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Old 9th December 2007, 04:42 AM   #1
kmtang is offline kmtang  Canada
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Default Cary Cad-300b Se

Hi,

I got a pair of CARY CAD-300B SE monos from my friend. I tried it out in my home. I noticed that the mid and high are pretty good. Unfortunately it does not have sufficient bass.

I checked the circuitry. It employs 6SL7 SRPP as driver. The B+ for 6SL7 is 450Vdc. The cathode resistors are 4.75K. The bottom cathode resistor is bypassed with a 500uF 25V electrolytic capacitor and a 2.2uF Solen film cap.

The coupling capacitor between the 6SL7 and 300B is a 0.22uF vitamin Q like capacitor. The 300B is with fixed biasing and the B+ is 440Vdc. It is set to 60mA bias current.

I am wondering if any one have worked on this Cary 300B and any experience in improving the Bass of it.


JOhnny
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Old 9th December 2007, 05:28 AM   #2
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60mA sounds like a very conservative bias current. i shoot for at least 70-75mA.
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Old 9th December 2007, 10:42 PM   #3
kmtang is offline kmtang  Canada
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The B+ is about 440Vdc with fixed biasing. Therefore, I am afraid to run the 300B to its maximum power limit.

Anyway, I placed a 0.22uF Jensen copper-in-oil in parallel with the Vitamin Q like coupling capacitor in the amplifier this morning. Both dynamic and bass were improved.

One other thing I found is that the B+ filtering is:-

5U4GB - 1000uF - 20-ohm - 1000uF.

The first filter capacitor is way way higher than the specification of 5U4GB. I just put a diry chip 5U4GB that I got from swap meet at $2 each. I saw sparks came off when first power on the HT.

I have also measured the primary inductance of the OPT which is 24H according to my DMM. May be this is one of the reasons why the bass is not powerful enough.


Johnny
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Old 10th December 2007, 01:51 AM   #4
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Quote:
Originally posted by kmtang

5U4GB - 1000uF - 20-ohm - 1000uF.

The first filter capacitor is way way higher than the specification of 5U4GB. I just put a diry chip 5U4GB that I got from swap meet at $2 each. I saw sparks came off when first power on the HT.

1000uf?

its crazy....
the 20ohm is the DCR of the supply choke? or a power resistor?
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Old 10th December 2007, 03:39 PM   #5
Sherman is offline Sherman  United States
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As Commstech says 1000uF after a 5U4G is way too much! That rectifier is rated for 40uF when used with a cap input filter. I find it hard to believe that it isn't arcing and self-destructing with 1000uF.
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Old 10th December 2007, 03:48 PM   #6
sgerus is offline sgerus  United States
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Quote:
It employs 6SL7 SRPP as driver. The B+ for 6SL7 is 450Vdc. The cathode resistors are 4.75K
Hi,

Are you shue that's 4.75K? 475R would sound more like it.
What is the voltage at the cathode of the bottom 6sl7

-Scott
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Old 10th December 2007, 04:03 PM   #7
kevinkr is offline kevinkr  United States
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440V fixed bias with any 300B is pushing your luck, the JJ and WE 300B can handle these voltages, but just a few years back none of the Chinese made ones could.

Unless you can get these for a very good price you can build a better amplifier, and probably for less money.

Cary has a long history of to put it nicely "questionable" design practices.

I have used 6SL7 SRPP to drive 45 and 2A3 since the 1990's - it does a great job with these tubes, but the 200Vpp swings required to drive a 300B at these plate voltages seems imo to be asking a bit much.

1000uF input capacitance is unconscionable with a 5U4, some of them will arc with this value of capacitance, and all of them will have considerably shortened service lives due to the very high repetitive charging currents, not to mention the inrush current as the rectifier warms up.

24H is not an unreasonable primary inductance for a 300B provided that value is maintained at the design plate current. This should give you flat response down to somewhere around 30Hz ime.. (Tamura F-5002 has similar primary Z.) Note also that your meter probably is not that accurate, and the inductance varies as a function of standing current and the ac signal current superimposed on it - in operation the inductance may be higher or lower than you measured.

Given the way Cary designs things I would not assume that the OPT has a lot of current headroom built into the design so I would not change the op point. (Verify that the output transformer is not saturating at full power when you do. If overall power increases and THD at the previous highest output power decreases them you have a little margin to play.)

Not my place to say, but were it my amplifier I would reduce the input capacitance to 50uF or less, (check winding dcr if less than 220 ohm end to end use <40uF cap) switch to a 5AR4 rectifier, install a nice hefty 10H choke with a dcr suitable to drop the plate voltage to 400V with a 60 - 70mA Q point, change the second filter cap to a 100uF PPIO motor run cap and I would boost the current in the 6SL7 SRPP to 2mA, and install a better and larger coupling cap.
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Old 10th December 2007, 04:11 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by sgerus


Hi,

Are you shue that's 4.75K? 475R would sound more like it.
What is the voltage at the cathode of the bottom 6sl7

-Scott

Actually I use 6SL7 in SRPP at 330V (I'm pushing it) - with 1K cathode resistors the q point is 2mA. I suspect the idle current here is under 1mA, and frankly the tube is running way beyond its safe voltage ratings for both max plate voltage, and cathode/filament insulation breakdown rating. Another fine example of the Cary approach - no respect for design maximums.

6SL7 specs here: http://www.mif.pg.gda.pl/homepages/f...3/6/6SL7GT.pdf
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Old 10th December 2007, 04:25 PM   #9
sgerus is offline sgerus  United States
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Kevin,

I was thinking that each tube would have its grids, plates and cathodes connected in parallel, so you could get more like 4mA through the srpp. And, the upper valve could have its own filament supply if needed, based on the voltage of the upper cathode.

In other words this is a 2 tube SRPP with each tube connect in parallel?
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Old 10th December 2007, 04:40 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by sgerus
Kevin,

I was thinking that each tube would have its grids, plates and cathodes connected in parallel, so you could get more like 4mA through the srpp. And, the upper valve could have its own filament supply if needed, based on the voltage of the upper cathode.

In other words this is a 2 tube SRPP with each tube connect in parallel?
Hi Sqerus,
Been there, done that - it does address the fundamental issues you raise, but frankly a 6SL7 or 6J5 driving a 6SN7 or 6J5 based SRPP will almost certainly sound better IME.. Run the 6SN7 at 10mA or so.. Because the 6SN7 is a medium mu tube another gain stage is required as noted above. Another option D3A or 5842 driving a transformer.
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