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KT61 Equivalent - Ping Waveborne

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gingertube

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Joined 2004
#1
I am looking for replacements for KT61
I am aware that these are equivalent 6P25 / KT61 / CV1853 / 6AG6 / EL33

Any of the modern Russian Tubes suitable?

These are for a vintage suitcase style guitar amp which runs 6J7 pentode into a single ended KT61 for 4.5 Watts out. Cathode bias resistor 100R for 40 to 45mA idle, at which operating point gm is 10.5 mA/V. (Va = 250V)

KT61 are now VERY expensive - all of those TL10 owners must have snapped them up.

Any hints gratefully received.

Cheers,
Ian
 
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#3
Data
Heater voltage 6.3V
Heater current 950mA
Max anode voltage 275V
Max screen voltage 275V
Max anode dissipation 10W
Amplification factor * 27
Anode impedance * 2750 ohms
Mutual conductance * 10.5mA/V
Capacitances
Cge 17.5pF
Cae 0.8pF
Cag 1.2pF
*: At Va=Vg2=250, Ia=40mA

Octal Base
Pin Function
1 n/c
2 Heater
3 Anode
4 Screen grid
5 Control grid
6 no pin
7 Heater
8 Cathode

Application Data
Class A S/E Va = Vg2 = 250V
Vg1 = -4.4
Ia = 40mA
Ig2 = 7.5mA
S = 10.5 mA/V
Rk = 90 Ohms (my amp has 100 Ohms)
Zout = 6000 Ohms
Pout = 4.3 Watts
THD = 8%

Best I could manage, problem I found in looking through my tube collection for a sub is the 10.5mA/V @ 40mA. I need that sort of sensitivity.

Cheers,
Ian
 
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#4
Leaks still in using must be swapped to 6L6 or EL34 by now.

IIRC, only minor tweaking is needed (cathode resistor?). It's said the linearity of KT61 is superior than those replacements. I haven't heard myself, only seen once.
 
#5
Ian, I believe the only current octal valve with that kind of transconductance is the EL34. However it's heater requirement of 1.5A may prove a problem in your amp. All other octals with similar heater current to the KT61 (6L6, 6V6 etc) have rather lower transconductance. Have you tried Evatco in Qld for a KT61??
 
#8
The first valve amplifier I ever made (about45 years ago) was a mullard 3/3 in which I replaced the EL84 with a KT61 and the EF86 with an EF36 (based on what was available). It worked fine. (The transformers weren`t to Mullard spec either).

Barry
 
#9
Ian,

If it's acceptable to put in a noval-tube with a socket-converter: EL821/6CH6.
It's a video beam-tetrode.

Google for "EL821 KT61" and you'll find some claims that these valves have
very similar characteristics.
Sometimes you happen to find them very cheap as nobody seems to use them.

Kind regards,

Yves D.
 
#10
Seems logical to replace this with a 6V6 as it was commonly used in a lot of SE guitar amps, and no it probably won't sound exactly the same as a KT61 and you will need to change the cathode resistor to 270 - 300 ohms, otherwise it will plug straight in.

Transconductance unfortunately is somewhat more than just half that of the KT61..

(I actually just built a 6J7/6V6 SE stereo amp so it is fresh in my mind)
 
#15
It should work like a charm if to connect cathode legs of 9 pin female and octal male socket by some resistor, I believe it should be 20 Ohm, but needs to be calculated properly. Also, an octal socket would have some space for bypass capacitor, if you want to shunt that resistor out (for PP amps I would not do that!).
 

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#16
dupyv said:
Ian,

If it's acceptable to put in a noval-tube with a socket-converter: EL821/6CH6.
It's a video beam-tetrode.

Google for "EL821 KT61" and you'll find some claims that these valves have
very similar characteristics.
Sometimes you happen to find them very cheap as nobody seems to use them.

Kind regards,

Yves D.
Click to expand...

If you decide you want to try that Ian send me a PM - I have four NOS 6CH6 doing nothing at the moment.
 
#17
807tubenut said:
....
worst case scenario, you buy them, pay with paypal, they don't send them, you get a refund from paypal...
I have bought tubes from them, there fine. shill sellers don't have 1000+ feedback.
Click to expand...

I wasn't suggesting they wouldn't send them. I -was- suggesting they are almost certainly fake, and convincingly so. A few months in China (HK's factory/warehouse) will cure one of all naivety and replace it with a healthy dose of cynicism.

..Todd
 
#18
Thank you all for your replies.
I'm onto a couple of KT61 from two different sources - surely one will actually come up with the goods.

When the amp came to me for repair it had an EL34 plugged in. Unfortunatey who ever had done that did not change the 100 Ohm cathode bias resistor and so had cooked the output transformer. Probably fortunately since EL34 in SE will produce 10.5 W RMS and the amp has a 10W PEAK rated speaker fitted.

The Russian 6P9 looks a close substitute - if a little lower power capability, may have to get a couple of these for an experiment.

If I can't come up with KT61 then I'm tempted to just put in a 6V6 particularly as I have 20 or more 6V6G (the old coke bottle shape) which would look the part in this vintage amplifier.

Before doing my homework (and working out that it should have had a KT61) I went out and bought a Hammond 1625ESE Output tranny. I'll put that aside for a future EL34 SE Guitar Amp.

The owner of the little Maton Amp spotted an 845 sitting on my shelf and now wants an 845 SE Guitar Amp - BUT thats another story.

Cheers,
Ian
 
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