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Old 23rd August 2009, 01:28 AM   #1
ppchiu is offline ppchiu  Canada
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Default 6sl7 srpp kt66 se

Hi All,

This is going to be my first tube project. I already have the tubes and transformers. I wanted to get everyone's opinion about this schematic.
http://www.analogmetric.com/forum/viewthread.php?tid=8.

The only thing i notice about this schematic is that it runs the KT66 at 370V which seems a bit high.

I'm really interested in the SRPP topology but I would like to know how to analyze it as I need to know which values to use. Can someone point me in the right direction?

Thanks in advance!
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Old 23rd August 2009, 02:27 AM   #2
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The voltage across the KT66 is not 370V. It is 336V (the 370 less the 34v on the cathode). This is all well within the KT66's range and is probably a bit low.
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Old 23rd August 2009, 05:42 AM   #3
ppchiu is offline ppchiu  Canada
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I'm looking at http://tdsl.duncanamps.com/show.php?des=KT66 and it shows quite a lot of operating points for the KT66. Can someone tell me what i should be looking for?
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Old 23rd August 2009, 07:51 AM   #4
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In SRPP mode u /mu/ is aprox. 30 % more than for one triod, and
in this case Ri SRPP is very small and this is good agreement with Your KT66.
I think Ia 0,5 ma first stage is small, I prefer You Ia=1-2 ma, and use two
resistors apr. 1-1,5 kohm in cathods instead 3,6 kohm.
I will use output transformer 5-6 kohm /Hammond 125 ESE or FSE-Canada - apr. 50 US$/ instead 3,5 kohm /it is small/ and I will remove 680 and 47 ohm
/cathod resistor to ground/.
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Old 23rd August 2009, 02:02 PM   #5
kevinkr is offline kevinkr  United States
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I used 6SL7 in SRPP configuration to drive a 45 in my first SE amplifier design about 12yrs ago. Azazello is right Ip should be in the range of 1 - 2 mA with my preference being around 2mA.

The operating conditions in this amplifier were 330V on the plate and 1K cathode resistors resulting in 165V across each triode at 2mA. I would probably use a 100uF electrolytic (Blackgate if you can get it) across the cathode resistor of the lower tube in the SRPP.

The 330V is convenient because you can use a 10K 3W power resistor to drop the 370V supply you are using to power both drivers. Use a 50uF/600V film cap (Solen/Aeon) for decoupling and you are good to go.

For general ideas refer to my site and look at the amplifier project section for the driver stage topology.

The circuit works well, and sounds good.
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Old 23rd August 2009, 05:48 PM   #6
ppchiu is offline ppchiu  Canada
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Thanks for all your input. How is Ia determined? Also, I've looked compared this SRPP circuit with others and this has a couple extra components after below the lower half of the 6SL7. (47ohm, 68k ohm, 220kohm and the 100mircoF cap). Does anyone know what it's for?
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Old 23rd August 2009, 09:23 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by ppchiu View Post
Thanks for all your input. How is Ia determined? Also, I've looked compared this SRPP circuit with others and this has a couple extra components after below the lower half of the 6SL7. (47ohm, 68k ohm, 220kohm and the 100mircoF cap). Does anyone know what it's for?

The 100uF/68K/220K resistor have nothing to do with the SRPP cathode circuit, the schematic shows them sharing a ground connection, but in fact this circuit provides a common mode dc voltage on the filament supply to prevent cathode to filament insulation breakdown in the upper triode of the SRPP. We refer to this as "elevating the filaments above ground" and it has been discussed a lot on other threads here.

The 47 ohm resistor is part of the feedback network returned from the output stage. IMO because the KT66 is very linear in triode connection I would recommend NOT using global feedback as provided by the 47 ohm cathode resistor and the 680 ohm feedback resistor from the secondary, at best this is just a few dB of feedback - and I have (arguably) found it does nothing good for the sound and is insufficient to significantly reduce output impedance or distortion. I like them better without.

I would delete the 680 ohm resistor from the secondary of the output transformer, and replace the 47 ohm resistor with a "link" - just a piece of wire. Replace the 3.6K resistors with 1K resistors and you are good to go.

Otherwise I would build pretty much as shown, except that the missing safety ground on the IEC power inlet must be present.

For the amplifier audio grounds do NOT use the chassis, use a heavy ground bus and connect that to the chassis at one point through a ground loop breaking network. (See below) Connect all of the other signals to that ground bus, however connect the center tap of the HV winding directly to the negative terminal of the first supply cap and from there to the ground bus, connect the second cap nearby, and nothing in between the two, the output stage cathode resistor and cap next, and finally the input tube cathode resistor, bypass cap and (isolated)rca ground return. Typically I connect the chassis to the bus right after the 2nd filter cap. (with the ground loop breaker)

Ground loop breaker: Referring to the ground bus single point that you connect to the chassis I usually use a 10 ohm resistor and a pair of anti-parallel diodes all in parallel to break potential ground loops while allow me to safely ground the chassis directly to mains earth.
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Last edited by kevinkr; 23rd August 2009 at 09:35 PM.
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