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5u4g for 5ar4?

12/15/1997 5:33 PM
paul kreimer	5u4g for 5ar4? My bluesbreaker profect calls for a 5ar4 rectifier but my sovtek one has turned up faulty, I know you can use a ar4 in an amp that calls for a 5u4g but what about the other way around. Do I have to wire up the #1 pin (what is pin one?), that isn't on the ar4 for the 5 pin u4g. Also what are the major tonal differences? Thanks Paul

12/16/1997 9:02 AM
Doc	Paul: The 5AR4 and the 5U4 have some differences. In some circuits they can be interchanged, but a few cautions should be observed. The 5U4 draws 3.0 amps of 5vac filament current. The 5AR4/GZ34 only requires 1.9 amps. If the power transformer's 5 volt winding is rated at 3 amps, it will support either tube. Some transformers are designed to be used with a 5AR4 or 5Y3 and will have a 2-amp rectifier winding, and may possibly overheat the windings with the extra current demand of the 5U4. The 5U4 uses the coated filament as its cathode, wired between pins 1 and 8. The 5AR4 has an indirectly heated cathode element, which is internally connected to pin8, and heater filament connected between pins 1 and 8. The positive rail of the rectified high voltage gets connected to the cathode of the rectifier. This means it should be wired to pin8 for the 5AR4, but could be wired to pin 1 or 8 in a circuit calling for a 5U4. If it is wired to pin8 in a 5U4 circuit, you can substitute the 5AR4 into the socket and not have to worry about either the tube or the transformer, but continue reading. The 5U4 has a greater resultant voltage drop across it than a 5AR4 does, due to the cathode construction and element spacings. The 5AR4 was developed to have a slow warmup time, use less filament current, and have a lower voltage drop than conventional filament rectifiers. For instance, the 5U4 may have 30-40 volts drop across it in a typical power supply, where the 5AR4 will have only a 10-15v drop. So if an amp was designed to obtain a certain high voltage with the lower efficiency 5U4 and you just plugged in a 5AR4, the power supply voltages may all be too high for filter capacitor rating and possibly for the power tubes & associated bias voltage. However, if the circuit was designed for the voltages produced with the higher efficiency 5AR4, and the transformer has a 5v/3amp filament winding, it is safe to operate the amp with the 5U4. The power supply voltages will be lower. The amp will sound slightly "browner", but only if the bias voltage is set for the new lower anode voltages. For a cathode biased (self biased) output stage, you can disregard the comments about bias voltage setting and run either tube for the sound you prefer. In general, solid state rectifiers have negligible voltage drop an give the amp a loud immediate response. The 5AR4 tube rectifier will sound somewhat softer and allow a small amount of voltage droop or sag, but will sound reasonably live. The 5U4 will sound softer yet, and a little rounder, and will exhibit more sag under high amplifier loads. The 5Y3 has lower ratings than any of the above types, with respect to maximum current it can supply and peak inverse voltage it can withstand. It has a 50-60 volt drop and appx. half the current rating of the other 2 tubes. Its use is in lower power circuits (10-15 watt amps) and it exhibits a large, easily noticeable (audible compression) sag. Hope this explanation isn't too confusing. Doc

12/16/1997 9:02 AM

Doc

Paul:

The 5AR4 and the 5U4 have some differences. In some circuits they can be interchanged, but a few cautions should be observed.

The 5U4 draws 3.0 amps of 5vac filament current. The 5AR4/GZ34 only requires 1.9 amps. If the power transformer's 5 volt winding is rated at 3 amps, it will support either tube. Some transformers are designed to be used with a 5AR4 or 5Y3 and will have a 2-amp rectifier winding, and may possibly overheat the windings with the extra current demand of the 5U4.

The 5U4 uses the coated filament as its cathode, wired between pins 1 and 8. The 5AR4 has an indirectly heated cathode element, which is internally connected to pin8, and heater filament connected between pins 1 and 8. The positive rail of the rectified high voltage gets connected to the cathode of the rectifier. This means it should be wired to pin8 for the 5AR4, but could be wired to pin 1 or 8 in a circuit calling for a 5U4. If it is wired to pin8 in a 5U4 circuit, you can substitute the 5AR4 into the socket and not have to worry about either the tube or the transformer, but continue reading.

The 5U4 has a greater resultant voltage drop across it than a 5AR4 does, due to the cathode construction and element spacings. The 5AR4 was developed to have a slow warmup time, use less filament current, and have a lower voltage drop than conventional filament rectifiers. For instance, the 5U4 may have 30-40 volts drop across it in a typical power supply, where the 5AR4 will have only a 10-15v drop. So if an amp was designed to obtain a certain high voltage with the lower efficiency 5U4 and you just plugged in a 5AR4, the power supply voltages may all be too high for filter capacitor rating and possibly for the power tubes & associated bias voltage. However, if the circuit was designed for the voltages produced with the higher efficiency 5AR4, and the transformer has a 5v/3amp filament winding, it is safe to operate the amp with the 5U4. The power supply voltages will be lower. The amp will sound slightly "browner", but only if the bias voltage is set for the new lower anode voltages. For a cathode biased (self biased) output stage, you can disregard the comments about bias voltage setting and run either tube for the sound you prefer.

In general, solid state rectifiers have negligible voltage drop an give the amp a loud immediate response. The 5AR4 tube rectifier will sound somewhat softer and allow a small amount of voltage droop or sag, but will sound reasonably live. The 5U4 will sound softer yet, and a little rounder, and will exhibit more sag under high amplifier loads. The 5Y3 has lower ratings than any of the above types, with respect to maximum current it can supply and peak inverse voltage it can withstand. It has a 50-60 volt drop and appx. half the current rating of the other 2 tubes. Its use is in lower power circuits (10-15 watt amps) and it exhibits a large, easily noticeable (audible compression) sag.

Hope this explanation isn't too confusing.

Doc

12/17/1997 7:18 AM
Jim S.	Slight correction to the last post: The cathode in these typical full-wave rectifier tubes (5Y3, 5U4G, 5AR4) is connected to pins 8 and 2 (not 8 and 1).

12/17/1997 12:19 PM
Doc	Thanks Jim, you're right. There's no connection at all to pin 1. I guess my answering mechanism got screwed up because the question asked about connecting pin 1. Filament pins are 2 & 8. Doc

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