DIASON HYBRID AMP

[TIMBO]

Hi guys,I decided that this power amp would be better used with a tube preamp that it originally had when made.
I traced it out to get a good look at its workings.
Seems simple till I drew out the power supply, then the LDR/globe setup has got me scracthin me head.
The power amp is seen here  http://www.ozvalveamps.org/diason.htm in the unknown 3 and SS40w.
I was told that it was part of a 4x12cab, this is the only info I can find, but found the same PCB used by another amp builder http://www.ozvalveamps.org/goldentone6.htm Hybrid 100
Looking at the head on the "unknown 3", two channels VOL, BASS and TREBLE, with REVERB and TREMOLO, connected the speaker cab via an umbilical cord.
This cord has one signal wire, two for the heaters, one B+ and one ground.
This is the Goldentone Hybrid http://www.ozvalveamps.org/goldentone/goldentone100watthybridtwincctgw.jpg it has the reverb and tremolo in the preamp.
So I'm trying to work out why the LDR/GLOBE is position where it is. 

From what I know Diason made there own amps but its good to see that they didn't mind shearing components as the PCB is marked with "ROSE MUSIC" who makes the Goldentone.

[PRR]

> why the LDR/GLOBE is position where it is.

I dunno why an INput is way over on the right. (I'm obsessive about signal flow left-to-right.)

The "globe" (lamp) is across the speaker (via trim resistance). When you play too loud, the lamp glows. When the lamp glows, the LDR drops in resistance. The LDR is wired to short-out signal coming through the 100K resistor. So it is a slow-acting compressor/limiter.

For this to work right, the "valve preamp" must also have an output resistance around 100K. And the preamp signal will also be limited.

The power amp design is old and unsophisticated. There is NO protection for the power transistors. A shorted speaker cable under power will try to fry the 2N3055s. The original lump-wafer '3055 would stand some astonishing abuse. All modern "2N3055"s are made of much thinner stuff and probably won't stand for much insult.

The gain is around 100 below 150Hz and around 31 above 250Hz, a 10dB shelf. This is odd for a guitar amp. I'd expect it to be boomy. Maybe the speakers used were boom-less and needed the push.

The input impedance is probably over 220K, which agrees with using 100K mix/limit resistors.

I suspect "INPUT" on the right with the 1Ks is to take the speaker level output of another power amplifier. With pot fairly high it would boost a sick Champ to large venue level; pot way low to accept the output of another 50W-70W amp and deliver an equal level.

There's much here that I think has been done better in the decades since it was penned. However if it is working for you, enjoy.
______________

> Diason made there own amps but ... the PCB is marked with "ROSE MUSIC"

It is not silly to imagine that Rose had the PCB process mastered, both guys knew each other, competed, but also asked (paid) for help like etching a run of PCBs, possibly the same PCB which both guys would flesh-out and upholster in their own style. And it really could be a hi-fi amp, and possibly taken from the good Australian electronics magazines of the day.

[TIMBO]

Thanks PRR, A comment from a local bloke. http://www.guitargear.net.au/discussion/index.php?topic=45043.0
Seems it had its problems 

[TIMBO]

Not a good startup, popped the mains fuse.
I disconnected the two rectifiers to get an idea of the health of the PT.
org/yel 176v
blu/yel 47v
brn/brn 6.7v
I was planning to transplant the components onto a bigger chassis to accommodate the preamp, so I will pull it apart and breadboard it to fix any problems.

I have not seen the two rectifiers connected this way, so a quick explanation would be great. Thanks

[sluckey]

I have not seen the two rectifiers connected this way

They are simply connected in series, like two batteries in a flashlight. The B+ voltage at the top rectifier output will be the sum of the two individual rectifier outputs.

Pull the two 2N3055s. Will the fuse hold now?

[TIMBO]

I did not have the PCB connected at the time as I was just wanting to see what voltages I was getting after rectification.
I have no way of checking the health of the 1000uf/63v caps as my MM only goes to 200uf, so starting from scratch is were I'm at.
I'll get the PSU sorted and then add the other bits. Thanks

[PRR]

The rectifiers for the 47VAC 66VDC supply must be rated at *least* 200V and 5 Amps. _I_ would just buy a 400V 6A or 10A Full Wave Bridge module.

(However it appears you *have* a FWB in there? The square 4-leg epoxy thing. But that does not look big enough to me; the wires also look like tube-scale instead of fat transistor scale. Also for good reliability this low-volt bridge should be heat-sunk to the chassis; whereas I dunno why they heatsunk the high-volt bridge.)

After all this time the big filter caps could be good but I would not hesitate to replace them. Two * 1000uFd @ 63V, when what they *really* wanted (maybe could not get at that time/place) was 4,700uFd (or two 2,200uFd) @ _100V_, which is readily available today. The mere 2,000uFd will play but with a nasty buzz when over-loaded (that is, if the limiter lets it overload).

[TIMBO]

The rectifier is a MB4 and on info at this stage.
I used a 4000uf/75v on another build and easily sourced. Thanks

[DummyLoad]

the 1000uF electrolytic cap vents are bubbled. the end is near or already there.


drill a hole in the open space next to the fuse and mount a 10A bridge to the chassis. use thermal grease or pad.


rectumfrier:


http://www.altronics.com.au/p/z0088-kbpc1004-400v-10a-metal-bridge-rectifier/
http://au.rs-online.com/web/p/bridge-rectifiers/8122240/


electro caps:


http://au.rs-online.com/web/p/aluminium-capacitors/8526544/

[TIMBO]

Hi guys, Some new parts have arrived and things are lookin good.
AS well as the head that goes with this power amp.
One of the local blokes had this head stashed away for a future project but didn't think that he was going to get around to doing something with it, I'M A LUCKY BOY.
The head is in great condition and I don't think i'll have to change the Ecaps as they all measure pretty good and are not leaking.
So a few final checks and a bit more soldering should be able to inject some life and some sounds.

[TIMBO]

The head.............

[DummyLoad]

very nice work tim! :-)


--pete

[PRR]

That power-amp is a much nicer build than the last you showed.

I bet the big filter caps are much more recent than the amp.

[DummyLoad]

That power-amp is a much nicer build than the last you showed.

I bet the big filter caps are much more recent than the amp.

looks like he replaced them. the fist set of photos they are gray cans.

--pete

[TIMBO]

Thanks guys,All Ecaps have been replaced throughout, with the caps on the board as close as I can get with todays values and higher voltage rating.
The filter caps are now 2x2200uf/200v and the output coupling 1000uf/200v, these are this size so I could reuse the external mounting brackets.
Fired up well, no issues.
Sounds great with lots of reverb and dreamy tremolo.
This thing is pretty loud and those that got to use them in the past complained that they weren't loud.
If I was to say I would think up to about 40w output through a 2x12 cab.
There is a bit of fizz but no where as much as I thought there would be, pretty quiet.

At first there was no changed when the adjustment was changed on the "limiter" cause I forgot to connect it. once connected it still didn't change things, thought the bulb was stuffed, tested good.
Trying the find a reason for this not to work, as I already tested the LDR and it reacted to a flashlight, I could not a voltage reading at the speaker connection and found this only to be a few volts and with the adjustment as a dead short there was a shimmer from the bulb but the amp WAS on "7" on the dial.
I think that this could be useful to cap the output and the only reason I think they added this feature was that the speakers may have only been 2x15w and when cranked I think this would put out +50w.
The power amp module may be a bit primitive but seems to work well, is there any protective circuitry that could preserve these old components.
I should be able to get some measurements of the original speaker box to complete the picture.

[TIMBO]

Some final voltage readings, all look good.

[TIMBO]

Hi guys, A progress update.
Cab almost done and found some nice vintage Aussie speakers.

[Ed_Chambley]

Really enjoyed watching this thread.  Timbo, you really do good all around work.  Amp and cabinets, plus creativity.

[shooter]

, is there any protective circuitry

one easy crude protection is a bi-metal temp switch mounted on the PA heat sink.  You'll need to know the *normal* operating temp, then hunt the web for one close, run your B+ through it so when she gets to hot the switch opens and removes B+.  The downside when it cools it closes again.

[trobbins]

Tim, if you put a resistance meter across the LDR (with one end disconnected) whilst playing, that can easily identify if all parts upstream are working.


A disadvantage with upsizing the bridge rectifier to something more bullet-proof is that it usually increases the parasitic capacitance across the diodes, and increases any reverse recovery current that may flow, so you end up with more noise floating around.  It may be more prudent to just use two or three UF4007 in parallel for each diode in the bridge.


 Also, taking the 'A' rail from the bridge terminal (cos its close), rather than from the capacitor terminal adds in rectifier noise.


Are you actually using the old CuO diode bridge?

[shooter]

There is a bit of fizz

parasitic capacitance across the diodes

you could try tacking .1 or .01uf across each diode to help with transient spikes.  You can also hang the same caps along the B+ rail to ground.

If you have a scope they are pretty easy to see.

[HotBluePlates]

At first there was no changed when the adjustment was changed on the "limiter" cause I forgot to connect it. once connected it still didn't change things, thought the bulb was stuffed, tested good.
Trying the find a reason for this not to work ...

See the pic below. Doe you have the holes shown plugged in some way? I'm just wondering if ambient light is getting in through those, reducing the change in light when the bulb shines.

[DummyLoad]

very nice work tim! 


--pete

[TIMBO]

Thanks guys, When I last fired up it all seem to work well and I do not remember it being too noisy.

I now see that the LIMITER is just that but you have to dig in pretty hard to get any effect. I feel that this was put in to protect the speakers from too much juice.

I'm waiting on some grill cloth so that I can complete the wiring to the speakers, these are the speakers http://www.etonesound.com/speaker_drivers.htm#241

I'm not too worried about the power amp too much and it is not up to the job I can always make a tube PA.

[trobbins]

Some alu foil around the back of the bulb, or some other jury-rigged reflector may assist in getting more of the available light to the LDR.  Do you know if the bulb you are using actually gets enough voltage on it - you may have a non-original bulb, with slightly different ratings. 

[TIMBO]

HBP, all holes have grommets for wire protection, so minimal light should enter.
Tim, if you are talking about the rectifier bolted to the side wall, then yes.
The bulb had a dob of glue on it, so I would say so.

[TIMBO]

Hi guys, With time over the Xmas break I was able to put the finishing touches on this one.
I think it has come up great and it sounds awesome as well.
Pretty clean, but with the BOOST on the treble and bass knobs give it a bit of edge.
It's not as loud as it should be, I think the limiter is doing its job.