Lowering B+ voltage

[blue lander]

Thanks to everybody's help my Tubelab SE is running nice and reliably, but I am concerned that I'm getting about 327 volts of B+ for my 45 tubes. The tubelab manual recommends 280 or 320 volts, is 327 close enough? Now that I think about it, I measured b+ about 5 minutes after powering the unit on, I'm going to measure again after letting the unit warm up for 20 minutes or so.

I'm using a couple cheapo old 45's off ebay (one Ken Rad and one Silvertone) so I'm not going to be out hundreds of dollars if I burn them out with too much voltage, but I would like to make them last as long as I can.

The manual says I need to reduce the value of C4 to lower the voltage. Is there some formula for figuring out what value I need, or is it trial and error?

[rknize]

I would say that you are OK. I get about 322V of B+ when using 45s. The vintage 45s seem like they can handle it. I'm using some older etched-base Sylvania 45s that were not the strongest tubes when I got them. I've put a few hundred hours on them maybe and they still bias about the same.

Edcor Universal Power Transformer for Tubelab SE

To answer your question, the best thing is to use PSUD. Measure the HT winding AC voltage when the amp is in operation. Plug in the voltage and your capacitor and choke values and then run the simulation. Plugging a 10uF cap in the C4 position should drop your B+ about 10V or more.

[Glowbug]

I'm not familar with the schematic, but you can enter it in a program like Power Supply Designer: PSUD2 and model what changes in cap values will do to the B+, roughly

[w5jag]

I omitted C4 entirely from my Tubelab SE, making the power supply choke input.

I'm running my 45's (and 46 and 47) at about 275 volts.

Win W5JAG

[rknize]

You will need a quality choke for such a use and you will want to add snubbers. I haven't tried it myself, but the cheap little Triad 6H choke probably won't cut it. The rule of thumb I have heard is the choke should weigh at least as much as the power transformer. Take a look at the Hammond 193 series for that. I used the 193J when I first bread boarded the Tubelab SE without C4. There was a little bit of 120Hz coming out of the speakers and the choke buzzed. It probably needed the snubbers.

[w5jag]

I hadn't heard that rule of thumb before, not that that means anything.

The choke, in a choke input power supply, certainly needs to be at or above the critical inductance, and able to maintin that critcal inductance while supplying the desired current, but that's about it, AFAIK.

I think I have a Hammond 10H, 100ma, choke in mine, and it is stone cold silent with 45's, 46's, and 47's into 96 dB speakers.

I do get a slight amount of 120Hz hum with 2A3's and 5930's that can be heard if you get within a few inches of the speaker drivers. I believe that this is because my 5 amp regulator chip, really isn't a 5 amp part,.

I rarely use 2A3's (because I don't care for the way they sound) so I haven't swapped in a replacement part to test this theory.

Win W5JAG

BTW, what's a snubber? Is that what hi fi people call a bypass capacitor?

[rknize]

Yeah. You bypass the choke at both ends with a small, high voltage film cap. This helps soften the choke's tendency to lash-back on the rectifier while discharging, IIRC.

[w5jag]

I don't want to sound like I'm advocating a change from the original design.

I think there is a lot of merit to choke input supplies, but I only did it because my power transformer is a hamfest refugee, and the only way I could get the B+ where I wanted it was to go choke input.

No doubt it's better to get a correct transformer, and build the amp as Tubelab intended.

[rknize]

I did it for the same reasons. All I could find at the time was a 700VCT transformer. Choke input is friendly to your rectifier and transformer, but it requires that you buy decent iron.

[Nikon1975]

Well, there is an easier way: if you are using a 5AR4 rectifier, try a 5UG-something. You will bring B+ down of 30 V approx.


D.