Some Help With Biasing A 2203x

[Sukkoi19]

Hi there. I have a 2011 2203X that has been giving me some issues. I traded for the amp and it works beautifully but in the tradition of screwing with everything until it is broken I wanted to check the bias from the factory. From what I can tell it is still all stock.

I have a Eurotubes Pro 1 https://www.eurotubes.com/store/pc/viewPrd.asp?idproduct=144&idcategory=6

I am sure that many people hate this device but it has always worked well for me. Within 1 percent of my Fluke in any case.

So with the stock tubes from the factory the plate current was 7.2. Adjusting the trim pot all the way gave range from 6.5-25.4. The plate volts were 452 at stock and all the way up 448 and all the way down 462.

Now in the past I had always done the calculation: tube dissipation/plate volts*.7. That gives me 38 mA assuming 455 volts at the plate and 25 watts for EL34. I have used this method many times in the past so I am kinda confused what is going on here.

So thinking I had a bad set of tubes or something I ordered a full retube kit. Obviously I was going to do this anyway but I hoped it would cure the issue. Well it didn't. Exact same issue.

Just for the sake of checking I plugged my bias tester into a 78 JMP that I just retubed for someone and it is showing correct readings for the indicated plate voltage so I don't think its the equipment.

I had a Hotplate set to load as the speaker load during the operation. Same as always.

Is there something different about the reissues versus the originals? This is the first reissue I have biased but I have done a few 2203 and 2204s as well as 2205 and 2210 in the past without any issues.

Thanks

 

[erkl]

I would first compare the bias section against the 2203 reissue schematic - http://schematicheaven.net/marshallamps/marshall_2203_reissue.pdf - make sure none of the resistors are off.

If you need any help with that, take a good high quality photo of the circuit board...

 

[Sukkoi19]

Checking now. Always helps to have multiple eyes though. Let me know if this big enough.

 

[Sukkoi19]

 

[erkl]

what are the colors on R28? Confirm that R29 is brown/green/orange? And R30... red/purple/orange?

 

[erkl]

And while your in there, just check the values on those resistors with a DMM...

 

[erkl]

annnnnnnddddd... what is your DCV reading on pin 5 of the EL34s?

 

[Sukkoi19]

R28 appears to be: brown, orange, blue green

R29: brown, orange, green, purple

R30: Red, purple, orange, brown

Values: R28-55.5/ R29-14.8/ R30-26.8

So except for R28 being a bit high they seem ok.

 

[Sukkoi19]

438 Volts on pin 5.

 

[Sukkoi19]

Again appreciate the help.

 

[erkl]

pin 5 should be negative voltage in the -30 to -50 VDC area..... Pin 5 is between the two pins connected zith the big cement block resistor...

 

[erkl]

Again appreciate the help.

No problem... I hope I am able to help, so far no go though...

 

[Sukkoi19]

Should that pin be checked with standby on or off. I will check again. The pins appeared to be labeled.

 

[Sukkoi19]

Ahh the labels were misleading. It read -43 VDC with the standy off.

 

[erkl]

Not sure if it matters, but in my 2204, R28 was actually a 56k when it actually needed to be 47k... once I swapped it out, I was able to bias my amp properly, otherwise I was sending 40mv to the tubes at the minimum setting... once I changed it I was about 22mV at the minimum setting which was a much better range... I can't see this being the case here because it is actually backwards of your issue.

 

[erkl]

Ahh the labels were misleading. It read -43 VDC with the standy off.

okay - that's perfect.

But something has to be out of spec for the bias pot to only give 6 to 25 at min-max.....

 

[Sukkoi19]

Do you think somebody changed that resistor or do you think it shipped like that? It doesn't look tampered with and they guy I got it from is the last person that would mod anything lol.

 

[Sukkoi19]

Well doing a google search with the resistor in the title brings up a thread where someone changed that resistor the correct value. Unfortunately I can only read the description in the search page as it won't load. Typical. I think I will swap that resistor and see if that fixes it.

 

[erkl]

Well, if that R28 is green/blue/orange (the 4th stripe is gold and is only a control value meaning +/- 5%) those colors mean that it is supposed to be a 56k ohm resistor, which it reads as also - but the schem says 47k goes there. Did someone change it? probably not, it looks untouched to me.

You could TRY to clip a 220k ohm resistor in parallel with that one, the two resistors should bump the over all resistance down to 45k ohms. This way you will at least know if it is the culprit. This is of course assuming you know how to do all that stuff??

 

[erkl]

Well doing a google search with the resistor in the title brings up a thread where someone changed that resistor the correct value. Unfortunately I can only read the description in the search page as it won't load. Typical. I think I will swap that resistor and see if that fixes it.

Must be thegearpage.... it's been up and down all night here.