3M3 and 10pF pass filter in Fender Reverb amps

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Firestorm
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Firestorm »

Kevin O'Connor just links the two channels with a straight wire after the coupling caps and cuts the connection from the Normal channel's 220K mix resistor. Dave Funk says this is wrong and adds 220K mix resistors at the 3M3/500pF junction (as you now show it). Funk is right, IMO. I don't think the presence of 220K in front of a 3M3 has much sonic impact. The original 3M3 were 10% resistors so the 220K is within that margin anyway.

On actual BF Fenders, the Vibrato channel is always hissier than the Normal channel, mostly because it has an extra, heavily attenuated gain stage. Both of your channels now share that.

There is also noise in the BF Vibrato channel because of really long leads connecting the tone stack. But this usually manifests itself as a parasitic fizz on top of notes rather than hiss at idle.

For comparison, find somebody with a 60s blackface. The reissues are almost all PCB and have very different "lead dress." PS is different on some, too.
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ElectronAvalanche
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

Hi Y'all,

so this whole hiss thing boils down to:

It is the mixer gain and the resistor (3M3/10pF) is responsible for the hiss. To test this I put a 3M3/10pF combo with a 220k to ground right on the grid and still the hiss with the floating side of the 3M3/10pF grounded.

Then I did a comparison with one of my Dumble clones and this amp also hisses with the Master maxed and the Preamp volume down, albeit a tiny bit less (which does not surprise since there is one gain stage missing when in clean mode on the Dumble clone).

The speaker also has some influence, using my trusted 2x12 (G12/65 Celestions) together with the Fender Vibroverb Clone had some tiny amount less hiss (there is a Jensen Neodym in the Fender combo), so the speakers ability to transduce slightly more high end results in more hiss (silly me).

So I am a happy camper now. I will still have to check this against a 65 Deluxe Reverb RI that is incoming, but I must say I am very pleased again with my Vibroverb Clone.

Thanks to all for the help.

Chasing my tail not anymore!

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pula58
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by pula58 »

Did you ever make more headway on this hiss problem you were experiencing?
Bombacaototal
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Bombacaototal »

Just wondering if anyone ever tried replacing the 3.3M / 10pF resistor by a pot 3Ma / 10pF to be able to control the gain (and consequently the amount of dry signal) on that part of the circuit wired as variable resistor

My concern is if the resistance will eventually become too small and then cause issues on the reverb (like oscillation)
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Mr. dB
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Mr. dB »

Bombacaototal wrote: Wed Mar 01, 2017 11:19 am Just wondering if anyone ever tried replacing the 3.3M / 10pF resistor by a pot 3Ma / 10pF to be able to control the gain (and consequently the amount of dry signal) on that part of the circuit wired as variable resistor

My concern is if the resistance will eventually become too small and then cause issues on the reverb (like oscillation)
Put a fixed resistor in series with the pot to prevent it going "too small".
Bombacaototal
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Bombacaototal »

Mr. dB wrote: Sat Mar 04, 2017 7:04 pm
Bombacaototal wrote: Wed Mar 01, 2017 11:19 am Just wondering if anyone ever tried replacing the 3.3M / 10pF resistor by a pot 3Ma / 10pF to be able to control the gain (and consequently the amount of dry signal) on that part of the circuit wired as variable resistor

My concern is if the resistance will eventually become too small and then cause issues on the reverb (like oscillation)
Put a fixed resistor in series with the pot to prevent it going "too small".
Thanks for the suggestion. What would you advise as a minimum value? 900k or 1M kind of range?
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Mr. dB
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Mr. dB »

Bombacaototal wrote: Sat Mar 04, 2017 7:29 pm
Mr. dB wrote: Sat Mar 04, 2017 7:04 pm
Bombacaototal wrote: Wed Mar 01, 2017 11:19 am Just wondering if anyone ever tried replacing the 3.3M / 10pF resistor by a pot 3Ma / 10pF to be able to control the gain (and consequently the amount of dry signal) on that part of the circuit wired as variable resistor

My concern is if the resistance will eventually become too small and then cause issues on the reverb (like oscillation)
Put a fixed resistor in series with the pot to prevent it going "too small".
Thanks for the suggestion. What would you advise as a minimum value? 900k or 1M kind of range?
That would probably have to be determined experimentally. As a seat of the pants guess, I'd probably start with a 1M pot and 2.3M of fixed resistance, just to see if that gives enough range of adjustment for what you're trying to do. But I really just pulled that out of my nether regions.
Roe
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Roe »

One possibility is to replace the 3m3 with a 330k resistor and to add a voltage divider at the preceding gainstage. Just replace the 100k plate resistor with a 91k in series with a 10k resistor. Feed the dry signal from the 10k and the wet signal from the 91k (not the 10k). This should reduce hiss considerably. You may need a 100pf bright cap across the 330k and a 330 or 390pf across the 91k to retain the original frequency response.

Another possibility is to add a gain boost by paralleling a resistor and a pot over the 3m3 resistor. I'd try a 2m2 no load pot in series with a 1m or 820k resistor
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