Conference napalm::guitar

    If you have made up your mind that you definately want a Solid state 
    amp, I would suggest you look at amps using MOST-FET technology.This 
    will give you the best distortion sound available from solid state
    amps.
    
    One of the most popular solid-state amps on the market is the Roland
    Jazz Chorus amp, but these amps do not have a good distortion sound
    built into them. They make great clean-sounding amps, but that's about 
    it. 
    
    Gallien krueger 250ML are extremely popular (at least they were a few 
    years ago) and put out a great distortion sound for SS. GK has a couple
    of 1-space rack mount guitar amps which have about 100wpc. They have
    two models like this, one of which includes lot's of built-in efx. 
    You'd have to have a separate speaker cab/cabs to go with these heads.
    
    Fender's little M80's and some of their other SS combos are said
    to have a great sound. Check em out.
    
    Of course, Peavey offers their Gain-Sat distortion in most of their
    solid-state amps. You may like it. Peavey has about a million differant
    models to choose from.
    
    This is just a small list. There are tons of other SS amps out there
    that you should look at. Carvin, Kustom, crate, etc. Check em all out.
    The best deals are on used equipment. Check the want ads.
    
    Mark
    

    Sounds like any good Peavey should do the trick.  Many a combo has
    reverb, many stages of eq, and their "sat" control is a very good
    overdrive.  Best of all, they can be had cheap.  Check Want Ad
    books, or hunt the music stores.
    
    AND, if you get into replacement of IC's, you can beef up
    the sound.  But that's another story.
    
    IF you can find one, grab a Lab L-5 combo.  Built by Moog, they have
    GREAT eq, compression, reverb, and 100+ watts rms.  Going price is
    anywhere from $200-$400.  A steal at that price.  Ave: $300
    
    Hope this helps:
    
    Jay Tashjian
    

    re .-1
    
    replacement of IC's to beef up the sound? Can you please fill me
    in a little? I've never heard of modifying a Peavey!
    
    - Dave (Special 130 owner)

>================================================================================
>Note 2034.3                  used guitar amp advise                       3 of 3
>DEMING::CLARK "psychedelic music fills the air"       6 lines  26-NOV-1990 10:12
>--------------------------------------------------------------------------------
>                                     -< ? >-
>
>    re .-1
>    
>    replacement of IC's to beef up the sound? Can you please fill me
>    in a little? I've never heard of modifying a Peavey!
>    
>    - Dave (Special 130 owner)

Hello Dave,

Here is my three penny worth.....

Early ICs (op-amps), just like early transistor amps, used standard (bi-polar) 
transistors and sounded "hard". Modern ICs used JFET's inside them, and just 
like the modern MOSfet power amps, sound warmer.

In addition, the JFET amps have MUCH lower input current ( higher impedance ), 
and therefore higher bandwidth and lower (odd harmonic) distortion. They are 
also much lower noise.

The result of all this is that front end stages are cleaner, filter circuits 
sound more natural and "open", and overdrive circuits sound warmer when using 
newer ICs.

The great thing is that the schematic designed around the IC does not need to 
be changed, so a plug -in replacement is all that is needed.  However care 
needs to be taken to ensure you are replacing a single op-amp version with 
similar ie. standard 8 pin DIL packages can come in single, dual, or even quad 
op-amps configured inside.

The standard op-amps I design and replace with are the TLO 71 (single) TLO 72
(dual) low noise JFET op-amps available in 8 pin packages.  There are others, 
but these are as good as any, and are cheap ( 18 UK pence..30 cents ?).

Now, do not expect to turn your cheap tranny into a Boogie, but if your 
current op-amps are non - JFET, then listen for a subtle change in tone 
sweetness, and a cleaner open sound, rounder bottom end, coupled with lower 
high frequency hiss.

Someone mentioned the Lab Series amps. I have built these, and agree they are 
great sounding amps with excellent EQ. A little noisy when turned up.

I have a couple of brand new PCB's for the pre-amp ( the power amp is just 
bi-polar and is nothing special, I always used MOSFET power stages).

I also have the full schematics. The compressor ( I think this is really a 
limitor ....sounds to me like early Fender twins on overload), is a real 
doll, and I have used this on it's own within a SESSION amp.

Get back to me if you have any further queries.

Regards.....Dave DTN 782-2314

Dave Litten @SBP
 

    You can go further with the ic replacements, I'll enter some tommorrow,
    as I REPLACE TL071/2's anyway, and EVERY x448!
    
    Jay
    

    I have found the BEST overall x448/072 replacement to be the AD712 from
    
    Analog Devices
    181 Ballardvale st.
    Wilmington, Ma.  01877     617-937-1428
    
    This is also about the quietest, most stable, most linear op-amp
    around.  Ya find folks replaceing hi-tech ic's from CD players
    with AD712s....!!!!
    
    When you remove any ic, replace it with a socket & ic.  A simple bag
    tie wraped around the socket & ic will hold it in place forever.
    
    I have a L5 with AD712s and let me tel ya, it rivals most TUBE
    combos, and has a eq system PAs would die for.  The compressor is a
    limiter in truth, but a great sounding one.  Overall, this is the best
    "do-all" amp I have owned.  The AD712s cost about $9 each (*!*) but
    are worth EVERY PENNY!!!!!
    
    Jay.
    

Jay,

	Thanks for the info on your choice of op-amp.  You are right that 
folks use them in CD players since they have been designed for D/A buffers
as well as low noise applications. They therefore have been designed to have a 
fast and accurate settling time.

They have identical noise specs to the TLO71, but a slight edge in terms of
slew rate and input current.

Other models of FET op-amps I have used and are superior specs to the 
AD712 are:

OP-42FZ, manufactured by PMI, also designed for A/D converters, 0.7 of 
the AD712 noise spec. Cost about 3:50 pounds

OPA121KP, manufactured by Burr-Brown. These beauties have a third of the noise
of an AD712. Cost about 4:50 each.

Still.......if you have done an A/B comparison and the AD712 sounds better.....
.....throw the theory books in the junk pile !!!

Out of interest, what IC's do the latest Fender amps use ?

You have got my juices going, and I will have to try some out !

P.S. the AD712 cost about 1:50 pounds each here in the UK.

P.P.S.  I note with interest the lastest MOSvalve power amp that uses the 
output stage open loop ie. no positive or negative feedback.  I reckon it 
would be fairly easy to convert a "standard" MOSFET output stage.  

Anyone got a schematic ?  I will publish the results here.

P.P.P.S.  put a bit of "BLUE - TACK" on the IC socket before you insert 
the IC (keep it away from the connectors), it will keep it in place forever.

Regards,

Dave 
 

   I've been following the recent discussion on IC replacement and 
   decided to try it on my own S.S. amps (Peavey).  Due to incomplete
   cross-reference guides I have the following questions which I hope
   someone here can answer.

   1)  I have two amps, a Studio Pro 40, and a Bandit 65.  The Pro has
       (3) "510A / RC4558P" op-amps in its preamp circuit.  The Bandit
       has (4) "549A / RC4558P" op-amps.  Are these the same units?  The
       "RC.." number makes me think so, but the TI number (5xxA) makes
       me wonder.  (The "RC..." number matches an SK3465/778A.) 

   2)  The Pro uses power transistors labeled "SJ6392 / 8426".  The
       Bandit uses p.t.s labeled "SJ6392 / MEX / 8547".  Both amps use
       two power transistors.  The "SJ" number makes me think they're 
       similar; the 2nd number makes me think they're different.  Are
       they the same or not?

   3)  Radio Snack has #1458 op-amp ICs (also labeled TO35AB) which seem 
       to be a direct replacement for the RC4558P/SK3465 units I currently
       have.  Will replacing my 4558Ps have the desired effect?  (I.E.  
       Are 4558Ps equipped with bipolars, whereas 1458s are JFET based?)  
       If the R.S. unit is not particularly great is there something else I 
       should use?  (E.G., are the Peavey parts as good as I can get?)  

   4)  Is it worth it to change the output transistors?  If so, are the
       above numbers interchangeable?  With what should I replace the
       current units?

   Thanks for any info,

   -- Sam

Hello Sam,

	I will take your questions in order........

1.) The RC4558P number in each case determines the type, and they are therfore
the same. The difference in the other numbers I would have to guess at, but 
are likely to be different distributors, manufacturing plant, production run 
etc. They may signify some small change in specifications, ie. BOSS pedals 
have their own run of lower noise variants of op-amps.

The RC4558P is simply a dual amp version of the 741 design, it is bi-polar and 
not JFET. 
Both of the versions will be plug (pin) compatable. In my opinion you 
will gain nothing from these variants, and I advise you to replace them with
the JFET versions that Jay and myself were advising.

2.)  I am not familiar with the many types of output transistors, and the 
numbers you have given do not ring any bells for me. I am sure that Jay 
through his extensive experience will be able to advise.  However, from the
numbers I would guess they are bi-polar power transistors. I do not advise tha 
you change them as transistors of different types my need slight changes of 
the components around them.  You will not gain from any sonic improvement.

3.)  The 1458 is a bi-polar dual op-amp and it is pin compatable with the 
     4558.  Again you will gain nothing from using this type.

There is nothing special about Peavey parts in themselves, unless Peavey have 
specified a quality op-amp. I am sure you will be able to get the JFET amps in 
your local electronic parts shop. If you are having problems, get a list of 
op-amps and publish it here, I am sure Jay and myself can advise the best type
based on what you can lay your hands on.

4.)  Unlike op-amps, you will not be able to replace power bi-polar 
transistors with power FETs. This is due to the different schematics used to 
drive them.

If you are capable of reading a schematic and carrying out a small amount of 
re-wiring, I would suggest you replace the output stage with a Power FET 
board.  These are available from many distributors as a PCB with the 
components already mounted. They would use the same power rails, so no other 
mods would be required apart from perhaps using a different heatsink.

It is worth mentioning a fundamental difference between bi-polar and FET power 
transistors. As bipolars get hot ( you running your amp at med to high 
volume), they reduce their resistance, this means they draw more current 
resulting in thermal runaway and eventually blow. Designers therefore go to 
great lengths to ensure adequate heatsinks and thermal feedback. As you have 
probably guessed, FET's do the opposite. They are inherently thermally 
stable. This means easier circuit design and smaller heatsinks. They also need 
less drive current ( as tubes do) and so the pre drive stage is far simpler.

All this means to you is that replacing the power output stage should be a 
fairly easy task if you have some previous experience and a little skill.

I seem to have gone on a bit, but hopefully the above is some use.

Dave

   Dave,

   Thanks _very_ much for taking the time to write that up, it's extremely
   helpful.  The only problem I have at this point is the relatively low
   number of well stocked "electronic hobbyist" stores around - Radio Shack
   seems to be more interested in selling electronic toys these days than
   parts, and there just aren't many other places one can walk into and
   get a half-dozen op-amps.  I'll certainly see what I can come up with,
   though.

   Thanks again,

   -- Sam

Sam,

	I have no idea of the names of hobbyist outlets in the US, we have 
lots of choice here in the UK.

Jay gave the address of Analog Devices, the TLO72 are made by Texas 
Instruments ( I hear tell you folks have a village called Texas...spend an 
afternoon wandering through it....you are bound to find TI !! [joke!]...well 
nearly....)

Perhaps some US readers can advise.

Perhaps we can do a deal......to save taking up noters space, mail me on
Dave Litten @SBP


Dave

    Sam,
    
    I've had good luck getting electronic parts and connectors from Sager
    Electronics which I believe is in Norwood, MA.  There are a couple of
    electronics distributors on the southern side of Boston, so I might
    be getting it mixed up with the one that's down of rt3 3 near Quincy.
    I'll check for catalogs tonight.
    
    Another possibility is You-Do-It (You-Blew-It 8^p ) which is off rte
    128 near Newton Highlands, MA.  This place tends to not have a wide	
    selection, but occasionally they're the only spot.
    
    Oh, one final name that just came to mind is Active Electronics, which
    is somewhere between Framingahm and Westboro, MA, just of rte 9.  Pretty
    good spot.
    
    None of the above spots gave me any hassle when I was buying just one
    or two of something, even though they're distributors.  Money talks, 
    no matter how quietly.
    
    Good hunting,
    John
    
    PS - For place like Sager, one calls ahead to place the order; you pay
         at pickup time.

re: .-1

Thanks for the info.  I'm already due to go to Active to check out some
other parts (replacement pots for the KH) so may just see what they have
to offer w/r/t replacement chips.  Also, I just ordered a catalog from a
mailorder parts shop (can't remember the name;  they're listed in the KH
note (1103)).  U-Blu-It is just a bit too far from Manchester, unless I 
stop in next trip to the Cape...

General question:  Are TL082s reasonbly equivilent to TL072s?  I found
a source for the TL082s, and checked the specs:  they're pin compatable
to both TL072s and 1458s, and have very similar characteristics.  They're
billed as "BiFET/JFET dual op-amps".  Are they useable for this applicat-
ion?

Thanks again for any/all info,

-- Sam

        Re:     <<< Note 2034.13 by SMURF::LAMBERT "Specialization is for insects" >>>

        TLO72s are low noise versions of the TLO82.

>
>General question:  Are TL082s reasonbly equivilent to TL072s?  I found
>a source for the TL082s, and checked the specs:  they're pin compatable
>to both TL072s and 1458s, and have very similar characteristics.  They're
>billed as "BiFET/JFET dual op-amps".  Are they useable for this applicat-
>ion?

Sam,

	The TL082's are physically compatable, but the TL072's are a LOW 
DISTORTION/LOW NOISE version of the TL082's.

The 1458 is a bi-polar op amp and I suggest you replace it with a TL072.

The TL082's are "useable", but.....

My advice is go for the TL072's. There is little (10 cents/5 pence) price 
difference and the higher spec is well worth it in a guitar/audio preamp 
application.

From my experience, the best upgrade anyone can make to their solid state amp 
is to replace bipolar op amps with the TL071/2 range. (the TLO71 is a single 
op amp version of the TL072).

Hope this helps.

Dave

    
    	Fascinating stuff.  Out of curiosity, how would you rate the SK7641
    	(858M)?  It is a dual op amp, JFET input (whatever that means).  I
    	*think* it's made by/for RCA.  At the place I bought it, this
    	turned out to be a cross ref for the TL072.  It made a big 
    	improvement over the 1458.
    
    	Thanks,
    	-pat

   To follow up:  I tried out the TLO82s available at Radio Shack.  Hey, for
   an $8 investment (for all 4 that I needed) I figured it was worth it, even 
   if they weren't the low noise TLO72s (which I haven't been able to find 
   locally, and which are rumored to cost $8 a PIECE...).

   Well, it _was_ worth it.  Nice smooth tone, much better sustain, higher
   gain (a LOT), and better bottom end.  You can really hear the difference
   on a long, sustained note ala Santana.

   So, I now have a Peavey Bandit 65 with an amazing sounding preamp stage.
   Alright!  

   One thing I found odd:  even with the volume controls turned all the way
   down a little sound "leaks" out.  I'm not sure if this condition was true
   prior to changing the preamp ICs or not.  It doesn't appear to be a fault
   though, as I've used the amp quite a bit and have had no problems.

   Thanks for the advice.  I'd encourage anyone to make this mod to their
   solid state amp.

   -- Sam

   p.s.  Now onto the Kitty Hawk mods!  :-)