Hi Everyone, Like I indicated I have compiled an overview of the Poddwatt type amplifiers. I will try to post the whole thing here.
EL84/Poddwatt Amplifier Update June 2019
This design has gone through 5 main versions. Each version has built on the strengths of the previous ones. For convenience I will call them all Poddwatts and label the versions A through E. I have attached generic / typical schematics for each version. As a diy amplifier begun in 2008, it has evolved in the past 10 years. It is impossible to list all the minor variations that have been built. As a result there is significant overlap between the types. I did not include the power supplies as they tend to be less variable. The latest ones use DC heaters and regulated B+ for the driver stage. Both features are desirable for the best performance but amplifiers without either will actually sound quite excellent. I recommend that the balancing modification in version “D” not be used as it can cause reliability issues. I encourage builders to use quality coupling capacitors as they have a significant effect on the sound. Transformers other than the Edcor ones are fine, but I cannot guarantee they will perform the same. I suggest using the NFB configuration indicated in the schematics, not because it will do much to the sound, but to guard against above audio band resonances that can color the sound and possible ringing in very high audio frequencies. It is under 3 db and IMO innocuous.
The first version (the posted EL84 Project) was quite good in performance and easy to build. It is characterized by the use of Edcor CXPP-25 watt size output transformers and the use of only one coupling capacitor in the output stage. It used 12AU7 (ECC802S) driver tubes.
Attachment:
Early Type A.jpg
The second version type “B” was a budget version of the amplifier and used Edcor GXPP10-6-10K output transformers. This reduced the cost significantly and maintained most of the performance of the originals. It also uses 5751 driver tubes in place of the original 12AU7 types for additional gain. Grid stopper resistors and a negative feedback network were added.
Attachment:
Early Type B.jpg
The third version (“C”) was set up to use either the original transformers, the GXPP ones or in final designs a custom Edcor transformer designed especially for the amplifiers. This was the Edcor CXPP10-MS-10K (also listed as EMO750). It used coupling capacitors on both output tube grids and a resistor from the slave tube’s grid to ground equal in value to the one on the active tube. This was done to make the resistance values match and slightly improve linearity. The second capacitor helps match the AC grid input impedance for each tube. It is not exact as it ignores the anode impedance of the previous stage but is sufficient to be effective. A few amps used the bias adjustment found in version “D”.
Attachment:
Type C.jpg
The fourth version type “D” (very few of these exist) overlaps the previous type but uses a variation of the adjustment procedure for balancing the output tubes. A DC voltage is taken from the driver tube anode supply, filtered and run through a network with control pot and fed to each output tube grid through high value resistors. This is similar to a fixed bias arrangement, but uses positive voltages not negative ones. The voltage applied is in the 5-10 volts DC range. This seems contrary to good practice but when examined carefully it is quite acceptable. The current and thus the voltages on the tube cathodes are controlled by the LM317. Increasing the positive voltage on the grids increases the voltages on the cathodes by an equal amount. The total current is unchanged. This has several effects. First it eliminated the variable resistor in the cathode circuit. Second it provided additional negative headroom for the output tubes (by raising the cathode voltage). This is important as the LM317s are not linear below about 3 volts DC. The added cathode voltage insures they will always remain in a linear range unless the amp is severely over driven into clipping.
Attachment:
Type D.jpg
The fifth and current version “E” has a number of new features. It standardizes the use of the Edcor CXPP10-MS-10K output transformer. The transformers were used in some prototypes of the earlier versions, but not standardized for them. In any of the versions the original 25 watt transformers may be used with virtually identical performance. They are physically much larger though. The “E” version is a dual mono configuration with separate power supplies for each channel. It standardizes the use of LR8 high voltage regulators in the driver anode supply. They were used in some earlier prototypes as well. It returns to a mixed configuration of an added positive voltage on the grid circuits, but goes back to the earlier cathode configuration with the variable 25 ohm resistor in that location. This apparent step backwards was done to reduce the noise level slightly in the amplifier. It also eliminated a possible reliability issue with the control in the type “D” positive voltage circuit. Failure or intermittent operation of the control could result in excessive DC voltage on one or more of the output tube grids and cause excessive voltage on that tube’s cathode and ultimate failure of the LM317 and possibly the tube.
Attachment:
Type E.jpg
All schematics are for only one channel and for simplification do not include the power supplies.
There are numerous DIY variations of the circuit and in particular the power supplies. As the circuit is quite versatile nearly combination of output tubes, drivers and power supplies will work well.
I hope this helps eliminate the confusion
Good listening
Bruce
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