Hello friends. A buddy of mine asked me to refurbish his twin reverb which happened to be a silver face type and see if I could black face it. I am attaching it’s schematic. When I studied it, I realized that it’s not your typical grid bias type amp... I mean it is the grid bias but it’s done differently on this amp. Any of you who had an experience working on this type of schematic, would you recommend black facing it at all or leave it as it is? Is it recommended to install more traditional 10k bias pot?
Lastly, The bypass capacitors Look different ( see photo, the white one) - They appear to be sealed. Would you recommend replacing them?
Questions about modifying 79’ Twin Reverb amp.
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Questions about modifying 79’ Twin Reverb amp.
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Stevem
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Re: Questions about modifying 79’ Twin Reverb amp.
Those white Mallory bypass caps are the ones used in the 70's amps.
What to do to mod the amp fully hinges on how your buddy will use the amp, clean and hot gained up solo sounds, clean sounds only, crunch tones and distorted sounds , blues playing?
These questions above need to answered to reply to your basic question as to what to do .
First off, yes it's high time to replace all the electrolytic caps in the amp and to test the tubes.
Blackfacing the amp is not the way to go for solo sound, as fender amps suffer from too much bass when cranked up and blackfacing a silverface amp can make that even worse!
Another thing that is certain is that those Utah speakers the amp was made with was done by the CBS engineers for bottom line cost factors, not for tone.
In short the best thing to do with one of these amps once it's in good basic condition is to get better speakers in it and if it will need to produce good solo / gained up sounds then need to do two mods to the amp.
1) make the normal channel into one with more Mid's, more gain and less Bass.
2) mod the amp so you can get reverb on both channels.
3) get a A/B foot switch so you can go between both channels on the fly.
Also on the list if the amp is played loud enough when soloing is to reduce the amount of global feedback in the amp and back down on the gain of the mixer tube, or the PI tube.
All off these and many other amps like Fenders have this feedback circuit and it helps to keep the amp sounding clean , but only up to a point!
Once the amp is played loud enough and that point is reached then the amp leaps right into distortion, so in short these is no middle ground unless you get such with a stomp box!
This is yet another reason why these amps are not great for solo work, remember Loe Fender them to replicate acoustic guitar sound and never dreamed player would Crank them up to the point of clipping / distortion, and to make matters worse the CBS guys on these 100 plus watt amps used a ultra lenear output transformer with further helps to keep the amp sounding clean !
Your buddy should first tract down a music store where he can play cranked up thru a Voc ac15 or ac30 to see what he thinks of a amp with more mids and no global feedback circuit before you nail down any mods to do to the twin.
Really if you ask me unless your buddy only wants to set this amp up for clean playing only and will drag around another amp to switch to for solo work then I think a different amp will be the best way to go!
These high wattage Fenders with the UL output stage are not collector items and never will be , so he might just be best off trying to sell it and then get a 30 watt RMS 2-12" speaker amp that will give him good solo tones and feel if that's what he needs !
What to do to mod the amp fully hinges on how your buddy will use the amp, clean and hot gained up solo sounds, clean sounds only, crunch tones and distorted sounds , blues playing?
These questions above need to answered to reply to your basic question as to what to do .
First off, yes it's high time to replace all the electrolytic caps in the amp and to test the tubes.
Blackfacing the amp is not the way to go for solo sound, as fender amps suffer from too much bass when cranked up and blackfacing a silverface amp can make that even worse!
Another thing that is certain is that those Utah speakers the amp was made with was done by the CBS engineers for bottom line cost factors, not for tone.
In short the best thing to do with one of these amps once it's in good basic condition is to get better speakers in it and if it will need to produce good solo / gained up sounds then need to do two mods to the amp.
1) make the normal channel into one with more Mid's, more gain and less Bass.
2) mod the amp so you can get reverb on both channels.
3) get a A/B foot switch so you can go between both channels on the fly.
Also on the list if the amp is played loud enough when soloing is to reduce the amount of global feedback in the amp and back down on the gain of the mixer tube, or the PI tube.
All off these and many other amps like Fenders have this feedback circuit and it helps to keep the amp sounding clean , but only up to a point!
Once the amp is played loud enough and that point is reached then the amp leaps right into distortion, so in short these is no middle ground unless you get such with a stomp box!
This is yet another reason why these amps are not great for solo work, remember Loe Fender them to replicate acoustic guitar sound and never dreamed player would Crank them up to the point of clipping / distortion, and to make matters worse the CBS guys on these 100 plus watt amps used a ultra lenear output transformer with further helps to keep the amp sounding clean !
Your buddy should first tract down a music store where he can play cranked up thru a Voc ac15 or ac30 to see what he thinks of a amp with more mids and no global feedback circuit before you nail down any mods to do to the twin.
Really if you ask me unless your buddy only wants to set this amp up for clean playing only and will drag around another amp to switch to for solo work then I think a different amp will be the best way to go!
These high wattage Fenders with the UL output stage are not collector items and never will be , so he might just be best off trying to sell it and then get a 30 watt RMS 2-12" speaker amp that will give him good solo tones and feel if that's what he needs !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: Questions about modifying 79’ Twin Reverb amp.
Restore it to good working order and then see what you have. Some people really like these amps. I’d replace all of the electrolytics, and look closely for other issues like drifted, burned, or cracked resistors, especially around the power tube sockets. Consider new power tubes if they are a mixed lot, and/or are not close in idle current. For the bias, I’d replace the 33k resistor (off the balance pot, going to ground) with an 18k in series with a 25k trimmer. That way you have both bias and balance controls.
Re: Questions about modifying 79’ Twin Reverb amp.
Stevem, thank you for your very detailed answer. That actually helps a lot. I will restore it to its original tone and then see what the owner wants. So far I found a signs of a short circuit on the 3 power tube socket ( replaced), two damaged rectifier diodes and shorder B+ secondary on the power transformer... Wow... while I am waiting on the replacement to arrive, I will replace those white mallory bypass caps and create this adjustable Bias pot. Thank you Martin for your advice!
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Stevem
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Re: Questions about modifying 79’ Twin Reverb amp.
Please note that Fender output tube sockets suck!
They loose there gripping power rather fast with heat loads and vibrations.
I good idea it to atleast replace that socket that had the failed resistors on it with a tan Marshall style socket as these have opposed grippers that rarely if ever get loose!
Also for how cheap they are I would concider replacing all the rectifiers in the amp with noise hash reducing UF type fast recovery diodes, like a UF4008.
They loose there gripping power rather fast with heat loads and vibrations.
I good idea it to atleast replace that socket that had the failed resistors on it with a tan Marshall style socket as these have opposed grippers that rarely if ever get loose!
Also for how cheap they are I would concider replacing all the rectifiers in the amp with noise hash reducing UF type fast recovery diodes, like a UF4008.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Questions about modifying 79’ Twin Reverb amp.
The bad diodes and HT winding imply possibly shorted main filter cap(s.) Make sure the fuse is correct. All those went bad on an amp I worked on and the fuse was way to high to prevent it.
If it says "Vintage" on it, -it isn't.
Re: Questions about modifying 79’ Twin Reverb amp.
it is time for the update. I received the replacement power transformer and successfully installed it. Next thing, I found had a short inside the stand by switch (!). I replaced it. The amperage was OK after that. I installed all the tubes in anticipation of the beautiful vintage guitar tone oozing out of a pair of a vintage speakers but all I heard just some hum then my fuse blew. I tried another fuse and then I saw my two outside power tubes red plating like everything. I stopped the voltage and checked the Negative Bias supply, but saw none. I checked the AC voltage before the Bias diode and saw 60V AC, but nothing after the diode. The schematic asks for .5A 400v diode, I had 1N5408 diode ( which is rated at 3Amps and 1000V). However, I still get may be just 5V DC on the Anode side of this diode. Does anyone know what the problem is?
Last edited by chikov on Fri Dec 14, 2018 4:50 am, edited 2 times in total.
Re: Questions about modifying 79’ Twin Reverb amp.
Blown cap? Measure other components in the bias circuit, too.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Questions about modifying 79’ Twin Reverb amp.
Wait, did you deal with filter caps?
I build and repair tube amps. http://amps.monkeymatic.com
Re: Questions about modifying 79’ Twin Reverb amp.
Yes, I replaced all the Filter Caps. I did replace the 70 uf and the 80uf caps in the negative Bias circuit. I need to check this crazy Output tubes matching pot...
Another interesting thing, while I have barely any voltage on the anode side of the diode, I have about 380VDC on the cathode side... Is it normal? Plus, without the output tubes my B+ voltage at the Standby switch is about 880VDC... I guess it is normal for it to be this high when the output tubes are not there to conduct the electricity...
Another interesting thing, while I have barely any voltage on the anode side of the diode, I have about 380VDC on the cathode side... Is it normal? Plus, without the output tubes my B+ voltage at the Standby switch is about 880VDC... I guess it is normal for it to be this high when the output tubes are not there to conduct the electricity...
Re: Questions about modifying 79’ Twin Reverb amp.
I suspect you have installed the wrong PT for this amp. This amp uses a FWB rectifier and the PT will have half as much AC voltage as the older TRs with a conventional rectifier.
You will either have to modify the power supply or get the proper PT. Meanwhile, you're killing your filter caps and tubes.
Re: Questions about modifying 79’ Twin Reverb amp.
Yes, my tubes were redplating and the voltages do not make any sense.
Here is the photo of the transformer:
Is it a wrong one? If so, how do I go about to modify it?
Here is the photo of the transformer:
Is it a wrong one? If so, how do I go about to modify it?
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sluckey
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Re: Questions about modifying 79’ Twin Reverb amp.
Yes! That PT is meant for the older amps such as AB763 that don't use a FWB rectifier. You cannot use that PT with that amp without modifying the power supply. Basically you need to replace the FWB with a conventional FW rectifier. You also need to make some subtle changes in the bias supply. Use the AB763 TR schematic as a guide.
Or, just get the proper PT. This may be easier. Here's a proper replacement for your PT... https://www.hammfg.com/files/parts/pdf/291ZZ.pdf
Either way, don't power the amp up again until you have this resolved. There's a good chance that those new caps are already damaged. They have definitely been stressed! The bias supply may also have some damage.
Last edited by sluckey on Fri Dec 14, 2018 12:20 pm, edited 1 time in total.
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Stevem
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Re: Questions about modifying 79’ Twin Reverb amp.
When you get back to checking the bias circuit do not reinstall the output tubes with out confirming that you have at least -48 volts on pin 5 of each tube.
Also note that without the output tubes in circuit and idling your V+ at the center tap of the OT will be near 500 volts.
Also note that without the output tubes in circuit and idling your V+ at the center tap of the OT will be near 500 volts.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Questions about modifying 79’ Twin Reverb amp.
Thank you very much for all your help. If I am to replace the diodes, What type would you recommend?