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Axis 30th Anniversary

HD-500 tube. What kind?

TSanders

TSanders

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Joined
Jun 9, 2004
Messages
3,535
Location
Columbus, GA
First, if I need to call CS for this, just delete the thread and let me know.

Second, Scott told me when I had the first one, but it slipped my mind. Sorry.:o

I popped the top off my HD-500 and the tube is unmarked. Need to find out what it takes, so I can get my "just in case" backup tube.
 
If you can find some NOS Telefunken, Mullard, Brimar, Silvania etc, it's definitely worth comparing different brands rather than popping in a run of the mill GT or Sovtek. NOS tube purchasing and trialling would add an entire new element to your rig GAS!
 
Bump.

Can anyone at EB help me?

Ive read up all that I could find. One review says 12AT7 while others say 12AX7. However, for some reason, I think when I called Scott when I had my last one he said it was a 12AU7. Dangit. I shouldnt have forgotten.
 
Travis, I just took the cover off the HD-500 that I bought from you and it has a Sovtek 12AT7.

I'm not an amp expert, but I really don't think that you'll hear a lot of difference by swapping out for a different brand of tube. The reason that I say this is because the tubes aren't running at high voltage - they seem to be there to get the feel versus the tone of the tubes. The nice red glow that you see is an LED that is positioned for just that effect, not the glow from the heaters of the tubes.

If you do change out the tube, and get good results in doing so, please share your findings here so those of us that are lucky enough to have one of these gems can benefit too.
 
The 12A*7 series are sort of interchangeable. I'd have to dig up some pin-out diagrams but I think they have all the same pin assignments. The big difference is in the gain factor. In theory I think you can pop any 12A*7 in there without harm to the amp but the sound will drastically change. I'll do some research tonight or tomorrow and get back to you.

Also, Tim (tadawson) is knowledgeable in the ways of tubes and should be able to help out if he comes across this.

If I recall, the 12AT7 is a high mu tube (hi gain) and the 12AX7 and 12AU7 are low mu tubes. So you may over drive or underdrive the power section switching to the different tube types. Also, without adjusting the bias points, you'll be in different parts of the gain curves.

You should be able to see the glow of the tube regardless of the led, although you may have to take the cover off to see it. I think you told me that you had witnessed the glow.

Regardless of voltage, tube rolling is fun and does provide a difference, all the way from preamp tubes to power stage tubes. Different factories at different times used different materials and construction. It has been my experience that it makes a difference in high end audio equipment. Don't know about bass amp.

I think the GK Fusion is begging for a home along with a berg' cab.

I'll make some time to look into this more this weekend, hopefully tonight .
 
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The gain structure of the 12A*7 series are as follows -

  1. 12AX7 - amplification factor of 100
  2. 12AT7 - amplification factor of 60
  3. 12AY7 - amplification factor of 40
  4. 12AU7 - amplification factor of 20

In practice, these amplification factors can be thought of as the overall gain of the tubes. The higher the number the higher the gain. They all have identical pin-outs and can be interchanged freely. There can be problems with some if you use them in an application that really pushes the tubes to the limit. I had no problems interchanging 12AT7s and 12AU7s in my amp - and I don't think there would be any issue with using a 12AX7 either - if I had one on hand I would have tried it.

I have tried a little experimentation with the limited tubes that I have on hand. Here's my observations:

  • Tonal variations are very subtle with tube swaps. I really don't think that you'd be able to tell the difference while playing with a band.
  • The 12AT7 is definitely much higher gain than the 12AU7.
  • The difference between the brands equates to how much high frequency is present in the sound.

I tried 3 12AT7s and 2 12AU7s - here's my order of preference.

  1. Sylvania 12AT7.
  2. Sovtek 12AT7
  3. Mesa STR 12AT7
  4. Sylvania 12AU7
  5. RCA 12AU7

The 12AU7s had nice tone, but they produced way too little volume comparatively. Also, the RCA 12AU7 seemed to roll a bit more highs off than the Sylvania.

The 12AT7s came down to tone. They were all relatively equal volume. The Mesa rated last because it rolled the most highs off. The Sovtek came in second because it seemed to have a little too much highs - almost brittle at times. The Sylvania was the winner because it seemed to convey the ideal amount of highs.

Again, I'd like to point out that the differences are very subtle - so subtle in fact that I decided to leave the Sovtek installed in the amp even though it wasn't the "best". My decision was based on a couple of things, but it was mainly because a Sovtek is easily replaceable - vintage tubes are not, and that the Sovtek is of a "long plate" design which is supposed to be a bit more durable - who knows if that's true or not? To see the difference in the length of the plates, look at the innards in the following picture -

DSC02796.jpg


So, I stand corrected on being able to hear the difference - it's there to be explored, but very subtle at best. Also, the tubes are definitely doing something, they are warm to the touch when I removed them, but not nearly as hot as one that sees full voltage.

Disclaimer - This is just my opinion on the subject - I am not a professional musician, amp tech or sound engineer, so those with better technique, better ears, or better equipment may find different results. YMMV.
 
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Motojunkie said:
The nice red glow that you see is an LED that is positioned for just that effect, not the glow from the heaters of the tubes.
Click to expand...

To clarify this, here's a couple of pictures of the LED that I'm talking about. You can see the LED right at the base of the tube socket.

DSC02793.jpg


DSC02799.jpg
 
Many thanks to Andrew.

He brought me a JJ Tesla 12AU7, and I actually noticed a pretty big difference in tone. What I thought was good is now even better.

Really is just warmer overall, while still retaining the clarity it had with the last tube in it.

Its rockin.
 
maddog said:
The 12A*7 series are sort of interchangeable. I'd have to dig up some pin-out diagrams but I think they have all the same pin assignments. The big difference is in the gain factor. In theory I think you can pop any 12A*7 in there without harm to the amp but the sound will drastically change. I'll do some research tonight or tomorrow and get back to you.

Also, Tim (tadawson) is knowledgeable in the ways of tubes and should be able to help out if he comes across this.

If I recall, the 12AT7 is a high mu tube (hi gain) and the 12AX7 and 12AU7 are low mu tubes. So you may over drive or underdrive the power section switching to the different tube types. Also, without adjusting the bias points, you'll be in different parts of the gain curves.

You should be able to see the glow of the tube regardless of the led, although you may have to take the cover off to see it. I think you told me that you had witnessed the glow.

Regardless of voltage, tube rolling is fun and does provide a difference, all the way from preamp tubes to power stage tubes. Different factories at different times used different materials and construction. It has been my experience that it makes a difference in high end audio equipment. Don't know about bass amp.

I think the GK Fusion is begging for a home along with a berg' cab.

I'll make some time to look into this more this weekend, hopefully tonight .
Click to expand...


Looks like everyone pretty much beat me to it, and not much more to be said. The 12A?7 series is interchangeable for the better part, and the gains as well as the power dissipations vary with the variants - the higher gain tubes typically having the lower power dissipations. In a preamp, you can swap at will to get a sound, but in a
driver stage, substitutions may not always be good . . . .

- Tim
 
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