Barn Fresh GA-35RVT came in this afternoon for evaluation(pixs of Fender/Gibson)

[Platefire]

When I say barn fresh I mean it becuase this has been in some storage shed for years. The customer is a friend who wants me to evaluate it and tell him if it's work resurecting? I found a schematic in Doug's Schmatics. It's pretty ragged out and just with a quick once over noticed:

1-What appears to be the OT is damaged from rat chewing. What ever was in there they liked and destroyed it.

2-It has 7355 tubes in it and the schematic shows 7591's

3-No reverb tank in cab but I see it has reverb.

4-No OA2 tube--what ever that is for?

Doing a short search for info on the net indicates the amps are odd, parts are hard to find & expensive and after you get them working right they still don't sound good. Looking at the schematic it does look like an odd ball. I have an output transformer for an fender Excelcior but I think a 7591 is more like a 6L6, so I guess that OT wouldn't work. I may have a used pair of 7591's, I'll have to check. Can somebody that has had experiance with this amp give me the low down on it? Thanks, Platefire

[eleventeen]

Weird circuit design, that's for sure. Certainly the lack of a phase inverter & the use of an interstage transformer to drive the outputs is notable. The 0A2 (in general) is a gas regulator which, once started, tends to keep a 150 volt difference between the plate and the cold cathode. They are *always* used as a voltage reference is some way. In looking at the circuit (in Doug's trove) it seems like it provides a DC offset to the trem oscillator (meaning the output of the trem oscillator) such that it can be applied to a grid and provide tremolo.

Seems like Gibson was so determined not to build a Fender amp...that they didn't.

[sluckey]

The 0A2 is just used to regulate the B+ supplied to V3B and V5B plates.

[SoundmasterG]

When I say barn fresh I mean it becuase this has been in some storage shed for years. The customer is a friend who wants me to evaluate it and tell him if it's work resurecting? I found a schematic in Doug's Schmatics. It's pretty ragged out and just with a quick once over noticed:

1-What appears to be the OT is damaged from rat chewing. What ever was in there they liked and destroyed it.

2-It has 7355 tubes in it and the schematic shows 7591's

3-No reverb tank in cab but I see it has reverb.

4-No OA2 tube--what ever that is for?

Doing a short search for info on the net indicates the amps are odd, parts are hard to find & expensive and after you get them working right they still don't sound good. Looking at the schematic it does look like an odd ball. I have an output transformer for an fender Excelcior but I think a 7591 is more like a 6L6, so I guess that OT wouldn't work. I may have a used pair of 7591's, I'll have to check. Can somebody that has had experiance with this amp give me the low down on it? Thanks, Platefire

The 7355 is similar to the 7591 as far as its specs...a different pinout of course. The 7591 is midway between a 6V6 and 6L6GC for power, but as far as power tubes go, they have a lot of gain....not as much as an EL84 but close. It is easy to make them super touch responsive and harmonically rich/juicy-sounding by modding the preamp and phase inverter to hit them harder than most vintage designs. For this particular amp though, it would require a lot of work to get there, and the owner probably wants to keep it closer to stock I'm guessing. A pair of 7591's with 450v plate and 400 screen, fixed bias and with the proper OT of about 5000 ohms primary will make 44 watts RMS according to the RCA manual, though that is probably optimistic by about 5 watts or so.

Greg

[Platefire]

I pulled all the tubes last night and fired it up. The Power Transformer is working, so that's a plus. I also found the missing OA2 tube in the bottom of the cab. The amp hasn't been hacked and looks like the original circuit and board is intact. The filter caps are the blue Sprague Atom, so at some point the oringals caps were changed out. Guess next step is locate and price a OT. Platefire

[PRR]

6.6K 35W.

[Platefire]

Thanks, PRR

Looks like the Hammond P-T1650G is a perfect fit. 35W, 6,600 CT 4/8/16 Ohm taps. Only thing is it's Ultra Linear but I guess you just don't use those taps and everything is fine.

I found some used USA 7591A tubes(brand has wore off) in my stock that came out of an old Heathkit amp, so if those are working up to snuff, I may have that covered.

I've attached the schematic for reference. The reverb connections and switch connects to chassis through a five pin socket. I suppose finding a tank, footswitch and connections to this socket would be almost impossible. I did do an ebay search but didn't turn up nothing on GA-35RVT. Platefire 

[TIMBO]

Hi Plate, you might find something amongst this lot. Found a match for my FI SONIC tank

http://www.amprepairparts.com/reverb.htm

[woolly]

hey Plate.  here's a Two part youtube vid on renovating a Ga-35rvt Lancer.

https://www.youtube.com/watch?v=m7lKbTJRo4o

[Platefire]

Hay Thanks Woolly(BTW Merry Christmas)

Very informative. I was hopeing that he would show more about the footswitch, reverb all tied in on that 5 pin socket. From what I could see it appears the wires came out seperatly from the male plug that would plug into the socket. Also from that I found out what the reverb tank input and output Ohms should be.

Also found out from a post on another forum that if the replacement OT is too big, the speaker and the OT clash.

Don't know how far the customer would want to take this restoration but trying to find out as much info about it before I talk to him again so he can make an informed decision. Platefire

[PRR]

It's ground (twice), reverb recovery, trem rate return.

Convert to a less-ancient TRS 1/4" plug, or two plugs, like Fender.

Ground the reverb line to kill reverb.

Ground the trem rate pot line to make it jiggle.

Not clear if you have the tank. If not, standard Fender probably works.

[P Batty]

I've got a Gibson tank, a Fender style is a different impedance.

I've also got a Gibson pedal you can have:

They're dirty but they did work 15 years ago when I pulled them from an Epiphone amp of the same era.

You can have them for shipping costs, IM me if interested in one or both items.

[Platefire]

Yes, Yes, Yes--Thanks P Patty. I sent you a PM regarding this.

One thing I can't figure out is the reverb tank in and out connections. There is one rca female plug on bottom of chassis that connects directly to C29 and pin 8 tube V3A. On the schematic I can't see where it shows that rca plug? I'm assuming this is reverb tank out return to recovery based its location, but where is the send connection/routing to tank in at??? I'm use to reading fender schematics and this gibson schematic is weird and hard to read to me!

A problem with the selected Hammond 1650G OT. The existing OT is right up against the speaker when chassis is installed. The hammond is 1 1/8" taller(3.81"tall), 1/4" wider and 5/8" deeper. No room to move the new OT backward to make room. About 1/2" taller and top would be hitting speaker. Really need an OT the same size. The existing OT is 2 3/4" high, 3 1/4" wide, 3 1/2 C to C mounting holes, 2 1/2" deep. Need to solve this OT issue to proceed. Platefire 

[P Batty]

The send to the reverb tank consists of two tabs (like the tabs on a speaker) on the reverb transformer. As I recall, the cable wasn't shielded, as per the schematic. I have that transformer as well...

[Willabe]

One thing I can't figure out is the reverb tank in and out connections.

Find the reverb transformer, T4, and follow the wirers.

On the bottom of the Gibson schemo they show chassis placement of transformers, chokes and tube sockets all are numbered.


               Brad     

[heresrobert]

Here's some 6.6k primary OT's from Magnetic Components that might work. The 18064 would need to be turned 90 degrees. I haven't used their stuff but their specs might work for you.

http://www.classictone.net/40-18064.pdf
http://www.classictone.net/40-18038.pdf

Robert

[Platefire]

Thanks So much For your help!!

P-Batty-The info on the connection terminals on the Reverb Transformer clears up a lot!!! At closer inspection of that transformer the rats also had their way with it. I see they even chewed into the windings and the tabs you speak of are gone. So if this project turns out to be a go, I could use that transformer also.

Willabe--I think I'm seeing it on the schematic. I've been looking at T4 but it didn't make sense because they don't have the reverb spring labled, so I wasn't sure it the curly cue I was seeing was the tank or not---but I guess it is. The double arrows must represent the tabs on the reverb transformer, next the tank input transformer, then reverb spring, then tank output transformer. The tank rca output is connected thru rca cord to chassis rca recovery jack in bottom of chassis represented by the other double double arrows grid line, then on to reverb recovery tube V3A. If I still got it wrong, let me know!

heresrobert--the smaller OT would fit but is only 20 watts. The larger 30watt one has the same problem as the hammon, to big for where it's located. I read the post of one guy who had to get a new OT for this amp and to get it to fit he was going to make a new speaker baffle with the speaker moved over. I really don't want to do that. Thanks for showing me these OT, I just wonder if there is any 35W 6.6K that are smaller??? Platefire

BTW-I think I'm getting close to be briefed on this amp well enough where I can almost give a satifactory status report to customer    If I could just come up with a satisfactory option to the OT issue.

 

[Willabe]

Willabe--I think I'm seeing it on the schematic. I've been looking at T4 but it didn't make sense because they don't have the reverb spring labled, so I wasn't sure it the curly cue I was seeing was the tank or not---but I guess it is. The double arrows must represent the tabs on the reverb transformer, next the tank input transformer, then reverb spring, then tank output transformer. The tank rca output is connected thru rca cord to chassis rca recovery jack in bottom of chassis represented by the other double double arrows grid line, then on to reverb recovery tube V3A. If I still got it wrong, let me know!

Yes I think you've got it right.

The reverb tank is in the dotted line box. The double arrows are where wires/cables go to connectors, RCA jacks/spade luges/tabs.

Note that the grid wire to the reverb recovery tube (V3A pin 8) is drawn as being shielded.


             Brad       

[smackoj]

I have used the Mag Component trannies and they are top notch in my opi.  I buy them from the Triode Store.

good luck, jd   

[PRR]

> send to the reverb tank consists of two tabs (like the tabs on a speaker) on the reverb transformer. As I recall, the cable wasn't shielded

Doesn't really to be. Reverb Send is a small Power Amplifier.

An un-shielded Send wire probably should not wrap around other tubes, but otherwise won't be any trouble.

Fender probably just got RCA plugs pre-mounted on shielded cables and used them for both.


> problem .... OT is right up against the speaker

Use a smaller OT. A 20W or something. Even a part for a DeLuxe.

It won't burn-up.

A 6K or 8K "mis-match" doesn't matter.

Undersized, it may not give the phattest bass around. But you also have a wrong speaker. And it isn't scheduled for his next World Tour. Get it working, pretty-near, then figure if it is worth a more-appropriate speaker and a crammed-in big OT, or if it will only sit pretty in the parlor for casual plinking.

I'd be real inclined to adapt it for 6L6. Break the power tube cathodes loose from ground, add a 150 ohm 5W resistor common to both tubes. That gets the idle bias up in the 35V-40V zone that a 6L6 will need to idle un-red at 400V. Put 470K 1/2W from pin 1 to pin 8, you can run EL34 also. (And a 7027 won't blow-up the power supply.)

[Platefire]

Thanks PRR, that gives me an idea how loose I can get with it. I had a DRRI OT but used it in another project recently. However I have located a OT at Mojotone with what they call TW35, Project Output Transformer that is 35 watts, 6.6K CT 4/8/16 speaker taps. They had a drawing showing mounting footprint but not overall deminsions--so I sent them a message requesting deminsions. With that I could run the 7591a's or convert to 6L6's. I think this OT is a little smaller than the Hammond with the same operating specs.

The original circuit is in good shape, not tampered with and who ever installed the new filter caps did a neat job. I believe this amp can fly with a little help. Once I present the cost of wings to the customer, he might keep it grounded--we'll see. Platefire

[Platefire]

Well I heard from mojo, that OT is too big also. 

[AZJimC]

I see Robert mentioned the Classictone trannys. I had space problems with a re-build, and ordered one of their trannys. It's a 20w+ according to them, 6K6 input, 4&8 out, Deluxe design, and it is smaller than any other in that range I was able to find. old trans was a 10w. I think you'd be okay with whichever of their product line that would fit. They list all specs everywhere I've seen them, and the price is very reasonable, near $35 for the one I got. American made too, if that matters to you.

Jim

[Platefire]

The Classic tone 40-18061(drawing attached) looks promising in that it's only 2 1/2" tall, existing is 2 3/4" tall and 3" wide is good. They just don't have the width dem. You can't scale it because the dwg says "not to scale". This is 35 watts, 5K CT and 8 Ohm output. I think this is the closest I've found. Just to illistrate how critical it is see pixs:

[Willabe]

Classic Tone made most of Gibsons iron and they still have the order sheets, several +thousand of them. The problem is when Gibson would order something they had their own number system that was not the model number of an amp.

I have spoken to Mark Sacketti their a few times and he is a very nice guy. He told me if you have the number stamped on the PT/OT or choke and they made it and they probable did, he can find the spec order sheet.

It helps them to know what iron was from which amp.


                 Brad       

[Platefire]

Willabe

On your suggestion I sent them an e-mail with pictures and OT number requesting assistance. We'll see what they say. Platefire


EDIT: I heard from Mark today. He said that he is out of the office but would be back Monday, would check then and let me know. He apparently checked the GA35RVT schematic and said the original OT, choke and reverb transformers were theirs. Platefire 

[Willabe]

Good news.


          Brad     

[jaymz168]

I just had one of these fall into my lap, I started a thread over here : http://www.el34world.com/Forum/index.php?topic=16473.msg161879#msg161879

I'll be watching this thread intently!

[Platefire]

Hi jaymz168

Welcome to the forum! Yeah I had just looked at your thread and thought Wow! another GA35RVT.

The best thing I ever found moving into a house that someone left was a record collection, would have much Prefered a vintage tube amp! I will be watching your thread also. Platefire

BTW--Here is some pixs of mine but not as good as your pictures. You take good pictures! 

[jaymz168]

Wow, this is incredible!  The timing on this is priceless!  You pics are incredibly helpful, my board has definitely been recapped in places and I wasn't sure if any mods had been done.  Now I have something to compare to other than the schematic!

[Platefire]

If there is any paticular part on it you need me to zoom in on, just let me know and I will post the pix. That is as long as I have it. It does belong to a customer. Platefire

[Platefire]

I got an e-mail from Mark with Classic Tone Transformers and he recomended their 40-18088 and provided a link right too that OT. It's a 6K primary, 40 watts and has 2/4/8 Ohm taps.

 http://www.classictone.net/40-18088.html

It looks like the footprint is identical and the mounting bolts would fit in existing holes without any chassis mods. Thanks so much Willabe for turning me on to this info or I would still be stuck. Now I can
notify the customer and see if it's a go. Platefire

[Willabe]

 

[Platefire]

drgonzonnm

Your looking at a previous selection. The latest is in post #31 that the link leads to the specs. It's not UL. Plate

[Platefire]

Talked with the customer today, gave him a status report. He said he wants to go ahead and install the OT and fire it up and see what we got and kinda go from there based on what we run into. Platefire

[jaymz168]

If there is any paticular part on it you need me to zoom in on, just let me know and I will post the pix. That is as long as I have it. It does belong to a customer. Platefire

Hey, I see that you still have the amp in your shop.  Could you possibly get a couple shots of how the front panel/tone stack is wired on your example?  I believe there are some differences in our units (which I suppose is to be expected).

[Platefire]

jay

can do but it may take me a few days. I've got the chassis mounted in the cab right now but will need to pull it out and want to pull that old OT before I order the new one. When I do that I will take some pixs and post them. Platefire

[jaymz168]

Excellent, thanks!  I'm taking it slow with this thing b/c I have a bunch of stuff going on right now and want to make sure I get this right.

[Platefire]

Ok here are the pictures. They start at the input jacks of the normal channel and move across to the rev chan and finally the reverb and trem controls. Hope this helps. Platefire

[jaymz168]

Excellent, thank you!  It appears as I expected, no center-tapped pots for Bass shelf.  Mine has center-tapped pots for some reason.... this is going to be a fun ride ...

[TubeGeek]

The Classic tone 40-18061(drawing attached) looks promising in that it's only 2 1/2" tall, existing is 2 3/4" tall and 3" wide is good. They just don't have the width dem. You can't scale it because the dwg says "not to scale". This is 35 watts, 5K CT and 8 Ohm output. I think this is the closest I've found. Just to illistrate how critical it is see pixs:

Not sure if this will help you but I have a ClassicTone 40-18061 in my hands right now…

The width is 3".

[Platefire]

jaymz168

Your Welcome! Glad that helped.

TubeGeek

Thanks, I ended up ordering 40-18088 and indicated in post #31. The bolt center 3 9/16 match the existing transformer. The drawing is attached, the width is 3 1/8". Platefire

[Platefire]

Got to say special thanks to "P Batty" where in post #11 offered parts from his old scrapped Epi amp of similar make and vintage for this project. I was short two chassis mounting screws, reverb transformer, reverb tank, some knobs and Reverb/vib Footswitch with 5 pin connector. Thanks to P Batty, I now have all those parts. I think most of you know how hard it is to obtain vintage parts for common main stream amps but even harder to find vintage parts for a more rare Lancer. To me this is in the Miracle category to have these parts show up from Hoffman Forum member. Hats off to P Batty 

[P Batty]

You're more than welcome, when ever I get rid of old stuff in the basement it makes me a hero to my wife too!

[Platefire]

Hay, I appreciate it but a little bit a bad news. Best I can tell the reverb tank is bad and the transformer may be bad. Still trying to determine the status of reverb transformer. The reverb and vibroto footswitch works. This testing was all done with a multi meter.

Reverb Tank: The reverb tank reads 175 Ohms on the input and "0" on the output. Should be 1475 input and 2250 on the output. Also you should get continuity from input to output on the main hot leads, but didn't. Even though I could not find the number on the tank, according to research I've done the tank number is a 4FB2A1C and are avilable new from Studio Sound Electronics for $33.99 plus shipping.

Reverb Transformer: I'm thinking the reverb tranny may be bad too from testing info I got off the net(but not totally sure). Maybe someone will verify it by these readings. I found a site where it said if doing Ohm reading between the two primaries if the reading was over 1000Ohms it was bad. The reading is 1343 Ohms? Also they said the reading between blue plate wire and secondary should be multi-meg readings I am getting no Ohms response on the secondaries??
   Also I did a low voltage DCV test injecting 18.58VDC to primary orange wire to B+ connection and got 18.58VDC to blue primary to reverb driver tube plate. That seems pretty good because on the schematic it shows 252VDC to B+ primary and 240VDC at rev driver tube plate. OK feeding the same 18.58VDC into the blue rev driver plate wire, I got 5 Mv DCV or .33 VAC at secondary output.  These voltage readings seem reasonable to me because the B+ at plate needs to be almost identical to incoming B+ orange wire and the grid rides on a low voltage AC. To me this seems to verify it's good---maybe someone can tell me from this what I got from the readings I posted?

Foot-switch: Foot-switch plugged into socket perfect. Check continuity with on/off connections shown on schematic and got on/off continuity between pins for both reverb and vibroto when hitting each foot-switch.

Also I'm I'm wondering about the PI transformer--any good way the test it with a MM before I install the new OT. A new reverb transformer and PI transformer may be more than the customer wants to endure. I'm hoping the reverb tranny will pan out to be good.

Thanks, Platefire

Platefire    

[Willabe]

Also they said the reading between blue plate wire and secondary should be multi-meg readings I am getting no Ohms response on the secondaries??  

The verb OT just like all OT's should be open/no reading/infinity or multi-meg reading from primary to secondary. I think it's your meter as different meters will show different readings for open/infinity.


            Brad     

 

[P Batty]

How could there be continuity between the hot leads on the input and output of the reverb pan? They are isolated by the driver/pickup coils on the springs, no?

I got a dc ohms reading of around 200 ohms on both the input and output of the tank before I sent it, I don't know about the data or how you would measure impedance on a reverb pan. It did work when I took it out of the amp (but that was twenty years ago!)

[Platefire]

Willabe--I got three multi-meters and it did pretty much the same on all of them except my Fluke which read OL(overload). On one of my wally world MM I got a reading of about 16M but the reading was jumping around so much, I would rely on it. So from what I posted, do you think the transformer is OK.

P Batty--you had the tank well protected outside and inside. Your right on continuity through the tank. That don't make sense now that I think about it. I tested a good tank in one of my working reverb amps and there was no continuity from in to out on the grid or hot. That would be an easy way to test a tank if it were true, but it's not. As I sit here with my Fluke in hand, on the input I get a 175 Ohm on the input and no reading on the output. I've read it a dozen times with different meters and it all comes out the same. No worry, you did your best and I did too. Old parts fail--only go so far and boom--thats it. I've checked continuity on the connections from jack to transformers and they are good. The problem has to be in one of the transformers. The springs are good. One spring has about 59 Ohms resistance end to end and the other about 74 Ohms. The breakdown is apparently in a transformer--I think the output one. Platefire 

[Willabe]

Yes I think the OT is good and I wouldn't count the tank out either.

I'd put both in the amp and see.


               Brad    

[Platefire]

OK, good on the rev transformer and I will try the tank and hope I'm proven wrong on my findings. Har! won't be the first time   I did the same type of low voltage DCV test as I did on the RT on the PI transformer and it seemed good.

I think is so cool that the new OT bolts in the existing mounting holes. Next step is just to install the new OT and the RT and see what I got. Platefire