guitar speaker/tube challenge redux

[Paul R. Higgins]

Boy, with all this talk of litigation, Eric B and his super-sized ego, etc. 
let's have some positive stuff about t*bes!  I thought I'd tie up a few loose 
ends from my last couple of posts:

1) I think I might have confused a few people in my discussion of Hammond 
transformers.  I didn't even realize this myself until I re-read some of the 
replies:

Don Tillman wrote:

>  The lowest note on a Hammond organ is 32.7 Hz, the lowest note on a guitar
>  is 82.4 Hz.  That's a sizable difference [re: the size of the respective
>  transformers].

AFAIK, the Hammond transformer company (Hammond Manufacturing) has nothing 
whatsoever to do with Hammond organs.  Sorry if I confused Don or anyone else.  
I can't believe I didn't see this the first time around...to quote Homer 
Simpson, "Doh!!".  

2) Martin Czech wrote:

>  How can one destroy a tube (without an elephant sitting on it)?  Too much
>  anode current will make the anode glow (good idea to look at it), but
>  amateur radio people said to me that the tubes can stand that for a
>  while (a bipolar transistor or mosfet will die very soon if the SOA is
>  left so far behind). Tubes can also withstand very large anode voltage
>  transients/overstress compared to transistors (2nd breakdown, snapback).

>  If you do something wrong to a tube amp, either a cap or a transformer
>  will blow, tubes die slowly (my own experience and that of my amateur
>  radio collegues, perhaps radio tubes behave different from "audio"
>  tubes?).

You can indeed destroy tubes.  You are right that in general, tubes are more 
rugged and "blow-up-proof" than solid-state parts.  (As an added plus for 
Armageddon/Millenium/survivalist types, they can withstand the EMP of an atomic 
weapon going off!).  However, most--if not all--audio power tubes should NEVER 
have glowing red anodes.  A cherry-red anode is a sure indication that the tube 
has left its SOA and is headed for an early death.  This is one of the ways a 
tube can be destroyed.  The red-hot anode leads to a "gassy" tube and then to 
ionization and arcing...kiss your output stage goodbye.  The most common failure
mode leading to overheated plates would be a loss of negative bias (or 
inadequate bias in the first place).  Paradoxically, some RF tubes (which your 
amateur radio friends may use) are actually INTENDED to have red-hot plates.  I 
forget the reason, I'll have to look it up...something about the "getter" (gas 
scavenging) material being bonded to the anode?

>>  Hence the rise of the "reactive load simulators", one of which I know for a
>>  fact is a copy of a speaker equivalent circuit in the "Radiotron Designer's
>>  Handbook".  You could probably build a reactive load of your own for far
>>  less $$$. 

>  That's interesting. Any schematics online?

I don't know.  I've seen a number of amp/speaker simulators out there (email me 
privately if you'd like me to send you one as an attachment), but I've never 
seen an online schematic for a reactive load simulator.  If you have the CD-ROM 
of the "Radiotron Designer's Handbook" (available from Antique Electronic 
Supply--www.tubesandmore.com), the circuit should be there.  It's in a 
discussion about how speakers differ from purely resistive loads.  The schematic
is an equivalent circuit of a speaker.  I think it's just a resistor, capacitor,
and inductor.  It's got to be one of the easiest things in the world to build.

3) JH wrote:

>  I have very limited experience in that; the only thing
>  I've built and still use is a preamp with a little single ended class A
>  (EL84) power amp, approx. 1W, with a transformer I "found" somewhere, and
>  a 4 Ohm resistor to terminate it...does a decent job on my CX-3 for
>  that fake Hammond sound; better than a preamp-only box like the
>  Hughes&Kettner Metal Master I bought by accident (;->).

I'd love to see the schematic for this--sounds pretty cool.  I bet your EL84 amp
puts out more like 5W.  That's what the specs in the RCA Receiving Tube Manual 
suggest.  Maybe 1.5W in triode mode.

>  I'm sure that there *is* some contribution to the overall sound
>  from a saturated output transformer, little "peaks" appearing on the
>  maxima of a sine wave, which is quite different from other types of
>  distortion. I always thought that this is part of "the" sound (not sure,
>  though). If so, wouldn't you lose something when you retrofit your
>  Marshall with a "better" transformer??

Maybe...but then, do you really want your amp to blow up at a crucial gig just 
because you wanted "that" sound?  Furthermore, there are lots of amp companies 
out there, e.g. Wizard, VHT, etc. that build amps largely based on the Marshall 
topology, but with massively over-rated parts and transformers.  Their amps 
sound even bigger and meaner than Marshalls.  (In fact, VHT's flagship amp is 
called the Pittbull...grrrrrr!!).  Mesa/Boogie amps, which are a more "American"
design, but far more conservatively rated than Fenders, also have a really 
intense overdrive.  It is possible to saturate the output transformer without 
destroying it, IF you have a good one.

4) Harry Bissell wrote:

>  Is everyone aware of the flyback diode modification for the transformer
>  primary??? A couple of 1000V diodes in anti-parallel from the B+ (center
>  tap) to each plate in the push-pull stage... This will "defend" against
>  this problem.

Can someone explain how this works?  I've seen a similar mod that uses three 
1000V diodes in series from each plate to ground; cathode of the diode string 
facing the plate, anode to ground. 

Regards,
-PRH

_____________________________________________
Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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