Advice on building Gibson ga-5t crestline

[rayandkerry]

Hi all. First real post here (other than an initial greeting for first time users).  For any people who are Watkins site users also, I've been over there for years under the same username...so say hey if you post/respond!

Ok.  I got an old webcor 333-2 record player at goodwill a few years back.  I've attached the original documentation with schematic. 

I basically only used the PT from the record player because the OT was shot.  I got the Classictone 40-18060 for the OT.  I've attached the diagram for it.  It's a 10w OT with an 8 ohm secondary.

I originally turned it into a howler monkey amp as documented at this link: http://www.solorb.com/elect/musiccirc/howlermonkey1/

It's an ok amp, but not great so I'm looking for another one.  I was interested in building a Gibson GA-5T Crestline amp.  I've also attached the schematic for the Gibson.

So...a few questions.

1.  I think using this PT with a solid state rectifier might be in order simply to get a few volts more B+.  I'd think this would get me in the ballpark of the Gibson's PT with a  tube rectifier.  Would folks generally agree it would work from a PT perspective?

2.  The gibson uses 6eu7 tubes in the preamp. I don't know much about them but I hear they are quite similar to 12ax7 tubes, but are wired differently.  Would it be possible to take the Gibson schematic and just substitute 12ax7 tubes for the 6eu7's (simply wiring the sockets differently)?  Or would I need to change additional components in the power supply or biasing of the 12ax7's?

I've got more additional questions I'll hold off on until I get some feedback on the above.

And thanks in advance for any help/advice/opinions you can offer!

[sluckey]

That PT with ss recto should be fine.

Just change the 6EU7 to 12AX7 pin numbers should be fine. No other changes.

Was there a question about the circled cap on the GA5T schematic?

[shooter]

Gibson made many variants, so a AX7 has been done

[rayandkerry]

That PT with ss recto should be fine.

Just change the 6EU7 to 12AX7 pin numbers should be fine. No other changes.

Was there a question about the circled cap on the GA5T schematic?

Excellent, thanks!   The red circle was already on the schematic I downloaded.  But come to think of it, why does it say 250uuf? None of the other capacitors show anything other than a value (which I assume is in uf)  Is that an indication it's a non polarized electrolytic capacitor?  Or something else?

Gibson made many variants, so a AX7 has been done

Hmmm.  I must have passed over that schematic because it's single ended.  But let me look.

Thanks both!

The howler monkey amp I made has paralleled 7 pin and octal sockets so I can use either 6aq5's or 6v6's in the output stage.  I'm wondering if I could do that with 6eu7's and 12ax7's in the preamp section.  Maybe even use single sockets with a switch to use either preamp tubes.

[sluckey]

why does it say 250uuf?

250µµF, pronounced micro micro or mickey mic, is the same as 250pF (pica Farad). µµ and p are both 10^-12.

Maybe even use single sockets with a switch to use either preamp tubes.

I like to keep things simple. Nothing to be gained here except a cluttered chassis.

[rayandkerry]

I like to keep things simple. Nothing to be gained here except a cluttered chassis.

Well I've found it helpful sometimes.  Plus, I am clutter personified, LOL.

I have my el84 amp wired with switches to use el83s if I want.  But that's got a lot less wiring changes.  I just looked at the data sheets...all 9 pins are wired differently between the 6eu7 and 12ax7.  Oh well, no biggie.  I think I have enough to proceed. Thanks!

[shooter]

use either 6aq5's

fwiw;

I've rebuilt or fixed 6-7 of the 5T
this one seems to be a favorite of the lot, but it's got a lot of stuff Gibson threw at the wall to see if it stuck 

I replaced the EU with AX on most, one guy preferred an AT 
I liked the IT enough, I built 2 amps using the tranny PI, both were well received

[bmccowan]

I have also rebuilt several of those. And have one of each variety. Agree that there is nothing to be gained by switching between 6EU7 and 12AX7. You will get more tone variation by subbing 5751, 12AY7, for the AX7, or even between the wide variety of 12AX7s now on the market, than you'd get switching to the 6EU7 which is the same internally as the 12AX7.
I favor the 6AQ5 version, it has a great overdrive at reasonable volume. Of course, conversely is does not do clean very well except at low volume. But that's why we build more amps. The white panel model to me sounds thin unless you modify the circuit. BTW, when you finish it, if you want to tweak the tone, there are a couple of good threads here about that amp with some simple mods.

[rayandkerry]

I've rebuilt or fixed 6-7 of the 5T
this one seems to be a favorite of the lot, but it's got a lot of stuff Gibson threw at the wall to see if it stuck 

I replaced the EU with AX on most, one guy preferred an AT 
I liked the IT enough, I built 2 amps using the tranny PI, both were well received

Thanks!  I saw that schematic too, and chose the other just cause it looked like it'd print clearer. 

I'm going to print this one and compare with the schematic I posted to see the differences and compare that to the components I have lying around.

I have also rebuilt several of those. And have one of each variety. Agree that there is nothing to be gained by switching between 6EU7 and 12AX7. You will get more tone variation by subbing 5751, 12AY7, for the AX7, or even between the wide variety of 12AX7s now on the market, than you'd get switching to the 6EU7 which is the same internally as the 12AX7.
I favor the 6AQ5 version, it has a great overdrive at reasonable volume. Of course, conversely is does not do clean very well except at low volume. But that's why we build more amps. The white panel model to me sounds thin unless you modify the circuit. BTW, when you finish it, if you want to tweak the tone, there are a couple of good threads here about that amp with some simple mods.

Sweet, thanks.  I'll poke around after building as close a possible to one schematic.  I have a feeling mine will tend towards the dirty side given the webcor PT seems just a tad underpowered compared to the gibson ones.  Not much, but still isn't going to help with headroom,  I'd think.

And you're right about the need for many amps.  Most of mine are dirty (Marshall 18 watt, tweed fender tremolux...pretty much a 5e3 with tremolo, modded epiphone jr.).  After the AC30 is done I should have some more cleans.  Then I'll build some kind of fender clean amp 👍

[rayandkerry]

OK, I've got an updated schematic using 12AX7s instead of 6EU7s and a solid state rectifier.  I also added a 3-prong mains cord with earth to chassis ground and a jack for a tremolo footswitch connected to a switch on the front panel that allows on-off-footswitch.

So few more questions...

1. Does it look right?  Especially the SS rectifier part?  I've never really used a SS rectifier yet other than the amp I'm tearing apart to make this amp.  And that amp had a different design with only 2 diodes.  So I'm not at all sure I've got this part right.

2. Given that my PT is 190v, I think I'd be getting about 266v B+ rather than the 240v in the original schematic.  Am I correct in my calculation?  I am assuming this would not be a major issue...perhaps a bit more headroom.  Is that correct?

3. What about a sag resistor? I've heard of them being used and can look up how to add one.  But does anybody have an opinion about whether or not it would be beneficial in this case?  This question is really an afterthought, but what the heck. :-)

Thanks in advance for help/comments/opinions/ideas!

[sluckey]

If the donor PT came from an amp that used a tube rectifier you cannot use that bridge rectifier. You would need to use 2 diodes and connect the PT center tap to ground.

[rayandkerry]

If the donor PT came from an amp that used a tube rectifier you cannot use that bridge rectifier. You would need to use 2 diodes and connect the PT center tap to ground.

The donor amp did have a tube rectifier (a 6x5).  And the schematic showed the PT as having a center tap.  But there's actually no center tap wire on the PT (for either the high voltage or heaters).  What to do?

[sluckey]

The center tap wire(s) are internally connected to the PT frame. Use your ohm meter to confirm resistance between both HT wires and a PT bolt. Should be about the same resistance for each wire, typically 100 to 200 ohms. Do the same for the filament wires but expect about zero ohms. The filaments winding may not have a center tap. Check your orig. schematic.

Oh, and be sure to use star washers on the PT bolts!

[rayandkerry]

The center tap wire(s) are internally connected to the PT frame. Use your ohm meter to confirm resistance between both HT wires and a PT bolt. Should be about the same resistance for each wire, typically 100 to 200 ohms. Do the same for the filament wires but expect about zero ohms. The filaments winding may not have a center tap. Check your orig. schematic.

Oh, and be sure to use star washers on the PT bolts!

Thanks!  You are correct!  The HT measures 81 and 89 ohms to a PT bolt (and I have star washers on all 4 bolts  ).  Let me redraw the schematic for this.

Any comments towards questions 2 and 3?

[rayandkerry]

OK here's the updated schematic based on slucky's comments.  Looking right?

[sluckey]

2. Correct.

3. Sag resistor. Don't need one. I think your B+ is fine for a 6AQ5 amp. I only have one SE 6AQ5 amp and it's happy with 320C B+. Actually the tube is a 6005, same as 6AQ5. Don't know if they are tougher than a 6AQ5.

Schematic looks OK.

[rayandkerry]

2. Correct.

3. Sag resistor. Don't need one. I think your B+ is fine for a 6AQ5 amp. I only have one SE 6AQ5 amp and it's happy with 320C B+. Actually the tube is a 6005, same as 6AQ5. Don't know if they are tougher than a 6AQ5.

Schematic looks OK.

Excellent. thanks so much for the help, verifications and opinions!!  I've got all the pieces laying out in front of me now.  Just plotting on how to lay them out on the board I made for the last amp.  It'll need some minor modifications... 

[DummyLoad]

250pF ceramic coupling cap feeding a bootstrapped cathodyne creates a very hi-z node. it will be prone to picking up lots of hum with that small value coupling cap - use a conventional poly cap with a value of .01uF or thereabouts. shave bass to the power amp or preceding stage. probably why it was circled in the schematic provided in the opening post.

http://www.valvewizard.co.uk/cathodyne.html

just my 2cents...

--pete

[rayandkerry]

250pF ceramic coupling cap feeding a bootstrapped cathodyne creates a very hi-z node. it will be prone to picking up lots of hum with that small value coupling cap - use a conventional poly cap with a value of .01uF or thereabouts. shave bass to the power amp or preceding stage. probably why it was circled in the schematic provided in the opening post.

http://www.valvewizard.co.uk/cathodyne.html

just my 2cents...

--pete

Thanks.  I'll swap it out sometime and see how it sounds.  U just completed the schematic version and it works.  So I'll use it for a while before switching to a .01 cap.

Question...when you say "shave bass to the power amp or preceding stage", do you mean the capacitor swap itself will do that, or that i need to somehow shave bass in addition to swapping out the coupling cap? 

Thanks for you comments!

[rayandkerry]

Ok.  It's up and running. I've only plugged into a cheap speaker for testing purposes, but I'm going to really give it a good listen tonight or tomorrow to see how it really sounds.

As is with the crap speaker it sounded really nice 👍.

Voltages seem close enough.  My reading is the firsts voltage, the schematic voltage is in parentheses:
B1 = 259v (240v)
B2 = 254v (235v)
B3 = 209 (200v)
1st preamp tube plate 1 = 124v (130v)
1st preamp tube plate 2 = 142v (150v)
 Phase inverter plate = 155v (150v)
Tremolo tube plate = 105v (110v)

One thing i noticed right off was the tremolo only operates if i set to the fastest.  If i slowly back down from there it will slow and continue working until a little before halfway down on the frequency potentiometer.  Then it dies.  I'll play some more but if anyone has any ideas about that behavior, please chime in.

I've attached a couple of pics.

Thanks all for all the help!

[rayandkerry]

One thing i noticed right off was the tremolo only operates if i set to the fastest.  If i slowly back down from there it will slow and continue working until a little before halfway down on the frequency potentiometer.  Then it dies.  I'll play some more but if anyone has any ideas about that behavior, please chime in.

I was browsing topics on the forum related to ga-5t tremolos and just realized the speed pot is supposed to be reverse audio.  I installed a regular 1M audio pot.  That may be part of the problem.   If I just reverse the wires will I have the right taper (although turning the pot up will slow the trem while turning it does will speed up the trem)?  Just to test before getting an actual 1M reverse audio pot?

[vampwizzard]

Thats awesome. You can reverse the wires to get a reverse voltage so the knob adjusts correctly but the taper wont reverse, if that makes sense. I'd replace it.

As far as the trem goes, this troubleshooting discussion may help: https://el34world.com/Forum/index.php?topic=9851.0

[sluckey]

Changing the speed pot will not help. All the reverse audio pot does is spreads the speed change more evenly across the pot rotation.

More gain is what's needed, but with that grid leak bias circuit about all you can do is roll a bunch of 12AX7s and keep the strongest one. Higher B+ would help but that's not an option.

Or, if you feel like modifying, you could change the circuit to the Princeton Reverb circuit. Then replace the cathode resistor/cap with a red LED. This is my choice.You could even put an Intensity pot in circuit if desired.

[PRR]

> with that grid leak bias circuit

Those values look goofy to me. The R-C products should be similar, not 10:1 apart. 5Meg will, as you hint, take the triode off the max-gain point.

I think we should tack 1Meg, even 330k, across the 4.7Meg and see how-much which-way difference that makes.

Yes the LFO starts as "zero bias" but when it clips the grid will go negative until the plate wave is about centered.

[rayandkerry]

Woo hoo!  Sounds great on a real speaker.  I tried through a couple of 10" speakers in sealed cabs last night, a Weber alnico blue and a celestion greenback.   Both were great, but I liked the greenback best.

Excellent tone! I played over an hour before I used any pedals, which all sound awesome through this thing.  Got many great sounds just driving it from one guitar with humbuckers I can use in series, parallel or single coil mode.  Really really fun.  So glad I tore apart the howler monkey and made this.

As of yet I've only used 2 new production JJ 12ax7s with nos 6005s in the power section.   I can't wait to roll some tubes in this (nos 12ax7s/5751s/12at7s in the preamp, other 6a5q variants in the 7-pin sockets or 6k6/6v6 tubes in the octal sockets)!

Other than rewiring the speed pot for now, I think I'll try DummyLoad's suggestion on the coupling cap and live with it for a little while.

And thanks again to everyone for the help and comments!

Thats awesome. You can reverse the wires to get a reverse voltage so the knob adjusts correctly but the taper wont reverse, if that makes sense. I'd replace it.

As far as the trem goes, this troubleshooting discussion may help: https://el34world.com/Forum/index.php?topic=9851.0

Thanks I will eventually get the correct pot, but only when I'm ordering parts for another project and don't need to pay extra shipping.  For now, I'll just reverse wire it to get a better feel across the pot.  I'll try and mess with some other values to improve the tremolo a bit later.

As to the topic link you provided, it seems like a different tremolo circuit. 

I found a topic here ( https://el34world.com/Forum/index.php?topic=20923.0) that mentions a GA-18T that has a more similar design.  That topic links to this page: http://djlservice.com/AudioGibsonGA18wPhotos.htm

In that last link the last schematic shows the capacitor values changed from .03/.01/.01 to .047/.047/.047.  My tremolo design is quite similar with .01/.02/.02.  Maybe tweaking these values might help?

Changing the speed pot will not help. All the reverse audio pot does is spreads the speed change more evenly across the pot rotation.

More gain is what's needed, but with that grid leak bias circuit about all you can do is roll a bunch of 12AX7s and keep the strongest one. Higher B+ would help but that's not an option.

Or, if you feel like modifying, you could change the circuit to the Princeton Reverb circuit. Then replace the cathode resistor/cap with a red LED. This is my choice.You could even put an Intensity pot in circuit if desired.

I think for now I'll stick with what's there and roll some tubes.  I'll pull up the Princeton reverb circuit though and have a look.  Is there a specific one?  I know there are many versions of most of their amps.

> with that grid leak bias circuit

Those values look goofy to me. The R-C products should be similar, not 10:1 apart. 5Meg will, as you hint, take the triode off the max-gain point.

I think we should tack 1Meg, even 330k, across the 4.7Meg and see how-much which-way difference that makes.

Yes the LFO starts as "zero bias" but when it clips the grid will go negative until the plate wave is about centered.

Hah.  Not claiming to understand all that or the overall tremolo design, LOL. 

But are you basically saying that by tweaking the value of the 4.7M resistor in the tremolo circuit I might be able to get the tremolo to work across the entire range of the speed pot?

What might you have to say in relation to my reply to vampwizzard (i.e tweaking the capacitor values in the tremolo circuit)?  The GA-18T circuit is close to mine, but not exact...

[vampwizzard]

The value of the caps in the trem circuit control the trem speed. Larger values for slower trem and smaller for faster. The issue seems to be gain relative to the insertion point, which is where PRR is giving advice.

[rayandkerry]

The value of the caps in the trem circuit control the trem speed. Larger values for slower trem and smaller for faster. The issue seems to be gain relative to the insertion point, which is where PRR is giving advice.

Great.   That's super helpful, thanks!

I did get stronger or weaker results using different tubes.

[rayandkerry]

In terms of providing more gain, where am I aiming to  do this?  In the first gain stage?  It has the lowest plate voltage at 124v.  Would I basically want to try and drop the 100k plate resistor to 82k or 68k?  And if I do drop the plate resistor value, do I need to also drop the cathode resistor to something less (1k or 820r)?

Or is it that, by changing the 4.7M resistor I the trem circuit to something lower (as suggested by PRR), I can change the gain of the trem tube?

[sluckey]

The gain of the tremolo oscillator is what's being discussed, not the gain of the amplifier. The tremolo signal needs to be bigger (ie, more gain) in order to produce a stronger tremolo effect. Do what PRR suggested first to see if you get an acceptable tremolo intensity.

PRR said, "I think we should tack 1Meg, even 330k, across the 4.7Meg and see how-much which-way difference that makes."

If that still doesn't get the results you want, then I again suggest to convert to the Prince Reverb tremolo circuit. It's really a simple mod to your existing circuit and it works much better. I can draw it up for you.

[rayandkerry]

I think we should tack 1Meg, even 330k, across the 4.7Meg and see how-much which-way difference that makes.

Hi all.  Sorry...dumb question.  When PRR says tack a lower value resistor "across" the 4.7M, does that mean "in parallel to" the 4.7M (i.e. to produce a lower resistance)?

[HotBluePlates]

I think we should tack 1Meg, even 330k, across the 4.7Meg and see how-much which-way difference that makes.

... When PRR says tack a lower value resistor "across" the 4.7M, does that mean "in parallel to" the 4.7M (i.e. to produce a lower resistance)?

Yes.

[rayandkerry]

Yes.

Sweet!  Because that's what I just did.  Hopefully it will help the tremolo function.

I also reverse wired the speed pot and swapped out the 250p coupling cap with a .01uf, as suggested by DummyLoad.  The amp would get a little woofy when playing low notes on the rhythm pickup.  Maybe this will tame that a bit too.

All in all, a really sweet little amp as is.

Thanks for validating my understanding 👍