Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: LHPcope on October 24, 2013, 09:18:26 am
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Hey all, I just got done with a Crate V100H repair and I thought I would put up a postmortem.
This is a tube amp with op amp switching, reverb driver, effects loop driver, etc. The unusual thing is that it uses a switch mode power supply; so no power transformer. The mains voltage is rectified directly and filtered. That then feeds a couple of power oscillators running at above 40kHz (well really they are digitally controlled chopper circuits with a feed back loop controlling dwell time to control output voltage.) Each oscillator has its own high frequency step up transformer, the output of which is then rectified and filtered into DC. Just like in your desktop PC, only with 420vdc, -63vdc, +-15vdc and 12.6vdc.
Oh, by the way, Crate designed and built this beast before being sold to Loud Technologies. Now Loud seems to want nothing to do with it; so they are no longer selling replacement power modules for this thing.
This particular amp came in not working. A tube test revealed two of the four power tubes bad, one with an internal arc. But at this point the power supply was dead. It turns out that all three of the mosfet choppers were open along with one surface mount diode in the -63vdc section.
Based on chatter on some other sites the most likely offenders in this PS are the choppers, all diodes need to be checked, and filter caps. Also a schematic is really necessary. The way the thing is set up there is a LM555 timer that will not start the 420vdc until the other voltages are up and running so if anything else is wrong there will be no high voltage.
The owners of these things would probably be better off using better quality tubes (if there is such a thing) to prevent smoking this power supply.
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If I personally owned one of these Crate amps and the PMPS was really totally toast I think I would replace it with a regular transformer, rectifier, filter power supply. There is the real estate on the chassis to do so.
The problem is that it looks like these things have gotten the rap of exploding and being difficult to fix so prices on the used market are in the basement even for working examples. The one I just finished with the owner payed $100.00 for when working. So do you drop another tidy sum into a power supply?? My labor is cheep for me, not so much for others.
But to answer your original question the issue as I see it in building power modules from scratch is the inductors and high frequency transformers in the thing; every thing else is off the shelf semiconductors, resistors, caps, etc. You could even have a suitable PC board constructed for not a lot of $$$$. Scrap the surface mount stuff go with strictly thru hole construction on a 2 sided board; that part is doable.
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LHP; thanks for posting your 'time alone' with the Crate Hundred. I picked up a Crate 50 watt SS amp that uses one 12AX7 in the preamp. I replaced all the power filter caps which was a pain with the flimsy PCBs. It still has too much hiss and hum for my tastes but the High gain input will scream and holler with the best drummers around. I mainly bought it because it came stock with 2 Celestion 10s that are really nice speakers. I also don't like the way the reverb circuit is always 'on' and passing the signal thru it....stupid idea but probably saved them 50 cents when they made it! and I think I paid 40 bucks for the whole shebang! couldn't pass it up at that price.
adios amigos; :icon_biggrin:
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Yeah, I've played one of their classic 30s (I believe was the model with 4 EL84s) that sounded pretty good to my ears, but it seems like they did too many $0.05 & $0.10 short cuts to think its an amp that would really stand up over time, if you know what I mean.