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180.200.147.106
Is there one that has lower Z-out - than a 12AU7 in CF mode?Why? Has to fit in a QUAD 22 box. ? After it was a QUAD 22 it was an early Allen Wright FVP, built by Allen for me way back when he was in Australia.
(Still is a four valve pre-amp, but only two 12AX7s in the phono stage. 12AT7 line-gain and a CF'd 12AU7. 50K blue ALPS stead of 250K, and a far better PSU.)
Searching shows 5687s are close, but just a teeny bit too tall at 1 15/16" max, and they are rare anyway.
We could add a second steel strip to mount more short valves.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 01/04/16Follow Ups:
the outer shell is pressed steel and wraps around the guts - attached to the front panel - by about 4mm.To take one apart one has to undo two long bolts at either side of the back. Then the outer shell pulls off backwards / or you pull the guts towards you.
The height limit I reported engages with that ~4mm wrap around.
So, it just is not as simple to put in a taller tube, by cutting a hole in the top of the outer shell - unless one is willing to wait for cooling to occur and pull it out - before dismantling the case.
But, given the slight projection - of about 5-10mm max - above the top with all the other tt options - I'm not real keen on trying to push in / pull out such a tube, with the box closed up.
IF something is wrong WITH the pre-amp - like say the CF output tube has died?
Well, I would have to wait until I - MIGHT be ABLE - to pull the longer and HOT tube out (which is never going to protrude enough to get a really good grip on it) vertically - BEFORE - I could take the pre-amp apart.
........................
Simply put, is the 5687 family going to give me a lower output Z in CF than any 12AU7?
If so, I'll be buying a good few that come in as short as any 12A_7.
TIA.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 01/05/16 01/05/16
Tim,
It just might be time for a little "sand". ;> ) An IRFBC20 MOSFET will get you a lower O/P impedance than the tubes mentioned. It also happens to be very volumetrically efficient. Low reverse transfer capacitance MOSFETs make very transparent voltage followers.
Eli D.
That's probably the ticket.
With the line-drive tube eliminated I will likely have enough room for an IR remote ALPS VC.
And, maybe room to put SE to balanced transformers just after the FETs. Or just behind the pre box.
That would allow a balanced in ADC kit to feed a DSP-Eq box at my seat, down the track.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
The 5687's that I have are all the exact same size as a 12ax7.
I did a search on ebay for the 5687 and got 400 auctions.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I will try buying a few 5687s.IF those short-arse 5687s will - in CF - give a usefully lower z-out than a 12au7?
How many can you spare?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 01/05/16
TubeCad says a CF with a 5687 has a Zout of 116 ohms vs. 432 ohms for the 12au7.Here's a pair of tested good GE 5687WA JAN for $13.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/05/16
How much high voltage current do you have to spare?
But I don't have an exact figure.
He might remember - he serviced the pre late 2015.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
The 5687 can be run at higher currents than the 12AU7 to accomplish what you're looking for. The ECC99 will plug right in for the 12AU7, but can be operated much like a 5687 (the 5687 has a different pinout).
The 6DJ8 family can technically get you pretty low (sub 100 ohms), but you'll need to know the maximum expected voltage you intend to deliver and the minimum load you plan to let the stage see to determine if this is a suitable design for you.
IF I have to I will go with the late UncleStu's idea and cut a hole above where the line-out tube socket is.A 12BH7 might be a drop-in as I have heaps of reserve heater current. An ECC99 might be too.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 01/04/16
Tim,
The ECC99 bottle max. height is 62.2 mm. = 2.45 inches. The max. bottle height of the 12AU7 is 1.94 inches = 49.2 mm.
If you cut a clearance hole, make its diameter large enough to hold a protective silicone rubber grommet. Silicone is more heat tolerant than neoprene.
Eli D.
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