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Old 29th Jun 2022, 11:56 am   #1
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Default Kit Guitar Amp Restoration

I picked this up in a scrap yard over 15yrs ago intending to "do it up" one day, an Oz designed, kit built guitar amp from the late 1960's running a pair of 6DQ6's in the output, there were 2 versions published 40W and 60W, this is the 60W version - in their day they were very popular, as time wore on though, they were mostly forgotten as guitarists turned to Fender, Marshal etc. In the last few years they have made a comeback as Oz guitarists are finding out they are every bit as good as the overseas gear. I recently pulled it off the shelf and saw the chassis rust had well and truly taken hold and was much worse than I recall, so rather than let it get worse I've started a full bare chassis rebuild.
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Old 29th Jun 2022, 11:58 am   #2
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Default Re: Kit Guitar Amp Restoration

Electroplaters advised against trying to replate as they can't guarantee all of the rust can be removed from the many deep pit marks so a coat if satin black should protect the chassis for another 50yrs.
The valve and input sockets and tag strips came up real nice after an hour or so in the ultrasonic cleaner, the pots are not worth the risk of trying to resurrect, the Tx's are all good and look nice with a new coat of paint, new electro's and pots have arrived so I can make a start with the wiring.
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Old 29th Jun 2022, 12:02 pm   #3
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Default Re: Kit Guitar Amp Restoration

Part of the wiring completed - I though this would a reasonably quick project but it seems as if it is nearly the same as a scratch build, I'm keeping as much of the original stuff as possible, moving the on/off switch back to the rear panel where it was originally, with a power indicator bezel light along side the standby switch, on the front panel - the original speaker socket was one of those 4 pin things that were popular in the 60's, the impedance was selected by wiring the speakers to the correct 2 pins, I'll deviate slightly by fitting a 3 way impedance selector switch and a standard 1/4 inch socket.
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Old 29th Jun 2022, 10:37 pm   #4
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Default Re: Kit Guitar Amp Restoration

Its another Playmaster design that I mentioned in another current thread. They are very good amplifiers that can pump out lots of volume for lots of hours without struggling. To be really excellent you would need the Rola 12UXG speakers that go with it. The last of aussie made Rola's still with alnico magnets. They are very similar in performance to Goldentones, another highly regarded aussie amp. Also used the same speakers.
Moody also made their own amps and were very good at it. I still have a rusted out like yours chassis somewhere with a brass nameplate stating serial number on it. I will try find it. It was PP 6V6's though.

Do you have the origional articles ?. Shoot me a PM if you need them.

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Old 30th Jun 2022, 12:39 am   #5
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Default Re: Kit Guitar Amp Restoration

A bit of an update:
Both circuits for the 116 and 117 Playmasters, with the notes and errata that followed. DO fit the diode to the bias capacitor!!. These came from a guitarist forum here in Aus. I do have the full articles, but I will have to go through my magazine collection to find them. I can do this if you wish.

A note to other readers, this amp uses a 6BL8 in the volatge amp phase splitter position.
It works exceptionally well.

Joe
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Old 30th Jun 2022, 11:26 am   #6
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Default Re: Kit Guitar Amp Restoration

Just a quick note, as I sprayed a Mullard 5-10 black recently, for the same reason.

Be aware that the paint interferes with any bolted grounds, particularly the body of the volume and other pots and the input socket, if applicable. If you find that they are not grounded (hum, made worse by placing your hand in close proximity) Dremel some paint off the area around the hole so that the bolt and washer make good contact with the metal rather than the paint
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Old 30th Jun 2022, 3:11 pm   #7
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Default Re: Kit Guitar Amp Restoration

Gabe:- Thanks for the tip - I did place small circles of masking tape over each of the chassis holes so each tag strips would make contact with the chassis this can be seen in the last photo, but didn't think of the pots.

Joe:- Thanks for the offer of the original articles, I do have both of them though - I notice that the articles are geared towards those with a knowledge of electronics and also some practical experience building gear, there is a partial layout diagram, but for the rest the builder is on his/her own working only from the circuit, compare this to a modern day Silicon Chip constructional article, while I wouldn't go so far as to say it's "spoon feeding" there is a lot more info and detail in the layout diagrams, one almost doesn't need to know how to read a circuit diagram.
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Old 6th Jul 2022, 3:14 pm   #8
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I've made more progress with the wiring but really needed to attend to the front panel, the original looked to have been punched from 1.6mm Al, screen printed and then covered with a plastic film for protection, it looked very knocked about and I'm not keen to use it unless I really have to. I did make an early attempt at engraving with my (new to me) CNC mill, it was a fail but did teach me a few things.
A collection of engraving/milling cutters I had ordered arrived so I had another go this time with a thicker piece of Al at 1.2mm rather than the 0.8mm piece I had previously tried. It soon became clear why those 0.5mm engraving end mills come in packs of 10 - they are VERY fragile. The engraving was 0.5mm deep and even with very slow feed speeds the first broke after engraving 'VOLUME' the second after 'BASS' the third broke after I caught it with a rag as I tried to brush some metal chips away etc etc - I changed tactics and resorted to a home made 60 deg D bit engraver, this throws up quite a burr - I sanded it back with some 800 wet & dry and looking at it under magnifier I could see that some of the burr had been pushed down into the engraving so I ran it through again - which needed more sanding which needed another pass of the engraver and so on, I ran it through 5 times in all.
After a full 8 hr. day the end result is passable, not a great deal to show for the time spent but I have learnt a lot about the engraving process which will help me for future jobs. The panel should have some sort of protective coating, after which I'll fill the engraving with paint. I'll investigate plain anodising, I can make an anodising tank from a length of 100mm sewer pipe, then it is just a matter of finding a long enough metal container for the final boiling step.
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Old 6th Jul 2022, 5:21 pm   #9
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Default Re: Kit Guitar Amp Restoration

Why don't you print a front panel graphics using a standard printer on photo paper, place it behind an acrylic panel (1mm), or sandwich it between 2 pieces of acrylic, cut out the holes for the posts and keep it in place using the nuts of the control pots?

Essentially similar to how dials in radios are recreated (in some cases anyway) when they're broken
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Old 6th Jul 2022, 10:33 pm   #10
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Default Re: Kit Guitar Amp Restoration

Or Goldentone amplifiers. Although they are printed onto acrylic.
Anyway, for a first attempt it looks promising. Paint filled and anodised nobody is gonna check it out with a jewellers loupe anyway.
Interesting, that I have been looking at engravers for exactly the same purpose. I have to save, but I will be looking at a laser engraver. Latest one is 20 watts laser and will engrave stainless steel
( apparently ! ). Will cut 15 mm plywood in one pass.

Joe
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Old 7th Jul 2022, 2:40 pm   #11
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Default Re: Kit Guitar Amp Restoration

I'm bemused by the power supply arrangement, with a full-wave rectifier for the output stage and a bridge for the earlier stages. As I understand it, the earlier stages thus get about twice the HT that the OP stage gets. Have I understood that correctly, and if so what was the rationale?
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Old 7th Jul 2022, 4:25 pm   #12
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Default Re: Kit Guitar Amp Restoration

Isn't the half-wave recified bit the grid bias for the output stage, and the bridge does HT for everything else?

Because it is push-pull - does this mean the hum-ey grid bias gets cancelled somehow? And why do this instead of using cathode bias resitors in the output stage - due to fluctuating currents (at different o/p levels) causing shifting bias conditions?
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Old 7th Jul 2022, 6:30 pm   #13
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Default Re: Kit Guitar Amp Restoration

Quote:
Originally Posted by RichardGM View Post
I'm bemused by the power supply arrangement, with a full-wave rectifier for the output stage and a bridge for the earlier stages. As I understand it, the earlier stages thus get about twice the HT that the OP stage gets. Have I understood that correctly, and if so what was the rationale?
The bridge rectifier supplies 380 V to the centre tap of the output transformer, so to the anodes of the power tubes. This same 380 V is used (with some RC filters) for the rest of the amplifier, except for the two screen grids of the power tubes. Those are being fed 190 V, which is derived from the centre tap of the power transformer (note that this 190 V is half of 380 V). If I understood correctly, dc-current will flow in the power transformer so it has to be up to the task.

This arrangement was not unusual in amplifiers which used valves that were primarily designed for line output duty in television. These valves function with relative low screen grid voltages.

The Philips series of PA amplifiers using the EL36 as power valves is an other example of this (attached is one of them).
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Old 7th Jul 2022, 8:52 pm   #14
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Quote:
Originally Posted by Robert Gribnau View Post
Quote:
Originally Posted by RichardGM View Post
I'm bemused by the power supply arrangement, with a full-wave rectifier for the output stage and a bridge for the earlier stages. As I understand it, the earlier stages thus get about twice the HT that the OP stage gets. Have I understood that correctly, and if so what was the rationale?
The bridge rectifier supplies 380 V to the centre tap of the output transformer, so to the anodes of the power tubes. This same 380 V is used (with some RC filters) for the rest of the amplifier, except for the two screen grids of the power tubes. Those are being fed 190 V, which is derived from the centre tap of the power transformer (note that this 190 V is half of 380 V). If I understood correctly, dc-current will flow in the power transformer so it has to be up to the task.

This arrangement was not unusual in amplifiers which used valves that were primarily designed for line output duty in television. These valves function with relative low screen grid voltages.

The Philips series of PA amplifiers using the EL36 as power valves is an other example of this (attached is one of them).
Thank you. Looking again I now understand that. I had misread where some of the connections went.
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Old 7th Jul 2022, 10:41 pm   #15
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That power supply design is in fact quite elegant!!. If the HT ( plate supply ) drops, so does the screen voltage. It's virtually impossible to overload the screens. These amps are very good units and still command decent resale prices. They are not quite as good as Goldentone, but are capable of producing lots of noise. They don't really have any "overdrive " or distortion from the power amp though, and I believe that's because of the power supply design.

Joe
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Old 16th Jul 2022, 2:12 am   #16
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A bit of an update - the saga of the front panel artwork continues - after watching a number of youtube videos on anodising I decided it was time - a week before I had welded up a boiling tank from 1.2mm sheet steel, 12cm wide and 59.5cm long, long enough to hold future front panels if needed, it just spanned 3 burners of our kitchen stove top - I used 1 in 10 mix of sulphuric acid /water as the electrolyte - a car battery charger 8 amps max as the power source and some lead flashing as the negative electrode, the process was to clean the panel with soapy warm water, a quick dunk into a 5% caustic soda solution (1 - 5 min) rinse and then into the anodising tank - I was sure the youtube videos stated that bubbles came off the positive electrode (front panel) and after a few minutes of seeing only bubbles from the lead I decided that I had done something wrong so I shut it down. I re-watched one of the videos and yes they did say bubbles but weren't specific about which electrode gave off the bubbles - I decided to do a test piece on part of the first panel I had made that had failed when I engraved it. A quick rub with steel wool, into the tank - at 12V the charger amp meter showed 2.5 amps - I left it for around 90min - the only bubbles were from the lead electrode - on pulling it out it looked no different - did it work ?

I had purchased 3 packets of Rit fabric dye to test as an anodising dye so I mixed up around a tea spoon of the Red Wine dye into 500ml of water and dunked the test piece in for around 10-15 min - the dye penetrated perfectly - after 20 min in a saucepan of boiling water the result as good as I could have hoped for, which showed my anodising process was working. The next day I had another attempt at the panel after cleaning it up and spending time getting a good stable electrical connection - the charger indicated around 4 amps - I left it anodising for approx 90min and then boiled it for around 20 min - on looking closely it did have a different feel - the idea of the plain anodising is to give the panel a protective coating that will not show finger marks. Next process was to fill the engraving with water based black paint - I first tried with a very fine brush under a magnifying glass - wiping away any paint that went 'over the lines', this proved tiresome and didn't work that well, so I decided to flood the engraving with the paint and wipe away the excess with a soft paper towel wrapped around a cork sanding block (every time we eat out I grab the unused napkins for use in the workshop), once the paint was fully dry I went over the panel with more paper towel this time moistened with acetone, the end result is ok but not great - I used an old piece of Al and the nicks and dents which previously were almost un-noticed now have paint in them - while they don't over whelm the panel they can be seen if you look closely.

In my anodising internet research I came across those that said fabric dye is no good and does not give good results, so I decided to try a bit of scrap Al with black Rit fabric dye - mine came out every bit as good as the red it is indeed black, maybe those that stated fabric dye was no good used a different process. For those that are interested I used a 1 to 10 mixture (by volume) of conc. (98%) sulphuric and water as the electrolyte simply because I have an almost full winchester of the stuff, the anodising current density should be approx 800 amps per sq ft. per min - so in my case with a 0.235 squ ft panel I worked out around 5 amps for 90min - I believe this is not that critical and can be varied plus or minus 10% so anything from 700 - 900 amps per squ ft per min. I used DC current but I did read that AC can be used if the negative electrode is made from aluminium rather than lead. Seeing the success of the black I may try again this time with a new piece of Al - anodising it black and then engraving, so the end result would be black background and silver graphics.

Younger forum members probably don't appreciate having the internet at hand to 'look things up' I can remember in the days before the internet catching the train into the CBD and spending a whole day at the library trawling through the industrial arts index and then trying to local the relevant trade journals, papers and magazines etc to look up things like parkerising steel, the internet is a truly wonderful thing.
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Old 16th Jul 2022, 3:39 am   #17
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Default Re: Kit Guitar Amp Restoration

NICE progress mate !!!. Thats a fine result. Thanks too, for sharing it, as that may induce us all to more anodising. I had a few jobs ( paid employment as a factory manager, I mean ) that used anodised panels, boxes and labels, but I didnt have any input as to the how, or why, or where.
Apart from your chassis paint colour, I think this will be a fine example of a very fine amplifier.

Keep up the excellent work !!.

Joe in freezing cold north queensland.
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Old 27th Jul 2022, 2:42 am   #18
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Finally finished the wiring (I think) - it was sort of routine but I did find the chassis a bit on the cramped side, it isn't that easy to make point to point wiring look neat - there is a lot to be said for a turret/tag board holding most of the components, easier to wire, and easier to service when the time comes. The chassis at 38cm is not that long and not really long enough for a turret/tag board to hold most of the components - it is quite cramped around the power switch and fuse holder, and also around the first preamp valve, which makes that part of the under chassis look untidy. Insulating mains connections was not that common back when it was designed it, one had to know 'do not touch' even though I'm no newbie when it comes to amp building but I'm not that comfortable with any exposed mains connections, not only for my safety but anyone that comes after me, the heat shrink on the power switch connections made it that bit harder to bend the wires around the fuse holder. Somewhere along the line I lost the top cap connections and had to make some temporary replacements.

I powered up for the first time with no valves - there was no smoke, which was comforting - with a new set of valves the voltages were all around 5%-7% higher than the design values probably due to my high mains supply - it is always over 240V - the bias needs to be trimmed by changing the value of R40 - I used the suggested 100K which gave -38.5V bias, this was supposed give a combined idle current of 95mA - with 370V HT the plate dissipation would be right on the the max of 18 watts for each valve, but on checking this I found I had a combined of 40mA, I have a few dozen 6DQ6's and have sorted them into matched pairs so I tried a few different sets and the idle current varied between 40mA and 65mA with each pair. I recall reading somewhere that 6DQ6's are 'all over shop' with their specs - I rigged up a temporary adjustable bias and testing a few different pairs of 6DQ6's found that some valves needed the bias voltage as low as -30V to give a combines idle current of 95mA - I have heard that there is a difference in the sound of an amp if the biasing is changed from 'cold' to 'hot', so it might be a good idea to provide for an adjustable bias within certain limits.
Leaving it at 65mA for now I gave it a short test run with a guitar, the amp is very quiet, at first I thought there was a bad connection in the speaker wiring - everything worked first up with the exception of the trem speed control which worked backwards - an easy fix.
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Old 27th Jul 2022, 5:46 am   #19
Robert Gribnau
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Default Re: Kit Guitar Amp Restoration

Looks very nice!
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Old 27th Jul 2022, 8:24 am   #20
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Well done. Looks great.

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