Dr. Z B+?

[Gaz]

Anyone know what voltages Dr. Z is using to get 35, even 45 watts out of 4 EL84s? Seems like that's pushing it considering the bias resistors I've seen, but then again I really don't have much experience with the tube type and it's limits.

If you know B+ of any of those amps, I'd appreciate it. Also, if you happen to have noted at Plate voltages that's equally helpful, but please note if they are with respect to ground or the cathode - that's something that's been throwing me off. Thanks a lot.

[XgamerGt03]

At idle the Vox AC30 is already around 45W. Each tube has an idle current of something like 45 mA, so assume 40mA with screen current taken away.

So 310V * 40mA is around 12W a tube, or a little over 45W total.

If I'm wrong about this somebody correct me, but last time I took a look at one of the Vox amps those are the readings that I got.

Edit: Read the later post, I'm going out of my mind here.... to close to graduation and having to get all of that stuff sorted out.

[philo43]

Plate dissipation at idle is not the same as output watts, however at close to class A operation it will be nearly the same. Going from 30w to 45w is going to be less than 3db increase in output. The important thing is to not exceed the max idle dissipation for an EL84 (12w).

[XgamerGt03]

Plate dissipation at idle is not the same as output watts, however at close to class A operation it will be nearly the same. Going from 30w to 45w is going to be less than 3db increase in output. The important thing is to not exceed the max idle dissipation for an EL84 (12w).

It seems that most amp manufacturers quote idle dissipation as output power anyway though. And like you said, in this case it is basically the same being almost entirely class A.

A number of amps using EL84s do exceed the 12w rating at idle though, and do it quite easily. I know that Vox typically exceeds the 12w rating, as do a number of matchless.

EDIT: I'm probably going out of my mind right now... I'm looking back at this and realizing that I'm quoting class A as something different now. Usually class A can achieve about a 50% efficiency, so the thing about a Vox AC30 is that it is class A up until right about 20-25W and then it transitions into class B to allow it to go up to 30W.

I'm willing to be the Dr Z amps run closer to class AB than the Vox does and this usually allows for higher output power, or the Doc is using idle dissipation as his output power...

[philo43]

I've seen scope shots that clearly indicate cutoff in an AC30 at higher output levels. It would seem to be the case with the the DR. Z, I would think. I often operate my own EL84's up to 14w. Some can take it, others not so well. I have no reservations about using modern tubes as perishable items if I can get them cheaply enough.

[XgamerGt03]

I've seen scope shots that clearly indicate cutoff in an AC30 at higher output levels. It would seem to be the case with the the DR. Z, I would think. I often operate my own EL84's up to 14w. Some can take it, others not so well. I have no reservations about using modern tubes as perishable items if I can get them cheaply enough.

I tend to use about 14W as well for EL84s. The modern production JJ tubes don't seem to have any problems running that hot, and often sound much better at that level too.

[philo43]

Love the way the JJ's sound when you push them.

[Gaz]

Curious because I have an AC30 style build with about 315 on the plates (with respect to ground), 11.5 at the cathodes, 1K screen resistors, and a 4k OT.

My tubes are idling at about 14W, which I understand as typical for an AC30. I'm only measuring about 28-30 watts before clipping, and I can't imagine running the tubes at higher voltages, and hotter to boot.

I know there's EL84 amps that run fixed bias with high plate voltages and low screens to achieve higher output, but that's why I was curious about Dr. Z, because he seems to bias 4 EL84s with a 50 ohm resistor almost exclusively, yet claims 36-45 watts!

[M Fowler]

Yeah I am not sure, the RxES is listed at 45w and Maz at 38w. Stangray and Zwreck are bottom feeders at 30w.

Doc has said he uses 290-0-290 PT with 5AR4 but maybe he decided to increase the PT voltage on these higher rated amps or decrease dropping string. The 36-38w range seems more consistent with what I know.

I always use regular TW Heyboer iron 300-0-300 no sense in special ordering just to get 290-0-290.

I did use a custom Heyboer 290-0-290 in my Zwreck build though because I got it from my friend sniffyduke and it was just sitting on my self.

[Gaz]

Yeah, the 5199 PT is what I'm using, but the 260-0-260 taps with a SS recto. I tried the 300-0-300 taps just for fun, keeping the bias the same. Man, it had way more balls, but I just couldn't keep it that way in good conscience, it being so hard on the tubes. I may try them again with a 'sag' resistor to the plate voltage closer to 330vdc.

Anyone have any practical experience they'd like to share with the limits of these tubes? Of course, I don't want to be burning though them left and right, but having them on the threshold would sound the best methinks.

[Ian444]

In the diy hifi scene there is a guy Tubelab.com who designed a simple PP EL84 stereo amp with cathode bias. I built one so am familiar with it. He said there are 2 sweet spots to run these tubes. One is 340 to 350V B+, cathode bias, (with 270 ohm resistor and 1000uF cap on each cathode). He likes the JJ tubes which he has run up to 420V+ I believe. The 2nd sweet spot was 400V B+ and 320V regulated on the screens, and the equivalent of fixed bias, giving 25W for a pair of EL84 tubes with low distortion. Who knows how long the tubes will last. Here's the actual post.

http://www.diyaudio.com/forums/tubelab/ ... ost2360430

He was extracting 60W out of 4 x EL84 on his test rig/PCB when something let go, can't remember the details. That would have been a "no mercy" 60 watts. I don't think it was the tubes that let go.