Another PA Amp

[TIMBO]

Hi guys, I had this one under my bench for awhile and now its time to "PIMP MY AMP"
There is no info on this one, only that it is Aussie made, fairly well made with good trannys.

I had thoughts of just removing the inputs and doing the minimum rewire but the more I looked at it was going to be a big job.
There is some burnt resistors, one screen resistor on the 6CM5 and the two 100r on the artificial CT for the heaters.

There does not appear the be damage to the transformers, thoughts????

With three 12ax7 in the lineup this gives me some options in the preamp.
With plenty of room on the chassis and having a similar PI to this PA AMP I think that the Marshall Specialist http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_specialist_25w_2046.pdf
looks like a good fit.

The PT has a separate bias winding (two wires) and cause to get a matched QUAD of these old tubes is impossible.
Does any one have a bias circuit that has individual pots for setting the bias for each tube??

[eleventeen]

Quite the R/C traffic jam around those preamp tubes!

[PRR]

It's got mike transformers. It may be 70 Watts. Make it your band's vocal amp.

[Platefire]

This is me, I would want to fix the bad resistors mentioned, pull the front left pot/wiring, install a 1/4" jack and run a fender type input to the most convenient 12AX7 pre before the volume/tone stack, off course disconnect the PA input stuff running to that pre. It may already have a master volume--that would be fun. Also hook a connector or install a jack for 8 Ohm speaker. Then fire it up and see what you got. To me this is always good fun for a quick trill--adventure on the high seas!  Then after you determine it sounds terrible for guitar you can plan what you will do with it  Then the fun and adventure is over and time to go to work! Platefire 

[mat janssen]

HI I think this amp is family of the EV4431 from Philips. The power-, the output-transformer, the capacitors and the resistors are a Philips brand.
The website www.amplifiers-with-valves.nl has diagrams and some pictures. Not ofcource from this amp.  but a simular one.

[TIMBO]

Hi guys, Adding a reverb and looking through my schems to find a suitable place to insert it, I found a 5E3 that has it in front of the split PI.
Can't say that I have seen inserted there but I'm up for it if its a good option, otherwise I would put it before V4b.
At some stage I would also add a master volume and it would be positioned before V4b.

[trobbins]

Can you pdf the schem Tim?

[TIMBO]

[trobbins]

You could try it between the triodes of V2 - that dc coupled section would need to be changed to suit.  You would have enough signal at V2 pin 6 anode to drive a transformer driver stage.  Putting it at your X point may have low signal level from the tone controls, and you want to buffer the tone output by directly gong to an amplifier input.

[TIMBO]

Hi guys,Taken onboard all the good info and juiced her up with a few minor problems.
With them sorted things are sounding good.
I opted for a new chassis/layout.
Again this was a PA amp and the multiple speaker connections usually are not very guitar speaker friendly.
I have connected an 8ohm load to the tap that was singled out as this had a standard speaker jack as a mod that was done at some time, this seems to work fine with no visual stress on the OT.
I would like to be sure but there is no info on this OT.

[TIMBO]

Hi guys, Got a weird problem happening.
When testing, I was just checking the MV and turned it up (volumes turned down) and was getting some signal through the speakers (very low volume distortion).
After just about rewiring the grounding and some of the shielded cable, the bleed through was still there.
I kinda isolated it to the volume pot and they didn't seem to be grounding out when turned to 0.
These are new pots.
I replaced one pot and things were looking good, it was killing the signal when at 0.
The other pot was still letting signal through, so I replaced it and it also was letting signal through when at 0.
I checked for stray strands of wire , nothing.
Schem page6

[TIMBO]

Hi guys , Hit a bit of a snag with this one.
I may be looking for a replacement OT as the secondary taps aren't that speaker friendly.

I'm not the best at reading these datasheets but from what I can see the plate-plate resistance is 3.5k so can I assume that for a QUAD of valves it would be half of a pair as in this data.
http://tubedata.milbert.com/sheets/050/e/EL36.pdf
So would a marshall 100w replacement with 1.7k plate-plate work. Thanks

[PRR]

Yes.

[TIMBO]

Thanks PRR 

[jjasilli]

You probably don't need to replace the OT.  You can spec out the existing OT and decipher its mysteries if you want to.  Procedures are on the internet; in amp books; and guys on this Forum can help walk you through it. 


Basically you apply a known AC voltage on one winding, and measure the result on the other winding.  From this you can determine the OT's turns ratio.  To the tube charts to find the power tubes' plate-to-plate load impedance = the primary impedance of the OT.  From that, and the turns ratio, you can determine the OT secondary impedance to match to a speaker load.  Something like that anyway.  The point is that it's doable.


EDIT:  Speaking of the devil!!!:  http://el34world.com/Forum/index.php?topic=18637.msg190497;boardseen#new

[TIMBO]

Thanks jjasilli, I found Ks topic and found it easy to understand.