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Wednesday, September 21, 2016

Cobalt Output Transformer

Great news. We have found 2 sets of Cobalt C core good for making 2 pairs of output transformers or high power interstage transformers.

Price is U$1390 / pair for interstage transformers or output transformers, Level 3.

This is chance of a life time since Cobalt C core does not come easily. They are refurbished units and no longer being made. We had some a few years ago and they have run out for some time - refer this link, link.

Now we found 2 sets. Get it quick or it will be gone again. Sorry, we do not sell just the cores.

Hear the Unheard!
J&K Audio Design

Thursday, September 15, 2016

Parafeed Output Transformer

Busy is the only word left on our dictionary for quite some time! Time to update our blog or else people might think that we are MIA.

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 Farad
L is in Henry
R is load impedance (transformer primary impedance) in Ohm 
John Broskie from Tube Cad Journal has the formula as follow: 

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
  • Etc
Although there are so many drawbacks, they can be individually tackled/resolved (with money), that's the beauty of parafeed configuration! You can have a high quality plate load choke, high quality teflon / oil / film coupling capacitor, and you are half way to heaven!

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
Hear the Unheard! 
J&K Audio Design

Thursday, September 8, 2016

Peerless Input Transformer Repair

We've repaired a lot of power transformers and OPTs in the past. Chances of repairing input transformers are not has high since the failure rate is less compared to others but we still do get them once in a while, especially those very sought after versions, such as input transformers from Western Electric or Altec Lansing, such as this one that we just received.

The data was quite hard to be found online for this Altec Lansing input transformers. The owner provided the specifications to us after checking with the new owner for Peerless transformers. 

Input impedance: 25K CT
Output: 600 CT / 150 Ohm

This is how it looks like before it was dismantled. It is sealed in some sort of rosin and requires heating before the transformer can be taken out. 

After the extraction, the core lamination (obsolete now) are then cleaned up before it can be reused. The winding and bobbin are discarded as they no longer can be used. Due to the non-standard size of the core lamination, a customer bobbin has to be made to get a perfect fit. Standard size bobbin will not fit properly and there shall be unwanted design deviation that will not get it back to the original condition and performance. 

The original bobbin / winding are slowly and carefully unwinded to record down the original turns and taps location for rewinding later. Luckily the damage was not that bad that prevents us from doing so. If not, another unit might be needed to complete the big picture, or smart guesstimate based on physical unit + theoretical data will be required if no references are available.

Another challenge was presented to us when it is time to rewind it - the wires used are of diameter of 0.05 with 3000 rounds of primary turns with a center-tap!!! It is quite difficult to rewind and had to be manually done slowly. If it is broken, what's done will have to be thrown away and new one redone. 

After a 3 day affair, finally it is done! It will be shipped back for testing before another 3 remaining units are repaired with the same methodology & processes. 

Refurbishing or repairing input transformers will be worth doing so if it is those vintage exotic ones such as those Western Electric types where a pair could be U$1K - U$2K nowadays, or for those looking to restore expensive vintage equipment to original condition that can fetch pretty good price in the market.


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