Wiring the Tubelab SE without a volume pot

[boywonder]

Since I'm not using a volume pot, I'm assuming that I just wire the inputs to R2 & L2 and let 'er rip.......

Assuming that my preamp has plenty of drive, are the values for R8 & R28 still appropriate? They are 121K.

[tubelab.com]

Since I'm not using a volume pot, I'm assuming that I just wire the inputs to R2 & L2 and let 'er rip

Yes.

Assuming that my preamp has plenty of drive, are the values for R8 & R28 still appropriate? They are 121K.

Any value under 500K is OK for those resistors. They are there to provide a ground path for the grid under all conditions (no volume pot or bad spot in the volume pot). If your preamp prefers a different load resistance, use it. The value of this resistor was scientifically determined by the fact that I have a large supply of 121K low noise metal film resistors in my stash.

[Evenharmonics]

Originally Posted by boywonder

Since I'm not using a volume pot,

But it's good to have adjustable input level for the "gain structure" option which is important for low noise system.

[boywonder]

Originally Posted by Evenharmonics

But it's good to have adjustable input level for the "gain structure" option which is important for low noise system.

Can you elaborate on the gain structure option........?

[Evenharmonics]

Originally Posted by boywonder

Can you elaborate on the gain structure option........?

It applies more for pro audio setting but it can also be used for home audio to an extent. It's basically a way to adjust level of various components to achieve low noise floor and good dynamic range without clipping. link

With a case of using preamp (AVR or processor) and amp, listener can adjust the output level of preamp and input level of amp to achieve low noise floor. It's not feasible for every home audio but for those with adjustable options, it's good to have a variable level control for amp.

[boywonder]

OK, I have one minor issue..........Everything is checking out fine on the amp, and it's making beautiful sine waves (the sq waves don't look too bad either..) but I am running out of pot adjustment for the 5842 driver tube anode voltage adjustment pot.

The driver anode voltage for both channels maxes out around 165V. Do I need to change the value of the CCS current set resistor (the 332R) to bump the anode voltage up?

Also, this appears to be the first amp I've had on the bench that cannot be driven to clipping with my function generator, so I'm assuming that the input sensitivity is rather low.......I haven't done any number crunching yet other than to determine that with the func generator pegged, I'm getting 4W output.

[boywonder]

Forgot to mention.......With a 5AR4 installed my B+ is about 408V with 65ma on JJ 300b's.......

With a clapped-out 5U4, the B+ is about 350V........

After debugging, I'm planning on popping in a GZ37....so hopefully, my B+ will be around 390V. I'm using the 5K taps on the OT.

[Evenharmonics]

Originally Posted by boywonder

The driver anode voltage for both channels maxes out around 165V. Do I need to change the value of the CCS current set resistor (the 332R) to bump the anode voltage up?

Which value did you use for R14 and R25. Tubelab website lists 36 KOhm for B+ voltage of 350 to 400V.

[rknize]

I don't think you need to worry about it. Just try to make them about the same. The 5842s seem to vary quite a bit.

[tubelab.com]

Yes the 5842's vary a lot. That is the reason for the pot in the first place. 165 volts is probably OK, but it is easy to change. Some 5842's are just "off the end of the pot" so simply increase the resistor in parallel with the pot (R10 and R21). I have used 680 ohms in some boards.

It is a popular tweak to use an LED for bias in circuits like this. I have been asked why I don't recommend an LED here. As many have found out 5842's vary a lot and so do LED's, so you need a big box full of LED's and 5842's to find a set that match and are near 175 volts. I have LED's in my 307A TSE. I went through about 20 5842's and big box full of LED's. Does sound good though.