LENARD SGB100

[TIMBO]

Hi guys, Got a nice 100 watter that needs some fixin.
It has had a partial recap at sometime, so all new filter caps, mains lead (IEC)....

I have in the past I have replaced the yellow coupling caps as these can be a bit dodgy.
But when removing the circuit board found the traces were very thin and in some places lifting.
A new eyelet board and orange drop coupling caps will be used.
 
A couple of things.....
The standby switch is on the supply to the screen resistors, I'm thinking that the standby would be better on the HT leg that feeds diodes.
At the moment there is a 100r/1w resistors on the EL34 screens (forgot to put on schem) wanting to reduce the stress on the EL34s what would be the largest value required, as it states that this amp is 130w into 4ohms. May have 8ohm tap.
It also has a conjunctive filter (270/2w and.001/3kv)across the EL34 plate leads, is this required?

[tubenit]

Timbo,

I love reading your threads! You get the most interesting amps there.

IF it were my own amp, I'd be inclined to try 2.2k/3w screen resistors (for a more compressed tone). 

And I'd lose the conjunctive filter which I personally thing is an awful tone killer.  I've tried those on three different amps (usually EL84 amps) and never left the filter on more than maybe 20 minutes because it sounded so bad to me. 

You could easily experiment with the "enhance cap" and maybe try 250p across the 47k LTPI input plate resistor instead which has proved to me to be a better "smoothing" solution for high end hash then the conjunctive filter.

Is there a cap going to the bass pot in the tone stack?

Thanks for sharing the schematic and photos!  With respect, Tubenit

[tubenit]

Looks like ALOT of space in the chassis.  Have you thought at all about adding an active FX loop or something like a D-style overdrive?   Because how this amp has a separate mid control similar to a Dumblish style, it might not take much to convert into a Dumblish tone stack.   You could have an overdrive channel &  mid-boost & PAB.

Is this your amp or someone else's?

With respect, Tubenit

[TIMBO]

Thanks T, It's my amp and as these are becoming quite rare i'll just do the needed things to get it up and running.

[tubenit]

these are becoming quite rare i'll just do the needed things to get it up and running

That makes sense to me!  I wasn't familiar with the amp at all or knew if it had more collector's value in original condition?

Again, I appreciate your sharing of the schematics and photos.  Always enjoy your posts.

Best regards and respect, Tubenit

[DummyLoad]

1K 5W will work well.


--pete

[kagliostro]

The standby switch is on the supply to the screen resistors

I don't think to that as a bad way to have standby

I've seen also G2 disconnected from their node on PS and connected to the negative Bias supply



http://www.chambonino.com/construct/constwire1.html

http://www.chambonino.com/construct/const8.html

Franco

[TIMBO]

Thanks guys, I have taken out the PCB to replace the coupling caps as they are known to be unreliable, but found that the very thin traces are lifting in places.
So a new board made.


I have left the old filter caps in at the moment as I am waiting for new ones.


The 6.3v heater supply is connected to ground. I will do an artificial CT.

[trobbins]

Tim, you may avoid some noise/stability quirks if you run the output speaker signal back to the feedback point as a twisted pair, such that the speaker wire that connects to 0V, does so at the 0V for the PI stage.


It's also a good idea to physically separate AC mains wires from any other wiring in the amplifier.  That can mean running a separate 'loom', or putting some heatshrink on the mains wiring where it is in contact with other wires, or using some 'external lead' for wiring.  And to be picky, the mains protective earth should be lugged to a separate bolt, not a transformer mounting bolt. 


It would be a pity for other DIYers to think you've modified that wiring appropriately 

[TIMBO]

Thanks Tim, All good points.
Just not sure what you are implying with the speaker wiring.

[trobbins]

The speaker output is also the raw feedback voltage.  It looks like the speaker output has its 0V connection made to the chassis at the speaker connector, or near an output stage valve (can't tell from the photo)?  If that is the case, then the voltage used for feedback may include hum and noise that is picked up from the speaker chassis connection back to the PI stage 0V connection.

[TIMBO]

Tim, This is how it is connected.

The grey wire is connected from the 4ohm tap to the speaker connector then to the 10k feedback resistor.
The black wire is connected from the 0v tap to the speaker connector then to a lug on the side of the power valve socket.

[trobbins]

If it was a hi-fi amp then the black wire would connect to 0V at the star ground used for the PI and output stage.  That amp appears to have a more 'spread out' grounding arrangement for all the stages.  If there is no noticeable noise or hum or instability then you're in luck.

[TIMBO]

The new filter caps have arrived.
So some rearranging of wires.


Some more zip ties are needed but i'll wait till it fire it up to see if there is any noise problems.