Thursday, April 16, 2015

1965 Gibson Hawk amplifier, 7591 model





  Here's a rare, um, gem? A 1965 Gibson Hawk Amplifier. They made several versions of the Hawk and I believe this one to be the rarest of them all. It's loaded with that typical AMAZING Gibson reverb circuit and unlike it's smaller brothers and sisters, it uses a pair of 7591 output tubes with fixed bias and rather than using a phase inverter circuit it simply uses an interstage transformer driven by a 12AU7.

  I ain't gonna lie to you all here, yes, this is the amp used by the band Ultimate Spinach to cut those groovy psychedelic tracks*, and also, I'm no fan of this series of Gibson amps. This job was a nightmare....

  Amp was brought to me cause it had low output and was distorted. It also had in intermittent reverb short and weak tremolo. On top of that it hummed badly. It also lacked that Gibson high end, I mean, no treble. How can you capture that awesome tone on "Vision of your Reality" in this condition? Simple enough. Tube and cap job. NOT! Ok so I did replace some filter caps. They were re-done by an amateur in the 80's, terrible messy job. But once I opened it I noticed the interstage transformer business. Hmmm.

  Well after a re-cap of the power supply, it still hummed. Not as bad but not acceptable. This is where the job gets risky. 7591 tubes are expensive. This had a nice pair of Ampeg stamped ones but they read about 18ma off from one another. In a Fender amp you may not notice a hum unless it's one of the awful later Silver panel ones. But on this amp this could be the culprit. But there is that transformer too which was replaced. That's expensive as well. I ordered parts but you know if it's not the transformer, who am I going to sell it to?

  Turns out it needed both. Once I replaced the transformer (Mojo part made for Musicmaster Bass amp! VERY NICE!) which was a pain to do, the hum came down but not where I like it to be. What did happen was the volume went way up and the high end restored. Whatever jive part Dr. Amateur threw in there it wasn't doing this amp any favors. No wonder this amp is so clean, it wasn't exactly useable.

  So next I threw in a pair of fresh JJ 7591 tubes. I've spoken about JJ tubes before: risky. These worked fine but the bias was super duper hot. So I threw in an adjustable bias trim pot. Hum is passable but not to my liking. Some Gibson amps from this era are just like that. It may need a DC filament but for now I'm simply done. I don't care to spend any more time on this. I'm losing dough and that's no fun.

  I suspect that it's still the interstage transformer, not that this part isn't right but because you need to position them just right for maximum quiet. There is no room in this amp to play around with it and though I needed to drill some holes to make it fit, it's oriented the same way as the original once was. Before I bolted it in I rotated it and yes, noticeable change in hum. Lordy this is a poor design. Heck, maybe it just needs the exact part? I used to have a box of vintage interstage transformers, Triad ones. Bought them at a Hamfest years ago and sold about one a year till they were out. All to Gibson owners and they were always dead quiet. Oh I just don't know.....

  Yes, the schematic above is the best I could find for this amp. GIbson changed their stuff so much it can be difficult to find the right sheet of paper.

  Other bugs: Reverb cutting in and out: bad cap. Tremolo weak: Home made 'bug' I suspect but also previous tech put in the wrong value cathode resistor. Ugh. And this brings me to another point: If you don't know what you are doing and want to fix amps, don't mess with something like this. A guy like me or someone else will spend days un-doing a hack job. It's cool to do your own thing but start on something simple like a Champ. Better yet, build one from a kit and trouble shoot it when it starts smoking. And by all means, BE CAREFUL! Voltages in these amps can be lethal. It's no joke.

  So like I said, I'm not 100% happy with the result of this amp. It sounds much better and can play a gig but I like near silence. I don't feel too bad, I even solved some problems for Dave Friedman he burned out on. We all have our limits and maybe Dave will get this someday and make it silent!

JB



*April Fools! Ultimate Spinach probably used something worse! I don't know what they played and don't care!!!
  

Tuesday, April 7, 2015

Fender Musicmaster Bass amplifier



Today we have a Musicmaster Bass amp from the 70's. The customer complaint was it smelled burnt and the volume was rather low. Also had a faulty input jack.

I used to live with a cat that played 2 of these. He was way too loud! These little cheap hand wired tube amps can pump it out even stock. With mods they can really do some damage.

So to start I changed all the electrolytic capacitors. Typical stuff. The burnt smell was most likely the 1k 1/2 watt resistor:



It doesn't appear burnt but it was reading 2.5k. It's the first power stage resistor between the output transformer and the screen grids. I upgraded to a 1k 5 watt:



Yes, I like overkill in my power supplies. This baby will NEVER burn out. I also changed the first filter cap to an 80@450V cap. Deep, rich tones with no hum. Oh, may lose a little sag compression but really, it's a silicon diode rectifier so why not. Besides, the owner needs this amp to play loud and kind of hold it together so it's a wise choice.

These amps stock are meant to be a cheap bass practice amp. I've used them before with good results in the studio for, yes, bass and guitar. They are only about 12 watts so most bass players won't touch them. Yes, they are considered 'vintage' (wank wank wank) but this is an amp where for guitar the tone is kinda limp. So I say: If your tone is a' starvin', git on in that amp and start carvin'!

First thing: The first gain stage cap is a .01 ceramic disc:



Change it to a .022 cap of your choice:



Ok, missed a good photo op, but it's the yellow cap on the right. I also changed the 2.2uf bypass cap to a 22uf.

Now looking at the above pic you see a 470k resistor connected to a .0047 cap. That's the input going to the first stage. Not acceptable! Get rid of that cap and that resistor.

Since we are changing the input jack I'll put the load resistor right on that. I prefer a 1 meg carbon composition:


Should look like this:


Gah! Again, sorry about the poor pic! But notice that the 470k resistor is gone and the wire going to your first gain stage 12ax7 is connected right to the 68k input resistors.

Next, the very tone robbing tone stack:



Get rid of that 100k resistor and .022 cap. I simply copied a tweed Deluxe:



I'm using a 250pf cap for the highs and if you notice, I'm re-using that .0047 input cap as the rolloff. I could sell the customer a new cap here but why? This one is perfectly good and I'm a reduce reuse recycle kind of guy. Seriously. These are made of plastic. If I drop a new one in there this either sits around my tiny space or becomes sea turtle food eventually.

So how does it sound now?

In a word: British. Jangly top end with that nice euphonic mid. Think 18 Watt Marshall with a $300 pricetag. The customer will be replacing the speaker, I would go with a Weber English series or a real Celestion. Great amp for a jazz player who doesn't need fusion volume. Rich, warm and solid. These amps can be a real treat for those on a budget or those who just like a wonderful tone!


Another nice thing about these is they use the rather easy to find 6AQ5 tube. Plenty of cheap NOS examples out there. The earlier ones used 6V6 tubes, I think these sound more English than a 6V6 model. I kinda prefer these. And, you can just buy a pair of tubes and drop them in. No biasing required.


JB