1974 Super Reverb

[mresistor]

I took in a 1974 Super Reverb and the owner says he would like me to check it out. I plugged it in and it sounds pretty darn good. I haven't checked bias yet and have only done some repair to the cabinet. Seems it was dropped or struck hard on the upper front right side and it cracked the wood joint cleanly at the seam. It's been glued and clamped back together. Thankfully the split only went about halfway to the back.
This has to be one of the cleanest Fender chassis I have seen. It looks almost new inside and it is hard to really tell if someone has been in there. It looks like someone has replaced most of the preamp plate resistors with 1 watt carbon comps. But they missed one, as you can see in the pics. The amp has had one 20uf PS filter cap relpaced. The other 4 are the old original Mallorys. Even the front panel has very little damage to it, it looks really nice with the engine turned effect clearly visible.


Honestly, my thought is to maybe just replace the PS filter caps and leave it as it is, since it is sounding very decent. And check bias. Note this has that bias balance setup. I am tempted to change it to AA763 style, but it is working with no noticeable hum. Just have to see if the bias is optimum.


Would like your educated opinions. Customer left it up to me, so I would like to know what you think. thank you .   


~Jim

[mresistor]

Aside from the fact that its that crazy CBS pastel wiring which apparently is all above board, and it has those little caps on the power tubes, this is a pretty nice amp.

[mresistor]

I believe this Super is an AA270 circuit. But the bias cap is 80uf 70v.  Strange.

[sluckey]

It is an AA270. There are two bias caps. The 100K/1W CCs are stock. I believe in the old rule, "Don't fix it if it ain't broke". I'd leave the bias circuit just as it is. It works fine. Changing filter caps (including both bias caps) is a good preventive maintenance task.

[mresistor]

I totally agree Sluckey..  was what I was thinking.  Still puzzled as to why an 80uf lytic in the bias circuit board.  But can find a suitable replacement.

[sluckey]

Still puzzled as to why an 80uf lytic in the bias circuit board.  But can find a suitable replacement.

They look stock to me. And they match the e-caps on the board also. But the schematic calls for 50µF. If I was gonna replace them I'd use 47s or 50s just to keep it in line with the schematic. But regardless, 50 or 80, not much difference.

EDIT... I just noticed that the earlier AB568 circuit used 80µF for the single bias cap. I bet Leo had some leftovers that he put in your amp.

[Ed_Chambley]

Yea buddy a Super Reverb sounds great.  I cannot see the ends of the filter caps to know if they have blisters, but a replacement is not a bad idea.  I do not like a Super Reverb biased hot so I change the Balance to adjust instead.


Favorite mod to a super is switching out the NFB.  On my main 73 I just disconnected it.  Makes the amp breakup at a level that is much more manageable.  I just recapped mine and used some Yellow Mallorys I had replacing some of the originals and a couple of 715 Orange Drops.


I changed the tonestack values to Marshall spec and lowered the Bypass cap on V1 so I can use that channel as my overdrive and the Vibrato channel remained stock for Tremolo and Reverb.


But it is not your amp so I guess it is up to the owner if he wants mods.  These couple of simple things makes the amp much more versatile IMO.  The best things I did was replace the speakers using 2 Weber 20 Watt Alnico 10 on top of 2 Ceramic 30 watts on bottom.  I really love these amps, but they are too loud for most places I play these days, but not for my Amp Room.


There is value in the aint works don't fix guide I must admit, but the Super Reverb is begging for mods.

[mresistor]

I hear ya Ed..  thanks..

[mresistor]

I recapped the PS and the bias board. New e-lytics. Played it myself and it sounds really nice. But, the client says he thinks it isn't as loud as it was. Maybe that is true, but I didn't dime it before, and we dimed it today. Honestly, I think my Hoffman Single DR is louder, but I'm running it through an EVM15L in a huge ported bass cab I think that speaker is way more efficient than the stock alnicos in a Super Reverb. I'm also using JJ6V6's in my Single DR too, and everyone says they are more like a 6L than a 6V. I've found that to be true as well. 
The bias voltage in the super was running -47v which is pretty spot on.
I'm kind of thinking this Super Rev will come back for some more work. Namely to get rid of the bias balance and change to AB763 bias setup - as well as recapping the preamp e-lytics. I asked the client if he has a spare 6L6 duet of a different brand to try them and see if there is a difference.


Anyone have any thoughts on what would cause a 1974 Super Reverb to lack apparent power?

[Paul1453]

Ed suggested the NFB.

A switch or switch/pot in the NFB might be something to experiment with.   

[P Batty]

Re: loss of power. You've got a pretty severe impedance mismatch between the EVM (8Ω) and what the Super Reverb wants (2Ω). The EVM is probably getting less than 10 watts. Also, the Super Reverb has a nearly open back, which gives a more "open" sound whereas your ported enclosure is probably more directional. You can re-purpose the death-cap switch for NFB- 3kΩ one way, 820Ω the other.

[mresistor]

PBatty, I am not running the SR into the EVM15L. I am running my Single Channel DR into the EVM15L. The SR is output to its cabinet and 4x10 Alnico speakers. I was just making a comparison; and you are absolutely right that it is not a "fair" comparison at all.  But the client said he thinks the Super does not sound as loud as it did. Granted the room acoustics are different, so he has taken it to check out in his environment(s) and will let me know. I was wondering if anyone had thoughts as to loss of "apparent" power in an AA270 Super Rev after a PS recap. Feedback alteration is a possible fix, but I am trying to stay with the stock circuit.

[mresistor]

 Also, there might have been more effect on perceived output due to the placement of the amplifier when we played it. We had it sitting right in front of the 2x15 bass cabinet. This could have subdued the back wave off the Super Reverb.

[P Batty]

Mea culpa. That all makes sense now. If it comes back you'll have to start looking at voltages.

[mresistor]

As I layed there last night I had that same thought, need to check all the PS nodes again and see if something changed.