HH Amps

Just finished working on a very good condition HH VS Musician from 1977.   Because it had its original vinyl cover it is in fantastic shape.   Sounds wise this amp was suffering from noise, multiple time.  I guess as things get older the problems just get on top of each other; a bit like life for humans too.

So there are noisy pots, that scratch and make the sound of the wind as they are turned.  Connectors that cut in and out, usually when you look away.

Then you have those annoying connectors on the rear that we almost never use – the effects loop.  Your signal goes through those unused connectors and they suffer terribly from dirt and oxisdisation.  Many techs will clean them and this might ease the symptoms for a while but I prefer to replace.

Most annoying is that scratchy sound that just won’t go away no matter which knobs you turn, which buttons you press or which connectors you fiddle with.  Now often probing with a chop stick can identify some bad joint on the PCB or a connector, but often it is something else and something so subtle to find that there is no way to identify it without isolation and tools like an audio probe and oscilloscope; I favour  the former as the scope requires eyes but the audio probe relies on ears – much better if you are not a multi-tasker. The fault that I am talking about is the  humble op-amp.  The little 8 pin chips dominated the HH  range ofamps and many others and to this day they are many designers first choice.  There seem to be three types of failure that I have observed:

  • total – no signal despite a clear input
  • severe distortion – where the signal turns into a robot like version of  the input.  Usually quite easy to identify.
  • subtle scratchy noise – hard to spot.  The amp works perfectly but you get a little scratchy signal where you expect silence.  It’s really annoying.  To identify which op amp you need to isolate sections of  the pre-amp and completely separate stages.

On the HH amps you need to be careful because turning the volume of say channel one does not isolate channel one, just the input coming in to it.  So if channel one op amps are playing up then there signal goes straight into the power amp and there you have it coming out of your speaker too.

Once the amp is put back together it sounds great again although the valve-sound never got me too excited.

HH IC 100

This HH IC 100 came to me with filter capacitors dried up.  Simply replaced with high quality modern ones from CPC electronics. Fortunately they were roughly the same size so I was able to use the existing rivet system for keeping them in place.  However solder tags are hard to come by these days and I had to make my own from tinned copper wire wrapped into a loop and soldered onto the PCB mount of the electrolytic capacitor.

Also found that the smaller filter capacitors that smooth the low voltage (+/-15v) power supply that is used for the pre-amp supply rails had dried up blue paper on them.  This seems to be a popular problem for this range of amps and maybe caused by the caps being so close the high wattage resistors nearby which run warm.

The sound of this amp is not particularly remarkable and in fact seems a little quiet compared with the other HH amps looked at recently.  I am not sure what  the reputation is with these amps.

UPDATE:  These amps are a little quiet when the switches are old and knackered.  They switch the sound between studio and stage volume and the difference is not surprisingly quite a bit.  It’s a little lesson really checking each switch out carefully and giving a it a firm wobble to see if there are any bad connections.

HH V-S Bass

This amp came in for a check up – nothing too much wrong with it from listening to it.  However upon opening it up found a few tell tale aging signs.

Another check I recommend on the HH amps is the smaller filter caps on the pre amp board that smooth the +/-15vdc supply.  They often have dry joints on them which you can see by gently wobbling the caps.  However in this amps case I found that rather than having bad joints the blue paper just fell off.  Another inexpensive replacement and recommended as a routine fix.

Another issue I have noticed with the HH amps is on the rotary switches used for bass-boost or valve sound.   They do seem to suffer from noisy contacts.  I replaced them on this amp as I found they click and popped when the shaft of the switch was moved slightly.  Also hard to get shafts that are compatible with the size of the old HH silver knobs. However you can trim the knob shafts and make shims to fit them together snugly.

All fixed up now and sounding pretty nice especially with the valve sound is on (as it always should be set – in one amp menders opinion).