Tweed Vibrolux 5F11 too Midrangy, Mods?

[AtomicMassUnit]

Hey guys, I'm messing around with my tweed Vibrolux. It has a really nasally mid (800-1khz I'm guessing). This is an amp I'm pretty attached to, but it's not that useful for me since my guitars and pedals tend to be very midrangy and sound better with blackface amps, which I normally use. I'd like to use the Vibrolux, but I need to reduce those mids somehow. I've been playing around with the caps in the tone stack, and when I changed the 500 to a 1000, it got the mids sounding better but lost treble.

Any suggestions? It's just the single tone control like the tweed princeton.

[txstrat]

Just a thought.
What if you leave the 1000 in there and put a 150k to 220k resistor in series with it. Then bypass both with a 250pF cap. That should scoop your mids.

[Bruce / Mission Amps]

Assuming it is a real 5F11 or the dims of a real one, the first thing to do is pull the baffle board and install one from a 5E3 Deluxe so you can have a 12" speaker (pick a good speaker).
Then use a 680pf treble cap and nothing larger then a .01uF grounded "bass" cap.
I like a 6800pF (.0068uF) bass cap and 680pf treble cap combo with smaller value cathode bypass caps on the preamp stages.... like 4.7uF.
A mid priced speaker, that I like the best, really will annoy some, ... but I think the 12" Eminence NEO magnet Tonker Lite kills in this amp.
Oh, using a 1000pF WILL NOT cause a loss of treble so something is wrong with what you are hearing.
Come on back after that and report as to how it is going.

[AtomicMassUnit]

I realized I mistakenly put a 47pf 1000v cap, not 1000pf! Here's where I'm at with this amp.

I replaced the coupling caps with sozo blues, and put the 250pf cap back in it. I have a tube set coming for it that will be considerably less midrangy, and got a Weber 12a150 speaker for it. Yes, it is an original 5f11, but it was in super bad shape when I got it, blow output transformer, all the caps were leaky. I tried a couple different output transformers in it, including an original brownface deluxe one, a mercury tweed vibrolux replacement, and a classictone tweed deluxe, which was the best sounding for this amp.

This amp is very close to being dialed in, and may finally reach its potential!

[barfoden]

Many of the amps that has no tonestack but just a single tone pot will be midrangy, if one is coming from a TB Vox- or Fender tone stack.

The midrange you are hearing is probably from 400 Hz to 1000 Hz where the Fender blackface will scoop at the lower end of that frequency range and the Vox at the higher range of that range.

Changing the tone cap values in the 5f11 and similar amps will just alter at what frequency the treble roll of should happen and how much is should throw away to ground.

The single tone pot amps needs scooped speakers to sound balanced. Many of the low and mid powered 10" and 12" Jensen speakers are scooped at 500 Hz compared to most Celestion speakers that are often flat between 200 and 800 Hz.
I kind of like some Celestions in blackface Fenders, but a scooped jensen with a 95db sensitivity will sound pretty when playing on your own, but will vanish in a band setting with a blackface amp.

TO get an idea of different tone stacks see this.
differences

This is just for the preamp. Recall that the VOX AC30 is cathode biased with no negative global feedback and the blackface fenders employ negative feedback which flattens the frequency response and extends the lows and highs. A Blackface fender tonestack might not sound great without any negative feedback, whereas the VOX's TB tonestacks high-mid scoop (900 Hz) is needed to lower gain in that stage before it hits the non-NFB power stage.

[pdf64]

'the blackface fenders employ negative feedback which flattens the frequency response and extends the lows and highs'

When driving a real speaker (whose impedance increases at bass resonance and in the treble range) the impact of the global negative feedback tends to be to reduce the level of bass and treble.
Pete

[barfoden]

You are correct..
What i meant it that it extends the extreme highs and lows but these are often beyond the frequency levels that are important for guitar range,, and as you said NFB actually increases speaker damping and flattens the resonance curve especially at the speakers bass resonance and treble range..

One of the major differences between the preamp (besides the center frequency of the mid scoope) of a top boost VOX TB and a Blackface TB is that the VOX really limits bass coming from the first gain stage (500pf coupling cap) and lower value bass cap in the tonestack, whereas the fender has a large 0.1µf coupling cap in the tonestack. The 12" speakers bass resonance (75 hz - 100 hz) is kept in control by the NFB, but when you really start overdriving that blackface amp, the NFB is starting to loose its role, and you really have to lower that bass pot. This explains that Albert Collins ran his quad reverb at full throttle and kept the bass knob at 0 or 1. Mids at maximum (10K to ground), bright switch off and treble almost full up (he was using a really long cable which decreases treble).
MAybe he was approximating a slightly more british sound out of his fender..