sound difference between fixed vs cathode bias?

a DIY, modified Mullard 5-20 monoblock design

Fixed Bias

Postby EWBrown » Mon Jan 19, 2004 6:16 pm

The low resistance cathode resistors in the Dynacos (except the ST- and SCA- 35s) is there just to make bias measurement easier. For some reason, way back in their early days, they chose 1.56 volts as their default "correct bias measurement" voltage, so some amps have 15.6 ohms, some have 12, etc. I just go with 10 ohms, and for the ST-70, set the bias at 1.00 VDC (100 mA through two EL34s). Eliminates the mental math.

I haven't done a trial run of the same amp using fixed bias and then cathode bias, for comparison purposes, so I cannot state which would sound bettter. I would side with fixed bias, but then it does have a little added complexity, as compared to the "self-adjusting" cathode biasing.
Fixed bias will give you a bit more power, just because those 13 to 28 volts DC aren't "wasted" in the cathode resistor. Some fixed-bias amps dispense with the cathode resistor altogether, either by switching it out (shorted) during normal usage, or bias is measured through the plate circuit. (caution: High Voltage!)

Either method, I do like having individual bias adjustment for each tube, as compared to all four together in the ST-35 or in pairs in the ST-70.

Eiclones in progress, will repoort once they are powered up and running.

The gold-plated high end amps like McIntosh do use cathode feedback, in addition to UL taps, as well as some global feedback. The Tamura trannies that were listed in E-bay also have the extra cathode feedback winding.
I have two small (10 watt?) Baldwin organ OPTs that have a cathode FB winding, though no UL taps. The primary impedance is approx 6600 Ohms CT, the cathode winding is approx 77 Ohms impedance, and the single output impedance is 8 ohms. The cathode winding is about 8 ohms DC resistance total, FWIW. I'll eventually figure out a simple 6AQ5 or SV83 amp circuit for these.

/ed brown in NH
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Re: sound difference between fixed vs cathode bias?

Postby Shannon Parks » Mon Jan 19, 2004 7:39 pm

john8417 wrote:If anyone has built both bias types, what is the difference in the sound and in the apparent power. I'm no so interested in the measured power difference, but if you have that ok. Has anyone tried some common resistance, say about 12 ohms, to reduce IM distortion ala Dynaco?


John,
I think I'm the only one who's really had either one of the Eiclone types going yet. I would refer to the common wisdom about fixed vs cathode biased amps, but that is just subjective hogwash - I want to experience it myself - err, my own subjective hogwash is what matters. I've been listening to a fixed bias pair lately and I've become familiar with 'their sound': dynamic, powerful, unrestrained, not sweet but full of you-are-there presence.

I was listening to some Leo Kottke last night. My wife and I had just seen him a few months ago and it felt like an encore performance. His frantic picking, percussive style came across in a very natural way. It sounded as my mind remembered it anyhow. Oh yes, the wife agreed and she doesn't hesitate to tell me when the tube 'magic is gone' either!

After I spend some more time with the fixed Ikes, I'll lift the grid resistors going to the negative bias, add the cathode-style grid resistors, pull up one leg of the C- cap, lift the 10 ohm resistors and add the cathode resistors and cathode bypass caps. Not a simple flick of the switch, but just 10 minutes on the work bench for a new amp. I would recommend going this route, too. Depending on your listening tastes and your loudspeakers, you may find a preference.

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Re: sound difference between fixed vs cathode bias?

Postby Shannon Parks » Tue Jan 20, 2004 1:09 am

separks wrote:...Not a simple flick of the switch, but just 10 minutes on the work bench for a new amp...


The transformer wouldn't be a good match, though. I seemed to forget that I switched outputs during these test. Sorry 'bout that.

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Postby erichayes » Fri Jan 23, 2004 9:30 pm

Hi guys

There actually was method in the apparent madness of 15.6 ohms for the cathode resistors in the Mark and ST70 amps. When Bruce DePalma and David Hafler were designing the MkIII, they discovered that a low value cathode resistor (<47 ohms, I think) in conjunction with a fixed bias supply improved the performance of the amplifier. Go to DePalma's website and click on Analog Amplifier Design at his "Other Papers" link for the straight dope.

They decided on 15.6 ohms because that was 10 times the value of a fresh carbon-zinc dry cell. This enabled the builder to use a cheap VOM to adjust the bias by checking the battery voltage, physically marking the meter face, and then adjusting the bias control until the needle matched the mark.

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Aha!

Postby EWBrown » Mon Jan 26, 2004 7:01 am

That would make sense, as the bias properly set would yield 1.56 VDC.
The other amps use different value cathode resistors to deliver 1.56VDC as well, except for the ST-35 and SCA-35, which have all four EL84s cathode common to a single 95 ohm 5watt resistor, which raises the cathode voltage to about 13.3 VDC above ground (assuming 35 mA each for a total of 140 mA).

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