EL34 Ultralinear DC Voltages

[PPL]

I have finished an EL34 Ultralinear push Pull Tube Amp and would like options on the Ideal DC Operating Voltages of the Amp shown in the schematic. Thanks for Looking

[VacuumVoodoo]

It violates the KISS principle (Keep It Simple, Stupid) because it simply is too simple.
Try the superpentode UL configuration from here:
http://plitron.com/wp-content/uploads/Atcl_4.pdf

[PPL]

Thanks for the Tip however I am not Interested in other Topologies I am doing this one at the moment?

[Cliff Schecht]

For hifi I think high voltages and low impedance OT. What little you lose from a low impedance you can make up for with B+. I'd go for 500Vish.

What OT are you using? The Hammond 1650P can handle 200mA bias so you won't be running into any problems with current if that's what you used but the impedance is higher than what I would use (6600 Ohms, I'd use maybe half of that).

[PPL]

Cliff Schecht, Thanks! Yes i am using the 1650P along with the 372JX Power Transformer rated at. 243VA, 300-0-300 @ DC ma 250, 5VCT @ 4A, 6.3VCT @ 8A. the 1650P secondary is connected for 16 ohms so This would make the Reflected Primary Impedance 3,300 Ohms with an 8 Ohm load, I am thinking that should be ok?? TIA.

One concern of mine is the rather High cathode Voltage on the EL34's Normaly one would expect to find 15-18 Volts Hear? We do have to subtract the voltage on G1?? TIA

[David Root]

The "standard" OT for a pair of EL34s in a guitar amp is 3400 primary, but with a bit less than 500 Vp, but not of course operated in UL mode.

Still, I think I agree with Cliff. The Hammond 1650K might be a better choice, 3400 primary. Typically the spec'd Zpri for UL operation will be lower than for pentode operation, check the old datasheets, which were designed for audio, not guitar amps.

For sure the two different primaries will give different tone and power.

[rocketeer]

I'm using a 1650K in a EL34 PP amp and have always wondered what UL sounded like...especially for a hyper clean sound. My PT is only putting out 420V after SS rectification and 1st filter cap. Is that a safe voltage to use in UL mode? Do I just need to move the screen connection from choke output over to UL taps? (keeping the same screen resisters). And then does feedback have to be changed? Just wondering...........

[TUBEDUDE]

Rocketeer, relocate the screens to the UL taps on the primary, remove the screen resistors as they are no longer needed, and will degrade the transformers control of the tube. As for the feedback, i like little or no feedback myself. I like the sparkle and immediacy of tone that feedback takes away. i've worked on several twin reverbs that had the UL circuit. these amps were considered to be too sterile or cold sounding. after removing the feedback, and tweaking the circuit some to adjust for the increase in gain, (and eliminating the goofy mixed bias scheme)they become tone monsters! clean, warm, responsive and toneful.

[Firestorm]

There are a few interesting things in the old Mullard EL34 datasheets:

For "maximum output" with distributed load (UL), they use a B+ of 450V, UL taps at 20%, Rk of 500 per tube, primary impedance of 7K, 55.2V RMS input signal (g1-g1) for 40W output with 4.5% distortion.

For "minimum distortion" in UL they show 430V B+, UL taps at 43%, Rk of 470 per tube, primary impedance of 6K, 50V RMS input signal, for 34W output with 2.5% distortion. In this configuration, if you drop the input to 35V, power out falls to 20W, but distortion nearly disappears (0.35%).

Note: in both cases there are 1K screen resistors even though the connection is UL. Some other sheets show Rg2 as low as 235. Personally, I wouldn't eliminate screen resistors completely even in UL.

So depending on what you're trying to accomplish, those are a few parameters to play with (along with primary impedance).

[Structo]

@ PPL

Is this a Hi Fi amp?

[rocketeer]

Thanks for the UL info on my project ...... I will try it here soon.

One more question is biasing now in UL mode. I've been biasing to 70% of 25 watts. At 420volts that would be around 40ma per tube (I actually have separate bias controls for each tube). Does this all change since it is capable of more power in UL mode? What per tube power output number do you now use?

Many thanks to this great board!

[Firestorm]

UL doesn't change the plate dissipation of the tube; it will always be 25 watts. That's heat, not output power.

UL doesn't increase the output power of the tube. With the taps around 20%, you can ALMOST get the same power as you would with conventional pentode connection (assuming the same Va and Vg2). But UL can reduce the amount of negative feedback you "need" so you can squeeze more power from the amp by using less feedback.

With cathode bias, UL and about 430V B+, some of the old datasheets show 'em running essentially in Class A. Probably not a good idea with fixed bias. 70% at idle should be fine.

[rdjones]

Strongly recommend using some protection for the screens, at least the Fender value of 470 Ohms.
You can get away with directly connected screens in a more conservative hi-fi design.
But for a guitar amp that has a much more likely chance of being driven into clipping screen "stoppers" are good design practice.
I use 1k or 1.5k.

Steve Pompura
RedDog Thermionics
Nashville, TN

[David Root]

+1 on that 470R is a good idea in a UL mode EL34 MI amp. Power rating is open to opinion, I would not go less than 1W and that is probably light.

[PPL]

The Amp I for Hi Fi use.
Screen resistors are really needed to help prevent instability. By using both cathode Feedback from the OPT Sec and screen UL Tap's on the transformer, The Tube and transformer are interlocked in a localized feedback loop around the Output stage so the screen resistors act as grid stoppers. I am using 1/2 watt (RN70) resistors with out issue