Another DC coupled amp (Ampeg bass combo)
 

Hi all, I have an Ampeg bass combo with a DC offset - and a twist?

The amp was making a very loud throbbing sound and if left switched on after a few seconds would blow it's T2.5A fuse. A quick test of the 4 Mosfets revealed 'Q5' to be dead short between gate and source, the rest tested good as did associated diodes.

So I replaced the IRFP140 and wound up the variac and the amp came to life but with a 500mV DC offset with no load connected. A voltage test across the four 0.33R source resistors showed one, 'R74' had 20V across it! The other had a few millivolts each, so I thought 'Q2', despite testing good must be leaking and turning on. So I replaced it (IRFP9140) but on turn on it's still measuring 20V across its resistor ???

I've yet to get to the bottom of the DC offset but am puzzled why Q2/R74 is conducting while the paralleled 'twin' Q7/R75 is apparently behaving as expected? The gate resistors and others are good

Many thanks

Re: Another DC coupled amp
 

Just a thought here, if R74 (0.33ohm) has 20V across it it will be pulling some 60A, that works out to approx. 1.2Kw, is the resistor hot? or am I missing something?

Andrew

Re: Another DC coupled amp
 

I agree Andrew, I had to keep recalculating 20V over 0.33 because I thought it must be wrong?! No it's not getting hot so maybe I'm missing something?

Re: Another DC coupled amp
 

Methinks R74 is O/C!

Andrew

Re: Another DC coupled amp
 

Where did I put my dunces hat? It is o/c Andrew :-] I reckon that is the cause of the offset too

I subbed a 0.33R but the offset is still there, still progress!

Re: Another DC coupled amp
 

Taking a careful look at that amplifier, you'll see that it's a peculiar design. Instead of the power devices being used as emitter-followers, or source followers with unity voltage gain after a voltage amplifier with large swing, this amplifier is a Blomley type, originated by Peter Blomley in Wireless world in the 70's. Peter Blomley only had plain old bipolars to play with in his output stage, but this follows his principle of using q4 and Q1 to phase split and switch the signal between positive and negative halves of the amp. The output transistors are used as common-source amplifiers with appreciable voltage gain.

This circuit has a number of attractions, but it has a nasty disadvantage. The output transistors try to control the CURRENT into the load, not the voltage across it. The voltage gain is strongly influenced by the impedance the speaker presents and so the speaker impedance strongly influences the stability of the feedback loop.

The thing can only be designed to have reasonable stability over a limited range of speaker impedance. With wider ranging impedances, the amplifiers dynamics change a lot and you either have to have less feedback than you'd like, or your amplifier can oscillate into some speaker. R28/C17 is an attenmpt to provide loading at high frequencies.

In the late 70's I built an amplifier to this scheme and got to fall into all its pitfalls. When things were favourable, it was enticingly good.

I think this circuit has deliberately little feedback to keep it tame.

One advantage is that they ar less prone to the firecracker sequence of destroying transistors that the tradition (Lin/Tobey/Dinsdale) architecture suffers from.

David

Re: Another DC coupled amp
 

Oh, and the low gain reduces the ability of the feedback to control offsets.

David

Re: Another DC coupled amp
 

Checking the +/- 16V rails to U5(NE5532 feeding Q1/Q4 phase splitter) the voltages are all over the place. Will it be safe to remove U5 temporarily and run the amp for testing, i.e. will that upset the DC conditions?

Re: Another DC coupled amp
 

With the internal PSU off, I lashed up an external +/-15V 1A supply (I haven't got anything that'll go higher at present)and clipped it to the appropriate zeners 'D11/12' in the power supply (sheet 1 in the schematic), 'U5' and the front panel display and tuner lit up and the ad-hoc supply remained at +/-15V without either rail being dragged down.

But when it's powered internally the +16V rail rises to +18V while the -16V drops to -7V? Also there is the distinct aroma of hot components but I can't find anything getting hot! >((

Re: Another DC coupled amp
 

Amp won't work without U5, as half of it is in the feedback loop. Actually, I don't think it would blow up unless you had a load connected as it might just swing to one rail or the other! If one of the 0R33's is toast then I would be looking for another dead one ...

Is U5 getting very hot? If you can keep your finger on it it's OK. An NE5532 is the kind of thing you might have lying around, so try substitution but usually they're quite reliable. I would check the voltages around R42/R50 and Q3. Q3 c-e should be between 3V5 and 8V5 depending on the pot and the temperature with the remainder of the 32V split near equally between R42/R50.Q1 and Q5 are operated as cascodes (some will point it out if I'm wrong) so the base voltage will be pretty static but tracking heatsing temperature.

dc

Re: Another DC coupled amp
 

Thanks Dave, I'm out of NE5532's and Maplin were out of stock too and didn't know when they'd be back in :-) So I'll get some from RS.

The only thing that are getting hot are the two 16V zeners (D11/12), but they do get hot in service, no?. The 3 other 0.33R's are intact

'U5' isn't getting warm, nor are any of the other opamps, at least in the short term, I dont like leaving it on in case something pops, although the amp is running through a 60W lamp limiter. Incidentally, if I connect an 8R dummy load to the o/p the DC offset swings to 7V!

As I say in post#9, the opamps run from an external supply seem ok, doesn't that preclude U5 being at fault?

EDIT: I just checked the 0.33R (R74) that I subbed and I read 125mV across it, the other 3 read 24mV, the schematic says to set the bias to get 15mV to 20mV.. The 16V zeners are definitely the source of the burning smell, they aren't happy!

Re: Another DC coupled amp
 

I think I may have discovered the problem? The image is of the two 330R dropper resistors that supply the 16V zener diodes for the +/- supply.

What do you reckon?

Re: Another DC coupled amp
 

That's the problem, one resistor is 0.33 ohm and the other is 330 ohm!

Re: Another DC coupled amp
 

Do those resistors look like they've been fitted at the factory? If so, though one is the incorrect value, the amp would have worked with them at one stage.

Re: Another DC coupled amp
 

That's the same value and wattage rating as the output devices source resistors.

Lawrence.

Re: Another DC coupled amp
 

Sortedradio, Michael, yes we must presume it was factory fitted 8-o

The poor zener diode D12, would have been sinking the full negative rail, 39V in this model, and some how held up at least for a while. The owner said it never worked right (!) and stored it for a couple of years before digging it out and giving it to me.

So 2 new zeners and a 330R 10W are on order and hopefully it'll be a fix...

Re: Another DC coupled amp
 

The downstream electrolytic on the -16v rail might have had a hard time.

David

Re: Another DC coupled amp
 

Good point David, although the zener seems to have gone short luckily, else it could have caused carnage with the opamps etc,. I'm of course assuming that this is the one and only fault?

Re: Another DC coupled amp
 

A 16V zener could not stand 39V applied through 0R33!
It must be open, then the cap would fail under excessive voltage - probably :)

The fact you powered up through an external 15-0-15 gives some hope!

dc

Re: Another DC coupled amp
 

Cripes, what a mess this thing is, it's been got at :censored:

I fitted the new zeners and resistors and powered it through the lamp limiter - same fault? So I decided on a radical approach, I removed the 4 Mosfets, powered it up but the lamp still glowed and the power rails were still all over the place?

So I took the limiter out of circuit and just used the variac on it's own and wound it up slowly. I reached about 200V when magic smoke rose from the newly fitted R74 (0.33R) the source resistor for Q2 - even though the Mosfet was removed??

So I traced some wire links from R74 across the board to jumper plug J29 - some wires were cut from the plug had been hard wired to the board, but whatever clot did it wired it to a pin that goes straight to ground! :censored: :wall: