On Monday, July 20, 2020 at 11:58:01 AM UTC-4, wrote:
On Sun, 19 Jul 2020 16:04:12 -0700 (PDT), John-Del
wrote:

On Sunday, July 19, 2020 at 3:39:04 PM UTC-4, wrote:
One of the EL34 tubes in my amp was making intermittent noise so I
bought a couple new matched EL 34 tubes and put them in the amp. They
sounded OK but not great. At first I thought it was the particular
recording that was sounding so bad because the recording was bad. It
was the Happy Trails album by Quicksilver Messaging Service. The
original recording just wasn't the best quality.
Then about a week later I was listening to some Herbie Hancock and
the sound was kinda muddy or muffled sounding. It did not have the
same clarity as it did with the first set of tubes. So I swapped in
the old pair and and there was a marked difference in the quality of
the sound. I was really surprised at the difference.
Now I gotta shop for some better tubes. What is done in the
manufacturing of tubes that can have such a big difference?
Eric

I can't answer your question, but would it have killed you to put the brand names of the original set of outputs and the replacement set you installed, or the amp you're using?

Also, did you bias the new tubes properly? If they're running too cold they'll sound like crap.

I never heard much difference between brands of EL34s, 6CA7s, and even 8417s I've used in my vintage Fisher integrated amp. They all sounded fine until they became flabby with age.
Yeah, I almost had a heart attack when I tried posting the name
brands.


How did you know I was being literal?


The better sounding
tubes are Psvane brand tubes. The worse sounding tubes are
Electro-Harmonics.

I've used EH EL34s and the 6CA7s in my audio amps and guitar amps, and I thought they were as good as a set of NOS Sylvania 6CA7s I keep on hand. Not to pick on your amp, but I'm not sure that amp is the right device to critique vacuum tubes/valves. Also, I've never listened to (or even seen) a single ended EL34 amp.



The amp has no bias adjustment. I know that different circuits are
used for adjustable bias and auto bias. Could it really make that big
of a difference? Should I consider modding the circuit so that the
bias is adjustable?
Thanks,
Eric

I've never actually owned any EL34 amp that was auto bias, so I never compared them. I do know that if you manually bias the outputs too cold, they become class B and sound like crap, which is why I asked. I bias my guitar amp a little hotter than design spec, and my vintage audio amps just a bit on the cool side, and they sound great, but I do rebias them every few months.

Before modding your amp, I would measure the cathode current and see where the tube is running. You should be running close to but not over 100% plate dissipation of the tube you are using. You may have to replace your cathode resistors to get there.