Starting to populate the chassis....

[erwin_ve]

While comparing wiring diagrams between the Dumble #124 and Jason Borders layout I noticed the wires on the "Presence" pot are switched. Should I switch them correctly or wont it make a differece?
Thanks, BrianL.

It doesn't matter: the presence pot is wired as a variable resistor, one way or the other, the resistance stays the same for the pot settings.

[BrianL]

Thnx Erwin, I wasn’t sure.
Easy fix for the FET switch.

[jazzbass]

Hi BrianL,

great idea, I have the same problem in making #102, I will adopt your system.

Franco

[BrianL]

Just an update for you guys....
Turn the amp on with no tubes, still burning that 22K resistor on the power board...not as fast as it was b4 but still smokes after a minute or two...I triple checked the board wiring and it looked good to me, all the jumpers looked good. I've stripped it to the bare board and will resolder everything. Is it possible to have a bad cap? How do I check the caps. I built the normal power board not the precession board, it shouldn't matter right, both should power the amp. Could be my lousy soldering too....or maybe my input power is too much, maybe I've got the wrong transformer wire on it, I'll recheck that, it wouldn't be anything after the resistor would it?
I have an extra cap set, I'll use those in case I had a bad one,
Looking for suggestions of what I may have done wrong so I don't do it again..
Thanks, Brian

[dbharris]

Hi Brian,

Are you able to post pictures of your power board as built? That would be helpful.

-Dan

[BrianL]

I took it apart already but I'll try to find my older pics...
Here's one:
Can't find a backside pic, sorry

[BrianL]

Rotate above picture clockwise,
This is how I'm seeing it, correct me if I'm wrong.....
I wired, starting at bottom left corner:

Bottom
Hole 1" From B+ on rectifier board (I haven't checked voltage here as of yet,,)
Hole 2: to left side of standby switch
Hole 3: from right side of standby switch
Hole 4: to output transformer
Hole 5: left side of choke
Hole 6: right side of choke
Hole 7: B+2 to V4 Pin 6

Right side:
B+2,3 ground is bottom hole of board to ground 2
next up is to B+3 on preamp board
next up is to B+4 on preamp board
next up is to B+5 on preamp board on to bottom right of FET board

Top
B+5,6 ground, to ground 5

left side:
B+1 to ground3

Jumpers:
B+ from rectifier jumps to left F&T cap(cap5)
B+ from rectifier jumps to left standby switch
right standby switch to B+1 to output transformer to left side of choke
right side of choke to B+2 Scr then to right end of 2K7 5 Watt resistor
left side of 2K7 5 Watt resistor to B+3 connection then to bottom of 22K resistor
B2,3 ground2 jumps minus side of bottom F&T cap(cap 1) and minus side of cap 2
B+4 jumps to top of 22K resistor
B+5 jumps to bottom of 2K2 resistor
B+4,5 ground 5 jumps to minus end of top 2 caps(cap 3,4)
minus cap5 jumps to plus of cap 6
minus cap 6 jumps to B+1 ground 3

[BrianL]

checked that B+ voltage, it's 640 volts, should be 450volts......I got 320volts on the two transformer inputs
top right of the rectifier board and 640volts coming out at B+. the caps arn't working correctly. I'll have to
check that out, maybe I got the wrong caps. too much voltage would fry that resistors.
Brian

[erwin_ve]

checked that B+ voltage, it's 640 volts, should be 450volts......I got 320volts on the two transformer inputs
top right of the rectifier board and 640volts coming out at B+. the caps arn't working correctly. I'll have to
check that out, maybe I got the wrong caps. too much voltage would fry that resistors.
Brian

With no load the high voltage is partially explained. Also I dont think the caps would take that much of a excess voltage.
Maybe a variac with light bulb limiter could protect your valuable parts.

[dbharris]

I agree with Erwin on the light bulb limiter and variac if you have one. With that resistor consistently burning up you have something in that part of the circuit pulling too much current. This is likely going to be a wiring error.

I know it is a pain, but if you can upload top and bottom board shots for at least your power board and how you have the wiring in the chassis currently, it would help us help you.

Also, I do not mean any disrespect, but some of those solder joints look a little suspect to me. Could be the pictures, or maybe you have some cold joints. I don't think that alone causes your current issue but reflowing now might save you time in the future chasing down others.

-Dan

[BrianL]

I rebuilt the power supply board, put the jumpers on the top so I could see them.
I know I don't solder great, my original solder was very rosiny, made a big mess. I'm glad I took the Power board apart and cleaned it.
Looks much better now.

[BrianL]

OK, I made a mistake reading the voltages.
Picture says 640 VAC in and 450VDC out. I measured VAC on both.
So I have 640VAC going in and 286VDC coming out. Do i need to adjust this to get 450VDC?
Thanks, Brian

[BrianL]

Other pics, messy in there, good thing it's not for a customer

[timrobertson100]

Hi Brian

I’m not anywhere near the level of others on this forum, but thanks to their help have successfully built a few amps.

It sounds to me like you have something pulling too much current (e.g. a wiring error with path to ground or low resistor value) that pulls down your B+ and the extra current pops that resistor.

If it were me, I’d follow a known startup procedure and report back which step I got to, the relevant voltages and problem. I’m not sure when reading this thread if you have tubes in for example so can’t say if the bias plays a part or not. All tubes out and your various B+ and Bias voltage would be useful.

There are a few procedures, but I use this one:

https://robrobinette.com/Tube_Amp_Start ... e_vignette

I hope this is useful

[BrianL]

Thank you for things to check and inspiration....
Alright, we're figuring stuff out now....
Reflowed the rectifier brd and now have that 450 VDC. I was looking for.
I have done all tests with no tubes in place, no speaker cab plugged in, and no guitar plugged in.
Current check results:
B+ 450 VDC from Rectifier Brd.
B+1 450 VDC to output transformer.
B+2 450 VDC to screen resistor V4P6.
B+3 450 VDC to first trimmer on left side of Preamp brd. (B+3)
B+4 450 VDC to Preamp Brd connection at B+4
B+5 450 VDC to Preamp Brd connection at B+5

Still smokes the 22K resister on the Preamp brd.
If I unhook the "FET" attachment wire it doesn't smoke,
So now I'm thinking something with the "FET" board or after.
I did add all the new stuff to make it foot switchable but it smoked before that
So maybe just the board. I'll take a good look at it, Probably clean and reflow it also.
When I started I had hardware store solder and compared to what I got from Tube Depot now
it was terrible stuff, made a big rosin mess out of everything.

Ain't this fun!!!

Thank you BrianL
pic 1 B+3 trimmer
pic 2 B+4 go thru
pic 3 B+5 go thru
pic 4,5 old picture, I have removed the trimmer for the foot switch mod.