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Post subject: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 5:58 am
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What I have is a 1970 Twin Reverb and the only thing not stock original are the Ruby tubes.

I'm not an amp expert and I want to make sure that I do not do anything to change the condition of the amp. In otherwords, what do I do with it now? It has all original wiring and caps, including under the dog house.

It has original speakers, and it also has the cloth cover over the reverb spring.

What are my options with this thing? Any ideas?

Thanks for reading,
Bob W


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 12:43 pm
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For safe operation if that is your intent, have a technician replace ALL the electrolytic capacitor's and check voltage level's, resistor's, non electrolytic capacitor's as well as bias supply and transformer's. Prolly cost about $200 unless a major component is faulty.

If you intend on selling it, sell it "as is" many folks prefer to work on their own amplifier's.

GL, we won't be around much longer to help unfortunately. Other forums are available- google search.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 12:48 pm
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First , I'll check if the fuse is the right one. With a too powerfull fuse, power transformer is not protected in case of major amp failure, mostly power tubes.

New filter caps in a dog house is good idea. Use only good quality like F& T caps because you have a great amp.
Bias filter cap, new one too.

I will not replace signal cap ( non electolytic) these are the tone of your amp. Unless they are défective; easy to test for a amp tech.
Bring amp only to a tech who know how to work on vintage amp to keep their tone an value.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 1:22 pm
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Foggy memory -- does a 1970 have a 2-prong power plug?

If so you'll want that replaced with a 3-prong grounded power cable, and bypass the polarity switch to bring it up to modern safety standards.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 2:11 pm
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strayedstrater wrote:
Foggy memory -- does a 1970 have a 2-prong power plug?

If so you'll want that replaced with a 3-prong grounded power cable, and bypass the polarity switch to bring it up to modern safety standards.


+1000
It is the fist thing to do, the most important.

Forum Fender is closing march 20 :shock:


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 2:11 pm
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These are great tips - It does have a 3 prong adapter, and I'm not sure which circuit this amp has, as the paper tube chart shows AB763. The fuse is a 125v Buss mdl 2 1/2

And it figures that as soon as I become a member they are shutting down the board.

Pic of reverb cover - https://www.dropbox.com/s/2byye3b596pgz5x/Photo%20Mar%2009%2C%205%2041%2051%20PM.jpg?dl=0

Pic of inside of chassis - https://www.dropbox.com/s/wxhapfrpgxf6nbs/Photo%20Mar%2009%2C%205%2049%2000%20PM.jpg?dl=0

Pic of the power cord - https://www.dropbox.com/s/ssukl7rf7bbf60q/Photo%20Mar%2008%2C%207%2027%2023%20PM.jpg?dl=0

And this - https://www.dropbox.com/s/tq6c3gti286coi9/Photo%20Mar%2009%2C%205%2052%2034%20PM.jpg?dl=0

Bob


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 3:31 pm
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Looks pretty clean from the pics.
I see 3 minor repairs/replacements, one of which I would address.
The bias supply resistor(68k) going to the main board from the left connection on the balance potentiometer- the lead dress should probably be covered or replaced w/ wire and a resistor w/ shorter leads(its carrying negative DC voltage to bias the power tubes).
Even the doghouse electrolytics look decent- not meaning they are good but no obvious signs of rupture(should be checked/changed).
If that 3-way is an adapter and not a fixed 3-wire lead, I would replace it also.
The white coupling capacitor's(electrolytic) should be changed also as already stated.
The rest is just testing.
It is not an AB763 circuit, most likely an AA270 circuit.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 3:41 pm
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Thanks for the response, the 3 way is a fixed wire, no adapter on it. For the 68k resistor, I could just put insulation around it, right, like either electrical tape or something, or do you really recommend replacing it?

Can you tell me by any chance what circuit type this is? I would appreciate an expert opinion on it. Got it AA270

Also - what are your thoughts on replacing parts, as I don't want to devalue this thing by making changes.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 3:56 pm
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Routine maintenance and servicing issues aside, the only alteration to the amp that I'd make is to deep-six those OEM Oxford speakers for something more appropriate. Vintage (Chicago-made) Jensen EM1202 Vibranto's or C12NA's would be an outstanding sonic improvement.

Arjay

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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 4:14 pm
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OK - so it sounds like you are saying that I can replace the speakers guilt free, as it doesn't affect the "collectable" value of the amp, is that right?

I will for sure replace them with one of the 2 suggestions you made. I mostly play a fender 65 reissue or a Gibson SG through this amp.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 4:44 pm
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Few silverface Fenders can be considered as "collectibles" since they represent the epitome of CBS's nefarious meddling and they made so damn many of them. Twin Reverbs take a double hit because they're so heavy and there are few venues left where a 100-watt combo makes sense (if indeed there ever were any). I wouldn't deliberately trash one but you can remain guilt-free if all you do is upgrade your drivers to something more tonally tasty. Save the OEM Oxfords for "posterity" if you wish.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Fri Mar 13, 2020 5:36 pm
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Quote:
For the 68k resistor, I could just put insulation around it, right, like either electrical tape or something, or do you really recommend replacing it?
I wouldn't recommend electrical tape, but any hardware/electrical outlet should have a good selection of heat shrink tubing. Just slip it on to the resistor leads and heat er up leaving room to re-solder the connections.
+1 on the speaker upgrade if you plan on playing the amp.


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Sat Mar 14, 2020 6:31 am
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ok, will for sure shrink wrap that wire. So I read a lot about people converting the circuit on these to black face, does that make any sense to do that, seeing as how this amp basically hasn't been touched since it came off the production floor?

I mean, if caps and resistors need replaced, should I make the effort to convert it to the AB763 circuit?


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Post subject: Re: 1970 Twin Reverb - What to do with it
Posted: Sat Mar 14, 2020 9:51 am
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It is what many people decide to do w/ silver face amps. I have two 70's amps, I black faced my super reverb to AB763.
The bias "balance" pot converted to adjustable bias pot, allows the user to adjust the bias voltage to their preferred level.
The driver network, consists of mostly resistors. Adding a couple and changing values of others. Increases stability and provides more gain to the output.
There are other mods that can be done to tweek the tone if thats what you prefer.
I would recommend, restoring to a reliable functioning amplifier using the current circuit. Listening/playing for a time then making circuit modifications if desired.
Upgrade the speakers 1st, if that is your intent. Then changing the bias balance to bias adjust 2nd. Then altering the output section if you feel the need.


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