• 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

Magnavox 8802 visits Kegger

On mine/Andy's because there was only a pot and it was originally wired this way
all heaters are from 1 set of heater windings, so each side of that heater winding
goes to the outer taps of the humm pot, then the junction of the 10ohm resistors
and the 330ohm resistor from just 1 pair(either pair) of outputs goes to the center
tap of the humm pot. (so it goes between the 10ohm resistors an 330ohm resistor).

And yes the 10ohm resistors on each cathode are for measuring the output current.

If you were to use 2 6.3v windings, say half of the amp on each winding then you'd
want a second humm pot and connect just like above using one output pair per pot.

-----------------------

To form a "virtual center tap" (if not having a pot or removing it)

You can also do away with those pots using 100ohm resistors from each side of the
heater windings an connect the output cathode to the center point of the resistors.

-----------

Just in case you or anyone else wanted to know, the reason for doing this is to float
the heaters on a small amount of DC voltage, to both help in humm, and because we
are direct coupling the input tube to the phase splitter we are raising the heater with
respect to cathode as to not violate the maximum spec allowed on the tube.
 
Ok....

So here's what I did. I put a 10 ohm 1 watt resistor between pin 8 and pin 1 (dummy pin) of each 6K6. Then I hooked each pin 1 together per pair of 6K6's. Then I ran the 330 ohm/330 uF cap from the shared pin 1 connection to ground. On the right channel I connected the wiper from the hum pot to the shared pin 1 connection. I then hooked the outside pot connectors to pin 2 and pin 7 respectively. Sound correct?

BTW why just one channel for the hum pot wiper? Is that to adjust the variable channel against the fixed channel?

32750c1c.jpg




I think I'm almost done except the RCAs, 5 way binding posts and the PS....:thmbsp:
 
You got it correct.

The voltage from 1 pair is just a voltage source, we could use a voltage source
from anywhere to send to that center wiper, that output cathode is convenient
is all, the reason for not both pairs is then we are tying all our 330ohm resistors
together as well and upsetting our bias scheme.

By the way what wattage is that 330ohm cathode resistor, it looks 1 or 2 watt?
(should be at least 2w, but more like the 5watter)
 
Almost done....

b1b09a11.jpg


Just need to hook up the speaker terminals and the 6BL8 filaments....:thmbsp:

Question for Kegger - can I hook up the filaments on the 6BL8's off the same tap as the 6K6s or should I use the spare 6.3V tap that's currently capped off.

And the connection is just a series loop in at pin 4 and out from pin 5, repeat on other channel, correct?

Oh and the 330 ohm cathode resistors are just 1/2 watt...
 
Couple things here.. (just for info and reference).. :)

I posted this earlier..
"Just in case you or anyone else wanted to know, the reason for doing this is to float
the heaters on a small amount of DC voltage, to both help in humm, and because we
are direct coupling the input tube to the phase splitter we are raising the heater with
respect to cathode as to not violate the maximum spec allowed on the tube."

I was trying to give info as to why we use the voltage reference, so we want heaters
on the 6BL8 to have a dc voltage on them, so yes you want them on the same as the
outputs, or you need to put the humm pot on the other 6.3v heater string an run that
to the 6BL8's. (were using a dc reference to elevate the heaters)

And no it is not an series heater string to them, you want 1 side of the heater hooked
to pin 4 on both tubes, then other side of that winding hooked to pin 5 on both tubes.
(all the tubes in the amp are on a parallel heater wiring) :thmbsp:

Also you asked previously what wattage the 330ohm resistor should be and I said a 5w
resistor should be fine, you'll be dissipating about 1w in that resistor, an 1/2w will burn
up in that spot, 1w will be at dissipation, so an 2w would work, but I say 5w or higher.

-----

You'll get it, just a bit of tweaking an doing more of it, only way to learn is by doing.. ;)
 
When you do hook up those speaker terminals don't forget to ground that neg terminal.
 
Kegger said:
Couple things here.. (just for info and reference).. :)

I posted this earlier..
"Just in case you or anyone else wanted to know, the reason for doing this is to float
the heaters on a small amount of DC voltage, to both help in humm, and because we
are direct coupling the input tube to the phase splitter we are raising the heater with
respect to cathode as to not violate the maximum spec allowed on the tube."

I was trying to give info as to why we use the voltage reference, so we want heaters
on the 6BL8 to have a dc voltage on them, so yes you want them on the same as the
outputs, or you need to put the humm pot on the other 6.3v heater string an run that
to the 6BL8's. (were using a dc reference to elevate the heaters)

And no it is not an series heater string to them, you want 1 side of the heater hooked
to pin 4 on both tubes, then other side of that winding hooked to pin 5 on both tubes.
(all the tubes in the amp are on a parallel heater wiring) :thmbsp:

Also you asked previously what wattage the 330ohm resistor should be and I said a 5w
resistor should be fine, you'll be dissipating about 1w in that resistor, an 1/2w will burn
up in that spot, 1w will be at dissipation, so an 2w would work, but I say 5w or higher.

-----

You'll get it, just a bit of tweaking an doing more of it, only way to learn is by doing.. ;)
Click to expand...


You're absolutely right, you did mention that specifically, I just brain farted. Guess that's what I get for not starting on this last night until 11 PM. I've got some 3W 330 ohm resistors, so I'll put those in, run the filament supply and wire up the speaker terminals. Should have it done this evening....:thmbsp:
 
Up and running....

Brought it by my buddy's shop for a little tweaking, and now she's running the Khorns beautifully. He said she's putting out 5 clean watts out of the 6K6's. Stuffed 4 matched frosted glass RCA's and a couple of Dutch 6BL8's in the sockets, and we're off to the races....:thmbsp:

Thanks for all the help everyone, especially the Kegmeister.....:thmbsp:

b87d5e41.jpg
 
Sweet, glad to hear you were able to get it all working, congrats!

I bet it does sound nice in that setup..
 
I just rolled mine back into the main rig and it's been turning my head all day.

That Kegger guy sure knows his stuff and does some darn nice work :D :thmbsp:
 
Ain't that the cuttist little face you ever saw.

I walked into my office yesterday and there stood Dave of Fantisy Football fame.
 
I've searched for the "triode strapped" curves for the 6GH8 to no avail.

Do you have a link for that data?

Thanks,

Scott

Kegger said:
The 6GH8's triode is beefier(higher transcon) for the "driving" then the 12at7.

But..
The reason started for using the 6GH8's was the mounds/piles of them laying
around, I mean you probably got stashes your tripping over, then I found the
"triode strapped" curves for the pentode section and decided they had a use.
(plus the 6BL8 is such a really really good sounding tube as well) :music:
Click to expand...
 
scitizen17 said:
I've searched for the "triode strapped" curves for the 6GH8 to no avail.

Do you have a link for that data?

Thanks,

Scott
Click to expand...

Thought I had saved them on my PC but I don't find them.

They came from Tom Schlangen's site I believe which seems to be longer available.
(he's on diyaudio you may be able to email him for them)
 
Thanks, I'll try to contact him.

Kegger said:
Thought I had saved them on my PC but I don't find them.

They came from Tom Schlangen's site I believe which seems to be longer available.
(he's on diyaudio you may be able to email him for them)
Click to expand...
 
OK. I'm curious now. I've got a chassis and some transformers here, that might well be begging for this circuit.

My one qualm, is that I have more B+ voltage. I'm probably going to wind up with about 340v- which I might be able to drop to 320 or so (using a 220 ohm power resistor- that's about the biggest one I'd be willing to stick in series). I'd PREFER to use it at 340v, since it'd be pretty tightly regulated that way.

Looks like that might be too much for the 6K6- but what would stop me from using 6V6s with the same driver circuit?

Though, if you do the math- 320v minus about 20v of cathode bias gives 300v plate-to-cathode. My outputs are higher-impedance (about 10K or so), so I could use a lower screen voltage (say, about the "standard" 280v, or maybe the 260v or so specified here), and that should more optimize the impedance match between output transformer and 6K6s...

Regards,
Gordon.
 
With that much voltage Gordon I would surely use 6V6 tubes, not so much because the
6K6 not taking it, but because the current would be so low to stay in dissipation ratings
that I'd prefer to use the bigger tube with some decent current.

But other then that yah the driver section is just fine for 6V6, I've built a couple an the
amps performed very very well, actually a 6V6 version is one of my favorite sounding of
the small push pull amps I've built, as it sounded better than my expectations could be. :)
 
The choice is between this, and some variation of the El Cheapo- since there has been quite a bit of discussion, about how well the 6AQ5/6V6 family "melds" with the 12AT7, in terms of distortion cancellation.

This would definitely be simpler- no CC source required. That may well be a deciding factor. Not much room in this chassis!!

Regards,
Gordon.
 
You must log in or register to reply here.
Back
Top Bottom