Gibson G20 schematic question and switch question

[yorgle]

I've started my first PP project based on the G20 schematic:
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga20.pdf
I'm using 12v tubes (trying to use up my old radio stuff) with a separate 12v filament transformer.  The schematic shows the output tube cathodes connected to the center tap for the heaters.  I assume that has something to do with reducing hum.  Can I just ignore that on my build or should I connect the cathodes to the CT on my filament transformer? 
Also, I like using a volume pot with a built-in on/off switch so I can have just one control, but that requires AC leads in proximity to signal leads.  Are these types of volume pots known to cause hum?

[shooter]

Personally I'd never run big dirty AC near any low volt signal, asking for hum.

I also won't do the "raised filaments" in this case.

[sluckey]

Can I just ignore that on my build or should I connect the cathodes to the CT on my filament transformer?

I would connect the ct to the cathodes. Or connect to chassis.
 

Also, I like using a volume pot with a built-in on/off switch so I can have just one control, but that requires AC leads in proximity to signal leads.  Are these types of volume pots known to cause hum?

Lot's of cheap electronics from the '60s, including the fender champ, put a power switch on the volume pot. Most had cheap small speakers that could not easily reproduce 60Hz hum, so no big deal. So, do you really want to build a cheap amp?

I will add that lot's of expensive hi-fi equipment also had a power switch on the volume control. But they went to a lot of extra effort to make sure hum would not be a problem. Usually the power leads to the switch would be shielded and the switch body would also be shielded.

[shooter]

with a separate 12v filament transformer.

never messed with 12v filaments, wasn't sure they used a CT

[Latole]

Heater are connected to cathode with a 220 ohms resistor to ground to have some DC to quiet the amp.
You can ignore that and put power transformer CT to ground ( chassis ) and use a 3 prong power cable to wall outlet.
 

https://www.tropicalfishvintage.com/blog/2019/7/4/on-vacuum-tube-heaters

On elevated heaters. Sometimes, it is advantageous to connect the center tap to a ground reference that is greater than 0v. On all tubes, there is a maximum heater-to-cathode voltage that should not be exceeded. Otherwise, the insulation between heater and cathode will break down, introducing hum into the circuit and eventually causing the tube to fail.

Some people achieve elevated heaters by connecting the center tap to the cathode of the power tubes, which is usually around 30v. Alternatively, you can create a voltage divider in the power supply. This allows you to fine-tune the elevation voltage, because you can set the voltage divider to give you a specific number whereas the cathode voltage is dependent on the bias of the power tubes.

[shooter]

Gibson had a way with making something from nothing, or what everyone else threw away.  I suspect since there isn't anything close to power supply filtering it was a MUST

[Latole]

For a first PP project I will use Fender schematic and layout ; easier and probably a better amp

[shooter]

stick with the Gibson, it's the ugly one you'll love WAY better than a car part 

[yorgle]

Thanks for the input, folks.  The volume/switch pot is getting tossed back into the drawer from whence it came.  Here's my schematic so far with my mods (ignore the power supply for the time being- I'm going with SS rectification on this one.

[bmccowan]

For a first PP project I will use Fender schematic and layout ; easier and probably a better amp

Assuming you have good reasons for wanting to build a GA-20 - I'd say go ahead. And the version you chose (there are a few) is a cool sounding amp.
I would agree with using a Fender style layout as it will allow use of the turret board. Not very hard to start with a 5C3 layout and convert to the GA-20.
The weakness of Gibson amps was their choice of lousy capacitors - especially the filter caps - so just use good stuff.
A suggestion I have is to use a power transformer that gets you into the typical Gibson B+ range which was much lower than Fender. It's part of the "brown" sound of these amps.
I have tried the center tap both to chassis and to the cathode on these amps - I did not notice much difference, but stuck with the original circuit.
added comment - I just saw the SS rectification note - that may make it hard to get into typical Gibby B+ voltage range

[shooter]

the Cathode R for the PA tubes should be 220 NO K
SS ps is the way to go, since it's PP, maybe the 40,20,20, (20)

Have a 47uf at least 50vdc on hand, might come in handy on that 220 OHM R

[yorgle]

Thanks Shooter for catching that error on the cathode resistor.  Bmccowan:  regarding that “low Gibson b+” just how low should I be shooting for?  Right now I’m at 392vdc at the first dropping resistor, but I anticipate having to increase those values to account for the missing rectifier tube.  My plan was to fire it up as is and see how it sounds before lowering the b+.  I want as much clean headroom as possible. 

[bmccowan]

If your goal is "as much clean headroom as possible." You are likely fine with that voltage. Gibson's of that era typically have a B+ in the 310-320 range. But, they do not excel at clean headroom.
However, I'm not clear about your statement regarding the rectifier tube. If you are at 390 with a 5Y3 and change to straight SS - you may end up near 450 but it depends on the particular 5Y3 and the amperage demand of the rest of the circuit.
There are many schematics of Gibson PP 6v6 amps of the era, and some of them have voltage charts. I'd recommend taking a look at some of those as I think you will be adjusting resister values throughout the preamp if you start with a 450 B+
I'd also want to hear from Sluckey and/or PRR on this subject

[yorgle]

Forgive my lack of clarity regarding the b+.  I'm actually starting with 410vdc after SS rectification- no tube. Not sure why it measured 392 yesterday and 410 today 

[shooter]

Not sure why

line voltage, variations in the analog world, this isn't IC's and precise clock rates 

I would expect a loaded PS to fall in a good value for 6V6's, maybe 350 to 370vdc

[bmccowan]

agree - if you have no tubes drawing current - when you load up the circuit the B+ will drop as Shooter suggests. But you still may want to download one or more of the similar model Gibson schematics with a voltage chart to compare where you end up on the preamp tubes.

[yorgle]

Everything is wired up, tubes in and it's LOUD! At least it was until the second filter cap put on a little light show and died.  Here's the revised schematic showing my voltage doubler/rectifier and fixing some errors in my first schematic (the way it was drawn had the grids of the power tubes tied to the plates).  I managed to get a couple of voltage readings on the output tubes before the filter cap blew- 380vdc on the plates, 173 on the screens. 

[bmccowan]

173 on the screens

Is that a typo?

[yorgle]

173 on the screens

Is that a typo?

damn these fat fingers- should have typed 273.

[shooter]

was the cap backwards or junk - junk-drawer?

[bmccowan]

plate and screen voltages - when you get a new cap in place - you should check voltages again.

[shooter]

tubes in and it's LOUD

take THAT car-part amps 
I fixed 2 dead G amps, one a PP skylark, the other a PSE GA-8
comment from the owners when I gave them back;
"man this thing is LOUD"
always a fun response 

[yorgle]

New cap is in and I’m back to 380 on the plates; 285 on the screens; 10 at the cathodes. I’ve got some buzzing/humming to work out, but at least nothing is smoking. 

[tubeswell]

I’ve got some buzzing/humming to work out, but at least nothing is smoking.

How have you wired the 12V heaters on the 12V6s? Ideally, these should be wired so that any transient noise(s) riding on the heater AC (which may appear in the plate signal on each tube) gets cancelled out between the two halves of the OT primary winding. To experiment, you could swap the wires to pins 2 and 7 around on one of the 12V6 sockets to see it its 'optimal'.

[yorgle]

I’ve got some buzzing/humming to work out, but at least nothing is smoking.

How have you wired the 12V heaters on the 12V6s? Ideally, these should be wired so that any transient noise(s) riding on the heater AC (which may appear in the plate signal on each tube) gets cancelled out between the two halves of the OT primary winding. To experiment, you could swap the wires to pins 2 and 7 around on one of the 12V6 sockets to see it its 'optimal'.

Does it matter that my heaters are fed dc? (see my revised schematic, posted above). Nonetheless, I’ll give your suggestion a try.  Thanks. 

Edit:  I see that my schematic (and consequently, my build) omits the 20uf bypass cap for the power tubes.  Could that be the problem?

[tubeswell]

I’ve got some buzzing/humming to work out, but at least nothing is smoking.

How have you wired the 12V heaters on the 12V6s? Ideally, these should be wired so that any transient noise(s) riding on the heater AC (which may appear in the plate signal on each tube) gets cancelled out between the two halves of the OT primary winding. To experiment, you could swap the wires to pins 2 and 7 around on one of the 12V6 sockets to see it its 'optimal'.

Does it matter that my heaters are fed dc? (see my revised schematic, posted above). Nonetheless, I’ll give your suggestion a try.  Thanks.

Okay you’re on DC heaters. I missed that information above sorry.


In that case make sure you’ve got uber filtering on the heater rail. Consider using UF diodes or snubber caps on the rectifier diodes to minimise diode switching transients.


As for other potential sources of hum, pay attention to good lead dress (shortening signal wires, avoiding unwanted induction/coupling, shielding sensitive grid wires etc), and get a low S:N ratio by maximising bandwidth in V1 and playing with attenuation and bandpass/HF/LF shelving in the subsequent stages. Also try some tube swaps to get ‘less-noisy’ tubes. Shield V1 preamp tube if you can.

[PRR]

> my heaters are fed dc?

Imperfectly filtered DC is worse than good AC. (Speaker and ear do not do 60Hz as good as 120/240/480Hz.) Tubeswell's comments apply, though I'd expect to need more than magic diodes (unless it was totally thin buzz with no hundreds-Hz body).

Me, I'm not SO worried about heater hum/buzz on Power tubes because the signal level (thus signal/crap ratio) here is 500 times bigger than your preamp stage(s).

[yorgle]

I'm suspicious that my buzz is heater related because it starts quiet when switched on but grows along with the tubes as they heat up.  I've been doing some reading on heaters and buzz and according to the great Valve Wizard, the "first rule" is that all heaters must have a DC connection to audio ground.  Assuming that rule applies even to separate heater transformers, I'm wondering if I should create an artificial CT and ground that to the chassis (the heater transformer is actually 24V (12-0-12) so the "real" CT is being used as one leg of the 12v circuit).  Or instead, could I ground the "real" CT to the chassis and use that as a common connection for half of each tube's heater and just run a single "hot" lead between each tube. 

As for the hum, that's definitely 60hz and starts only after the volume is turned up so I'll start moving some leads around to see if it's my rats nest approach to layout.

[shooter]

see if it's my rats nest approach

just because Gibson did it that way doesn't mean you have to 

[tubeswell]

...(the heater transformer is actually 24V (12-0-12) ...

Another possibility (using a 12V 150mA lamp to balance the load on the winding) is to just revert to plain old AC. Then you could use a bit of CT elevation if desired (or even dispense with the CT and use a 200R wirewound pot for hum-dinging)

[yorgle]

Adding an artificial CT to the heaters makes no change- still buzzes and hums like a happy honeybee.  Swapping or pulling the preamp tube doesn’t do anything either.  But just touching the SL7 tube changes the volume of the buzz/hum noticeably, and better yet, grounding pin 1 of that tube kills both buzz and hum altogether.  Bad phase inverter? Unfortunately, I don’t have another 12SL7 to swap in.   

[tubeswell]

... better yet, grounding pin 1 of that tube kills both buzz and hum altogether.  ...

Pin 1 is one of the grid pins according to GE

https://frank.pocnet.net/sheets/093/6/6SL7GT.pdf

and Tung-Sol

https://frank.pocnet.net/sheets/127/1/12SL7GT.pdf

Have you wired it up properly? (Your schematic in reply #16 shows the pin numbers all funny. Also that schematic doesn't show a correct DC wiring arrangement for the heaters. You do have filter capacitors in there don't you? What Vdc do you get on the heater rail?)

[yorgle]

I see I do indeed have pins 1 and 2 mixed up on my schematic, but the amp itself is wired correctly.  The filter caps are 68, 22, and 22.  I’m getting 11.6volts on the heater wires. 

[tubeswell]

For heater DC filtering, the filter caps need to be in the order of 2200uF to 4700uF. 50V caps will be fine.


So Pin 1 is the 12SL7 is wired correctly. Seeing as grounding Pin 1 stops the hum, then that stage is not where the hum is coming from (because Pin 1 is where the signal goes in to the PI). Got another 12SJ7 you could try?

[yorgle]

Good news.  Following ideas from you guys, I started to rewire the heater circuit and as a test, I momentarily lifted a heater lead from its pin to open the circuit while the tubes were hot and the amp was buzzing. The buzz/hum changed somewhat, but persisted despite the lack of juice in the heater circuit.  For the heck of it, I tried the same test with a primary lead to the filament transformer and as soon as I did, both the buzz and hum ceased instantly.  Swapped the noisy transformer out for a 12v wall wart and she now runs nice and quiet. 

[tubeswell]

Check your wall wart has enough capacity to supply the required filament current. Each 12V6 sucks 225mA and the other three tubes suck 150mA each. In parallel, that's 900mA.

[yorgle]

Check your wall wart has enough capacity to supply the required filament current. Each 12V6 sucks 225mA and the other three tubes suck 150mA each. In parallel, that's 900mA.

It’s rated at 1.5 amps and as a bonus, it takes up much less space inside the chassis.   

[yorgle]

Even though this build is nearly done and working great, I've still got a nagging question about the original schematic the maybe someone here can answer:  is that 4700R from the PI in the correct place?  The way the original schematic is drawn, the 4700 looks to be in series with the 220k grid resistor right above it- but that means 225k more or less for one tube, but only 220k for the other.  Is there a reason the tubes aren't treated the same?

[shooter]

I'm not good with PI terms but the "bottom" tube is fed from the plate of the top, so it's an attenuated signal going to G1.  Hence the split load plate setup, I think 

[DummyLoad]

Even though this build is nearly done and working great, I've still got a nagging question about the original schematic the maybe someone here can answer:  is that 4700R from the PI in the correct place?  The way the original schematic is drawn, the 4700 looks to be in series with the 220k grid resistor right above it- but that means 225k more or less for one tube, but only 220k for the other.  Is there a reason the tubes aren't treated the same?

paraphase inverter - voltage divider to feed non-inverting (lower) triode so that output of the lower triode is the symmetrical to the (upper) inverting triode. resistor is in the right place.

yes, there is imbalance in the grid-leak load for the phase inverter, however, at 4700R it's about 2% difference and not enough to be concerned with. if you are really bothered by the slight imbalance, add a 4700R in series with the other grid leak.

some more reading and calculator to help solve for value of the divider network.

https://www.ampbooks.com/mobile/amplifier-calculators/paraphase/ 


--pete

[tubeswell]

Despite the way its drawn on the Gibson schematic, a paraphase inverter is really just a couple of cascaded inverting* gain stages (where the plate of each stage feeds one or the other sides of the output stage).

*All ordinary ole gain stages invert the (phase of the) signal between the grid and the plate (so that the signal coming off the plate is in opposite phase to the signal at the grid). The paraphrase is a simple PI that makes use of this principle.

Since the signal coming off the 1st plate is feeding the 2nd stage’s grid, this signal needs attenuation so that you don’t get a huge imbalance between both sides of the output stage. So that’s why there’s a voltage divider to cut down the signal going into the 2nd cascaded PI stage.

[yorgle]

...if you are really bothered by the slight imbalance, add a 4700R in series with the other grid leak.

Hey, if you guys are ok with it, I'm certainly not going to let it bother me.  I'm beginning to see that I tend to look at individual stages of the amplifier in isolation, when I should be looking at the ways they interact with each other.  Lots of learnin' goin' on here. 

[yorgle]

One step forward, two steps back.  Not content to leave well enough alone, I decided to tweak the V1 tube's plate and screen resistors to match the V1 in my Les Paul Jr. amp.  I like the slightly brighter tone of the Jr., but apparently the G20 does not and it's letting me know by emitting an AM radio static noise that comes and goes as I play.   One or both of the 12v6's must have choked on the extra treble because the static is unaffected by the volume pot and is present even with the guitar unplugged and both input and PI tubes removed.  I tested the speaker on another amp and it's good. Unfortunately I don't have any spare 12v6's so I have to wait until I can order some to do any further testing.  I hate waiting.

[tubeswell]

Not content to leave well enough alone, I decided to tweak the V1 tube's plate and screen resistors to match the V1 in my Les Paul Jr. amp.

Which iteration of the LP Junior Amp have you got? '54-'57? - The 6SJ7 in that is set up to directly drive the 6V6 grid

[yorgle]

Yes, the 54-57 version. 

[tubeswell]

Check that you didn't accidentally leave a cold solder joint in there when you changed the parts (and reflow if necessary)

[yorgle]

Problem solved!  I followed your advice Tubeswell and started to reflow some of the solder joints.  I found some questionable ones, but I also discovered that a tiny tone cap that I thought had fallen onto the floor never to be seen again had instead wedged itself amidst the wires and lugs of the PI socket, tickling some of the pins with its legs.  Of course, I discovered this after I had already ordered two new output tubes.