Hi I wonder is this is normal. On the power amp part of my Dynaco ST-70 the power transformer at the end of the night when I use it is too hot to put or hold my hand on it. Should the power transformer be this hot on this onld amp or do I need to replace any part?

Hi

Power transformer will get very hot. You should be able to touch the transformer without burning you hand.

The output circuit of your amp may be drawing excessive current? Is there a way to adjust bias? Caps may also be leaky reducing bias. High line voltage can cause a transformer to run hot.

Norm

:Hi I wonder is this is normal. On the power amp part of my Dynaco ST-70 the power transformer at the end of the night when I use it is too hot to put or hold my hand on it. Should the power transformer be this hot on this onld amp or do I need to replace any part?

:Hi
:
:Power transformer will get very hot. You should be able to touch the transformer without burning you hand.
:
:The output circuit of your amp may be drawing excessive current? Is there a way to adjust bias? Caps may also be leaky reducing bias. High line voltage can cause a transformer to run hot.
:
:Norm
:
::Hi I wonder is this is normal. On the power amp part of my Dynaco ST-70 the power transformer at the end of the night when I use it is too hot to put or hold my hand on it. Should the power transformer be this hot on this onld amp or do I need to replace any part?

hi:

I've owned this amp for 37 years and its power transformer has always felt very hot after only a few minutes of running. Hasn't seemed to affected it in all this time! So, I always presumed that all is OK!

Hi,
The cilter caps that were around when that amp was new were still a bit leaky. While those caps may be "okay", I would still have some concern if my speaker load impedance were correct. If everything checked okay, about the only other precaution I would consider is to never use metal tubes in that chassis, just in case.

Only the original designers could state whether or not that the transformer heating is to be expected to the extent that you could not touch the transformer after only a few minutes- and if it were factored into the design.

:hi:
:
:I've owned this amp for 37 years and its power transformer has always felt very hot after only a few minutes of running. Hasn't seemed to affected it in all this time! So, I always presumed that all is OK!
:

:Hi I wonder is this is normal. On the power amp part of my Dynaco ST-70 the power transformer at the end of the night when I use it is too hot to put or hold my hand on it. Should the power transformer be this hot on this onld amp or do I need to replace any part?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
B Hedberg's :
I have used one of those units that I built in '64 with absolute MINUMAL maintenance required.......I
believe to be attributable to the installation of
a whisper ventilation fan at the center rear of its
top cage/cover on day one. Of course nowadays I would probably use a pair of 12vdc computer brushless fans.
The units rectifier tube tends to deplete its emissive
coating over time depleting B+ output and yes there is
bias adjustment.Check the output tubes bias + coupling caps. As well, electrolytic filters may be aging.I'm sending 2 helpful URL's, primarily the first.I don't know your degree of expertise but this info should get you well started on the unit.

http://www.curcioaudio.com/dynadr_3.htm

http://members.bellatlantic.net/~vze2w7wc/projects/amp_page/st70amp/st70.html

73's de Edd

:Hi I wonder is this is normal. On the power amp part of my Dynaco ST-70 the power transformer at the end of the night when I use it is too hot to put or hold my hand on it. Should the power transformer be this hot on this onld amp or do I need to replace any part?

Bob, that is typical with ST-70 power transformers - they get really hot, and everyone worries about that.

However - leaky coupling capacitors and the filter supply electrolytics will not help anything at all if they are going south and are a whole lot cheaper than new power transformers out output transformers - for peace of mind I replaced the 4 electrolytic sections with modern 600 volt TVA Atom axial capacitors,the 'black cat' paper capacitors on the board, the bias supply electrolytics, and the selenium rectifier. Still gets PFH, as in "too hot to hold" but no worries. Set bias to 1.56 volts - a fresh "D" cell was utilized by design as a reference voltage for this amplifier.