The Amp Garage

From the pictures, it looks like all three relays are plugged into their sockets backwards. Please confirm. It also looks like the OD entrance trim pot (TR2) is wired to a panel mounted pot with non-shielded wire. This will likely prove to be problematic, and you should consider replacing the three ...

I can attest to pocket screws, though I have only used them on 3/4” plywood. But the tools do accommodate 1/2” material, so I’d say it should work well. You have to clamp the two pieces together really well, especially if you’re using glue, which makes everything more slippery. Since the screw goes ...

This is really awesome! Thanks to everyone who pitched in with suggestions; especially pdf! You guys are the best.

That sounds logical. However, how would you test if 1 interacts with 3 or 2 interacts with 4? First the short answer: By opening up all the controls to 10, with nothing plugged into the guitar input, and observing the PI input with a scope for signs of oscillation. When nothing is plugged in, the f...

Lou, On the main layout for #183 it's tied to the 4 ohm not 8 ohm. M Hi Marcus, This is the schematic I referenced; it is in the the first post of The 183 5th Generation EL-34’s (High Plate Skyline) topic in the Dumble Files section. Lou Edit: and here is the schematic I forgot to attach....:-) htt...

What does the collective think is the best thing to do? ( moving it is still an option, but I'm hesitant to basically gut the whole board so I can get in there with the greenlee punches or?) ~Phil I think before that can be answered, it would be good to know if the oscillation is occurring from the...

Hi Phil, The more I think about it, I am thinking it’s odd that the amp does not oscillate with all the controls dimed but with no instrument plugged in. This seems to imply the feedback loop involves the first and third gain stages, rather than the second and fourth stages. The next time you are sc...

Phil, Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it wil...

glad to hear your's worked well, now I'll go cry in my beer :) I'm still going with the idea that Ive got a component wrong somewhere that's pushing hte V1 gain a bit too high. I'll be able to do some more troubleshooting this weekend. ~Phil Hang in there, Phil. There are a lot of astute builders o...

I’m pretty sure it’s a layout issue caused by the discrepancy in the relative position of V1 between the HRD and the Blues Deluxe. There looks to be maybe 1” between the input stage grid terminal and the wiring for the 2nd plate of the overdrive circuit, so 4 cascading CC gain stages totalling mayb...

Phil, Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will...

Hi Phil, I think it may be advantageous to replace the wire that goes from the board to V1, pin 2 with shielded cable. Ground the shield at one end only. Also, starting at the input jack, can you tell me the functions of all the switches and pots? And if there is shielded cable used on any of these ...

You might try adding a small capacitor from V1A grid to GND; something in the 10 - 47pF range. Does the oscillation happen all the time now, even with nothing plugged into the input?

The small cap should be added right at V1 socket, not at the same net on the board.

I can’t figure out how to post the schematic from my iPad, but I will post it here when I am home later tonight.

Latest schematic is attached. Page 1 represents the PC Board only and Page 2 details the chassis power amp wiring.

Cheers,
Lou

Additionally, the bias point of the preamp stages will have an impact on the perceived headroom, or cleanliness. If the bias point sits either hot or cold of center, the stage will break up into distortion earlier, or at a lower signal voltage. If the design intent is for a high headroom, clean soun...