• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

6l6 mystery amp

You generally want a ground reference for the heaters. Ground one side of DC as a way to do it. Using either a winding center tap or using two resistors across a filament winding and ground the midpoint are other ways.

Here's a good writeup on heaters: https://valvewizard.co.uk/heater.html

The other pages on this web page have a lot of good information too.
 
ok so I banged together a test circuit for that inverter. I made the assumption that the B+ supply is 300 volts, mostly because that seemed like a reasonable enough number.

I rigged up a 6M pot to bias the grid since I had no other way of doing it.

With the pot set so that about 1/2 the plate voltage drops from plate to cathode and 1/4 across the plate and cathode resistor I get the following voltages

1 - grid 2 -3.5 volts from grid to cathode (can't measure this directly, meter loads it down too much)
2 - plate 2 +225v
3 - cathode 2 +75v
4 - grid 1 +225v
5 - plate 1 +300v
6 - cathode 1 +235v

Its not completely unreasonable, but the current is a little on the low side for a 6sn7. The inverter side is about 1.5ma and the cathode follower side is about 2.3ma. With a 6SN7 at those voltage levels closer to 10ma would be typical.

I didn't pass any AC through it but its not going to have any gain. Actually it will have slightly less than unity gain, so for every 1v in about 0.9v will come out as measured to ground.

I didn't have one to try, but this may fare better with a 6SL7.


Still not sure what to think about the input, I'm guessing it is actually direct coupled to a 12ax7 plate since thats the only thing that really makes any ind of sense here. If the plate of the 12ax7 is about 71-72 volts, that would work fine. Thats not a lot of overhead though.
 
gadget73 said:
ok so I banged together a test circuit for that inverter. I made the assumption that the B+ supply is 300 volts, mostly because that seemed like a reasonable enough number.

I rigged up a 6M pot to bias the grid since I had no other way of doing it.

With the pot set so that about 1/2 the plate voltage drops from plate to cathode and 1/4 across the plate and cathode resistor I get the following voltages

1 - grid 2 -3.5 volts from grid to cathode (can't measure this directly, meter loads it down too much)
2 - plate 2 +225v
3 - cathode 2 +75v
4 - grid 1 +225v
5 - plate 1 +300v
6 - cathode 1 +235v

Its not completely unreasonable, but the current is a little on the low side for a 6sn7. The inverter side is about 1.5ma and the cathode follower side is about 2.3ma. With a 6SN7 at those voltage levels closer to 10ma would be typical.

I didn't pass any AC through it but its not going to have any gain. Actually it will have slightly less than unity gain, so for every 1v in about 0.9v will come out as measured to ground.

I didn't have one to try, but this may fare better with a 6SL7.


Still not sure what to think about the input, I'm guessing it is actually direct coupled to a 12ax7 plate since thats the only thing that really makes any ind of sense here. If the plate of the 12ax7 is about 71-72 volts, that would work fine. Thats not a lot of overhead though.
Click to expand...

Wow ok you really went to town! I do have 6sl7’s and I can put a capacitor at the 12ax7 plate
 
hopefully one side of it is not. Tone controls are a mess but mostly need to know whats going on with whatever is directly connected to pin 1 of the 6sn7 to figure out if that makes any sense. There has to be a voltage amp stage somewhere. The expected setup would be 1/2 of the 12ax7 as the voltage amp and what feedback comes into. The other half can be tone control stuff.
 
On V2
pin 1 has to be missing a resistor from B+. Cap out to the tone control I can believe but its getting B+ from somewhere.

2 looks reasonable

3 also makes sense.

4 and 5 make sense if its in 12v mode.

so 6 is a plate, direct coupled to the 6sn7 grid. That makes sense. The 47k resistor is the plate load resistor, so that also fills in a blank. Its just connected at the 6sn7, though electrically it goes to the 12ax7.

Pin 7 doesn't quite make sense. Resistor from B+ must be to bias it, but with no resistor to ground its just going to be at B+. Normally when you do stuff like this it ties to a resistor in the cathode circuit so it self-biases. It can be done from B+ to make it fixed bias but the grid in and of itself doesn't draw any sort of current so it needs something to pull it down if its wired this way.

pin 8 going to the bass control doesn't really make sense either.

Feedback on 9 makes zero sense. Thats the center tap for the heater. A coupling cap from there to pin 7 is also not sensible. The only thing that should happen at pin 9 is either it ties to one side of the heater for 6v operation, or its left disconnected for 12v mode.

the resistor, cap to ground, and larger un-bypassed cap would make sense at pin 8. Tie the resistor at pin 7 to the junction. Coupling cap on 7 to the bass control would also make some kind of sense. I don't think a 47k plate resistor and a 400k un-bypassed resistor sound right but its at least a closer to sane configuration.


gonna go eat dinner, I'll think about V1 after
 
I should mention that when I say b+ I don’t mean directly from b+ but just high voltage. Most of the resistors are also out of spec as well.
 
Yeah i know, a voltage source. I'm just working on basic topology problems, the fine points can come later when its basically logical

I think what they meant to do is probably something like this.
 

Attachments

  • 20240617_195232.jpg
    20240617_195232.jpg
    59.1 KB · Views: 13
The first side is a split load inverter. In normal form one grid would feed from a plate, the other from a cathode. This one someone decided they wanted lower drive impedance, so its got a cathode follower direct coupled to the plate output. Output from a cathode follower is always off the cathode.

I don't know that there is much advantage to this but its probably not hurting anything either since its direct coupled.
 
gadget73 said:
The first side is a split load inverter. In normal form one grid would feed from a plate, the other from a cathode. This one someone decided they wanted lower drive impedance, so its got a cathode follower direct coupled to the plate output. Output from a cathode follower is always off the cathode.

I don't know that there is much advantage to this but its probably not hurting anything either since its direct coupled.
Click to expand...

So do you think it’s worth pursuing whatever this guy was after or just start over with a fresh pallet and schematic?
 
I took the liberty to rotate and crop the schematic. ;)

20240617_195232.jpg
 
Carbsandcaps said:
So do you think it’s worth pursuing whatever this guy was after or just start over with a fresh pallet and schematic?
Click to expand...
Ultimately, that's your decision. :thumbsup:

Maybe you could try a couple more things and decide where to go from there. Do you feel you understand this circuit design's philosophy a little better now? :idea:
 
If you shuffle the stuff around on that one side of the 12ax7 so it makes sense, I think it will function. How well is hard to say but wired properly with some voltage readings will give a decent clue about how good or bad it is.
 
Should, and if the tone controls are wired right you ought to be able to feed it in at the volume control


I will say that if that one resistor really is 400k, thats way off. 4.7k or so might make sense
 
gadget73 said:
Should, and if the tone controls are wired right you ought to be able to feed it in at the volume control


I will say that if that one resistor really is 400k, thats way off. 4.7k or so might make sense
Click to expand...

I was going by color stripes so I could be wrong. That sounds like an odd number anyway.
 
You must log in or register to reply here.
Back
Top Bottom