Best way to use a PAIR of 6SL7’s to drive a 300B?

[sgerus]

I’ve been doing some research on high end 300B amplifiers and I noticed that both the Cary CAD 300SE and the Canary Audio CA-308 use a pair of 6SL7’s for the input/driver.

Does anyone know how they use the pair of 6SL7’s

IE
SRPP, mu-follower, both tubes and both sections in parallel?

 

[kevinkr]

The Cary design uses the 6SL7 in SRPP, and (in my opinion) to get the drive voltages required exceeds the design maximum limits for plate voltage and probably filament/cathode insulation as well. IMHO this is about the worst way to do it that I can think of.

See my comments here: www.diyaudio.com/forums/showthread.php?s=&threadid=113306

I have used the 6SL7 in SRPP at 2mA for well over a decade with great success to drive 2A3/45 which require about 80Vpp for full output, however depending on OP point the 300B requires up to 200Vpp for full output and this is really pushing it.

 

[Miles Prower]

I’ve been doing some research on high end 300B amplifiers and I noticed that both the Cary CAD 300SE and the Canary Audio CA-308 use a pair of 6SL7’s for the input/driver.

Does anyone know how they use the pair of 6SL7’s

IE
SRPP, mu-follower, both tubes and both sections in parallel?

Best way to use a pair of 6SL7s to drive a 300B? Connect 'em to a Source Follower, or Cathode Follower. The 6SL7 has a very limited current capability that is suitable for driving a high impedance, low capacitance load. That does not describe the grid circuit of a 300B. Additionally, these DHTs also tend to start pulling grid current even before the Vgk goes positive. There have been lots of reports going around that 300B's have given disappointing results. That, I believe, is due to inadequate grid drive. And inadequate grid drive is what you're gonna get if you connect that 300B to a 6SL7.

SRPP: Fugedaboudit! This is a trendy circuit used by folks who don't understand what it is or how it works. The SRPP is an active pull up/down circuit. Due to the lack of a "P-Channel" tube, it's an asymetrical combination of grounded plate and grounded cathode amps. It will give balanced currents for one specific load impedance only. It works just fine as a line driver operating into a low VSWR T-line at its characteristic impedance, which is what it was originally designed to do. Let the load impedance vary, and the SRPP goes out of balance and distortion rises. Constant load impedance does not describe the grid circuit of any DHT.

 

[sgerus]

Thanks Miles

That is one of the best explanations of SRPP I have ever read. I like the idea of using the 6SL7 as a grounded cathode input stage, and then a 6SN7 as a 10mA cathode follower.

I’ll give that a try.

 

[gofar99]

Hi Everyone, Another thing to watch for if you should use an SRPP is the heater to cathode voltage limits. I cooked a pair of ECC802S tubes that way. The upper cathode can easily be above the rating. The heaters will have to be raised to around 1/3 of the VCC to keep them within the safe zone. BTW I'm using a 6SL7 in SRPP to drive a pair of 6L6GCs in my current project. Within the ratings they work fine.

 

[Eli Duttman]

Sgerus,

Both Kevin and Miles made good points. You should be able to get the voltage swing needed from a choke loaded 6SL7 SRPP. Use a CT 6.3 VAC filament trafo and bias the CT to +95 VDC. The SRPP feeds an IRFBC20 MOSFET source follower, which is DC coupled to the 300B's grid. MOSFET Follies describes the technique and our own Tubelab's power drive is a good implementation.

Yes, "sand" in a SET amp's signal path! SY will tell you just how good MOSFETs are for this job.

Using the direct follower driver is absolute insurance against blocking distortion and you have the option to enter Class "A2" territory, should you so choose.