First post!

[ThunderOne]

Thanks for the info, Doc! Mine is #2740, so they are real close and should have identical wiring. I spoke to the former owner and he told me that he got the amp that way and that he tried to wire it back like yours but that the result was a horrible screeching sound...

I also found this schematic on Mark Huss's website. It might be for some different model though, because at those positions you see 47n V4 coupling caps.

Franz

Early 70s 4input Tag1Annotated.gif (72.38 KiB) Viewed 1568 times

[ThunderOne]

Here is the serial tag:

DSCN5639a.JPG (310.97 KiB) Viewed 1562 times

[ThunderOne]

It seems that this mod is not familiar to those I asked about it. In any event, idc because the amp works great as is. I just might get curious enough someday to find out what this is all about. In the mean time, I'll just enjoy playing through it!
Btw, I'm still looking for a nice original '70s 4 x 12!

[mikhailwatt]

I spoke to the former owner and he told me that he got the amp that way and that he tried to wire it back like yours but that the result was a horrible screeching sound...

I also found this schematic on Mark Huss's website. It might be for some different model though, because at those positions you see 47n V4 coupling caps.

I haven't seen a schematic of that DR201 preamp, but my semi-uneducated guess would be that the polarity on the OT primary is reversed (was the OT replaced at some point?) and, instead of swapping the OT primary leads, the tech swapped the preamp output jumpers on the board.

[Stelle79]

I spoke to the former owner and he told me that he got the amp that way and that he tried to wire it back like yours but that the result was a horrible screeching sound...

I also found this schematic on Mark Huss's website. It might be for some different model though, because at those positions you see 47n V4 coupling caps.

I haven't seen a schematic of that DR201 preamp, but my semi-uneducated guess would be that the polarity on the OT primary is reversed (was the OT replaced at some point?) and, instead of swapping the OT primary leads, the tech swapped the preamp output jumpers on the board.

That would be my guess as well. As for the V4 coupling caps, they're located at the power board on the 200's, not at the pre-amp board as in the 50's and 100's.

[ThunderOne]

Thanks for your feedback, guys. I actually tried rewiring as per original specs and the result without speakers (using just a dummy load) was the "motorboating" effect typical of reversed OT primaries. So your guess seems on the spot. It doesn't look like any of the transformers or choke have ever been changed. I'm no Hiwatt expert but the wiring, harnesses, marker checks, etc...all look like they've been there forever. Why then, would there have been a reason to disconnect the OT primaries?? Mystery...

[ThunderOne]

I forgot to ask if you could please tell me which are the OT primary wires on my DR201?

[mikhailwatt]

I forgot to ask if you could please tell me which are the OT primary wires on my DR201?

The wires from the OT that connect to pin 3 on the first (inner?) output tube socket of each pair. There might not be enough slack to swap them, though.

[ThunderOne]

Among the things that were obviously changed, I noticed the green standby pilot lamp. There was some resoldering done on the switch. Could that be where to start looking from?