One Side of my 295 is wrecking speakers any ideas ?

[japster]

Hi,

I've got a 295 PowerAmp on which Channel A so far has :-

Melted 1 EVM12L
Blown 1 EVM12L
Deformed Voice Coil on another EVM12L so it now makes a horrendous noise.

Incidentally Channel B seems to be working fine (taking the same punishment and speakers are fine)

Things I have noticed (apart from blowing speakers up)

Channel A takes turning up to 2 before it produces any sound (whereas Channel B starts at 1.5)
Channel A doesn't sound so harmonically pleasing at any volume.
I've ruled out the my Studio Pre channels by swapping the inputs to the 295.

What Valve / Component in Channel A could be producing such violent distortion / transients to break these speakers ?

Any help appreciated

 

[crane]

You have to measure output voltage (AC and DC). The output tranny may bad and galvanic isolation not perfect, so your speaker take tooooo much power.

 

[japster]

Hi,

thanks for your reply, I will measure AC & DC voltages at the Output Transformer tonight, shall I measure the Primary and / or the secondary ?

This Channel A also suffered from blown coupling cap on the Phase Splitter stage earlier in the year would this be linked at all ?

Cheers,

 

[crane]

Compare the AC and DC voltage of the secondary.
But with that blown coupling cap I would compare every point (anode/cathode/screen grid voltage for each preamp and power tube, grid voltage, grid leak resistor value, ....) from each channel to see if their's any differences.

 

[japster]

Hi there,

Great thanks for the advice, I've spent the whole night measuring and learned a lot about where to measure. Luckily I managed to pull a schematic off here to guide me.

Each Channel goes like this :-
Channel A (bad channel)
Power Section
Plate Voltage (all power tubes) = 535vdc
EL34's
Screen Grid = 529vdc
Control Grid = 51vdc
6L6's
Screen Grid = 535vdc
Control Grid = 64vdc
Phase Splitter Section
1st 12AX7
Plate 1+2 = 389vdc
2nd 12AX7
Plate 1 = 220vdc but Plate 2 = 233vdc

Channel B (good channel)
Power Section
Plate Voltage (all power tubes) = 530vdc
EL34's
Screen Grid = 525vdc
Control Grid = 51vdc
6L6's
Screen Grid = 530vdc
Control Grid = 64vdc
Phase Splitter Section
1st 12AX7
Plate 1+2 = 381vdc
2nd 12AX7
Plate 1 = 214vdc but plate 2 = 229vdc

Component values are comparable between both channels both output trannies measure similar on both primary and secondary.

Bit stumped here really :roll:

What do you think ?

 

[elvis]

Looks like the bias is OK and you didn't electrocute yourself. Both are good news!

With a transformer, power out cannot be more than power in. So if you're blowing the speakers just because of power, then the tubes are producing more power than is theoretically possible.

I think a fault is more likely. A short in the output tranny windings could allow high DC voltage to get to the speaker. Also, asymmetrical clipping due to a mismatch of some sort in the circuit could cause heating of the speaker, because it effectively produces DC power. The output of the tranny should be AC only with no DC component.

A fault or mismatch may not show up unless you drive the amp with an input. If you have access to a sine wave generator and an oscilloscope you could run a test signal in the amp and see if it changes significantly as you change amplitude. You can also see if anything weird happens when the signal clips.

You could also be getting a high frequency oscillation in the circuit that you can't hear, but that delivers power to the speaker. An oscilloscope can help you find stuff like that. It would be easy enough to run the same test tone into both channels of the amp and measure both outputs on the oscilloscope and see if they are the same over frequency and amplitude.

 

[crane]

Yeah everything looks fine there.
I can't say it better than elvis, check the output speaker jacks for any DC component. Great if you can have an oscilloscope for that.

And be carefull with those high DC voltages ! it can kill you.

 

[japster]

Hi Guys,

Thanks for the input, it all helps me understand how the thing works

I do have access to a quite old oscilloscope which is unfortunately in Japanese, but I guess I can fiddle with it until I get some signals displayed.

Connecting both channels to loads is a bit of a problem as I'm worried I will blow speakers, which has become an expensive hobby. Testing at half Poweramp volume in simul-class mode nearly takes the roof off the building :lol:

I did measure the output tranny resistances on Primary and Secondary on both channels and the results were the same on both :-

Output side (secondary) = 0.6ohm
Input side (primary) = 106ohm from end to end
All with approx 3Mohm to ground

What frequency test tone do you suggest ?

Great help guys

 

[crane]

1KHz will be close enough to a guitar signal.

If you're worried about your speaker, build a load resistor: female jack with an 8ohm (or 4ohm depending of the output used) accross tip and sleeve. Use min 100W resistor. It's also easier to measure AC or DC voltage accross it.

 

[japster]

Great,

I've searched for 100w 8ohm resistor but they are not that common and mostly out of stock.

So if I connect 4R7 (50W) and 3R3 (50W) in series this should give me 100W 8ohm - right ?

Just checking, it's a long time since I learned about this stuff at school

 

[crane]

Right, but power will not be equally shared as they are not of the same value.
Better put two 15-16 ohm in //.
But if you're not pushing the amp to it's maximum it'll be okay.

 

[japster]

right got it,

glad I didn't order the wrong ones then

thanks

 

[japster]

Well I eventually got around to setting up my dummy load resistors and measuring across the speaker jacks with an oscilloscope

Immediately I can see on both channels the output wave is squaring off on the top and bottom. So I altered the preamp and poweramp levels and can only get a nice undistorted waveform at really low levels on both.

So I'm thinking, that can't be right

I recall when I purchased the Amp it came with an octet of 6L6's instead of the usual simulclass quartet of EL34's and quartet of 6L6's. So I replaced the outside 4 tubes with E34L's.

I tested the 295 in both Class A and Simulclass mode and the waveform was pretty much the same, won't go much above 3 on the preamp sends and 1.5 on the poweramp dials before clipping. I can see why my speakers have been getting such a hard time. It's virtually squarewave at times :roll:

Anyway I substituted the original 6L6's back in the EL34 sockets and ta-da !!!! The waveform is normal, I can run the poweramp at 5 and the preamp at 5 without clipping.

So what in the hell is going on here ? Aren't these amps supposed to be run with EL34's and 6L6's combined ?

What can I do to alter the circuitry to allow me to use EL34's in the outer sockets ?

Any help appreciated, I've learned a hell of a lot looking into this

 

[japster]

oops I need some bigger resistors, just blew my dummy load up

On Channel A actually, the Oscilloscope started doing something freaky then pop / bang bits of resistor everywhere. Hope I turned the standby off quick enough :roll:

Channel B still working fine, maybe I've found the problem ?

 

[elvis]

The outer sockets may be intermittent, or you may have an intermittent bias issue. I believe that because you changed the tubes in and out and then got the clipping to change. It could have been oxidation that built up in the sockets over time, so you should clean them. Or it may be a bad wire.

The weird behavior prior to exploding the load R is probably oscillation. The circuit could be on the edge of stable, and when it heats up, it goes unstable. Could be related to the bias issue, or a bad capacitor or resistor.

May also be the output transformer. If it was overloaded at one time, it may have gotten hot and damaged the insulation. Now, it may start out fine because the wires don't touch, but when they get hot and expand, they may touch and short across the windings, or worse, between primary and secondary.

I would send it to a tech. If you are really handy with electronics, and know how to discharge the high voltage caps, you could try swapping the output trannys. If the problem moves with the tranny, then it's bad. If it stays in channel A, then the tranny is OK.

 

[crane]

+1 for the trannies. Invert them.
Caps automatically discharge on that amp (and a lot of Mesa's) because of the resistors in // with the huge blue caps (as the blue caps works in series the resistors are there to equilibrate the charge, and they also serve to discharge the caps).
Just be sure to mesure voltage accross the caps before working in.

 

[japster]

Hi thanks for the reply,

Think the difference in the tubes were because they are different types, my 295 came with 8 x 6L6's not the designed 4xEL34's & 4x6L6's. The tube sockets seem fine I've cleaned them before anyhow with contact cleaner.

So basically all I did was take the EL34's out and put the old 6L6's back in the EL34 sockets. The EL34's must have been breaking up earlier than the 6L6's.

I did note as I tried the channels individually that Channel A did make a lot more noise (as in I could hear the output tranny vibrating / humming) a lot more than channel B. I knew when it was being driven hard as it kinda "whined". Channel B was mostly silent.

The Oscilloscope did display some terrific peaks in the waveform just before the dummy load exploded, maybe this is the oscillation you refer to.

It seems like a lot of work to swap the output trannies over as they are multi tap on the primaries, Suppose I could just extend the cables and link them to the other output valve set ? This would apply to the feedback circuit coming of the secondary coil too.

Any comments on my idea above ?

 

[elvis]

I'd go all in and swap them properly. It's a big job, but the last thing you want is to do it halfway and either blow something up (maybe even yourself) or just not be sure that you really ruled them out. Take photos and post them, too!

This problem has gone on a long time. Time for the big guns.

Also, not too late to send to a tech. If the CIRCUIT is the problem, it may have damaged the A-side tranny, and may then damage the B-side after swapping them.

 

[japster]

Yeah you are right, no half measures

I've priced a replacement output tranny from the UK distributer £140 - ouch :shock:

Just one more question, this sound I was getting coming from the amp guts while it was on dummy load. Like a high pitched whining. I guess it was coming from the output transformer of Channel A (the suspect one) What is this sound ? and why didn't I get it on Channel B ?

I believe the oscillation occurred on Channel A when I was driving the output quite hard, around 7 on the controls. Thinks were really hotting up then.

Although after the blowing up of the dummy load on Channel A I touched the Output Trannies and the main power tranny to see if they were hot and got these results :-

Channel A output tranny - cold
Power Tranny - Warm to the touch
Channel B output tranny - warmer than Channel A

What does this tell us ?

 

[crane]

Tranny shortcut ? No resistance, no heat ! You can mesure primary and secondary resistance value.