Magnovox Phono Console
- Thread starter Beatnik
- Start date
Don't give up.
If you removed the balance pot, you need to install the two 22K resistors shown in post 71. You need to "replace" the balance pot with something - that woudl be those resistors. They can be anywhere around 20K to 30K in value, 22 is commonly available.
I have played with several Magnavox amps. My suggestion would be to remove (but save) everything except the sockets and transformers. Check each piece with a meter, then reinstall following the 8601 schematic (8six01, not original 8three01). That is the simplest schematic to follow that uses the component values you have on hand.
I have restored a 8601 as a 8601 and have also rewired a 8601 into a RH84. The RH84 is a highly regarded amp using the same tubes -you can google the schematic. Either way, they can sound really good when you get things sorted.
Try to find the cash to order two EL84/6BQ5 tubes. The russian equivalents are available as 6P14P. A cheap but ok-sounding Sovtek can be found from on-line retailers for 10 to 12 bucks each. And there is nothing wrong with using cheap radio-shack chinese resistors. The expensive stuff sounds better, but not that much better. You can always upgrade later once you know that things are working properly.
If you removed the balance pot, you need to install the two 22K resistors shown in post 71. You need to "replace" the balance pot with something - that woudl be those resistors. They can be anywhere around 20K to 30K in value, 22 is commonly available.
I have played with several Magnavox amps. My suggestion would be to remove (but save) everything except the sockets and transformers. Check each piece with a meter, then reinstall following the 8601 schematic (8six01, not original 8three01). That is the simplest schematic to follow that uses the component values you have on hand.
I have restored a 8601 as a 8601 and have also rewired a 8601 into a RH84. The RH84 is a highly regarded amp using the same tubes -you can google the schematic. Either way, they can sound really good when you get things sorted.
Try to find the cash to order two EL84/6BQ5 tubes. The russian equivalents are available as 6P14P. A cheap but ok-sounding Sovtek can be found from on-line retailers for 10 to 12 bucks each. And there is nothing wrong with using cheap radio-shack chinese resistors. The expensive stuff sounds better, but not that much better. You can always upgrade later once you know that things are working properly.
janikphoto
Lunatic Member
Two things:
1.) it's weak, not week.
2.) these problems sound like resistors. From the experiences I've had, bad caps cause hums and oscillations, bad resistors cause your symptoms.
1.) it's weak, not week.
2.) these problems sound like resistors. From the experiences I've had, bad caps cause hums and oscillations, bad resistors cause your symptoms.
knockbill
Lunatic Member
That's too bad, take a breath and re think it... Should be a fine sounding little amp, SE 6BQ5s and 6EU7 driver is a pretty good design... listening to my RCA right now, similar set up,,, Maybe you can pass it forward...
Info for future projects,,, I usually replace PS caps, and any out of value resistors, and get V readings,,, then get it running properly in stock form before making mods and changes...
Hope you get back to it!
Regards,
John
Info for future projects,,, I usually replace PS caps, and any out of value resistors, and get V readings,,, then get it running properly in stock form before making mods and changes...
Hope you get back to it!
Regards,
John
Beatnik
What's this ?
I still need a pair of tubes, a 6EU7 socket and strip it down to the chassis and start from scratch.
I paid 20 for it, got 15 in caps on the way, spent 30 on various resistors switches, knobs, caps, bulbs, power cord, etc.
My voltage was 348 on pin 7 of the 6BQ5, says 235 on the schematic. I'm guessing because the house runs 123/124 more $$$
I really appreciate all the help, but I can't afford this for a little table amp.
I paid 20 for it, got 15 in caps on the way, spent 30 on various resistors switches, knobs, caps, bulbs, power cord, etc.
My voltage was 348 on pin 7 of the 6BQ5, says 235 on the schematic. I'm guessing because the house runs 123/124 more $$$
I really appreciate all the help, but I can't afford this for a little table amp.
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If one output tube is not there, its going to be weak. It relies on current flow through BOTH tubes to set the bias. Half the tubes will make it bias up wrong, and output will be feeble as a result. There is a single common cathode resistor on this. Also, without the load of both tubes, the voltages are going to be wrong. You need a full tube compliment for this to have any hope of working correctly.
cademan
Lunatic Member
Really!
Typo! DUH!
Please, you don't have to point out things like this! It pisses people off including me!
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Yes to the Variac startup.
Yes. It certainly can't hurt to use a variac. Put a meter on the AC input side of the amps power transformer. Run the variac up to half-normal-voltage. Your amp was designed for 117 vac. Run it at 58 or 59 vac. Take some readings and compare to the schematic. Stuff that should be at 6.3v should read 3.15, stuff that should 150v should be 75 etc, etc, etc. Its a quick check of the wiring. Then run it up to 117 and give it a listen. Its a good idea on any of these vintage amps to find a way of reducing the supply voltage a little. My house is at 124vac, most old equipment is designed to 117vac. I have employed unused 6.3v filament windings (that used to power tuners in now defunct consoles) and separate cheap 6.3v transformers to "buck" the supply voltage. It makes the 124 an apparent 118. But really, it doesn't hurt much to run at higher voltages on simple circuits like yours. Just be aware that the readings you get willl be a little higher than the values shown in the schematic. Filaments/heaters in tubes are rated at plus or minus 10 per cent on the voltage anyway.
Yes. It certainly can't hurt to use a variac. Put a meter on the AC input side of the amps power transformer. Run the variac up to half-normal-voltage. Your amp was designed for 117 vac. Run it at 58 or 59 vac. Take some readings and compare to the schematic. Stuff that should be at 6.3v should read 3.15, stuff that should 150v should be 75 etc, etc, etc. Its a quick check of the wiring. Then run it up to 117 and give it a listen. Its a good idea on any of these vintage amps to find a way of reducing the supply voltage a little. My house is at 124vac, most old equipment is designed to 117vac. I have employed unused 6.3v filament windings (that used to power tuners in now defunct consoles) and separate cheap 6.3v transformers to "buck" the supply voltage. It makes the 124 an apparent 118. But really, it doesn't hurt much to run at higher voltages on simple circuits like yours. Just be aware that the readings you get willl be a little higher than the values shown in the schematic. Filaments/heaters in tubes are rated at plus or minus 10 per cent on the voltage anyway.
Beatnik
What's this ?
I'll dig out the variac while I'm waiting for the tubes to show up. I'm glad those will work, I don't think I can get much cheaper than 5 each to work on this.
cademan, I did make up some 30k resistors last night and added them in place of the balance pot. Seemed to do away with the channel bleed. Hopefully the volume will come up with the pair of tubes.
cademan, I did make up some 30k resistors last night and added them in place of the balance pot. Seemed to do away with the channel bleed. Hopefully the volume will come up with the pair of tubes.
Tubes are actually easier to learn than solid state in that regard. Theres just less of it. Look at the schematic of say a 30 watt tube amp, and then a similar power solid state amp. THe parts count difference is tremendous. The 5 watters are even more simple since its usually got half the tube compliment. A little SE amp like you've got is probably the easiest place to learn how this stuff works.