GA-8 pa section ??

[shooter]

Hey, I have a question on the Gibson ga-8’s PA section, specifically the input section.
After the coupling cap (.022) the top tubes grid is connected to ground via a 220k.  The bottom tubes grid is connected to the coupling cap via a 470ohm resistor.  Are the two resisters connected?  Either way I’m having a hard time wrapin my crusty mind around the tubes being in parallel with the schematics configuration.  
Thx

[sluckey]

Are the two resisters connected?

Yes.

The tubes are in parallel. Both plates are tied together. Both screens are tied together. Both cathodes are tied together. Both control grids are tied together to the same signal input. The 470R provides a small degree of isolation between the two grids, but they are both looking at the same input.

[shooter]

thx man, hopefully my second build will go as easy as the first, then i can pass it all to my musicial son n get back to the future, has been fun tho!!

[shooter]

Ok, i got another question.  I've been googlin around n the consences seems to say I need an output tranny with an input imedence of 3-4000 ohms.  I checked on Dougs site n he lists a pt#022848 as 4000 in and 4 out @ 50w.  A little overkill on power, but will that tranny work in a se parallel configuration for 2 6V6's?
thx
dave

[sluckey]

No. That OT is designed to be used with a pair of push/pull 6L6s.

[HotBluePlates]

You're building this amp from scratch? Not repairing an existing one, right?

If you can't find anything else, a Hammond 125CSE or 125DSE should work for you.

The primary will be 5kΩ with an 8Ω speaker if you use the yellow and black secondary wires, or use the green and black secondary wires for a 4Ω speaker load.

It's not worth worrying about 4k vs 5k. I assume the Hammond transformers are rated for sufficient unbalanced d.c. for these tubes, as they don't provide that spec. However, they know their business, and have likely considered the probably tube types that would be hooked up to their transformers.

[mimungr]

they don't provide that spec.

They do: http://www.hammondmfg.com/125SE.htm

[PRR]

_IF_ the build matches the old voltages:

304V across 12W tubes, max 40mA per tube, 304V/40mA suggests about 7.7K per tube or 4K for two tubes.

What you want is "6L6 Champ" OT. This is typically an oversize 6V6 Champ OT with an option for lower impedance to suit a 6L6 working near twice the current of a 6V6. Two 6V6 come close to one 6L6 so it will work.

The nominal 40+40= 80mA total current suggests Hammond's 125ESE. But that is an absurdly large and costly lump for two 6V6, and it does not really offer a load near 3.8K-4K. 5K might be close enuff for rock, but look at 6L6 Champ options first.

Two tubes in parallel *can* do some screwy supersonic things. A few K between grids and 50r or so between plates usually kills this. It is very dependent on layout, tubes, and OT; often you can get away without all four resistors. But also, resistors make handy jumpers between sockets. So it may be a good idea to just do it. Grid resistor value is un-critial, a few hundred to several thousand. Plate resistor value is a little more critical: 10 ohms may not kill oscillation and 100 ohms starts to hurt audio power. 47 ohms 2W Carbon is popular.

[HotBluePlates]

they don't provide that spec.

They do: http://www.hammondmfg.com/125SE.htm

  Looks like I need to clean my glasses off! Thanks for getting me to re-look at that page.

[shooter]

You folks r great, thanks!  it is a scratch build, my 6v6 champ was a big hit with my rock son n crew, BUT, they want the same sound with a little more headroom n i was looking at ga-8 already.  The schematic already shows a 470ohm across the grid, i'll play around with the plate resister n see what i get.  So what specs am i looking at in trannys besides impedence?, what makes a PP tranny vs a SE tranny? I understand the dif in bias class's etc but It's been 30 years since my analog theory days. 

[Colas LeGrippa]

Good-a morning-a to all-a 

Dear PRR, do you mean to insert a single 47R resistor between the plates, in parallel wit the OT connection or 2 in series between the OT and plates?
And between rhe grids too, one single  (x) K resistor in parallel, right between the control grids ?

Thanks

Colas

[HotBluePlates]

what makes a PP tranny vs a SE tranny?

Read the 4th full paragraph in this article.

I don't know if it will turn on the lightbulb for you, but all the basic facts of the difference between push-pull and single-ended transformer are there.

It will help if you know that single-ended operation was the first known way to operate a tube. So the later discovery of push-pull operation was likely especially valued in how it could provide economy over single-ended operation (smaller transformer), in the same way as the later-discovered class AB/class B allowed economy over class A (more power output form the same set of tubes).

[PRR]

> It will help if you know that single-ended operation was the first known way to operate a tube.

It may or may not help to think about mechanical systems.

The earliest steam engines were single-acting. Steam moved a piston one way. Some other mechanism was needed to return the piston the other way.

This worked real good when the load was a heavy vertical water-pump. You could rely on the pump-rod pulling down.

When the load had no self-return feature, double-acting steam engines were tried (VERY sucessfully). Steam is admitted to one side of a piston, then the other side. Or two pistons are used, steam to one and then steam to the other.

Or it may help to think of a one-legged bicyclist. We had one riding around the school. Two legs makes a smoother motion.

[shooter]

i'm lovin the one-legged biker!  while i was touring pompii I had a 3 legged dog become friends with me, the chief wouldn't let me bring her onboard tho!  I did look at the hammond page n agree with the dse or ese tranny, n i will read the princeton link cuz i'm trying to understand how hammond says the input is 2500 to 10k depending on output tap selection.  anyway, thanks again

[HotBluePlates]

... i'm trying to understand how hammond says the input is 2500 to 10k depending on output tap selection. 

Lever-action.

The transformer has an apparent primary impedance determined by the load impedance (how much speaker) multiplied by the turns ratio between the primary and secondary.

Hammond devised a clever arrangement of primary and secondary sections to allow several wirings to give an appropriate turns ratio to give your desired primary impedance when a suitable secondary load is attached and you use the correct secondary wiring.

Versatility is improved even if you do have to mess with it a bit more than a typical guitar-amp transformer.