Converting a Newcomb 1625 TR Help Please

[RWood]

Yes. Two 8Ω speakers in parallel results in a 4Ω load.

If your unit is like the one in Tonewood's schematic, you'll see that your switch is a SPDT with the outer legs going to the OT 4Ω and 8Ω secondary wires and the center terminal goes to your speaker hot terminals.

Is there a difference in volume between the two settings, when using an 8Ω speaker?

RWood

[wrathfuldeity]

RWood,
Thanks, that makes sense. There is not a difference (perhaps very slight decrease) in volume between the two settings, when using an 8Ω speaker.
Tim

[wrathfuldeity]

Tube masters...yet another noob question. So yesterday, dug it out and having some fun and seemed to be fine...except when going through the 12ax7 input and only with the volume pot at and above about 6 of 10 started to get some motor boating. No motor boating through the mic and low volume phono 6av6? inputs. So what is it? A bad solder joint, some wires/layout too close together, caps or something else. It's pretty cool amp and just don't want to smoke it before its time. Thanks for all your generous expertise.

clip thrown up yesterday

YouTube - newcomb tr 1625 m guitar amp conversion

[RWood]

The oscillations could be caused by any of the things you mentioned. What I'd do is chopstick the amp, focusing on just that channel. Turn it up so you can hear the motor boating and tap on every joint in that part of the circuit, from tube socket connections to cap and resistor joints. Listen for anything that reacts to your tapping. Note which one(s) so you can later reflow those joints when the amp is off and drained.

Also use the chopstick to gently move around wires, particularly grid wires, to see if they are picking up interference from other nearby wires. If no love, change the grid wires to shielded ones, grounded at one end only.

Let us know what you find.

[tboy]

Motorboating is often indicative of weak decoupling caps in the power supply. That 4uf 450v cap near the bottom of the diagram would be a likely suspect.