6V6GT build thread - my first build

Plimpington2 said:
2). Now HERE is something: There are a lot of these 620 schematics available on the internet, some hard to read tho. Here’s a good one:

View attachment 2017891

And here is the one I have been using:

View attachment 2017892

Notice that the resisters on the voltage sources for the 5879 are reversed. The other schematic puts the 180k on the plate and the 120k on the screen. The schematic I have been using puts the 120k on the plate, 180 on the screen.
Click to expand...

While looking at this last night I patiently collected all the schematics I could find. When I get a chance I will collect them into a single document and put them in the library.

On the drawing you have been using the B+ that feeds the 180K and 120K should read 178V rather than 78V. I noticed this on one of the original Ampex schematics which appeared to have the "1" partially obscured from what was probably a photocopier error. (There is nevertheless quite a spread for all voltages depending on which schematic is being referred to.)

The SAMS schematic shows the LTP with 150V on both plates and 110V on the cathodes. 105V on the plate of the 5879.

There is another schematic I located which shows revised circuit values to give the circuit a wider bandwidth and more headroom. At a glance it looks like the original schematic. Whoever revised it did so with a paint program.
 
When you're down in the weeds sometimes the easiest things float by while you're looking for something complicated instead. It's a continuous learning experience.
 
Ok, started on the other side. And a funny glitch emerges - because of the way the tubes are arranged, I cannot use pin 6 on the 6V6 as a tie point like I did on the first channel (it’s too far away). However, pin 1 is close enough.

Yet, I’m unsure if pin1 is internally connected. Oddly, SOME of my 6V6GT’s have a pin 1, and some don’t.

Here is the data sheet:

5F487651-A93B-40ED-9EFA-76B2694B8189.png

The one on the left (which I THINK) is the mental tube version says pin 1 is “s”. The one on the right (glass) says NC. All my tubes are glass (and I’m sure I’ll only buy glass). But if it’s really NC, why do SOME of my tubes have a pin 1, and some don’t?

Can I use pin 1 as a tie point?
 
The original metal 6V6 had pin 1 tied to the outer shell for shielding. Non-metal ones don't tie it anywhere so far as I know. Using it for ground is perfectly fine, just don't use it for HV and use a metal tube in there.
 
I have made a huge rookie mistake. My front end tubes are arranged one in front of the other, but my power tubes are side by side. I installed my power tube sockets so they are “mirror images” of each either. Ie., my locating slots, for instance, point to the left on the left side of the chassis, and to the right on the right side of the chassis.

Now, as I work on wiring up the other side, some of the pins on my power tube sockets are facing the rear of the chassis instead of the front, and some of my component leads (ie., coupling caps) aren’t long enough to reach from the PI plates to the signal grids on the power tubes.

I’ve already wired up all my heaters and HT lines . . . But I think I gotta turn those sockets around :mad::mad::mad:.
 
Improvise and overcome. Layout is the artistic part of this hobby for me and it is something that becomes improved thanks to past mistakes. I just try and make sure they're not lethal or too expensive.:)
 
JohnT123 said:
Improvise and overcome. Layout is the artistic part of this hobby for me and it is something that becomes improved thanks to past mistakes. I just try and make sure they're not lethal or too expensive.:)
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I’m extending the leads on my caps. Life’s too short . . .
 
Consider this. You will have this amp for a very long time (supposedly). The extra effort to turn those sockets around is nothing compared to the length of time you will enjoy the amp. And after a while you will have forgotten that it was extra effort.

Do it right. But you define what right is, of course.
 
+1 for doing it right.
If you have blank pins on a 6v6 socket, you can use them as a tie point.
Extending leads causes components to have more freedom of movement resulting in potential problems down the road.
If you are using a steel chassis, and don't want to drill for whatever reason, i have successfully soldered terminal strips to steel.
It allows you to mount the components securely and nobody has to know.
The old timers even did it from time to time.
Aluminum chassis?
Drill and add a strip...
 
kward said:
Consider this. You will have this amp for a very long time (supposedly). The extra effort to turn those sockets around is nothing compared to the length of time you will enjoy the amp. And after a while you will have forgotten that it was extra effort.

Do it right. But you define what right is, of course.
Click to expand...

You give me pause. I can't disagree with you.
 
Other side wired!! Went up much faster than the first, which stands to reason. Tested without a hitch. And like the first, quiet as a mouse. I can’t even get my rehabs this quiet. I suspect it’s the single point ground bus.

97518B18-15AB-4D2D-903C-28DFFE135233.jpeg

All that left are the inputs and the pot.

Which I’m not quite sure how to handle. Preliminarily, my RCA jacks are not insulated from the chassis. That is to say, the ground is in contact with the chassis. Perhaps that is my first problem.

But let’s presume that that does NOT represent a problem. I had intended to run shielded line from the RCA jacks to the pot. I know I would solder the shield to the ground of the RCA jack. However, what do I do with the shield at the pot? The ground leg from the pot will go right to ground. Do I just send the shield to the same point?

And from the pot to the signal grid, should I also run shielded wire? If I do, do i also ground both ends of the shield? I feel that ground loops that way lay. . .

You could hear a mouse fart with this thing running. . . But I feel like the hard part is coming.
 
the ground buss should only tie to chassis at one point. Insulate the RCA jacks. shield (or twisted pair) grounded at both the pot and input jack, and carry the ground through the pot to the ground buss at the input tube. Can use shielded or twisted pair from pot to grid, your call on that.
 
gadget73 said:
the ground buss should only tie to chassis at one point. Insulate the RCA jacks. shield (or twisted pair) grounded at both the pot and input jack, and carry the ground through the pot to the ground buss at the input tube. Can use shielded or twisted pair from pot to grid, your call on that.
Click to expand...

I ran shielded from jack to pot, and soldered shield at both ends (one end to the jack, one end to the ground lug on the pot). Then I ran a line from the ground lug of the pot to the ground bus (which was just one inch away). Wiper to grid just one line, unshielded.

Dead quiet. Super happy.

IT IS DONE!!!!!!!

1821D1E7-0843-454A-9695-9EB3D120EB0D.jpeg

It’s running!! Testing and fine tuning tomorrow.
 
In terms of testing, i can say this as a preliminary matter - The schematic says that the screens are supposed to be about 10 volts lower than the plates on the output tubes. But mine are identical.

It’s a 1.5k dropping resister. So the screens should be drawing about 6.6mA. But they seem to be drawing nothing.

What would cause that?
 
gadget73 said:
the ground buss should only tie to chassis at one point. Insulate the RCA jacks. shield (or twisted pair) grounded at both the pot and input jack, and carry the ground through the pot to the ground buss at the input tube. Can use shielded or twisted pair from pot to grid, your call on that.
Click to expand...

oh, I see what you mean here. I like what you suggest better than what I did. I think I’ll make that change tomorrow.
 
Plimpington2 said:
In terms of testing, i can say this as a preliminary matter - The schematic says that the screens are supposed to be about 10 volts lower than the plates on the output tubes. But mine are identical.

It’s a 1.5k dropping resister. So the screens should be drawing about 6.6mA. But they seem to be drawing nothing.

What would cause that?
Click to expand...

check the voltage drop directly across the 1.5k resistor. There is some drop in the output transformers so you probably do have voltage drop on that resistor, just not as much as expected. Whats the voltage at the cathode and what value cathode resistor?
 
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