Help on new #40 build

[martin manning]

In the end that OD entrance network as-drawn looks like a 500p to ground followed by a 182k-5k divider.

[andresound]

Installed the trimmer! Works great. Thank you Martin.

Still no joy with FX loop. I am beginning to suspect a bad tube socket (they are 50 yrs old). Have ordered new ones. For now, I can only get ceramic basses. Are these inferior?

Have bypassed the FX loop for now.

Without ANY tweaking, votages, PI, tube rolling etc, the amp sounds very promising and surprisingly noise free considering the lack of space! Need to:

1) check why accent switch has no effect? Have an electrolytic there
2) Voltages
3) PI balancing
4) install FET input board
5) Deep switch and bright switch also seem to have no or little effect
6) refurbish head cab

Will report

[andresound]

Another worthwhile spot for a trimmer is in the 1M2 spot. You put a 1M trimmer in series with a 680K pot, so at half rotation on a lin pot, you have the stock 1M2. Different pickups like different values there. That's a tweak courtesy of ToneMerc.

Yes, I have used either a 560k or 680k resistor in series with a 1M trimmer as a substitute for the 1.2M resistor. That is my my preferred method for deploying that "funky OD entrance".

TM

So I can employ both methods. The OD entrance trim pot, (the 100k trimmer to ground as a voltage devider), and substitute the 1M2 with 560k and 1M trimmer? Or are they both achieving the same outcome?

[martin manning]

Two ways to accomplish the same goal. You have a 1M2-33k voltage divider. To make it adjustable you can make either the 33k or the 1M2 variable. The trimmer will adjust the OD volume, but it will also affect the available range of OD distortion. To adjust only the relative volume of the OD vs. clean you should adjust the divider at the output of V2b (which is the Ratio control on later amps as I marked on the schematic).

[andresound]

Thanks Martin,
I think for now, I will concentrate on the voltages while waiting for the tube basses.

[andresound]

Also, I n ow understand what a voltage devider is!!!

SO chuffed the amp is up and running. Once again a big thank to you Martin

[martin manning]

You're welcome Andre, good to hear you're having some fun.

On the recover stage, your DC voltages seem ok (maybe you could recheck them?), which says the socket is making contact. I'm inclined to think there is a wiring error or fault somewhere.

[andresound]

Will do Martin,
As I said, I pulled the tube and bypassed the FX loop (clip jumper cable) for now. I have checked the wiring SO many times my head is spinning. It it also not a very complex circuit to follow! I will try the amp with my band at practice this coming week and see what happens.

Put in the Ratio pot a 1hr ago and it works. SO I have a 100k trim pot at OD entrance and a 100k trim pot (ratio), at OD exit.

These are my latest voltages
AC primary 640v
Heaters 6.29v

B+1 = 425.3
B+2 = 424.1
B+3 = 404.7
B+4 = 302.1
B+5 = 298.2

V5 pin 3 = 427.9
4 = 425.9
5 = -47.6
8 = 34mv
V4 pin 3 = 427.9
4 = 425
5 = -47.4
8 = 33mv
PI pin 1 = 281
2 = 66.9
3 = 105.5
6 = 281.5
7 = 69.4
V2 pin 1 = 202.8
3 = 1.542
6 = 219.9
8 = 1.519
V1 pin 1 = 208.3
3 = 1.402
6 = 203.5
8 =1.467

[martin manning]

Overall those voltages look good except the V1 plates should be around 180-190 (at least that seems to be the preferred range for ODS). Don't do anything yet, though; adding the FET board that will take care of that.

How exactly did you do the Ratio trimmer?

On the loop tube voltages, most recently you had:

FX pin 1 = 250
pin 2 = 16.2
pin 3 = 30.75
pin 6 = 230
pin 7 = 30mv ?
pin 8 = 1.67

These look good, except for the 30mV on the recover stage, which should be closer to zero. The driver stage works, right? You were taking output from the send jack before. There is no obvious reason you shouldn't get sound through it.

[andresound]

Ratio was wired ;

1M to outer leg, output (centre leg) to relay and removed the 180k to ground. The remaining outer leg went to ground.

Should I replace the missing 80K to ground?

Just a thought, If I dont install the FET board and adjust the dropping string accordingly, I could use the remaining hole for a front Ratio pot?

Or is the FET input good for electric guitar as well. I understand it as being used for acoustic guitar mainly.?

Regarding the FX loop. I will have another look and post voltages.

[martin manning]

Ratio was wired ;
1M to outer leg, output (centre leg) to relay and removed the 180k to ground. The remaining outer leg went to ground.
Should I replace the missing 80K to ground?

The loss of 80k from the coupling cap to ground probably doesn't matter much (it's <7%), but you've cut your maximum available OD signal by approximately 40% that way. If you can't get enough OD volume to balance the clean, then you'll have to change it.

Just a thought, If I dont install the FET board and adjust the dropping string accordingly, I could use the remaining hole for a front Ratio pot?
Or is the FET input good for electric guitar as well. I understand it as being used for acoustic guitar mainly.?

The FET can be useful as a boost for single coil PU's, so it's worth having, but a boost pedal could do the same.

[andresound]

Martin,

Revisited the FX return, and noticed, I NEED MY EYES TESTED!!! I had 4.7uf Lytic coupling cap instead of 47uf for the this stage. Had to get out the magnifying glass to verify

My wife has promised one of those workbench magnifying glass lights for my birthday month end.

Could this be the problem?

[martin manning]

You must be referring to the 47uF bypassing (in parallel with) the cathode resistor. That certainly won't prevent the signal from passing through, it will only reduce the low end frequencies a bit.

I'd start by verifying that the DC voltages on the loop tube are still good. Then see if you can jump from the send jack to the power amp and get sound. Do this by plugging a cable into the send jack. Then put a patch cable from the send jack into the return jack. That should get around any problems with the switching contacts on the jacks themselves. If there is still no output, jumper from the return tip to the power amp with that patch cable still in place. If that works, then it's down to just the recover stage.

[andresound]

Martin,

I found the problem with the FX loop!! VERY embarrassing

Because of space constraints in the loop area (bulit around the tube socket), I had used a tag board (with three tags) for securing components around this area, not realising that the center tag was the part securing the entire tag board to the chassis, and of course TO GROUND!!!! STUPID, STUPID, STUPID.

Martin you did suggest a short to ground a gazillion pages ago, and I looked for it. Cant wait for that workbench magnifying glass/light!!

Amp now sounds great.

OK..... FET input next.

[martin manning]

I found the problem ... VERY embarrassing

It always is!

Which configuration of master and level controls do you have now, and how is it working out?

I guess you already have a board and components for the FET stage?