GA-16/18T in a Stout chassis?

[Blooze]

I'm gonna try this again.

First off let me apologize to everyone for the last two thread getting corrupted.  I have no idea what is causing it unfortunately.

So I've become enamored with the sounds I've been hearing from a 6V6 version of the Gibson GA-18T with 6V6's.  It is essentially the same circuit as the GA-16T. Evidently the 18T was produced in both EL84 and 6V6 with little to no change in the schematic.

I want to try and fit this into a Stout chassis (for $$ sake as they are half of a fender style chassis). So my limitation is really the Stout sized board. I would use a 18W Marshall power transformer as that should give pretty close voltages. The Gibson's tended to use lower B+ than Fender as far as I can tell (~300V). I was looking at a Musical Power Supply OPT12PP+ (8K/4-8-16 ohm 18VA) as well.

I've added a MV as I like having that option. I've also changed a few of the caps in the tremolo that are supposed to make it a little wider/deeper.

On the previously corrupted thread, bmccowan suggested I go back to the original parts values, so that's what I've done with the exception of the MV and the tremolo caps.

I'm going to try uploading some jpgs of the schematic and layout. Be kind, I'm trying to learn 

[Blooze]

Right off.  in the schematic the screen resistor should 470R. Input jack resistors on layout should be 56K/56K/1M. Coupling cap between V1B-V2A should be 0.0047 and the tone cap should be 0.0047.

I can't use DIYLC at work (no java) so those will have to wait to get corrected.

[bmccowan]

I was wondering what happened. Re-attaching the version of your schematic that I had adjusted - if its any help.
I think you should be able to get it all in a Stout chassis, as the Gibson is a low parts count amp - and you are right, that chassis is the best deal out there.
A few space saving ideas - mount the power tube screen resisters on the tube sockets - check out Fender layouts. Mount the B+ dropping resistors right on the multi-can. And look at Sluckey's American Dual Lite for a great example of cramming a lot onto a turret board.
When building with turrets, it is super easy to modify component values which is part of the reason I suggested sticking close to the original design for the first go around.
I can't start sniffing the solder on this one until after Labor Day, but when I do I'd like to share notes and see what we come up with.

[Blooze]

Anyone see any issue with the layout per se? If not I’m going to try and order some parts this weekend.

I do appreciate you experts taking the time to give a new guy advice.

I haven’t ordered the resistors I need to modify my Vibrochamp yet as I was waiting to get this amp and the last bits for a Hagerman Clarinet preamp all in the same orders. I just don’t have a lot of stock outside of pedal stuff.

[tubeswell]

If you can find an RA pot with a SPST switch built in (for the trem depth control), you can use that to short the 6V6 grid leak resistor directly to ground - to switch off the trem via the pot (as an alternative to using the foot switch).

[bmccowan]

Your layout looks efficient - couple of things: The original has NFB with a 47K resistor. It could easily be added if you want by soldering the resistor right on the speaker jack and running a lead to the V1b cathode, and maybe put it on a switch. Unusually, the V1B cathode bypass cap is 0.1uf on the original schematic - yours is 10uf. Either will work of course. I do not know if the 0.1 cap is a typo?
I'd be tempted to extend the board close to the PT and use an F&T 22uf and a 2x16 axial filter caps. But that's because I find the cap cans a pain. Maybe there is not room for it. If you use Doug's DIY board analyzer you'll be able to easily figure it out.
Last thing - I suggest buying a bunch of extra resistors of common values. They are cheap and it makes it easy to modify the circuit. Same thing for .01, .02, and .047 film caps. If you like this hobby - you will certainly use them.
Fun project!   

[Blooze]

I’ll most likely be adding NFB on a switch. The Hoffman Stout chassis just isn’t going to have the room for separate filter caps I think. Here’s a pic of the 18W it’s designed for. You can see there is little room. My layout above has the same board dimensions as in this pic. The Version 2 chassis has three octal and 3 9-pin holes as compared to this older version. Although, I’ve thought of mounting the can cap in the first octal hole and drilling out one of the 9-pin to octal so I don’t have to mount the can on the inside of the chassis.

Edit. Added a topside shot of the V2 chassis for reference.

[bmccowan]

Yup - tight fit. The older my eyes and fingers get, the more I like working with a large chassis and board.
And for larger chassis that are similar build to the Stout - try Zach on Ebay. I have used several, and they have always been willing to customize things.https://www.ebay.com/sch/i.html?_ssn=zachmdhunter&store_name=zhmusicparts&_dmd=2&_oac=1&_trksid=p2047675.m3561.l2562

[Blooze]

Yup - tight fit. The older my eyes and fingers get, the more I like working with a large chassis and board.
And for larger chassis that are similar build to the Stout - try Zach on Ebay. I have used several, and they have always been willing to customize things.https://www.ebay.com/sch/i.html?_ssn=zachmdhunter&store_name=zhmusicparts&_dmd=2&_oac=1&_trksid=p2047675.m3561.l2562

I’ve had him bookmarked for a year now, LOL. Nice to know he customizes as well and the prices are reasonable.

I have a space of 1.5” x 3” between the PT and the OPT on top. I thought about brazing together a little aluminum doghouse and putting the two 16uF up there instead of a can cap. 

[Blooze]

Any benefit to having individual cathode resistors/caps on the 6V6's?

[sluckey]

Any benefit to having individual cathode resistors/caps on the 6V6's?

Probably no sonic benefit. But having individual resistors means you can see how each tube is biased.

[Blooze]

Thanks Sluckey.

Another question. It was mentioned above about using a pot on the tremolo with a spst switch to turn it  into on/off without a footswitch. I’m not finding anything but typical value linear or audio taper pots with a switch. No reverse log pots at all (both the trem controls are reverse).  Any suggestions besides just adding a toggle on the amp someplace?

Fwiw I just need transformers pretty much at this point!

[sluckey]

Another question. It was mentioned above about using a pot on the tremolo with a spst switch to turn it  into on/off without a footswitch. I’m not finding anything but typical value linear or audio taper pots with a switch. No reverse log pots at all (both the trem controls are reverse).  Any suggestions besides just adding a toggle on the amp someplace?

A 1M-A pot will work just fine for your intensity pot. Easy to find with a push/pull switch...

     https://www.tubesandmore.com/products/potentiometer-cts-audio-1-m-spst-push-pull-solder-lug

[Blooze]

Alright, time to choose the iron for this thing. Looking again, it seems similar to a lower voltage 5E3 (B+ 290-310V)

I think the Marshall 18W power transformer (580VCT) will work, albeit maybe a bit high on the B+. A Hammond 270DX (550VCT) would be better, but it’s an upright. The 290HAX is similar and a laydown, but all those Hammonds are too small for the cut hole, so I’d end up putting some sort of support or plate there anyway.

I’ve been looking at the Musical Power Supplies OTP12PP or OTP20PP for the output transformer (both 8K/4,8,16 ohm with 12PP being a 20W and 20PP a 25W). Matt says these are good for 12W to 20/25W range on his page). I've read the original OPT was woefully undersized, but was this a benefit to the "sound" or a deficit?

Any other suggestions?

EDIT: I ordered a OTP20PP output transformer and a PT275.2 (550VCT) power transformer from Matt. The power transformer is a stand up but I can slot out two of the Stout chassis holes to match the transformer feet as they almost match the Stout bolt holes and put a 1/8" 4"x4" plate on the inside of the chassis to cover the square hole. Much cheaper than any laydown model.

[Blooze]

Here's what I currently have for a "layout". I would appreciate any suggestions.  This is my first guitar amp build. The grounding scheme is always a little tenuous for me, but I've used star grounds in the hifi amps I've done. There's a lot more room in those enclosures for all the extra wires though.

[bmccowan]

Hey Blooze,
I have not been home much as I'm rebuilding a porch on the family camp - its on a lake in Maine, so no complaints.
But I was back for a few days and wired up the chassis and tested it before heading out again.
Recap (no pun) - built in 5F11 chassis because I wanted to fit a Gibson cabinet I had gathering dust. And the sockets & pot locations are the same.
Your layout looks fine to me. At first I was confused at your gain pot, but then realized you were showing a shielded cable. My ground scheme is similar - preamp buss grounded near the input including the preamp filter cap. Other filter caps grounded with the power tube cathodes and HT center tap. Heater center tap elevated at power tube cathodes. It is quiet and the amp sounds great. I did not have RA pots on hand so I used linear pots temporarily- they work, but not the best, will change.
Pt I used my last ClassicTone PT designed for Valco 2x6v6 and 2x6973 amps. Gave me 304 volts - the right ball park. I had planned to use a PT from a Hammond A0-39 chassis, but it required cutting the chassis more than I felt like doing. I did use an A0-39 OT. Sounds fine to me. I would not sweat the exact voltage, but staying under 325 or so should give the tone of that amp. If seeking more clean punch go higher, but that's what Fender amps are for
I detached the NFB just to see if I wanted to put it on a switch - too noisy for me, but I may add a switch that ups the resister from 47K to 100K and try that - or use a pot. Trem will be switched at one of the unused input holes - no footswitch.
Doug made the turret board from my DIY layout - perfect. I have always enjoyed making my own boards - but this was painless. And Doug is FAST!

[Blooze]

Hey Blooze,
I have not been home much as I'm rebuilding a porch on the family camp - its on a lake in Maine, so no complaints.
But I was back for a few days and wired up the chassis and tested it before heading out again.
Recap (no pun) - built in 5F11 chassis because I wanted to fit a Gibson cabinet I had gathering dust. And the sockets & pot locations are the same.
Your layout looks fine to me. At first I was confused at your gain pot, but then realized you were showing a shielded cable. My ground scheme is similar - preamp buss grounded near the input including the preamp filter cap. Other filter caps grounded with the power tube cathodes and HT center tap. Heater center tap elevated at power tube cathodes. It is quiet and the amp sounds great. I did not have RA pots on hand so I used linear pots temporarily- they work, but not the best, will change.
Pt I used my last ClassicTone PT designed for Valco 2x6v6 and 2x6973 amps. Gave me 304 volts - the right ball park. I had planned to use a PT from a Hammond A0-39 chassis, but it required cutting the chassis more than I felt like doing. I did use an A0-39 OT. Sounds fine to me. I would not sweat the exact voltage, but staying under 325 or so should give the tone of that amp. If seeking more clean punch go higher, but that's what Fender amps are for
I detached the NFB just to see if I wanted to put it on a switch - too noisy for me, but I may add a switch that ups the resister from 47K to 100K and try that - or use a pot. Trem will be switched at one of the unused input holes - no footswitch.
Doug made the turret board from my DIY layout - perfect. I have always enjoyed making my own boards - but this was painless. And Doug is FAST!

I decided to go with a JTM45 chassis as I just didn't like all the signal wiring going over the tubes. I have everything here, I just need to order a board and drill some front face holes.

[Blooze]

Also, would it be a good idea to do a chassis lug for grounding at the inputs? 

[bmccowan]

Love that JTM chassis - lots of room and works well with a lot of circuits.  I've built a Matchless Clubman, a Dr Z-28/Gibson GA-40 mongrel, a Valco/Gibson style 6K6 amp, and a Little Wing with those JTM chasses. Maybe I should build a JTM? Preamp ground - a separate lug for the buss is what I usually do. But this time I used a toothed washer on the input jack and tightened it down really well. Both approaches seem to work well. When I get home again I will share some pics of what I have going on. Have fun with the build.

[bmccowan]

The RA pots for the trem arrived and I went back to the amp. At first I followed Sluckey's lead and installed a switched 1MA pot on the depth, and a RA 500K on the speed. I did not like the 1MA pot on the depth. The taper was too quick and turned to full the trem did the putt-putts. So I changed that to a RA 250K as the schematic calls for and it was perfect. So I may add a switch, or just leave it. With the depth all the way down there is no trem and sounds fine. The only advantage I can see to adding the trem switch is to save a favorite depth setting, and to fill an empty hole. Next I will try adjustable NFB; 25K to 150K for starters. The amp has great tone - I've always liked P-P 6V6s at around that 300v mark. If you have not seen it, look for a Bill Frisell documentaryhttps://vimeo.com/ondemand/billfrisellaportrait. He plays a lot through one of these amps. And he's a riot to watch - like Mr. Rogers playing great, inventive, guitar. He does not tour often but I've been fortunate to see him 3 times - small venues full of guitar geeks.

[bmccowan]

I've now played the amp a bit into a cab with a 12" Greenback. Awesome with P90s. Have not tried other guitars or pedals yet. Like many amps, the trem is too fast for my taste. I searched the forum and found that PRR a while back had recommended upping the value of the 3 trem caps (2-.01s and 1-.03) to slow it down, so I went with .02s and a .047 - nice change. He also suggested upping the value of the cathode bypass cap on V2. I have not done that yet, but probably will try it. Searching old forum threads is a great learning tool.
I also lowered the bright cap value to 86pf which put the tone control into a more useful range for me (I have Tinnitus and highs bother it.) Adjustable NFB is next - Tinnitus be damned!

[Blooze]

I was planning on using all 0.047uf caps. Based on a couple of things I found on the interwebs. Hopefully order the last bits and board from Hoffman this weekend. My priorities (and work load) have been taken elsewhere so I’m shooting on having this done by the first of the year.

[PRR]

...suggested upping the value of the cathode bypass cap...

Mainly if it wants to "stall" at the lowest speed. We know what caps to use for 50Hz or 20Hz bass, but a slow-slow-vibrato is even lower. And rather than just being gutless, it can quit oscillating. If it works for you, don't tinker.

[sluckey]

upping the value of the cathode bypass cap on V2

Replace the cathode resistor AND bypass cap with a red LED. Don't look back.

[bmccowan]

Always learning - even at 69 years. The trem does not stall, so no need to change. But Sluckey, what is the "theory" behind changing to a red LED?

[sluckey]

http://www.valvewizard.co.uk/trem1.html    Read the last paragraph.

The LED will produce a much larger oscillator signal that will result in a stronger tremolo effect. And because of the larger signal there is less chance of dropout on low or high end speed. This means you can use a larger cap(s) to produce a slower speed without dropout. And the blinky LED looks neat when mounted on the front panel.

[Blooze]

I love this forum. As a new tinkerer I’m learning new stuff all the time.

[bmccowan]

Thanks PRR and Steve. PRR's advice: "If it works for you, don't tinker" is good advice. But I'll ignore it anyway and try the LED. Tinkerers can't help themselves.

[bmccowan]

Finally back at my bench.

Sluckey; Replace the cathode resistor AND bypass cap with a red LED. Don't look back.

So I did that and had to look back - because it did not work. The LED lights steady and the voltage at the cathode is 2.2 and no trem. So being a confessed tinkerer, I tried various combinations and found that I needed capacitance in parallel with the LED. 25uf worked best. Got trem, got pulsing LED. I'm not sure it's any better than the resistor & cap - but I like the pulsing light. So I am confused. The Wizard and Sluckey are typically right. Thoughts? What might I be missing?

[sluckey]

The LED alone should flash. The LED cathode must connect to ground. Did you use a 5mm red LED.

[bmccowan]

I do have the LED cathode connected to ground. Its a 5mm LED, but its one of the bezel mounted panel LEDs - Internal current limiting resistor in that screwing this up?

[sluckey]

I do have the LED cathode connected to ground. Its a 5mm LED, but its one of the bezel mounted panel LEDs - Internal current limiting resistor in that screwing this up?

Yes!

[bmccowan]

Thanks - back to the tinker bench!

[bmccowan]

Yup - works. Like I said, you and the Wizard are typically right.
Cheers!

[sluckey]

Thanks - back to the tinker bench!

I use these LEDs and bezels. Scroll down a bit...

     https://hoffmanamps.com/MyStore/catalog/parts20.htm

[bmccowan]

Yup, that's what I found in my parts bins. I don't remember buying them, but at 69 Y.O. there are a growing number of things I don't remember. I didn't think about the current limiting resistor in the other style LED assembly until you asked me if it was a 5mm.
Next question coming up.

[bmccowan]

And here goes that next question. Most vintage Gibson amps have no global NFB. The 16/18T and a few others have 47K NFB resistors with modest resistors to ground - 1.5K in this case. With many other amp circuits, Fenders for example, the NFB resistor is much lower. Changing them, or making them adjustable has significant effect. Changing this amp's NFB resistor (25K to 100K) has very little noticeable effect. Eliminating it, increases the background noise, as I would expect. Is the small amount of NFB that resistor is allowing through, likely only intended to limit the noise? I'm not trying to fix a problem, the amp sounds great with that stock NFB - just looking to better understand.

[PRR]

...Changing this amp's NFB resistor (25K to 100K) has very little noticeable effect. ...

There's not a lot of NFB there. The 6V6es+OT have gain of about 0.7, cathodyne unity, 12AX7 about 27 (no cathode bypass). Open-loop gain is say 20. Closed-loop goes toward 100K/1.5K or 32.

If you reduce 47k to 10k or 4.7k it will make a difference, both in tone and sensitivity (it will be a pig to play because so insensitive). 

Steal volt-amp cathode network from AA-Champ. (yeah I know, SE vs P-P; stay with me.) The 1.5k is fully bypassed. A new 47 Ohm inserts NFB from something like 2.7K? That added just enough gain for Leo to put a full BT tonestack in the classic Champ. It's hardly Hi-Fi damping but without TS it may be an interesting path.

And put a 5k-10k pot across the OT secondary and take NFB from the wiper. This lets you dial down to zero NFB for full funky tone. Pick the 47k or 2.7k for "very tame" high-NFB heavy damping. Now all inbetween NFB tones are on-tap easier than beer at the bar.

[bmccowan]

Thanks PRR - I think I get it. But, speaking of beer at the bar, I am sipping a wonderful 16 year old Lagavulin single malt right now. Post that buzz, I will sketch out what I think I understand. I have used pots in that fashion a few times for adjustable NFB, rather than the "presence" pot approach which I find very subtle.

[Blooze]

Interested in what you end up with this n the NFB arena.  I got my chassis all drilled out tonight. Of course the last hole my step bit grabbed and made the 1/2” hole for a fender style jewel light to big. I’ve got the hole epoxied up with PC-7 and I’ll sand and redrill. Trying to decide if I’m gonna paint it or just do a DIY Plexi type faceplate.

Quick question: my board mounting holes are very close to 3 of the turrets so a M3 bolt head is within 1/16 of the turret. Is this an issue, or should I get some nylon standoffs to be safer?

[bmccowan]

I think you could get by, depending on what connections are at those turrets - top hole only? high voltage? But I would choose to re-drill the mounting holes. Even if I could get it to function ok - it would always bug me. nylon hardware is of course the other option - all hardware store stuff. And speaking of hardware stores - You may want to look for a 5/8" ID washer for that pilot light hole.

[Blooze]

I think you could get by, depending on what connections are at those turrets - top hole only? high voltage? But I would choose to re-drill the mounting holes. Even if I could get it to function ok - it would always bug me. nylon hardware is of course the other option - all hardware store stuff. And speaking of hardware stores - You may want to look for a 5/8" ID washer for that pilot light hole.

First filter cap, unfortunately. I think I’ll get nylon standoffs as the chassis holes came predrilled. I meant to scoot everything over another 1/8”, but forgot.

I filled the hole with epoxy then sanded flush and redrilled. Seems to work perfect.

[bmccowan]

On PRR's suggested variable NFB. Looked at the AA764 champ - this sketch is how I interpret it

[PRR]

Ground the pot.

[bmccowan]

Okey doke - thanks

[bmccowan]

wired it up with the 47K resistor - sounds good, but very subtle effect, if any, from the 5K pot. To be expected? Or is it just that my hearing is shot?

[PRR]

wired it up with the 47K resistor ..... very subtle effect, if any, from the 5K pot...

With the 2.7k (or less) resistor. (Yes, my mind got ahead of my fingers.)

[bmccowan]

Ahh - that makes sense.

[Blooze]

Thanks - back to the tinker bench!

Is there much difference between this and stock, other than the cool blinky factor 😁

[bmccowan]

Thanks - back to the tinker bench!

Is there much difference between this and stock, other than the cool blinky factor 😁

Isn't that enough? Trem is a bit stronger - voltage on that half of the tube are higher. But I think the reason for doing it is to make sure there is enough drive for the increased cap values.