Edcor Transformers for Mark lll

I am curious about the disadvantages of using Edcor transformers for a Mark lll. Edcor seems to manufacture transformers to the general Dynaco specifications, and the cost is significantly less than Dynakitparts or Triode. Other than authenticity, are there other disadvantages?

TN Allen wrote:I am curious about the disadvantages of using Edcor transformers for a Mark lll. Edcor seems to manufacture transformers to the general Dynaco specifications, and the cost is significantly less than Dynakitparts or Triode. Other than authenticity, are there other disadvantages?

You cannot sell the amp as a genuine dynaco anymore.

I'm more interested in technical differences or disadvantages. I think I recall correctly that Dynaco had an improved transformer design that provided improved audio. Do the Dynaco replacement transformers from Dynakitparts and Triode match the original transformer designs making them superior in some way to Edcor, and possibly Hammond transformers?

in my experience the Edcors are superior to Hammonds at about half the price, as long as you don't mind waiting 4-6 weeks for them.
They are physically larger and won't fit the mounting holes of the stock chassis, but provide excellent performance, made with high quality materials and methods

TN Allen wrote:I'm more interested in technical differences or disadvantages. I think I recall correctly that Dynaco had an improved transformer design that provided improved audio. Do the Dynaco replacement transformers from Dynakitparts and Triode match the original transformer designs making them superior in some way to Edcor, and possibly Hammond transformers?

Are you looking only for an output transformer, or are you also interested in a power transformer? For an output transformer the Edcor CXPP100-4.2K is very similar in size to the Dynaco A-431-S sold by Triode Electronics. The mounting holes of the A-431-S are 3" x 3 3/8", the CXPP100-4.2K is 3" x 3 5/16". Both are just over 4 1/2" tall.

Regarding the power transformer, I am not sure which one you would be looking at. There is only one Edcor 800VCT power transformer, the XPWR201, that has a Bias output. It has a 12.6VCT heater. The Edcor XPWR110 seems a better fit spec wise, but it doesn't have a Bias output. Both of these power transformers have 3" x 3 5/16" mounting holes. The Tridoe Electronics P-782-S has 2 7/8" x 3" mounting holes.

Is there an easy way to get the required Bias voltage if the power transform doesn't have a tap for it?

I looked at the CXPP-60-4.2 and the XPW011, they seem close to the Dynaco specifications. If I do a Mark 3, I'll mill a chassis similar to the first Mark 3 I did. One attraction with doing a second is to improve on the first layout, a second is to use a Mullard GZ33 I recently bought.

TN Allen wrote:I looked at the CXPP-60-4.2 and the XPW011, they seem close to the Dynaco specifications. If I do a Mark 3, I'll mill a chassis similar to the first Mark 3 I did. One attraction with doing a second is to improve on the first layout, a second is to use a Mullard GZ33 I recently bought.

I agree that the XPWR011 is the best fit spec wise, but where are you getting the Bias voltage, 50v to 60V, from?

I see what you mean, good point. I'm learning about most of this. Other than the 300mA(XPWR201) vs. 200mA (XPWR011) and the bias tap, are there other differences in the specifications that would make the XPWR201 an unsuitable substitute?

TN Allen wrote:I see what you mean, good point. I'm learning about most of this. Other than the 300mA(XPWR201) vs. 200mA (XPWR011) and the bias tap, are there other differences in the specifications that would make the XPWR201 an unsuitable substitute?

The XPWR201 does not have a 6.3V CT heater output. Instead it has a 12.6V CT heater output. This is not a huge issue, as the two 12.6V leads can be tied together and then you will have 6.3V @ 6A available for the heater. However, this heater supply does not have a center tap, so a virtual center tap should be created with a couple resistors. Do some research, but I think a couple of 100 ohm @ 1 or 2 watts would be good. The center of those two resistors would connect to the .02uF capacitor, just as the center tap did on the original transformer. This is used to reduce hum.

There are two other options.

Use the XPWR011 and use a voltage divider off of the 400V high voltage output. This type of bias supply has been used in Marshall guitar amps.

The other option is to have Edcor add a 55V bias output to the XPWR011. They will do this, but they have a design charge.

I hope someone else with more experience will comment. I know Bob has recommended the XPWR011 as a replacement previously. He probably has a recommendation as the best approach to create a bias supply.

We're I to proceed with a second Mark lll, it is probably more practical to buy the P782 from Dynakitparts and the output transformer from either Edcor or Dynakitparts. I looked into the virtual center tap, it seems like a practical solution. I have also sent a message to Edcor about adding a bias tap to the XPWR011.

As you suggest, perhaps Bob Latino will provide futher information. It would be interesting to know what he has suggested in the past regarding using the XPWR011.

Edcor has responded regarding adding a bias tap to the XPWR011: "Cost would be $82.28 each plus a one time design and setup charge of $150.00."

TN Allen wrote:Edcor has responded regarding adding a bias tap to the XPWR011: "Cost would be $82.28 each plus a one time design and setup charge of $150.00."

I guess this will convince you get install genuine dynaco transformers then ?
( dynakitparts, triod or similar)

The XPWR201 is $79.76.

BryceJ wrote:The XPWR201 is $79.76.

P-782 from dynakitparts is $120
A-451 from dynakitparts is $150, Exact drop-in replacement for Dynakit MKIII

A kit ( P782 + A-451 ) is $260 from dynakitparts

You can install a bias circuit on any power transformer that doesn’t have a bias tap by installing a resistive voltage divider tapped off either high voltage lead to ground. Add a diode, another resistor, a couple of electrolytic caps, and a pot. Use ohms law to calculate the resistor divider values. The diode, capacitors, and pot specs will be typical of bias circuits used for the given output tubes. Ampeg and a few other amp makers used this type of bias circuit in their designs.

The Dynakitparts transformers are an easy solution, but the Edcor transformers are still an interesting possibility, given there are alternative ways to provide either a bias tap, or the 6.3V filament voltage. My interest in starting this thread was primarily out of curiosity, given the Edcor transformers have a good reputation, and cost significantly less.

I haven't decided to build a second Mark 3, though it is tempting. And, I have several other projects to finish before starting another.