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In Reply to: RE: I have the opportunity to buy an ARC VT100MkIII - for QUAD 63s - any issues?!?_Issues posted by Timbo in Oz on June 06, 2016 at 02:54:09
Tim
The amp is a 100 watt class A/B amp. I repaired and recapped an MK2 version for a guy in Indiana 3 years ago but this one used 4 6922s per channel as opposed to the 6H30. It was actually hit by a lightning power surge and State Farm insurance covered it. I see very few ARC amps and this one uses a mosfet for the screen regulator which I like instead of the 6550 they used years ago. I ended up replacing it along with the OP amp
and several zeners.The amp sounded very good once I rebuilt the power supply and put in better coupling caps but it was kind of funky to work on. I had to completely disassemble the amp by pulling the face plate and side panels to work on it being the boards are mounted vertically on the sides of the unit.Once it was apart,it was pretty straight forward and I understand why they had to do it this way because in order to set the boards flat,it would take up much more room.
The reason I told you all this stuff is because I want you know what's involved if you ever need to service it. It's a good sounding amp but if you are going to roll tubes,I wouldn't buy an ARC amp..This is the type of amp that likes to be put in service and left alone.If you keep yanking tubes in and out of it,you are going to have problems with loose socket pins and then it has to be taken apart to access the pins to tighten them back up.Find the tubes you want,put them in and bias them.
Are you using your stereo 20s strapped in parallel? Like Chip said,you could be very disappointed if you really like the more purist Leak sound and the ESLs are not very forgiving.
I have modern Martin Logan ESLs and I drive them most of the time with a pair of McShaned Citation 2s strapped in mono.I tend to like the wide bandwidth vintage amps on ESLs.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Follow Ups:
And, see my detailed response above.
If I were to strap them - to drive one QUAD each - which OPT taps to use? They have 3 taps 4,8 and 16 ohms. I'd like to have the lowest possible output impedance.
And I'd have to turn them matching amps, which they aren't in their PSUs. The bass duty (Pentode) amp has 4 of the 470uf Nichicons
Currently one is in pentode giving 20WPC 20-20k and one in UL 16WPC 20-30K. So pentode mode or UL?
And if I was going to pay for PSU work and re-wiring I might as well go with plastic PSU caps - to replace the 400V 470uf Nichicons, and use at least one choke.
What choke (or chokes) IF CLCLC OR LCLCC, and which Polypropylene caps?
We'll be staying with four SR SS diodes in the bridge rectifier. I do believe there are more recent and quieter ones than what were around in the 1990s.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
If I were to strap them - to drive one QUAD each - which OPT taps to use? They have 3 taps 4,8 and 16 ohms. I'd like to have the lowest possible output impedance."
To connect for the lowest impedance,you would would parallel the 4 ohm taps which would give you two ohms effective impedance.
"And I'd have to turn them matching amps, which they aren't in their PSUs. The bass duty (Pentode) amp has 4 of the 470uf Nichicons"
The 470uf Nichicons are quite good if they are the 105 degree caps and they even have a few of the film cap characteristics especially if they are the switch mode type power supply caps.
"Currently one is in pentode giving 20WPC 20-20k and one in UL 16WPC 20-30K. So pentode mode or UL?"
Pentode runs best when you regulate the screens and lower the screen voltage in reference to the plate. They would have had to be modified from the original Leak circuit which was UL,but pentode is easier to drive and this one of the reasons you are getting more power in pentode.
If I was using the original circuit but with simple upgrades,I would stick with UL. If you want more power in UL,a 12AX7 will struggle so I would redesign it for a 7247 which is half of a 12AX7 and half of a 12AU7.
Are you bi-amping with these amps now?
"What choke (or chokes) IF CLCLC OR LCLCC, and which Polypropylene caps?
We'll be staying with four SR SS diodes in the bridge rectifier. I do believe there are more recent and quieter ones than what were around in the 1990s."
I love the Schotky bridge and if you did that,good on you and you get the losses that you get with tube rectifiers and they are dead quiet.
Using polys,I use the CDE,Panasonics,or Wimas,and even electronic concepts.. Being you are using the Schottky bridge setup,you can go as high as will fit without harm but I typically like the charging cap to be smaller to have a larger conduction angle.You don't need a lot when you use film caps and even if you kept the original circuit and went with 35uf polys on all stages,the amps would be fantastic as that's what I did on my Leak Stereo 50.
For chokes I would use Low DCR and low hy around .32hy to .50 hy. I have just fallen in love in those chokes in the Mac Mc30s in place of the 150 ohm resistor and it keeps the DC voltage the same believe it or not.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Their current state is a reflection of a previous plan using a three sphere stack with a Manger driver from Germany in the middle smaller sphere running from 150Hz @ 1st order with the two bass spheres tried at 3rd and 1st for LPass.
But the idea of a 1st HP & 3rd LP crossover works quite well with dipole planars, too.
I will go with both the LEAKs in Pentode. I should then have about 40 watts pc into the 63s, and with the 150Hz high pass that should be just enough power. We will just add a 4th PSU cap to the one with just three.
We aren't using 12AX7s anywhere in the LEAKs. The shared input valve is a 6DJ8 and the drivers are 6CG7s with a FET CCS. See URL below my signature block.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
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