new build 36lite with vvr

[chaliapin]

Couple of Qs regarding the centre tap. I have the solder strip, planning on drilling it out a little so it sits on one of the PT mounting bolts as in picture.

Im also planning on connecting the screen to the tag strip. On one schematic it shows a fuse marked HT 500mA. On my other layout and schematic using that exact transformer there is no fuse. Shall I proceed without?

[Daviedawg]

You should have (at least) two fuses in any build, the incoming mains power fuse and the HT fuse protecting the power distribution circuit after the PT. Some circuits have it before the rectifier and some after.

Dd
Edit: to be clear you need a fuse after the power transformer but it can equally well be before or after the rectifier. The value in ma to be based on the current at that point.

[chaliapin]

Great thanks, so a half amp fuse for the screen wire running from the pt. Should the centre tap connection go through that fuse as well or centre tap directly to ground?

Is it another slow blow fuse?

[Daviedawg]

In the situation you are asking about the fuse needs to go after the rectifier on the B+. Slo blo.

Dd

[Daviedawg]

After a quick look at the schematic I believe you are using, I would put it after the standby and before the drain (220k) resistor. That way the caps will drain if it blows.

Dd

[chaliapin]

Thanks this is great, I've included a pic of the relevant schematic part. So I'll keep what I've labelled as f2 straight to ground using my solder strip with the CT resistors as a handy place to hang it. I'll use a fuse holder not mounted in the chassis unless that's a problem.

Then what ive labelled f3 after the rectifier and standby switch, I can use a chassis hole for this but what value fuse here?

Also what does the HT mean in HT 500mA?

Update

Before I posted the above I realised Josh would tell me to look away the other schematics... both his layout and Ritchie( I think) only show what I've called f2 in the same place as my schematic and also not chassis mounted on Josh's so that's all good...

[Bieworm]

HT is the high tension secondary voltage.

[chaliapin]

Slow progress on the board. Obviously not very professional but im happy with the look and the feel of the joints.

[chaliapin]

I know the wire colour scheme has gone wild, I ran short of blue for the grid connections.

[chaliapin]

Not connected anything to the board as still need to get sockets and do the heater wiring but it's starting to look and feel like it might be an amp one day. Shout if anything looks REALLY bad and I'll re do it.

Any pointers on which wires are friends or enemies with which other wires appreciated. I read a post of Josh's but I can't find it again...

[JMPGuitars]

Any pointers on which wires are friends or enemies with which other wires appreciated. I read a post of Josh's but I can't find it again...

Plates (anode) shouldn't run parallel to control grids ("the grid"). My method (and color scheme) is, from bottom (chassis) up:
Plates (red), then cathodes (black), then grids (blue).

In this image, you can see where there's a couple spots that were hard to completely avoid parallels, so I created right-angles (and distance) where I could:

_JMP3283.jpg

Thanks,
Josh

PS. It's hard to inspect your soldering from those photos. Side-angle images would make that easier. Make sure your soldering is smooth, shiny, and concave.

[chaliapin]

Great thanks Josh, I sort of intended my soldering wasn't too clear on the picture but I suppose im here for the punishment...

The ones without solder err, haven't been soldered yet...

[chaliapin]

Couple more

[JMPGuitars]

Great thanks Josh, I sort of intended my soldering wasn't too clear on the picture but I suppose im here for the punishment...

The ones without solder err, haven't been soldered yet...

I love that response, and I don't blame you. I knew what was going on though.


I'll try and be nice. Clean it up! You need to make your wire wraps TIGHT on the turrets. They're big and floppy right now.

You also need to work on your soldering technique - I'm actually glad a lot of it isn't soldered yet. You need to work on creating a heat bridge between the iron and turret before applying the solder. That means you put just a drop of solder and wait until that drop melts into the turret before applying the rest of the solder to create the bond between the turret and leads.

Smooth! Shiny! Concave! Those are the things you need for good solder joints.

solder1.jpg

These solder joints are clumpy (not smooth), not concave, and only partially shiny. You can also see large space between the lead and the turret filled with the solder.

solder2.jpg

These are convex and clumpy. Those little indentations usually mean the solder was overheated.


Watch the videos linked in the soldering thread in my signature. Then watch them again. Then desolder anything that doesn't look awesome, and try again. Focus more on those heavy resistors as they can take more abuse. Use heatsinks when you can to save the components from damage.

_JMP1289.jpg

I'm not pretending to be perfect, but this is a previous build of mine, and an okay example of what you want to do. There's no solder blobs, wires are tight, soldering is smooth, shiny, and concave.

One other trick I do is to wire up the amp before installing any components. That way they're less likely to take any heat damage. For example:

prewired.jpg

Thanks,
Josh

[chaliapin]

Thanks Josh. Id hate to catch you on a bad day not being nice...

Do you use the solder sucker things? I have a sort of braid stuff as I was told it was better but it doesn't seem to do much...

[JMPGuitars]

Thanks Josh. Id hate to catch you on a bad day not being nice...

Do you use the solder sucker things? I have a sort of braid stuff as I was told it was better but it doesn't seem to do much...

Hahahahahaha! Don't worry, the end result is that you'll produce higher quality work, with less need for troubleshooting.

I use an Aoyue 2703A+ solder rework station (currently $299 from SRA through Walmart and they're selling it for $379 everywhere else), and it has a dedicated desoldering iron that works GREAT. Usually. Right now the airflow is a little weird on mine (it's over 5 years old, and has been abused regularly). I'll either fix it or replace it. It's about 1/5 the cost of similar machines. I had to desolder a couple tube sockets on a mic I'm building right now (I didn't realize I had better sockets to use), and it took a few minutes to get the air flowing again. Once it was going, it drinks up all the solder like nothing.

I have used those solder sucker pumps. They're tedious, but they usually work better than the braid on turrets. Those braids also depend on what brand. I've had some generic brand desoldering braid that did nothing but waste my time. I have braid from NTE now that works well, but I only use it if my desoldering gun is inappropriate (like when cleaning up PGA113 VSSOP chips).

Thanks,
Josh

EDIT: Looks like I fixed the soldering station. I cleaned the air pump and connected it to a variac, and it worked as expected. So I connected a bigger compressor to the tubes, and found the pressure gauge ball was stuck. The fix was…special. I put a few drops of sewing machine lubricant in the tube leading to the pressure gauge ball, and connected it to a compressor. Then I wacked it with a rubber mallet, and now it seems to be working. Though sometimes I still need to whack the ball with a mallet. I'm gonna see if I can get a new front panel.

[Bieworm]

I think the braid is nice on desoldering PCB components. On turrets I still use the good old pump

[chaliapin]

Thinking I might as well incorporate the MV now while I'm ,ahem, tidying my turret board. Is the type 2 the same as the drive control in the downloads bieworm? Interrupt C7 and C8 (coupling caps?) With 1m audio taper potentiometer, replace r12 and 13, currently 470k with 1m resistors. Continue as before to R14, R17, R19 and R15 (grid resistors?) To pin 2 on the power tubes. Handwritten schematic attached.

[chaliapin]

They good enough or still a bit crap?

[Bieworm]

Thinking I might as well incorporate the MV now while I'm ,ahem, tidying my turret board. Is the type 2 the same as the drive control in the downloads bieworm? Interrupt C7 and C8 (coupling caps?) With 1m audio taper potentiometer, replace r12 and 13, currently 470k with 1m resistors. Continue as before to R14, R17, R19 and R15 (grid resistors?) To pin 2 on the power tubes. Handwritten schematic attached.

That 1M pot is a dual ganged pot. But it should not be 1M.
The dual pot replaces the grid load resistors of the power tubes. On the 36W those are 220k each.. meaning the dual pot should be 250k audio taper.

On your schematic you parallel 1M resistors with the pots, but that is not correct. Those resistors are safety resistors in case the pot fails. They also should be 10x larger in value than the pot, so they don't influence the sweep too much while the pots still work properly. In case the pot should fail, then the grids wouldn't have a ground reference anymore and that would be a bad thing.. hence the 2M2 resitors across the 250k pots.

LarMar MV type 2.png