Ok it worked a bit better, but it still cuts out..I think I'm goin to throw away that power amp..it's not enough for real bass.

Quote from: phatt on October 14, 2016, 11:12:45 PM
You could try a simple voltage divider at the output of your preamp.
If the volume control is at the end of your preamp and it's say 100k pot then change it to a 10k pot, that will limit the maximum swing. or just bridge a 10~20k resistor across the existing volume pot.
Phil.

This partially answers why it was completely unusable when I initially put a buffer after the master pot (wich actually is 100k indeed).
After I removed the buffer I went where I'm now, 1/4 of the travel of the pot. Now I got to check it with a 10k pot .

There's a reason of why shunting the signal down with diodes didn't worked in my case?

Thanks!

The problem is that  it stops  even at 1/4 of the  volume..The preamp is really "old school" ,  something beetween an old peavey and a sunn..It even has a sort of distortion inside.. for sure is bit brutal..

Hi there, I'm basically an FX pedal builder but lately I developed a lot of preamps ideas so I moved on amps. (well I'm trying to)

So now, I'm kinda stuck in this situation:

I designed a bass preamp, it's an idea of mine from start to finish and I'm very very happy on how it sounds when I try it on the PA of my 400w Ashdown. The whole thing was meant to be placed in a box with a connex Class D module wich doubles also as regulated power supply but, as I play it (even at very low master levels) the board goes into protection and I must restart it.

I contacted Connex and they told me that it goes into protection with every signal bigger than 1.5V RMS and it supports 4V p-p maximum. I know those are the standard values for line-in signal. Bass/guitars preamps anyway are always stronger than this and that's how I minded mine. I've tried to clip my signal with everythin, even with German diodes, but it still seems to much for the Connex.

Is there some kind of rule I'm missing? Or I should buy/build a whole different kind of power amp?


Thank you for any kind of help,

Ciao!

Quote from: Roly on February 15, 2014, 02:15:09 AM
Yeah, well that looks like about a 200VA core to me.

What am I missing here?

Let's go back to the OP.  If the amp is "underpowered" then that is a function of the power supply capacity, and changing the power amp to Class-D, Project 27, or anything else, isn't going to make any difference at all; the power supply is delivering what it can into the load (apparently without catching fire).

Changing the power board might make it more reliable (if the basic cause of blowing transistors is original mis-design; possible but unlikely), but things like faulty speaker leads or connectors have to be considered and eliminated first.

Currently it has "two BDW52C/BDW51C couples".  These are each 100V/15A/125W transistors and it has 2 pair - so why has it "busted the output section so many time(s)"?  At face value these should be more than capable of delivering 100W into 4 ohms (given sufficient heatsink).

Upping the power output would require some major changes, such as a heavier power transformer (but to me that core looks quite sufficient for 100W output); but again I would want to eliminate the possibility it has inefficient speakers and could possibly be improved by changing the speakers for more efficient ones.

Now if it has the original power supply and the original speaker load it can only perform as original, and it seems the problem here is blowing transistors, not blowing the power supply, so the supply must therefore have sufficient capacity to drive the original load.

Until we change the basis of this discussion to "more volts" or "fewer ohms" the power supply won't need changing.

220v * 1A = 220 watts in, about what you would expect for a nominal 100W output amp.

35VDC implies ~25VAC * 2.5A = 62.5 watts.  But it has two such secondary fuses and there would seem to be no reason for that with a single winding into a bridge, so (without a circuit) we can guess that it has two fuses because it has two secondary windings to protect, and that the available DC power will be around 125 watts.  Fuse blowing isn't the complaint.

So;
- what is the original nominal output power?

- what is the speaker cab impedance? (or simply measure the cab ohmmeter resistance and post that)

- well, its original nominal output power is un-mentioned on the chassis, back in the days , when I was not involved with electronics, the tech said it was like 70W..
- the cab instead was 8 ohm, wich makes sense of why it blew so many times..most of our cabs are 4 ohm, and this head has litteraly no heatsink apart from the chassis itself..

The output section looks very strange anyway, I got to post a pic.. it has a quad op-amp, at its center and it includes the power supply capacitors wich are 4 and each one is located next to one output transistor..

I'm a newbie here, I'm a DIY fuzzbox maker, but it looks to me like faulty design/unrealiable..probably not so efficient too..

well the Lem has, 4 output devices as well (2X couples of BW51C/52C) but as you say, it seems that the whole trasformer is scaled down to one half of what I need...

Dunno about the current rating (you're talking about VA right?) but the transformer is quite big if this means something

No  indications are written on the chassis or over the trafo..

Hi guys! I got this early 80's amp made by LEM
It sounded nice back then but it was too underpowered and running it at max busted the output section so many time years ago, so my mates stopped using it and it got shelved...

I decided to give it another chance but I want to replace the output board with something more efficient..maybe D-class??
I measured the transformer voltage and I got +/-32-35V wich is not that much..

The original has two BDW52C/BDW51C couples and BD317S/BD318S as drivers..

ideas??

it looks like this!

Hi all!

My bass player blown his BassKick 707 twice in this late month. We all know it's a cheap head from the '90s, but we love how it sounds when maxed! It's only 250W, so it's easy to squeeze it to hell. He also push it with a booster and a very hard distortion.

Now, most of all, will jump in to tell me "you don't have to run such a small head so hard", but that's what we did in the last 4 years, touring the whole Italy and Europe with our noise-rock band, so if it lasted so long, why it's so fragile now?

It blows the fuse each time you power it.
So I read that when it happens it's beacause of bad Output Trannies, so I replaced all of them, squeezed new thermal paste and BLING it ran again. (this was the first time). It lasted two days, just in time to record the new album (PHHHEEEW) and then it died again some days ago, during the first rearshall since then.

WTFF???

Useless to say, inside it looks perfect. 

-------------------------------------------------------------------------------------------------------

USUAL STUPID QUESTION

While looking for new heads I saw a nice Peavey bass series 400 from the late '70s. They rate it 210W at 2 ohms, I looked at the back and it has 4+4 output transistors, our Hughes & kettner has only 2+2 (MJ11016/015) and it puts out 250w at 4ohms.
So how the f*uck the peavey is so weaker in comparison?
I suppose it has more modern trannies or something like that, SO my question is:
Is there any way to replace the older trannies in the Peavey with more modern and efficient ones?

Thanks to everyone who'll chime in!

Emanuele.

Hey guys! I completely forgot about this post, since I spent a lot of time on recording with other gear..yeah the +/-15.7 volts are on the preamp rails, I haven't checked the voltages on the power amp rails...when I bought it 5 years ago, the seller said it was fully revisioned at MAJ electronics (the official english H|H repair center) so I suppesed the big filtering caps were good.. Now I still have to re-open it, but I recieved a bag full of electrolytics for the whole circuit.. they looked worse compared to the big ones..then I got also sockets and opamps to replace originals 741s, yeah the 4739s are too rare to replace, maybe should I make a conversion socket??

Another thing, this head has a big hiss from first time I bought it..is that normal?

Ok, now it's clear and it is even simpler than I supposed!!

Gimme some more spare time, and I will let you hear my impressions!  <3)

P.S.
I got some difficoulties reading all that graphics by Roly, I actually can't understand the schematic.. the 1M/12K5 resistors are the voltage feedback ones?

Quote from: Superfuzz on October 11, 2013, 08:19:56 AM

Quote from: J M Fahey on October 11, 2013, 06:38:14 AM
Interesting.
Why those values?

So you're telling me of, reducing the sensing resistor to 0.1 ohm and then take the signal directly from the node beetwen speaker - and resitor, without any resistor inbeetwen?

>>>>>>>>>>>calling mr. J M Fahey  <<<<<<<<<<<<<<<<

Quote from: J M Fahey on October 11, 2013, 06:38:14 AM
Interesting.
Why those values?

Mm, being just a "ssguitar apprentice" made me copy in some way, something that I saw on some Peavey designs and stuff..then I read here about a member modding his alesis Power amp and..you know how it goes when you don't know so much about electronics

So you're telling me of, reducing the sensing resistor to 0.1 ohm and then take the signal directly from the node beetwen speaker - and resitor, without any resistor inbeetwen?

Ok, here's the mod I did to my Carlsbro SLAVE 1000 power amp.. you can see I used a rotary switch (2X4) to vary the amount of CFB, from stock (nothing) to "max", as the CFB increases the overall gain increase..maybe it does too much gain per step...this may be the problem.. the original gain is 10k/220r (there's also the real schematic attached below)

I can see clearly the Current Feedback path, wich actually is DC coupled, then I got some more troubles identifing the Voltage Feedback network, but it must be that 680k res parallaled with the two caps and resistors..