18 Watt Lite Test Readings?

[tatter75]

Hi everyone!

First post from a novice so please be gentle!

I have just completed building an 18 watt Lite kit (normal channel only and tube rectified) which was bought for me about 15 years ago from Aston Electronics in the UK. They no longer sell kits and I'm sure they used to provide some test voltage readings to verify the build but I can't find them.

Is there a list of expected voltage readings for certain parts of the circuit?

I do seem to have a couple of issues with my build which I will look into but would like to check voltages:
1. The power light doesn't light up (should be an easy fix)
2. If the volume is on zero and the tone is on full I get a loud white noise/hiss sound
3. There's some low volume hum but I reckon that is probably down to wiring

The amp seems much louder than I expected which I suppose is a good thing.

I will attach an image of the layout that I have used, and some photos.

[tatter75]

Just found the test voltage charts in the downloads section!

Any pointers on the white noise issue much appreciated.

[Bieworm]

Just found the test voltage charts in the downloads section!

Any pointers on the white noise issue much appreciated.

Personally I don’t like the layout plan. Ground return wires are way too long imho for instance. If the wires from the input are not shielded, well change them to shielded wires. Ground the shield on only one end at the input and seal off the other end. Let’s start there. Also a sound clip would be helpful

[tatter75]

Thanks, currently the input wires aren’t shielded so I will do that.

I did wonder why the ground point was so far away from the source of the wires! If I still have issues at a later date could things be improved by just drilling a hole in the chassis nearer the board and shortening those 5 ground wires?

[Bieworm]

Shielded input wires are pretty crucial when you want the best signal to noise ratio. Remember that this is an amplifier. It will do its job and amplify the signal you put into it. The noise generated in the early part of the circuit will be amplified several times along with the guitar signal. You might want to try different 12AX7 tubes in the first socket. Some tubes are more quiet than others. Also a 5751 tube is a good and quiet option. But you lose some overdrive, which might be good or bad, depending on what you aim for

There are ground schemes that have proven to be the best for these amps. Follow the link and read up on the advice. Best chance on good results

viewtopic.php?f=4&t=25372

[tatter75]

Finally got round to doing voltage tests and some chopsticking.

Found a cold solder joint which seems to have sorted out the crackles I was getting.

The pilot light I’ve ordered is the wrong size so I need a new one.

Some of the voltages seem a little high in places, I’m assuming there’s a safe tolerance level? If I’m supposed to be getting 320 but I’m getting 340 is that ok?

I will get around to shielding the inputs soon but most of the noise was stopped by moving the high input wire to over the top of the board rather than under it.

[JMPGuitars]

We can only give voltage answers in context. If you have a specific question about a specific voltage: tell us what it is that is off target, or share a voltage chart from the downloads section.

Thanks,
Josh

[tatter75]

Voltage readings:

V1 12AX7
1. 137.6
2. -
3. 1.21
4. 3.3H
5. 3.3H
6. 137.7
7. -
8. 1.21
9. 3.3H

V2 12AX7
1. 254
2. 64.7
3. 86.3
4. 3.3H
5. 3.3H
6. 218
7. 57
8. 86
9. 3.3H

V3 EL84
1. -
2. 22mA
3. 12.56
4. 3.3H
5. 3.3H
6. -
7. 358
8. -
9. 338

V4 EL84
1. -
2. 22mA
3. 12.6
4. 3.3H
5. 3.3H
6. -
7. 357
8. -
9. 336

V5 EZ81
1. 309
2. -
3. 364
4. 7.2H
5. 4.8H ???
6. -
7. 307
8. -
9. -

Some seem to be on the high side compared to the chart in the downloads section.

I'm in the UK so not sure if that makes a difference to some readings. My amp is a dual input lite version with no tremolo channel.

[JMPGuitars]

I see your concern now. Pin 3 of the EZ81 is your B+ voltage which is high. If you like how it sounds, it's okay. However, if you're after the 18W tone specifically, you'll want to drop your B+ about 20V. That should also help with some of the voltages that are a little high down the line. See my signature link about voltage dropping.

Are the heater readings for the EZ81 errors? I would see what's going on there. Make sure you're reading AC voltage reference to ground.

The attached voltage chart is a good reference, though I target 345DC for B+ node A in most docs now.

Thanks,
Josh

[tatter75]

Thanks! I think the amp sounds pretty good but I may look into lowering the voltage using the methods you’ve suggested later on down the line.

I was having trouble getting a reading on the EZ82 heater pins for some reason. Maybe I have the MM set incorrectly. What should I be seeing on those pins?

I retried and got 4.9v (pin 4) and 6.3v (pin 5). I would’ve thought they both should be the same?

[JMPGuitars]

I retried and got 4.9v (pin 4) and 6.3v (pin 5). I would’ve thought they both should be the same?

It sounds like you might be wired incorrectly. Many PTs have rectifier taps that say 5V, 0, 6.3V. You should be using the 6.3V and the 0V. Check the PT connections.

Thanks,
Josh

[tatter75]

Weird, the connections are definitely right, they are the only two wires coming directly from the transformer and not on terminals. Should both pins read 6.3v?

[JMPGuitars]

Weird, the connections are definitely right, they are the only two wires coming directly from the transformer and not on terminals. Should both pins read 6.3v?

Share some photos of the PT please, and the make & model.

Depending on the PT, you might see 0 on one and 6.3V on the other, or 3.15V on both. You should not see 6.3V on both.

[tatter75]

Thanks again for the help.

I’m not sure of the make, it just came with the kit.

It looks like I should be getting 0 and 6.3v from the labeling but I’ve definitely got a reading on both pins. So perhaps I have a short in the wiring at the valve socket?

I will desolder it and try again.

[JMPGuitars]

I would suggest desoldering the leads from the PT and testing it directly. You could also remove the EZ81 and test the heater pins without it. If it's still weird, then test the PT.

[tatter75]

The voltage is still present when the rectifier tube is removed, so I'll desolder and check the PT.

[tatter75]

Desoldered and checked the PT and voltage is present on both the red and black heater leads.

Does this mean the PT is faulty? Could I just use a different ground lug from the PT or is that asking for trouble?

[JMPGuitars]

It's not a ground lug, it's supposed to be 2 points in a coil.

If the voltages are 6.3V and 5V directly on the PT, then yes, I would replace it. Looking at the picture of it makes me think you should replace it anyway. It doesn't look particularly high quality.

What are the voltages you read there?

[tatter75]

Ah, I see, apologies for my stupid questions.

I'm getting about 8.4v on the red (should be 6.3v) and 6.1v on the black (should be 0).

The lugs that say they should be 290v are actually about 330v, which seems rather high also.

I can get a replacement transformer from Modulus in the UK who are well respected. The company I bought this kit from are long gone.

Wondering if I should swap the output transformer while I'm at it. It looks a similar quality.

[JMPGuitars]

Secondaries will usually be higher than rated unless the PT is accurately spec'd for your wall voltage. Mine PTs look for 110VAC, and they see around 120VAC. My secondaries are 290, but usually see around 310. That's normal.

If you get a different PT, preferably get one with two 6.3V heater taps, and hopefully there's a real center tap on one of them. The regular (non rectifier) tubes get the center tapped heater wires.

If you can't find a good PT with separate heater supplies, I'd consider a solid state rectifier instead. Or do that anyway.