6GM8 <-> 6DJ8

I've come across a stash of 6GM8 (ECC86) tubes and have a PCB which uses the 6DJ8 (ECC88 / 6922).

Max plate voltage of the 6GM8 is 30v DC.

Is anyone able to assist with the changes that would be required to the resistors and capacitors in order to adjust this circuit for use with the 6GM8?

 

what ecc88 u got?

 

Haha good pick-up!
Original post now edited.
Any tips on adjusting the circuit greatly appreciated!!

 

Here's one option.



You may need to tweak the value of R2. This sets the current running through the tube, and therefore how much voltage is dropped across the tube vs how much is dropped across the resistors R2 and R3. I wouldn't fret too much over the 30V rating. Do note that the circuit will start to distort significantly with signals greater than 2Vpp.

The input is at approximately 0V, so I would consider omitting the input coupling cap C1. Even if your source introduces some offset, the output is cap coupled; so it will not introduce offset to your downstream gear.



If you don't mind keeping the input cap, you can run the circuit off of a single supply, like so. This would allow you to use a single 48V switching power supply, though I'd recommend adding a regulator.



One last option, that nobody asked for: replacing R3 with even a modest constant current source would improve distortion.

 

Thanks @peef!

This is great - some good food for thought.

I have a decent regulated 24V DC power supply so will use that for the B+.

Some comments/queries on the options:

I note that if we remove C1 then (according to your schematic) R2 no longer is required?

Lastly, if we replace R3 with a CCS such as LND150 then what is is the best way to 'attach' it to the circuit? The PCB itself obviosly only has two solder pads for R3.

 

You'll need two in a +/-24V arrangement if you want to DC couple the inputs.

That's right. The original schematic runs the tube in cathode bias (or self bias). By contrast, if you take out the cap, the grid is set to 0V; so the circuit is now running in fixed bias.

I drafted some daughterboards that I use. This is probably the cleanest solution.

https://oshpark.com/shared_projects/NoRbHbHh

That said, you can connect the LND150 exactly as shown in the schematic above (no source resistor, no gate stopper) and get 1-2mA without oscillation. I'd probably do this point to point with a TO92 part, and try to extend the pins to where the resistor pins would go, as the CCS functions as a two terminal device. Take care to get the polarity right.

 

Wow how did you draft those boards so quickly? Would it be difficult to do a board for this whole project using LND150 with a single 24V DC power supply (ie no separate terminals for the filament)?

I've got enough 6GM8 tubes to share around if anyone else is keen?

 

I've been using the LND150 boards for the better part of the last decade. They've just been waiting for their prime time.

It shouldn't be too difficult to make an all-in-one board for the 6GM8s. In the spirit of DIY, I'll suggest you grab KiCAD or an old version of Eagle to give it a go. But if you have boards already, it's probably worth building those to work out the values and tweak things to your liking.

 

If you have a stash of the tubes, consider the aikido

https://www.tubecad.com/2006/04/blog0059.htm

.. dB

 

Thx mate I in fact built this one but the gain is too much for systems these days as the DACs routinely have at least 2vrms being pumped out of them. Hence the search for a unity gain buffer

 

https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/

This could be easily adapted too, gain is adjustable

 

This is another unity gain buffer i built many years ago but sounded great ( still does )



it was developed by Alex Cavalli

complete schematic:

 

This is quite a walk down old DIY

 

wow nice build @dBel84 - can't go wrong with a design by Alex Cavalli!

I've got a few 6H30Pi tubes which might make for a good buffer so I've asked in the thread over at DIYAUDIO you posted as well, looks like a very flexible circuit with the jumpers for various gain settings etc

What's the volume pot you've used in your build there?

 

Nothing fancy, probably an old alpha. I tend to use TKD in current builds but back then it was whatever I had on hand