another project looms - HH performer

[Bushbaby]

Hi All,
Occasionally I've had the opportunity to pick up some weird 'bargains' as electronic to add to my 'round to it' pile (hence why my garage is full!) I couldn't resist this one which I picked up locally for the sum of £25. Its is working (of sorts)  and I have obtained (with difficulty) a schematic for it. This model was one of the last produced by HH amplifiers in Cambridge and dates around 1980. I am led to beleive that some of the circuitry is a nightmare although I do have a modification sheet for it as the design has no gate protection for the input FETs for example.

Its a 150watt 2 x 12 combo so I think its unlikely that it will be played at much volume at home.

Rob


   

[ppppenguin]

More about HH: http://www.hhamplification.co.uk/ https://en.wikipedia.org/wiki/HH_Electronics

Not to be confused with HHB who also make amplifiers: http://www.hhb.co.uk/about_directions.php

[Bushbaby]

Hi all

On the service sheet it mentions in several places the following protection measures - add 2 x2V7 400mW Zener diodes wired back to back in series across R16. To this end I have purchased some BZX55 Zeners at 2.7V 500mW to use.  I assume that this is how its connected and polarity will need to be considered. Am I correct in my thinking?

Best Regards

   

[pwdrive]

With stuff I've seen, the back to back connection was with the two anodes connected together, the cathodes being across the circuit.

Lawrence.

[Bushbaby]

Hi Lawerence,
Excuse my naivety (i'm no electronics expert by any means) - what does this practically look like?

Many thanks

Rob

[Mark Hennessy]

Just flip one of the diodes.

Having not seen any schematics (always useful if people post them when asking questions), I'm assuming it's bring used as a limiter - not allowing an AC signal to exceed a certain value.

So on the positive-going peaks, one zener will be acting as a regular diode, with 0.6V dropped across it. Meanwhile, the other zener will be being a zener, holding 2.7V across it - giving a total of 3.3V, give or take.

On the negative-going peaks, the diodes switch roles...

It's crude, but basically works.

[Bushbaby]

Hi Mark et al
I do have the schematic which I shall be happy to share via PM but I cannot post due to copyright. Apparently, unlike previous HH amps of note (VS series, IC series) this one was not well designed and has series of 'flaws'. I bought this as a 'working amp' and I'm conscious that I'd really like to keep it that way. You never know I might rejoin another band one day (lol).

Thanks

Rob

[Mark Hennessy]

Hi Rob,

An extract from the schematic would be fine - it would be considered "fair usage" for the purpose of this conversation. Or, we could move this thread from public view. so you could upload what you have. But totally understand if you're not comfortable about those options though...

Cheers,

Mark

[Bushbaby]

Hi Mark

Does this help?

Regards

Rob


  HH P schematic sample.docx (Size: 109.12 KB / Downloads: 25)

[Mark Hennessy]

Ah yes - it's as I guessed earlier.

For a bit more background, this looks promising: http://www.261.gr/limiters.html

Figure 5a is what you're doing...