Help identifying MKIII power transformer

Let me start by saying Hello to everyone since this is my first post. I've been in the hobby for about 10 years, but only recently chasing Dynaco gear.

On to the question, I recently purchased a set of 4 transformers that were supposedly used in a pair of MKIII's. The all important outputs appear to be original with the woven insulation on the leads, however the power transformers appear to be a more modern replacement of unknown make. The size, shape, and windings all match the common P782 replacements with the exception of the bias tap missing. There is no stamping or stickers/labels anywhere, and I pulled the covers to see if they were maybe painted and they looked as though they were grey before. The covers are shaped the same as the originals. I was told that the gentleman who owned these was into building his own amps 20-30 years ago, so these are likely from around that time.

I apologize for having no photos to assist in the ID, but because I'm so new I cannot post any pics yet. I was hoping that the old heads of this forum may have known of a replacement that was once offered without the bias tap.

In case these numbers help, here are voltages from my test,

Primary
BLK-BLK - 122vac applied from wall

Secondary
YEL-YEL - 5.5vac

RED-RED/YEL-RED - 440-0-440vac

GR-GR/YEL-GR - 3.6-0-3.6vac

Thanks in advance for any information you guys can help me gather about these.

Nick

Sorry but not encouraging news. Those are not the correct MK II/III power transformers. All of the larger Dynaco tube amps used fixed bias. They supplied a bias tap from the power transformer to power that circuit. The transformers you have may make a nice pair of amps but not real original Dynaco circuits. You did not state how much of a DYI/tech guy you are. So here are some options…

1 Sell these and buy the correct transformers. A little expensive and may not be your best choice.
2 Use these and make a pair of custom amps. If you are planning to use the Dynaco chassis, bottom plate and covers it may be a tight fit. One way is to purchase a small transformer that will power the bias circuit. It can be pretty small and you have to do some figuring on the bias circuit to get the right dropping resistors in the circuit. If you decide to go this way I can help..
3) Use these transformers but mount on a different chassis that has more room where you can make a completely different circuit pair of amps. A lot of work and not much resale value.

Another caution. The winding voltages you show are not to bad but you do not have the current limits. The high voltage seems a little high but you have 122VAC for line voltage. A choke input may be needed but to early to tell. There may be enough power from each winding to run the amp but nothing is for sure.

WLT wrote:Sorry but not encouraging news. Those are not the correct MK II/III power transformers. All of the larger Dynaco tube amps used fixed bias. They supplied a bias tap from the power transformer to power that circuit. The transformers you have may make a nice pair of amps but not real original Dynaco circuits. You did not state how much of a DYI/tech guy you are. So here are some options…

1 Sell these and buy the correct transformers. A little expensive and may not be your best choice.
2 Use these and make a pair of custom amps. If you are planning to use the Dynaco chassis, bottom plate and covers it may be a tight fit. One way is to purchase a small transformer that will power the bias circuit. It can be pretty small and you have to do some figuring on the bias circuit to get the right dropping resistors in the circuit. If you decide to go this way I can help..
3) Use these transformers but mount on a different chassis that has more room where you can make a completely different circuit pair of amps. A lot of work and not much resale value.

Another caution. The winding voltages you show are not to bad but you do not have the current limits. The high voltage seems a little high but you have 122VAC for line voltage. A choke input may be needed but to early to tell. There may be enough power from each winding to run the amp but nothing is for sure.

Thank you for the quick response.

I figured that the missing bias tap meant these weren't really meant to be used as a replacement in the MKII/III. In my hours of reading that lead up to my original post, I saw all manner of methods to reintroduce the bias voltage including the addition of a separate transformer to adding a voltage divider from a leg of the HV secondary. The purpose of my posting was more or less to understand if these "bias-less" transformers were a well known sighting in the Dynaco circles, or if I just happened across some generic transformer that shared remarkably similar dimensions and specs to the P782.

The fact that these came in a box and not mounted to a pair of chassis leads me to think that someone was gathering bits to build one of the MKIII derivatives in a DIY chassis and not for a restoration. I suppose that puts me at the crossroads you mentioned where I either sell the power trans and head towards a new MKIII chassis build, or if I just DIY my own and finish what this guy started. I purchased a quad of JJ EL34s and a quad of EH EL34s along with 4 Mullard CV4004s as part of this package so I can only assume this was destined for whatever amp he had in mind.

And lastly, as to my tech/diy knowledge, I have been getting my hands dirty with it for about 8 years now. I do repairs and upgrades as a hobby for maybe 8-10 or so audio folks on my local area, and have found that I seem to have a natural aptitude for it. Admittedly though, my experience thus far has been 90% SS, and only about 10% tube. I know that SS and tube are as different as EFI and carb in many respects, and that circuit design and troubleshooting/repair are again very different animals.

OK. The EL34s were used with the A430 output transformers in the original MK IIs. Schematic and manual located on this site. A few things to consider. Chassis and covers can be expensive so finding some good salvageable used ones (Dynaco or custom) can be a plus. Dynaco used a circuit board to mount the front end of the amp. Highly recommended to do the same. I sell an early MK II style on Ebay. Vintage Electron and Dynakit parts.com sells the narrower MK III style. Any of these would work and saves time, space etc for DIY projects. A small bias supply transformer should be easy to obtain. Just pattern the circuit after the MK II resistor string. Use about .002 amps for the load. Set the center of the adjustment pot for about -60/55VDC and it should be fine. If not enough adjustment then change one of the resistors. Good luck with your project.