Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Hoodnight on August 10, 2021, 08:58:09 am
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Hey folks, first proper post so apologies in advance if I've missed some rule or something.
I've been working on my DRRI '65 trying to track down a hum, found a couple of dodgy coupling capacitors. While I was in there I did Rob Robinette's mod to add reverb and trem to both channels.
When I reassembled though, the reverb had died. Swapped the 12AT7 for a different working one, still nothing. If I knock the tank I get splash coming through. Have checked continuity on the RCA plugs and they're all good.
Tremolo is working in both channels, and I've confirmed continuity on the path between the two channel's mixing resistors and the grid of the reverb driver.
When I had it up on the bench I disconnected the reverb tank (and my brain apparently) and was testing without it plugged in for quite a while, including turning up the reverb to track down the hum. Didn't think about the reverb transformer needing a load.
I'm guessing I've blown the reverb OT, but I was wondering if there was any good way to confirm that it's busted before I go ordering a replacement?
Cheers in advance.
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Check resistance of the primary and secondary windings.
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Ah yep! The primary is coming in at 19Ω and the secondary at 1Ω.
I'm assuming the primary has shorted out, as on the datasheet for the Hammond equivalent it's listed at ~1KΩ DCr and ~1Ω for the primary and secondary respectively. Ah well, it's a cheaper lesson than burning out the output transformer.
Thanks Sluckey!
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I just happen to have a RT (part number 037099) from a '65 DRRI on my desk. Primary = 1237Ω and secondary = 1Ω. You can have it if you come get it. :laugh:
EDIT... Be a good idea to check that 2.2K/1W cathode resistor on the 12AT7 driver tube.
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That was bad luck, it typically isn’t a problem.
It’s good mitigation to pull the reverb driver valve (V3) when operating the amp with the reverb unplugged.
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I just happen to have a RT (part number 037099) from a '65 DRRI on my desk. Primary = 1237Ω and secondary = 1Ω. You can have it if you come get it. :laugh:
Australia has still got its border shut at the moment, so plane tickets are out of the question unfortunately, but thanks for the offer. Picked one up on ebay for AUD$30 though so not too bad.
That was bad luck, it typically isn’t a problem.
It’s good mitigation to pull the reverb driver valve (V3) when operating the amp with the reverb unplugged.
Have ordered some high-wattage resistors to make some dummy loads, will be handy regardless. Was just my laziness in not wanting to have to take the tank out onto my bench! Also my preamp tube voltages are all pretty high, somewhere between 10-15 over spec, I'm guessing because I'm on 240/50hz. I think this might have contributed to the failure.
I've been thinking about just gutting the DRRI and putting in a turret board anyway, so will probably end up turfing a lot of components.
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Hi Hoodnight,
My apprentice (son in law) rebuilt his DRRI with a 2 ch Deluxe Turret board, supplied by Doug.
It was a great project, really quiet amp but Gutsy and Clean.
The preamp voltages were a little higher, well within +/- specs by Fender. Also our mains runs typically around 245V AC.
Recommend your replace the first 2 caps in Dogbox with higher voltage F+T or something, as the Fender arrangement has proven unreliable in Aussie (fixed a few).
see attached pix.
Kind regards
Mirek
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Ah very nice, looks ace! And will heed your advice on the filter caps. My DRRI's only a 2014 so wasn't intending on replacing them, but might just do it when I've got some spare silver. Cheers Mirek!
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Modern Fenders can typically be configured for 230 or 240V, but 230V is the default for such regions.
Certainly with the UKs 240V, it’s well worth checking and reconfiguring, everything runs happier.
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Modern Fenders can typically be configured for 230 or 240V
Oh interesting. Just checked with the schematic and looks like mine is already set up for 240V.
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Just as a final follow up: reverb issue has been resolved. Thanks to everyone for the assist!
Reverb driver transformer arrived this morning, chucked it in and turned it on - all working perfectly, and the loud hum caused by the leaky coupling cap prior off the plate of V2B (the instigating issue) is no longer occurring after replacing that cap.
Still hunting down a late chain crackle unfortunately, but will make another post about that after some more investigation.