Since a customer from down South in Australia asked for a pair of parafeed output transformer (parallel-feed), we might as well do an introduction here. Parafeed output transformer is not something new.
Below is a sample schematic from diy-audio-guide. It is a parafeed tube amplifier designed to operate with 2A3 / 6B4G / 6A3 vacuum tube.
How it works is that the tube load, which is originally a single output transformer, is now split into 3 parts - the load inductor (can be resistor, or CCS too), the parafeed coupling capacitor, and the parafeed output transformer. The parafeed capacitor can be placed on the receiving arm of the parafeed output transformer, or the grounding arm. Both way works but if the parafeed output transformer do not have high insulation strength, it is better to place it at the receiving arm to isolate DC from the parafeed output transformer.
The parafeed coupling capacitance for flat frequency response can be calculated (estimation) as follow:
C = 2*L/(R*R)
C is in FaradJohn Broskie from Tube Cad Journal has the formula as follow:
L is in Henry
R is load impedance (transformer primary impedance) in Ohm
C = 159155/(rp + Rload)/Frequency,
C is in µF
rp is the tube plate resistance.
Frequency is the cut-off
Rload is the primary impedance of parafeed output transformer
The most frequently faced problem with single ended tube amplifier is always the output transformer capability (bandwidth). As we have always mentioned, the design constraints of output transformer are always contradicting each other. In this case, the air-gap required for SE operation will work against the primary impedance that is required for better low frequency extension/responses. With parafeed configuration, this is eliminated where we do not need to have air-gap for the output transformer due to only having AC going to the parafeed output transformer, which in turn, make it smaller and better optimized for lower capacitance and leakage inductance. Benefits to output transformer design one of the most important benefit for parafeed configuration.
Of course, there are other benefits as well such as better PSRR due to having a choke as load (hum reduction), having the opportunity to use exotic core (nickel/permalloy, cobalt, nanocrystalline, amorphous and etc) for the output transformer since DC saturation is no longer an issue.
Parafeed is all pros but no cons? Nuh... don't be happy so soon. There are drawbacks of course:
- High quality high value load inductor will be needed since it will experience full audio frequency spectrum instead of 120Hz or 60Hz mains frequency, and provides high impedance at all audio frequencies
- Additional parafeed capacitor is in the signal path
- Tuning may be needed to optimize the inductor / capacitor
- More tube amp real estate is required
We had tried parafeed amplifier and they do have excellent low frequency response, coupled with sound that feels louder than the Watt rating. Of course, the sound quality will be dominated by the 3 parts now (parafeed load choke, parafeed capacitor and parafeed output transformer) instead of 1 single output transformer.
For those that are interested to build a parafeed 300B / 2A3 amplifier, you can consider the following output transformer options:
- Level 1 - 10W Parafeed Output Transformer
- EI Z11 - U$190/pair
- EI Z9 - U$240/pair
- Level 2 - 10W Parafeed Output Transformer
- Permalloy OPT - U$640/pair, potted
- Level 3 - 10W Parafeed Output Transformer
- U$890 for EI Core potted
- U$1090 for Nanocrystalline Double C potted
J&K Audio Design