Greetings Friends, hope the holidays are treating you well. I've had a bit of a lull recently and had some time to think on my 6B4G project amp. Amp started life as a Lacewood 2.0 but I replaced the OPTs, added two DC heater supplies and installed 6B4G power tubes, now I got a SET.
The amp chassis only has 3 octal sockets, so my options for driver tube are rather limited; I've been using a 6SL7 with 100K on the Plate and 2K2 on the Cathode with 303v supply. Here's the schematic with voltages.
Being a tinker, I can't help wonder if this is the absolute best way, and some reading through old forums has given me the idea to try a 6SN7 as driver with 27K on the Plate and 1k5 on the Cathode. A look at the charts in the RCA manual tells me to swap out the following grid leak resistor with 220k, or perhaps smaller? Would my bypass cap need to change as well? Coupling caps?
Here's my proposed changes.
Will it work? Is it worth it? Should I change anything? Any Miller Cap issues to worry about?
Here's the power supply, if you're curious. This won't change. Thanks Matt for posting all the great designs. Hope you don't mind what I do to em.
thanks for taking a look!
w
The amp chassis only has 3 octal sockets, so my options for driver tube are rather limited; I've been using a 6SL7 with 100K on the Plate and 2K2 on the Cathode with 303v supply. Here's the schematic with voltages.
Being a tinker, I can't help wonder if this is the absolute best way, and some reading through old forums has given me the idea to try a 6SN7 as driver with 27K on the Plate and 1k5 on the Cathode. A look at the charts in the RCA manual tells me to swap out the following grid leak resistor with 220k, or perhaps smaller? Would my bypass cap need to change as well? Coupling caps?
Here's my proposed changes.
Will it work? Is it worth it? Should I change anything? Any Miller Cap issues to worry about?
Here's the power supply, if you're curious. This won't change. Thanks Matt for posting all the great designs. Hope you don't mind what I do to em.
thanks for taking a look!
w
I have a few 6C4C tubes and was wondering about a similar circuit. At the moment I have trioded PSE 4P1Ls, 4 in total, as my output and I'm driving those with an ECC40. I'm loving the sound - very nice tonality on acoustic instruments and especially piano. So in my case I'd use an ECC40 to drive the 6C4C. It's now a neglected valve since it has a rimlock base, but it used to be used in high quality European amplification in its time. Apart from the good sound, which is the main thing that matters to me, it has a gain of x32 which gives you a bit more gain in the amp than you would get with a 6SN7. Not difficult to change the socket if you were interested.
It depends. Not a bad idea, but first, you will need a preamp that can swing more volts than the typical 2 V rms max. With a maximum voltage gain of about 10 times with a single 6SN7 you will need a hefty input voltage to drive the 6B4G to full power at 45 Vrms at the control grid.
This driver stage has about -a little more than- 23dB gain (a little more than x14 gain), so for 45V peek output requiring 3.14Vp input signal (2.22V RMS) ... but at not low distortion.
If you use about double anode current (near 10mA), the distortion will significantly decreasing.
If you use about double anode current (near 10mA), the distortion will significantly decreasing.
6SL7 for voltage amplification plus a 6SN7 for cathode follower? Plenty of gain with drive capabilities, and still four tubes for stereo.
That's what I kinda figured. Need a fancy preamp to make enough drive to max out the output stage. But maybe that's ok. I keep the volume below 50% most of the time. That will change when I upgrade my speakers, of course.
It does sound interesting, but I know very little of this new-fangled Silicone. I see you've started a thread or 2 on the subject. Maybe I need to check the MJ book. He loves the little sandy bits.
Another way to play here is to make use of a full 6sn7 in a plate follower arrangement.
Volage gain can be adjusted by the ratio between the two resistors between the two plates.
Have fun.
Volage gain can be adjusted by the ratio between the two resistors between the two plates.
Have fun.
The Get*Set*Go amp was my starting point. It had 300k plate resistors on the driver tube. I'm down to 100k.
How did you choose 27K for your plate load on the 6sn7? That's only half of the lowest example with a 100v plate supply. And not near 100K for 200v. The lower your plate load, the lower your gain will be.
If you are interested in this direction (i.e. keeping the 6SL7 and adding a MOSFET follower) you could check the Tubelab TSEII design for how George Anderson implemented it in a similar amp. Schematic and BOM are in the first post.
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The MOSFET follower is not hard to implement. I am sure you have found this internet link: MOSFET Follies
For 6SL7 just use ZVN0545A. Make sure to use a 15V zener diode to protect the gate, and try not to exceed the 700mw power limit to the ZVN0545A.
It seriously takes up almost no space, and yes, you will hear the difference right away. Even a wimpy 12AX7 can effectively drive a 300b when using a source follower.
I suspect that's how AmpsandSound uses a single 12AX7 to drive a pair of 300b tubes. thanks for the info!
So it's creating another gain stage using a MOSFET and I don't have to drill any holes in the chassis. cool.
Something like this?
I assume the resistor controls the current thru the device and should be measured under use to optimize. I see 20K 3w in the TSE BOM.
EDIT after looking at the TSE schematic again I see the above drawing is wrong. standby.
Something like this?
I assume the resistor controls the current thru the device and should be measured under use to optimize. I see 20K 3w in the TSE BOM.
EDIT after looking at the TSE schematic again I see the above drawing is wrong. standby.
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Ok, is this it?
If I'm reading the schematic correctly the TSE-II has a negative voltage rail that supplies the MOSFET Source. My amp doesn't have a negative supply. Is this necessary?
I see another node in the power supply was created to feed the Drain of the MOSFETS. Simple enough.
I'll stick to a traditional plate resistor for now.
Is that Zener the one you were saying would protect the gate?
thanks for taking a look!
w
If I'm reading the schematic correctly the TSE-II has a negative voltage rail that supplies the MOSFET Source. My amp doesn't have a negative supply. Is this necessary?
I see another node in the power supply was created to feed the Drain of the MOSFETS. Simple enough.
I'll stick to a traditional plate resistor for now.
Is that Zener the one you were saying would protect the gate?
thanks for taking a look!
w