Magnavox 8802 visits Kegger
- Thread starter Andyman
- Start date
jaymanaa
RIP 1961-2018
Schematic is on page one, 7th post..
How do I make the pages go backwards.:scratch2: I'd like to hear your guys impression of this same amp with a couple SS diodes clip leaded across that 6CA4 socket (and the tube pulled of course). The 6CA4 is a mighty little rect., but I wonder if the SS diodes would improve the bass any? Might also put the voltages more in favor of the 6V6 too, while lightening the PT load even further. Just random thoughts.
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This build looks interesting...
Got a "new" 8802 amp in the back of the truck, and stacks of 6BL8's/6GH8s/6K6s to play with. I got a few Kegger questions.....
1) I don't see the volume/gain pot in the schematic. Am I missing it?
2) It looks to me as if you're using different wattage resistors in the front end vs. the output stage. Is there a reason for this, or were you just using the parts you had on hand? Is there a minimum resistor wattage value that works thoughout, other than the specialized locations? 1/2 watt?
3) What wattage is the big 330 Ohm cathode bias resistor? How about the 10 Ohm resistors in front of the cap/resistor bias pair? I assume you're using those for current measurements?
4) Are you using any other voltage taps off the PS other than the 275 B+ leads? Is one of the tied off set of leads from the PT another 6.3V source that could be used to lower line voltage?
5) Do you have to use CL-80's on the AC line in? I think I have CL-70's in my parts box, but not 80's....
6) Where do you like to source your oddball valued resistors from? I don't know if I have any 510 or 390 ohm resistors in the ol parts box....:scratch2:
Thanks....:thmbsp:
Got a "new" 8802 amp in the back of the truck, and stacks of 6BL8's/6GH8s/6K6s to play with. I got a few Kegger questions.....
1) I don't see the volume/gain pot in the schematic. Am I missing it?
2) It looks to me as if you're using different wattage resistors in the front end vs. the output stage. Is there a reason for this, or were you just using the parts you had on hand? Is there a minimum resistor wattage value that works thoughout, other than the specialized locations? 1/2 watt?
3) What wattage is the big 330 Ohm cathode bias resistor? How about the 10 Ohm resistors in front of the cap/resistor bias pair? I assume you're using those for current measurements?
4) Are you using any other voltage taps off the PS other than the 275 B+ leads? Is one of the tied off set of leads from the PT another 6.3V source that could be used to lower line voltage?
5) Do you have to use CL-80's on the AC line in? I think I have CL-70's in my parts box, but not 80's....
6) Where do you like to source your oddball valued resistors from? I don't know if I have any 510 or 390 ohm resistors in the ol parts box....:scratch2:
Thanks....:thmbsp:
Kegger
R.I.P. 1/12/1966 - 6/1/2017
It's not a volume pot, it's the original humm balance pot. (connected like original)
The smaller wattage resistors you see are the grid resistors, no current or voltage
on those so 1/2w or even 1/4w is fine, any of the resistors that have "voltage" on
them I tend to use at least 1w there, the 10k and 1k on the amp schem 1w there,
the 51k 2w, the 22k 1w, the 330ohm output cathode 5w is fine, all the rest there
1/2w, the 2 50ohm in the supply 7w is more then enough.
And yes the 10ohm are for measuring current, and 1/2w is fine there.
No no other taps, that tied off 1 is 6.3v, don't need to use it to lower voltage with
the thermistors in there(you wouldn't do both) an the CL-70 should be fine as well.
Actually 510 an 390 are "standard" value resistors, should be able to get them any
place you get the rest of your resistors from, 390 is usually pretty plentiful for me.
(the 510 could be as small as 100ohm, I like CC in that spot and I have 510's here)
Anything else..
The smaller wattage resistors you see are the grid resistors, no current or voltage
on those so 1/2w or even 1/4w is fine, any of the resistors that have "voltage" on
them I tend to use at least 1w there, the 10k and 1k on the amp schem 1w there,
the 51k 2w, the 22k 1w, the 330ohm output cathode 5w is fine, all the rest there
1/2w, the 2 50ohm in the supply 7w is more then enough.
And yes the 10ohm are for measuring current, and 1/2w is fine there.
No no other taps, that tied off 1 is 6.3v, don't need to use it to lower voltage with
the thermistors in there(you wouldn't do both) an the CL-70 should be fine as well.
Actually 510 an 390 are "standard" value resistors, should be able to get them any
place you get the rest of your resistors from, 390 is usually pretty plentiful for me.
(the 510 could be as small as 100ohm, I like CC in that spot and I have 510's here)
Anything else..
Thank you sir!
Just need to get started....I need to see if I can come up with 4 reasonably close 6k6's out of the pile....:thmbsp:
Just need to get started....I need to see if I can come up with 4 reasonably close 6k6's out of the pile....:thmbsp:
cubby01
aka Buck
Got a question about thermistors and their use here. I know a little but not enough. I know they increase resistance when the current goes up so they can work as an inrush limiter. Is that the purpose here? I don't see a fuse on the mains side. What happens if something on the secondary side goes terribly wrong, if you had a fuse it would blow, what would happen to the thermistors. Does resistance go up to a point of failure? And would that be enough time for more damage than if a fuse?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Thermistors have a high resistance when cold, that is why they work as
inrush limiters for first turn on, as they heat up there resistance goes to
a point of just a few ohms, so when warm only drop a "few" volts across
them, in the case of vintage power transformers tends to be just enough
to drop that wall voltage down to what the transformer was designed for.
(so you kill 2 birds with 1 stone)
inrush limiters for first turn on, as they heat up there resistance goes to
a point of just a few ohms, so when warm only drop a "few" volts across
them, in the case of vintage power transformers tends to be just enough
to drop that wall voltage down to what the transformer was designed for.
(so you kill 2 birds with 1 stone)
Anything else..
Actually, yes. Voltage rating on the 330 uF bias cap. I have a couple of 100V 330 uF caps. can I safely assume those will do?
Also, I can come up with about 200 uF at C3 in the PS. Is that good enough?
Now I gotta go through the pinout of the 6BL8 to make sure I don't F up the connections.....
Thanks.....:thmbsp:
Off to the races....
Well the 6BL8 pinout is so different from the 6EU7, that really the best way to get started was to completely tear down the 9 pin sockets.
Went quicker than I thought.....
Well the 6BL8 pinout is so different from the 6EU7, that really the best way to get started was to completely tear down the 9 pin sockets.
Went quicker than I thought.....
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Actually you can tell by going back to the schematic, look at that voltage
listed at the cathodes, 17.5v, so anything say 25v or higher is good there.
200uf is good enough for C3 as well..
Yah stripping the socket is best way to proceed, looks like yur on yur way.
No way I can match your work, but I think since I'm doing it, I'll document the restore and mod job in this thread,
Already have found 4 RCA frosted glass 6K6GT that test new and match within 10%....:thmbsp:
Well the 6BL8 pinout is so different from the 6EU7, that really the best way to get started was to completely tear down the 9 pin sockets.
Went quicker than I thought.....
Suggestion: I'd replace those 9-pin sockets with something newer. The dielectric (wafer phenolic) is quite possibly 'leaky' given the age of the amplifier and the pin contacts are bound to be loose. Get some good ceramics or better yet a set of Beltons in there - it's cheap insurance and you'll be happy at how tight the Beltons grip the tubes.
-D
Slow, like any other thing in my life....
I decided to raise the bar one notch higher this time, and paint the chassis and transformers. I also bought some gold plated ceramic sockets for the front end tubes.
These aren't great pix since all I had was my cell phone, but hopefully they convey the general idea....
I found this black hammer tone paint with copper specks that I really liked, so I gave it a try. Here's the close up as it is....
Lastly, a picture of the more or less finished product with the iron added. I think I'm going to modify the PS transformer to eliminate the purple sheen I put on the top plate and go with 100% of the same color as the chassis. Otherwise, I'm pretty happy with it.....
Once the cosmetics are done, I'm back to the rewire....:thmbsp:
I decided to raise the bar one notch higher this time, and paint the chassis and transformers. I also bought some gold plated ceramic sockets for the front end tubes.
These aren't great pix since all I had was my cell phone, but hopefully they convey the general idea....
I found this black hammer tone paint with copper specks that I really liked, so I gave it a try. Here's the close up as it is....
Lastly, a picture of the more or less finished product with the iron added. I think I'm going to modify the PS transformer to eliminate the purple sheen I put on the top plate and go with 100% of the same color as the chassis. Otherwise, I'm pretty happy with it.....
Once the cosmetics are done, I'm back to the rewire....:thmbsp: