• 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

First tube amp purchase... what have I done?

lattiboy

lattiboy

Well-Known Member
Hi all,

I am going to have a lot of free time over the next few weeks as the Mrs is starting a new job and I have some vacation to burn. In preparing for this, I decided I wanted to check out an old tube amp to possibly rebuild or just toy around with.

Rather than researching and educating myself in these matters, I made an impulse buy and picked up this guy today. I kind of figured for under $70 with shipping I could live with the mistake.

It appears to be a late-50s pull from (I'm guessing) a portable record player or console radio system.

I have access to a variac and have read this super helpful post by Thatch_Ear, so I don't think I will blow off my fingers or burn down my house.

Was this a poor first purchase for somebody with an interest in tubes and literally no skill?

Thanks!
 
Register to hide this ad
You did really well on that deal I'm sure as It looks like it came from a stereo console (and has a big PS transformer) and is likely SE by the number of tubes. Some cheap amps didn't have a PS transformer so you lucked out there.

The 2 output tubes are likely EL84/6BQ5 , but could be PP with rare combination driver & output tube combined in one envelope.

Maybe it won't be hard to find a schematic and it should be recapped which you will find easy on that small amp. Looks like it's similar to the popular Magnavox except it has the tone controls on it.

The 9 pin output tubes max out at maybe 4W/CH in SE for use with pretty efficient speakers necessary. Many favor SE sonics.
 
Last edited:
So it's an RCA and has the mentioned output tubes. Should sound nice!

Can cap could be left in place (unhooked) for looks and to save $ individual caps could be used and strapped under chassis etc paying attention to polarity.

Another option is to re-stuff the can capacitor as some do. or get a new one made by hayseed hamfest. Coupling caps to output tubes and bias caps would have to be changed for sure.

Recapping ensures many more years of use as caps dry out and can short ruining power or output transformers and sometimes a tube(s).

Taking lots of detailed pics and doing one at a time saves mixups.

Sure one can use the variac & dim bulb tester first to see what you've got.
 
I had one just like it. It is a stereo amp, single ended 6BQ5. Good for about 4-6 watts per channel. Sounds pretty good with efficient speakers.

Not an RCA but Symphonic. Symphonic was its own brand.

Uses a 6CA4 rectifier, 12AX7 AF amp, and one 6BQ5 output each speaker.
 
Last edited:
Hi , nice clean amp. It does look like it came from a console stereo and is a single ended amp where each power tube is for one channel. So, you need to decide how much of the amp you want to keep as is and how much you want to add on to make it work in your system. For example the outputs to the speakers are those round plugs with the 3 wires. If you have the pair of those plugs you could probably add speaker wires to the plugs and make it work for the amp. Or, you could put in some modern 5 way binding posts on the back panel . the input jacks need to be added as well since they are none that i see. But, before that you need to know if the amp is working specifically the power trans and the output trannys. So , you can do that check with the amp as is with the variac. Ideally, you would want to use a SS rectifier to replace the tube rectifier just so you can start at a low AC voltage like 5vac so as to reform the electrolytic cap in the power supply. Which is the tall round can next to the power tx. If you are going to replace that cap then you can use the tube rectifier and start the variac at 40vac which should start the tube conducting after about a minute or 2. With your DMM on ACV check the 2 red wires coming out the PT that connects to the rectifier socket. There should high voltage probably over 100v as high as 200vac. There will be one wire going to the ps cap. Switch your dmm to DCV and check the voltage there. It should be at least as high as the AC volts nearly twice as much. If you are getting reasonable voltages you can check the dcv at the power tube sockets. Not knowing what kind of power tubes you have i will guess el84. If that is true then pin 7 which is the plate or anode of the tube is connected to the output transformer. You should be reading a slightly lower dcv at that point. At this time the tubes should be heating up and the voltages will start to drop as the current load is increasing. This procedure will tell you if you should go on with your rebuild or start looking for some new trannys. cheers
 
Nah, I wouldn't worry about all that ^. As cheap as filter capacitors are, just replace the can and the coupling capacitors and you're good to go.
 
Just one tip--take lots of pics of the underside and every wiring connection before you disconnect anything. That will help you to remember what goes where when you replace parts.

As century tek says just replace the electrolytic filter capacitors in the can (with similar values and voltages; these are polarised, so make sure you get the positive and negative connections right) and the coupling caps (e.g. with orange drops; again similar values and voltages; these are not polarised, so no positive and neg to worry about) and you're away. Unless it's been abused, it should work fine. Tube amps are tough.

Other problems may need sorting out later, but this will get you started.

p.s. I've never baked a tube amp in my life. Not many cakes either.
 
I think I'm with Dandy on this one. I also have never baked an amp. I'm not saying it never happens, but with all the amps I've had run through my fingers, maybe one that I can remember, had a shorted power transformer but it was already shorted when I got ahold of it.

Most of these old console or suitcase record player tube amps are pretty much bullet proof. The transformers, resistors, capacitors and tubes are readily available on the internet so it's not like you can't repair it if something smokes. Power transformers are easy to find for that particular amp, and if all else fails, even I have spare output transformers for ya! :thumbsup:

But........always be safe when powering up for the first time in who knows how long. If you are a novice and don't have the right equipment such as a meter, scope, variac, or dimbulb tester, just watch for anything out of the ordinary for the first few seconds. If nothing starts smoking, its probably ok to test temporarily with a pair of speakers. Then think about replacing capacitors, resistors, etc., to make it safe and reliable in the long run.
 
Last edited:
Thanks so much for the input!

So, the consensus is to replace the coupling capacitors, hook it up the variac, bring it up slowly and see if anything smokes or smells? After that, throw on my cheapest speakers and see if sounds emerge?

My only concern is wiring in an input, but that seems to be pretty basic. Where is the best place to grab the capacitors?
 
HI good score imho. but you know this is the start of a sickness called vacuum tube ampites and the is no know cure.lol

just be careful while poking around in there a lot of high voltage in there and the caps store it too.

old trick is to stick one hand in your pocket as you work on it. it harder for electricity to pass through your Heart if you get your hand where it does not belong.

good luck and enjoy.

Don
 
You must connect your cheapest speakers prior to wake up with a variac, not playing amplifier with no load, or you may damage output transformers..
 
One thing on this amp to be very careful about. The white powder looking substance on the chassis may have some component of cadmium in it.
You can wipe it off with stuff like Clorox wipes, but please wear rubber gloves. If you touch the stuff wash your hand before touching food , etc.
don't breathe in the dust, best bet is to take it outside to clean it. Normally cadmium oxidizing producing a yellow or green looking "fuzz" on the chassis.
it may be in there so please be careful. you can look up cadmium poisoning on wikepedia to read about it
 
As century tek says just replace the electrolytic filter capacitors in the can (with similar values and voltages; these are polarised said:
Y'all talk like changing the power supply and coupling caps is as easy as going to the 7 !! and picking up a 6 pack. Unless you know the history of the amp then why take chances of doing all the work you recommend and then end up sucking your thumb when nothing works. It only takes less than 10 minutes to check if the power and output trannies are working. If one doesn't check these most basic elements first then you are not working very methodically.
Click to expand...
 
It's just a cheapo amp. I think the Magnavox 8601 (and other SE 6BQ5 stereo amps) is similiar. Even if no one knows the history of the amp, it's still not really worth spending a whole lotta time ramping up voltages to see if the electrolytics are any good. I myself would normally just plug it in and see what happens after the first few seconds as I know what I am doing and what to look for. In my experience, 99 out of 100 are going to work ok for general testing purposes and usually that is what I would do for the first few seconds or a couple of minutes.

Ok, so quick and simple testing should be in order. I believe that lattiboy says he has access to a variac but not a volt meter. So get a cheapie one from rat shack or Harbor Freight. Test the power transformer and output transformers without the tubes in it. Should only take a couple of minutes.

If a novice doesn't know what to expect or what to watch for, then it is highly recommended to simply put in new capacitors. It looks like the amp still has all of its original capacitors. We all know by now (most of us anyway), and as old as it is, it needs to have them replaced anyways. So, 3 or 4 in the can, and a couple of coupling caps? 6 capacitors can be easily had for under $20.00 at places like AES, Just Radios, and other such places on the web.

I think we all know this by now especially if we have been working on them for over 30 years like me and lots of other members here. There are soooo many threads about capacitor replacement on original equipment over 40 or 50 or more years old, that it isn't even funny.
 
There's a lot to be said for making and using a DBT (dim bulb tester) for the first power up test. If it shines brightly this is a warning of a short somewhere that needs to be located.
 
lattiboy said:
Thanks so much for the input!

So, the consensus is to replace the coupling capacitors, hook it up the variac, bring it up slowly and see if anything smokes or smells? After that, throw on my cheapest speakers and see if sounds emerge?

My only concern is wiring in an input, but that seems to be pretty basic. Where is the best place to grab the capacitors?
Click to expand...

Sorry, just to answer your first question: replace the electrolytic capacitors that smooth out the DC from the rectifier. These are in the can. You can replace them with individual caps of a similar value (e.g. 47uf for 50uf) and the same or higher (never lower) voltage. It will have a common negative and then you need to note carefully which wire goes to which positive to each cap. There are four caps inside the silver can, with values marked in MFD(= uF). The voltages go down from 350V to 25V (I can't make it out clearly). So for the first three at least go for electrolytic caps rated at 400V or higher. Just Radios would be a good source for all your caps. For the can, you could buy a new can that has maybe two sections. That would leave more space for fitting in the other two underneath.
 
Even if it works the reason to replace the coupling caps and all power supply caps is you risky any or all the transformers
because those caps can suddenly short out.

Also never short cycle a tube amp (let the tubes really cool off for 5 min. or more before restarting) as the tubes don't like that &
always have a load (speakers or big 16W resistors) as the output transformers can be damaged from that.
 
You must log in or register to reply here.
Back
Top Bottom