recapping BF Twin

[HTH]

I'm recapping a BF Twin Reverb and it's got a pair of 220uF caps in series first, then three 20uF caps (all on the top of the chassis) then a 60uF inside the amp.

Looking at the AB763 schematic, its got a pair of 70uF's in series, then three 20uF's and no 60uF inside the chassis.

I don't see that many vintage Fenders here in the UK, so I'm wondering if you guys in the US with more experience with these amps can chime in and let me know what the most common arrangement was on these amps. I'm thinking of going with the vintage specs as shown on the AB763 schematic but am concerned about the two 70uF's in series at the first node, thats only around 35uF of filtering before the choke.

Thanks in advance guys.

[Jazz P Bass]

(2) 70uf/350V caps in series is how it's done.
Don't forget, there are (2) 220K 1 watt resistors in parallel with the caps.

[g1]

Does the rest of the circuit look AB763 (is that 673 a typo JazzP? ) ? The only Twin I know of with 220uF's is the 135W SF with the push/pull master.
If the 60uf is not the bias cap, where is it connected to?

[HTH]

the reissue Twin has the two 220uF's in series at the first node, thats why I'm asking really - I know the old BF Twin AB763 schematic has the 70uF caps shown in this position, but was wondering whether Fender actually used these values in practice - have you guys seen the 70uF value used in an actual amp? The amp I have here has 220uF caps and they look stock.

the 'added' 60uF Mallory cap inside the chassis is tagged onto V2's plate feed, which in turn means it's connected to everything in the first preamp stages prior to the PI.

[Jazz P Bass]

Does the rest of the circuit look AB763 (is that 673 a typo JazzP? ) ? The only Twin I know of with 220uF's is the 135W SF with the push/pull master.
If the 60uf is not the bias cap, where is it connected to?

Yeah, that PDF was a typo.
I fixed it.
I do know that in the mid 1970's that Fender used (2) 100uf caps.
Then, when they changed to the 135 watt amp, they used 220uf.

[g1]

the reissue Twin has the two 220uF's in series at the first node, thats why I'm asking really - I know the old BF Twin AB763 schematic has the 70uF caps shown in this position, but was wondering whether Fender actually used these values in practice - have you guys seen the 70uF value used in an actual amp? The amp I have here has 220uF caps and they look stock.

the 'added' 60uF Mallory cap inside the chassis is tagged onto V2's plate feed, which in turn means it's connected to everything in the first preamp stages prior to the PI.

The 70uf's were used in practice in the old amps, the stock units I have seen use them. 65 twin reissue uses 220uf's. Is yours original or reissue? If it's original, someone may have wanted stiffer filtering and changed the 70's to 220's and added the 60uf.
The 135watt Twins came out in the late 70's. Someone could have seen that schematic and recapped yours to that spec. 30+ years ago.

[HTH]

Thanks for the replies guys, I went with the BF spec and used a pair of 70uF F&T's in series at the first node.

There are some oddities in the amp though, some look stock, some not - this one (stock) looks the weirdest... the trem intensity pot usually connects to the 220k mixer resistor at the end where the two resistors DON'T join, however in this amp the trem intensity pot joins at the junction of the two 220k channel mixer resistors. The 220k mixer resistor for the trem channel is also paralleled with a 15k resistor.

Gonna have to check this one through and get it singing sweetly again (its not that far off to be fair).

Once thing I'm not 100% decided on is the normal channel - the owner doesn't use it and we've discussed modding it so he gets some use out of that channel. I initially thought of 'Tweeding' it, or even cascading it into the trem channel. Not sure yet, what say you all?

Cheers :0)

[HTH]

here's the pic...

[g1]

I'm wondering how you are deciding what is stock and what is not. In the picture I can see quite a few things that are not stock. I'm pretty certain the blue plastic wire and the gray shielded wire are not stock. So the fact that the gray shielded wire is moved to the other side of the 220K implies to me that someone wanted the tremolo to work on both channels. I suppose the 12K across the 220K for the vibrato channel was an attempt to increase the signal from that channel to the phase splitter.
Here is the stock layout: http://www.prowessamplifiers.com/sch...763_layout.pdf
From the amount of work that appears to have been done to this amp, I would assume anything odd to be non-stock (modified).
To repeat what I mentioned earlier, this amp could have been modified 25+ years ago, the components used would look stock, but the circuit would no longer be stock.

[jrfrond]

I recap all vintage and reissue TR's with 80uF/450V caps for the B+, which is pretty close to original and makes for 40uF worth of filtering. I really don't like much over 50uF for BF/SF/RI amps because then they are too stiff-feeling. In amps that are driving stack cabinets, like Marshall, etc., you need the stiffer filtering to achieve the "thumpy" attack we all know and love.