Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on July 20, 2025, 04:59:18 am
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The Bugera is essentially an SVT based amp. On power up Im getting no filament voltage on the first two preamp tubes: V10 and 11. All other valves heat up fine.
From looking at the schematic it appears that the preamp tubes use rectified DC for the filament supply, and there’s some relay switching (X50 connects to X63 in reality) applied.
Before I delve further (there are a myriad of clips and bolts and screws to get to get to the various boards and components), am I right in what I think I’m seeing, and I’m looking for a fault in the DC rectify/supply area or the relay switching?
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Hi Jennings
You need to see Heater volts across R77 and R73 )probably around +7.5 DC or so)
This transfers to Connector X76-1 and X76-2.
The components past R151 are irrelevant!
Horrible circuit complications :l2:
Regards
Mirek
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Yep, it’s a royal pain with all of the added bells and whistles…sounds great when it works, but a beggar to work on…plus the arrangement of the PCBs means that I’ll have to probably remove all twelve of the output tube socket and spring retainer bolts, two PCB retainer bolts and twelve cable clips just to turn the PCB round and access the rectifier component if they’re faulty.
A quick check of the DC at X77 and 76 reveals no DC supply.
Pic attached shows what I’m staring at…blue ring are the connectors which lead off to the X77 cable clip on the preamp chassis. Green ring shows the area of the output tube board being used to also house the DC rectification circuitry, I think (haven’t peeped underneath yet). Purple ring shows the main power in board, which feeds the power and standby switches and filament tx etc.
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I’ve identified the legs of the diodes in the DC filament rectifier area using a dentist mirror, so could try a quick test without speculatively removing the board at this stage. All four test low volts in diode test mode both directions with the meter. Circa 0.3V on one meter and 0.5V on my backup. Both directions. So I’m figuring the first thing is going to be to pull that output tube PCB after all. That’ll have to wait for a weekend, I think.