Fender Twin Reverb (1972, CBS) - suggestions for “refurbishment”

[ccaudle]

I've found values as high as 47uF, 25v.

I found the data for that specific combination:
https://www.murata.com/products/productdetail?partno=RDEC71E476MWK1H03B
You can see that at 10V DC the capacitance is already around half of the rated value:

Just something to be aware of when using ceramic capacitors.

 

[Alva Goldbook]

That datasheet does not have a graph for capacitance vs. bias voltage, but you might be able to find that in a sheet for the specific part number (that datasheet covers a family of parts).
This page describes the effect:
https://www.murata.com/en-us/support/faqs/capacitor/ceramiccapacitor/char/0005

Well, if I recall correctly, the bias voltage was about 1.3VDC, so it should have a negligible effect. Within the tolerance range of most components.

 

[Alva Goldbook]

I found the data for that specific combination:
https://www.murata.com/products/productdetail?partno=RDEC71E476MWK1H03B
You can see that at 10V DC the capacitance is already around half of the rated value:
View attachment 142781
Just something to be aware of when using ceramic capacitors.

Interesting. So these will work fine for most bias arrangements for 12ax7s, but for 12au7’s, 12at7’s and other high current triodes, this effect will have to be considered. Thank you for letting me know!

 

[Whoops]

Hi guys,

I’ve picked up a 1972 fender twin reverb, in good nick and entirely original (down to tubes even)

Anyone have any suggestions of things to look over, be aware of?

I'm late to the party.
I've serviced many Silverface Fender reverb amps.

- Filter caps need definitely to be replaced, if they didn't failed yet they will fail, after all these years they are not good. Seen countless of failed Filter caps in those amps over the years. Modern Electrolytic caps (from a quality brand) are much better than any Lytic from the 70s.

- Check if you have the Death Cap installed there, if you have remove it and change the power wiring assembly with a grounded 3 prong mains cable

https://robrobinette.com/Death_Cap_and_Ground_Switch.htm

- Modify the Bias circuit from the worthless "Balance Bias" in silverface amps to the standard Bias as used in Blackface amps. You will not be able to set the Bias properly with the "balance bias" circuit. The conversion from Balance Bias to Blackface Bias is quite easy

https://el34world.com/charts/bias_conversions.htm

 

[13engrsapper]

and get earmuffs

 

[Silas]

I'm late to the party.
I've serviced many Silverface Fender reverb amps.

- Filter caps need definitely to be replaced, if they didn't failed yet they will fail, after all these years they are not good. Seen countless of failed Filter caps in those amps over the years. Modern Electrolytic caps (from a quality brand) are much better than any Lytic from the 70s.

- Check if you have the Death Cap installed there, if you have remove it and change the power wiring assembly with a grounded 3 prong mains cable

https://robrobinette.com/Death_Cap_and_Ground_Switch.htm

- Modify the Bias circuit from the worthless "Balance Bias" in silverface amps to the standard Bias as used in Blackface amps. You will not be able to set the Bias properly with the "balance bias" circuit. The conversion from Balance Bias to Blackface Bias is quite easy

https://el34world.com/charts/bias_conversions.htm

Cheers!
yes all electrolytics being replaced - just waiting on a part to finish the filter caps - the others are done

It’s an export with selectable voltage and no death cap - wired three prong with proper earth as standard, so one less thing to sort.

 

[Quantum J Audio]

and get earmuffs

Nothing like the break up of a Twin at full pelt. I tend to use an attenuator to get the sound. Crank it up and run back to the control room! When I build my studio I'm planing to remove all chassis from cabs and have them in the safety of the control room.

If you want to use in a house with other people or neighbours close by, worth building an attenuator.

 

[13engrsapper]

Nothing like the break up of a Twin at full pelt. I tend to use an attenuator to get the sound. Crank it up and run back to the control room! When I build my studio I'm planing to remove all chassis from cabs and have them in the safety of the control room.

If you want to use in a house with other people or neighbours close by, worth building an attenuator.

I lived next to one on stage for about a decade.

 

[Whoops]

Cheers!
yes all electrolytics being replaced - just waiting on a part to finish the filter caps - the others are done

It’s an export with selectable voltage and no death cap - wired three prong with proper earth as standard, so one less thing to sort.

Nice, now modify the Bias Circuit from the "Balance Bias" as used in Silverface CBS amps, to the standard Bias adjustment as used in most tube amps and used in the Blackface amps, so you can be able to set the Bias properly.
I don't do it for a while now, but there's plenty info on the net about it, also go to the el34world forum as it's the best forum on the web for tube amps, a lot of help over there specific to Tube Amps, specially classics like Fender/Marshall/Vox.

I will search also search on my Gerald Webber books the notes for that mod.

 

[Silas]

Nice, now modify the Bias Circuit from the "Balance Bias" as used in Silverface CBS amps, to the standard Bias adjustment as used in most tube amps and used in the Blackface amps, so you can be able to set the Bias properly.
I don't do it for a while now, but there's plenty info on the net about it, also go to the el34world forum as it's the best forum on the web for tube amps, a lot of help over there specific to Tube Amps, specially classics like Fender/Marshall/Vox.

I will search also search on my Gerald Webber books the notes for that mod.

Thanks Whoops, yes looking at either blackface bias, or bias trim additionally to the bias balance as on Rob Robinette’s notes. Any reason not to do it as suggested by Rob? : https://robrobinette.com/Silverface_Amp_Mods.htm#Convert_Bias_Balance