Why is the Wheatfield so hard on tubes?

[Dusty Chalk]

One reads your link more closely. Since the standard tube supplied was the 6080WC (for power)...

Well, if you read the manual I linked to, it says 6AS7G.

If you read the tuberolling FAQ, it says 6080/6AS7G in the subject header, then goes on to say that they've tried 6AS7, 6080, 5998, 7236, and 6528.

If you read this page or the specs page, it says 7236 or 6080.

So I already read them pretty carefully, Mr. "one reads more carefully". I was looking for a more definitive or carefully thought-out answer. He goes on to say that they started out with new production tubes and moved to NOS tubes due to noise issues. So I wouldn't trust what it came with, neither.

[hirsch]

So I already read them pretty carefully, Mr. "one reads more carefully". I was looking for a more definitive or carefully thought-out answer. He goes on to say that they started out with new production tubes and moved to NOS tubes due to noise issues. So I wouldn't trust what it came with, neither.

Sorry. I followed your link. I thought "We now supply an NOS JAN 6080WC tube manufactured by GE as standard equipment in the HA-2." and "For the 5U4G: Leave the Sovtek tube in there!" to be reasonably unambiguous. Silly me. Yes, he may have started with new production 6080's, but the new ones are indeed crap. It's still the tube the amp was made for, which is the answer to what you asked. You want a thought out answer, ask a different question.

[Dusty Chalk]

I thought "We now supply an NOS JAN 6080WC tube manufactured by GE as standard equipment in the HA-2."... to be reasonably unambiguous.

Well yeah -- quote anything out of context and of course it seems unambiguous. I thought I had already pointed out that there were several different isolated sentences, any one of them out of context would have seemed unambiguous. It's the fact that there were several conflicting answers amongst the documentation that makes it confusing. I would ask a different question if I knew what questions to ask. The thing is, I know very little about this sort of thing. I'm trying to ask the sort of questions that will lead to the kind of discussion that will lead to me asking more intelligent questions and ultimately, an education. I'd like to understand what Pete Millet is telling me when I start talking to him. Quoting from the manual without explanation is not helping. Silly you.

This helped:

The 6528a, along with the 6336, is the most powerful tube in the series @ 30 watts dissipation per triode section vs 13 watts per triode with the 6as7/5998/6080/7236, .... and the 6528a also has the lowest output impedence. The trouble with the 6528a and 6336 is the tube demands 5 amps of heater current per tube vs 2.5 amps for the other tubes.

So what parameters do I need to look at to determine how many amps of heater current the Wheatfield is capable of generating? Is there a way to figure this out from the schematic? Is there a way to figure out the bias voltage that's being placed on the tubes from the schematic? Presumably, the bias voltage is constant, so sticking different tubes in with different parameters will run them at relatively higher and lower percentages of their specs, yes? Are there any other parameters with which I need to be concerned?

[Large4mat]

The Wheatfield was a "premium set of tubes" which included the 6528A........

http://www.pmillett.com/Wheatfield/tubes.htm

[Large4mat]

"The 7236 is our overall favorite, but it is difficult to find."

http://www.pmillett.com/Wheatfield/headphones.htm

I've got to believe the amps XFMR's are capable of feeding the 6336, 6528, and 7236. Pete is a bright guy, and you can bet he did the math before making the tube recommendations. That being said, the amp does run warmer with the 7236 than it does with the 6080. The 7236 also gives the amp a more tipped up top end than the 6AS7 and 6080 which is not complimentary to the AKG K701 or Grados IMHO.

Also see this link for comments on headphones for use with the Wheatfields:

http://www.pmillett.com/Wheatfield/headphones.htm

[hirsch]

Quoting from the manual without explanation is not helping. Silly you.

OK, let me rephrase what I said at first in a different way. Forget the tubes. The Wheatfield was designed to drive high impedance headphones. Used with an optimal tube set, and there are many, it will sound great as long as you don't go playing about with low impedance headphones (and yes, that includes the L3000). The Wheatfield can be made to work with low impedance headphones, but the tube search is a pain and I never found a sound I liked when it did work. Consequently, I don't recommend the Wheatfield for use with headphones under about 100 ohms or so. Try running low impedance headphones out of the HAP-03 without the impedance matcher. Same issue carried to a greater extreme.

If you really want to push low impedance cans with 6080 and related tubes, get your Supra tuned for this:

http://www4.head-fi.org/forums/showpost.php?p=2505812&postcount=8

Parallel plates mean half the impedance and twice the power (more or less).

[Dusty Chalk]

OK, let me rephrase what I said at first in a different way. Forget the tubes....Consequently, I don't recommend the Wheatfield for use with headphones under about 100 ohms or so.

Probably good advice. I may or may not heed it.

Try running low impedance headphones out of the HAP-03 without the impedance matcher.

Yeezh. My HAP-03 with anything other than 600 ohm headphones sounds like crap.

If you really want to push low impedance cans with 6080 and related tubes, get your Supra tuned for this:

http://www4.head-fi.org/forums/showpost.php?p=2505812&postcount=8

Alright, I'll bite -- what's that?

And my Supra already pushes my low impedance cans adequately. Do I really need to change it? I mean, yeah, it may need some fine tuning, but does it really need to be rewired for different tubes etc.? We'll talk about that the next time I see you (and have some time -- not this weekend).

[Dusty Chalk]

Alright, I'll bite -- what's that?

Uh, never mind, I went ahead and read the thread.

[pmillett]

Peter -

Interesting thread. You should have emailed me sooner

The amp does run very hot (temperature-wise)... not much to be done about that, I'm afraid.

I'm surprised you say it burns through tubes quickly (or at least more quickly than any other tube equipment). Never had that issue, nor heard it elsewhere. What tube(s) are getting killed? And how do you tell when they're dead?

If your AC voltage is high this may contribute to both heat and shorter tube life. Measure it; if it is much over 120V you could maybe get a transformer of some type to drop it. Clean or dirty (noisy) power probably won't have any effect.

It's also possible that there is something wrong with the amp, maybe the electrolytic caps are getting leaky (they do get hot inside the chassis), which compounds things and makes things even hotter. Depending on how many hours of it's life the amp has been turned on, the HA-2 has been out there for a while so it's possible some of the components could be getting stressed.

As far as tubes, the good old 5U4 is quite rugged, probably the best choice. An old US manufacture one is definitely better than Russian or Chinese as far as reliability goes; I doubt that it will affect the sound.

For the output tube, the 6528A runs very hot and heats up the whole amp due to it's 5A filament. And yes, the transformer is rated for it (6A, if I recall, so 5A for a 6528A and 600mA for a 6SN7), but it does get VERY hot with that load. A 5998 tube is almost as good to drive low impedance phones, it's easier to find than a 7236, and I think it sounds better anyway.

It is true that the HA2 was designed to optimally drive phones over 100 ohms. To tell the truth the lower-Z phones was a bit of an afterthought. But I've always thought that most low-Z phones (Grados, at least) sounded better with some damping (higher output impedance). So to me, an HA-2 with a 5998, 7236, or 6528A tube sounds OK. But it is a matter of taste.

Along those lines: the reason you can substitutes any of these seemingly dissimilar tubes in the same circuit is just due to the topology. The output stage is a direct-coupled cathode follower, so the grid voltage is fixed by the 6SN7 stage, and the bias current is set by that and the cathode resistor (which burns ~ 20 watts of heat per channel!). So swapping tubes in and out does change the stage current a little, it turns out that all of these tubes are within a normal operating range under those conditions. I did do testing with 6AS7, 6080, 5998, 5998A, 7236, 6336A, and 6528A. The amp shipped initially with 6AS7, then 6080. The "Premium" version shipped with 7236 then 6528A tubes. I dropped the 7236 tube after I had one unit suffer heater-to-cathode breakdown - the resulting pop in the headphones was very painful (I had them on my head at the time!)

You can estimate Zout by looking up the tube's voltage gain (mu) and plate resistance (Rp). Zout is approximately Rp/mu. So you can lower Zout by lowering Rp or raising mu. That's why the 6528A has the lowest Zout of the group; it has higher mu (9) than the 6AS7 (2), and an Rp of 245 ohms vs. 280 ohms for the 6AS7. The 5998 gets you a mu of 5.4 and Rp of 330 ohms, which is not quite as good as the 6528A, but better than the 6AS7/6080.

I'll post this to the forum so others can read. Another forum... I can't keep track of them all any more! So feel free to email me if there's something I should read.

Pete

[Dusty Chalk]

Thanks for that. I'll look at the caps. It's too bad it don't have a "capacitance" reading on my little ol' ratshack multimeter...

[n_maher]

Pete (Dusty),

Did you ever end up opening your amp to have a look inside? I had noticed a bit of low level hum with my Wheatfield and after talking with Mr. Millett we'd narrowed it down to a couple possibilities all of which involved visual inspection. Opening the amp was no problem (8 torx screws) and what I found inside was definitely worth the time.

That's the primary power supply filter cap and it's pretty much ready to blow. We think that the amp was probably exposed to some sort of over voltage event during its life. I'm going to replace it along with the other 3 electrolytic caps in the power supply along with a resistor. I'm also going to change out the primary transformer which should help the amp run a little cooler.

So if your amp is still chewing through tubes it might be worth having a look under the bonnet. If you do let me know what you find.

Nate

[thrice]

Thanks for that. I'll look at the caps. It's too bad it don't have a "capacitance" reading on my little ol' ratshack multimeter...

Peter, my meter has the ability to measure capcitance...let me know if you want to get together sometime and use it.

[Dusty Chalk]

No, I never opened it up -- it got kind of shoved to the back-burner. Thanks for the "bump" -- I'll definitely get around to it shortly. And thanks again for the offer, thrice.

And thanks for the pic. I'm kind of a newb, so started to ask what was wrong with it, until I looked at it compared to the ones next to it. OMG!

[n_maher]

Well, $50 in parts from Digikey and my HA-2 is now back up and running pretty much silent. There is the slightest, and I mean slightest hum in the background but it is so low that there has to be no signal present and you have to concentrate to hear it. WUHOO! My new trafo for the amp should be in next week if all goes according to plan and while that will probably have no effect on the remaining hum I'm hopeful that the new trafo might be quieter than the current one. Now that the hum is gone the trafo noise, that is to say the actual noise that the trafo makes while it's working (sort of buzzish) is now what I hear more than anything. I tell you, living in the country with almost no ambient noise can be awful hard on the old audio system!

[n_maher]

Another Wheatfield update and this seemed like the place to post it. On Pete's advice I decided to mod the amp further, what the hell it's only money. So I ordered up a high-spec Hammond trafo (not cheap, $100+) and prepped for surgery. Replacing the trafo required not only drilling two new holes in the chassis since the new piece is even larger than the original but also enlarging two other existing holes to allow for the wires to be more easily fed down into the belly of the beast. The operation was a complete success and the new trafo is not only silent while it's running and runs much cooler but it looks like the tits with its matte black paint job.

I also replaced the stock POS rca connectors with some bling Cardas rhodium bits from the parts box.

Time to get back to some quality listening time!

[deepak]

Nice Nate, that amp is a looker.

[hirsch]

Another Wheatfield update and this seemed like the place to post it. On Pete's advice I decided to mod the amp further, what the hell it's only money. So I ordered up a high-spec Hammond trafo (not cheap, $100+) and prepped for surgery. Replacing the trafo required not only drilling two new holes in the chassis since the new piece is even larger than the original but also enlarging two other existing holes to allow for the wires to be more easily fed down into the belly of the beast. The operation was a complete success and the new trafo is not only silent while it's running and runs much cooler but it looks like the tits with its matte black paint job.

Nate, what's the part # for that transformer? That looks like something I should do to my own HA-2.

[n_maher]

Nate, what's the part # for that transformer?

Hammond 370HX. Best price I found online was through Angela Instruments, took about a week and a half to get it which isn't bad in my book.

That looks like something I should do to my own HA-2.

You've got email.

[n_maher]

Well, my Wheatfield tube sampler arrived today from Radio Electronic Supply so I now have all of Pete's recommended output tubes. Included was a Tung Sol 6528A, TS 7236, RCA 6080, TS 5998 and an RCA 6AS7G, wuhoo!

[JBLoudG20]

When I had my 6SN7 -> 6AS7G amp, I was a big fan of the Tung-Sol 6080.

[n_maher]

When I had my 6SN7 -> 6AS7G amp, I was a big fan of the Tung-Sol 6080.

Thanks man, I'll look into that tube. The good thingi about 6080's is they're pretty cheap (generally speaking). I like the NOS RCA that I've got in there now.

In bad news it would appear that the 6528A that I got is unhappy. I put in the amp last night, powered it up and it makes this occasional pinging sound. It's like of like the sound your car's engine might make while cooling off. I let it go for a little while hoping that it was just the tube warming up but it kept going for 20min. at which point I shut it down for fear of damaging the amp. I'm hopeful that I'll be able to return it otherwise I just flushed ~$65. Grrrr.