About

Typical modern Fender designs force the zener diodes to dissipate so much heat they burn the PCB.

This at first causes dry joints, and thus lots of horrible loud noises, eventually causing the amplifier to die altogether

Simply resoldering these joints does not solve the design problem for very long.

What you really need to do is to add heatsinking in the form of adding wire to the PCB tracks, and lots of solder.

This problem will never now happen again.

Hartke HA3500.

Apparently it was turned on after being stored a while, and it went "pop".

I found the mains fuse had been removed, presumably it had blown.

Now, the one thing you should never do in this situation, is replace the fuse and turn it on, hoping for the best. This could cause a huge amount of further damage.

You really do not want to now have to replace a very expensive Mains Transformer, which is the very thing the fuse is trying its best to protect.

The old "100 Watt lightbulb in series with a Variac trick" is the method I use. It gives a visual indication of current draw, and should things suddenly go horribly wrong, will limit the current drawn by the amp to 250mA or so, so nothing should blow up that hasn't already.

In this case, it was immediately obvious from slowly winding up the Variac, that the lightbulb was going to be constantly on full brightness, indicating a serious short inside the amp somewhere.

Turned out to be a shorted capacitor just downstream of the Bridge Rectifier.

The photo shows the repaired amp back in series with the lightbulb, glowing happily as you might expect on idle, as a precutionary measure before full testing.

I measured over 400W out of this into 4 ohms, despite its conservative claim of 350W.

Not a bad Bass Amp at all IMHO. Passed its PAT Test, no issues.

Well here we have a Blackstar HT60.

Apparently 60 Watts from a pair of EL34.

The Blackstars are kind of a hybrid amp range in that they mix well proven valve technology with contemporary Microcontroller technology, with combined transitor and valve drive circuitry for the output valves, if all that makes any sense.

So it's a valve front end supplying a hybrid drive stage incorporating Mosfets and valves, with a conventional valve output stage, and the whole thing is ultimatley governed by a Microcontroller which switches and mutes stuff, depending on what you have plugged in, or havent.

So you can't blow this up if you have accidentally forgotten to plug in your speaker cab.

A new set of matched output valves was all that was reqired, along with the usual cleaning and fettling you would always do once inside an amplifier.

The new valves needed biasing of course - that's not a good sine wave, way too much crossover distortion.

That's better. And over 22v RMS into 8 ohms.

So that really is more than 60 Watts from a pair of EL34.

Plate voltage was higher than normal, so perhaps this is how they get 60 Watts out of it.

This shows why poking around valve amps is so dangerous unless you really know what you are doing.- you do NOT want to pass 550v DC through your heart by having one hand on the chassis and the other on that pin at 550v.

I did that once with a Fender Twin and could have died.

Now I ALWAYS keep one hand in my pocket.

Mesa Boogie Mk IV. Very hand wired, very solid.

And a load of output configuration options from Class A to full Push/Pull, with options of 6L6 and EL34 combinations to boot, and a huge amount of EQ options as well, this is a really flexible customisable and VERY loud little combo.

Interestingly though, these don't have ajustable bias.

Like most Mesa's, you are supposed to choose the output valves to match the fixed bias, by buying the appropriate Mesa valves. Cuh.

In this case it means the present output valve set is apparently useless, even if they are just a little worn, but still perfectly useable.

Well I think I know a way or two around that problem.

Parallel a resistor from the Bias supply to ground to take the bias into a far more reasonable ballpark.

Resulted in pretty much all configurations now being free of the previously hideous crossover distortion, whilst ensuring maximum life out of the valves.

(And by the way, there is no such thing as "Watts RMS". That's pretty much an Oxymoron - a Watt is a Watt)

Another box of 6L6 and E34L valves arrived from the JJ Tesla factory today.

Now I just have to match them all up ......

Marshall Plexi 50W. Late 1968 to early 1969.

Been messed about with.

Came in with a fried Mains Transformer.

Not the original, this has been replaced at some point with a different transformer, and the amp modified to take it.

At some point the amp has also been converted to Master Volume, hence the extra pot beside the switches, not now in circuit.

Found a burnt output valve socket that had arced between HT supply and heater. This is most probably what took out the transformer.

Fortunately a new mains transformer can be bought from TAD that will work in this.

It is considerably larger than the original and will only fit by filing the chassis hole slightly larger, but it does eventually go in.

This amp came in with 2 x Mullard EL34, pre-amps ECC83 1 x Mullard and 2 x Brimar. All tested good on my digital tester, so no need to change these.

Caps all test good, no need to change these either.

This has been messed with a lot, I had to reconfigure the new mains transformer supplies and add appropriate diodes and stuff, but it did eventually accept the new transformer and all was good.

An unoriginal treble boost cap modded accross the treble boost volume pot was removed, as this was ridiculous and way too much treble. Caused oscillations and all sorts of nasties when the treble pot went past two thirds.

Owner now thinks this sounds fabulous, which it does.

if you drive it hard so the output stage clips, you are getting into proper Angus Young territory.