Marshall 3203 problem

[jgcable]

Re: Marshall 3203 problem

Could I just throw a spare 12AX7A in it temporarily and see if the volume level fluctuates or will I damage something? ?

[sambencuda]

Re: Marshall 3203 problem

Just be careful with that soldering iron buddy! [img]/images/graemlins/poke.gif[/img] [img]/images/graemlins/laugh.gif[/img]

Seriously, I hope it's just the phase inverter. Funny, this problem never happened once while I had. Well, I only played it a handful of times. That little amp rips.

Good luck bud!

[jgcable]

Re: Marshall 3203 problem

[ QUOTE ]
Just be careful with that soldering iron buddy! [img]/images/graemlins/poke.gif[/img] [img]/images/graemlins/laugh.gif[/img]

Seriously, I hope it's just the phase inverter. Funny, this problem never happened once while I had. Well, I only played it a handful of times. That little amp rips.

Good luck bud!

[/ QUOTE ]

I am sure I will figure it out.

[jgcable]

Re: Marshall 3203 problem

I tried a different tube that is in good shape and the fluctuation stopped completely. Now I know I need to get a new balanced phase inverter tube. Yippee!! What a simple fix thanks to all the help on the wonderful informative JCF.

[DonP]

Re: Marshall 3203 problem

[ QUOTE ]
Could I just throw a spare 12AX7A in it temporarily and see if the volume level fluctuates

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yes.

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or will I damage something? ?

[/ QUOTE ]
no.

I think you are going a bit far looking for a balanced 12AX7, but if it helps you sleep at night, go for it. I'm sure Marshall wasn't searching out balanced phase inverter tubes when they were selling these.

There are so many other things that could be out of balance in the power section, that if you aren't going to blueprint the entire circuit and use calibrated resistors, caps, and output trannies, I think you are wasting your money. Also, the power section has the negative feedback loop to correct these out of balance issues.

[Soloist1]

Re: Marshall 3203 problem

Phase inverters tubes are always tested for an even balance between the triode sides. At least by anyone that does it right. Do a search on the subject. If you buy matched tubes for the power amp section, where power tubes are concerned, why would you want a phase inverter that isn't matched as well?

[DonP]

Re: Marshall 3203 problem

Waste of money IMO.

Do you check the PI load resistors to see if they are balanced for gain? For bias? Are the coupling caps checked to make sure they are passing the same frequencies to the grids of the power tubes? What if one coupling cap is .018 and the other is .022, one side of the power amp would be getting a lot more bass than the other side.

What is the feedback loop for? To correct these issues.

[jgcable]

Re: Marshall 3203 problem

What differences will I notice between a 12AX7A and a 12AT7?
The schematic for the amp calls out a 12AX7A but there was a 12AT7 in it. I threw a stock Sylvania 12AX7A in it and it is working perfectly. It really is a great sounding amp.

[DonP]

Re: Marshall 3203 problem

The 12AT7 has less gain (30-40 vs. 100) - I said that already. If it is working perfectly, you are done.

[thebisch]

Re: Marshall 3203 problem

I can see a balanced PI being of marginal advantage in a technical sense (ie: HiFi), but in a guitar amp, it is a total wast of money IMO. Don't buy the hype.

[Soloist1]

Re: Marshall 3203 problem

Tell you what. Get a tube that is say 1100/800 and use it as a phase inverter. Then try one that is 1100/1100 and see which one sounds better. The loop is ment to correct those issues alright. Specifically for some of the reason's you mentioned. But to a degree. The components should still be matched as closely as possible. You'll never get perfectly matched tubes period. The idea is to get ones that are close as possible and let the loop correct the small differences. The difference is a amp that sounds like ass and the same amp that doesn't with better components. Plenty of opinions out there. It boils down to some people want max performance and tone out of their amp. And others don't.

[thebisch]

Re: Marshall 3203 problem

Got any double blind, SPL matched (with a meter), listening testes to validate this?

[jgcable]

Re: Marshall 3203 problem

For the cost of a balanced tested tube I might as well replace it even if I can't tell the difference. Bottom line right now.. the amp is working great and I owe it to you guys. Thanks. I appreciate the tube lesson.

[Soloist1]

Re: Marshall 3203 problem

[ QUOTE ]
Got any double blind, SPL matched (with a meter), listening testes to validate this?

[/ QUOTE ]

Yeah right here in my back pocket. First off if you buy tubes from a place that gives a shit they won't charge you for balanced tubes. Secondly do you also just throw power tubes that aren't balanced in your amp because balanced are more expensive? If your putting matched power tubes in an amp why would you put an unbalnced tube that drives them in front? Because your cheap? That's the only reason I can see. And do you have any tests that validate that an unbalanced one is just as good as balanced?

[thebisch]

Re: Marshall 3203 problem

I just looked up the data sheets/application notes for GE, RCA and Mullard 12AX7s. None of them mention anything about balanced PI tubes in their PI designs. The tube engineers from back then have forgotten more about tubes and analog electronic design than most "pesudo enginners" on the net have ever known or ever will know. After spending many years in the high end audio hobby, I have heard all sorts of claims and snake oil. Unfortunatly, this kind of hype has been hitting the guitar/amp end of things lately. The snake oil is why I left the audio hobby and started playing guitar again. Oh well, I guess I can't escape it! I guess all those classic vintage tones everyone chases their tails to obtain really wern't good...they weren't using balanced PIs! LOL!