Vibrolux build sounds woofy and congested

[Toxophilite]

I built a AA964 vibrolux reverb about a year back. It's true to the original circuit except i have a bias vary treme life the hoffman amps. It also has a multitap Hammond OT. this was so I could use it with a JB: D13O . I found the JBL to be too low mid heavy in this amp so I switched to amore conventional 2X 10 setup. This was a little better but i still found I was running the mid control(which seemed to work) at zero.. I've tried different 10" speakers, and even different power tubes(in case there was something wrong with the ones in there).
I checked the circuit a zillion times, all seems to be well, voltages are within spec and the amp is biased properly.


I'm wondering if it could be my cabinet layout as I actually have the amp orientated like a tweed amp hanging of the back of the top of the amp. The amp is 24" x 19" x 9". the gap between the two back panels is about 4.75"
Could some of my woof/low mids be from the not being fully open backed?

[SILVERGUN]

Did it just get woofy all of a sudden?, or it has always been and you're just noticing/caring now?

true to the original circuit except

I don't see a "mid control" in an AA964 

The cabinet shouldn't be effecting the EQ to a point that would make you change the cab, so, I would focus on fixing the circuit

[tubenit]

What kind of caps did you use in the amp?  Any chance they were Mallory 150's or were they the typical Orange Drops? 

Did you use shielded wiring much?   Is there a bright cap cross the volume pot?   

Have you tried a 12AY7 in V1 and a 12AT7 in the LTPI?

I would doubt the issue would be the cabinet.

With respect, Tubenit

[jjasilli]

I agree with silvergun & tubenit that the cab should not be an issue.  But you could test it by pulling the amp from the cab; or running the amp into a different cab. 

[Toxophilite]

Thanks
I forgot to mention the mid control, indeed
It if anything allows me to have less mids than the original circuit
I've never different caps make a significant difference in amps. As long as the value is right.
I've certainly been wrong before This amp has orange drops (and sometimes solens) as do most my amps, at least in the audio path.


Thanks I'll
try it with my super's speakers and see if I get the same results. That's a good idea
And I'll go over the circuit yet again. It really has been pored over alot.

[mresistor]

That JBL should have had tons of treble and upper mid response. The lows could have been adjusted downward to accentuate the mids and highs. So the probem seems to me to be in the amp circuit. I wonder if bypassing the tone stack would help in trouble shooting?

 

[Toxophilite]

Well I went over it again and everything looks good
hmm

[tubenit]

Does it sound woofy with all your guitars?

IF you take the chassis out yet plug into the cab, does it sound woofy?   I still can't imagine it being the cab?

Have you tried a 12AY7 in V1 and have a 12AT7 (not 12AX7) in the LTPI position?

You could change the post LTPI coupling caps from .1 to .047 (or .033) to see if that helps?

You could change the cathode caps on the first two gain stages to 5uf instead of 22uf or 25uf.

With respect, Tubenit

[SILVERGUN]

You could change the post LTPI coupling caps from .1 to .047 (or .033) to see if that helps?

This will tighten up the bass response and really serve you well seeing how you seem to lean towards 10" (or smaller) speakers.
Rather than relying on the speakers' response to act as the final "crossover", you should eliminate excess bass before it gets there.
You should be able to go down to .02, if that helps

 
If you still find that it's the "low mids" that are truly haunting you, you should be able to shape them in the tone stack(s)

[Toxophilite]

HI thanks for the idea. It does sound better through my super's 4x10
Though that could be the 4 X 10 thing


I like a nice round bottom on the amp.
It's the low mids than seem to be bothering me. Muddies up the sound. not unlike the proximity effect you get with microphones.

 I was monkeying with the tone stack calculator and it looks like if I up the value of the mid cap (.047) I can move the scoop a little lower and deeper .For fun I'll try a .02 across the .047 cab to bump it up to .057


Out of curiousity (and I think I've asked this before) on my UL super there's a .01 cap in parallel/bypassing the 820 ohm feedback resistor. This seems like a tone shaping cap. What exactly is it doing? Is it allowing highs through? thus damping the top end some? or the opposite?...need more coffee!

[SILVERGUN]

I like a nice round bottom on the amp.
It's the low mids than seem to be bothering me. Muddies up the sound. not unlike the proximity effect you get with microphones.

I would still support tubenit's suggestion of lowering this post PI caps. Guaranteed less mud at the output.
Also, his suggestion of lowering the bypass cap values on the preamp gain stages would apply.

I was monkeying with the tone stack calculator and it looks like if I up the value of the mid cap (.047) I can move the scoop a little lower and deeper .For fun I'll try a .02 across the .047 cab to bump it up to .057

That's exactly where I was looking. You could experiment all the way up to .1 on the mid cap to shift the low mid scoop downward. (obviously, you meant .067, when you add on the .02 to the existing .047)

I'd be interested to know the value of the mid control pot and how it was wired.

Out of curiousity (and I think I've asked this before) on my UL super there's a .01 cap in parallel/bypassing the 820 ohm feedback resistor. This seems like a tone shaping cap. What exactly is it doing? Is it allowing highs through? thus damping the top end some? or the opposite?...need more coffee!

Like any bypass cap it will allow some highs to get around the 820 ohm resistance, so those highs will remain in the feedback loop, and result in less highs at the output than if it wasn't there.
Conversely, if you were to put the cap across the 100 ohm resistor to ground it would result in less highs in the feedback loop, which would result in more highs at the output.

[drew]

I think if you posted a schematic of the amp as-built, a voltage chart, and hi-rez pictures of the guts, it would put you and everybody else in a better position to avoid tail-chasing and to understand why the amp isn't sounding like you want it to. One picture of the back of the cab and a handful of conclusory statements are not exactly a solid analytical foundation. My 2¢

[Toxophilite]

HI
I don't know if there's any tail-chasing going on
This is a one channel AA964 vibrolux reverb build with bias vary tremelo
It also has a 10 k mid control (instead of 6.8K resistor to ground)
a line out ala my UL super reverb
and a SS diode rectifier


Here is a schematic, ignore the additions in red (though I did ask about the one on the negative feedback)
None of these have been implemented.


However the schematic does not ensure that my amp is identical too it, though it should be
I will try to get some high resolution photos up too.
This is built using hammond cone boards and was rebuilt as it originally had a pitch shifting vibrato in it which i removed and replaced with a bias vary tremelo
Also it was built inside a Hammond AO-29 chassis and uses the PT from that
because of this there is a small bias steal circuit for the Power tubes

[tubenit]

The schematic and voltages look fine ........... IF you ignore the additions in red.  I am presuming the 5/25 cap that looks like it could be something like a "conjunctive filter" has not been implemented. 

I have removed every conjunctive filter I've tried because it made the amp sound like a wool blanket had been draped over the cab.  Very "woofy" or muffled sounding to me.

With respect, Tubenit

[Toxophilite]

yeah none of the stuff in red has been implemented.


i was wondering about the .01 bypass cap on the negative feedback (also in red)

[sluckey]

yeah none of the stuff in red has been implemented.


i was wondering about the .01 bypass cap on the negative feedback (also in red)

If you put that cap in the amp it will increase the high freq. feedback which will decrease the high freq. sound. IOW, the amp won't sound as bright.

You have some red numbers (150) written near the cathodes of the output tubes. What do those numbers represent?

[PRR]

The 0.01uFd across 820r works at the top of the audio band, well above the guitar band. If it does anything, it band-aids a supersonic stability problem. You should not need or want that.

Follow on down from the 820r. There is a dot where this lead crosses the grid line to the PI. This dot should not be there! I doubt you wired it this way-- gain would be way-way low. But another reason to remember the schematic is NOT the amplifier.

[Toxophilite]

Thanks!


Here's some actual voltages. As I said and PRR reiterated the schematic is NOT my amp
My voltages these days seem to higher than I remembered them
A couple things:
-I can't seem to get the tremelo tube to not oscillate without the footswitch on, maybe one of my pots is less than perfect.


Here's some pictures.
Discalimers:
The lead dress camp will not like mine(I've been told this before, however my amp isn't noisy)
the amp has been reworked 3 times I believe
)originally pitch shifting vibrato, then replaced with bias vary, then position of reverb and bias vary swapped on board to emulate standard fender layout.)
and my layout is the reverse of a fender amps due to chassis orientation
Also there's .02 cap across the .047 mid cap as stated above.





voltages
                1            2          3          6         7         8
V1 pre  -  193        .2        1.67      195       .2     1.63


v2 rev  -  418      17.8mv   7.74


v3 rev  -  199        0?      1.77      201                1.77


v4 trem   217        1.1     1.86      420      217     230


v5 PI      175          54      82.9     172      56.4    82.9




                1          3       4            6         8
V6 PA -   -40       435    428       432       40mv


v7           -40      436    427        430      39mv

[sluckey]

-I can't seem to get the tremelo tube to not oscillate without the footswitch on, maybe one of my pots is less than perfect.

That's the way a 6G16 Vibroverb tremolo is supposed to work.

[drew]

One other thing: is your impression of the sound this amp makes based on a comparison with a real Vibrolux? In other words, is the problem that it doesn't sound like a Vibrolux, or that you're just not a fan of how it sounds?

[Toxophilite]

Well let's put it this way.It's less scooped than my 1x 12" deluxe reverb build, or my 2 x 10 deluxe reverb build
It should really be pretty close to the 2x10 deluxe sonically, except louder
As the circuit is pretty well identical except with different output tubes and slightly higher voltages on the PA