or EY84 as the rectifiers.

John