CSA PA AMP

[TIMBO]

Hi guys, Another local built amp has come my way.
This one IS a ROSE MORRIS built amp and flew under the banner of "Communication Systems of Australia"
It has had some mods done but nothing too drastic.
Things look in good order and so not too much slicing and dicing.

[DummyLoad]

cool! sweep tubes.     


in the first pic, what's the tube off to the right without a plate cap?


--pete

[TIMBO]

Hi DL, I love these old amp, there is always a bit of mystery to them.
Roly, was the first to solve it, it's just a spare socket that was probably used as a connection for another input source and the original owner was kind enough to stick a backup tube in it. 

[TIMBO]

Hi guys, I was able to lite this one up and we have sound.
Voltages look good, just a bit low on the preamp.
There is some hum with nothing plugged in.
The original power tubes are a mismatched brand and are mismatched on the cathode readings.
How much is too much, when the readings are a good 10mA difference.

[kagliostro]

To Caesar what is Caesar

I've find the connection of the path signal to that socket to be odd to me

Roly was the first to say it was probably the input for an external signal

I only confirmed I've think the same referring to some old Geloso PA amp

K

p.s.: For those that don't know him, Roly is a friend at AGGH (http://www.guitargear.net.au)

[TIMBO]

Your right K, anyway its not going to be used so I not worried about it. Thanks

[DummyLoad]

maybe socket was for lo-Z matching transformer.


--pete

[kagliostro]

@ Pete

Yes, may be, I've a pair of old RCF PA amp that has also input transformers

@ TIMBO

You have two stages preceding the Tone Control and one that follows, I think there is enough signal to experiment with a  TS also an FMV

Ciao

Franco

[TIMBO]

Hi guys, I have tidied up some of the wiring and added some better inputs but the only place to put them was over the preamp valves, so I don't know if that is where the hum is coming from.

[kagliostro]

Have you think to open holes for the jack on the front panel or go with DC heaters ?


Franco

[TIMBO]

Hi guys, I got another pair of power tubes and these also were said to be a mismatch.
When fired up, the SPARKS flew in one tube, these are NOS and probably 50yo.
Am I safe to say its cactus.
I have approx. -32v for my fixed bias and although the tube brands are not the same, if the cathode readings are similar, is this odd pairing going to be ok.
I don't have a tube tester so this is the only way I can match them. Thanks 

[kagliostro]

An amp and a meter are a tube tester 

don't care about different brands, look to the current consumption

if they are within 5mA from each other they are a perfect match

if you want to refine, double the bias control circuit

Franco

[TIMBO]

Hi guys, With some mismatch power tubes I was able to get a close match.
I couldn't find the source of the HUM, although I narrowed it down to V3.
For a suck and see I rewired V3b to be similar to the Goldentone amps, this reduced the hum greatly but introduced a FIZZ as well, I tried paralleling a cap with the plate resistors and this did nothing so I'm thinking it may be coming from the rectifier.
With a very clean sound, it has lots of volume, so much so that I had to turn down the guitar to 3 and the amp to 2, anything above these settings was causing blocking distortion.
Could some of this volume be caused by the OT tap not being 8ohms and using an 8ohm speaker.
I can only ASSUME that the tap being used is 8ohm.

http://www.ozvalveamps.org/goldentone/1757bassmaster.jpg

[TIMBO]

Hi guys, with some things to try to tame the gain, I have settled on a james TS and a master vol.
Tweaking the values in the TS I tried to cut some of the BLAT I was getting in the bottom end, I'm still getting some but with the master vol seems to help.
The amp has turned out great and even adding some extra components I was able find some old bits that are working well.