• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Vintage Magnavox tube amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Ok man this is really hard for a newbie , Don't forget I do get sound and sounds darn nice allot better then my B&K st120 .But again, do I need to replace them if there all good ? Do u thing the noise is coming from the big silver cap? If It needs to be replaced is there anything I should know ? Right now Im running it with a tube pre but it only has a ten db gain and JBL LX44 witch Im trying to sell, I would like to find a another small pair of Snells but I do need something with high db like 95,98or more .
 
Last edited:
.But again, do I need to replace them if there all good ? Do u thing the noise is coming from the big silver cap? If It needs to be replaced is there anything I should know ?
Yes the silver cap needs replaced. Look up cap explosions on google, and think that the slime it throws everywhere is corrosive. 10-12 years max for cans, in my 50 years of tube experience. Don't buy a can cap if you don't want to do it again, buy 3000 hour up rated radial lead caps. Do buy a can cap if you have $30 to burn and plan to sell it next year anyway. Yes there is something you need to know before you replace the can- read post #7, and read High Voltage for newbies above in the sticky section. No, I don't think it all needs replaced. The resistors don't look burnt as far as I can tell. I replace tubes only after I have measured the output and proved it is bad. My 1968 organ has 22 of 23 tubes original. My 1969 organ had 3 tubes replaced by the sleazy organ serviceman that was too lazy to replace the can caps at 35 years, and replaced some tubes to get a little more volume. The 1968 organ had a cap short and cause a wiring fire in 1997, which is when the caps should have been replaced by lazybones in the identical 69 organ owned by a church. No, you don't need to put fuses on the output transformers. Tubes can take a shorted speaker for minutes. (Unlike cheap transistor amps that go PHHHHT immediately when a speaker wire is shorted). The 6EU7 tubes are signal tubes, and typically those don't go bad much if the unit is not carried around like a guitar amp. The 5u4 was already replaced, that is the most likely one to need it; the 6BQ5 need replaced at 5000-10000 hours life, or when power out goes down. Most signal tubes in my PAS2, and ST70 are 50 years old, and the signal tubes in my organs are all original at 42+ years.
The black caps (plate-grid couplers) are the ones I didn't replace on my organs at all or on my ST70 until 2010. Those were black, and GI brand. So there is the second kind of 600V cap you don't have to worry about much.
Have fun. Glad you like the sound.
 
Last edited:
OK so 6BQ5 are the output tubes and they will put out less power with age? The the silver can cap is the one that I need to replace, and Yes I plan to keep it! Thats where Im confused . Dont put the ?same type in the amp? Do you think the noise is coming from that cap? Last night I turned the amp on and there went the noise then powered down , then back up and and listen to all night unless I run wide open , I think? Sometimes in one or the other speaker . Ok and now back to the knob with hum next to it, Is is something I can remove because when I turn it I dont here anything different! What do u think it was for ? One more thing The main power supply , I put a fuse in the line, is there a wright and wrong way to wire that? Should it be wired in the line with a resistor at the switch or the other just between the power supply itself? Im sorry Iv always wonted some to gear but man ... Thanks for your expertise
 

Attachments

  • PB300252.jpg
    PB300252.jpg
    511.2 KB · Views: 458
The fuse should be the first thing the "hot" wire sees when it enters the chassis.

6BQ5 are the output tubes and will wear out with USE not AGE.

Replace the silver can cap. If it's not already gone, it will be soon. Replace with individual caps underneath, leave it place or throw it out, it's worthless. This is usually the part that makes old radios sound like old radios, replace it, they make noise if not, or much worse.

Leave the hum balance for now, there is a mod you can do to remove it, I forget were it is at the moment. Worry about the other stuff first. It's hard to put the smoke back in.
 
I can't read the schematic to see the actual can cap values, it is too small even blown up. But here are some typical long life radial caps available in NC, USA today. This is a 22 uf 450 V 10000 hour cap PANASONIC|EEUED2W220S|CAPACITOR ALUM ELEC, 22UF, 450V | Newark.com this is a 33 uf 450v 10000 hour cap RUBYCON|450TXW33MEFC12.5X30|CAPACITOR ALUM ELEC, 33UF, 450V | Newark.com and here is a 2000 hour 40 uf 450v cap. MULTICOMP|MCAX450V406M18X36|CAPACITOR ALUM ELEC 40UF, 450V | Newark.com You can look at these on newark.com, click passive components, click capacitors, click aluminum electrolytic. Click your desired size and minimum voltage. Mouser.com is similar but they don't have the life rating in the table, you have to download the file from the manufacturer. Most of the brands have cheap and long life caps, it is up to you to choose. As so see here, none of these is over $2 each, so choosing long life is not an expensive option. You have to install these on solder terminal strips like from tubesandmore.com. Their website doesn't have links like this so you will have to look it up for yourself. Tubesandmore does have 20' hanks of 600v rated plastic insulated wire at attractive prices, which is handy for extending the wiring to reach your replacement caps if the old ones are too short. I screw the terminal strips to holes in the chassis with #6 machine screws. I plug the old can hole with steel mesh, some junk I found laying by the side of the road and cut up in squares with tin snips.
I use a 130 watt pistol grip soldering gun to get the can tabs out, and use a 25 W WP25 iron to put the wires back. Use safety glasses, solder splashes coming off. Measure the cap at safe voltage before touching any metal under the deck.
I pay $7 UPS surface NC to IN, but if you live in a residence you might save money USPS priority shipping. I don't pay residence charge because this house used to have a body shop in the back. If I get the order in by noon sometimes I get the order the next afternoon, surface UPS.
 
Last edited:
Man thats great I really do love my amp but have not done to much mods, being its confusing to me. I did have a pair of Kllls there where the pettiest sounding and looking speakers . I should have never sold them! I would like to listen to my amp but with the noes ones in wile Im afraid to. Now Im using a pair of JBL lx44 and Jolida jd-5t pre amp.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.