50C5 amp

[dkevin]

Greetings to all,
I recently found a Symphonic brand record player in a thrift shop and discovered it had a tube amp inside. It's a cute little thing so I spent the $30 and took it home. Looking closer at the tubes I noticed 2 ea. 50C5 tubes and 1 ea. 12AX7 . There was no tube rectifier and no power transformer.
Now, I have read quite a bit about the horrors of such amps. But I wonder, do they pose the same risks if they are used as designed-as consumer electronics? I am leaning toward installing an isolation transformer and a 3-wire cord that grounds the chassis. I think I can do this fairly inexpensively.
I am a bit perplexed by the schematic, which was thoughtfully included inside the case. Comparing it to several similar (guitar) amps some of the resistors used are quite strange. Also, for those of you who are Gibson amp fans, there are two sealed networks (similar to the dreaded Sprague networks found in Crestline series amps) inside . I would like to work on this amp safely and plan to do so while it is powered up on my variac. Will that suffice until I get the isolation transformer installed and the three-wire cord attached? I know I could ditch the 50C5's and get a power tran. and choose better tubes, but I am just looking to have a small record player for the living room. I remember seeing many similar design record players in the early 60's and would like to keep it original.
Got any advice for me???
dkevin

[in a vacuum]

These old radios and record players with the serial filaments (50+50+12=112 volts=line voltage) would not make UL listing because of shock hazards. The reason this type of amp is so dangerous for guitar is that the strings and bridge of the guitar is attached to the chassis ground. As long as the operational knobs for the record player are plastic (not metal) it is probably pretty safe for those of us with common sense. It wouldn't hurt to install a two prong plug with the hot side of the plug NOT going to ground. Do a recap and all the standard, "its more than 30 years" old maintenance, then spin them records.

[Enzo]

Used as sold for the original purpose, it is not such a hazard, though not up to modern standards either.

But I must point out that as valuable a tool as it is, a Variac offers ZERO isolation from the mains. You can dial the voltage down, but you are still connected directly to the mains.

[lesliengland]

My cherry Alamo Capri guitar amp (1967?) has a 50C5, 35W4 and 12AU6, hums and is probably dangerous. The quick replys of others are similar to comments elsewhere on the web. Two questions. 1.) Can I make it safe? 2.) Can I eliminate the hum with the isolation transformer strategy and a three prong cord?

Will "ditching the power tubes" for a better variety and adding a power transformer make it appreciably louder? Otherwise, I thru away $108. I am into great sounding old amps, not great looking ones!

[Enzo]

It probably hums because it desparately needs new filter caps. Does it hum even without the guitar plugged in?

Looking at the schematic, this particular amp with the tube complement that usually is direct off the mains actualy has a power transformer. SO install a three wire cord, and ditch the death cap. (lower left in the drawing.)

Those tubes have to work as a set. I don't mean they are matched or something, but there has to be a 50v power tube, a 35v rectifier, and a 12v preamp tube. SO you can;t really slap a EL84 in it or something. There is not a lot of upgrading tubes you can do. You can replace old worn ones with new old stock tubes. No one makes those types any more, but they were made by the millions in their day. TVs and table radios were full of them.

I think it would be a fun amp to fix up. You probably should change every cap in it. Fortunately that is not very many.

Schematic taken from Schematic Heaven.

[Glasgow]

The first tube amp I ever messed around with was a '59 Califone portable record player. It had a power tranny and 6.3 volt heaters.

Man, was that fun and a great sounding amp. I would jump on the chance to score one of those again. I think I found it at a flee market for like $5. I usually only see the solid-state ones now.

I have never used an isolation transformer, but I understand it is a good idea. Could someone please explain the principles behind using one and how to treat your DUT when using one? Maybe also using one in conjunction with a variac?

Thank you,
Brian

[Enzo]

The whole problem with power transformer-less circuits is that they rectify the mains voltage DIRECTLY to make your B+ for the tubes. If you do have a power transformer, then it supplies the isolation. With a powr transformer, touy may have several hundred volts in the circuit, but none of it is connected DIRECTLY to the mains. Now there are some small amps that have a power transformer for the heaters yet still directly rectifies the mains for B+. Those are still dangerous.

An isolation transformer is simply a 1 to 1 transformer. WHatever voltage goes in is what comes out. So instead of plugging into the mains, you plug into the iso transformer. The amp still gets the same mains voltage as always, but now it is supplied by a transformer instead of the mains directly. There is no longer a direct connection between the circuit and the mains.

If I am working on a hot chassis amp, I cannot count on the chassis being at earth potential. it may be the circuit common, but it could also be at mains voltage above ground AC. I cannot then safely ground my scope to the chassis. And I don;t want to ground my guitar to it either, not unless I am standing on insulated flooring. By inserting the iso between the DUT and the mains, I break the connection. The whole circuit now floats, allowing me to directly earth the chassis.

I then treat the DUT as I always did, with the added ability to connect it to the rest of the world.

The variac doesn;t change the formula in any way it doesn't change the formula normally

[Glasgow]

Thanks for the explanation Enzo.

So, by isolating an amp with the isolation tranny, you can safely establish a known, true earth. I guess you must ensure that you *do* establish a ground with this setup. Does the isolation of the mains power limit catastrophic currents? Maybe sacrificially?

Brian

[Steve Conner]

I have a box with a variac, isolation transformer and a couple of sockets on it, that I use for things like troubleshooting switchmode power supplies and hot chassis TVs.

The theory is simple, you plug your "DUT" into the output of the isolation transformer, and then connect its chassis to ground. Or in the case of a switcher, the negative terminal of the big DC bus filter cap.

If you try this without the isolation transformer, there will normally be some sort of explosion. I've seen the ground clips blown clean off scope probes this way. If you understand why this catastrophe happens and why the isolation transformer prevents it, then you understand the "principle".

If you don't understand it, then you shouldn't be playing with hot chassis equipment, and you certainly shouldn't own a transformerless guitar amp.

PS: Americans always get confused between "neutral" and "ground" in their wall outlets. Neutral is not ground. OK, so 99 times out of 100 neutral and ground are connected together in the breaker panel, and so are de facto the same thing. But it's that 100th time that could fry you like a corndog. This is the main reason why transformerless amps are unsafe: there's a risk of the chassis, and hence your guitar strings and yourself, ending up connected to hot rather than neutral.

If your amp has a power transformer and a three-prong plug, then it doesn't matter if hot and neutral get swapped. But if Igor the venue electrician wants to kill you, he can still do it by getting hot and ground mixed up. As a test, you can rub your guitar strings against the microphone, and if they explode in a shower of sparks then there is most likely a wiring problem. :-)

[Jag]

Sure beats the "kiss the microphone quickly" method.

[Glasgow]

Thanks Steve.

It seems as though even a power-transformered, three conductor corded amp could benefit from isolation since that ground conductor goes right to the chassis from what you *hope* is grounded to neutral in your panel.

Probably not a problem at home if you know your wiring is correct, but elsewhere???

Brian

[Enzo]

Adding an iso to that scenario doesn't change anything though. The regular power transformer offers all the isolation you need. Your chassis is grounded to the ground pin either way - or should be - so if the outlet is miswired leaving the ground pin hot, then that will remain even with an added iso.

[Glasgow]

I think I understand now.

With a transformer, be it an isolation tranny or the power tranny, the DUT common is always tied to the secondary of the transformer and therefore can't be at mains hot potential.

BrianB

[Glasgow]

PS: Americans always get confused between "neutral" and "ground" in their wall outlets.

Yes, some of us do. People that work on their home wiring or are technically minded usually get it.

Is your system more straight-forward? I know you guys probably have 'funny' looking plugs and sockets. ;-)

Thanks,
Brian

[Prairie Dawg]

PS: Americans always get confused between "neutral" and "ground" in their wall outlets. Neutral is not ground. OK, so 99 times out of 100 neutral and ground are connected together in the breaker panel, and so are de facto the same thing. But it's that 100th time that could fry you like a corndog.
:-)

Didn't know that you could get corn dogs in Glasgow. The way I see it, the only possible way that a Glaswegian could know about corn dogs is if they took a wrong turn at Dubuque along about the middle of august and ended up at the fair.

If you're looking for a midwest simile concerning series string amps consider using the term "bug zapper". It's far more descriptive.

and yes, this is