DIY Audio Projects Forumhttps://diyaudioprojects.com/Forum/ Direct-Coupled Audio Amplifier with Cathode Followerhttps://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4576 Page 2 of 3

 Author: Gab [ 11 Jun 2019, 07:14 ] Post subject: Re: Direct-Coupled Audio Amplifier with Cathode Follower Suncalc wrote:Gab wrote:As you can see in the schematic description, both impedance and resistance are specified. I assume that since the transformer replaces the cathode resistor, it must have a precise resistance (dc). Not quite. The 6V6 cathode load is the series combination of R3 (1000Ω), R5 (355Ω), and the transformer primary dc resistance (250Ω) for a total of ≈1.6kΩ. This (along with the B+ voltage) sets the 6V6 quiescent plate voltage but NOT the grid bias. The grid bias is set by the resistive voltage divider formed by R3/R5/R6/R7. The 49.5mA cathode current drops 12.4 volts in the transformer primary which adds to the voltage set by the R6/R7 divider to set the cathode at 85v against the grid at 73v. Total grid bias is therefore -12.4v. (i.e. essentially -12.5v, the most common 6V6 grid bias.) Also note that the plate voltage works out to be 355-85=250v, the most common plate voltage for the 6V6 tube.Thanks for sharing your knowledge. 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. Thanks again for your help ! It is very appreciated !

 Author: Gab [ 11 Jun 2019, 07:15 ] Post subject: Re: Direct-Coupled Audio Amplifier with Cathode Follower This is a very uncommon and interesting circuit topology. I wonder why so little people have tried to build it !

 Author: Suncalc [ 11 Jun 2019, 19:27 ] Post subject: Re: Direct-Coupled Audio Amplifier with Cathode Follower 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.Attachment:Screen Shot 2019-06-11 at 5.17.47 PM.pngThe 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.

 Author: ILoveHiFi [ 11 Jun 2019, 19:56 ] Post subject: Re: Direct-Coupled Audio Amplifier with Cathode Follower Quote:"Would the impedance of the output transformer go up if a resistor was added in series with it ? Since mine are 5k, they would get closer to the specified 6k."No it will not go up, all your doign is reducing the power output into the speakers when you do that. Further more you reduce the maximum voltage swing possible on the speakers.Transformer impedance stays the same, but you just lose extra power on the 1k resistor to get 6k.Should be able to do 5k instead of 6k, minor changes dosen't matter too much and if you know how to fine tune stuff thats even less of a plorbem. But you'll probally design your own amp anyways if you know to design.The transformer resistance is specified in scheamtic, because that will effect the operating point in DC. All you do is just fine tune the resistors to controll grid cathode voltage to compensate if you have diffrent resistance.Using Power formula I^2R to find power accros the resistor, dudes in the old days can't even get utmost basics right, R5 355R 10w should be only a 5w resitor.(52*10^-3)^2*355 = 0.96W of power dispation in R5(52*10^-3)^2*1000 = 2.7W of power dispation in R3

 Author: Gab [ 12 Jun 2019, 06:44 ] Post subject: Re: Direct-Coupled Audio Amplifier with Cathode Follower ILoveHiFi wrote:Quote:"Would the impedance of the output transformer go up if a resistor was added in series with it ? Since mine are 5k, they would get closer to the specified 6k."No it will not go up, all your doign is reducing the power output into the speakers when you do that. Further more you reduce the maximum voltage swing possible on the speakers.Transformer impedance stays the same, but you just lose extra power on the 1k resistor to get 6k.Should be able to do 5k instead of 6k, minor changes dosen't matter too much and if you know how to fine tune stuff thats even less of a plorbem. But you'll probally design your own amp anyways if you know to design.The transformer resistance is specified in scheamtic, because that will effect the operating point in DC. All you do is just fine tune the resistors to controll grid cathode voltage to compensate if you have diffrent resistance.Using Power formula I^2R to find power accros the resistor, dudes in the old days can't even get utmost basics right, R5 355R 10w should be only a 5w resitor.(52*10^-3)^2*355 = 0.96W of power dispation in R5(52*10^-3)^2*1000 = 2.7W of power dispation in R3I clearly don’t know how to design complete circuits yet. I wish I could though ! I need a lot more reading and practicing before I’m comfortable enough with vaccum tubes principles. But, i’m getting closer to this goal each time you guys share your knowledge with me on the forum ! I really appreciate it !