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Old 3rd September 2006, 07:21 PM   #11
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More on microphonics -

I've had very good results with diff pairs in the line stage, which were mounted on a 4mm thick alu top plate - simple as that, just mass, no rubber damping.
I'm getting worse results with a SE setup in a normal type preamp case, 2U high with a 3mm alu bottom plate - seems to ring more, even when I put the tubes on a teflon sheet decoupled with rubber from the chassis. that did help, because it was worse before, but I'm going back to thinking you need mass. Like thick alu, granite, whatever. I'm starting to believe that the whole chassis must be quite dead - no ringing anywhere.
Otherwise the microphonics comes though the volume control, cables, all sorts of things. Anything that rings seems to set it off.
If anybody has got further than I have with this, do let me know. I was in the process of selling a line stage to a friend which we both agreed sounded great and I ended up putting it on a cushion and chucking another cushion on top of it (that silenced it!!). I took it back to the bench to solve the problem, so have to make some progress to make the sale. Andy
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Old 3rd September 2006, 07:47 PM   #12
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My question re microphonics was actually meant to mean wheather there was anything one should do with a DHT as opposed to other tubes. In other contexts, to reduce microphonics, I have lined the inside of a chassis with dynamat which worked pretty well.

As for transformers, thoughts on these? http://jianshin.myweb.hinet.net/PRE_OUTPUT_TRANS.htm
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Old 3rd September 2006, 09:56 PM   #13
kevinkr is offline kevinkr  United States
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Hi Dave,
You probably want to look at the local James distributor's web site if you can find it. (I had it, but with the last crash some months ago lost all of my bookmarks.)

I assume you are talking about the 71A as a candidate here, in which case 5K is a good choice, however it is not clear to me whether these are SE or PP transformers. Unless building a PP pre you don't want a PP transformer as it is not designed to cope with the quiescent dc in an SE application.

Don't expect much voltage gain with a 71A and an output transformer as opposed to a choke. You will however get much lower output Z.

Heavy mass loaded chassis, possibly shock mounting the dhts, and power supply remotely located work well.
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Old 3rd September 2006, 10:08 PM   #14
Sherman is offline Sherman  United States
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For James transformers check out Euponia Audio, that's where I got mine!

(No association, just a satisfied customer.)
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Old 3rd September 2006, 11:18 PM   #15
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More on microphonics:

A friend just reminded me NEVER to go over the rated filament voltage, and in fact to stay 5 per cent at least under it. Reason being that DHTs can start to scream above their design filament voltage. I'll have to go back and check mine. He also thought it was possible that in diff pairs the difference in frequency of ringing might cancel out to some degree - certainly was less of a problem with diff pairs. All this is worth noting - you can have some nasty surprises with DHTs in line stages, like really crazy feedback so you have to quickly shut the amp off.
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Old 3rd September 2006, 11:20 PM   #16
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Quote:
Originally posted by kevinkr
Hi Dave,
It's Doug by the way.


Quote:
You probably want to look at the local James distributor's web site if you can find it. (I had it, but with the last crash some months ago lost all of my bookmarks.)

I assume you are talking about the 71A as a candidate here, in which case 5K is a good choice, however it is not clear to me whether these are SE or PP transformers. Unless building a PP pre you don't want a PP transformer as it is not designed to cope with the quiescent dc in an SE application.

Don't expect much voltage gain with a 71A and an output transformer as opposed to a choke. You will however get much lower output Z.
The plan is to try 71a with a plate load of either a ccs or a choke, or maybe just a resistor in the first go to see what happens. I am investigating transformers in case I decide I want to go that route with a different tube. And, for now, I have some edcor 10K:600 transformers that I can use to experiment. Just figured I'd do the reserach to figure out where to get thjem, how much they are, etc. so I can plan a bit.


Quote:
Originally posted by Sherman
For James transformers check out Euponia Audio, that's where I got mine!
I do have a pair of james transformers from euphonia for a headphone amp (still waiting for some parts to complete the assembly) but they don't seem to have the preamp type in stock. I have seen a few posts that James will begrudgingly send you some at a good price if you bug them enough, but otherwise I can't find any other distributors.

-d
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Old 3rd September 2006, 11:31 PM   #17
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by kevinkr
The 71A has a cgp of 7pF and a mu of slightly less than 4 which would result in a Miller capacitance of worst case about 28pF
Your mind must have been on other things; if Cag = 7pF and mu = 4, then at best the Miller capacitance will be Cag(1+mu) = 35pF. Plus Cgk, probably about 5pF, means 40pF is a likely figure. Not a problem provided that the volume control pot is 100k or less.
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Old 4th September 2006, 02:12 AM   #18
kevinkr is offline kevinkr  United States
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EC8010 It must have been one of those days I guess, you are of course absolutely right Cag(1+mu)=Cmiller.. Ouch....

And Doug my apologies for calling you Dave, having been suitably chastened I won't do it again..

You might want to take a look at some of the Magnequest wares as I suspect by the time you factor in shipping by anything other than surface a pair of James transformers shipped from the Far East is going to be in the same price range. Mike's plate to line transformers are very good. (from experience)
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Old 4th September 2006, 08:34 AM   #19
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Quote:
Originally posted by kevinkr
Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.

A lot of these tubes (30, 26, 01A at any rate) seem to have very similar operating points but different filament voltages (the datasheets all suggest 135V, a grid bias around 8-10V, and a plate current between 3 and 5.5mA). Is it reasonable to build an amplifier for one or another and (adjustng the filament supply) simply swap between them? Obviously I am getting ahead of myself here, but I am curious if there is some clever bit I am missing.

Doug
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Old 4th September 2006, 01:53 PM   #20
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Quote:
EC8010 It must have been one of those days I guess, you are of course absolutely right Cag(1+mu)=Cmiller.. Ouch....
Well, since we’re getting picky about the definition of the Miller effect, the multiplier is really based on voltage gain, not mu, and it is 1-Av, where Av is the voltage gain (note that Av is negative for common-cathode stages). Only if the plate load is >>rp, AND if the cathode resistor is bypassed, will Av approach -mu. In those cases, 1-Av = 1+mu, as you said. Otherwise the magnitude of Av will be less than mu and would have to be calculated using the usual equations. For quick and dirty estimates of Miller capacitance, I agree that 1+mu is a conservative guess. I know that EC8010 and Kevinkr know this, but just to make sure the record is straight for others… And I’ve had nothing else to post this morning yet
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