AWA: Model PA806, 4w Valve PA Amplifier

Such an awesome looking little amp from 1954.  Photos from an online listing, I am yet to track one down in person.

It's an interesting circuit, that's for sure.

AWA MODEL PA806 PA AMPLIFIER

AWA MODEL PA806 SCHEMATIC

VASE: Trendsetter 60, Vintage Australian Guitar Amplifier

This amp is a bit of a classic from my hometown, Brisbane.  Vase Trendsetters were very common back in the day, and many are still floating around.  If you couldn't afford a Marshall, there was a good chance that you had a Vase.

They were mostly fairly simple amps, and pretty well made - I'd say they were all designed for working musicians, as I'm not aware of any budget low-wattage combos being produced.  I've only ever seen Vase heads and cabs.

VASE TRENDSETTER 60

This amp has already been very well docuemented here on on Aussie Guitar Gear Heads (thanks Mike), but as a recent acquisition, I thought I should add it to my blog anyway. 

It’s working well and sounds great, but it needs a bit of attention to the wiring where the old spaghetti insulation has crumbled away.

Photos to be added as I commence work.

SCHEMATIC

Excellent trace by Mike - so good to have all of the voltages etc on hand like this.  Vase are also known for a lot of minor variations on their amps - so if you have a Trendsetter, this is probably a really useful guide, but don't be too concerned about little differences, as it may have left the workshop that way.

FURTHER READING

https://ozvalveamps.org/vase.htm

https://guitarnerd.com.au/2010/10/vase-amplifiers/

https://reverb.com/news/a-history-of-brisbanes-vase-amplifiers

GOLDENTONE: Model 1755 Slimline

I saw this one pop up recently and decided to take a punt on it (thanks Damian).  For the money, I figured it was worth a look even if it only ended up being a decent chassis and set of transformers.

Turns out that this amp is a testament to the quality work that Goldetone produced - despite what has clearly been a very hard life, she works just fine (apart from the heavily corroded reverb tank). 

image from the eBay listing

Cosmetically, it has had a bit of a rough life (perhaps an understatement).  Where to begin? The original combo was cut down to a head, tolex removed, it was painted yellow, then black, and at some point it was covered in 1970s or 80s woodgrain vinyl.  The head still feels solid, so there’s something to work with.

I didn’t think the original faceplate was on the amp, so I was pleasantly surprised when the silver paint started wiping away with isopropyl alcohol, revealing the original panel underneath.  It's still in good shape too - maybe the dodgy paint work protected it?  

The reverb tank, on the other hand, was absolutely covered in heavy corrosion- no chance of salvage.

Inside the chassis was a much nicer.  It looks to be all original, with no obvious signs of previous work or failed parts.  It was dusty as hell, but I’ll take dust over a butchered circuit any day.

GOLDENTONE - 1755 SLIMLINE

I believe this is a Goldentone Model 1755 Slimline, based on the schematics I've seen.  Circa 1965, serial number 3967.  

The 1755 is regarded as one of the classic Goldentones - after a bit of time playing it, I can understand why.

FEATURES

Two channels, both have bass and treble controls.  One channel has reverb and trem.  

40watts

The original open back cab would have had 2 x 12" alnico speakers.

PREAMPS

Miniwatt 12AX7s.  

Tone controls

James style, bass and treble

Reverb / trem channel

First gain stage

Ra 220k

Rk 3k3, 25u bypass

Second gain stage

Ra 220k

Rk 1k8, 25u bypass

Normal Channel

First gain stage

Ra 100k

Rk 3k3, 25u bypass

Second gain stage

Ra 100k

Rk 3k3, 25u bypass

All pots 1MC (Volume, bass and treble)

REVERB

Valve driven, Miniwatt 6GW8

Pioneer EAV-201 reverb tank fitted (very small, 2 spring)

Rola LRH3 transformer

Transformer bench measurements

10.4v, 440Hz sine wave applied to primary:  result, 3.3v on secondary

That gives a turns ratio of:  10.4 / 3.3 = 3.15:1

Because impedance ratio is the square of turns ratio:  3.15² ≈ 9.9:1

So the transformer reflects about 10 times the tank/input impedance back to the 6GW8 pentode.

As a replacement, I'm using an old reverb tank from a Peavy.  Part number 710-1111.  DC resistance measures:  input ~50 ohms, output ~250 ohms.  It's probably an Accutronics 4EB2C1B, 600ohm input impedance, which will reflect 6k to the 6GW8.  That’s a very reasonable load, and it sounds great.   

Reverb level is completely independent from the preamp volume control.  i.e. the channel volume can be all the way down, but you can still hear the reverb.  The reverb send is post tone controls - before the volume control (attached to pin 3 of the volume pot).   

Reverb can be turned off via a foot switch.

TREM

12AX7 oscillator, modulating the cathode of a preamp triode.  

No depth control, just speed.

The trem pot is also the power on / off switch for the amp.  The trem is turned on / off by a foot switch.

PHASE INVERTER

12AX7, longtail pair

100n coupling caps

22k negative feedback from OT secondaries

POWER AMP

2 x Hitachi 6DQ6-B fitted - although schematics show 6DQ6-A (6GW6)

47 ohm resistor to screen

10k grid stopper resistors

470k grid leak resistors

Fixed bias -30  Bias not adjustable - no pot

OUTPUT TRANSFORMER SPECS (OT)

A&R 2700

Primary 3k3

Secondaries 16, 8 & 4 ohms

Output transformer bench measurements

84 ohms DCR primary

Test conditions:

10.5v, 440Hz sine wave applied to primary:   result, 0.58v on secondary

Turns ratio 18.1:1  

Primary

2.6kΩ at 8 ohms

5.2kΩ at 16 ohms

This is different to the specs above - possibly the result of construction variations and a low-voltage test.

POWER SUPPLY

A&R 5892 (which is different to schematics)

Silicon bridge rectifier

Rola choke TV301, May 1965

B+ about 360v

My little moment of joy, finding the original faceplate under the silver paint.  They may be original knobs, but I'm not sure. 

The 6DQ6 valves have their anode on a top cap - hence the warnings. 

A&R 5892 power transformer &  a big Ducon filter cap for the power supply.

Choke & reverb transformer& 6GW8 valve for the reverb

Prior to dust removal - all the mustard caps look fine, no cracks.  Can't see any resistors that have been heat-stressed.

They used an interesting method to mount components on the preamp and phase inverter.  There's a thread through the valve base, with another valve base on the other end of it.  It kind of makes a little tree for components to be vertically mounted.  

It is kind of difficult to get to the valve socket end, so maybe not the most practical method when compared to the tag stip mounted components.

This was a reverb tank at some stage in life.  

This is after most of the horrific vinyl has been removed, showing the remnants of various paint jobs underneath.

SCHEMATIC, GOLDENTONE MODEL 1755

Compared to some schematics floating around, mine has a few differences.   Worth noting that there are some major changes between the 1755 and some later Reverbmasters.  

• Some preamp plate and cathode resistor values differ   
• Reverb send and return paths are different   
• 47Ω screen resistors, not 47kΩ as shown on the schematic
• 10k instead of 180ohm resistor used for screen power filtering
• 4n7 capacitor across the reverb level control
• different power transformer is used

The measured voltages in my amp are very close to the schematic values overall, although the screen voltages are slightly lower, possibly due to the larger resistor in the filter section.

I liked the look of this schematic, so I made a few rough and ready modifications to match the values in my amp.

GOLDENTONE CATALOG IMAGES

FURTHER READING

https://www.aggh.net/discussion/index.php?topic=54693.0

REFERENCE: Power Transformer Testing

Testing an Unknown Power Transformer

Every now and then I end up with a transformer on the bench that I know came from valve gear, but I did not take enough notes when I pulled it.  That was the case here.  I knew this one came out of an old Zenith radiogram, but that was about it, so the job was to work backwards and identify the windings properly.

Naturally, anything involving a power transformer deserves caution.  Unknown windings, mains voltage, and potentially lethal secondary voltages are not something to rush into.  It is worth stopping and thinking before connecting anything, or even attempting this.  

What I Knew Going In

In this case, I knew the donor unit ran on 240V and used a pair of cathode-biased 6V6, along with preamp and phase inverter valves, plus a valve rectifier.  

That gives a rough idea of the sort of transformer it is likely to be, but only in a broad sense - it does not confirm the actual heater voltage, HT voltage, or current capability, nor whether it actually works to begin with.

What a Typical Valve Amp Transformer Looks Like

A typical valve amp power transformer usually has a primary winding connected to the wall supply, one or more low-voltage secondary windings for heaters, and a higher-voltage secondary for the HT supply.  

Where I Start

First up is a visual inspection.  Before I go anywhere near it with power, I want to know whether there are any obvious signs of trouble - burnt areas, dodgy repairs, brittle or damaged insulation, loose leads, corrosion, or anything else that makes the transformer look questionable.  Basically, I am looking for any reason not to go any further.  Now is not the time for blind optimism.  

If it passes that first check, I start with the multimeter on continuity, just to sort the loose leads into groups that appear to belong together.  Often the original colour coding will help here too, if it is still clear enough to trust.

A low-resistance winding is often a heater winding, and a higher-resistance winding is often HT, but resistance on its own does not prove the voltage.  Winding resistance mainly reflects the wire size and the number of turns, and practical transformer behaviour also depends on inductance, losses and loading.

So I use resistance readings to sort and narrow things down, not as final identification.

Warning: It is also worth checking each winding to the transformer core, frame or bell ends.  Normally, I want to see open circuit there. 

Spotting a Likely Centre Tap

Where resistance readings are especially useful is in spotting a likely centre tap on a HT winding.

If one winding measures, say, about 74 ohms end to end, and about 38 ohms from each end to a third wire, that strongly suggests a centre-tapped winding.  Each half measures about half the total, which is exactly what you would expect.  A large resistance also suggests that it is not a heater wire.

In the case of this transformer, the HT is the only winding with a centre tap.  The 6.3v heater does not have one, which is not uncommon on older transformers.  Trying to confirm a heater centre tap using the method is not reliable, due to the very low resistances that will be involved.  

The Better Test

Once the winding groups are sorted out, the next step is the one that really matters: apply a small known AC voltage to one winding and measure what appears on the others.

That is the proper way to identify an unknown transformer, because the measured voltage ratios directly reflect the turns ratios. 

It is also the point where you stop guessing and start mapping what the transformer actually does. If you put a known AC voltage on one winding and get a much higher voltage on another, you are clearly looking at a step-up relationship. If you get a much lower voltage, it is a step-down relationship.

This is the step that tells you which winding is the likely primary, which winding is the HT secondary, whether a winding is centre tapped, and what the heater winding voltages actually are.

That is a much firmer basis than ohms readings alone.

Bringing It Up on Mains (really, really carefully)

This is the point where things can get dangerous in a hurry.

Once I think I have identified the primary, I power it up and check the secondary voltages with no load connected. I use a Variac with a current meter, start with low voltage, and keep a close eye on the current draw. Some no-load current is normal, but I do not want to see anything excessive.

This is a completely hands-off test. The leads need to be properly secured on a clean bench before anything is powered up.  I want a clear view of the multimeter and the current meter, and I do not want to be moving probes or wires around while it is live.  

The Variac starts at zero and switched off.  Then I bring it up slowly, while watching, listening, and paying attention to how the transformer behaves.  Once I have the readings, the Variac goes back to zero and gets switched off.  Then I double-check that before touching anything.

At this point, I am not expecting the transformer to draw zero current  - some no-load current is normal. 

What I want to see is sensible voltages and sensible behaviour. 

If the transformer stays reasonably quiet and cool, and the secondary voltages look believable, that is a good sign. 

If it draws obviously too much current, gets hot quickly, or starts buzzing and complaining, I would stop immediately and reconsider all previous steps and the safety of the transformer. 

What the No-Load Voltages Tell You

The unloaded voltages are important because they confirm the winding functions far more reliably than resistance ever can.

A low-voltage winding that sits around the expected few volts AC is very likely a heater winding. 

A winding that gives you two equal voltages from each end to a centre lead is very likely a centre-tapped HT winding. 

Once that is established, you can start thinking sensibly about whether the transformer is suitable for the sort of amp you have in mind.

There is also a practical reason for checking the transformer unloaded before going any further.  Transformer behaviour changes under load. Whitlock points out that when a load is connected, the secondary current opposes the excitation flux and the primary draws additional current accordingly.  So if all you want at this stage is identification, unloaded testing keeps the picture simpler.

A Couple of Easy Mistakes

One easy mistake is to overstate what a resistance reading tells you.  It can suggest a heater winding or suggest a centre tap, but it does not confirm the operating voltage.

And one more: if the transformer came from old or unknown equipment, do not assume the original primary was intended for today’s wall voltage, or is in fact even vaguely suited to your wall voltage.

Bottom Line

For me, the sensible order is:

1. visual check
2. continuity,
3. resistance,
4. small AC voltage test to confirm,
5. then a careful mains bring-up and unloaded voltage check.

That way you use each test for what it is actually good at.  Continuity tells you what belongs together. Resistance helps sort likely windings and likely centre taps. AC voltage testing tells you what the windings really are. Then a careful live test confirms whether the transformer behaves sensibly at its intended input.

Resistance gets you close. Voltage confirms it.

⚠️ Caution — High Voltage:

Transformers can produce dangerously high voltages, even when powered by a seemingly low-voltage AC source. The windings in tube amplifier transformers are capable of stepping voltages up or down significantly. Always assume exposed leads may carry lethal voltages, and take proper safety precautions. Use insulated tools, never work on live circuits unless qualified, and ensure your test setup is isolated and safe.

AWA: Model PA872, 20w Valve PA Amplifier (Serial Number 94)

I posted another AWA PA872 a while back, but realised I never actually shared this one.  It had been sitting on the shelf for months - one of those projects that was nearly there, but never quite finished.  I finally got around to wrapping it up this weekend.

This looks to be an earlier unit, although schematically the same.  The serial number is much lower (#94), and the general aesthetic appears earlier.

Someone had already had a go at converting it - let’s just say what I found inside is a good reminder to be cautious with amps of unknown history.  I never even turned it on - deathtrap.

This one came to me as a friendly trade. The previous owner hadn’t worked on it themselves, and was unaware of the condition - they were just clearing out a few projects they knew they were never going to get around to.

With no real baseline to work from, I stripped it right back to the sockets and started again.  It’s now rebuilt as a 5E3-style Deluxe, with a few tweaks to taste.

The stock 5E3 uses 100n coupling caps on the preamp.  I dropped the bright channel right down to 22n, and trimmed the normal channel slightly to 82n.  For the tone control, I’ve gone with stock 5n on the bass side and increased treble from 500pf to 1n, for a slightly different response.  The results are good - two distinctly different, but useful channels.

I chased a pretty stubborn hum for a while - in the end, it came down to grounding.  Moving the power transformer ground to the same point as the filter caps fixed it immediately.  We're only talking a distance of about 80mm.

There was a previous attempt to “fix” the hum, which involved adding 2 x 220uf filter caps to the power supply - so it’s no surprise the valve rectifier was dead on arrival.  Yes - 220uf, not 22uf...  

AWA MODEL PA 872 - 20 WATT PA AMPLIFIER

I really like the blue hammered finish - someone in the design team really nailed the look of this one.  

Schematic screened onto the back of the amp - a nice touch for serviceability.  Serial number 94.

The amp can run a 15 ohm speaker without any dodgy rewiring of the output transformer, so that's always handy.

I patched over some holes for a couple of can capacitors, as they were dead, and not conveniently placed for what I wanted to achieve.

Not my most amazing work to date, but it is solid, and it sounds great.

FURTHER READING

Tim Robbins PA872 conversion 

My previous post for the 872, which has full tech details 

FORTIN: Fuzz

Request - here you go & I hope this is what you were after.  Could shave off a column or two if that's a bother.

The Fortin is a pretty stock silicon fuzz face with an input blend, a 10k trimmer in place of the usual 8k2 resistor and a 100n cap on the output.  A few smallish caps on the collector and base of transistors to tame the fizz.   It's not unlike the D*A*M Drag n Fly DF-05 and no doubt a few other fuzz faces with an input blend.

Schematic can be found here

FORTIN FUZZ