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Fender Deluxe Reverb Reissue Modifications April 8, 2001

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Fender Deluxe Reverb Reissue Modifications April 8, 2001 Fender Deluxe Reverb Reissue Modifications April 8, 2001 Posted by swingin.little.guitarman on Ted Weber’s Amp Board 10/8/2003 Part 1 External Modifications Tube Replacement, Bias, Speaker Replacement I just finished modifying a Fender 65’ Deluxe Reverb reissue amp, and had promised to share the details. This amp was a 1994 blonde tolex reissue. Serial number was BLD-00021, one of the first of the reissue amps. It belongs to my good friend “The Hof.” The 65’ Deluxe Reverb is Fender’s attempt to bring back the famous blackface amp of the 1960’s. The general complaint concerning the reissues is that they don’t sound like the old 60’s amp. They tend to be overly bright and brittle sounding, particularly at low volumes. The overdriven harmonics tend to be harsh instead of smooth and compressed. Player touch sensitivity and dynamics are virtually non- existent. These are all valid complaints on the reissue amps I’ve played. The construction of the reissue amps versus the point to point wiring of the original amps makes these amps a bit more difficult to modify. Many people shun these amps simply because of the circuit board construction. I’ve found the circuit board construction to be a very small factor in the overall SOUND of the amp. The choice of components and their values is the biggest factor in determining the overall character of the amp. By modifying or replacing selected components I was able greatly improve the sound of this Fender 65’ Deluxe Reverb reissue. In my opinion, this amp sounds BETTER than many of the original blackface Deluxe Reverbs I’ve played. The goal of this project was to evaluate the amp, and bring back as much of the old 60’s sound without a total rebuild. The maximum parts and materials budget was $100, not including labor. The cost of the replacement speaker used can substantially increase the cost beyond $100. New replacement speakers can be purchased in the $35~$100 range. All of the speakers used for testing in this project were used. The standard precautions apply when working on tube amplifiers apply to this project. 1. The high voltages in these amplifiers can be dangerous. Make sure the amp is unplugged and all power supply capacitors have been discharged to prevent nasty and potentially deadly shocks. 2. The circuit board construction requires patience and a delicate touch to prevent damaging the board traces. A good 20 to 30 watt soldering iron and solder sucker are required. NO WELLER SOLDERING GUNS! 3. MOST IMPORTANT. If you are not familiar with and had experience working on electrical circuits STOP. It is better to pay an experienced tech to make these modifications for you than to risk injury or damaging your amp. Before disassembling the amp, I evaluated the amp and tried replacing a few external components to improve the overall sound. I removed all the preamp tubes from the amp. The stock Fender preamp tubes were Chinese made 12AX7’s and 12AT7’s. None of the tubes were noisy or micro phonic. After trying a variety of tubes I settled on a combination of NOS, new manufacture, and stock tubes for best tone. The normal channel preamp tube was not used. The socket was left empty (farthest on the right when viewed from the back of the amp). The owner seldom uses this channel so why burn the tube? Removing the tube from the socket changes the voltage/current of the Vibrato channel preamp tube. It provides a little more warmth and a slight increase in gain. If you regularly use the normal channel replace the tube. The change is tone is slight. The vibrato channel tube (second from the right when viewed from the back of the amp) was replaced with a NOS Phillips 5751. These are great sounding tubes with a little less gain than the stock 12AX7, but the tone and harmonics are superior to the stock Chinese tube. Also excellent was the GE 5-Star 5751 tube, however these are getting rather pricey. The Phillips 5751’s are still available for about $15. The reverb driver tube (third from the right when viewed from the back of the amp) was replaced with a RCA 12AU7. This tube is a low gain tube that is typically used as a driver in high-end amps. The gain is about 20% of a 12AX7 and 28% of the stock 12AT7. This particular amp had a bid of a mushy overdriven reverb sound. Beyond 4 on the reverb knob was about useless. This tube helped to tame some of the gain and cleaned up the reverb. There is a bit more detail to the reverb that was lost with the original Chinese 12AT7. The reverb return tube (fourth from the right when viewed from back of the amp) was replaced with a new JJ 12AX7S. This is JJ’s latest 12AX7 version and is an excellent all around tube. The vibrato tube (fifth from the right when viewed from the back of the amp) was left stock with the Chinese 12AX7 tube. This is the least critical tube in the amp. Any good, 12AX7 free from micro phonics is acceptable. Changing to a higher quality tube had virtually no audible impact on the sound of the amp. The power tube driver/phase inverter tube (sixth from the right when viewed from the back of the amp and next to the large power tubes) was replaced with a JJ 12AT7. Some people like to replace this tube with a good 12AX7 for more gain and drive. The 12AX7 gave a little more warmth and drive which was welcome when playing at lower volume levels. It’s certainly an option. As a complement to the new power tubes the JJ12AT7 was judge best for overall sound. The power tubes (seventh and eight from the right when viewed from the back of the amp) were replaced with new Electro Harmonix 6V6’s. These are the best tubes for the money that I’ve found. They are designed to handle higher plate voltages than the stock Sovtek tubes. Most of the other new replacement tubes weren’t even close to the Electro Harmonix 6V6’s in detail and harmonic content. I think they are the best of the new 6V6’s available at this time. I installed a set of my RCA black plate 6V6’s as a comparison. The RCA’s are still the best sounding of the 6V6’s. Definitely, the best choice if you’re not on a budget. However they are now $95 a pair and I wanted some money left in the budget for a replacement speaker. I have tried the Brimar 6V6’s in other amps with excellent results. I would rate them on a par with the RCA tubes. I did not have any Brimar 6V6’s available for testing during this amp project. The rectifier tube (farthest tube to the left when viewed from the back of the amp) was left with the stock Sovtek 5AR4. These are good tubes. Some people replace this tube with a NOS 5A4-G to reduce the voltages in the amp. Deluxe Reverbs are designed with the plate voltages on the power tubes exceeding most manufacturers’ design limits for the tubes. The higher voltages can produce some problems when used with many of today’s new 6V6 power tubes. I have not experienced any problems with the higher plate voltages and the Electro Harmonix 6V6 tubes. The plate voltage in this reissue amp was only a modest 407 volts. This is really quite low compared to many of the Deluxe Reverbs I’ve worked on. Many have plate voltages in the 450~460 volt range. This was probably an intentional design change on the part of Fender to allow for the operating parameter of the new 6V6’s. Since Fender has a short warranty on their amp tubes, this may have insured the stock tubes would last the new owner a reasonable period of time. The factory bias setting for the power amp tubes left a lot to be desired. I attributed much of the amps brittleness and lack of warmth to under-biased power tubes. I checked the plate voltage of the amp by removing the power tubes, turning on the amplifier, turning off the standby, and measuring the voltage from the number 3 pin on the power tube socket to ground. As stated above, the voltage measured 407 volts. I turned off the amp and reinstalled the power tubes. I turned the amp back on, turned off the standby, and allowed the tubes to warm up for approximately 5 minutes. Using my Allesandro bias meter I measured the tube current at 20 mA. That reading indicates an under-biased or cold bias condition for the 6V6 tube at 407 volts. A 6V6 tube has a maximum designed plate dissipation of 14 watts. In a fixed bias, class A/B amp the standard practice is to bias the tubes for 70%~80% of the maximum plate dissipation. The stock tubes were biased at 58% of maximum plate dissipation. With the new Electro Harmonix tubes installed I adjusted the bias for 27mA, yielding 11 watts of plate dissipation (0.027 amps * 407 volts = 11.0 watts) That works out to 78% of the rated maximum. For amps that are used at lower volume settings I’ve found biasing the tubes closer to 90% of maximum gives a little more warmth. With the Electro Harmonix tubes as well as the RCA black plates I have biased some amps as high as 100% with no detrimental effects.
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