JCM Build - Voltage problem!

[shunt94]

Hey,

I am building a JCM 800 2204 from scratch and just got to the fun part of voltage testing!
So, I tested all of my valve holder pins and they turned out a bit different to expected! 😕

I understand that after I plug in valves the voltage will decrease, But Im really not sure if they will decrease by this much!
Here is the voltage chart that I am following:
My 50 Watt JCM800 - Model 2204
Schematic: http://mhuss.com/MyJCM/JCM800_2204.gif



Here are my results:

An externally hosted image should be here but it was not working when we last tested it.

The voltages which are bold are the ones that are different, the voltages in red are, in my mind, Really quite different!
The voltage in orange seems like it should be negative from the actual voltage chart, but my pin was positive 😕

I've checked that non of the pins are touching and I was almost certain that all my wires were going from and to the correct place!

Here are some pics of the amp (its a bit messy cause its my first amp )

http://i41.tinypic.com/2ns1l05.jpg

http://i42.tinypic.com/2ldg8o.jpg

http://i43.tinypic.com/2s1xb1y.jpg

http://i39.tinypic.com/xc6t74.jpg


Any help would be greatly appreciated!

Many Thanks!

 

[DF96]

Voltages look fine; some will change a lot when the valves are in place and taking current. Check that V4,5 pin 5 (grid bias) can adjust over the required range.

 

[paulrerrie]

Quite ambitous for a first amp! I'm afraid thats not an excuse for messy wiring - its a lot easier to check/doublecheck before first switch on, if its tidy.

I would adjust your bias pot to suppy minimum bias current (or max negative volts) before plugging in valves and switching on.

 

[shunt94]

Thanks for the quick replies!
I shall check the grid bias and try adjusting the bias pot tomorrow!
I haven't ordered valves yet, so there's no particular rush for me.

And Yea, My wiring is awful! - To be honest, this is one of the first electronic projects of mine! - Still getting the hang of it all!.. Though I've been building this for ages; just doing it bit by bit and researching things as I go along. 🙂
I may invest in a bigger chassis if I ever build another one! - I just find working in such a small space a bit difficult!

Still, Many Thanks for replies!

 

[speakerfritz]

do you have a variac? It would be helpful to bring the power up slowly and take measurements as you go ........

 

[cerrem]

Voltages will be drastically different with tubes in the amp...
Keep in mind that your filter caps may take a beating with no tubes in the amp...since the voltages are excessively past the ratting of the filter caps..
Also check your bias diode may be in backwards...since your bias voltage should be negative....
Also when the amp is in working condition....and you adjust the bias....all your voltage will fluctuate substantialy on where you bias the output tubes...Since the power supply in guitar amps is not only unregulated but suffers from very poor regulation, which makes them sound soo appealing...
The mains voltage will also throw off the DC voltages....also the quality of the mains voltage will have a drastic affect on the DC voltage depending mostly on 3rd harmonic polution in your mains....
Voltage charts are just a rough approx since many factors can influence thefinals DC voltages...
Oh..and one last most important thing...Have fun playing your amplifer..

Chris

 

[shunt94]

Hey,

Thanks for the info!

I've solved the problem now! - It was a CT on the P.T that I accidentally didn't ground!

So everything works! - I've plugged in tubes and all voltages are next to perfect! - I still need to fully test the amp as I was only playing at Very low volumes.. But I think all should be fine!

Thanks for helping me out everyone!