r core or toroidal output trafo for dynaco st70

[srinath]

OK next stupid Q about my dynaco 70.
I have been seeing on fleabay none the less R core and toroids as Output trafo's.
I've heard EI core works the best for output, totoil power trafo is fine, but EI for output.
Has anyone heard or known of R core or toroids that sounded great in a amp like an st70.
I am asking cos I am eventually going to hot-rod the thing, wanna get the trafo to account for it to start with ...
Thanks much guys. You are phenomenal.
Cool.
Srinath.

[taj]

Some people would say the stock output transformers are the best part of an ST-70. People regularly swap out the driver circuit board, and the power transformer, and other parts of the power supply to improve the amp, but you'd be the first I've heard of that kept everything else and replaced the OPTs.

Those stock ones will sell quickly on eBay if you go that route, assuming they still work properly.

..Todd

[quadtech]

You may find this interesting -

diytube.com :: View topic - New Amp - ST-70 Driver + Toroids

[LinuksGuru]

I use output 60W toroids of my own design, in KT88/6550 push-pull amplifier with CFB. Not likely to use anything else in the nearby feature.

Contrary to popular belief, properly constructed output toroids can handle output current imbalance of 7mA without any problems, even at max power / 20Hz.

R-core is not very well suited for audio transformer application, it is rather exotic and not optimized design.

[analog_sa]

Originally Posted by quadtech

You may find this interesting -

diytube.com :: View topic - New Amp - ST-70 Driver + Toroids

Not sure if i got this right but is this amp a Dynaco clone using a different driver stage, different output tubes and different output transformers? What exactly makes it a Dynaco?

Have to agree with taj, the only parts worth keeping are the outputs. Upgrades are certainly possible but pointless while keeping the driver stage and power supply.

[bear]

It seems to me that China is making R core and toroids... so you'll see that on ebay...

Imo, toroidal xfmrs for outputs are asking for trouble, keeping the DC imbalance below "7ma" is going to be a problem, unless you check it every time you run the amp... or set up a "servo" type offset adjust... or keep the DC off the output iron.

I worry about the toroidal output transformer mfrs that keep the DC imbalance specs in very tiny print... The other thing is where does that "7ma" spec come from? What happens to the bass/inductance with DC? Where is the curve showing inductance vs. DC and/or distortion?

Imo, you want to upgrade get a gapped core xfrmr (or gap the existing one?) but then ur not running a Dyna, you're into a custom amp, so start from scratch would be my advice.

_-_-bear

[LinuksGuru]

Originally Posted by bear

Imo, toroidal xfmrs for outputs are asking for trouble, keeping the DC imbalance below "7ma" is going to be a problem, unless you check it every time you run the amp... or set up a "servo" type offset adjust... or keep the DC off the output iron.

I worry about the toroidal output transformer mfrs that keep the DC imbalance specs in very tiny print... The other thing is where does that "7ma" spec come from? What happens to the bass/inductance with DC? Where is the curve showing inductance vs. DC and/or distortion?

7 mA comes from real world bench testing, and it is more then enough, typical idle current is about 50 mA, so if transformer works fine with 43 mA for one output tube and 57 mA for another, its OK. Crossover distortions raise with imbalance because of tubes (not core), if core go into saturation area, it can be easily spotted on oscilloscope. Inductance must not fall below certain value on low frequency only. Calculating inductance on 20Hz with and without DC doesn't exhibit very complex problem.
Core with micro air gap is one of the possible workarounds, but IMHO it is good solution only for amorphous/nanocrystallnie based trafos, for CRGO/GOSS it doesn't make any sense.

[bear]

actually that is 14ma... fwiw...

_-_-bear

[Shoog]

Do not attempt to use power toroidals as outputs unless the output tubes are below 2K rp, the bass tends to distort easily.
Imbalance is an issue but Broskie has some nice relatively simple servo circuits. Otherwise Garter bias is a simple way to keep DC out of the transformer if you have some voltage to spare.

When they are set up correctly they sound absolutely fantastic and will better any EI put up against them.

Shoog

[amptramp]

Give these guys a call:

Vacuum Tube Audio Output Plitron

They are in the Toronto area.