? What's the difference btw bandmaster and vibroverb circuitry ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbruffey, Jan 19, 2006.

  1. mbruffey

    mbruffey Supporting Member

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    Hi All!

    Is there any significant difference in the circuitry of a mid-60's Blackface Vibroverb (64) and a mid-60's Bandmaster? I'm trying to get a nice blackface sound and there are quite a few re-done bandmasters where the seller indicates vibroverb in the topic line.

    Even if you could just point me to a site that compares the innards and tone of the various fenders, I could do my own research.

    Thanks,

    Mark
     
  2. mbruffey

    mbruffey Supporting Member

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    While I'm waiting for help I'm doing a little exploring. It _appears_ to me that models having the same "board number" (?) might have similar circuitry, e.g, [SIZE=-1]AA-763[/SIZE]. Is this true, or does it just mean the various models use the same chassis or circuit board, but may have different components placed in the chassis or on the board? Please forgive me if this is a really dumb question, but I realize I still have a great deal to learn about fender (and other) amps. Thanks for all (nice[!]) comments. Mark
     
  3. Matt H

    Matt H Guest

    once you get to the blackface era, there are really only two categories of amps... reverb and non-reverb (bassman being the odd one, since, ding ding ding- was designed as a bass amp... that being said- it's not all that different either). (i should state, there are also the champ/princeton little amp things, but ignoring those too for now)

    anyway, since this question is about a vibroverb (a reverb model) i'll ignore the nonverb models.

    the biggest difference between any of these amps are the transformers (which can give different voltages- more voltage=more headroom/power, and the output transformer size determines bass response) and the type/number of speakers.

    there's one other ingredient- rectifier type (this is really related to transformers, but i digress).


    the vibroverb had a pair of 6L6's, a tube recto, and and a single 15".

    the amps "most similar" to this are the super reverb (4x10, but with beefier OT), the pro reverb (2x12, similar OT), and later in '68- the bandmaster reverb.

    So, the pro/bmr had the most similar OT, but the super reverb had slightly more similar voltages (higher).

    the most famous "vibroverb" tone I'm aware of is SRV- but keep in mind his OT was changed out to a twin's (which gives the right impedance match for a pair of 6l6's into an 8 ohm load). So, it didn't have the "wimpy" OT of the bandmaster/pro/vibroverb. (the vibroverb OT wasn't the same model, but was more similar to the pro/bmr than to the super/bassman type of larger, beefier one).


    really- i'd say if you get any of those amps, play it through a single 15- you're there. if you want to tweak voltages and the OT- go for it.

    the BMR is a great amp because compared to the others- it's cheap!!! great platform to swap the OT into... want hgiher voltages/ stick in a GZ34 instead of the 5u4GB. won't be a huge difference- but get you close.
     
  4. mbruffey

    mbruffey Supporting Member

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    Hi Matt!

    That's a pretty nice reply. I think the essence, if I understand you, is that SRV took a transformer meant to drive four (4) output tubes and put it in an amp with only two (2) output tubes, yielding . . . more clean headroom than you would usually get in a two tube amp (?)

    In this case, I would expect the SRV Vibroverb to have the tightest bass, etc. Am I on the right track yet?

    Also, if I want the cleanest "vibroclone" kind of amp, I need to be sure the cloner has either upgraded the output transformers, or cloned it using a super reverb (?) or a twin chassis/transformers?

    What should the output of the SRV transformers be?

    Let me be specific. There's a fellow that makes blackfaces (allenamps.com). I don't think he speaks of SRV, for the record, but are any of the transformers shown here:

    http://allenamps.com/parts.html#transformers

    of sufficient "SRV" power for a 2 tube amp?

    I'm specifically considering the Accomplice (http://allenamps.com/accomplice.html)
    His upgrade for that model is the TP40D, etc.


    Thanks,

    Mark
     
  5. mbruffey

    mbruffey Supporting Member

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    or, I take it, a "showman" chassis/transformer would do the job, too . . . .

    Also, do the '64 SRV Vibroverb RI's have the beefed-up transformers, then?
     
  6. Matt H

    Matt H Guest

    ok, the "Twin" reverb transformer thing i was talking about...

    i think the "bassman" or "super reverb" sized transformer would work (push enough bass) but it's not an issue of pickign a transformer based on the number of output tubes you're using.

    output transformers have two major features that make them "applicable" for a certain application. The two are current (or power) rating and the impedance ratio.

    current/power rating is a pretty easy one to get.... you generally want an output transformer that's designed to handle the amount of power you plan to run through it. transformers that handle more power are generally larger. it's pretty logical.

    ok- as for impedance ratios.. you've probably heard people talk about "primary impedance". anyway- an OT doesn't have a primary or a secondary impedance... what it DOES have- is a ratio between the two.

    i've mentioned that the super OT and the bassman OT are almost the same... one of the biggest differences between them is the impedance ratio. they're roughly the same size, both used in 2x6L6GC amps. A super, however, is run with a 2ohm load on the secondary, whereas a bassman was designed around a 4ohm load on the secondary.

    now, let's say that the primary impedance that the transformers were designed to have to run a pair of 6L6GC's is 4k.

    now, the bassman is going to match the 4k primary to a 4ohm secondary. The ratio (the only thing the transformer itself has) is 4k:4 ohm (or 1000:1 if you like).

    the super, on the other hand, has 4k:2 ohms (or 2000:1). See WHY they're different transformers now?

    so here's the cool thing- stick an 8ohm load on the bassman's secondary. what happens? well- you're doubling the secondary impedance. Since the transformer only provides a RATIO- what is the primary impedance being reflected on the tubes? TWICE AS MUCH! (8k).

    ok- why did i bother going into that? To explain WHY the twin transformer is a good fit.

    now, we know that a pair of 6l6GC's in push pull "wants about a 4k primary impedance" (bunny ears used generously... talking round numbers, fender circuits, and, uhm any other disqualifying comments you can think of). So we can then see that a QUAD of 6l6GC's would want an impedance that's HALF of that, or 2k.

    now- the twin transformer is designed to provide an adequate ratio such that the primary impedance is 2k (for the four tubes) into a 4ohm secondary load. So, this transformer is 2k:4ohm (the ratio? 500:1)

    Now- what happens if you put an 8 ohm load on the secondary of that twin OT? You got it- the primary impedance that is reflected back to the tubes is doubled as well. so you've got a transformer that provides a 2k:4ohm OR a 4k:8ohm load (or, an 8k:16 ohm... get the picture?)

    Now- think back to the design of the vibroverb. a pair of 6l6GC's (which we know "wants" a 4k load, at least as designed by fender) into a single 8 ohm speaker. So, 4k:8 ohm... where have we seen that before? THE TWIN OT!!!

    So- that's one of the reasons the twin OT was "Desirable" in this application.

    The other, as you innocently asked, was that it was designed for 4 tubes, but only ran 2... remeber when we were talkign (briefly) about matching a transformer to the power the amp would be putting out? yup- same thing. You use an "oversized" OT, and it won't saturate. (more clean headroom, stronger bass). However, cheap little OT's (undersized) are also desirable in some contexts (it's comparing a twin reverb OT in a vibroverb amp as your "overkill" example... to say, a tweed deluxe- which is a dirty little monster, partly due to it's undersized OT- it chokes the bass response down making it a screaming lead monster).


    the SRV vibroverb fender RI's do a couple of things. I believe they do, in fact, use an oversized OT (perhaps not a twin sized one, but larger than the original vibroverb called for... probably more like the super/bassman type. I honestly don't "know", just seem to remember- don't quote me on that part). But- they have a few other "mods".

    tube rectifier is switched to solid-state when it's in the "mod" mode (the diaz mods as per SRV's amp). This is going to give more headroom and less sag (which you may have read about, if not, check out the www.aikenamps.com tech section about sag).

    the other thing they do is switch out the 25uF cathode bypass cap on the input gain stage(s) to a smaller value, .68uF if i remember correctly. This will limit the bass (and reduce the "Fender flab). People often forget that reducing bass earlier in the signal path can actually lead to the net result of TIGHTER bass...

    so, those three things (reduced input bypass cap, SS recto, oversized OT) are crucial in giving the amp a ton of headroom.

    The last ingredient, obviously, is a big clean 15" speaker. The JBL D130 is the most famous- a good modern replacement is the Weber California. I've never played through a JBL or an alnico Cali- but I do own a ceramic cali- and it sounds great driven by fender amps. (i've hit it with a bassman 50). I really like how the bassman through the 15" handles single coils. you've heard of big "piano" low notes? yeah...

    I think that any of the amps in question, with a few tweaks, will get you a very similar results- and i'd hesitate to say exactly which would give the most clean headroom if 1) all had SS rectos, 2) had their bass limited through cap changes, 3) ran two power tubes into a "not-undersized" OT.

    I think if i was going to mod an amp, I'd probably pick the BMR because they are cheaper, and I'd feel less bad modding it. :) (versus a mint '66 super... hehe)
     
  7. Matt H

    Matt H Guest

    oh, forgot to discuss the allenamps transformers.

    i notice he doesnt' have a twin OT available, but if i was picking one for a vibroverb (srv-style) project, i'd go wtih his TO2/4/8. It appears to be a 50W OT that has a multi-tapped secondary (2, 4, and 8 ohm secondary reflecking back 4k to the primary).

    keep in mind that the vibroverb/pro/bandmaster OT was rated to about 40W. it's "just about right" for those circuits, but many people prefer having the 50W OT- it gives a little more headroom and low end.

    (for instance, a common mod for a pro reverb is to install a bassman OT... and as we kinda mentioned before, the pro reverb isn't all that different from the vibroverb- mainly just the speaker selection was different)
     
  8. mbruffey

    mbruffey Supporting Member

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    These are great responses, Matt. I'm gonna print 'em out and file them! Thanks for your time and for sharing your knowledge! Mark
     
  9. Matt H

    Matt H Guest

    hey- no prob. :) that's why the internet is here, right? sharing of knowledge?
     

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