Hoffman Princeton build in a PRRI Chassis

[Jrusto]

Just finished the last solder joint and wanted to pass this picture by for an initial check before I apply power.  Not quite as good as I wanted but ....  Did one check but will double check again tomorrow. 

Thanks for your help.

Rusty

[sluckey]

May I suggest... Connect that red/yel PT wire DIRECTLY to the ground terminal on that multi-cap can.

[Jrusto]

Thanks for the suggestion and I will certainly do that.  For my education, what is the reason to connect that directly to the MultiCap Can.

Rusty

[sluckey]

Every bit of DC current in the entire amp must flow between the PT red/yel wire and the negative terminal of that cap can. As is, that current must pass through 3 solder joints and 2 mechanical screw/nuts and a couple inches of chassis to get from the cap can to the PT red/yel center tap. Lot of potential possibilities for loose/bad connections there. Just makes good sense to me to just connect the red/yel directly to the cap can, especially since it's so convenient. Now there's only one solder connection. Make it a good one.   

[Jrusto]

Thanks

[WimWalther]

For my education, what is the reason to connect that directly to the MultiCap Can.

It is a *very* good practice. And fwiw, you often see this done on vintage commercial equipment that used the FP type can caps.

[Jrusto]

Well Crap.  I went through Robinnettes startup proceedure and things were great until I reached the power tube section.  As soon as it warms up a get a hum regardless of where the volume is.  I am hoping all I have to do is swap the output transformer wires on the power tubes but I will await some valaditation.  Here is a link to a video of the recorded Hum, https://1drv.ms/v/s!AnZDbFFZGsVLh94LjwXvalVOogwY3Q?e=Xol6U1 .  I think the picture of the build is up above.

Again, thanks for all your help.

Rusty

[stratomaster]

Reversed primaries result in positive feedback and a high pitched squeal or whine.  That sounds like a filtering problem to me.  Check grounds and cap orientation. Check sections in the multi section cap for bad sections by measuring AC at each node.  Confirm the 1k resistor is the correct value off the B+.

Also check the ground connection on your heater virtual CT. Looks like no solder in the photo.

[mresistor]

I would take that black ground wire from the pot ground wire off of the transformer bolt and make sure the other end of the ground wire on the back of the pots is grounded to the input jack. You have all your pots grounded to the power section and the preamp ground should be far away from that.

[Jrusto]

Ok, I have checked the AC Voltage on the MultiCap Can and there is no AC present.  I have moved the ground wire from the Pots to the transformer and grounded the pots to the Input jacks.  I resoldered all the MultiCap Can terminals, the virtual CT and confirmed on the schematic that the first resistor on the B+ is 1K.

The voltage going down the B+ is 470v at A, 440 at B, 356 at C, 273 at D.  When I touch the probe to the turret especially at B, I hear static from the speaker if I move the probe.  The power section schematic is attached for letterd references.  Seems like the voltages are high according to both the Reissue Schematic and the Robinette schematic.  The Reissue had a Thermistor and a Black Block before point A.  I dont know if those will make a difference in the voltages. 

The only change I made from the original Hoffman Schematic was to replace the board mounted BIAS pot with the one that came with the Reissue.  Both measure between 10 and 11K.  Could I have wired it backwards. 

Again thanks for any help in solving the problem.

Rusty

[sluckey]

get a hum regardless of where the volume is.  I am hoping all I have to do is swap the output transformer wires on the power tubes but I will await some valaditation.

Disconnect the negative feedback wire from the board or speaker jack. If the hum goes away, then swap the OT plate leads.

[Jrusto]

Sluckey, You nailed it.  Disconnected the NFB and no hum.  Reversed the OT wires and we're rockin and rollin.

Seems like I got a little hum as I turn the volume up but I will work on that. I can shorten up some wires and work on the dress a little more and it will probably clean up. 

Now to set the BIAS.  Fender recommends 23mV.  Is that a good number.

Thanks for your help.

Rusty

[HotBluePlates]

Now to set the BIAS.  Fender recommends 23mV.  Is that a good number.

Plenty good.  So is 21mV.

[Jrusto]

Finally got about 3 hours playing/running time on the new build today.  Sounds really good and I appreciate the help getting it going. 

Questions for the day.  I did not have my rev/trem pedal plugged in so I plugged it in while the amp was on and got 2 loud pops as I plugged it in.  Won't do that again but all was fine and I continued playing for another hour and a half.  I noticed on the PRRI that Fender has 2 caps on botht he reverb sockets and the footswitch sockets going from the pin to ground.  Is that a good idea and could it stop that loud pop.  I have included a capture of that part of the PRRI.

2.  Seem like the chassis is getting pretty damn hot near the IEC power socket. I never really checked it when it as a PRRI.  I am soldered to the connectons.  Could be coming from the rectifier tube.  I just noticed there is a difference in the Rectifier Specs from the PRRI which calls for a 5AR4 and the Hoffman schematic calls for a 5UG4.  I am using the 5AR4 Rectifier.  I'll look up the specs for both tubes but could use some confirmation here.

Agian Thanks for your response.

Rusty

[sluckey]

Caps may help. But how often will you be unplugging the footswitch and reverb cables? Most people don't mess with those.

5U4 or 5AR4 are both used in the PR. Use whichever came with the PRRI.