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!!!
  

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 later 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

Alamo Montclair Reverb

  Had a rare old amp on the bench today, this little single 12" Alamo Montclair Reverb. Nothing special about the job, just a power switch replacement. But the amp itself is really cool. It's an early hybrid amp, 1/2 tube and 1/2 transistorized. The power amp is loaded with a pair of 6V6 tubes with a single 12AX7 phase inverter. It's about 15 watts and packs a mean lil' punch! The preamp is all transistor and has a nice reverb and decent tremolo. This one was loaded with a Celestion Vintage 30, one of the finest speakers out there in my opinion. Great live speaker. I'd love to hear this with a trashy old Jensen though. That's more my taste....

1998 Fender Hot Rod Deville 3/13/15

Need a good, well made amp that is LOUD for cheap? You still can't beat these. This is an earlier version. Easy to work on, mod, bias and make music with. Tonally? Kinda harsh for my taste but I have had success with changing signal caps and resistors in the signal path. Heck. I once changed every signal cap and resistor plus the harsh, terrible modern Jensen speaker for a customer out west at his request. I thought he was nuts as it was a $350+ job. In the end though it sounded like an old amp. I was quite pleased with the results and so was he!

  This amp just needed tome new tubes and routine maintenance. Every few years a bunch of little solder joints inside need to be touched up, they simply come un-done and cause noise and dropouts. This was no exception. Also, I've replaced more load resistors on the phase inverter in these amps than any other. If yours sounds particularly harsh and lacks volume, there is a 50% chance that is the problem. Easy fix for the experienced tech. Don't try it if you don't know what you are doing!

  Great amps overall. They get my thumbs up. One of the only modern amps I enjoy working one. Many modern amps I simply won't look at cause the construction is so poor and they are not made to be worked on at all. These are no problem!

JB

All Test Devices Wah/volume pedal. 3/13/15

Here is a rare bird. An All Test Devices Wah Volume pedal. Yeah, it's really just a Maestro Boomerang in disguise. Nice wah, very nice indeed, but these pedals aren't my favorite. Why not? I'm just not a fan of the volume wah arrangement to begin with. And these really eat your dry signal alive. This one was purchased at a junk shop for $10 last week. The pot was broken, or at least the housing had come undone. I found the perfect replacement for it but still, these have a nice lil' flaw. When you adjust the volume for maximum sound (which also gives you the brightest top end for the wah) it has a 'bump'. In other words a pop happens. The old pot did this and so does the replacement. I needed to back it off a bit to get it to behave. Sounds like a parasite of some sort to me. I'll mess with it to find a solution but I think anything that will help may rob even more tone.

I wish this were mine cause the wah is really good. I would gut that stupid volume part and throw in a true bypass switch. Yup. That would make me happy.

JB

Alamo Fiesta! 3/13/15

 Today we have an Alamo Fiesta. I re-capped this amp last year and also modified the tremolo to play slower. Today it was in for a simple re-tube. Output tube was making horrible howling sounds and the main preamp tube was microphonic. These use a 50L6 tetrode for power yielding about 4 watts. Easy to find NOS examples on the cheap. Nobody makes them any more. For the preamp tube I installed an RCA 12AU6. I'm usually not a fan of amps with these tubes, they have a cheap arrangement for the filaments. They really are kinda crappy when you put them up next to a Fender but, this little amp kicks. Plenty of volume.

  I recommended the customer buy my personal favorite 8" speaker, an old Jensen C8R, one of the brown ones you see pulled from an old organ where they were usually used as midrange drivers. I find these on a regular basis in great condition since they weren't being abused in a guitar amp. And they are usually obtained at a good price. And of course, they just have a lovely sound. Better than anything being made today.

This amp also boast a lovely tremolo. Very rich sounding amp overall, more of a darker sound which I like. Warm like a tube amp should be.

It's always helpful to have a schematic handy. Thanks to whoever drew this one and I hope it's okay to feature it here!

To slow the tremolo down I simply increased the value of the 3 in series tremolo capacitors. The bigger the caps, the slower the trem. Experiment! Start with the one on the plate of the 12AV6 first. Try a .047 or a .1 and proceed from there.

JB

1959 Fender Champ, signal cuts out. 3/6/15

Quite a beauty this amp. This is the kind of project I really enjoy, it's small and simple with a beautiful voice. These amps are so well made, Fender really raised the bar higher than anyone else did.

This one belongs to a really fine jazz player I know. He uses it for solo gigs. Yes, it can happen. A 5 watt tube amp for jazz! A little breakup and warmth makes music sexy. An Ibanez Ts-9 with a Roland Jazz Chorus? Um, not sexy.

So what was the trouble? The amp would work for about 20 minutes then crackle out and die. He would plug into the other input and it would die after a few more minutes. The client was frustrated as this is his favorite amp and it's been to another tech who couldn't find the problem. Truth be told, I couldn't either till tonight and it took me a week. Oh, I did everything: the ol' chopstick test, tested the caps, resistors, cleaned and tightened everything, checked the sockets. I even changed the cathode caps only because on the 6V6 tube there is 23 volts sitting on a 25 volt cap. I don't like that. I prefer a 50 volt or better cap there. I've seen those lil' Sprague Atoms pop like a firecracker.

I saw this baby was loaded with a JJ 12AX7 and 6V6. Their preamp tubes in my experience have all sorts of reliability issues. So I put that tube in my tester and let it cook overnight just to see if the heat caused any shorts. Nope. Not even tapping on it caused any trouble. This one was made on a good day.

So I could not get this amp to not behave like a champ! (Yes, stupid pun intended!). I did find one disconnected ground wire flopping about which could reach the input wire by the 12ax7 tube. This wire once was connected to the pin 9. The heaters were improved in this amp, lifting that side from ground and 2 100 ohm hum balance resistors were installed. Well done but why leave that wire?

This brings me to my point. I spent a lot of time I can't justify billing for on this amp due to another techs sloppiness. I replaced the cheap signal caps and the ones what were in there were barely soldered. So I figured maybe this was the problem. But once I soldered the new caps in it finally quit on me! YES!!!! If it ain't broke I can't fix it. Now I had something to fix! I hate turning a project in that I don't feel 100% right about.

THE CULPRIT:

Yup. A washer. Flopping about like that drunk and annoying hobo asking for a sandwich. Right under the solder joint where the first .022 cap and wire connect. When the amp was taken apart it must have come loose and gotten lost in there.


And here is is back in it's proper place:

Man this kind of thing....well, yeah, job done.

This is a great little amp. The power transformer was replaced some time ago with an early 70's part so the B+ voltage is a bit higher which I like. Also the speaker is replaced with a Kendrick. I don't really dig his speakers but in this amp it's a great choice. I installed a NOS GE 5751 tube so it has a nice darker and warmer voice to it. This is a terrific amp. Sad to see it go home tomorrow but happy to get paid and have another satisfied customer!

JB