Best choice of ST70 output transformers

knowledge base for the classic Dynaco ST70

Postby LarryDA » Thu Apr 23, 2009 12:41 pm

I am winding my own output transformer, and I don't see much difference between how they were designed in the 50's and today. ( Please don't get me wrong on this one). Material wise, especially the insulation have improved tremendously over the years. We don't use wax anymore, and hopefully the iron quality have also improved over the years ( I hope this is not being outsourced from .. you know where).

Manufacturers, and transformer builders have its own "secret" in distributing layers on the bobbin. Some uses bifillar, trifillar, single etc...so many options available.

In regards to Dynaco output transformer ONLY (the power transformer needs to be redesign and recalculated, so that it will have a descent TPV Vs. the most common 2.5. A TPV of 3.8 is ideal in today's tap voltage), I think they are well built comparable only to the expensive brand available today. Dynaco output transformer have a better primary distribution for each EL34. The primary has been arranged in mirror image like winding and then connecte in parallel. Very similar to the McIntosh approach (not the triffilar though). This technique allows an evenly distributed primary between tubes with almost the same MTL between the lower and the upper sections of the primary windings. The same is true with the secondary. Although there are only two sections of secondary layers, these layers are always active, without having to worry connecting terminal to terminal just to achieve a better match. The only minor down side of Dyanco transformer is the winding loss of more than 3.2% or maybe higher. Increasing the wire diameter, reduces the winding loss to the most acceptable value of 2.5% (at least). Please make a note that increasing the wire diameter, increases the number of turns needed to meet the 4.3K. The lamination stack must also increase in height ( Only to some extent though. Improperly calculated stack height can also deliver an audible problem.

-->> Dynaco output transformers are calculated to operate between 6hcs to 60Kc ( Hz). Actually this is really good, that gives you a constant of at least 295 to calculate your core area requirement.

-->>Higher output? It is all up to the builder. If you think it is sounding great, why not. Mathematically, NO..Why? Increasing the output at let say 40watts requires about another 1/2 inch of iron on the core to achieved this output. This Justify the mono connection on Dynaco manual. By setting it up in mono mode, the two transformer are in parallel mode still 4.3K, but double the iron stack. Whenever the core height is altered or changed the number of turns must compensate either higher or lower depending on the wire size used.

In contrast to my statement above, the most transformer design in output transformer calls for at least 4 sections of secondary layers between the primary sections. This is the most commonly used layer distribution today.

Running KT88, 6550 on A-470 why not. I tried it, and I did not noticed much difference . Maybe it is just my hearing.
LarryDA
 
Posts: 1
Joined: Thu Apr 23, 2009 11:14 am

Postby kheper » Thu Apr 23, 2009 2:40 pm

LarryDA wrote:Running KT88, 6550 on A-470 why not. I tried it, and I did not noticed much difference . Maybe it is just my hearing.


Running KT88s in an st70 with an old power transformer would not make much of a difference in audio output, because the tubes must be biased no higher than what EL34s are. The power supply was the bottleneck. A new power supply with higher voltage and current capabilities would raise the output. The A470s were advertised as being capable of 40 watts continuous in the MKIV. I do not think that the old st70 produced more than 30 watts per channel, probably less.
User avatar
kheper
KT88
 
Posts: 1252
Joined: Wed Dec 21, 2005 10:14 pm
Location: Philly, PA

Postby Slartibartfast » Fri Apr 24, 2009 8:17 am

LarryDA wrote:
In regards to Dynaco output transformer ONLY (the power transformer needs to be redesign and recalculated, so that it will have a descent TPV Vs. the most common 2.5. A TPV of 3.8 is ideal in today's tap voltage), I think they are well built comparable only to the expensive brand available today.



Triode Electronics sells a replacement PT for the ST-70. It is a lot better than the original.
User avatar
Slartibartfast
KT88
 
Posts: 458
Joined: Sun Apr 15, 2007 9:40 pm
Location: Columbia S.C.

Previous

Return to st70

Who is online

Users browsing this forum: No registered users and 18 guests