KT120 Tryout as SE

[JamesD]

Neat!

I particularly like having nearly twice the current on the first stage rather than the driver - nice way to get the dc voltages to work and sort of twists convention on its head

J

[Cressy Snr]

Neat!

I particularly like having nearly twice the current on the first stage rather than the driver - nice way to get the dc voltages to work and sort of twists convention on its head

J

Aye, it worked out quite nicely doing it that way round and a further bonus is that both plates are dissipating just about exacly the the same power.

[Cressy Snr]

Ok,
I have converted the KT120 SEUL to use EF184 pentodes as input/driver valves.



As you can see by the pair of OA2 glow tubes, I am running the drivers in pentode mode with screens regulated to 150V with 190V anode.

First impressions are favourable.
There appears to be more detail in the high frequencies than with the old Brit triodes and the amp is quieter, in fact it is completely silent when idling, even with an ear to the speaker, easily the equal of the push-pull amplifier when it comes to S/N ratio.
My main concern at the moment is that the tonal palette seems a bit bleached. Bass tone is as good as the MH4 but the mid seems a bit less rich and rounded. I'll bet it's H3 making itself known, but without a spectrum analyser I can't say for sure.

For information, I'm using Pete Millett's settings for the pentode driver as detailed here:

http://www.pmillett.com/pentodes.htm

See pentode mode...6EJ7/EF184 for the area they are working in.

I did not expect these cheap pentodes to equal to the MH4s but they are not bad. As you can see in the above picture, the B5 bases are still there

One thing it does do with the pentode driver, is kick total butt.

Hmm..... pentodes driving Ultralinear finals? That smacks of too much of the same characteristic, I'm going to try the KT120s as triodes and see what happens.

[IslandPink]

This is all very subversive Steve !
Now, how difficult is it to drive a KT120 in SE-UL ?
I mean - is it more like a triode than a pentode in terms of Miller capacitance etc . ?
Only the EF184 pentode only has 2.5 W dissipation, is it running 10mA anode, or less ? Might be a bit weedy, especially with 12k load or whatever . I had poor mid tone from lower-power pentodes driving the 300B , eg. ( especially ) the WE310B variant with the low-current 4328D's . I know it takes 17 or 18mA to sound really good when driving a 300B with a pentode.

[Cressy Snr]

The MH4s were only running 4.65mA into the KT120s operating in UL mode.

The EF184 is running at about 11mA.

I've just finished converting the outputs to triode mode and we are now back in MH4 territory regarding mid tone....wonderful

I had a hunch that would be the case, the harmonic blend was incorrect with what was in effect, pentode driving pentode.

Before it was triode, driving UL. That setup gave great tone and ease of listening.
Now we have pentode driving triode, and hey presto! the same good tone, same characteristics as we had with the MH4s.

It is now perfectly livable with. Is it as good as with MH4s? a tough call. If pushed I would say the MH4s just edge it, but it is, by no means night and day.
I think we have success

[JamesD]

Wonderful!

But don't tell anyone how good the EF184 is!

J

[Cressy Snr]

Wonderful!

But don't tell anyone how good the EF184 is!

J

Nah!
EF184? Rubbish! Worst valve I have ever heard.

If anyone has any, the best thing to do would be to send them to me for safe disposal!

[grog]

all this talk of KT120's is making me think of grafting them into the Kat88 i have.....

[pre65]

all this talk of KT120's is making me think of grafting them into the Kat88 i have.....

Just make sure the transformer can handle the increased heater power.

[grog]

good point, i could always use another small transformer for the KT120s,
given the current HT, is there any benefit to going kt120,
i could increase bias..

[grog]

now at 510 volts 55ma

[JamesD]

There is anecdotal 'evidence' that the KT120 sounds better than the KT88...

I would take that with a huge pinch of salt as my experience of the KT88 is that its sound very much depends on how it is used - so I would finger the circuits it is used in rather than the beast itself...

SO it may well be that the KT120 is easier to deploy than the KT88 but I haven't tried both together to see...

Steve, any idea?

J

[Cressy Snr]

now at 510 volts 55ma

Have a look here
http://www.tubedistinctions.co.uk/kt88.htm

Scroll down until you get to the KT120 valve.
There is some good information about how to go about replacing KT88s with KT120s and some of the issues to be aware of.

Hope that helps.

[cressy]

ive used the kt66, 88 and 120 in my amp, thats running all triode strapped, at about 60ma with 350v ht, and 5k output transformers. the driver is a cascoded 6h1n.
the 66 was a bit bass light compared to the 88 and 120 but had better treble extension and sounded overall abit sweeter. the 88s sound nice and balanced but the treble isnt quite as convincing as the 66. the 120 is about the same as the 88 but the bass end is better

the psu in my amp needs to be better, it always has done but ive not done anything to it since it was built so there has been no development to it at all

personally in mine i favour the 88, but thats what i developed the amp to use in the first place

[Cressy Snr]

Here is the pentode drive version schematic.



The output stage, local cathode feedback is still in place.
I did try removing it, but the amp sounds better with the feedback applied.

Grid and screen stoppers have been used as a precaution against the VHF oscillation, these TV pentodes can exhibit given half a chance.