Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Throstur on July 28, 2010, 06:15:24 pm

Title: GA-20RVT Minuteman (White Panel)
Post by: Throstur on July 28, 2010, 06:15:24 pm
I bought a 1966 GA-20RVT Minuteman (White Panel) few months ago.
The speaker has obviously been changed.  All electrolyte caps seem to have been replaced (pretty neat job).
I need a good diagram and a tube chart. It would be nice to hear your opinions on this amp, and if and how itīs possible to make it sound good/better.
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Geezer on July 28, 2010, 06:49:28 pm
Does this look like yours?

http://www.el34world.com/charts/Schematics/gibson/GA-20RVT.pdf

Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Throstur on July 29, 2010, 05:11:39 am
Yes this is probably the right schematic, I will compare it to the amp. Unusual design. 
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: bigsbybender on July 29, 2010, 10:54:37 pm
I've repaired a few of these. The wiring is a mess, I have spent time straightening the wiring just to be able to repair them. The thin trebly sound is typical for these, the output transformer is very small for what this amp is. A typical mod that I do for these is add a pot to lift the tonestack from ground in a variable fashion. It really fattens the amp's mids and lows.
The Tremolo is pretty good on these amps, the reverb is weak when compared to a Fender from the same era. The reverb is usable and great for people that don't want Dick Dale, surfy reverb.

-j.
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Throstur on July 30, 2010, 03:47:21 pm
Thanks bigsbybender.  The wiring in mine is pretty much OK, has probably been tidyed up by the same person that changed the electrolytic caps.

What kind of a pot do you use to lift the tonestack from ground? I would like to do this mod and put in a decent speaker and see what happens. How about putting in a bigger OT?
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: angushung on July 30, 2010, 07:08:09 pm
Do you have any gut shots? I would love to see yours. Looking inside mine makes me not even want to work on it. It looks like a plate of spaghetti dumped in a box on there...
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: bigsbybender on July 30, 2010, 08:07:30 pm
I've never replaced the OT on these, the tonestack lift was usually all it needed to sound good.  None of them were very loud, and that interstage transformer instead of PI kept this thing pretty tame. I have played shows with a GA15RVT from that same era at several bars and kept up with stage volume (after I did the Tonestack Lift).

If I remember correctly, I used a 1M Audio Taper Pot to lift from ground...  I experimented with some pots on each of them before I settled on that.
I eliminated a line out  or "booster amp" jack or whatever they called it and used that hole for the ground lift pot. That jack was next to the input jacks. Yours may or may not have it... Gibson did some subtle changes to these in the mid-1960's

j.
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Throstur on July 31, 2010, 06:22:57 am
Quote from: angushung on July 30, 2010, 07:08:09 pm
Do you have any gut shots?
I will take some for you soon.
 :smiley:

Quote from: bigsbybender on July 30, 2010, 08:07:30 pm
If I remember correctly, I used a 1M Audio Taper Pot to lift from ground...  I experimented with some pots on each of them before I settled on that.
Thanks bigsbybender, I will make this mod, switch the speaker and post the results here. I will not have time to work on it until after one or two months though.
 :blob1:
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Throstur on August 07, 2010, 01:39:35 am
Quote from: angushung on July 30, 2010, 07:08:09 pm
Do you have any gut shots? I would love to see yours. Looking inside mine makes me not even want to work on it. It looks like a plate of spaghetti dumped in a box on there...
Follow this link and you will see a lot of gut shots :smiley: http://s415.photobucket.com/albums/pp231/throsturv/Gibson%20Minuteman/
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: angushung on August 07, 2010, 06:15:08 am
Awesome! What did you use to replace the big 3 way 300v 20mfd caps with? Mine is pretty much ripped apart from old age...
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: Throstur on August 07, 2010, 08:33:03 am
I would have to check for you, this was all done before I got it. I put in a new (used) Weber Classic speaker yesterday and it sounds much better although a little bit on the bright side. If you need some information or more pics Iīll open it up for you and check.
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: angushung on August 13, 2010, 02:31:35 pm
can I replace my 20x20x20 300v cap with the Hoffman 40x20x20x20 500 volt and just not use the 40uf connection? will this produce good results or should i go with individual caps that match the original specs? It's surely cheaper for me to go with individual caps but with the wiring nightmare that is the Minuteman there is little room for a terminal strip. I would have to just add to the pile I guess...
Title: Re: GA-20RVT Minuteman (White Panel)
Post by: HotBluePlates on August 13, 2010, 09:29:21 pm
can I replace my 20x20x20 300v cap with the Hoffman 40x20x20x20 500 volt and just not use the 40uf connection?

Sure you can, as long as the new cap fits in the space allocated for the old part.