The Amp Garage

Thinking about building a low wattage (5-10 watts) single-ended amp using a KT88. What is the preferred Output transformer impedance for that power tube? There is no "right" primary impedance. If nothing else, it's going to depend a lot on the operating conditions (i.e. plate and screen voltage.) E...

If we're getting technical.... There's (at least) two kinds of shield that you might find on a transformer. One kind is actually around the outside of the coil and core. It goes on after the transformer is more or less fully assembled, but possibly before paint. They're sometimes called a flux band ...

When testing for continuity, the meter will beep only of the resistance is low enough. The secondaries of OT's are often low enough to get a beep, but not the primaries. Set the meter to measure resistance and just see what the actual resistance is in ohms. ClassicTone puts the expected DC resistanc...

Maybe this is obvious, but low wattage bulbs limit current more than high wattage bulbs. A 200W bulb might not protect a small amp that has problems. I use them jut for first power up. If the bulb burns brightly, then I shut everything down and look for the problem. A low wattage bulb (40W) can be u...

Most bias supplies come up pretty quickly (less than one second) so, as much as I admire Merlin, I wouldn't worry too much about tubes redplating before there's adequate bias. Just be careful not to go nuts adding massive amounts of capacitance to the bias supply; that will slow them down. It's poss...

Looking again at the schematic, if pin 9 on V2 is accidentally grounded, then the amp would show the symptoms you describe.

Look around the tube socket for stray crap, maybe carbon...

With that much resistance (even 150 is a lot) the B+ becomes very sensitive to load current.

The GZ34 is not a linear resistance (not "ohmic") but it will drop around 15V at 185-200mA, so you might simulate it with a 75 ohm resistor. One for the whole B+ or one in each diode leg. Either way.

V1 and half of V2 don't light up. (12ax7) I swapped the tubes with other 12ax7's that were lit up in the amp, and they don't glow either when put in the V1 & V2 positions. The heaters are all in a series string like old Christmas lights. That means they all light up or none of them do. The fact tha...

Tried it on PSUD and found that it requires 150 ohms to give 292VDC @ 195mA.

edit: changed the load slightly to get 296VDC @ 184mA. Closer to your numbers. Resistance is still 150 ohms.

Fireworks don't bother me. But, opening day of deer season is nuts. My house is surrounded (on three sides) by state land and I stay inside all day. It's amazing that nobody gets killed.

Phil, I can't even imagine. I have a good friend that is a Nam vet and all I can do is watch...

My wife and I have been fighting 3 years to save our oldest son from injecting crap into his veins and it's been quite the struggle for him and us. Mark I feel for you, Mark. Obviously, he can't clean up if he doesn't want to. But, when/if he does, then you being there to help will make the differe...

It has its upsides too.

Ha! No I enjoy your vids. They move along at an easy pace.

You should make a YT vid of the build, but spend a lot of time talking about irrelevant shit so that the vid ends up 6 or 7 hours long.

Just kidding. I hate those vids. Don't do it. But do keep us posted with pics. Doesn't matter how long it takes.

Nice job. I hope you and your dad enjoy it.