power amp influence on sound?

[add4]

Hello,

I'm quite new to amplifier tweaking/building. I recently serviced a polytone i bought used for 25 euros and that got me thinking about building a polyclone.

The amp i have does follow exactly the schematics on the murchmusic site.
However, i read that this power amp was difficult to bias and was not really easy to build. Another guy that build a polyclone told that he replaced it by a FET power amp.
How much does a power amp change would change the sound of the amp?
Are there classical and good sounding power amp designs around 80 or 100 Watts that would preserve the polytone sound?

In general, are power amps transparent and does the color of the amp comes from the preamp?
Thanks in advance for your help

[Enzo]

Just my opinion, others may disagree:

In general, I don't see the power amp of a solid state instrument amp being a real part of the tone shaping.  I expect that to come from the preamp.  In the case of a combo amp, the single thing that has the most effect on the sound is the speaker.  And that includes its enclosure.

If I were cloning a Polytone, I'd probably just dummy up the preamp, and either just buy a commercial power amp or if I built something, I;d probably go with a chip amp like an LM3886 based project.

Or I might take an old workhorse like a Peavey head with a 400BH power amp and build a preamp on the front of it.   Or take the 400BH board from it and use it in my clone.

[add4]

that's what i read earlier and the confirmation i wanted.
So there is no hearable difference between power amps or different class and using different technologies (FET vs BJT vs chips vs ... )?

so basically, the great sound of a polytone comes from a super simple opamp preamp with a bandaxall eq, and a crappy speaker with no brand and name written onto it? could it just be the bandaxall eq with flat mids that make a big part of thatgood sound?

[Enzo]

Yes, I think as an overly simple generalization that is the case.   I will say there are some subtle differences in amps, but not really the tone thing.  I like the effortless sound of the old Trace Elliot heads, which I attribute to the MOSFET output stage and its low resistance.   But overall, I can;t hear the difference between the Hartke 7000 MOSFET or the Hartke 7000 bipolar amps.  ANy difference between those power amps in the otherwise identical amps is tiny compareed to the preamp.

[Roly]

This is a bit of a can of worms and harks back to the transistors vs tubes wars in days of yore, but I agree with Enzo and would add that the output stage in a typical solid state output stage has so much excess gain and so much negative feedback applied that below overload/clipping it makes very little difference what circuit or devices are used; they are all effectively transparent.

Once they clip it's a whole other story, but it is normal for solid state amps to introduce clipping in a controlled way in the preamp (or as a stomp) and to operate the output stage below clipping - then the tonality (if any) is determined by the preamp.

Looking at the circuits (attached) this looks like a pretty conventional amplifier.

Their power amp is quite conventional apart from the application of current feedback (R20 to R23), and while I'm not mad on the method of bias adjustment it should still be straightforward.

The only really notable things about the preamp is the use of thumpless LDR channel switching, and a fully asymmetric clipper in the drive channel.  The input impedance is 150+680= 830K for Hi and around 600K for Lo.

Of all the stages the Drive clipping stage, here U4, plays the major role in the tonality of a preamp; the number and type of diodes used, and how they are applied to the guitar signal.

Two other important factors in the overall sound are the speaker cabinet, and preamp input impedance.

It is fairly easy to cross-connect different amps and speakers, and you quickly discover that, generally speaking, different cabs make much more difference to the sound than different amps.

The preamp input impedance is important when a passive guitar pickup is used straight in.  Again as a generality, most amps (and stomps) have an input impedance that is too low and damps the natural resonances of a passive guitar, thus tending to make all guitars sound similar.

The tonal character of the reverb is determined to a large extent by the reverb springline tank itself (normally a third-party item), and to some extent by the drive circuit used (voltage or current drive).

HTH

[add4]

wow thanks for the great answer.
So in this case (my polytone doesn't have the drive section, nor the reverb section, only the basic amp/EQ preamp, and the exact same poweramp that is on the schematic). it is safe to assume that most of the tonal characteristics of the amp come from the bandaxall eq with flat mids, the bright switch, the speaker and cab ?

I'm curious about the heatsinking of this amp.
The 2 power transtore are mounted on 2 heatsinks, it seems ok to me, but the whole amp if filled with insulating foam, very light. i confess i worried about the heat resistance of this foam and wondered if it wouldn't start to burn if the power transistors started running too hot. is that standard practice?

Also, how do you guys make chassis? the biggest problem for me in this hobby is the hardware part :-)

[Loudthud]

The problem with most transistor power amps is that the Safe Operating Area protection circuits get activated when the amp clips with a speaker load. Driving a resistor dummy load is not a problem. The Polytone has no protection circuit other than a simple fuse. Drive it hard and it will eventually toast the output transistors. Most protection circuits are too simple to allow a big enough operating area unless the instantaneous voltage across the output transistor is limited to about 50V. There are some exceptions, and MOSFETs don't have this problem. Chips like the LM3886 have a much more complex protection circuit and they perform better. There is a graph on the LM3886 data sheet that shows the SOA. It's pretty impressive. Current limiting sounds bad. Voltage limiting is good. You need to stimulate a speaker with voltage and let the current go where ever it wants.

One other thing to look out for is overdriving the input of any chip. On the chip data sheet there will be an Input Common Mode range. Don't drive the inputs beyond it or the output will not obey the input. The output usually slams the opposite rail. It sounds really bad but you won't be able to diagnose it without an oscilloscope.

[J M Fahey]

Custom chassis making is the most complex task, because tools are "industrial" and not available for the home maker.
Some suggestions:
1) forget steel, work with aluminum sheet.
1.25mm thick is a good homemaker compromise.
1mm is way too flimsy and 2 mm, although desirable, usually can't be bent at home.
2) it *can* be bent to right angles, using 2 pieces of 2" x 2" angle iron as clamps, the whole pressed in a shop vise, and applying judicious pressure (or some hammer blows) through a piece of wood and a hammer.
3) I worked for ages using just 2 drill sizes: 10 mm for old style jacks (the ones you find in old Fenders and such), "large" (25mm) pots, even found a switch which fit 10mm holes, power cables (with a rubber grommet) and 4 mm for everything else: 1/8" screws which held PCBs, transformers, power transistors, you name it.
Primitive but functional.
If, say, I fly anywhere with my hands empty, I can start building "sellable" amps in a couple days, with stuff bought at a hardware store.
This is an example of a 1972 200W RMS amp made with such poor tools:


All holes either 10mm or 4 mm, except power fuse holder and switch (13mm), which required a couple minutes of filing.

[Roly]

I'm curious about the heatsinking of this amp.
The 2 power transtore are mounted on 2 heatsinks, it seems ok to me, but the whole amp if filled with insulating foam, very light. i confess i worried about the heat resistance of this foam and wondered if it wouldn't start to burn if the power transistors started running too hot. is that standard practice?

Also, how do you guys make chassis? the biggest problem for me in this hobby is the hardware part :-)

Lacking an internal pic I can only go from your comment, but some people seem to think that heatsinks somehow magically dispose of heat.  A heatsink is a coupler between the case of the device and the surrounding air, and it requires a constant supply of cool air to heat up to dispose of the generated heat.  If it is not mounted somewhere it gets a constant supply of fresh air it will simply recirculate the already warm air making it hotter.

Because heatsinks tend to be both expensive and a bit problematic to mount Murphy's Law of heatsinks is that they are often too small; you can't have too much heatsinking.  Note the ones in JM's pic above, lots of fins and covering the entire back panel.

I'm at a bit of a loss to understand "the whole amp if filled with insulating foam" and can only imagine that this is some sort of misguided attempt to make the electronics more robust for transport, and I would expect thermal problems to follow.

Can you post a pic please so we can see what you are talking about?

A couple of alternatives to metal bending are to use flat sheets joined with L-section, and have a look at the range of ali extruded sections that are available, I've seen chassis built up of various sections bolted together.

[Enzo]

The Polytone is a very compact combo amp, and closed back at that.  The preamp is a small cut in panel at the rear of the top - controls face up.  The power amp is on a small chassis in the bottom.  Shielded cables connect the two.  The speaker fills the front wall of the thing.  The entire cabinet is stuffed with common pink fiberglass insulation.  This basically a powered speaker.  The insulation is stuffed in this amp for the same reason it is stuffed in any speaker.  Nothing to do with anything thermal or with any sort of padding to the electronics.  SImply damping material for the speaker.

Pain in the ass to work on?  Yes, indeed.

Oh and chassis?   They make them commercially too.  I have never made a chassis for anything in my 50+ years of electronics.  I always bought either ready made chassis, or I repurposed some existing one.  COnsider that option.

[add4]

What enzo described is exactly what i found into my polytone.
i posted gutshots here :http://www.ssguitar.com/index.php?topic=2771.0

No distortion or reverb in mine, just the very simple basic preamp. the preamp pcb has a lot of free traces and spaces and i guess i could add a reverb from this, but i don't see where i would place the pot and the reverb unit

I'd be curious to know where i could buy chassis, enzo, could you share some links?
Thanks to you all for your help !

[J M Fahey]

You show the PCBs but not the fiberglass  
So I guess I'll have to show mine  :lmao:
This guy clearly had an obsession with fiberglass.
Some people think that not only heatsinks magically draw heat away, but also that fiberglass magically turns a bad speaker into a good one. :loco :duh
This is a "big" Polytone (by their standards), 100W into 2  10" speakers.
*Very* heavy for its size, which sort of defeats its purpose.
In fact, I think they do not blow often simply because they are used by gentle Jazz players, but when driven loud, yes, they die.
Notice the ton of free space available on the PCB and missing parts ... and this is 2 channel, with distortion and reverb.
I guess they designed *one* big PCB covering all bases, and mount just what's needed.

EDIT: just for you to feel better: the Factory original chassis shown, also has "DIY" characteristics.
By itself it's the simplest one, a "C" folded sheet of thin aluminum.
Since it probably was too flimsy, they added on the ends the two silver shiny blocks ... made out of a square piece of plywood covered in kitchen type aluminum paper, glued with contact cement, how's that ?

PS: try to find some local tinshop, the kind who custom make gutters, air conditioning tubing, pig or chicken feeding trays, etc.
They have the required shears and folders, and maybe some punching machines.
Lightweight type, because they are meant for 18 Ga or thinner galvanized sheet, but that means they can easily process up to 2mm aluminum.
The chassis I showed above was made that way: "they" cut and folded it and I did the rest.
Never went empty handed.
I was "this weird nerdy kid with crazy ideas" but visiting them with a couple "longnecks" (good quality wine bottles) in my rucksack always made me welcome by the blue collar guys in the shop

Or go the extruded aluminum route, visit some shop which makes custom windows, doors, office separators, etc.
Look at the catalogs, there are infinite cool shapes, pop riveting them (can't be soldered) will offer many possibilities ,,, and anodized, polished , scratched or blasted aluminum looks *good*.

Or go the Enzo route; get for peanuts or for free some dead amp and gut it.

[Enzo]

Aluminum - and steel - chassis boxes are an industry commodity.  Brands that come to mind are Hammond (the transformer people) and Bud Industries.  But i am sure ther are many others.   I spied a whole passel of them at Angela.com, a supplier to this industry, but general parts houses like Mouser, Allied, Newark, and most others will have them as well.

Chassis come in all shapes and sizes.  Look at the power amp chassis in your Polytone, is it not just a rectangular box?  And the preamp, the same, another rectangle?  Chassis have an open side, but you can either cut sheet metal for your own cover, or they usually sell covers too.  Chassis also come as project boxes.  That would be a two part (typically) box that completely surrounds the innards - no open side.  I am thinking basic sheetmetal chassis, but they also come diecast - like a lot of efect pedals are built in - rounded corners and a screw on bottom.  And there are more decorative things like to put finished goods into

Here is a page from Mouser with a large selection of Hammond chassis.  These are aluminum, but many come in steel as well.
http://www.mouser.com/catalog/catalogusd/645/2079.pdf

Here is the chassis page from Angela, note the plain boxes, as well as those ready punched out for guitar heads, and fancier stuff even.
http://angela.com/chassis.aspx

Here is the chassis page from AES
http://www.tubesandmore.com/products/enclosures


Those are just a few of many.  I was buying aluminum chassis 50years ago, and they were not rare then either.

[add4]

Great, thanks!
Another question : do you have a magical solution for producting small batches of PCB for a good price? do you etch at home or do you send them for production ?

[phatt]

Yep no worries,,,,,IF You've got a couple of million dollars to spare :lmao:
Might be cheaper and you will learn more if you take it all in steps.
you have to crawl before you can walk,, if you catch my drift?
Phil.