my new old GRO100

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my new old GRO100

Postby Pacafeliz on Sat Apr 29, 2017 10:19 am

OK so while i was in Mexico a couple weeks ago, my friend from Bavaria was able to drop off the old GRO100 i had asked him to pick up for me in Munich, where he works.
It's a pics-only plexi front head, parts are marked early 1970, serial number 464. All complete except for one big knob ("fist") and back panel.

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Whilst trying out here at home (at very low volume) i seemed to get the impression that the treble/bass knobs were not really doing much, also the "fist" (presence?), but that one would change the sound a bit when i pressed it - i guess it's a bad/dirty contact or so.

Yesterday i took it to rehearsal, connected to a Marshall 4x12 at 4ohms. It sounded amazing, even though i had the volume at about only 7:00!
After about half an hour i wanted to see how much i could/would have to push it to get it to overdrive. In fact, at some point (around 12:00) it didn't get any louder but dirtier = heaven!
...then, all of a sudden, a massive volume drop, and very "fizzly" sounding. :dumass: :thehorror:

I turned it off. Nothing smells burned or such.
Today i checked the fuses, and the "upper" one looks as if it has sand or something in it. Is it blown?
Might i have damaged something, or could it be a fuse? I mean, an almost 50 year old amp with all original parts will need to be serviced, of course (will do ASAP)!, but is there a chance i messed up something bad?! :skeeered:

I'll be out for the weekend, but will check out the tubes and chassis... and will try to organize some helf from s/o with a car, to take it to the tech.
Thnx,

Pat.
oaklandanny too wrote:Pat...
This is way late in posting... but I want to say that she's beautiful, man!!! You are blessed and the world will be a better place because of your little girl. Mark my words... She will live in a better world than we live in now... because she will help change it. God bless!!!!

RIP OaklandDanny, thanks for EVERYTHING! God bless you, always!
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Re: my new old GRO100

Postby Tauntonhobbit on Sat Apr 29, 2017 11:57 am

Take it to a competent tech right away.
DO NOT attempt to power it up beforehand.
Keep everything crossed (!)
Please let us know how this turns out,
I wish you all the best- nice amp.
8)
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Re: my new old GRO100

Postby Unit_1 on Sat Apr 29, 2017 6:43 pm

I had an amp that was doing this very predictably right around 8 on the knob. It turned out to be a wire that needed to be moved as the second stage of the preamp was feeding back into the first stage causing runaway feedback.

On the oscilloscope you could watch the wave turn into a squiggly line filled box after being a perfect sine wave. I could move the wire back and forth and watch the wave go from one state to the other. Pretty cool. Anyway just moved that wire so it was at a perpendicular angle from the wire in the first stage and all was fine.

If you see any wires running parallel to each other in the preamp stage then that would be the first place to start. Also if you turn the volume down, and it sounds good, then you turn it up and it gets quiet but fizzy, this is the classic sign of improper wire dressing (parallel wires).

WARNING: amplifiers have lethal voltages inside and should only be worked on by a competent tech! :Zap:
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Re: my new old GRO100

Postby Gladmarr on Sat Apr 29, 2017 10:38 pm

Yeah, you've got to find a good tech close to you. Don't risk killing that amp! That's a rare beast there. You've probably got some old caps in the power supply mucking things up or bad old tubes or something. Also, I didn't realize I need to tell people this until I started seeing problems again, but make sure you are using SPEAKER CABLES for speaker cables. Also, make sure you are plugging into a known good speaker cabinet. I have just fixed a friend's amp for the fourth time after finding out he's been usin instrument cakes for speaker cables and hooking up to a broken speaker cabinet at his practice space.
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Re: my new old GRO100

Postby Pacafeliz on Sun Apr 30, 2017 9:27 am

Thanks guys, I'm happy to hear no "oops, sounds like big trouble..." *phew*
Yes I'll take it to the tech and keep you updated asap.
Thnx, rock on!!!

Pat.
oaklandanny too wrote:Pat...
This is way late in posting... but I want to say that she's beautiful, man!!! You are blessed and the world will be a better place because of your little girl. Mark my words... She will live in a better world than we live in now... because she will help change it. God bless!!!!

RIP OaklandDanny, thanks for EVERYTHING! God bless you, always!
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Re: my new old GRO100

Postby Gladmarr on Sun Apr 30, 2017 2:19 pm

I have a checklist of sorts when I get a new/old amp that I don't have 100% confidence in. I'll check for busted transformers, leaking capacitors, tubes that have lost their vacuum, anything that can be ascertained before applying power that might blow the amp up. Then I'll pull all the tubes and apply power looking for any weird current draw or stuff getting hot or smoking. If the filament and various plate supply voltages look good with no tubes, I'll shut it off and put tubes back in. It's very rare that I'll get a tube amp home and just turn it on and start rocking. Especially with vintage amps, that can be very dangerous.
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Re: my new old GRO100

Postby Pacafeliz on Sun May 21, 2017 3:48 pm

will take it to the tech as soon as a friend offers me to drive me there... :oops:

in the meanwhile, some more pics!

Image

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i love how EVERY part in the amp has been serial numbered!
oaklandanny too wrote:Pat...
This is way late in posting... but I want to say that she's beautiful, man!!! You are blessed and the world will be a better place because of your little girl. Mark my words... She will live in a better world than we live in now... because she will help change it. God bless!!!!

RIP OaklandDanny, thanks for EVERYTHING! God bless you, always!
Pacafeliz
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Joined: Tue Feb 17, 2004 8:01 am
Location: Cologne, Germany

Re: my new old GRO100

Postby Pacafeliz on Sun May 21, 2017 3:49 pm

with its newly arrived younger brother!

Image

Image
oaklandanny too wrote:Pat...
This is way late in posting... but I want to say that she's beautiful, man!!! You are blessed and the world will be a better place because of your little girl. Mark my words... She will live in a better world than we live in now... because she will help change it. God bless!!!!

RIP OaklandDanny, thanks for EVERYTHING! God bless you, always!
Pacafeliz
King
 
Posts: 6967
Joined: Tue Feb 17, 2004 8:01 am
Location: Cologne, Germany


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