1979 Hiwatt

[C Moore]

I have a hand wired, 1979, Hylight, 2 Hole, DR504. It follows most of the 4 hole schematics pretty faithfully, there are a few exceptions.
There are 4 wires from the OT primary. Black and Gray are for the power tube plates. I am having a hard time figuring out what the yellow and brown wires are. One of them must be the CT, but what would the 4th wire be?
These two wires are connected to the turret that is in between the 100R 5 Watt resistor on the layout below. That layout does not show these two wires. I know it must be something simple/obvious, but I do not recall seeing 4 wire on the OT primary before.
Thank You

http://hiwatt.org/Layouts/PowerAmpLayout50.pdf

[selloutrr]

is it part of the bias? --- stupid suggestion - my mind was thinking PT thought my eyes saw OT. - sorry

what is the make and model of the transformer?

[C Moore]

is it part of the bias?

what is the make and model of the transformer?

Here is a picture of the yellow and brown wire. There connection is that turret between the 100R 5 Watt resistor. They come out of the same hole, from the OT primary, that the black and gray plate wires do.

[Dr-Joned]

I'm not exactly sure on the partridge transformer, but I have seen on others. The transformer, instead of having a center-tap, would have 2 primaries. A negative phase from one winding and a positive phase from the other winding would be grounded just as a center-tap would be.

[David Root]

That makes sense. Mark Huss could tell you for sure I would think.

[selloutrr]

does this help? OT wiring diagram
I know it says soundcity but take a look
http://www.soundcitysite.com/sc_webpages/sc_120_ot.jpg
http://www.soundcitysite.com/sc_webpages/sc_120_pt.jpg

I can't find any Technical data sheet of the TH7751 online. I'll check the shop tomorrow see if I have anything on file.

I just stumbled across this hiwatt forum maybe they can help?
http://www.vintagehiwattconvention.com/ ... 1c1d69db9b

[paulster]

Sounds like two discrete primaries to me since the two wires are terminated at the same point, rather than a single primary with a centre-tap.

Odd though, since you're hardly going to ever use them in any other configuration than as a centre-tapped primary.

I can only imagine it made winding it easier with the way the primaries are interleaved.

[sliberty]

Could it be a UL trainee?

[C Moore]

Could it be a UL trainee?

I do not have much experience with those. But don't they have 5 leads? 2/Plates, 2/Screens, and a CT?
That sure is a good guess though. I don't know........

[C Moore]

Electrically, those two wire (brown and yellow) are the junction of the SB Switch and the 100R 5 Watt resistor. If I measure from that point to each power tube plate, I get about 40 ohms.
I will have to plug in a speaker and turn the amp on, but it seems that those two wires make up the OT Primary CT. I do not know if that is possible, or if so, why it is built that way. This stuff is all new to me.....
Thank You

http://hiwatt.org/Layouts/PowerAmpLayout50.pdf

[paulster]

It can't be ultra-linear without 2 screen taps, so I think it's certain they've wound 2 identical primary windings based on how it's connected. You'll probably find they started both windings at the same point on the bobbin and then finished each at the same point, so the centre tap would have to go from one end of the bobbin to the other and they brought it out on two leads instead.

[ToneMerc]

See schematic

[C Moore]

Tone Merc -
That sure looks like it.
What does the OL stand for?
Thanks

[ToneMerc]

I've always took it to mean Overdrive Lead because of the extra gain stage, but I might be wrong at it's actual connotation.

TM

[gldtp99]

I just checked my 1980 Hylight DR504 (PCB model) and it also has the twin yellow/brown OT primary CT.
Mine also has the OT secondary taps of only 8 and 16 ohms--- same as the schematic----- the 8 ohm OT secondary tap is soldered to both the 4 and 8 ohm tabs on the impedance switch.
The amp sounds very good, just like a 50 watt 2-Holer should----Blackburn Mullard ECC83 in V1..............gldtp99