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In Reply to: RE: Decipher This posted by GEO on November 17, 2014 at 15:40:53
GEO,
The big problem with mesh tubes is they tend to draw more grid current for some reason. The Moth's and other directly coupled amps that did not have good time constants for the power supplies, would tend to push the grid forward when turned off and this would cause the output tube to arc over.
There was tons of stories like this with the EML and other mesh plate tubes. But I think the problem was the design of the tubes. I have a boat load of NOS mesh 45's and never had problems other then just dieing. But the newer ones I had customer problems and tons of people they just died and I don't direct couple them.
I think they are using gold grids now for these tubes to better insulate them from drawing grid current.
Thanks,
Gordon
J. Gordon Rankin
Follow Ups:
Hi Gordon. I fondly remember you from a show or two...
I had 40 of those AVVT mesh plates. They had hand-picked them over in Germany, so the ones I got were all later types and were nice. (I had bought some of them earlier in the USA, but those were "earlier" batch-- and had the grid problems). They were 100% reliable for me anyhow, but I didn't like their hum levels.
The newer ones had far lower hum, the plant had gone to work to better balance the filament. I suspected that because I was using balanced A.C. to run the filament-- the difference between the two batches was huge.
I have put many thousands of hours on these tubes with zero failures and zero "wear-outs". The plate currents I was using-- I found out later from JAC, were what he always used. About 2/3 "normal". Plate voltage was set up as 490 ON the plate, and 240 on the cathode, giving 250 VDC plate voltage. (D.C. amp).
What I love about these tubes is their transparency. They can SOUND "superconductive".
Kevin Hayes (VAC) had coached me into trying JJ's, which he had fallen in love with (big, rugged, easy to drive, (bold!), powerful sound). Idealize an amp for those and the mesh sounds a little "off", certainly not very "off"-- still more than acceptable in the tube amp world.
I would run those old mesh tubes for thousands of hours, running loud levels (I love it-- it stays clean) just running them hard, cooking, doing dishes, working outside. I just let them run like an appliance.
For shows or visitors, I always had a super-good pair, one that I had hand-picked, balanced in every way, including the resonant frequency tests. Those would have had about 1000-1500 hours on them, (broken-in) and them they were stashed, and only run every 6 months or so, for a short period.
Interesting tube (last batches). Idealize an amp for that tube, and you'll never forget it.
---Dennis---
Do you mean shaking them and listening to what they sounded like to determine if the sound matched?
RFT is used in many industries.We use it in the lab where I work on some of the test gear, heavy industrial applications.
Link to AN 8.1 Microphonic tubes.
If DF does any advanced analysis beyond tapping on the tubes is unknown to me.
Edits: 11/20/14
Is it done by ear?
I've seen DF tap on uninstalled tubes at multiple RMAF shows, with the tube being up to his ear. He seems to know exactly what to listen for !!
Sorta amazes me. I wanted to giggle, but I am sure he is serious.
Jeff Medwin
OK. If you don't have a good mike and a spectrum analyzer present, then you can still do what it takes.
The tube that resonates identical to another, and their currents and internal capacitances are equal also, are a good pair. Is this important in S.E. amps where only one output tube is used? (normally-- one could have parallel S.E.). Sure it is-- you're trying to get one amp to track with the other.
If you have two tubes that have the same internal capacitances and draw equal currents at idle, but they resonate differently from each other, usually because their filaments are different-- one is stretched tighter than the other-- then they will sound VERY different from each other on equipment that is good enough for you to tell!
So--- WHY NOT balance the two channels (in Stereo playback mode) sonically with each other? It sure does sound more coherent when you do.
-Dennis-
So that is why my quad of 45s sounded funny as two pairs I thought were identical were not.
It was only when I matched 1 tube from 1 pair with another that I got magic.
And it didn't correclate with the specs on the boxes either.
The other pair sounds half way between the best I have and the worst I had.
Trust your ears as Jeff keeps telling me.
Smart
Tubes and capacitors! Best thing we can do with both is have a box full of them (in caps, you need different brands as well), and play.
Let THEM tell you what is the correct applications and values. You will have far more useful "data" than just theory to figure it all out with-- later. Keep the Science & Theory HANDY, just don't FALL IN LOVE with it!
I see you're a thinker:
You've seen all the criticism on my old amp on here. Take another look at the picture posted by GSH and see if you can figure out why the main ground bus wire went straight to the IEC socket ground, and why it was routed that way instead of going around the other side of the amp where all the Power transformer A.C. lines are.
People like to make amps look pretty inside. I do what I can and is practical, when that works. Can you figure out why there's a "bird's nest" wherever many capacitors are paralleled? Why are the leadouts (from each cap) not straight, why are they coiled, or bent? (doesn't look as pretty!).
Why are leadouts from the power trans left as tinned-copper, but elsewhere in the amp, a lot of silver is used? How did I get away with a Hammond, unshielded power trans? Why was it used?
Why is so much brass used? What fasteners must be brass, and what ones can just be steel?
What spacing is necessary when you bundle caps, and why is silicone glue used to bundle them? Why aren't they allowed to touch each other-- or be near anything else? (In each bundle, they're separated from each other by glue pads).
Back to the "bird's nest": The photo is 2-dimensional. What is happening in 3 dimensions that is important in mounting parts-- sonically, and, since they can't be allowed to touch anything or be too close to it, what anchors them? Why is the chassis as deep as it is?
Where is most of the A.C. hum in this amplifier coming from, and what efforts were made in later years to cut it back, but still not affect the sound qualities?
What did the manufacturers of some S.E. triodes do in the last 3 years to reduce hum levels?
Why can't Zip-Ties or cable bundling (to look neat) be used to anchor parts or wiring?
What wires CANNOT be anchored or bundled with, or be near-- anything?
What happens when grounds are "dressed" close to the chassis metal, and how is this different from signal-carrying wiring?
What about B+ wiring? How is it different from the other two?
What happens if one of my old amps falls off a table onto the floor-- and what protects them if one does? How does this relate to shipping them around, using UPS or FEDEX?
What happens when even the best of modern capacitors are used to bypass a cathode "fully" (that is with both the cathode resistor value and the tube's calculated "resistance" rolled-into one, and that figure used to determine the cap's value). What happens sonically?
What can you do to inch closer to the calculated cathode cap value and get away with it sonically?
If you put two resistors in series on the cathode, and bypass only one of them, why will that "sound better" than having only one resistor fully bypassed?
What is there about putting two resistors in "the tail" (cathode) in series, and bypassing only one of them that will wreck your signal linearity and timing accuracy on Hi-Eff. speakers? Why can't you get away with this, and what works vastly better?
These questions are open to all members-- GSH opened things up, so let's figure it out.
I like your system ideals, and I think you're going to be a Happy Camper!
---Dennis---
The problem is Dennis, you don't know the answers yourself. And what theory you do post is hogwash to anyone with an engineering background - no matter how it was obtained. IOW,it's not just the professional engineers here that debunk your fantasy. It's most of the experienced and competent hobbyists as well. Anyone who knows how these circuits actually work see through all your hype.P.S. And no matter what sonic benefits you think you reap with your IEC ground wiring, I can tell you it's not approved by UL or any other world safety agency. You are not permitted to ground primary power in that fashion.
Edits: 11/21/14
LMAO, first you say "good questions", and then....."DF doesn't know".
What you failed to realize, conceptualize, is that DF has already asked ALL these questions to himself, likely years ago, and found the answers that worked well for him. He was simply "opening up" the conversation on this Forum to people who wanted to discuss it, in a legitimate non-offensive way .... on a higher level than what we generally see in this thread.
Are Nuns all just whores to you Gusser?? Is white really black, and red really blue ??
Jeff Medwin
I'll bet he has some input to add when it come to the electrical engineering skills you and Fraker posses.Let's get another professional opinion on this build.
Edits: 11/21/14
Henry's no problem, except for his bias and dislike of DF ( and I ) which clouds his even-handedness.To his credit, it was Henry who very nicely explained the superior regulation obtained by a less than L-Critical L1.
Search word is Flywheel. Its a real shame hes never been to a show, etc. to hear the amps perform. Same for you, countless others.
Jeff Medwin
Edits: 11/21/14
Not seeing exactly where the IEC ground is connected to the chassis.
Perhaps the photo does not show it clearly.
What problems are you finding, Gusser?
1) It's not good engineering practice to run a dirty AC line ground back right through the input stage components. Furthermore UL and European codes dictate the power safety ground must be bonded by a dedicated purpose means. That means it cannot share with any other wire or component mounting means. Now we surely don't have to follow that rule for DIY but Dennis sells these amps to the general public. They should at least meet the basic electrical safety standards.2) The negative side of the main filter cap bank is also run back under the input stage. This lead will have substantial 120hz spikes from the rectifier action. Now the use of a tube rectifier does tame these pulses quite a bit. But it's not good engineering practice at all.
But it still might work fine in this amp. Ground loops and EMI/RFI are quite difficult to thoroughly analyze. Even experienced RF engineers as is my area of study, often joke about the witchcraft needed to tame EMI/RFI. But be assured it can be done through mathematics.
So Dennis either got lucky, which is very possible, or there is 120hz buzz on his output.
Also keep in mind that the LSES is deficient in it's filtering ability. So the additional 120hz buzz due to this poor lead dressing may in fact be masked by the amps poor PSRR.
Edits: 11/21/14
The IEC ground wire is firmly bonded to Chassis metal BEFORE it goes anywhere else.
You don't see that in the photo. Your claim is understandable-- from just looking at the photo, but it is wrong-- safety has always been the first concern-- circuitry comes second.
In other words, chassis cannot be disconnected from IEC ground regardless of any component failure(s).
---Dennis---
A
Exactly. But it's good to understand why. Easier to duplicate if you like that kind of sound. Like changing the tint setting on your tv. Good to know where your settings are when using tone controls.
Maybe he could share what he is listening for with the rest of the world. Thanks.
See above, he does.
Jeff
Must have good hearing.
Thanks Gordon. I hope life is good in the Queen City. I recall those AVVT 2A3 problems. I was just trying to make heads or tails out of the explanation for why mesh plates sound good. I used to use mesh 27 and solid plate philco 27s. I preferred the ST philcos even after I got around the microphonics in the globe mesh 27s, but to each his own, unless you disagree with a couple fellows here.
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