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Post subject: Amp tech question: SF vs BF twin
Posted: Fri Mar 11, 2016 2:18 pm
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Hi all,

I have been reading on line (And we all know how everything on line is the truth) about my new to me amp, a 1973 SF twin.
I keep seeing that this amp can be modded to be the same and a BF by removing the master volume and changing a few caps to lessen the bass response.

My question here is, if you dime out the master volume with the boost left off, does this not do the same as removing the master volume?

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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Fri Mar 11, 2016 6:50 pm
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Not sure what sonic differences the MV makes on the Twin Reverb. It is basically a clean amp throughout the volume range. Not much OD setup in the gain stages. Dunno for sure, never had one of these amps. I'd keep the MV. Changing the coupling caps will have greater tonal effect. Perhaps heading to the endpoint you desire.

For everyday use, converting the circuit to a bias pot (versus balance pot) maybe helpful. Still need to buy matched quartets.

I think pedals will achieve the tonal outcome you seek. More than circuit tampering, at least in this amp.


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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Sat Mar 12, 2016 1:37 am
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BMW2002Ti wrote:
I think pedals will achieve the tonal outcome you seek. More than circuit tampering, at least in this amp.


+1

The only outcome for a Twin Reverb is LOUD/CLEAN.

Any expectation to the contrary is pure folly. Get some decent pedals.

Arjay

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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Thu Mar 17, 2016 5:54 pm
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Hi all,

I get that these amps will never on there own make any sort of gain, that is why I got it, I love the clean lush tone it has.
I have boost pedals and two other amps (A Peavey classic 50 212 and a Peavey Ultra 212). The classic can do clean and dirty blusy/classic rock tones, and the ultra can melt your face off in the ultra channel. But as with all two or three channel amps, they're a compromise, they do something better then others and the clean well, it's not where they stand out.


The question I was asking is if the BF (Non master volume) is better sounding then the SF (Master Volume) then by turning the master up to 10, and using just the channel volume to set the output, does this not make these amps equal to one another to tone quality?

So I have been trying this on my own and BMW, if you could try this on your to see if it's just mine or not I would very much appreciate it.
With the MV at 3 and the tremolo channel set at 5, the reverb at 3-4 and trem speed 4 and intensity at 7, I hear a nice clear tone, it response well to how hard you pick, the reverb seems to fuzz out at the end and the tremolo is subtle.

With the MV at 10, and the channel set at 2-3, the volume is about the same, but the effect became more pronounced and the overall tone, fuller and deeper and the reverb no longer fuzz's out at the end of it's range.

It is similar to my ears like the Peavey Presences control bringing in the drive tubes in tone, but I was surprised by the effect being effected.

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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Thu Mar 17, 2016 6:14 pm
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I don't presently own a MV Fender Twin Reverb or any MV Fender amp. I can't remember having that subtle tone change with MV and volume settings. It's been a long time. I kinda remember diming (or close) MV and playing with volume and guitar controls.

You may have a weak reverb and/or tremolo tube --- so gain stage control is effecting the linear (or non-linear) quality of those tubes. Try your setting with known strong 12AT7 and 12AX7 tubes. See if you have same results.

Good luck!


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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Mon Mar 21, 2016 1:20 am
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Thanks for the reply BMW, the amp has new Fender tubes in the drive and the 12at7s. The guy retubed it and it is why I got it so cheap, in V5 (The tremolo drive) when he installed the new 12ax7, he bent over one pins, and he thought the tremolo was bad.
I had a set of new Peavey 12ax7 out of my ultra as back ups, I replaced all four with these for now, until I get my matched set from Eurotubes.

I was hoping that you still had one to see if this is normal or if I have a cap going out. (It is a 43 year old amp and all).
Anyways, I am very pleased with the tone it has and I will just enjoy it for now.

Thank you again for your time.
JT.

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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Mon Mar 21, 2016 8:13 am
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You can check for a leaky coupling cap by measuring the DC voltage on each side of the cap. One side is usually connected to the anode plate and typically has several hundred DCV. The other side is connected to the next stage's signal grid. And should not have much more than 0.1 DCV.

If the grid side voltage is above 0.1-0.2 DCV --- replace it with a new cap of same mfd and VDC value.


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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Mon Mar 28, 2016 9:24 am
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The MV presents a 1meg load that lowers the available signal. It's a slight difference.
The biggest difference between SF & BF is the size and inductance of the OT.
Some of the SF have ultralinear transformers. These will have a couple striped wires coming out of the OT.
In the BF, the low inductance OT at 6 Henrys limits power and saturates out the bass before it gets too boomy.
In the SF, the high inductance OT at 19 Henrys allows sub-sonic bass to be fully amplified.
You can watch the speakers waffling around. This robs power and clarity from the midrange.
The simple fix is to replace the V1 & V2 cathode capacitors from 25uF electrolytic to 2.2uF Poly caps. This will limit the bass response.
A closer emulation could be had for the price of a new OT from Fender for the 65 Twin reissue. That one went back to the 6 Henry inductance. It would need new mounting holes drilled to accomodate it.


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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Mon Mar 28, 2016 4:53 pm
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"The simple fix is to replace the V1 & V2 cathode capacitors from 25uF electrolytic to 2.2uF Poly caps. "

TimsAudio, I've heard of some folks cutting one lead of the 25/25 bypass cap (to equal no bypass) and being happy. 2.2mfd poly caps can be kinda $$$. What type do you use?

Thanks!


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Post subject: Re: Amp tech question: SF vs BF twin
Posted: Tue Mar 29, 2016 8:50 am
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The cathode capacitor adds a lot of AC gain to the circuit. The cap acts as a bypass for the AC signal. Cutting it out lowers the AC gain to the DC gain, which is much lower, with no bass limiting capability. There are applications for that, but a SF Fender, IMHO isn't one of them.
I use Solen Fast caps. 2.2uF/250V at a few bucks each. They are marked as Solen/ MKP though. The similar MKP 2.2/400V cap is less than a buck. I have also used 2.2/50V poly from Mouser. The poly caps can track the signal much more accurately than electrolytics. The better caps make a noticeable difference over the electrolytics.
It effectively cuts subsonic bass by about half. The 2.2 is a noticeable drop in bass, but a 4.7 doesn't make much difference at all. If the application is lead on the Normal channel, I might go down to 1uF or even .68uF.


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