SF Deluxe Reverb filter cap mod

[Jazz-O-Sonor]

I wish to upgrade filter caps in my late ‘70 SF Deluxe Reverb modding it for more bass response and clean sound using two 100mdf-450w caps with two 220k resistors and another three 20mdf-500w with 1k and 4.7k resistors.
Actually in the filter can I have five 20uf-500w and they are all positioned with the positive polarity on the same side. All layouts of bigger fender amps shows the filter cap on the far left positioned backwards from the others .

So please I have some questions:
1)Should I change the polarity of the cap on the far left ( 100mdf-450w ) positioning it backwards from the others?

2)Or I can just do the cap mod respecting the same actual polarity?

3)In order to get a clean sound (jazzy) , about the two caps on the left is better to use 100mdf-450w or 100mdf-350w? Or even 220mdf-300w?

I am using 6L6’s and already replaced the original OT with a 8ohm vibrolux/pro-reverb style one and I will switch for a GZ34 instead of 5U4GB.
The speaker is a ‘12 Eminence Legend 1218.

thank you in advance , John

[Groovey Records]

Originally Posted by Jazz-O-Sonor

I wish to upgrade filter caps in my late ‘70 SF Deluxe Reverb modding it for more bass response and clean sound using two 100mdf-450w caps with two 220k resistors and another three 20mdf-500w with 1k and 4.7k resistors.
Actually in the filter can I have five 20uf-500w and they are all positioned with the positive polarity on the same side. All layouts of bigger fender amps shows the filter cap on the far left positioned backwards from the others .

So please I have some questions:
1)Should I change the polarity of the cap on the far left ( 100mdf-450w ) positioning it backwards from the others?

2)Or I can just do the cap mod respecting the same actual polarity?

3)In order to get a clean sound (jazzy) , about the two caps on the left is better to use 100mdf-450w or 100mdf-350w? Or even 220mdf-300w?

I am using 6L6’s and already replaced the original OT with a 8ohm vibrolux/pro-reverb style one and I will switch for a GZ34 instead of 5U4GB.
The speaker is a ‘12 Eminence Legend 1218.

thank you in advance , John

I would not change the polarity.

I don't know which cap value to suggest, The value swap you are suggesting seems very radical

[jhundt]

I'm a little confused by your description.

1) don't just change the polarity without understanding the circuit!! If you don't understand the circuit, you should NOT be doing this work!!!

If you wire 2 x 100mF caps in parallel, you will have a first-stage equivalent of 200mF. That won't work with a GZ-34.

If you wire two x 100mF caps in series you will have a first-stage equivalent of 50mF. You better check if that value works with a GZ-34. Rectifier tubes have a limit on the first-stage filter cap value, but I can't find the GZ-34 documentation at the moment.

Fender wired the first stage of many amps like this: 2 x 20mF in parallel to equal 40mF at 1050 V.

2) like I said above - you better learn more about the circuit before you go messing with this stuff!

3) "is better to use 100mdf-450w or 100mdf-350w? Or even 220mdf-300w?"

If you do this, use the 100mF/450. Higher voltage rating is safer. Forget about using 220mF caps. That will be too high a filter value for your tube rectifier.

I know that you are looking for a big, clean jazz sound. I would suggest that you forget about the tube rectifier, get one of those solid-state diode inserts (Weber makes them, or you can make one yourself for a few Euros). Go for 40mF at 500+ V in the first stage and 20mF at the later stages and you will be OK.

[Scott S]

Max input capacitor for a 5AR4/GZ34 is 60uF. I could be wrong, but I don't think the voltage rating doubles when you wire two caps in parallel, just series.

I wouldn't expect a huge increase in clean headroom just by bumping up the filter caps -- after bumping up the first cap to 60uF and the second (6V6 screens) to 47uF or so, the benefits will diminish. I agree with jhundt that if you want the cleanest, tightest sound possible from that amp, go with a solid-state rectifier before going crazy on the filtration.

- Scott

[celeste]

If you want tighter bass and more head room from just PS mods, ditch the GZ34 and put in SS diodes. UF's or schottky's will be quieter the the 1N variety. That will give you about 20v more on the B+, so make sure all your caps can handle that. I do not know the voltages in the circuit off the top of my head, so here is how to tell it the caps have a high enough voltage rating to use. Pull all the tubes, but the GZ34 and measure the voltage at the first cap. If it is under 420v, then your 100uf/450v will work at hte first and second power supply nodes. After that , play the amp and see if you are going in the right direction before you replace the other caps.

The higher voltage will give you more head room, but you will have to REBIAS!!! The extra capactiance on the plates and screens of the output tubes will help bass.

[jhundt]

@Scott S

I could be wrong, but I don't think the voltage rating doubles when you wire two caps in parallel, just series.

you are probably right; it's been a long time since I thought about series vs. parallel caps, and I never had no education no-how!