Fender blues jr - convert to cathode bias?

[wagdog]

Has anyone here ever converted a blues jr to cathode bias?

Easy enough to do, just curious what the sonic results were.

If you've done it - thoughts on the tone would be appreciated. Sound clips would be awesome if you have any!

[Chuck H]

What is your goal? Sonically that is. I ask because I had a 2Xel84 amp that I fitted with a fixed/cathode bias switch. The results were very anticlimactic. Since el84's require very little bias voltage there isn't as much affect of the attack envelope or power as you might have with other tube types. Almost none. A little extra crossover disrortion when clipping is the biggest difference. Maybe a tad less punch and power. But nothing you would ever notice is a band mix. IMHO it's really more trouble than it's worth. But if the subtle difference pleases you, great. If not, it's not hard to reverse and forget about it. Put it on a switch so you can hear the real side by side difference without just believing you hear an improvement.

[soundguruman]

Has anyone here ever converted a blues jr to cathode bias?

Easy enough to do, just curious what the sonic results were.

If you've done it - thoughts on the tone would be appreciated. Sound clips would be awesome if you have any!

If you are making the amp class A with cathode bias

The first thing is to take out the stock output tube sockets and replace them with ceramic sockets.
Sockets that are wide will help block some of the heat from rising into the chassis.
Socket - 9 Pin, Miniature, Ceramic, PC Mount, Gold Pins | Antique Electronic Supply LLC
this is what I use, and it's not expensive. The plastic sockets are not worth a hilber...

Another thing is the circuit board where the screen grid of the output tube may start to fry the fiberglass.
This makes the board discolor around that pin..sometimes it starts to burn and the burn starts an arc / carbon trace on the fiberglass...
I have been cutting out the burned fiberglass and leaving an airspace around that socket pin.
This is only if your board is burned. That one screen grid pin seems to run pretty hot, especially with those wimpy plastic sockets installed.
The ceramic sockets I mentioned are beefier, and this seems to help dissipate and reflect heat better than the plastic ones.

You are following? When a tube is upside down, the heat is rising into the base of the tube, and the chassis.

Anyhow, if you change to cathode bias you are going to be getting a bit more heat.
The tube is shut down between notes when you have a grid bias voltage, and it runs class AB.
When you cathode bias, the amp is class A, and is no longer resting between notes.
That's why it's going to run hotter, the tubes will be on all the time.

[wagdog]

What is your goal? Sonically that is.

Good question.

Was just thinking that it might have a bit more compression when cranked, spongier, more singing sustain.

I have a twin project amp that I built and for grins cathode biased it - was pretty interesting sounding, but in the end I wound up going back to fixed bias cause it lost some of that twin punch.

You are following? When a tube is upside down, the heat is rising into the base of the tube, and the chassis.

Anyhow, if you change to cathode bias you are going to be getting a bit more heat.
The tube is shut down between notes when you have a grid bias voltage, and it runs class AB.
When you cathode bias, the amp is class A, and is no longer resting between notes.
That's why it's going to run hotter, the tubes will be on all the time.

Check, gotcha. Will definitely take that under advisement. thank you for that.

[Chuck H]

if you change to cathode bias you are going to be getting a bit more heat.
The tube is shut down between notes when you have a grid bias voltage, and it runs class AB.
When you cathode bias, the amp is class A, and is no longer resting between notes.

I think your suggestion of running the bias up to class A for a real difference is a sound idea. And in that light I think, but I don't know, that making certain to upgrade for the additional heat consequences with a class A bias by upping the spec on the sockets and traces is also a good idea. But you seem to be implying that cathode bias means, automatically, that the amp will be class A. Though there is a little qualification in there with "and it runs class AB." You also say "When you cathode bias, the amp is class A, and is no longer resting between notes." And this is entirely untrue on all fronts. The fact that there is bias current to measure in AB indicates that the tubes are not "shut off". And the operating class is an entirely different issue from bias method. It's true that cathode bias is self adjusting, for better or worse. But that alone doesn't dictate class A. Nor does fixed bias dictate class AB! I would caution against such blanket statements. The likelyhood of being wrong goes up enormously. But you've never shown any fear of that!?!

[frus]

It won't be exactly class A without changing the plate voltage and/or OT primary impedance, but heatwise it will be real close to class A (if biased hot like most of the cathode biased EL84 amps seem to be), so SGM's advices are definitely sound

[Chuck H]

Agreed. I tried to qualify it as it pertained. But I also expect accounability. It's entirely common to cathode bias in class AB (with a resulting cold bias when clipping). Even the venerable VOX AC30 shifts into AB on heavy clipping!!! Biasing too cold when using cathode bias is a crossover distortion disaster waiting to happen! WRT el84 tubes... Just bias at 100% dissapation. Not accounting for screen current. The resulting bias will be class A on clean tones and shift into class AB as the power tubes begin to clip. Crossover distortion should be acceptible. Biasing colder is buzzy suicide!!! I can't account for the Blues Juniors OT's ability to stand up to such constant current. Most class A push/pull OT's are a little more beefy. Primary impedance not withstanding.

[kleuck]

Stock blues Junior are already biased very hot, too hot actually, making one cathode biased won't make the amp hotter.

[Chuck H]

That WOULD depend on the resistor value and idle current!?!

[kleuck]

They are already about 13 watts per tube.

[Chuck H]

Well... That would be class A then. In fixed bias. Thank you.

[kleuck]

Not necessarily, 100% of dissipation doesn't mean that the signal can go 360° for each tube, depends on the load, and voltage. AC30 run tubes even hotter, and they are not true class A, nor the BJ.

[Chuck H]

No. Not true class A. Depending on the load. But the tubes are idling in class A. Even if the tubes DON'T swing 360*, where do you go from there for output? Many AC30 discussions have ended with the same conclusion. Which is, the amp IS class A but voltage rise on the cathode during clipping cools it down to AB1. Besides, I was really only referencing whether the OT could take the current. Which it clearly can. Voltage spikes when the amp is in AB1 are another matter. But I don't suppose those amps blow up OT's any more often than others. When you consider that there are so many out there is use and the abuse they take. It would be easy to be mislead by the frequency of posts. I'm always amazed by the abuse that el84's will take too!

[frus]

Which is, the amp IS class A but voltage rise on the cathode during clipping cools it down to AB1.

well it's kind of circular definition, because if it WAS class A, then the plate/cathode current in full tilt wouldn't increase and voltage on the cathode wouldn't rise... OK, it would a bit because of the screen current.... maybe even considerably...?

I have built an 18W and experimenting with cathode resistor noticed that the max cathode current in full tilt doesn't change at all with bias change. btw I settled on 100 ohm cathode resistor for 2 EL84's

so the max current through the OT wouldn't change with hotter bias, I wouldn't worry about the OT

[frus]

Most class A push/pull OT's are a little more beefy

Most near-class A EL84 push-pull amps have no NFB, apparently beefier OT attributes to improvement of bass response; blues junior has the NFB