Suncalc wrote:
Gab wrote:
Could you just confirm what I understand from what you have said : since the primary resistance of the transformer isn’t the only thing setting the cathode load, a small difference won’t make the entire circuit go wrong.
Yes that's correct. A small difference really won't matter. But the bias is partially set based on the primary resistance so it will change things a little.
Gab wrote:
This is a very uncommon and interesting circuit topology. I wonder why so little people have tried to build it !
Using resistance ladders to set circuit voltages was a common method of design in the very early days of tubes. Here is an example of a two stage, direct coupled amplifier using this approach.
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The reasons for this were two fold. First, many early radios were run from batteries since electrical power was not ubiquitous. And since batteries were expensive, it just made sense to specify one with a high enough voltage for the B+ and find ways to use resistors to get the lower biasing voltages. Second, this method worked well with direct coupled amplifiers. In the early days of tubes, capacitors for coupling stages were expensive, and of rather poor quality. They were prone to leakage and early failure.
This was an interesting circuit in its day. However, given that we now have access to very high quality, inexpensive signal capacitors, this circuit is more of a curiosity than anything else. IMHO.
To me, the interesting parts of the circuit are the use of direct coupling and the cathode follower output stage.
My main system is direct coupled and I really like the sound of it. The power amp is an Adcom GFA-555 with an Adcom GTP-500II preamp, and both are direct coupled (if you use the "lab" output of the GTP). I believe that there is no such thing as a perfect capacitor, or any electrical component for that matter. It may only have a negligible impact, but I’m a firm believer of the less is more philosophy. I don’t have all the knowledge to back this statement, I just see this as something logical.
The other interesting part, the cathode follower output stage, leaves me puzzled. I thought that a cathode follower didn’t provide any voltage gain, but this amp has an output of 4.5w according to the author. Also, it is not clear to me if the 6V6 is used as a pentode or a triode.
Finally, your comment confirm what I had on my mind : looks like an old fashion way to get this amp working. Power supply parts are very minimalistic, one more time because of the availability, quality and price of capacitors in that time. I’d like to build the amp, but taking advantage of the availability of modern parts. The circuit would become more of an inspiration than anything else, but i’d like to use something like an ef86 with el84 output stage, and a power supply with multiple voltages output replacing the resistor ladder. The power transformer I plan on using have multiple secondaries, so it could make sense To build the supply that way. On the other hand, it would only remove the resistor ladder to add many parts in the supply, so that is not simpler in any way.
Let me know what you think !
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