How To Bias A DSL5C?

[The Ballzz]

Hey Folks,
For the past few years, all of my amps have either been cathode biased, except for my JTM30 which is "pre-set" fixed bias. The last time I owned an adjustable fixed bias amp was long before I started playing around with building amps, which so far have all also been cathode biased.

Well now I have a DSL5C which uses the two triodes of a 12BH7 as the power tube in a push pull, adjustable fixed bias configuration. I have read many of the various "biasing" threads, but have never had occasion to check or set the bias on anything. I have also been led to believe that Marshall is not very critical about proper bias setting before sending an amp out the door and I would like to check the bias on this amp to either confirm that it is set to it's most optimum specs and/or adjust it to be so.

I am quite familiar and respectful of proper safety concerns/procedures when working inside an amp and would appreciate some guidance and possible hand holding to perform this task. From all that I've read it appears that bias readings CAN be accurately taken with nothing more than a good multimeter and that although the math gets a bit more complicated, no added resistor is required. Yes, I know that a nice bias probe makes it all more convenient, but I can't see spending the money at this point for one out of six amps that will occasionally need the bias checked and set. The other amp that uses fixed bias is an octal tube amp that would require an additional probe, so for my situation it's simply not worth the $$$ just for a bit of convenience. Not to mention, being armed with the proper biasing procedure would allow me convenient "tube rolling," as the power tube overdrive and distortion plays a large part in the overall sound picture in an amp like this, that is usually used at maximum volume settings. I just LOVE the sound of properly tuned tubes screaming "Mastah, please, PLEASE don't beat me no more!" which, of course, instigates me to mercilessly continue the beating until all involved entities have been sufficiently and properly flogged!
All Help & Guidance Appreciated,
Gene

[xtian]

Aiken's page is the place to read up:

http://www.aikenamps.com/index.php/the- ... on-biasing

Specifically:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

Stuff these values (resistance and voltage drop) into Ohms Law:

http://www.ohmslawcalculator.com/ohms_l ... ulator.php

That will get you the dissipation in mA for each triode/pentode. You need to know the plate voltage, and then you can get the target dissipation here:

http://www.ax84.com/biascalc.html

[matt h]

(deleted)

[The Ballzz]

Matt,
On this amp both cathodes are tied together to a common 10 ohm/1 watt resistor to ground. Remember this is a single 12BH7/dual triode used in a push/pull configuration.

The only other commercially made amp I've found using this configuration is Blackstar HT5 series and there is much debate and argument on websites of their fans as to proper biasing.

General concensus at the Marshall forum has brought one esteemed member to this conclusion:

"The TAD 12BH7a-STR is a very lo-priced tube:
https://www.tubesandmore.com/products/T-12BH7A-TAD
And here are the specifications:
https://www.tubesandmore.com/sites/defa ... 7a-tad.pdf
As you can see, the tube costs about $15 or so, and is available almost everywhere. You can also see from the spec sheet that the max plate current is 20ma. Remember, this is the maximum... 70% of that is 14ma. Personally, I feel it is a great little tube.

The test points are on the mainboard, and the bias resistor is already installed.
In the case of the DSL5c, only 1 test point is needed, and only 1 bias resistor is installed. You will be measuring total current for both sides of the tube, as they are tied together at pins 3 & 8 of the tube, which then feeds the 10-ohm bias resistor to ground.

Yes I said 10-ohms...

So guess what. Instead of reading 20mv max, you would read 200mv max. Basically 10 per side. Hence the single adjusting trimpot. So to bias at 70%, one would adjust for 140mv. But wait! Does that include the screen current? To be honest, I don't really care. As long as I stay under that number I think I am doing OK. Besides, only a cork-sniffer is gonna ask you that anyway...

Marshall sends these amps out biased rather cold, mine was set for 109mv as you can see from earlier in the thread. Personally, I feel anywhere between 100-140 (10-14 ma) is fine, the cooler you bias the more clean headroom you get. And with this amp, you need all the headroom you can get...

So I set mine to around 110mv and let it run for a while to check for drift (there wasn't any) and put it all back together. It is that easy.

With only one trimpot, you don't need to balance sides, you just set it and go. And for $15 bucks each output tube, you can afford to burn these up all day. So if you want lots of early breakup and fizzy gain, crank the bias up to 140 or so. If you want this amp to sound deep and rich with lots of Plexi-style breakup at higher volumes, put it around 100-110 or so. It is up to you, but the important part to remember is that you are measuring across a 10-ohm resistor so all bias readings need to be divided by 10 to get the actual milli-amp values."

FWIW, plate voltage is around 450V. I guess may main question is, "is this assessment on the right track or am I/is he missing something?"

I won't post/share a schematic here, but I have a copy I would be willing to email to anyone who is interested and willing to help me with this. These amps are pretty new to the market and there is not a lot of tech info out there on them, unless of course, you are an authorized Marshall service center!
Thanx 4 All Your Thoughts,
Gene

[martin manning]

I did a quick google on this and found someone saying that the trim pot is a balance control. Now I'm curious about how it's set up, and I'd be glad to look at the schematic and give you my opinion on how to measure and adjust it.

At 450V and 3.5W max plate dissipation, max current would be 3.5/450 or 7.8 mA/section, 15.6 for the pair if they are tied at the cathode. Over 10 ohms that would be 156 mV for 100% plate dissipation. The "esteemed member" you quoted above doesn't appear to understand the limiting values, and talking about screen current in a triode is nonsense.

[matt h]

(deleted)

[jazbo8]

Yes I said 10-ohms... So guess what. Instead of reading 20mv max, you would read 200mv max. Basically 10 per side. Hence the single adjusting trimpot. So to bias at 70%, one would adjust for 140mv.

Well no, a reading of 140mV means each tube idles at 7mA, with B+ of 450V, Pda = 3.15W or 90% of the maximum plate dissipation allowed.

But wait! Does that include the screen current? To be honest, I don't really care. As long as I stay under that number I think I am doing OK. Besides, only a cork-sniffer is gonna ask you that anyway...

Really? What screen current?! They are triodes...

Marshall sends these amps out biased rather cold, mine was set for 109mv as you can see from earlier in the thread. Personally, I feel anywhere between 100-140 (10-14 ma) is fine, the cooler you bias the more clean headroom you get. And with this amp, you need all the headroom you can get...

Nope, Marshall got it right, 109mV means each triode idles at 2.45W, or exactly 70% of the 3.5W max Pda for the 12BH7. If you think the amp sounds better with a hotter bias, that's fine, but do expect a shorter tube life.

FWIW, plate voltage is around 450V. I guess may main question is, "is this assessment on the right track or am I/is he missing something?"
Gene

Yup, he is missing something...

[matt h]

(deleted)

[Reeltarded]

Clean headroom doesn't come from cold bias, nor does it come in a micro amplifier.

[jazbo8]

109mV... volts.. .not Amps there. Yup, I'm being a nitpicker, but this is an important nit to pick! Jazbo, I'm sure this was a braintraincrash on your front--no judgment. I needs meh coffeh.

you got me! Correction made to the above post... what I meant to say was, 109mV/10 = 10.9mA so 5.45mA each.

[martin manning]

Here are voltages posted in the Marshal Forum thread where the quote above came from, with my Pa calc added. Ballzz, I'm guessing your line voltage is a bit higher than when these measurements were taken.

[The Ballzz]

Hey Folks,
I hope I didn't confuse things here by mentioning the Blackstar amp. The Blackstar does indeed have two trim pots and apparently one is for balance and the other is for bias. I can't find a schematic for it either. My questions are about the Marshall DSL5C, which is a bit different than the Blackstar, but uses the same unusual power tube. I only mentioned that amp as an example of the only other amp I've found using that tube. I do have the schematic for the DSL5C!
Thanks,
Gene