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'LIVEWIRE?' 1st Nov 2006 8:41 am

KT61 alternatives.
Is any problem likely to arise due to the substitution of a KT61 with a KT66 in an HMV1117 radio?

In theory, I suppose the O/P Transformer matching is incorrect, for one thing, since the KT66 has different ratings/characteristics to the '61.It's anode & screen current ratings are higher for one thing.

Ghostuser 1st Nov 2006 9:00 am

Re: KT66 as substitute for KT61
You can make almost any output pentode or beam tetrode work, provided that you change the cathode bias resistor to suit. Unless you actually measure the power output at full drive, you will probably not notice a degree of mismatch from the O/P transformer. However, you need to be careful not to overload either the transformer primary or the HT supply by drawing excessive anode and screen current. The easiest way to ensure this is to set the cathode bias resistor to give the same nominal HT current as the valve you are replacing. I would guess that you need to drop the KT66 anode current to about 40 mA. I think I had a U/S KT61 in one of those sets and just plugged in a spare 6V6. There was some loss of audio gain due, I presume, to a lower mutual condictance. Why use a KT66, when there are so many cheaper alternatives or when you can find a good used KT61 from our usual suppliers?


ppppenguin 1st Nov 2006 9:03 am

Re: KT66 as substitute for KT61
The KT66 is a much beefier valve than the KT61 so the valve itself is not going to come to any harm. The pinouts are the same. You will need another 300mA of heater current. The KT66 has lower gm so you may be a bit short of gain. You may need a bit more bias to get the anode current down to something sensible. That's probably the major worry if you just plug a KT66 straight in and end up cooking the output transformer. I'm not sure the output mis-match will matter too much since you are not trying to extract maximum output.

PS: Nigel replied while I was still typing. I agree with everything he says.

Tim 1st Nov 2006 10:15 am

Re: KT66 as substitute for KT61
Stick an EL33 in.(Or a 6V6 if you're desperate.)
The Mullard alternative will probably be much cheaper too, as anything with a KT prefix seems to attract a premium.

paulsherwin 1st Nov 2006 11:32 am

Re: KT66 as substitute for KT61
I agree with everybody else. KT66s cost a fortune and there's little point using one. If you have a 6V6, 6L6 or EL33 in the junk box try them. The 6V6 probably won't need any component changes but the EL33 or 6L6 may need the cathode resistor bumped up. If you need to buy a valve, a 6V6 is as cheap as they come and should work fine despite the lower slope.

Check the current after making the substitution.


'LIVEWIRE?' 1st Nov 2006 11:36 am

Re: KT66 as substitute for KT61
Thanks to Nigel, Jeffrey, and Tim for help. I must admit I never thought about using an EL33. I had a KT66 to hand, thats' why I checked that out. 40mA seems about right, so if I go with the 66 I will change the Cathode bias resistor. Will also check out< >

Skywave 1st Nov 2006 3:20 pm

Re: KT66 as substitute for KT61
Hi all.

Just a small contribution: KT66 specs.
My valve data book specifies for a single-ended class A amp.:

heater current at 1.27 amps,
Va at 250v, Vg2 at 250v, Vg1 at -15v, Ia = 85 mA, Ig2 = 6.3 mA, gm = 6.3 mA/V, and just over 7 watts O/P @ 9% distortion.
EL37 listed as equivalent.


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