Gibson GA19RVT Tweed v. Crest ... compare/contrast

capnjuan

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Hi Jon; the pin #s in my mistake ... was wrongly thinking the tweed ran on 12AX7s and the crest 6EU7s forgetting that both models use 6EU7s ... whose pins 4 and 9 are cathodes.

6eu712ax7.jpg



In B'rer Rabbit, the tigers (amp owners) ran around the (Gibson schematics) tree until they turned in maple syrup ... let's not do that but instead ... let's review the bidding.

In the tweed schematic I have, the only " ... 2.7K resistor, but with a .02uF cap ... " is on V3A ... the other half ... V3B ... at least on the schematic, isn't shown. Go you Gibson engineers ... :evil: V3 in the tweed model is a voltage amp downstream of the vol/tone controls.

In the crest schematic I have (with the tube ID #s reading from R to L), V3 ... the 6C4 .. a single triode ... is just ahead of the reverb transformer and can ... the reverb driver. V2A ... just for distinction, is the reverb recovery section amplifying the verbed signal and returning it to the line.

V2B (with its 4.7K R / .1uf C) does what V3A did - act as a voltage amp downstream of the controls and it mixes the wet and dry signals ahead of the phase inverter. I'm not engineer enough to understand why one R/C combination is 'better' than the other. It may also be one of those itty bitty changes that somebody made because they thought it would sound better ... it being that the market was heading towards the cleaner, brighter sound of 7591s/EL84s ... who knows.

My now-gone tweed Gibson GA19RVT w/ Weber 12A125 speaker:


GA19back02-1.jpg



Congratulations on your back-dating the amp ... maybe a little darker with slightly earlier breakup ... a little more compression in the mids ... pretty much the signature of all the Gibson 6V6 amps ... a sweet, golden, crunchy-brown tone.
 
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Hi,

I'm new here, and this is my first post (bought a '62 M65 3/4 freshman about a month ago, and I love it ;) )
And I've just bought a '66 V1 Thunderbird amp that's waiting for new output tubes...

I've just been lurking about a lot, having not much to add to most conversations.
But reading through this I think I spotted another small change to the mods for the crest.

Just as CapnJuan mentioned in a later post that he had overlooked the plate resistors in the preamp, the same goes for the Phase Inverter.

The place where the original post mentioned changing the voltage divider from 3,3K/100K to 1,5K/47K. Look at the plate side of the tube, in the crest-version the plate resistor is 100K and in the tweed version it's 47K, to balance out with the bottom resistor.
If you don't change both the top and bottom resistors the PI will be out of balance and the result should be more assymetric breakup from the output-stage and more hum from the imbalance...

So, I don't know if you should change it or not, it might sound good with an unbalanced PI, but if you want to get closest to the tweed sound you should at least check it out. To just check out if it makes a meaningful change to the sound you could just use crocodile clips to clip in another 100K in parallel with the existing plate resistor.
Then you could easily change between the two and see if it's worth it.
Oh, if you plan on doing this be very very careful, or use thick rubber gloves ;)

If you're going for earliest breakup the imbalance may actually be a bonus, but I just thought I'd point it out.


Thanks for all the inspiring discussions going on in this forum!


Greetings from Belgium,
Kenny
 

capnjuan

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Hi Qwi; welcome to LTG and thanks for the very useful comments! If it's any help at all, the tweed GA19 RVT pictured above had all the passives replaced with stock values and no circuit revisions; it broke up pretty easily ... it was one of the things I like most about it. :D
 

honkyjonk

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Wow. thanks guys.

That amp is beautiful.

Thanks so much CapnJuan for the help.

So, I received a replacement reverb transformer, a fender version. I'm getting 1.6K ohms on one side and 1.5 ohms the other. I'm guessing the primary is the 1.6K side?
 

capnjuan

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honkyjonk said:
...So, I received a replacement reverb transformer, a fender version. I'm getting 1.6K ohms on one side and 1.5 ohms the other. I'm guessing the primary is the 1.6K side?
Hi HJ; the primary side is usually blue and red and the secondary nearly always green and black. I wouldn't put too much weight on measuring resistance ... it's not that simple ... it's the same issue with output transformers ... can't necessary tell impedance by measuring the resistance of the winding ... only that the winding isn't open.
 

honkyjonk

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Hey guys,
Just bumping this one as I'm slowly tweaking my physically horrible looking GA19RVT into a physically horrible looking amp that sounds awesome.

Anyway, I got a new 5Y3 rectifier tube, and I'm getting 300V after the 470ohm 2 watt resistor. On the schematic it shows that this should be 290V. (I actually had previously strung together about 2.9K worth of 5 watt resistors in series in place of the 470ohm resistor to get it down to 290V) Now with the new GE tube it's up to 300V.

Is it just a matter of adding more resistance? I haven't worked on tube amps much before, but I understand the schematic fairly well. I just want to make sure that something's not going to go terribly wrong if I keep adding a lot resistance where it was once a 470ohm resistor.

Also, I rewired the 6eu7 tube sockets for 12ax7's because I had a few of those. I never checked the heater voltage prior to switching, but I'm getting 19V for the heaters for these 12ax7's. Too much! Should be 12.6V correct? (I made sure and wired the heaters as recommended by a number of folks I found via googling.

Like this:
6eu7 12ax7
heater1 pin1 > pin9
heater2 pin2 > pins 4+5

Anyway, again, to get the voltage down to 12.6V, is it again just a matter of adding resistance after the transformer so that the tubes see the right heater voltage?

P.S. I've also replaced all out of tolerance resistors in the entire amp, so that's not an issue.
 

capnjuan

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honkyjonk said:
... Anyway, I got a new 5Y3 rectifier tube, and I'm getting 300V after the 470ohm 2 watt resistor ... Now with the new GE tube it's up to 300V. Is it just a matter of adding more resistance?

Hi HJ: no, just leave it alone ... it's fine the way it is. The voltage at the tap after the 470 R is for the 6V6 screens; all that's important is that the voltage at the point be measurably less than at the output of the rectifier. You don't have to do anything else.

Also, I rewired the 6eu7 tube sockets for 12ax7's because I had a few of those. I never checked the heater voltage prior to switching, but I'm getting 19V for the heaters for these 12ax7's. Too much! Should be 12.6V correct? Yes.

Like this:
6eu7 12ax7
heater1 pin1 > pin9
heater2 pin2 > pins 4+5

I don't think I understand this. See below.

If you are using 12AX7s; one side of the heater circuit goes to pin #9, the other side goes to pins 4 and 5 which are jumpered together. Assuming one of the wires is black, if it hits pin 9 on the first 12AX7 socket it comes to, then it has to hit the other pin 9s ... the other side hits all the pins 4/5. You can screw it up by crossing them over. Same thing goes for the 6V6s; use the same side of the heater circuit for the same pin #s.

Otherwise, the 12AX7 wiring is; pins 1 and 6 are the plates (power in, signal out), pins 2 and 7 are the grids (signal in), and pins 3 and 8 are the cathodes. See if these help:

6EU7:
9ls.gif


12AX7:
9a.gif
 

honkyjonk

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Hey Juan,
I wired the conversion exactly as you said. Sorry if that was confusing. But yes, one side of the heater hits all the 4+5's and the other hits all the 9's.

What would be a proper way to reduce the voltage to the 12ax7 heaters? As well as after the rectifier? Just resistors in series like I've been doing?
 

capnjuan

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If you have it wired correctly, then something else is wrong; you should be getting 6.3 volts AC between pins 4/5 and ground ... same between pin 9 and ground. Can you post some pics of the wiring?
 

capnjuan

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Hi HJ: diagrammatically, this is what you should have. Don't know what colors your heater wires are ... back in the day, black and gray were common. If you have this but don't have the correct voltages, don't manipulate the voltage ... find the wiring error.

ga19crestheaters.jpg



Now that the cow is out of the barn, I wish you'd said something before converting the sockets ... I have several working 6EU7s that you could have had for the cost of postage. Anyway ... check your work ... good luck.
 

honkyjonk

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Thanks Juan,
Yup, it's wired exactly like your diagram, and I'm getting 18-19V on both heaters.
My heater wires are black and white.

I'm getting 19V directly out of the transformer too, where it connects to the 6V6's. I hope this doesn't suggest I have a bad power transformer.

The other thing it could be is that a couple of these 12ax7's aren't actually 12ax7's. They WERE in a 12ax7 (ecc83) box. I can't remember where they came from though, and they didn't have legible markings on them. Doh!
Hmmmmm. . . .
 

capnjuan

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Hi; read you note ... will reply tomorrow.
 

capnjuan

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Hi HJ: can't really go too much farther without pics; in-focus and with plenty of ambient light; you can see what you're looking at ... but you're the only one who can.

12AX7/ECC83: ECC83 is the European designation for a 12AX7. Get to know this page and how to use it: Duncan Amps Tube Data. If your tube isn't marked ... well ... who knows what it is but in any case, it doesn't matter ... we're measuring pressure, not current ... it could be a Martian tube and so long as it fits the socket, it doesn't matter.

One more time: with your meter set to AC voltage, leads in their proper red/black jacks, amp on, tubes in their sockets, volume turned down, rectifier in: at the 12AX7s; touch the red lead to pins 4/5 and the black to chassis, repeat for pin 9, repeat for all the preamp tubes. Same with the 6V6s; red to pin 2, black to chassis .. repeat for pin 7 and the 6C4; red to pin 3 black to chassis repeat for pin 4. Pilot light: red to one side of the light, black to chassis switch to other side of light, black to chassis.

Since I can't see what you're looking at, I don't know where the heater supply goes after it comes out of the transformer. In the old tweed models, they terminated the heater supply on unused posts on the 5Y3 socket; two conductors leading away to the pilot light and two more going to the tube sockets. How is yours terminated? 12V center tap; usually the heater circuit has a center tap / commonly a white wire; where is it terminated?

Bad transformer: if it were bad and I don't think it is, you'd be more likely to get no voltage or unstable voltage - not too much voltage. Can you tell if the transformer is original? ... pics?
 

capnjuan

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For anybody following this thread; recent pic of a '61 Gibson tweed GA19RVT; 15 watts of reverb'd smoky 6V6 goodness (rare to see the plastic amp badge still intact as well as original knobs and handle):

Falcon.jpg
 

honkyjonk

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Here's some pics. One mistake I've made is I've been measuring in DC not AC. Duh. Of course it's not rectified if it's coming straight from the transformer. Anyway, AC voltage for both heaters is 3.12V, which interestingly is roughly half of what we want. This was measured at every pin 4+5 conjunction and every pin 9 on the 12ax7's, as well as pins 3 and 4 on the 6C4, and all the way back to pins 2 and 7 on both 6V6's.

First pic you can see where the heater wires come directly from the transformer, and then go to 2 and 7 of the first 6v6 tube. It's kind of hard to see in the other pics, but the white and black wires definitely are wired to pins 2 and 7 of both 6V6's.

AC transformer appears to be original.



Uploaded with ImageShack.us
12ax7wiring.jpg

powersection.jpg

connect6v6.jpg
 

capnjuan

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Thanks for the pics ... and yes ... AC v. DC makes a difference :p Leave the heater voltages alone; it'll be fine the way it is. In the pic below; the black cap is the 6V6 cathode resistor bypass cap and the resistor above it looks like the 270 ohm / 2 w cathode resistor. Confirm that that's the value of the resistor; if not, where is the cathode resistor?

There should be 4 of the black electrolytic 20uf caps in there: V1, V2, V4, and here on the 6V6s; if the others have been changed, fine - otherwise they all have to go ... use Sprague Atom 25uf/50V This is what you're looking for - 25V version okay too.

hjga1904.jpg



The center tap for the heater winding is terminated under the nut of one of the transformer hold-down bolts.

Does the amp work?
 

honkyjonk

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Yes. The amp works. I guess I thought it was a little more vital to have the full heater voltage. I'm sure this is a bad idea, but, hypothetically, what if I connected the the two heaters together at the transformer and then supplied all the heaters with that? Theoretically that would be really close to what we want right? (3.12VAC x 2)

Also, yes, that is the 270R 2 watter. I couldn't find any carbon comp for that size, so that one is metal film.

BTW, incase you couldn't tell, the big green sleeve is my string of 5 watt resistors to get the rectified voltage down to 290VDC. It's actually 300VDC right now with the new 5y3. (and with 2.9K of resistance! instead of originally 470ohm) Should I just add a little more resistance to get it right?

Anyway, thanks for all the help, capn'

Jon
 

capnjuan

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Hi HJ: Sorry ... I went braindead when I said 12VAC ... it's not ... it's 6.3VAC; see image below. That transformer has the same windings as yours does including the 6.3VAC for the heaters. That means that at any given time - since this is AC and it cycles back and forth - there's 3.XXVAC between one hot side and ground (the center tap) and the same between the other hot side and ground (the center tap). There's 6.3VAC across both. Your heaters are fine like they are.

025130sch.jpg



Okay on the metal film; 2W is fine ... some people like the comfort of 5W but it's not mission critical. Yes; I couldn't help but notice the green shroud over your resistor strings ... it's nice if the work is pretty but nicer if it works correctly.

You're right about the cosmetics; the chassis interior is a little rough. If you have a wood chopstick, you can trim the end into a bevel edge ... like a chisel ... and shove the chopstick at an angle and you can get some of that grunge off. No WD40 but isopropyl alcohol will help and also help cleaning up some of the tone cheese round and about.
 

honkyjonk

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Hey Capn'

Just wanted to get back to this thread and tell you thanks. This amp sounds really awesome now. It REALLY needed a new 6c4 because the reverb sounds like God now.

Anyway, it sounds so good running at 300VDC (post resistance after the rectifier) I think I'll probably just leave it like that. It's only 10VDC over, and it certainly doesn't need any more dirtyness as I can get fuzzy around 12:00.

I ordered the last caps to be replaced (those little electrolytics) and then I should be good to go. It's been a long haul. I actually did a number of the changes that you originally mapped out in the beginning of this thread to convert it to a Tweed style amp.

One thing that is a little curious that hasn't been brought up concerning the differences between crest and tweed is the presence of a 500pf cap between the volume pot and tone control pot in the Tweed schematic. I actually added a mica 500pf capacitor to my amp (crest) and it definitely makes a difference. Seems to give it a little more bite.
 

capnjuan

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8) 8)

You're right on the spec'd voltage; my copy of the schematic indicates 300VDC at the rectifier output. It doesn't give the voltage at the screens but the only thing that's critical is that there's at least some voltage differential between the plates and screens ... pins 3 and 4 respectively.

Those small caps are cathode resistor bypass caps and if the ones that are in there are older and tired, you'll get another modest performance bump once they're changed out.

500pf: Good eye; I missed that earlier. My GA19RVT was pretty dark and smokey ... maybe made a touch darker by the Weber 12A125 ... I don't know but in reading W. Marx book on Gibson amps, Mr. Dick Evans, who took over the Gibson amp line in early '62 ... I guess he though he needed to tweak the circuit to match the tastes of the day which were moving towards brighter, gainier, and maybe a little more twangy.

Speaking of speakers; what do you have in there and are you going to change it out?
 
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