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.)

SCA-35 perf table

Dynaco Z-565 xfmrThis 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.

Heathkit AA-151 ampA 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 51-29 xfmrHeath 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 having traditional resistive cathode bias.

Happy Listening!



Reader Comments

Posted by Dave January 08, 2020 - 11:02 pm
Hi Meyer -- The short answer is that the taps are placed at 25% of the winding on either side of the CT lead. Unfortunately however, simply having the correct turns ratio information will not necessarily produce a transformer capable of a successful outcome in this case.

In any Negative Feedback Amplifier, the sub and supersonic characteristics of the output transformer become critically important in producing a proper replacement transformer. Such things as the amount of inductance, amount of interleaving used, and type of steel laminates used all play a huge factor in the performance that a given output transformer displays, and determines (at least) the power response, stability, and transient performance of the amplifier. To achieve maximum stability, the response of the amplification circuits are tailored to mate perfectly with the response characteristics of the transformer, making the relationship between circuit and transformer on par with a prearranged marriage. I would strongly suggest that you at least obtain one example of an original transformer, so that you (or your winder) can see first hand how the transformer was constructed so as to produce the exemplary performance they are capable of.

I hope this helps, and good luck in producing an accurate transformer clone for your project!

Best --


Posted by Meyer Rochwerger January 08, 2020 - 04:25 pm
I bought a Dynaco SCA-35 amplifier missing both output transformers type Z535.

So I need to build a new pair of transformers from scratch due to the high values of transformers that are for sale on the web plus the high postal rates for Brazil.

Searching the web, I found some characteristics of the windings:

The primary has 8000 ohms CT to plate

But the information about screen winding CT to screen is missing.

If anyone has this information to provide me so that I can proceed to the work.

I'd be very grateful..



Posted by Dave February 28, 2019 - 03:02 pm
My apologies for such a delinquent response, but I only became aware of the question just now.

The type of bias system that can be used to advantage is definitely very dependent on the output loading conditions, and how the screen grids are treated. For example, due to the greater current flow produced with lower impedance plate impedance values, cathode bias always requires a higher plate load impedance (all else being equal) because that type of bias system inherently limits the amount of current that the output stage can flow. Similarly, UL operation also typically restricts maximum possible current flow through a tube, so the plate impedance is elevated in that scenario as well, again, all else being equal.

As for "actual bias", the optimum operating point for a given scenario will always yield the lowest amount of distortion. Therefore, it is always best to use load lines as a design tool for a number of reasons, from ensuring that the optimum load and bias is used, to ensuring that a tube is not operated beyond its dissipation ratings, and that it (or they) will actually deliver the expected performance.

I would encourage you to study and ultimately learn to understand load lines and the different classes and types of output tube operation available, as these principles are at the heart of proper operation for any audio amplifier design. A good place to start is the RCA Receiving Tube Manual, which is written on an engineering level, but which is also written clearly enough for even the layman to help understand these concepts.


Posted by Tom Dickinson January 20, 2016 - 11:45 am
Hi Dave,

Just trying to 'bone up' on my understanding of EFB prior to my next project. Just a couple of quick questions here: 1) In your article above, is any or all of this applicable if one is NOT using UL type OTs? and 2) Is there a particular guideline for establishing that 'actual' bias for a given Tube/OT as based on the PI of the OT itself? In this last question, I'm assuming you're going to point me to the Tube Data Chart itself, and tell me to look at the load lines at the various PI ratings....which if so, once I actually understand how to do that, I guess I'll be able to answer that question across the line for all tubes and OT PIs. But, if that's NOT the right way, then I'm wondering what is? Thanks Dave. TSD

Posted by Dave July 21, 2015 - 01:11 pm
Hi Timbo -- You can't answer the question you've posed until you know for certain what class the output stage operates in. That is turn is largely determined by the load presented to the output tubes by the output transformer, and the quiescent current that the output tubes operate at.

If the tubes are operating in true Class A fashion, then it matters not whether a single bias resistor is used for each tube, a common resistor is used either for each channel or both channels, or even if fixed bias is used as provided an EFB cathode regulator -- they will all produce the same levels of power output -- this because overall current draw through the output stage remains rather constant in Class A output stages.

While the use of individual bias resistors aids in helping to achieve a consistent quiescent current between randomly picked output tubes, it also prevents the sharing of cathode currents that can happen when a common cathode connection is used. Under dynamic conditions, this can help lower distortion, but again, this too is somewhat dependent on the loading conditions that the stage operates into.

While I have helped others with Leak equipment, I have never personally work on one to have any first hand experience with them or how their circuits operate. But a good place to start would be determining the output impedance that the output tubes operate into, and from there, the class of operation that the output stage operates in.

I hope this helps!


Posted by Timbo in Oz (from AA) July 20, 2015 - 09:56 pm
hi there,

My rebuilt modified LEAK St20s have EL84Ms each with its own cathode R.

while a fixed bias circuit might give me more power I think the bias is stable unlike with a singe R across all four tubes?


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