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Anyone Tube Roll with the ZMA yet? (Read 90768 times)
NormD
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Re: Anyone Tube Roll with the ZMA yet?
Reply #550 - 11/09/18 at 03:10:19
 
I learn something new every day.  I'll pull the tubes and take a look one of these days.
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Archie
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Re: Anyone Tube Roll with the ZMA yet?
Reply #551 - 11/09/18 at 15:34:53
 
I looked at both my Tung Sol KT66s (2004)and 7581As (2012) and they look completely different inside.  Also the bias window for the 7581As is wide and goes into the 80s.  I don't remember the KT66s going that high but it's been years since I had them in.
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stone_of_tone
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Re: Anyone Tube Roll with the ZMA yet?
Reply #552 - 11/09/18 at 22:35:55
 
LR wrote:
"Sorry if I stepped on your d**k there and got you all bent out of shape"!

No need for sorry. I had that coming ~ touche'.

Very interesting info and appreciated.
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Lonely Raven
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Re: Anyone Tube Roll with the ZMA yet?
Reply #553 - 11/10/18 at 07:18:13
 

Archie, cool, that's what I was trying to investigate. I think there is a difference with the newer tubes, but photos online aren't helping to prove or disprove this.

Does your 7581A innards look like this pic, or does it have 3 holes instead of 2 squares?




I know for a fact the older 7581A looked just like the KT66. I'm betting that now they look more like the KT88

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stone_of_tone
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Re: Anyone Tube Roll with the ZMA yet?
Reply #554 - 11/10/18 at 14:42:12
 
The bias window is enjoyable Arch. Thank goodness, they sound nothing like KT66's or my KT88's. Interesting though, the innards might be KT88 stuffed in the smaller envelope. Cool, if so. .....all about the sound and they are special in the ZMA. IMO.

I look forward to hearing the 7581A's with my ZMA 25th. Also, Archie, was it you whom mentioned with your 25th, KT88's are running the power trannies cooler too? Subjectively, to hand touch. I don't believe you said you measured the temp. If bypass is bringing 88's more in congruence .....all for that!
J of A, have you noticed this? I would like to not be reticent about the KT88's and really enjoy them. However, if I never put KT88's in my ZMA again. I can be happy with the other three quads I roll and will find a fourth quad I bet.
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Archie
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Re: Anyone Tube Roll with the ZMA yet?
Reply #555 - 11/10/18 at 17:44:25
 
LR,

My Tung-Sol KT66s from Steve look just like yours.  The 7581As have 3 holes but the grey thing (whatever it's called) is a completely different shape.  I don't have any Tung-Sol KT88s to compare with but given the high bias, maybe you are right?  Although plate dissipation should be the same between the KT88 and 7581A.



Stone,

I think the 7581As with bias set to 68mA are running cooler compared to other tubes with bias set mid 50s and pre-mods.  All purely subjective but J of A measured a 5 degree temperature drop, I think he posted.

I hope you get your ZMA back before you have to pull the trigger on the Rachael.  It might make you change your mind.   Wink
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stone_of_tone
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Re: Anyone Tube Roll with the ZMA yet?
Reply #556 - 11/10/18 at 18:03:17
 
Hi Arch, Timeline on the Rach' bypassed would be next Sept/Oct for order. Right now the ZMA 25th'd and its enjoyment. ZBIT in February too, coming.
Plus, Steve's Power Conditioner 10 Amp.

Good news on the 7581A's running cooler. Should bode well when I get the 88's in. Yes, the high bias of the A's like a 88 could be 88 in there. The 88's like the 66's though, stay in a tight margin bias, as where the 7581A's have a beautiful swing Bias I love to utilize.
Even if the 88's don't run cooler trannies... still will use for 4 to 6 hour sessions. All day on my other quads of course.
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Lonely Raven
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Re: Anyone Tube Roll with the ZMA yet?
Reply #557 - 11/10/18 at 22:43:33
 

I'll be digging out my IR Thermometer as well. I have some of the new 7581A on the way, and I'm listening to NOS GE 7027 right now. I'm hoping I like the new tubes as much as you guys do. The GE seem to be throwing a very large image compared to the stock KT66.

Beatles White Album (Super Deluxe 2018 Mix) spinning now.

I got some unusual preamp tubes on the way - I'll share what they are if they work well in the ZMA.
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stone_of_tone
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Re: Anyone Tube Roll with the ZMA yet?
Reply #558 - 11/11/18 at 17:33:30
 
LR, look forward to reading.

I'm suffering. My ZMA has been gone 8 days. I usually get a great Saturday morning and Sunday session in. Well, you guys suffered 3 to 4 weeks...so must I.
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Lon
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Re: Anyone Tube Roll with the ZMA yet?
Reply #559 - 11/11/18 at 18:03:01
 
I totally understand that feeling Larry, I've been through it before. Be strong! You'll get through!
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Jeff of Arabica
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Re: Anyone Tube Roll with the ZMA yet?
Reply #560 - 11/11/18 at 22:12:34
 
Hey Guys,
Sorry for the late reply..  been one of those weeks.

Yes, based on my random observations using my IR thermometer, I do believe my trannies are in the ballpark of 5 degrees F cooler after power supply mods.

Still have my Phillips ECG N.O.S. tubes in play and digging them tremendously. Even found a single for sale on Ebay to replace the one tube gone bad in one of the quads I purchased.  So, now I have a full backup set again.

LR, I cant speak to the GEs but this Phillips set of 7027As are my favorite tubes I have rolled thus far. I have not tried the 7581As though, to compare.  It seems there may be some similarities in SQ between the 7027As and 7581As based on Stones and Archies observations. The 1 and 6 pins of the 7581As and 7027As are unused, but unlike the 7581As, those pins on the 7027As are tied to the screen and control grids - therefore acting as heat sinks for improved heat dissipation.
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stone_of_tone
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Re: Anyone Tube Roll with the ZMA yet?
Reply #561 - 11/13/18 at 21:25:21
 
So true Lon, you've been thru it a few times.

J of A, cool! A little lower temp on my Cryo'd GL KT88's will be nice. +, the other quads.
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Lonely Raven
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Re: Anyone Tube Roll with the ZMA yet?
Reply #562 - Yesterday at 00:25:29
 

How about some old Brimar KT8C tubes for the ZMA?

I got 12, randomly grabbed 4 and put them in the Mystery amp, and they were within 5% of each other!



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Archie
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Re: Anyone Tube Roll with the ZMA yet?
Reply #563 - Yesterday at 20:01:56
 
LR, I saw your tube reply in the other thread but I figured this is a better place to continue.

Those Brimar KT8Cs are interesting. What class of tube do they correspond to, if that makes sense? There aren't many listed on ebay and they aren't cheap too!

Do you have a ZR2? With a ZR2, I'm wondering if the other tube bass might catch up with the KT8C or if the KT8C bass might go through the roof!  (or floor)
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will
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Re: Anyone Tube Roll with the ZMA yet?
Reply #564 - Yesterday at 23:10:07
 
Archie,

These tubes LR pictured look like variations on British KT66 and Brimar 807s, Brimar 807s looking like some Russian 807s. When you read up on them, this makes some sense. Here is one explanation of the KT8C http://www.r-type.org/exhib/aab0080.htm
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Archie
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Re: Anyone Tube Roll with the ZMA yet?
Reply #565 - Today at 00:13:14
 
Cool, thanks.
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Lonely Raven
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Re: Anyone Tube Roll with the ZMA yet?
Reply #566 - Today at 21:12:23
 
Yes, what Will posted.  These set is supposedly the very last of the KT8C done to Milspec for the British Navy. The note that the seller found in the box of them said they are direct replacement for the Ceramic versions and to bias like KT66. They are ruggedized with ceramic standoffs in the structure and (obviously) the classic curvy "coke bottle" shape vs the larger KT66 shape. Milspec typically requires smaller, ruggedized versions of our audiophile tubes that bias up lower and are pre-matched in batches so once the equipment is biased, the radio tech can just swap out tubes quickly and get back to work.

I'm typically really skeptical of tubes that are "milspec" unless they come in large batches and are re-matched. So I was very pleasantly surprised when I grabbed four from the 12 I picked up and they were so closely matched I hardly had to tweak the ZMA. Usually I'm balancing each pair (on the ZMA) for 2-3 minutes as they warm up and tweaking the overall bias. With these, they warmed right up, settled, and were all within a half-space of each other on the gauges. I sat there gawking for a minute waiting for something to change, then laughed and went to listening.

I don't have a ZROCK, I tend to be a purest and go as direct as possible, so I'm XLR out of my DirectStream directly into the ZMA XLR. The DAC is at 100% and I use the attenuation on the ZMA which is typically at 85% all the time, though with these tubes I lowered it a couple times during my listening session! LOL

I do have some Milspec CV124 which are the earlier versions of these tubes that are much more like the 807 tube. These KT8C sound similar, but BIGGER. I'm listening to the CV124 as I type this in fact.

Later this week I'll have different adapters in hand, and I'll be trying out some 6BG6GA, some of which are supposed to have the same guts as the 7581A that Jeff likes.

Speaking of, here is a tip for those of you looking to try out the Phillips ECG but can't find nor afford the 7581A - look closely as these.

https://www.ebay.com/itm/Lot-of-Eight-Heavy-Duty-JAN-6L6WGB-Tubes-By-Philips-USA...

These are the Phillips ECG 7581A guts stuffed into a smaller "ruggedized" envelope and relabeled as 6L6WGB for military use. I was told by some old timers that they might not get the full 35watts of dissipation due to smaller envelope, but they are basically the same tube as the 7581A. The link posted above is a rare find someone should jump on!
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Lonely Raven
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Re: Anyone Tube Roll with the ZMA yet?
Reply #567 - Today at 21:55:22
 

Is there a secret magic password trick to getting these NOS boxes open without tearing the damn flap?  I've been taking a butter knife and carefully prying the flaps open and still tearing the bastards. Grrr.

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