Express with 6V6's and 12K Primary OT?

[fusionbear]

Just curious if anyone has built an Express with 6V6's and a 12k primary OT in order to match what is going on with the standard EL34 version with 6.6k OT?

Just thinking out loud to see if there is a way to duplicate the magic with a slightly lower volume threshold.....

thoughts?

[andresound]

Forgive the stupid question,... But wouldn't the adjustment of the impedance selector achieve similar results?

[martin manning]

You are right Andre, but 6k6 is a pretty good match for a pair of 6V6 at 400V, and running it at 13k2 would probably be very hard on the screens. Also, it is a beam tetrode where EL34 is a true pentode, so it would not be quite the same.

What might be interesting is a 6K6 Express with the impedance selector down one click for 13k2, to get a bit less than half power vis-a'-vis the EL34's. 6K6 is a true pentode with the same pinout and 8.5W Pa max, and they are available NOS at a reasonable price. Could be risky running them at the Express' 400V Va, but it just might work.

[fusionbear]

What voltages would be better for the 6K6?

I think I found a data sheet 315+ on plates and 250+ on the screens?

[martin manning]

You see similar voltages on 6V6 data sheets, but they are routinely run at over 400V. I imagine something under 350V would be pretty safe for 6K6. You could bring them up on a Variac and increase voltage slowly, playing through them, and see if they will take the full 400V.

[andresound]

You are right Andre, but 6k6 is a pretty good match for a pair of 6V6 at 400V, and running it at 13k2 would probably be very hard on the screens. Also, it is a beam tetrode where EL34 is a true pentode, so it would not be quite the same.

What might be interesting is a 6K6 Express with the impedance selector up one click for 13k2, to get a bit less than half power vis-a'-vis the EL34's. 6K6 is a true pentode with the same pinout and 8.5W Pa max, and they are available NOS at a reasonable price. Could be risky running them at the Express' 400V Va, but it just might work.

I normally run my express with the VVR at 1-2pm. So... At lower voltage. Will I need to do any mods to the circuit to implement the use of 6K6? (never heard of this tube!!)

[martin manning]

Check the bias and set it for 70% of 8.5W, but that's it. Some people will recall that 6K6 was the original driver tube in Fender's 6G15 stand-alone reverb.

[fusionbear]

Thanks Martin and Andre.

I think an Express with this tube at 350+ will be my next experiment.....

I will report once I get started....

[RockinRocket]

But now wouldn't the problem be the 35 watt output transformer is way over spec for 6k6?

[RJ Guitars]

IMHO - Over spec is not a negative in this case since the output transformers are not typically pushed to the limit in an EL34 Express. So my view of it is that you are just pushing it less toward the limit but your tone should be similar. In some of the early Marshall's they used undersized iron and that transformer saturation was a big part of the sound but not so much with the Express design.

I tried the 6K6 tubes in a build with a 5K OT: https://tubeamparchive.com/viewtopic.php?t=27513 This started out as a triode powered express and it had little to offer as far as tone and character So the testing of alternative tubes became a salvage mission for my efforts.

The 6K6 tubes sounded decent but not radically different than the 6V6's.

It was during this build that I discovered the 6Y6 tubes and they were to my ears the best alternative (in sound) to an EL34, without the ear shattering volume. They require a big heater current and can't take much B+ voltage (rated at 180V Max but IIRC I ran mine at ~240V) so if you have a typical Wreck amp with a VVR they will drop right in if you keep the voltage dialed down. Among the other great things I can say about them is that they are very available and affordable.

YMMV

[martin manning]

arm if you were running 6K6 into a 5k load that would be way too low. Running them in an express with the impedance selector down one click so they see 13k2 would be much better I think.

[RJ Guitars]

If I understand how things work, could we alternatively use a higher impedance speaker load to get that ratio correct (somebody help me if I have that backwards). I have a 16 ohm cabinet that is handy for this option.

I am also curious about what we might expect sonically as we deviate above or below the "ideal" impedance match. Gerald Weber published in his book that you can typically have up to a 100% impedance mismatch with no risk of damage to your OT. Assuming this to be true, what would we hear?

[martin manning]

Sure, anyone with an Express and a pair of 6K6's could give this a try. Connect a speaker 2x the impedance setting, VVR or Variac to be cautious. With a 5k load I's worry about over dissipation, but 13k2 looks good. In general I would say that a 100% miss-match is tolerable IF the nominal load line is in a nominal place, like the typical Fender. Others may not take it without red-plating or arcing somewhere. For sonic qualities, if you can switch the impedance selector up and down one click you can hear the effect of varying the load impedance.

6Y6 is a beam tube, so it's similar to 6V6 in that respect, but it seems to be specifically designed for low voltage operation. The high heater current would be closer to the EL34 and bring the heater voltage down accordingly, and VVR'ing the B+ down would keep anode voltage within limits, but you'd like to dial down just the anode and screen voltage and keep the preamp where it is. Looks like a 2k-2k5 load would be about right for push-pull.

[Roe]

I prefer reducing the volume with inefficient speakers (greenbacks), attenuator (alex/scumback) or fryette power station

[RJ Guitars]

... 6Y6 is a beam tube, so it's similar to 6V6 in that respect, but it seems to be specifically designed for low voltage operation. The high heater current would be closer to the EL34 and bring the heater voltage down accordingly, and VVR'ing the B+ down would keep anode voltage within limits, but you'd like to dial down just the anode and screen voltage and keep the preamp where it is. Looks like a 2k-2k5 load would be about right for push-pull.

Thanks for addressing this, your input is a great addition to understanding how to use this tube.