Brand new to the forums, hope this is the right place for my question. I have my parents 45-50yr old Magnavox console. It has the 8608-10 tube amp in it & the radio & turntable play - so I assume it's in working order. Been reading up a lot about caps drying out & tubes needing replacing. Too many posts and options. Can anyone point me to a good starting point for these amps....Thanks
The best source of info is the yahoo magnavox group.
otherwise it is a matter of searching the web for a schematic as a starting point.
While it may play, the front left aluminum cylinder is the main filter cap. It should be replaced as a safety precaution.
First is adding a three prong safety cord with ground to the chassis.
From this side it looks like a stereo 6BQ5 single end amp. what are the tubes?
These are nice little amps.
otherwise it is a matter of searching the web for a schematic as a starting point.
While it may play, the front left aluminum cylinder is the main filter cap. It should be replaced as a safety precaution.
First is adding a three prong safety cord with ground to the chassis.
From this side it looks like a stereo 6BQ5 single end amp. what are the tubes?
These are nice little amps.
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I'm guessing a 12AX7 and 2X 6BQ5/EL84s. Looks like the B+ rectification is SS of some kind, on the underside. 5 WPC is about right. The multi-wire umbilical is probably carrying power to and/or from the tuner. Unfortunately, the phono cartridge rates to be piezoelectric and anything but good. The turntable is probably no better, sorry. Look for a place where the turntable is connected to the tuner. You could pop those RCA males out and plug a CDP in there.
IMO, the I/P RCA jacks need to be replaced with something of higher quality that's spaced for modern "fat" interconnect cables. Mouser part numbers 568-NF2D-B-9 and 568-NF2D-B-2 (1 each) will do that quite nicely.
IMO, the I/P RCA jacks need to be replaced with something of higher quality that's spaced for modern "fat" interconnect cables. Mouser part numbers 568-NF2D-B-9 and 568-NF2D-B-2 (1 each) will do that quite nicely.
Tube in the back next to the power transformer is probably a rectifier. So not SS but tube rectification.
Single stage 12AX7 is about right for a decent SE amp with 6BQ5 output. Realistically about 3W.
That said, 3-5 W is a nice SE amp for driving from a computer, MP3 player etc for office or desktop usage.
Power isn't everything.
My grand-daughter and wife are happy with 3.5W.
Those old Magnavox amps are nice starter systems.
What are the speakers I see in teh pix?
Single stage 12AX7 is about right for a decent SE amp with 6BQ5 output. Realistically about 3W.
That said, 3-5 W is a nice SE amp for driving from a computer, MP3 player etc for office or desktop usage.
Power isn't everything.
My grand-daughter and wife are happy with 3.5W.
Those old Magnavox amps are nice starter systems.
What are the speakers I see in teh pix?
Thanks for the info guys....this is part of a console my parents had while I was growing up. They didn't want it - I did. The rectifier shows 6CA4, then two taller tubes show 6BQ5, then a smaller one in front of the filter cap shows 6EU7. The console is in good shape, but I have a vintage pair of Chorus II that would really like this amp I bet. @Eli - do I need to find it coming from the tuner because there is a pre-amp involved? Sorry - I'm not versed in the way these amps work.
If I had one looking that good, I'd keep it stock and enjoy it.
6EU7 is essentially a 6 volt 12AX7 with a different pin out. It was spec'd as a high grade low hum audio amp tube.
Is that a schematic in the lower left, or just a wiring diagram?
That may use the same schematic as the 83XX series for the amp.
6EU7 is essentially a 6 volt 12AX7 with a different pin out. It was spec'd as a high grade low hum audio amp tube.
Is that a schematic in the lower left, or just a wiring diagram?
That may use the same schematic as the 83XX series for the amp.
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The 6CA4/EZ81 is good for up to 150 mA. of B+. The 2X 6BQ5/EL84s draw a total of approx. 80 mA., operating in Class "A", as they must. It's almost certain that the power amp is providing B+ for the tuner/control section.
The triodes inside a 6EU7 are identical to those in a 12AX7. Only the basing is different. It's easy enough to rewire that socket and install the very nice, current production, Sovtek 12AX7LPS. Like the 6EU7, the 'LPS has a spiral wound, hum bucking, heater.
It is possible to use the power amp in combination with a CDP, without locating the phono I/Ps on the control section. You will need additional volume control capability. Pop the signal (RCA) cables from the control section out of the power amp and plug the cabling associated with the CDP in. A portable CDP, with its built in volume control, is plug and play. To use a full sized digital player, with it's better sound, construct a passive control box around a ganged, stereo, 10 KOhm log. taper volume potentiometer. Mouser part # 313-2420F-10K is dirt cheap and pretty good sounding. DigiKey part # KKA1031S28-ND is not especially costly and sounds VERY good. Use short, "captive", low capacitance, cabling between the volume control box and the power amp. Low capacitance means unshielded and braided.
BTW, look at the piece of clear plastic where the cheapy dipole antenna stapled to the console's frame connects to the 300 Ω twin lead from the tuner section. That point can be used to connect a high performance antenna and improve the utility of the tuner.
The triodes inside a 6EU7 are identical to those in a 12AX7. Only the basing is different. It's easy enough to rewire that socket and install the very nice, current production, Sovtek 12AX7LPS. Like the 6EU7, the 'LPS has a spiral wound, hum bucking, heater.
It is possible to use the power amp in combination with a CDP, without locating the phono I/Ps on the control section. You will need additional volume control capability. Pop the signal (RCA) cables from the control section out of the power amp and plug the cabling associated with the CDP in. A portable CDP, with its built in volume control, is plug and play. To use a full sized digital player, with it's better sound, construct a passive control box around a ganged, stereo, 10 KOhm log. taper volume potentiometer. Mouser part # 313-2420F-10K is dirt cheap and pretty good sounding. DigiKey part # KKA1031S28-ND is not especially costly and sounds VERY good. Use short, "captive", low capacitance, cabling between the volume control box and the power amp. Low capacitance means unshielded and braided.
BTW, look at the piece of clear plastic where the cheapy dipole antenna stapled to the console's frame connects to the 300 Ω twin lead from the tuner section. That point can be used to connect a high performance antenna and improve the utility of the tuner.
1. Install a fuse and 3 prong power cord to bring the unit up to modern safety standards.
2. Replace C1. You won't find a direct replacement as the cathode cap (30uF) is included and no one makes one like it now days.
So replace the 30uF cathode bypass cap on the output tubes with a separate cap. I recommend 330uF to 470uF at 35V or 50V. The larger capacitance will slightly improve LF response.
For the filter cap you can get a 40/20/20/20 @ 500V JJ from Antique Electronic supply. Tie two 20s together to get a 40 and you are set. The biggest issue with replacement caps is fitment. They won't plug in the slot directly. You need to open up the hole slightly, and use a clamp to hold the cap.
Replace C7 and C10 with either PP (Polypropylene) or PIO (Paper In Oil, inexpensive soviet units from ebay). They need to be rated at 300V or better.
C6 and C9 are feedback caps. I'd replace them with PP caps. They can be low voltage as they are coupling the output to the cathode of the input stage and no high voltage is present.
Measure all the resistors and replace any that are over 5% out of spec.
2. Replace C1. You won't find a direct replacement as the cathode cap (30uF) is included and no one makes one like it now days.
So replace the 30uF cathode bypass cap on the output tubes with a separate cap. I recommend 330uF to 470uF at 35V or 50V. The larger capacitance will slightly improve LF response.
For the filter cap you can get a 40/20/20/20 @ 500V JJ from Antique Electronic supply. Tie two 20s together to get a 40 and you are set. The biggest issue with replacement caps is fitment. They won't plug in the slot directly. You need to open up the hole slightly, and use a clamp to hold the cap.
Replace C7 and C10 with either PP (Polypropylene) or PIO (Paper In Oil, inexpensive soviet units from ebay). They need to be rated at 300V or better.
C6 and C9 are feedback caps. I'd replace them with PP caps. They can be low voltage as they are coupling the output to the cathode of the input stage and no high voltage is present.
Measure all the resistors and replace any that are over 5% out of spec.
Thanks so much for the info, I appreciate your knowledge. I have a degree in electronics - but I don't use it daily (I fly helicopters), so....not sure I'm capable of all that fabrication. Replacing caps, tubes, checking resistors - I can handle that. Is there a tutorial handy explaining rewiring for the 12AX7?
AFAIK, no rewiring tutorial is available, but it's not at all difficult. Just review the data sheets. The pins are counted clockwise, while viewing from the tubes' bottoms. I can give a hint about running a 'X7 on "6" V. heater power. Bridge pins 4 and 5. Apply the "6" V. heater power to the 4/5 bridge and pin 9.
The 6EU7 data sheet is here and the 12AX7 data sheet is there.
Constructing a passive volume control setup is easy. Start with an aluminum "bud box". Five holes get drilled: 1 for the ganged potentiometer, 2 for the I/P RCA females, and 2 into which grommets are inserted. The O/P cabling is passed through the grommets and soldered to the potentiometer, which makes them "captive". Strain relief is accomplished inside the box by mounting a short length of doweling on each cable using a figure 8 knot.
The 6EU7 data sheet is here and the 12AX7 data sheet is there.
Constructing a passive volume control setup is easy. Start with an aluminum "bud box". Five holes get drilled: 1 for the ganged potentiometer, 2 for the I/P RCA females, and 2 into which grommets are inserted. The O/P cabling is passed through the grommets and soldered to the potentiometer, which makes them "captive". Strain relief is accomplished inside the box by mounting a short length of doweling on each cable using a figure 8 knot.
Some mods may be necessary for stand-alone use of the amp - as I recall, there is some EQ in this to compensate for the tone controls. I'd leave the 6EU7 in place (probably a Mullard). They aren't that hard to find anyway (at least in MY basement...). I think I have the schematic for the amp, maybe tuner too - what's the chassis number (CR-xxx)? email tbavis(at)rochester(dot)rr(dot)com.
I agree Doz. That is the 6BQ5 for one channel. They are inexpensive and currently in production.
What Doz noticed is the white film on the top of the tube. Compare to the other 6BQ5 and it is black and reflective (almost silvery). This is the "Getter", which is a vapor deposited reactive metal (often barium) used to scavenge air that gets into the tube via the seals for the leads, etc. Over time tubes can go bad just setting.
It is best not to operate the amp until the tube is replaced.
What Doz noticed is the white film on the top of the tube. Compare to the other 6BQ5 and it is black and reflective (almost silvery). This is the "Getter", which is a vapor deposited reactive metal (often barium) used to scavenge air that gets into the tube via the seals for the leads, etc. Over time tubes can go bad just setting.
It is best not to operate the amp until the tube is replaced.
There is a schematic taped to the inside opposite the amp, I'll get a picture of it later today. Thanks for telling me about the white tube being bad & not using it. I haven't been playing it for extended periods...just showing it off really. That will cease now till I replace the tubes, caps, resistors out of tolerance, and go to a 3-prong power cord. Do I just ground it to the chassis of the amp itself? Also thinking about putting an in-line switch on the phono inputs to allow the iPod.
Both 6BQ5/EL84s should be replaced, in tandem. Save the "good" specimen in your possession for use as an emergency spare.
I suggest you buy a couple of JJs, as they are reasonably tough, have pretty good "tone", and are modestly priced, from Jim McShane or other reputable vendor.
Rectifier tubes tend to have long services lives. A Mullard made 6CA4/EZ81 can easily work hard for 20 years. So, get your rectifier tested. If it's good, as can be the case, use it until it dies.
I suggest you buy a couple of JJs, as they are reasonably tough, have pretty good "tone", and are modestly priced, from Jim McShane or other reputable vendor.
Rectifier tubes tend to have long services lives. A Mullard made 6CA4/EZ81 can easily work hard for 20 years.
So, get your rectifier tested. If it's good, as can be the case, use it until it dies.What I thought was a schematic was only a wiring flow chart....no real help there. Thanks for the link Eli, that's more than a reasonable price. I haven't had a chance to take this out & get underneath to check resistors...but that will be next. Tom (or any else), if you have the schematic specific for the 8608-10...greatly appreciated. Thanks
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