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Post subject: 1975 Twin Reverb Rebuild
Posted: Sun Jun 16, 2013 3:00 pm
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I bought a 1975 100W MV Twin about a year ago and I could never get it to sound right. The bottom end gets really nasty when it's turned up past 4 and I have tried so many things to fix it. After some posts on other forums someone suggested I try this one. Here is what I have tried so far:

new tubes
new speakers
converted bias balance to bias adjust
removed push/pull master volume control
changed filter caps and electrolytics
removed faulty hum balance pot and replaced it with the two 100 ohm resistors
I also tried the blackface values in the bias and phase inverter circuits

I took it to a tech and he said that the output transformer might be bad. The chassis is a little bit dirty, the circuit board is a little warped, and the tube sockets are a bit shoddy, so I decided to rebuild the amp.

I was wondering if I should build the amp back to blackface specs or silverface (without the parasitic suppression caps). I like good clean tones and while I don't mind a slight bit of breakup, clean is what I am looking for. If I decide on blackface and go all out, which transformers would need to be changed to complete the conversion? Any advice is welcome as I have never done anything more than changing components and simple mods.


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jun 16, 2013 3:18 pm
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Try a new choke. Those trannies are often overlooked when troubleshooting and they really play a bigger role in amp performance than many realize.

Arjay

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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jun 16, 2013 6:11 pm
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Any recommendations for a new choke? How about transformers?


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Mon Jun 17, 2013 12:20 am
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JZL, the choke is an inductor and easily checked for perfomance( Arjay has a good point). The power and output transformers are probably the last pieces of the puzzle, I'd mess with. They're built like tanks. You might want to retension- deoxidize all of the tube sockets, check all grounds, and clean all pots and jacks. If you can optimize this amp, as-is, I think you're better off, than a total rebuild. Seems as though you've done the most important work already. Art

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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Mon Jun 17, 2013 9:22 am
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aclempoppi wrote:
Seems as though you've done the most important work already.


Perhaps so, but the amp is still not operating correctly. Which leads me to believe further troubleshooting is warranted.

Arjay

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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Mon Jun 17, 2013 4:52 pm
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So I am going to replace the circuit board hopefully this weekend, and I was still wondering about the AB763 circuit. Does this circuit still work well with the bigger power transformer? What would be the tonal differences between an original AB763 circuit and an AB763 circuit with the 100 watt silverface transformers?

Also, how should I go about checking the transformers to see if they are operating correctly?


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Mon Jun 17, 2013 5:32 pm
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My experience is the "AB763" tone that is so hyped is largely in smaller Fenders (dual 6L6GC or 6V6GT DR --- running tube rectifiers.

The AB763 TR is powered by six heavy duty solid-state diodes in the rectification section. I don't think you'll hear too much difference between this circuit and the AA270 or later circuit. With both amps R&R'ed up to snuff with new 'lytics, good coupling caps, and steady bias supply.

Only thing of advantage maybe the bias pot in the AB763 versus a hum/balance pot in the AA270 or later circuit.

As for the output tranny, I beleive the BF and SF (up to the 100watt and the ultralinear TR) used the same 125A29A (022689) tranny.

BTW... best best diodes to replace the stock OEM ones, are good UF-5408. Fast switching, very quiet. Don't need bypass caps to cut the solid-state hash.

Good luck on your amp.


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Wed Jun 19, 2013 1:09 am
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Thanks for the advice guys. I'm gonna redo the circuit board, install new tube sockets, and then test the transformers. I'll post some pictures along the way. Any tips on the lead dress?


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Wed Jun 19, 2013 2:23 am
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Lead dress as in soldering and wiring of sockets?


:?: :mrgreen: :?:


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Wed Jun 19, 2013 12:34 pm
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BMW2002Ti wrote:
Lead dress as in soldering and wiring of sockets?


:?: :mrgreen: :?:


Well there are some long wires going to and through the main board. I guess the stock layout the way to go.


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Tue Jun 25, 2013 2:39 pm
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So I have finished soldering the new tube sockets and I am waiting on a new circuit board, in the meantime I have an interesting question for you guys. I always use the vibrato channel on my twin for guitar and I like the sound of the 100 watt Super Bassman for playing bass. Would it work to wire the normal channel like the Super Bassman and have a hybrid amp head? There are minor differences between the TR "normal" channel and the SB "bass instrument" channel:

no mid pot
bass pot has 8.2k resistor to ground
cap from treble to volume pot
no cap coming off of pin 6 of V1

There are some differences in component locations as well. Has anyone successfully made a hybrid like this before? These are the schematics I am referencing:
Twin Reverb AB763
Super Bassman CFA7002


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Tue Jun 25, 2013 3:07 pm
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JZL, the hybrid channel design is possible. Although, I've never tried a guitar-bass amp mod. I have rebuilt a '68 Bassman to my preferences. The bass channel is an 864 Bassman design, the normal channel is a generic AB763(BandMaster) with a little more mid-range. No harm in trying!!! Art

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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Tue Jun 25, 2013 3:13 pm
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Since you do in fact have a middle tone control in your "NORMAL" channel, follow the schematic for a late-model Bassman 135 and configure the tone stack accordingly.

Arjay

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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Wed Jun 26, 2013 1:39 am
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Thanks for the help guys, I am really excited about this project. I was comparing the two schematics and noticed that in the Bassman the 220K resistor (red) comes before junction X instead of after, replacing the .047 capacitor (also red) in the AB763. Should I add a capacitor there in series to get it to the Bassman specs, or does it even matter? Is the difference mostly in the tonestack?
Image
AB763 vs CFA7002 by esbkeysfan, on Flickr


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Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Wed Jun 26, 2013 2:27 am
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JZL, my take on this mod would be that the power amp section will remain the same(763). It's the pre amp sections that you're looking at. The usual bass tonestack would run three .1 caps(with deep switch engaged). Maybe I'm way off base from your intentions! Art

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