Cathode or Fixed Bias?
It Depends on Your Output Transformer!

Performance comparison of Dynaco’s Z-565
and Heath’s 51-29 Output transformers

By David C. Gillespie

This is a follow-up piece for those Dynaco SCA35/ST35, Heath AA151, and diy stereo 35 aficionados (like myself!), to help reinforce which type of bias is best suited for the transformers you are planning on, or currently are using in your ultra-linear (UL) 6BQ5 amplifier.

In the article where I introduce Enhanced Fixed Bias (EFB™) operation for the Dynaco SCA35 & ST35 amplifiers (“A New Look At an Old Friend”), I show how any stereo amplifier with a common power supply and common cathode bias resistor between the two channels actually causes each channel to operate under conditions of regulated B+ with fixed bias operation when only one channel is driven. But then, it reverts to cathode bias operation powered by a traditional power supply, when both channels are driven by a similar signal. I also show that when only one channel is driven in the stock Dynaco amplifiers, the resulting performance is excellent, yet when both are driven, the performance falls off significantly. (See table below.)

This was all due to the compromised bias system used for the operating parameters produced by the output tube/output transformers employed, and is the basis for why a significant performance upgrade is possible with these amplifiers. Specifically, the Dynaco Z-565 transformer (at right) is likely one of the best transformers ever made for UL operation of 6BQ5 tubes, but its lower primary impedance and screen tap percent requires the tubes to be operated in fixed bias to achieve their best performance. The EFB circuit was specifically developed to capitalize on the unique performance potential these amplifiers possess. In the SCA35 and ST35 amplifiers, not only did it provide a very simple way to produce fixed bias operation, but the Enhanced element of the concept makes the performance improvements for both channels largely invisible to each other. The result of the EFB modification is that now these amplifiers can produce their full rated power output with both channels operating, at the lowest possible distortion.

So if converting an output stage that is optimally suited for fixed bias operation from cathode bias to fixed bias operation produces such positive performance improvements, how does performance change when an output stage with a higher impedance load is changed from cathode bias to traditional fixed bias operation? All else being equal, from the perspective of power output, there will be a slight increase in available power (about 1.5 watts in this case) due to the recaptured B+ that was originally lost across the cathode bias resistor. But for the effort involved, that small increase in power amounts to little success, and overall, other performance parameters (distortion levels) may hardly change at all.

A stock Heathkit AA-151 amplifier (at left) can serve as a good vehicle to help with this exercise. Like the Dynaco amplifiers, this is a stereo amplifier that uses UL connected 6BQ5 output tubes, a common power supply, and a common cathode resistor bias system for both channels. As a result, the same changing conditions are created in it depending on whether one channel is driven, or both are. Also like the Dynacos, the Heath unit employs high quality 51-29 output transformers (at right) of similar capability, except that the impedance of the Heath transformers (10K ohms with screen taps at 50%) allows the tubes to have a more neutral position with regards to the type of bias used, whereas the impedance of the Dynaco transformers (8K ohms with screen taps at 25%) is clearly optimized for fixed bias. Since the same changing conditions are present in both units regarding channel drive, but the loading conditions of the Heath amplifier make it more immune to these changes, it implies that a stock Heath amplifier should not suffer nearly the same loss of performance as the stock Dynacos do when both channels are driven. Under certain conditions, it may actually improve as conditions return to pure cathode bias operation.

Tests were set up to investigate these facts, and conducted under the same conditions that the Dynaco amplifier was measured under for the EFB article. All power was measured at the onset of clipping, and distortion was measured at 1dB down from rated power output in accordance with the Dynaco tests. Since the AA-151 is rated at 14 watts per channel, the 1 dB down point represents a power output of 11 watts for distortion measurements. The results are as follows:

At 20 Hz:
Each channel driven = 17.22 watts RMS. THD = 1.45%
Both channels driven = 14.25 watts RMS. THD = 1.25%

At 1 kHz:
Each channel driven = 18.06 watts RMS. THD = 0.32%
Both channels driven = 16.81 watts RMS. THD = 0.46%

At 20 kHz:
Each channel driven = 17.22 watts RMS. THD = 1.20%
Both channels driven = 13.69 watts RMS. THD = 1.75%

In comparison to the Before data provided in the Dynaco SCA35/ST35 article, it is obvious that the difference between single and both channels driven performance is greatly reduced in the Heath amplifier with the 51-29 transformers. Also, notice that the performance under true cathode bias conditions (both channels driven) is superior to the stock Dynaco amplifiers under the same conditions. However, comparison to the After data provided in the same article shows how the best performance is still ultimately obtained with Z-565 transformers, and the EFB modification. The Heath’s performance can be further analyzed as follows:

At 20 Hz, there is a drop in distortion, as cathode bias operation is achieved with both channels driven. At low frequencies, the loading conditions these transformers offer slightly favor cathode bias operation, although fixed bias operation still provides very good results.

Moving up the frequency range, the reduction in distortion continues until mid-band frequencies (1 – 10 kHz) are reached, which represents a turning point in performance. Here, performance is very nearly the same for all intents and purposes, as distortion performance is changed very little whether the tubes are operated under fixed bias (a single channel driven), or cathode bias conditions (both channels driven). This underscores the neutral position the output tubes have towards the type of bias used with the loading conditions these transformers provide.

At 20 kHz, the performance discrepancy is greater, and distortion clearly increases when both channels are driven. But it is hardly a catastrophic rise, and is primarily due to the winding capacitances reducing the load (numerically) on the output tubes as frequency increases. With this reduced load, the fixed bias operation of single channel drive conditions affords more peak current capability for the tubes, so that distortion is minimized with fixed bias at this frequency. When both channels are driven, cathode bias conditions return, peak current capability is reduced, and distortion increases.

So what is to be made of all of this? Ultimately, it is further documentation showing the importance of providing the correct bias conditions for the transformers you plan to use, and therefore, how to get the very best possible performance from your amplifier. In summary:

1. Use of the Dynaco Z-565 transformer, or clones of its specifications, requires fixed bias operation for best performance with 6BQ5 tubes. Using the Enhanced Fixed Bias modification with this type of transformer provides the best possible UL performance these tubes are capable of. Using cathode bias for these tubes with this type of transformer significantly deteriorates performance as previously documented.
    
2. Use of the Heath 51-29 or similar transformer provides excellent performance with either fixed or cathode bias operation. Traditional fixed bias will provide a very small improvement in power output, and some improvement in HF distortion. EFB operation with these transformers will not improve power output significantly, but will provide at least the same improvement in HF distortion, and further improvement in overall performance since the Enhanced portion of the design will compensate for any power supply droop as power increases.

The EFB modification will improve the performance of any amplifier using either of these type transformers. By themselves, the Dynaco and Heath transformers  are  likely two of the best UL output transformers for 6BQ5 tubes ever made. However, since fixed bias operation is optional for the Heath type transformers, that makes them, and other transformers of similar specifications, ideal for the diy stereo 35 amplifiers, or any other UL 6BQ5 amplifier employing traditional resistive cathode bias.

Happy Listening!

Dave

Reader Comments

Posted by Bob H

March 12, 2012 - 11:24 am

Dave, It was a pleasure meeting with you the other day. Give me a call at the shop in Ball Ground when you get a minute. Thanks, Bob

Posted by Dave

April 18, 2011 - 11:36 pm

Thanks for your interest! I hope I am understanding your question correctly, so please let me know if I have not. In both cases (i.e. the SCA-35 and Heath AA-151), all distortions tests were all made at 1db below rated power output, so the tests would remain in keeping with those used to produce the original published specifications. Therefore, with the SCA-35 tests, the before and after distortion tests were made at a consistent power level between tests, since the "rated" power level did not change. The power output figures shown illustrate the increased maximum power output available at the onset of clipping, as a result of the EFB modification. I hope this helps! Dave

Posted by guitbanger

April 18, 2011 - 10:17 am

With the EFB mod installed how would the distortion figures compare, running at the same power output as the both channel measurements? Wouldn't that comparison be useful? The bump up of a couple watts is nice to know the figures improve (2 out of 3) but I'd bet that at the same wattage out, the modded figures might show the improvement is significant too?

Posted by Steve L.

December 06, 2010 - 08:14 pm

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