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How Doubling Guitar Amp Power Output Affects Speaker Performance?

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  • pdf64
    replied
    Originally posted by Helmholtz View Post

    Unbypassed cathode resistance is common practice with LTPIs. I don't think V4 in a Fender compares to a differential amp.

    Some literature: http://www.valvewizard.co.uk/SelfSplit.pdf
    I remember servicing a Gibson GA20 with a paraphase that had a really vicious overdrive character. A temporary type 1 master vol revealed that the paraphase was spitting out parasitic blips when it was being (heavily) overdriven. With a bypass on the shared cathode, the blips were gone and the amp behaved nicely. A paraphase is a pair of cascaded stages, so the ‘V4’ positive feedback mechanism should apply?

    But for the self split p-p pair, as the common grid section doesn’t do a polarity flip, that mechanism can’t operate

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  • nhbassguitar
    replied
    Originally posted by pdf64 View Post
    A JMP Super Lead was too much for me. I certainly wished that the other guitarist didn't also have one, or at least didn't insist on 'natural' valve overdrive, ie ear bleeding volume, for his solos
    Too funny. I've been down that same road dozens of times. Typically every time it happens I politely explain what a volume control is, and I insist the player stand directly in front of the cab rather than off to the side where he typically is. I've even had it happen with Super Reverbs. Those four tens can be very directional -- the array effect. Players sometimes don't "get it" until they're precisely on axis. And then if they still don't get it, I'm out of there. I've got better things to do with my time than share stages with the clueless.

    I once heard Robin Lane and the Chartbusters live. I'm pretty sure it was Leroy Radcliffe up there, playing a Tele through a Twin with JBL D120s. I was at the back of the club and even back there it was torture, especially when the lead break in "When Things Go Wrong" came around. They typically weren't mic'ing instruments back then, so the amps had to carry the room.

    But anyway, one of the most responsible players I shared a stage with ran a 50 watt EVH into a Mesa CabClone. That guy had tone to die for. Total perfectionist. You'd think it would occur to some of these volume freaks to try something like that. I don't know. Maybe I expect too much of humanity...

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by pdf64 View Post
    Crikey, working that stuff out is tough and I’m outta practice. I worked it out previously as being the same mechanism as V4 on a Fender reverb channel; if the shared bypass cap goes high esr, high reverb settings cause electronic (rather than mechanical) oscillation.
    But I’m stuck trying rationalise that at the moment
    Unbypassed cathode resistance is common practice with LTPIs. I don't think V4 in a Fender compares to a differential amp.

    Some literature: http://www.valvewizard.co.uk/SelfSplit.pdf
    Last edited by Helmholtz; 06-12-2020, 09:08 PM.

    Leave a comment:


  • pdf64
    replied
    Crikey, working that stuff out is tough and I’m outta practice. I worked it out previously as being the same mechanism as V4 on a Fender reverb channel; if the shared bypass cap goes high esr, high reverb settings cause electronic (rather than mechanical) oscillation.
    But I’m stuck trying rationalise that at the moment

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by pdf64 View Post

    Have you ever tried that?
    No, just remembered some old time amp circuit.
    How would you explain a positive feedback effect?

    Leave a comment:


  • pdf64
    replied
    Originally posted by Helmholtz View Post
    BTW, cathode biased (no bypass cap) class A push-pull stages can be operated in self-split mode, thus avoiding the need for a separate PI and allowing for a smaller, more efficient OT.
    The operating principle is the same as with a LTPI or differential amp. In its simplest configuration only the upper tube is driven by the preamp while the grid of the lower tube is grounded.
    Have you ever tried that? I wonder how the arrangement would cope with being overdriven
    The concern being that paraphase splitters that share an unbypassed cathode can have a positive feedback mechanism that can cause them to overdrive horribly.
    I've got a suitable amp chassis / PT to try such experiments out on; one day I'll have to get it actually wired up and try these things out

    Leave a comment:


  • Helmholtz
    replied
    BTW, cathode biased (no bypass cap) class A push-pull stages can be operated in self-split mode, thus avoiding the need for a separate PI and allowing for a smaller, more efficient OT.
    The operating principle is the same as with a LTPI or differential amp. In its simplest configuration only the upper tube is driven by the preamp while the grid of the lower tube is grounded.
    Last edited by Helmholtz; 06-12-2020, 02:13 PM.

    Leave a comment:


  • K Teacher
    replied
    Thinking a little bit more on the subject... what limits the VC power handling is heat...
    If the VC is wrapped on a heating generating element, like the FC, its power handling capability will be diminished compared to a PM (permanent-magnet) speaker...

    Leave a comment:


  • K Teacher
    replied
    I did not get any info on the F10-U VC diameter either... maybe 3/4"?

    BTW, F12-N power handling is only 18W, NOT 50W like modern P12N...
    Also, want to point out that all of these power figures are rated power, that is RMS...
    I estimate musical power (instantaneous peak) to be about 2x rated power...

    Leave a comment:


  • K Teacher
    replied
    Hi g1,
    Thanks for the info... I never paid attention to the EH-150 before...

    Hi pdf64,
    I have been looking for info on the Jensen F10-U speaker...
    I even wrote to Jensen, but they said that they do not have information on speakers this old on their files anymore.
    If I find anything I will share here...

    Looking on several places for info on the F10-U speaker, I noticed that, at that time, Jensen had the same field-coil speaker type with different field-coils (more current, less volts and vice-versa, all requiring the same power) to accommodate the amp manufacturer's design.

    What I got on the Jensen F10-U speaker used on Gibson's BR-9 & earlier GA-9, so far, is this (not sure if it is accurate):

    - 10 in field coil
    - Smooth cone (like current production P10Q)
    - 8W handling power
    - 3~4 ohm VC (voice-coil) impedance
    - VC DC resistance ??? ohms
    - Sensitivity ??? dB (as far as I know, they did not pubish sensitivity back them...)
    - FC (field-coil) DC resistance -- 850 ohm +/- 5% (NOT 1000 ohms as annotated on the schematic, that is for the F12-N)
    - FC power -- 5.5W (for Jensen F12-N is 14W) (this implies minimum required current for FC to operate)

    NOTE: Confined resistors need to have a de-rate of at least 5x, that is, in this case (F10-U), 850 ohm - 30W FC replacement resistor.

    About the design configuration... you are right... the Gibson's BR-9 circuit is very similar but is push-pull and uses transformer PI (phase-inverter)...
    additionally there is an earlier GA-8, also push-pull self-split PI.
    To my surprise, Gibson's latest GA-8/ GA-9 design is PARALLEL SE !!! Improvement ???

    Leave a comment:


  • pdf64
    replied
    Originally posted by K Teacher View Post
    ... the Jensen F10-U needs around 80mA to work... add another 6V6 wired in parallel to the first one...
    If you find a site with relevant info, especially if it’s somewhat arcane, please share a web link

    A 2nd power tube could have been in push-pull, rather than SE parallel. For a given amp performance, that could save cost on the OT and HT filtering.

    Leave a comment:


  • Chuck H
    replied
    Sorry, no specs on the f10-U, but...

    The parameters you're questioning regarding field coil speakers are entirely ratio dependent. As to impedance and frequency response relative to inductance these parameters were all able to be satisfied by even a single valve single ended amp in the era referenced. There are a few hi fi designs that demonstrate this.

    It's more likely the economy and lack of stringencies of the market that dictated available designs.If the stringency wasn't there, they didn't bother to meet it. But there's another aspect...

    considering guitar amplifiers we have to keep in mind that they are tone PRODUCERS, and not tone replicators. Anything coming out of a guitar amp would have been recorded and reproduced by circuits that are purpose built for THAT aspect. Since only studios and aficionados needed the highest reproduction accuracy this luxury would have been fudged or even omitted from instrument amplifier criteria.

    That isn't to say that guitar amps from any era don't sound good. This is the very thing they are purpose built for. They evolved with guitars and electronic music. But they AREN'T reproduction amplifiers. So...

    Don't worry so much about what is "proper" and maybe focus research on how players like the sound of any given design. How well the speaker is accommodated for absolute accuracy is a problem for recording and reproduction. Which aren't guitar amp tone considerations.

    Leave a comment:


  • g1
    replied
    Some of the earlier Gibsons used a ballast resistor (to ground) after the FC to pull extra current if that was required. (early EH150 for example)
    So that would be a much cheaper option. Not saying your theory is wrong, but Gibson did a lot of weird things and changed up mid-stream often. Maybe they thought it was saving the cost of a PI compared to a push-pull 2x6V6 version.

    Leave a comment:


  • K Teacher
    replied
    Of course, you can use the same configuration for PM speakers, but the benefit is not there... perhaps this is why this guitar amp output configuration is not used anymore...

    Leave a comment:


  • K Teacher
    replied
    So, I guess my initial hypotheses didn't worked-out...

    Leave a comment:

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