Blown H&K BASSKICK 707 (again) + usual stupid question

[Superfuzz]

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. 

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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.

[phatt]

These things (powerstages) are mostly DC coupled so if the power transistors blew then it's likely because a predriver transistor is also failing  hence it did not take long to kill the new parts.

You need to go back in and find the Real cause of failure before you waste another set of power transistors.

As to stupid Q,, 210 Watts at 2 Ohms is closer to 2 times the power of a unit that produces 250 Watts at 4 Ohms.
This subtle trick is used a lot to fool people into thinking they have a 100 Watt Amp until you read the fine print,, which says into 4 Ohms which is only about 60 Watts into 8 Ohms.

Tricked me many times when I wus young.

Meantime have a hunt for a schematic and link to it will help.
I have no doubt it's fixable but don't do anymore until someone here chimes in with good help.
Phil.

[J M Fahey]

It looks like *just* replacing the power transistors was not enough.
Start goggling/digging/begging for that schematic.
Write H&K with model and serial nº and they will probably send you a copy.
Probably with a warning "don't post it in the Net" but showing it here is fair use, "showing it to a fellow Tech who is helping you repair it to get his opinion".

[teemuk]

So how the f*uck the peavey is so weaker in comparison?
I suppose it has more modern trannies or something like that...

Indeed something like that. Modern semiconductors can easily feature higher power dissipation and SOA.

Is there any way to replace the older trannies in the Peavey with more modern and efficient ones?

Yes. You research what replacement devices work, fit them in (with minor circuit modifications if neccessary), adjust, check and monitor bias, and finally monitor for flawless circuit operation (no increased ditortion, hum et cetera., no oscillation, no instability when clipping, matches designed-in output power rating, etc.)

[Roly]

only 250W

"only"?   

twice in this late month

Then it is likely that blown output transistors are the symptom, not the basic cause.

My first action would be to go over the speaker leads and wiring with a fine tooth comb, and it's a fair bet you will find an intermittent short inside one of the connectors, or something similar.  If you have been touring for four years, when was the last time it (or all your gear for that matter) got a good going over from a professional tech?


Also what @phatt said, with a clarification; to compare two amps you need to do so into the same load.

In round numbers;

210W at 2 ohms, 105W into 4 ohms, 52W into 8 ohms

(500W into 2 ohms), 250w at 4 ohms, 125W into 8 ohms, etc

The design of an output stage is complex and power output has rather more to do with the power supply than the transistors (which are basically selected to handle the traffic).

You could, with equally dubious validity, reason that with twice the transistors for half the power the Peavey has four times the margin to failure of the H&K.  What Peavey might have been thinking is that more transistors gives a better distribution of waste heat into the heatsink.

To a first approximation all transistors have the same "efficiency" and changing them won't make a jot of difference to how the amp performs.  What it might do is alter the margin to failure, for better or worse, depending; but there is nothing to be gained and quite a bit at risk, so I'd advise against messing with the output transistor types at all.

Unless you want to go through the process @teemuk outlined, the short answer is no.