![]() Mods that keep the amp's original tone in place. The AB763 circuit is considered by most to be For information about theĪB763 model differences see this. Vibroverb, Tremolux and no-reverb Deluxe. The "763" in the model name comes from the circuit change dateĭeluxe Reverb, Twin Reverb, Super Reverb, Concert, Band-Master, Showman, Pro, Vibrolux, 'AB763' was Fender's internal model designation for the 1963Ĭircuit. ![]() Use just one hand when working on a powered amp. Lethal shock can run between your arms through your heart. Never touch theĪmplifier chassis with one hand while probing with the other hand because a High voltage then have an amp technician service your amp. If you have not been trained to work with Tube amplifier chassis contains lethal high voltage even when unplugged-sometimes EDIT: Wikipedia says that some older meters use moving coils, etc., while DMMs typically use shunt resistors see below.Includes modifications for the 65 Deluxe Reverb Reissue and Either way though, the user is given to understand that it is a current measurement, not a voltage measurement thus one doesn't have to do that nasty math of dividing voltage by one to get current. Plus in the case of my low-end EuroTubes probe that behaved so oddly, it was actually my DMM that was reading the current once the socket doohickie was in place so the sensing device would have been in the DMM, whether sense resistor, half-effect sensor, or the somewhat rare tootsie roll sensor. Interesting to hear these terms, but not really getting at the key point - which is, that by whatever means a bias probe of this sort measures current, the process seems to be disruptive to some amps. I will say, though, that in past threads on bias meters on this forum, many users have reported satisfaction with EuroTubes probes. Other than that, I wouldn't recommend the EuroTubes probes, based on both my experience & Lord Valve's warning. The user manual can be found here, but I don't see a description of its internal workings - so as suggests, you might want to ask. I don't know how the TAD BiasMaster works, but if it were me, I would make sure it uses the 1 ohm resistors. So I bought & assembled a Bias Scout probe kit courtesy of TubeDepot the Scout does use the 1 ohm resistors, and it works far better - with my DVM set up to read voltage, I get a stable reading from the probe as soon as the tubes are warmed up. The probe would settle down after maybe 10 minutes, but I didn't trust it. Something of the sort happened when I would use my Eurotube Octal Bias probe (plus my DVM set to read current) on my innocent little Laney Cub 10 amp - it was difficult to get a stable reading, because even after the tubes were warm, the readout on my meter would move up and down across a fairly wide range, as if at random, for many minutes - not quite what I'd call oscillation, since to me "oscillation" means something insanely rapid this was merely a constantly changing value, but even so it was unsettling. ![]() This would be fine, except that probes which measure current directly in this fashion can weird readings, including oscillation. My first probe was their low-end model, the "Eurotubes Octal Bias Probe," and it operates by measuring current and evidently the Pro One does as well see this brag quote from the EuroTubes product page: This definitely includes the various probes marketed under the EuroTube brand. Some operate differently: they measure current directly, not voltage, and are proud of it. In fact, all bias probes do not put a 1 ohm resistor in the cathode path. And I am not alone in my mistrust here is Lord Valve, who will give the closing argument for the prosecution - his biasing FAQ can be found at Duncan's Amp Pages at : I did not find this a satisfactory response. I wrote EuroTubes and they denied that customers had ever reported oscillation problems, let alone the slower instability I'd experienced. So I bought & assembled a Bias Scout probe kit courtesy of TubeDepot the Scout does use the 1 ohm resistors, and it works far better - with my DMM set up to read voltage, I get a stable reading from the probe as soon as the tubes are warmed up. Something of the sort happened when I would use my Eurotube Octal Bias probe (plus my DMM set to read current) on my innocent little Laney Cub 10 amp - it was difficult to get a stable reading, because even after the tubes were warm, the readout on my meter would move up and down across a fairly wide range, as if at random, for many minutes - not quite what I'd call oscillation, since to me "oscillation" means something insanely rapid this was merely a constantly changing value, but even so it was unsettling.
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