Tone stck mods, slope resistor change? Twin Vibrosonic

Steve Waltz · From: USA

Long winded question here. I'm not in love with the sound coming out of this 72 vibrosonic. It's not bad and I wouldn't notice it much if I didn't A-B it to my 59 Bassman RI with a single 15" Altec 421 and new old stock OT at 8ohms. The bassman has more tone to it. Maybe call it high fi?Natural? It just sounds like the guitar plus all good stuff where the Vibrosonic sounds a bit like there is a box infont of the speaker and the sound is traveling through that first. What can I do?

I've tried different speakers. No change of this "boxy" issue. Changed tubes-still there! All eletrolytic's have been changed to Sprauges. All tubes are new JJ's except for a few preamp tubes changed for taste. Speaker is now a Atec 421 as well.I've tried a JBL D140F- still there! Orange drops are on the 1st channel and and a number of Hovlands are on the 2nd. I kept all the old stuff to put back if I needed to. I've added a few silver Micas in a few places including the bright switch. My pickups are different than most I guess with True Tones at 16K and 11K and I tend to use the 11K. It's a sho bud guitar. It has had the bias changed to bias adjust like the AB 763 as well as PI resistors. There might be a change in the location of the wires for the master volume but I'm not sure.

I just don't want to carry the bassman and reverb unit to every show. The vibro has two separate channels which helps when I need to have a guitar as well and the vibro has vibrato. So I want it to sound as good as the bassman.

I'm thinking I just don't like the way the tone stack works. I set T-5, M-2 B-3. There is little to no room in these numebrs to get the sound I like. I want to drop the mids but then the treble needs to be turned up which seems to increase boxyness. The bass has little effect after 3. The issue seems to be in the midrange for me. I have read that the slope resistor can be changed to 56K for 100K but doesn't that increase mids? And if I feel like my mids sound more clear as I turn down the mid shouldn't I be going higher on the slope? 220K? If All I ever do is go between 1 and 3 on my mids is there a pot change that would allow for more choices in that range?

Are the AB763 PI resistors causing some of this issue. Should I change those to Vibrosonic specs. I can't find that layout to see what they should be.
BTW-I have two vibrosonics and one has this boxy sound twice as much as the better one. Is it just Vibrosincs?

Thanks for reading this far and thanks for any suggestions.

Steve

Rick Abbott · From: Indiana, USA

Steve,
The issues you describe are related to circuit design. The two amps can never be made to sound exactly the same. The Bassman is a very old circuit from the Westinghouse days, the late 40's and early 50's. The later vibrosonic tone stack is different, as are the gain and voltages. I'm limited on time at the moment but we should talk more about this, I think that I can help you out. Have you done all these mods yourself? If so this could be very interesting!! E-mail me, or something.
_________________
RICK ABBOTT
Sho~Bud D-10 Professional #7962
Remington T-8, Sehy #112
1975 Peavey Pacer 1963 Gibson Falcon

Steve Waltz · From: USA

I didn't do the blackface stuff, That was there before I got it. I have two vibrosonics. The one that I use for bass is the one that I checked against the schematic that I have and it is the one with all of the blackface changes.It sounds fine for bass on channel 1 but it sounds much more boxy than this second vibrosonic. I just checked the one that I have changed caps, which is the one that sounds better and it actually doesn't have the blackface changes. The bias is also still a bias balance. I thought the blackface changes made the amps warmer but from the two that I have, I don't see it. But maybe that isn't what is causign the problem. I also noticed that each vibrosonic has the master volume wired differently. The better sounding one has one wire going to the top of the first .1 cap at the far left of the board. The second wire goes to the .1 cap just to the right of the first one. The worse sounding vibro has the master wired closer to the tremlo area. I didn't write it down before I sent that amp to a local tech to check out and bias for me. So it seems that the good sounding amp has been altered less if at all other than my cap job.

The book that I have does not have have a schematic for a vibrosonic so I can't see what things should have been from the factory.

Bill Hatcher · From: Atlanta Ga. USA

Jerry Erickson · From: Atlanta,IL 61723

There are a couple of schematics over at scehmaticheaven.com and they both show the master volume control in the same place, pre phase inverter.

Brad Sarno · From: St. Louis, MO USA

Steve.

You probably do want to mess with the slope resistor, but from what you describe, it sounds like you want to clear things up instead of thicken things up. That would mean that you'll want to raise the slope resistor value to something above the stock 100k, not lower it. Maybe try 120k or even 140k. You'll probably have to mix some resistors to get the right values. This will sweep the midrange dip frequency downward, clearing out the "boxiness". I'm not exactly sure what's causing what you're hearing, but you may want to try this simple experiment. Also make sure that the tonestack caps .1uF and .047uF aren't accidentally swapped. This resistor mod will also let you turn your treble down a bit. I'd expect you to want to dial in a setting where the treble is more like 3.5, the mids around 5, and the bass around 4 to 5. Let us know what happens if you try this slope resistor change.

Also, when you pull the stock slope resistor, measure its real value and see what it is. Rarely will it actually be exactly 100k. Those components in a Fender can be all over the place, often by margins well over 10%. That's why these amps can all sound so different. Dozens of components with widely varying values from one amp to the next. If you find that the stock slope resistor was way off, like too low, then maybe just try one that's truly 100k.

Brad

Dave Zielinski · From: Pennsylvania, USA

Well, the obvious answer is you have two completely different amps from two completely different eras.

1-15" speaker and silverface tone circuit will never sound close to 4-10s and interactive vol/tone controls.

The first option is to leave the Vibrasonic stock, unload it to someone who is in love with it and go buy yourself what you really want.

Make sure you check bias and measure plate voltage before doing any mods. You amp just might sound lousy cause it needs tubes, or is running a little cold or hot. find that sweet spot first!

The second option is to change the tone stack, put a 250K pot in place of the slope resistor and dial in where you want it. Then either leave it there or take it out and measure it- then solder a resistor in its place. experiment with the caps in the tone circuit- 0.02s and .01s... I don't have a schematic in front of me, but you will find the caps for the tone circuit can and will make a difference.

Then plug a tweeter into the extra speaker jack.

You might find it sounds pretty good. But the vibrasonic wont get the compression and dynamics that the bassman gets- tube rectifier, 10" speakers, don't be afraid to play with plate voltage. You can drastically change the dynamics of an amp by a matter of 20-30 volts on the plates.
Weber makes a device called a "Copper Cap" that is the rectifier and allows for plate votls adjustment. this may be an option for your amp... (if your map hasd a tube rectifier..which im not sure about)

Steve Waltz · From: USA

"The Vibrosonic Reverb is the same amp as a Twin Reverb. Your book have a Twin schematic??"

It's the Dave Funk book and it does have schematics for Twin's but for some reason it didn't have ones for amps with master volume. I did download the schematic for Schematic heaven and now I have it. That advise was helpful.

My bassman has a single 15", not four tens. I don't think these amps can sound exactly the same. I was using the bassman as an example of how I would like the clean to sound. I wouldn't expect the vibrosonic to have the same break up caracterisitics as a bassman. I know what bassmans are known for. Vibrosonics are know to be good steel amps with good clean tone. It seems like it isn't there in mine so I'm looking for steps to take rather than selling it off.

I've checked all resistors to see any were more than 10% off and the only ones that were, were on top of the 6L6's. I replaced those a few months ago. I'll try Brads suggestion of slope resistor change. At least I can put that back if I don't like it.

I'm thinking of changing the other Vibrosonic back to stock since it seems that those blackface changes might be causing so problem.
Steve

Dave Zielinski · From: Pennsylvania, USA

Steve,

I'm sorry, I can't read... Ok, your bassman has a 1-15" however, the preamp circuit in that bassman is a complete different animal. as you know, the volume controls are interactive- plugging into Normal-1 and adjusting the Bright volume- even though you aren't plugged into it- it affects your tone. Thats a major difference and one of the reasons the tweed circuit rocks.

but you know that too...

Put a pot in place of that slope resistor and dial in, see if you can fix it. Just keep it in the middle of the range at first. try 12AY7 preamp tubes. Their low gain characteristics will make you turn the amp up more. And verify your bias on that Vibrasonic- 34-ish mA per side? (with 6L6) try 5881s?

I can't say enough about plate voltage too.

I would recommend against reversing the blackface mod back to silverface, as those mods typically make a silver amp sound better! but who knows!

I've admittedly run into this problem too- side by side comparisons of totally different amps. ONE ALWAYS sounds better. Or at the very least, your ears wanna here one more than the other. I've tried to change amps trying to make them more "toneful" to less than satisfactory results too. I inevitably ended up putting them back to stock and moving them on to make room for something I liked more.

Hey, what about trying a different speaker? try plugging a horn in the ext. jack too. it makes a difference in the treble department.

good luck.

Dave Zielinski · From: Pennsylvania, USA

you can tell I've been thinking about your tone problems all day... (I'm an amp nut)

It is a master volume amp??

How do you run it?? My reccomendation is to drop the master volume all together. Or at the very least, turn the master all the way up, then use the regular volume to adjust the loudness.

Hmm, I'm still thinking here.....

David Doggett · Posted 1 Mar 2007 2:47 pm

I have a silver face late '70s master volume Pro Reverb (same as Twin family, but with two 6L6s instead of four) that came out sounding "boxy" after an amp tech made some blackface mods on it without my asking. Mad

Also, the JBL140 is a bass speaker, not the usual JBL 130 guitar speaker. The 140 is more mellow sounding without the same sparkly highs of the 130. Another observation is that open back amps sound more boxy to me. I use a Dual Showman Reverb (same as Twin and Vibrosonic) head with closed-back ported speaker cabs with 15" JBL130s. Another observation is that, with 15" speakers, on Twin family amps I put the mids way up around 8, and the treble and bass around 2 or 3.

Steve Waltz · From: USA

This amp is a master volume. I usually set the master at 10 or close to it and then use the volume on the individual channels. I thought about having the master removed but the tech working on one amp right now didn't think that there would be much diference considering how I set it.

I've tried a JBLD140F, an Altec 421, a Califorina Weber and this amp had an Altec 515 in it when I bought it. I also have a showman closed back single 15" cabinet with the tone ring and I have tried the same speakers in that. That same issue always seems to be there, back there...but there. I'm going to try the slope resistor change tonight if I have time.

The Altec 421 is in the bassman so I know that speaker can do what I want it do but it doesn't do it in the vibrosonic.

Steve

Bob Metzger · From: Waltham (Boston), MA, USA

Steve,

You need to get this amp into the hands of a tech who is very talented for voicing amps for steel guitar use. Depending the part of the country you're located in, they're out there. You just have to turn over a few rocks.

These amps being discussed here are all open architecture and can sound any way that you can think of. You need to find the guy with the proper experience. It's not just tubes and speakers, you need to find someone who is really adept on the component level to get this correct.

Bob M.

Steve Waltz · From: USA

I checked the 100K slope resistor and it was around 107K. I pulled it and put in a 150K. I haven't checked it against the bassman but it did seem to help quite a bit. The tone controls seem to have a more gradual effect which feels like I can dial in the sound that I want more easily. I'm going to try the amp out at a show this week and see how it sounds turned up to a normal operating level. It still needs to be biased properly so I'll send it out after that. I want to put a dwell on the reverb and put it in the external speaker jack location before I send it to the tech since I want him to double check my work. I put in a new 3 spring reverb tank and replaced the reverb tubes with lower gain ones.

That was an easy change which made a difference. Easy to put back. It's interesting that I like the sound of taking the slope up when most people seem to bring it down. I'm also thinking that the other Vibrosonic sounds worse to me because it has been so blackfaced. Maybe I just don't like that sound. I also like 11K pickups which seems different from many people who use 16K- 20K.

Thanks for the advise.

Steve

Jerry Erickson · From: Atlanta,IL 61723

Steve, You might check out Randall Aiken's website for his article "the last word on biasing". It's under the tech info pages.

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