Tube Amp Archive

Have you tried changing the tube?

R Dekker has said that the uT3 is fine for the great majority of users. uT6 was created for people demanding higher voltage and current to test what I would consider special purpose tubes. From the comparison chart on his web site: "Application range The uTracer3 is in my opinion the ideal test...

I looked back in the web log pages, and there was no QT in a screen shot of GUI 3.5. IIRC there may have been a simple quick test feature by the time I bought the kit, but it was not that great, and definitely low priority. It had some issues too, as documented earlier in this thread.

I wouldn't worry too much about 120 vs 115, that's less than 5%. Some places in the US see 125 or more. Not a bad idea to bias on the cool side, if you expect to encounter higher voltage in the future.

Illinois makes film caps too.

Pfarrell, the drawing specifies 30 mm and 16.5 mm diameter caps. Are yours larger than that? Is the eyelet spacing the same? The 470k’s dissipate at most 0.11 W That’s for the screen, at 450V, and those are on the end. More spacing would be a bit better for cap life. You can offset the 270k’s a litt...

The bias voltage on the output tube pin 5's is much too low. It should be -45V or so. If you can't adjust it to that level there is something wrong with the bias circuit.

Probably what you are looking for is here: https://www.ampbooks.com/mobile/amp-tec ... synthesis/

That's what I was thinking, a DPDT that connects the screen supply to the tube bases as they are now, or routes it through the MOSFET reducer. I'd leave the dropping resistors as they are, and just insert the reducer in place of the wire that goes to the screen pins. That Zener conducts when the com...

You're right about the importance of the phasing wrt nfb. A multi-tap secondary must have the common lead going to ground so that the 4-8-16 taps will have the correct impedances. I've never seen anything like the dot markings on the diagram attached to the box, but white is clearly the CT since it'...

So many questions... I don't know too much about the uT6, so I'll answer based on my experience with the uT3. After nine years, it's good to take a look back. will the uTracers be able to replace my Hickok's? I am not so much worried about the gas test or life test, I can see how those are available...

I drew this up a year ago for a KT88/6550 screen supply. I'd use fixed resistors instead of a trimmer, and a big MOSFET since screens can draw significant current. I'm showing another part number, but it's basically the same as the one you have for the plate supply in your recently restored VVR. Thi...

Is this effect something we could hear while playing? The curves look a little more pentode like with the magnetic field present, but they're not nearly as rounded as a true pentode like an EL34. Clearly the magnetic field changes the curves to some extent, and so there will be some effect on the s...

Doubling the load to 4k by miss-matching would work for 2x EL34 or 2x KT88, but with the KT88's you need to lower the screen voltage. If you had a MOSFET VVR on the screen, you would just turn it all the way up or bypass it to use EL34's.

A quick experiment tracing curves on a 6L6GC with and without the presence of a 2 x 3 cm neodymium magnet placed 17mm (one toothpaste cap) from the tube envelope, at approximately mid-plate height: I tested with the magnet axis in-line with the electron flow (magnet axis aligned with the flanges), a...