Replaced my Genalex kT66 with the newer Tuns-sol 7581A tubes

Well I bit the bullet and changed my tubes in my VTA At-70 amp and so far so good. I surprised how similar they sound to the KT-66 but with about 5 watts more power which make a difference. Bass is tight and midrange is up front also. Highs aren't to bright  and still have break them in. But so far so good for the $100 cheaper price is good too.

Anyone else using the Tung-sol 7581A tubes in their amp want to comment?
Phil

I'm using GE 7581A's in my Craftsmen C550 amps. I have 4 real English Mullard KT66's that came with the amps when I bought them years ago, but for some reason, I'm saving them for the future. I had to reset the balance and bias voltage on the amps a bit when I switched to the 7581A's, bit other than that I really don't hear much of a difference in the way the amps sound.

yes thats what im experiencing right now with the new tubes. so for so good.

The main reason I went to the 7581's in my C550 Craftsmen amps is that the 7581's have a lower filament current draw which I thought would put less of a load on the old power transformers that are now 68 years old and probably not replaceable.

the bias are now needing lowered a little so the tubes are breaking in nicely. Pleased so far and will see when they get fully broken in.

Anyone know or recommend the proper bias setting on the 7581A on a VTA St-70 amp. I have then set at .425 to .450 right now but don't know if that is correct or not.

ok I have the tubes broken in and I doubt I will go back to the Kt-66 unless prices come down. The 7581A tubes sound as good but about $100 cheaper.

truebluephil wrote:Anyone know or recommend the proper bias setting on the 7581A on a VTA St-70 amp. I have then set at .425 to .450 right now but don't know if that is correct or not.

The bias you have selected should work fine. The data sheet says that the 7581 has a max. plate dissipation of 30 watts. I use 6L6GC in my ST70 and bias them at .450. Since the 7581 is a drop in replacement for the 6L6GC according to the spec sheet, you shouldn't have any problems.

I emailed Bob and he also said .450 will do fine. Nothing wrong with these tubes. Balanced and plenty of bottom end.

truebluephil wrote:about 5 watts more power

The 7581A is capable of more power than a KT66. It will make the same power with the same transformer

truebluephil wrote:which make a difference.

New tubes provide more power at the same input, so they will be louder. They are also probably more closely matched and thus the amp has lower distortion from more even drive on the OPT.

5 watts of more power makes a huge difference. I should have changed years ago to these. Prolly not going back to the KT-66

Like Seamus says, you have fresh 7581's which will sound better especially if your old KT66's have a bunch of time on them. When I swapped 7581's for the Mullard KT66's in my Craftsmen C550's I didn't hear any appreciable difference as the KT66's were low time tubes. Someday I may put the KT66's back in the amps to see if I can hear any difference.

I wasn't comparing them to old tubes but I have replaced the Genalex KT-66 at least six times in my amp and I know what they sound like new.

truebluephil wrote:I wasn't comparing them to old tubes but I have replaced the Genalex KT-66 at least six times in my amp and I know what they sound like new.

What @Seamus is trying to explain to you , is that the tubes are capable of dissipating more power than the EL34 if they are biased a bit higher . But if you leave them as is , they may last a little longer than EL34 because  they are not working as hard .

As tubes wear down over time transconduction (release of electrons from cathode to anode) decreases . This is the reason that the new tubes sound better , not because they are more powerful . That blue glow you see for a while with new tubes in your amp indicates strong transconductance - so pretty .

Here is a book published by Hendrick van der Bijl in 1920 , van der Bijl was part of the team under Lee de Forester lauded as being the ones to invent the triode .

http://www.tube-books.com/Books/intro_Vanderbijl_thermionic.pdf

I get it

Tubes that have sat for some time have significant gas which can cause changes in transconductance, i.e. gain - usually more.

NOS tube should be evaluated after it's had 24 hours on the heaters and allowed the getter to reduce the gas. Transconductance can drop by 20% on old NOS tubes.

New tube transconductance can drop 5% or more by about 500 hours. If a closely matched set drifts unevenly, a new set will have more gain and a change in distortion characteristics. Very few tube fans have a valve or distortion tester suitable to rebalance a set of tubes with some time on them.

From The Tube Store:
Blue Glow and Undertanding Blue Glow

Great post. I replaced mt GL KT-66's last Christmas since the previous ones had 8years on them. Nice to have the original sound back BUT moving forward I will get a quad of 7581's as spares. 5 extra watts gives me a bit more headroom. Thanks

Brap wrote:... I will get a quad of 7581's as spares. 5 extra watts gives me a bit more headroom.

Power is determined by the supply. The tubes control the current through the output transformer. A 'more powerful' tube is able handle more current, but the power supply remains the same, so the net power is unchanged.

Old tubes have a lower transfer function, so new tubes sound 'better' because they may be louder at the same input.

Tubes sound different due to their transfer function. Old tubes require more drive for the same current output.

There are 256 ways to insert 4 tubes, each of which will have slightly different response, some better than others.

A 'matched' set is only 'close' and begins drifting from the moment they are used.

Tubes do not age at the same rate, so it can pay dividends to rematch a set after a few hundred hours.

truebluephil wrote:Well I bit the bullet and changed my tubes in my VTA At-70 amp and so far so good. I surprised how similar they sound to the KT-66 but with about 5 watts more power which make a difference. Bass is tight and midrange is up front also. Highs aren't to bright  and still have break them in. But so far so good for the $100 cheaper price is good too.

Anyone else using the Tung-sol 7581A tubes in their amp want to comment?
Phil

I just recently added the TS 7581A's to my Dyna/VTA 70 (single channel mono blocks).  They replaced 2 year old GL KT66 with a guestimated 1000 hours on them, and so far they sound really nice.....only ~50 hours on the 7581A's so far.  

My observations are very similar to yours, in that they sound very similar to the KT-66, but are a smidge darker in the top end, which I'm actually liking quite a bit.  Midrange might be ever so slightly clearer, but the KT66 are likely closer to the end of life than the beginning, which adds a variable.  I haven't had a chance to push them much yet, so can't comment on perceived power.  Overall, I'm very pleased with the sound quality, and they appear to be an excellent value in this current crazily overpriced environment.

Edit: I did have one 7581A of the quad go bad, and it needed to be replaced. That weighs heavily on any value claims. Still like the way they sound with the amps.