First Amp: Bassman Micro LTP Build/Mod

I need to go check my BOM. Hopefully gonna have everything to start this build by Christmas.

 

There is no reason for embarrassment. I just appreciate all of the information you share! I suspect most folks just build from the layout and don't look at the schematic too close unless they have problems. That's probably why it went so long without being noticed. My build is more or less a mirror image of your's so I mostly worked from the schematic. I soldered in the two 10nF coupling caps and started on the LPT AC ground cap when things just started looking wrong to me. 10nF seemed way too small for the AC ground, at least compared to most circuits I'm familiar with. Going to the layout reassured me that 100nF was the right value.

 

gmdatillo. It would be awesome to see some pics of your build. Especially the dual V1 (Bassman/JTM45) setup. Assume you went the LTP PI model with the separate PI and pre-amp master volume?

 

I didn't know how much free time I had until i had a family.. my god i squandered all of my younger years....

 

Not true, you can wire many different combinations. Download the PDF.

http://www.hammondmfg.com/125a.htm

 

Yes, the Hammond 125x transformers are very versatile in setting impedance.

 

I've not been in the forum in a few months so I missed the request for some pics of my build. I'll pull some stuff together in the next few days and post them. Some of you will find them good for a laugh. I have a "unique" way of building my "chassis". In the meantime I'd like to share an option that I found for the micro's output transformer. Its not a versatile as the Hammond but its a little cheaper and you get normal 4/8/16 ohm output taps with 22.5k p-p impedance. Take a look at musicalpowersupplies.com or muzicalmatt on Ebay. He makes and sells his OT5PP that's perfect for robrob's micro amps. I bought a choke and the output trans and he combined shipping. I was very pleased with my purchase.

 

My schematic is attached as a PDF along with a handful of photos. As I mentioned in my post last night, my "chassis" is "interesting".

The switching between the two V1's (V1 & V2 in my schematic) is pretty crude. If you copy my design know that you will need to adjust your level controls when you throw the switch. The 12AX7 is much hotter than the 12AY7 so the volume will change. You must install R35 & R36 or you will get huge pops when you flip the switch. As me how I know... Those 2 resistors give the coupling caps a charge / discharge path when they are not selected.

Sorry about the rotation for the 2 internal pics.

 

So I think I did something stupid... I was messing around with the Bassman Micro in the pics above and I decided that the 470k grid stoppers on the power tube were way too big. I changed them to 47k thinking that there must be a mistake in the design. Surely 470k was was too large for a grid stopper (he thought). We'll now the amp is making some scratching noises, maybe like AM radio interference. I did have the amp torn down for the other messing around i was doing so maybe I've got a bad solder joint but maybe not. Hoping that RobRob will see this and comment on his choice of the 470k grid stoppers on the "power" tube. They are easy enough to change back to 470k but not as easy as asking for experiences. I did pull each of the other tubes (not power tube cause of course that would stop it) and the scratching continued regardless of which i removed. Thanks in advance for the help...

 

So I've had this Marshall style chassis kicking around for nearly a dozen years ... planning on some sort of Bassman variant which never happened. It has octal holes for the rectifier and output tubes, and I'll likely build this out as a 6SN7 output stage instead of a 12AU7 (I know ... I can get step down socket holes). I'm pondering an experiment with 6K6's as well, but first things first.

I'm still about a month out from starting the project, and I'll update with photos and such. A big thanks to Rob for his contributions to the community.

Funny you (@gmdattilo) should mention grid stoppers, as I've been thinking about this (here's a good read from Valve Wizard on the topic: http://www.valvewizard.co.uk/gridstopper.html).

Regarding grid stoppers, I see where guitar amp builders in general are locating the grid stopper resistors on the turret board. Isn't it best practice to have it right at the tube socket with minimal lead length on the tube socket side of the resistor?

... Thom

 

Sorry I didn't see this earlier. 470k is correct for these tiny little power tubes with their tiny little grid currents. Sorry if I got too technical there