Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 61

Dear Gibosi,

Top News!  Gold discovered in US tubes!

Came across a fascinating document about the GE 5 Star tube program. From a cursory reading these tubes were developed to be the ultimate in reliability for the airline industry, and yes, they feature gold (or silver) grids! (page 16) Here is the reference from the early 50's:

http://www.tubebooks.org/tubedata/GE_5star.pdf

BTW, this document shows you all the tube parts with excellent pictures and explanations.

So now my question:

I am looking to buy the GE 6AU6 tubes that are top rated on this forum. Do you think that the military version GE 6AU6WA is a better choice? In addition, there are GE 5 Star 6AU6WA tubes available as well. Would they be even better sound wise?

Quote:

Originally Posted by mordy 

Came across a fascinating document about the GE 5 Star tube program.

 

So now my question:

I am looking to buy the GE 6AU6 tubes that are top rated on this forum. Do you think that the military version GE 6AU6WA is a better choice? In addition, there are GE 5 Star 6AU6WA tubes available as well. Would they be even better sound wise?

What a great document! Thanks for sharing! :)

Your questions are great questions. However, this forum's collective experience with 6AU6s is still tiny. To my knowledge, only Audiofanboy and myself have installed 6AU6s in our LDs.

(If there are others out who have, please tell us!)

Based on the experience of only two persons, the GE 8425A/6AU6As are highly recommended. Audiofanboy did report about his experience with a pair of 1978 GE JAN 6AU6WC and he did not recommend them over the 8425As.

All I have at present is a pair of 8425As. I do have a a pair of 1960s GE JAN 6AU6 WB / 6136 on the way, but won't get them until some time next week. I also have a pair of RCA Command 6136s, 1950s mil-spec Tung-Sols, and of course Russian 6J4Ps on the way. :)

I think it is worth pointing out that just because a particular pentode is considered one of the very best for RF purposes, we cannot then conclude that this same pentode will excel when converted into a triode for audio purposes. After all, the GE 5 Star 6AK5s are generally not ranked as one of the very best 6AK5s...

So my recommendation is that you purchase one pair of each! 8425A,  GE 5 Star, and standard mil-spec! And then tell us what you think! LOL. (And if you actually do decide to do this, I would recommend nothing later than the 1960s.)

But if you want to buy only one pair, the 8425As are a very safe bet.

I just ordered some 1960's GE 6AU6A tubes, do I need to jump with pins on the socket in order to use them? And should the the 6AK5 setting, correct?

Quote:

Originally Posted by mab1376 

I just ordered some 1960's GE 6AU6A tubes, do I need to jump with pins on the socket in order to use them? And should the the 6AK5 setting, correct?

Yup, you can just bridge socket pins 2 and 7 like on the picture I posted two pages back. And yes, 6AK5 jumper setting or explosion. I am sorry for your GEC CV4014 and your near-deaf-experience, but I 'd recommend trying them again, as I haven't found another awesome tube like that one with as much gain (CV4014 has a gain of 75, whereas 6AU6A has a gain of 36), so it's still a very useful tube to have on hand if just for that reason.

Quote:

Originally Posted by Audiofanboy 

Yup, you can just bridge socket pins 2 and 7 like on the picture I posted two pages back. And yes, 6AK5 jumper setting or explosion. I am sorry for your GEC CV4014 and your near-deaf-experience, but I 'd recommend trying them again, as I haven't found another awesome tube like that one with as much gain (CV4014 has a gain of 75, whereas 6AU6A has a gain of 36), so it's still a very useful tube to have on hand if just for that reason.

I emailed the eBay seller and asked if I can get a replacement, worst case I'll just pickup another. any sellers you recommend?

Quote:

Originally Posted by mab1376 

 

I emailed the eBay seller and asked if I can get a replacement, worst case I'll just pickup another. any sellers you recommend?

As far as permanent buy-it-now auctions go, I've only seen the two sellers that are in the UK -and quite expensive if I may say so myself.

Hi All,

Found some comments on the military spec tubes re the 6AU6WA and now 6AU6WC tubes. Does ruggedness in construction affect the sound?:

"W is ruggedized. C is "improved ratings/performance". That doesn't necessarily mean they will sound better. Tubes manufactured for portable military applications may not produce better audio quality. They were much more concerned with reliability then sound quality. Then again, maybe better mechanical design will also produce better sound. Only one way to find out.
 

If it's ruggedized it might not sound better, but it'll sound the way it does a lot longer than a standard version. "

I think that this pretty much sums it up.

Re the GE Five Star tubes I have a couple of the 6AK5 variants and I was not so impressed by the sound. In addition, I also had a pair of 6AK5 1980's tubes that sounded quite poor. It looks like the golden age of tubes was in 50's to 70's, and for Russian tubes up to the fall of Communism; the consensus seems to be that Russki 70's - to early 80's tubes are the best.

Re the gold grids in Russian tubes: The Voskhods I have from the 70's sound different with the gold grid ones sounding more mellow. (And yes, both pairs went through a grueling 120 hour break in). Likewise, the 6N6P with gold grids sound nicer than the regular ones, and these are the power tubes I use. Good to know that the Russians copied us on the gold grid...

A pair of 1964 RCA Command 6AU6/6136 arrived today and are now burning in. It is my understanding that these are the best consumer tubes that RCA made, comparable to Sylvania's Gold Brand I assume (but no gold pins). First impressions (but remember I have rusty ears and cheap gear!), these sound very good. :)

I will report back later.... 

After 10+ hours, the sound of these 1964 RCA Command 6136s is indistinguishable from my GE 8425As (according to my rusty ears with cheap gear :). All the many good words written about the 8425As apply to these RCAs. They are very quiet, very musical and very enjoyable. And it seems that RCA's best consumer-grade tubes hold their own with the best of them. They will stay in my LD as my primary tubes until... the next new tube shows up my mailbox. :)

Quote:

Originally Posted by gibosi 

After 10+ hours, the sound of these 1964 RCA Command 6136s is indistinguishable from my GE 8425As (according to my rusty ears with cheap gear :). All the many good words written about the 8425As apply to these RCAs. They are very quiet, very musical and very enjoyable. And it seems that RCA's best consumer-grade tubes hold their own with the best of them. They will stay in my LD as my primary tubes until... the next new tube shows up my mailbox. :)

Interesting, I would have expected some house sound differences at least, like warmer or darker or more bassy, but alright. Did you ever feel a difference between the GE 6AU6A and the 8425A btw?

No, I wasn't able to discern any difference between the GE 6AU6A and the 8425A either. That said, I can easily hear differences between the various 6AK5s. And recently, it was easy for me to detect differences between major tube families, for example, 6CB6, 6WE6, 6GX6, 6GM6, 8136 and most recently, 6AU6. So I have to assume that any differences that might exist between these 6AU6s must be fairly subtle, requiring better ears and better gear than I have.....  and more experience...

The LD1+ is my very first headphone amp and I have had it for only three months. I bought my first full-sized budget headphones shortly thereafter. Before that, I listened to Pandora on a cell phone with IEMs... 

I try to contribute what I can, but I am truly a freshman in the company of graduate students. :)

Quote:

Originally Posted by gibosi 

No, I wasn't able to discern any difference between the GE 6AU6A and the 8425A either. That said, I can easily hear differences between the various 6AK5s. And recently, it was easy for me to detect differences between major tube families, for example, 6CB6, 6WE6, 6GX6, 6GM6, 8136 and most recently, 6AU6. So I have to assume that any differences that might exist between these 6AU6s must be fairly subtle, requiring better ears and better gear than I have.....  and more experience...

 

The LD1+ is my very first headphone amp and I have had it for only three months. I bought my first full-sized budget headphones shortly thereafter. Before that, I listened to Pandora on a cell phone with IEMs... 

 

I try to contribute what I can, but I am truly a freshman in the company of graduate students. :)

Well, I guess not all tube families show as much variety as the 6AK5 or EF91 families, or even all the different types we've been testing on lately.

In the meantime, I had ordered and just got in the mail 3 pairs of 6AH6 tubes. Not a single plain Jane one (for $2/3 a tube who cares?), and not just GE for once:

- JAN Raytheon 6AH6WA - 1973

- JAN GE 6485 - 1978 (that was before my conclusion about 70/80s GEs, oh well, we'll see if these are good or not...)

The 6AH6WA and 6485 are, as usual, the special quality versions of the 6AH6, which is considered and even recommended as a replacement of the 6AU6 in many circuits. So if the 6AU6 works so well in the LD amps, why wouldn't these be as good or better? Note that the 6AH6 has similar recommended plate voltage and cathode resistance triode-strapped compared to a 6AK5 -just like the 6AU6- which means the amp should be very well suited and biased for this type, unlike many types I tried in the last two weeks, that had different cathode resistance recommendations and other recommend settings (could explains why some tubes didn't fare so well).

I put the Raytheons for burn-in this morning and first impressions are very favorable; I'll wait the usual 10h to give more details.

The 6AH6 are sharp-cutoff pentodes with a 450 mA filament current (6AU6 has a 300 mA heater). These should have almost the same voltage gain/amplification factor/mu (40 instead of 36) as the 6AU6, so I didn't expect much more volume and didn't find a significant difference in level compared to the 6AU6; and they have more than double the transconductance/gm (11000 mmhos instead of ~5000) the 6AU6 has, which I'm still struggling to find out what effect that would have on the sound, should mean more mA on the anode -and into the power tubes- for every V on the control grid, so higher output current and lower output impedance if I understand... Apart from that, these tubes have similar characteristics and are known to work well in audio circuits triode-strapped (the datasheets even have triode curves for both), so it should interesting to see how they fare compared to the 6AU6.

I also ordered and got a -final- 6AG5 tube with the 3 pairs of 6AH6, last try for this tube family. A Sylvania JAN 6AG5WA/6186 dated 1967, how bad can it be really, it's mil spec, special type and from the 60s (so final improved construction and getter structure). Just so I can reach some kind of closure on the 6AG5 family after the disappointment the Russian 6J3P-E were (not 6AG5 really, but the point remains)...

I'll try to put some pics up at some point when I start reviewing these tubes.

And I have a pair of 1961 Tung-Sol JAN 6AH6s burning in:

Interestingly, these have a very open cage assembly compared to the 6AU6s, and in a dark room, they light up very nicely. First impressions, they look good and sound good. :)

Quote:

Originally Posted by gibosi 

And I have a pair of 1961 Tung-Sol JAN 6AH6s burning in:

 

 

 

Interestingly, these have a very open cage assembly compared to the 6AU6s, and in a dark room, they light up very nicely. First impressions, they look good and sound good. :)

So sexy ouah !