Hiwatt DR-504 oscillation; presence control

[Chevy]

Hi guys,

I did a cap job on a DR-504, got the F&T caps kit from VHR. Sounds fine with a guitar input, but putting it on the scope, I can see that there is a bad oscillation just on the signal peaks of the output waveform, on input frequencies below say, 1khz or so. More pronounced down around 100-500hz. This oscillation is there ONLY when the presence pot is somewhere OTHER THAN either extreme end of rotation. Make sense? (ie At extreme full "presence on" and extreme full "presence off", there is no oscillation.) I added a pot for bias adjust, but didn't move any existing wiring or anything like that.

I measured for any DC on the presence pot, and there isn't any. The pot values and associated 470R seem to be pretty close to spec. Poked around with a wood stick, moving wires, and got no readable change on the oscillation. Changed from 4 ohm dummy load to 8 ohm, but problem persists.

Not sure how the presence control on this amp works electronically, so am having a tough time troubleshooting this... seems to incorporate xformer feedback plus preamp signal.

Any ideas?

Mucho appreciated!

Chevy

[booj]

A schematic would help. Try posting a request.

[Chevy]

Hiwatt DR-504 schematics

OK, here are a couple of schematics... this is the actual preamp configuration for this particular amp (there are different variations):
http://www.schematicheaven.com/newam..._50w_dr504.pdf

But it doesn't show the feedback resistor clearly, so I've included this one:
http://www.hiwatt.com/manual/DR504_P...eamplifier.pdf

Not sure how the presence control pot and it's circuit on the input side of the phase inverter works.

[Lee_ranaldo]

hi..
i had a hiwatt here and the owner installed a 12AX7 as a phase inverter
the service manual says that you MIGHT HAVE oscillations if u use a 12AX7 due to it's higher gain

check that or replace ur phase inverter tube with a 12AT7

U can check the diagram for the tube chart to locate that phase splitter tube.

[Chevy]

Lee,

Awesome!!!! Thanks so much... didn't even think to look there at the original Hiwatt schematic to see the tube types. Guess what? The owner has a
12ax7 installed for the PI ! Man, I think this saved me lots of grief in troubleshooting. I don't think a 12ax7 would cause oscillations in a Fender or Marshall style PI, so didn't even occur to me.

Didn't get a chance to dig into the amp yet, but this sounds right on to me.

Thanks again!

Chevy

[Chevy]

Lee, that was it... thanks again!
Chevy

[Lee_ranaldo]

thanks mate u're welcome!!