Newbie here.

I have this question and a couple of others.

Why do EL84's sound bad in my SE84UFO!?
I recently picked up a SE84UFO to pair with my Lii Audio Crystal 10's.
Sounds amazing with the standard tubes provided.
Had hoped to roll some of my EL84 collection. TFK, Phillips, Mullard etc.
All sound muffled and horrible. I was under the impression they were a normal switch out for this amp?

I currently have the 6P15P Russian tubes in with a Phillips Metal base GZ34 and a TFK ECC88. And am pretty stoked with the sound.

Secondary questions.

What are the grail 6P15P to look out for?

In other ECC88 input tube amps I have I can also run a ECC189. Is this the same for the SE84UFO?

Thanks in advance.

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I will plus one what Lon said. My experience with EL84s is not good, and I think you will find several other posters with similar stories. EL84s are electrically compatible with the UFO amp, but that doesn't mean they sound good.

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Good question. The short answer is I don't think there is one. The long answer is a long story. Lets start with this: there are 3 different versions of the 6P15P: the "plain" 6P15P, and the 6P15P-EV and -ER. The latter two are ruggedized, long life versions. Back in the old days when you could get these tubes a for a buck to two I bought a bunch of them, and the sound, and the test values were all over the map. Even tubes form the same factory and with the same date code. Back when you could by 6P15P tubes from Decware that was a safe choice, but Steve doesn't sell them any more. So you're pretty much left with buying from ebay and trying them in your amp. I think you'll have better luck with the -EV and -ER versions, but you pretty much pays your money and takes your chances.

For me, I've tried a number of different power tubes. I didn't like EL84s but I did like EL83s. You need an adapter, I had custom adapters made that maintained the Hazen grid mod. But even better than EL83s (for me) are EL822/EL821/6CH6 tubes. They are an upspec'd EL84 and also need an adapter, but that is the best sound I know from both my pervious UFOs and current UFO25s. You used to be able to get 6CH6 -> EL84 adapters on ebay, but now you need to dig a little more.

Another option are the Cryotone tubes. Personally I'm not interested (long story) but several posters swear by them.

One other thing that I think is safe to say is the power tubes have less impact on the sound then the rectifier and input tube. It looks like you have a grail rectifier already, and pretty nice input tube, and I'm not surprised you're getting great sound. IMO finding the "perfect" power tube is likely to only be a marginal improvement from what you have now.

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Yes! Even better for me (and others) are both the PCC88 and PCC189 (the 7 volt versions). They seem to sound better than the 6V.

There are many many threads on the site about tubes, way more info than I can put in a single post. I'd encourage you to spend some time looking around see what others have to say.