SCA-35 B+ current

After replacing a fried power transformer with an original which i was very lucky to find i now find that after about an hours use the power transformer is still much too hot to touch. I powered one half of the amp with an external AC filament supply with no real change, still too hot. I then disconnected the HV leads and provided an external source of B+ and the transformer is noticeably cooler after two hours of running.
The amp is drawing about 100 ma of current.
Is this number somewhat in the right area for this amp or do these power transformer just run really hot?? I am ready to shelve this thing before another power transformer fries.

dockbuster wrote:After replacing a fried power transformer with an original which i was very lucky to find i now find that after about an hours use the power transformer is still much too hot to touch. I powered one half of the amp with an external AC filament supply with no real change, still too hot. I then disconnected the HV leads and provided an external source of B+ and the transformer is noticeably cooler after two hours of running.
The amp is drawing about 100 ma of current.
Is this number somewhat in the right area for this amp or do these power transformer just run really hot?? I am ready to shelve this thing before another power transformer fries.

https://www.vintageshifi.com/repertoire-pdf/Dynaco.php

a) How much current on the B+ winding?
b) Something is seriously off on your measurements. The amp is rated at 110 watts per the nameplate, or about 0.92 A @ 120V. More like 90 watts in-use. So, if you are reading only 100 MA, your instrument(s) is(are) bad.
c) I am running a ST35 here in the office, and the power-transformer is barely warm after a full day of operation (PB028). Sure, it is carrying two fewer tubes, but that is not enough to create what you describe.

It is difficult to do diagnostics-at-a-distance, but try running the amp with no tubes in it. It should draw significantly less current, and the transformer should run entirely cool. If not, you have a partial physical short somewhere, which could be anything from a capacitor shorting to a pinched wire or similar. If this turns out OK, then you will have to check the tubes as well. Do you have a tester that can handle Shorts and Gas? These are only the most basic tests looking for obvious faults. And I suggest starting with them as this, clearly, came on slowly as the amp was running for some time before failure.

Getting a bit more complicated here - is the amp 100% OEM? Has it been modified in any way? Have any parts been changed as maintenance? The closer it is to OEM, the more likely you have a component failure, or an open/intermittent trace on one of the boards or similar. Or a partially slagged tube. More information, possibly some pictures (above and below) would be useful.

dockbuster wrote:After replacing a fried power transformer with an original which i was very lucky to find i now find that after about an hours use the power transformer is still much too hot to touch. I powered one half of the amp with an external AC filament supply with no real change, still too hot. I then disconnected the HV leads and provided an external source of B+ and the transformer is noticeably cooler after two hours of running.
The amp is drawing about 100 ma of current.
Is this number somewhat in the right area for this amp or do these power transformer just run really hot?? I am ready to shelve this thing before another power transformer fries.

Hi,
do not use HiV input. Use an external source.
Pavel

dockbuster wrote:After replacing a fried power transformer with an original which i was very lucky to find i now find that after about an hours use the power transformer is still much too hot to touch. I powered one half of the amp with an external AC filament supply with no real change, still too hot. I then disconnected the HV leads and provided an external source of B+ and the transformer is noticeably cooler after two hours of running.
The amp is drawing about 100 ma of current.
Is this number somewhat in the right area for this amp or do these power transformer just run really hot?? I am ready to shelve this thing before another power transformer fries.

The transformer in these is hot since they are mounted close to the el84 tubes.
That said, the majority of current is consumed by the EL84, and they share
a common 100ohm resistor. According to the manual 16V is expected at idle
on the el84 cathodes ( and 100ohm resistor) giving 160mA consumption for
the EL84 alone. total shoule be app. 180mA Do check this !
A bad can-cap may draw current and load the transformer. Replacing them
seems unneded, better to install D.Gillespies EFB board that includes all
needed B+ caps.

I guess i need to be a little more specific here.

The 100ma i am referring to is the total DC B+ current draw. The primary current is .7 amps AC at 120vac. These values don't change over an hour.

I am using a B&K model 1602 to provide B+. I have set it to provide 360 volts on the plates of the EL84's. Both PC10 modules are new and i have installed the EFB mod. The voltage across the 5 ohm resistors are within 10 mv of each other and are set to .270 volts.

Bottom line: the amplifier works perfectly but after about an hour the power transformer will burn my fingers.
I still don't know if this is considered normal given its close proximity to the EL84's and their output transformers.

dockbuster wrote:I guess i need to be a little more specific here.

The 100ma i am referring to is the total DC B+ current draw. The primary current is .7 amps AC at 120vac. These values don't change over an hour.

I am using a B&K model 1602 to provide B+. I have set it to provide 360 volts on the plates of the EL84's. Both PC10 modules are new and i have installed the EFB mod. The voltage across the 5 ohm resistors are within 10 mv of each other and are set to .270 volts.

Bottom line: the amplifier works perfectly but after about an hour the power transformer will burn my fingers.
I still don't know if this is considered normal given its close proximity to the EL84's and their output transformers.

As you already have en EFB module the amp will consume less than stock, this
increases your margins ( 100 vs 160mA ). The old cancap's are removed thus
no problem. Given that no extra load on the filament windings your should be safe.
Is the transformer a P028 with 2 primaries ? They should be connected in paralell
as per the manual p.15

dockbuster wrote:I guess i need to be a little more specific here.

The 100ma i am referring to is the total DC B+ current draw. The primary current is .7 amps AC at 120vac. These values don't change over an hour.

I am using a B&K model 1602 to provide B+. I have set it to provide 360 volts on the plates of the EL84's. Both PC10 modules are new and i have installed the EFB mod. The voltage across the 5 ohm resistors are within 10 mv of each other and are set to .270 volts.

Bottom line: the amplifier works perfectly but after about an hour the power transformer will burn my fingers.
I still don't know if this is considered normal given its close proximity to the EL84's and their output transformers.

Using an inexpensive heat gun on my office ST35, as follows:

Left OPT - adjacent to EL84 output pair: 124.9 F (51.6 C)
Left EL34 (either) at top: 250F (121.1 C)
Power Transformer 134 F (56.7 C)
Right EL343 (either) at top: 250 F (121.1 C)
Right OPT - as with Left: 133.9 F (56.6 C)

Placement is such that the left side gets more direct air from the vent than the right, which would explain the different transformer temperature.

After doing a little research around this board it seems that burning hot power transformers on SCA-35's is not uncommon.
I guess i will just live with my "mini office heater" and hope for the best but if it fries again i will part it out and move on.
Thanks everyone for your comments.

T.

I will de-mothball my own SCA35, run it for several hours and see what I find. This is not an overnight project as it is (was) in the process of being entirely rebuilt and so is in pieces at this point. But I do not remember it running that hot.

And I still owe some tubes to others here. Work is nuts, weekends are nutsier if that is a word.

A small ( 5') fan mounted to the right . blowing into the amp might be beneficial here. doubling
one of the filament AC will supply the needed 12V ( a series resistance may reduce noise )

Thanks Peter W. I will look for your findings when you are able to post them.