Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gregaussie on June 17, 2012, 06:55:27 am
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i have built a valve amp before, but not tried to clone a 'name' amp.
I want to try building a "fender 65 twin reverb".... looked at the various versions of the circuits... some are the reissue...some allow for 240v (which i need: australia) but none seem to match the parts listed...
Doug mentioned an AB763... but i am not sure about the transformers.... he mentioned 2 extra tubes needed... but what ones? and is there a circuit diagram that works as a 65 twin reverb from that parts list??? the numbers don't add up....
my other interest is in creating a vox ac30 clone... i notice doug has a shopping list for an ac30.... is it intended that this is the same as the vox?
same question about 240v
sorry if this all seems dense.... but some help would be gratefully received...:)
greg
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I don't know about " Doug shopping list" , I never read it for AC 30 but IMO a Vox AC 30 vintage is most desirable than Twin Reverb . TR is too powerfull , heavey . Vox AC 30 have a nice natural distortion, not too much powerfull but enough for most use.
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I have built both of the amps you are talking about.
The twin reverb circuit is AB763. It's loud and clean. A very nice amp to have around for getting the clean fender tone with reverb/tremolo.
Doug's AC30 is really cool also. It has the normal and top boost inputs minus the tremolo/vibrato circuitry found in a real ac30. I never really liked the vox tremolo/vibrato so when I found Doug's layout it was just what I wanted.
As for voltage selection....make sure you purchase a power transformer that has the primaries you require.
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I want to try building a "fender 65 twin reverb".... looked at the various versions of the circuits... some are the reissue...some allow for 240v (which i need: australia) but none seem to match the parts listed...
That amp is a late model REISSUE amp that is based on the original blackface AB763 Twin Reverb from the '60s era. There are a few differences, mainly the PT and the bridge rectifier circuit. The amplifier circuit is identical. I don't recommend you copying this amp because of the pcb construction. I would clone the original amp. But here's a service manual for the reissue, including parts list, if you wish to build the reissue.
http://www.el34world.com/charts/Schematics/files/fender/65_twin_reverb_%20manual.pdf (http://www.el34world.com/charts/Schematics/files/fender/65_twin_reverb_%20manual.pdf)
Doug mentioned an AB763... but i am not sure about the transformers.... he mentioned 2 extra tubes needed... but what ones? and is there a circuit diagram that works as a 65 twin reverb from that parts list??? the numbers don't add up....
Doug's AB763 is not a true AB763. The basic circuit is the same, but he modified it to use a different tremolo circuit. The two extra tubes he mentioned are 6L6s. The AB763 Twin Reverb uses 4 output tubes, while the other AB763 amps only use two output tubes. Doug sells an output transformer for the TR but I don't believe his TR power transformer has a 240VAC primary. Better to ask him. You cannot build a 65 Twin Reverb Reissue from Doug's AB763 parts list. If you want to use Doug's parts list, you'll have to build Doug's AB763 amp because his AB763 is a hybrid amp.
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Hammond makes a power transformer for overseas voltages. 290FEX.
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I've built both, and really like both. You can also easily change Doug's AB763 so the reverb is on both channels; this also allows you to jump the channels without phase cancellation. I just recently finished the TR and am playing it as is for a while, but I may eventually modify the normal channel in some way for more variety.
I live in Germany, and use the Hammond PT. Works fine.
Either way you will have a great amp, but they are different great amps.
steven
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... Doug's AB763 is not a true AB763. The basic circuit is the same, but he modified it to use a different tremolo circuit. The two extra tubes he mentioned are 6L6s. The AB763 Twin Reverb uses 4 output tubes, while the other AB763 amps only use two output tubes. ...
One other caveat regarding the AB763:
Almost all of Fender's blackface amps from late '63 - '67 were labeled "AB763" as well as by the name of the amp. The ones that don't specifically say "AB763" still carry very similar (if not identical) circuitry. Fender evolved to a single basic circuit, adding or deleting reverb/tremolo, with different number/types of output tubes for varying power, and differing speaker setups.
Where the key changes reside are in the power supply dropping resistors, and the feedback shunt resistor. Hoffman's AB763 gives one set of values, but the exact Fender blackface amp you're trying to recreate may have slightly different values. Be sure to cross-check Fender's original schematic (from the 60's) against Hoffman's layout and parts list to verify correct parts for your particular build.
Also, if you don't want a big, heavy clean amp, you might consider the other amps in the blackface lineup. A Pro Reverb is essentially a Twin Reverb with a half-size output stage (2x 6L6GC), the Vibroverb takes the Pro Reverb formula and swaps 1x 15" speaker for twin-twelves. The Super Reverb is 4x 10" speakers and not typically used for clean tones. The Deluxe Reverb downsizes to 1x 12" speaker and a pair of 6V6 output tubes; it's a good portable toneful package for clean or distortion, as long as you don't need 50w or better. And there are head-n-cab variations of some of these (or you can build your own fave circuit as a head/cab setup).