Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Deric on November 15, 2023, 01:24:23 pm

Title: DC Filaments and Transformer Center Tap Question
Post by: Deric on November 15, 2023, 01:24:23 pm

Working on an Orange Rockerverb 100


https://el34world.com/charts/Schematics/Files/Orange/Orange_rockreverb_50w.pdf (https://el34world.com/charts/Schematics/Files/Orange/Orange_rockreverb_50w.pdf)


The preamp tubes run on DC.  PCB is burned up under the bridge rectifier (D3, D4, D5, D6) and the 22000uf cap (C33) is bad.  Pulled the diodes and all tested fine so cleaned up the board best I could and put them back in.  D3 and D4 now test shorted.  Scratching my head for a minute and I notice 2 100r resistors from the input side of the bridge to ground.  The negative side of the bridge is also connected to ground.  This is a bad thing, right?


The 100r resistors are not on the PCB. They are wrapped around the terminals where the PT connects.  Definitely an after thought but looks like it was done at the factory.  Should they be removed?

Title: Re: DC Filaments and Transformer Center Tap Question
Post by: PRR on November 15, 2023, 03:58:20 pm

The 1N5408 were marginal to begin with (as shown by char). I don't know how you got shorted but they should be replaced with 6Amp (>50V), probably a pre-made bridge bolted to the chassis. I doubt the 100r hack has anything to do with it.

Title: Re: DC Filaments and Transformer Center Tap Question
Post by: kagliostro on November 16, 2023, 01:33:15 am

As they acts as resistors, traces of burning on PCB MUST be completely removed, they are the way for further problems

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Looking to the way Mesa Boggie used DC for heaters on the Solo 50 and the Tremoverb I see that they use the trick od the two resistors connected to ground as virtual ground reference but the exit of the FWB Is manteined floating, no ground connection at the negative


Don't know if this is mandatory but they do it this way

Franco

Title: Re: DC Filaments and Transformer Center Tap Question
Post by: Deric on November 16, 2023, 12:46:20 pm

Thanks for the replies.


Checking again, the diodes weren't shorted they just failed a diode test on my meter when in circuit.  Checking actual reistance they mearure ~48r to ground.  Makes sense with the 2 100r resistors.


Planning on going with a premade bridge mounted to the chassis if I can find room to mount it - it's pretty tight in there.


Still curious about the 100r resistors.  On this page https://www.valvewizard.co.uk/heater.html (https://www.valvewizard.co.uk/heater.html) Merlin writes...


"A common beginner's error is to try to add a ground reference on both sides of the rectifier. This will short out the rectifier!"