All these wires and voltages,so little knowledge

[Mrcads]

Hi guys, new to the forum, don’t see an introduction section.
I have gotten some tips from this forum, but I ran into a wall.

I got the bug of building a JCM 800 clone.

Things were going fine, I thought until I flipped the switches.

I cannot find the high voltage on the amp. It gets up to 53 VDC. I have a feeling that the power section got wired wrong.
This is the PT:
http://www.classictone.net/40-18095.pdf
This is the layout:
https://tubeamplifierparts.com/layouts/ ... layout.pdf
This is the diagram:
https://goo.gl/images/DTCFNT

I wired a red and a red/yellow to the standby switch as shown in the layout. I put shrink tubing on the other red. I get 420 VAC on the standby switch, top 2 terminals. However, I can’t seem to find DC voltage greater than 53 Volts.

Any help is appreciated!

Thank you!
Gabriel

[danman]

The PT that you used has a red/yellow striped wire that is the centertap for the high voltage secondary. This wire should be grounded to the chassis. The layout that you linked to does not show the centertap going to the standby, but instead shows a black wire going to ground. This black wire will be the red/yellow in your PT. The two red wires in the layout are the high voltage AC wires that run to the standby switch and then the recifier diodes. Anything after those diodes will be DC voltage and heads to the first filter cap.

The PT that you chose offers three different high voltage AC options which can be confusing for a first build. The two red wires are the highest voltage, The two purples are slightly lower and the two grey wires are the lowest option. You can use any one pair at a time and heat shrink the ends of the others. If you use the red, B+ will be around 450vdc and with the grey it will be about 380vdc. The purple pair will give you something in between.

Theses are the wires that need to be connected to the standby switch. Choose which color pair you want to use and solder them to the switch. Take the red/yellow centertap wire and connect that directly to the chassis. There will also be a green/yellow centertap wire for the filament circuit that connects to ground also. You also need to be sure to have the primary side wiring correct. If your house has 120v, you will be using the blue and black wires from the PT. We can help further if you could post some pics of the amp.

[danman]

Also, did you use the Watts turret board and chassis for the build? Trying to build with this layout using a different board or chassis can be confusing because there are several versions of this layout that work with different company's boards and chassis. Mixing them can make the build more difficult.

[Mrcads]

Hi.
Thanx for the reply.
I was looking at the schematic and it dawned on me that the center tap should be grounded. I did that and I got DC.
The interesting part:
I had to use the gray pair of wires, cause everything else would give me >500 VDC. The gray wires give me 460 VDC.

I did the biasing and it sounds good, but it only sounds one way, meaning that the preamp knob does not seem to be doing much(sounds like it is up all the way) and the low sensitivity input does not work. Other than that, it sounds like a beast!
I got the chassis from ebay, cause I didn't want the retro look of the narrow chassis. I got the board on ebay also, and it looks very professionally made.
https://www.ebay.com/itm/Hand-Wired-JCM ... 41e4f89bbc

I will try to post some pictures.

Gabriel
PS: agreed on the transformer. Too many options for a beginner.

[danman]

Is that 460vdc reading with all tubes installed and biased properly? If so, there may be a wiring error on the primary side of the PT. If the 100vac tap was used with 120v wall voltage, you will see higher than normal B+ voltage. I have that same PT in my 2204 build and the gray wires should be delivering about 380vdc final B+ voltage. The fact that the low input is not working most likely indicates an issue with the low jack. They can be a bit tricky to wire up so I would look closely at both jacks as the signal from the high channel travels through the low input before moving on to v2.

[Mrcads]

You were right about the wiring of the primary.
The voltage selector is 120/220/240. The wires are 100/120/240.
I got that straightened out.(switched the selector so i use the 120 feed) I went back to the red wires and I have 450 VDC after biasing.
Thank you for pointing it out.


This is the wiring that I have seen in many layouts. Am I missing something?

Thanx again for your help.

[Mrcads]

You were right about the wiring of the primary.
The voltage selector is 120/220/240. The wires are 100/120/240.
I got that straightened out.(switched the selector so i use the 120 feed) I went back to the red wires and I have 450 VDC after biasing.
Thank you for pointing it out.


This is the wiring that I have seen in many layouts. Am I missing something?

Thanx again for your help.

[danman]

In your picture of the jacks, you have a bare wire jumper running left to right and I can't make out what terminals it is actually soldered to. If you look at this layout...http://site.triodestore.com/JCM8002204Stock.pdf you will notice that there is no jumper wire connected across like yours. There is also a 470k/470pf treble peaker circuit that runs between the low jack and the gain pot that I do not see in your pic. Some builders will omit this treble peaker and instead use a 68k in it's place. This only affects the amp's high end and would not stop the low jack from working properly. I believe the extra jumper you have soldered may be what is preventing the jack from working.

[neikeel]

In your picture of the jacks, you have a bare wire jumper running left to right and I can't make out what terminals it is actually soldered to. If you look at this layout...http://site.triodestore.com/JCM8002204Stock.pdf you will notice that there is no jumper wire connected across like yours. There is also a 470k/470pf treble peaker circuit that runs between the low jack and the gain pot that I do not see in your pic. Some builders will omit this treble peaker and instead use a 68k in it's place. This only affects the amp's high end and would not stop the low jack from working properly. I believe the extra jumper you have soldered may be what is preventing the jack from working.

Good job danman is on the case!
Treble peaker is on the board (470k cc bypassed with yellow silver mica). Personally I have tried the other input variations
but always go back to the stock layout (including the 1000pF on the pot) as all the other versions lack a little bit of edge and sizzle but it will depend on your guitar/board/cables and speakers, YMMV.

[Mrcads]

In your picture of the jacks, you have a bare wire jumper running left to right and I can't make out what terminals it is actually soldered to. If you look at this layout...http://site.triodestore.com/JCM8002204Stock.pdf you will notice that there is no jumper wire connected across like yours. There is also a 470k/470pf treble peaker circuit that runs between the low jack and the gain pot that I do not see in your pic. Some builders will omit this treble peaker and instead use a 68k in it's place. This only affects the amp's high end and would not stop the low jack from working properly. I believe the extra jumper you have soldered may be what is preventing the jack from working.

One jumper goes from the tip on the low to the sleeve on the high. The other goes from the sleeve on the low to the tip of the high and to the sleeve of the high to the front of the 1M resistor. The picture gives the impression that the nearest tips and sleeves are touching. They are not.
The bright cap is on the board in my case, turret 4 from the top right.

[danman]

There are a couple more things you can check then. Since the preamp volume pot is not functioning properly, reflow all solder joints on the pot and be absolutely sure that the ground wire from the pot has a solid connection. A missing or bad ground would cause the issue you are describing. For the low input, doublecheck all solder joints and be sure that none of the lugs on the jacks are touching each other and shorting the signal. Just a single stray wire strand touching the wrong spot could ground the signal and cause the jack to not function. Also check the leaf switch inside the jack. Be sure that the switch opens fully when you insert the cable. I have seen where these would not open completely and cause signal issues. The issue has to be in the wiring around the jack and volume pot because the amp performs correctly on the high channel. If there was an issue with the low channel further downstream, the high channel would sound weak because it uses the other half of v1 (low channel) to increase the gain before mixing the signal at the two 470k resistors and then on to pin 2 of v2.

[danman]

In your pics, it looks like you have the cable shield soldered to the left lug of the volume pot but I do not see a ground wire attached to the pot. Check this area first.

[Mrcads]

Hi!
I did a cleaner job, this time with some braided wire.

Here is the back of the Volume pot, with the shielded wire removed. The braid gets snipped off at the V1 end from my understanding, right?

It sounds very anemic (you can barely hear it) with the Master And PV cranked up all the way.

[danman]

I see the ground wire soldered to the volume pot now so that looks fine. Are you saying that the whole amp sounds weak on both channels or just the low channel? The low channel on these amps is quite a bit weaker than the high channel because it is missing one gain stage. It is hard to follow the wiring because the Watts layout is slightly different from other 2204 layouts such as this one...https://drive.google.com/drive/folders/ ... llPYTc5Wms If you notice, v1 is rotated differently in each layout which can make wiring it tricky. Just be sure to trace all wires to the correct pin numbers and you should be fine. If you feel comfortable doing it, we need a full voltage chart. Measure the AC filament voltage at all sockets and then check DC voltage on all other pins. Hopefully we can spot a clue in the voltages.

[Mrcads]

It sounds good on the high sensitivity input.
I was expecting it to be quieter on the low sensitivity, but this one I would say it’s unusable. Also the preamp knob not doing squat makes me wonder. I will check voltages and post back.

Thanx