Recent content by Bauhausler

Not around here much any more. Traveling 200 days/year. Drop in when work allows.

Why?

Agreed that Gm varies with A-K voltage and current. That's true with all tubes. So the chart on the tester should indicate what transconductance should be 100% for the test conditions applied by the tester. But for every other new small signal triode I get the specified target Gm with the...

So it might be oscillation that causes the Gm measurement to be off. i usually don't work with high Gm tubes, but I'm sorting 6DJ8s to make a White cathode Follower OTL headphone amp. I could make up a 9-pin plug/socket adapter, but I have never used ferrites. Any references for where to buy...

Thanks. That makes a lot of sense. When I test high value tubes like a batch of MOV KT88 I make up my own test rig so I can measure bias voltage, Gm, etc. But I've had pretty good luck with all other 9-pin dual triodes on this tester. I'll look into it some more.

Inaccurate roll chart data was my first thought. I know people have documented errors in Hickok settings before. Probably the Bias knob setting is wrong. If I discover there's a mistake I'll post here.

If you want to go full DIY this thing I designed for Make Magazine sounds good. http://makezine.com/projects/bass-bump-headphone-amp/ To run it from a line input rather than from a lower impedance source change R8 from 100 Ohms to about 5K.

Maybe someone knows the answer to this. I use a Hickok 533A tester. I believe it's pretty accurate because new tubes always test near the target transconductance. That's for small signal triodes and power tubes. But I always get out of range readings for 6DJ8/ECC88 tubes. The tester says...

Cleared out, all the way back to 2006.

It's important that those 330 Ohm cathode resistors be matched within 1%. The actual power dissipation there is 0.045A^2 X 330 = 660mw each, so a 7W part is way overkill. Probably chosen for stability rather than dissipation. I had one broken one so I subbed in matched 330 Ohm 3W metal films...

Thanks, Doug. Your word carries a lot of weight with me because you're an engineer and you measure things. $12 is pretty cheap for a quality 6L6 type. Before I used any of these in an amp I'd run each sample up to maybe 450V/50ma cathode current and monitor the G2 current. Any units with...

Yes, I read about them. The 6BG6GA swap has been around for over 10 years. As I said, these particular tubes may be fine at higher voltages and powers. But as much as possible I prefer to run things within manufacturers' specs when expensive transformers are at risk. I recognize that actual...

If you're concerned about long term reliability another thing to consider is the line voltage. A lot of gear designed 50 years ago assumed a nominal line voltage of 115VAC or lower. Today many places have line voltage 120VAC or higher. This could result in higher power dissipation in the...

Thanks, I found it on the SAMS page and will purchase the download. I don't know when I will get to these amps. I have literally dozens of restorable tube amps and receivers and have to be fussy about which one I tackle first.

SND is selling Philips 6BG6GA tubes as 6L6GC/7027 replacements. Like a lot of TV tubes, this tube has a screen voltage rating (Philips spec) of 350V. In the W-5M the screen voltage will be 450V. Also the plate power diss rating is 20W rather than 25W for original 6L6GC and KT-66. Technically...