Can Someone please offer some suggestions

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10/11/1999 3:12 AM

Darren

Can Someone please offer some suggestions Hi guys,   A little while ago I built myself a bit of a frankenstien tube amp from bits I had lying around the place. It is based on the marshall 2204 (I think it's 4, the 50W version anyhow) amp with MV preamp. It has 6l6's instead of EL34's and it has a low B+ (370VDC) and an output tranny from an old tube bass head that sported KT88's.   Now what I'm wondering is this, what sort of bias current should I be looking at with 6L6's ? I have shockingly mis-matched tubes right now and I'm measuring (using the 10ohm res between cathode and ground type method) 35Ma and 50Ma.   Also, Should my significantly lower B+ be considered as a factor when setting my bias current?     By the way it goes (which surprised me as this is my first home built tube amp) But the low frequencies sound horrid. my best description would be that it sounds farty, particulary when playing open E or chords up that end of the neck. But i think getting all my biasing and such sorted will go a way towards improving that.     Does anyone know where I might find a voltage chart or a circuit with the voltages on it for the Marshall 2203/4 and MV Preamp ?   Just want to check mine with the original and adjust anything that needs to be adjusted to compensate for a lower B+.     Thanks for reading   Darren

 

10/11/1999 3:41 AM

Randall Aiken

Darren,     Go to http://www.aikenamps.com and check out the "Tech Info" page. It has several technical articles, but the two that would interest you the most are entitled "The last word on biasing" and "What is blocking distortion". These articles should answer most of your questions.     The "fartiness" is usually caused by coupling caps and cathode bypass caps that are too large. .     Low B+ is not a detriment, in fact, I prefer lower B+ voltages on my amps. They allow you to run higher currents, for less crossover distortion, and a warmer tone. My best-sounding '72 50W Marshall has 380V on the plates.     BTW, if your plate currents are too far mismatched, the offset DC current in the transformer primary will cause transformer core saturation, resulting in loss of low frequency response.     Randall Aiken

 

10/11/1999 5:13 AM

Darren

Thanks Randall,   I went and read all that info and that gives me a lot of things to check and adjust when I get home. By the formula on that web page I should be running my 6L6's at around 57mA. Looks like I will be modding the bias circiut first up. I think whilst I'm weilding the trusty soldering iron I'll add a pair of trim pots (whatever value I have lying around) in series with each of the 220K "bias" resistors to offer a balance adjustment to help even out the tube current mismatch.   Using the formula on that page I'd say you would be running your EL34's at around 46mA Is this about right, cause it sounds like you are happy with your sound.     Anyway, thanks again.     Darren

 

10/11/1999 5:35 AM

Randall Aiken

If you are running 370V using 6L6GC's, 57mA would be at the upper end of the acceptable range. I like my EL34's around 43mA at 380V in that particular amplifier. The actual value is subject to taste; the important thing is to make sure you don't exceed the plate dissipation at idle or at full power.     By the way, Duncan Munro has a cool Excel spreadsheet tool for calculating the acceptable current range for biasing. It is at http://duncanamps.simplenet.com/ under the "software" section, and is entitled "Anode load calculator".     Randall Aiken

 

10/12/1999 6:20 PM
jason	Randall,   Thanks for the pointer on the spreadsheet. I just downloaded it and must say it looks rather handy.   Thanks,   jason

 

10/11/1999 6:06 AM
Bob	I've posted a schematic at http://tmxnet.com/2204mv.zip . It doesn't have an actual voltage chart, but voltages are listed at various points of the circuit. Note that this is not the JCM800 version.     =Bob

 

10/11/1999 7:03 AM

Darren

Thanks for that Bob, makes for very interesting reading.   My Amp, although it has a lower B+ seems to have radically higher voltages all over the circuit compared to that schematic. For eg. cathodes of phase splitter should be 39V I have 93V. Anodes of phase splitter should be 87V I have 239V and 298V. Bias voltage should be -63V (well it says 63V but that can't be right) and I have -34V.     Either my multi meter is having issues, or I have some serious hassles here.     I'm off to re-measure everything, cause the Amp goes, it sounds farty yes, but it goes, I can't see it working with voltages that out of range.     Darren

 

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