HI-FI Console Amp

[custom10]

Hi All

New to this forum and to the world of tube amps. Basically I am looking for any advice that can be offered here even if it is not in my tread questions. I have a 1965-67 Model 4078C Fleetwood HI FI stereo console in fine working order. I have changed out the 4 EL84 output tubes and the 5 12AX7 pre amp tubes. It is used primarily for us to play our old and new LP's as it has a nice garrard type A turntable which I refurbished with some ebay parts. A friend of mine suggested testing/replacing the caps and setting up the tube BIAS. Is this just a matter of removing the amp and discharging caps, then replacing with the same uf values of caps. Also should the new tubes be biased as soon as possible to avoid problems? As well as the amp section there is a AM/FM tuner section to this unit, about 7 type 6 tubes (6C9,6AV6,6AL5,6BA6,6AU6's) Can I replace these with new ones without having to worry about setting up bias and other adjustments or are they going to work straight out of the box. The system works pretty good maybe I should just abide by "if not broke don't fix"?

I can't find much info on these fleetwood consoles on the net, who made the amps for these? I have a tube layout card that was inside the unit and can post some pics.

Thanks

[DonMoose]

Definitely replace the electrolytic caps. The ones in cans are often multi-section with 2 to 4 caps in one package. You don't HAVE to replace the cans with new cans - modern caps of similar value and same (or higher) voltage are often much smaller and can be tucked inside the chassis with a bit of RTV or hot glue.

I have a Bell (later TRW) receiver that uses half a heater in a 12xxX tube as the bias resistor for a quad of 6V6 outputs - in other words, your bias setup may not be all that adjustable - but you can get an idea where it's running by measuring the OT's DC resistance from where the B+ goes in to each output tube's plate (power OFF). Then, with power ON, measure the DC voltage drop from the B+ on the OT to each plate. V/ohms = current in Amps.

Use extreme caution taking those voltage measurements - that's the highest voltage in the amp and will very likely be over 300V.

You will also want to measure the voltages from each output's Grid terminal to its Cathode terminal - this should be a NEGATIVE reading. Bear in mind that the Cathode may not be grounded - if you're Cathode biased, like I am, there will be some resistor, probably in parallel with a cap between the cathodes and ground. You WILL need to replace that cap.

Hope this helps!

[Structo]

I won't offer any technical advice pertaining to this amp but please learn how to properly drain the filter caps before working inside the chassis.
That is not shorting a cap to the chassis with a screw driver.
Not only is that unsafe it can damage things as well.
Like a piece of hot slag hitting you in the eye.

You should always measure for voltage across the filter caps to verify that there is indeed no voltage there.

So learn how to use a test lead and a plate resistor to drain the caps or make a discharging test lead out of an insulated test lead and a 100K 2 watt resistor.

As you probably have read or heard, these large caps can store a lethal charge, even if the amp is unplugged these voltages could be lurking to shock the first unsuspecting person that sticks there hand inside.

So Safety First!

Then you can start troubeshooting with advice from the forum.

[MBD115]

+1 on the safety warning!!!!!!

Like said above the bias may not be adjustable. If you could post the schematic I'm sure someone here could tell you if it is.

As far as the caps goes..... I agree with what you said above, "if it ain't broken - don't fix it". If you have a bad cap you will also have some indication of it in the quality of the sound (to some degree I would think). If it sounds good as is - then I'd leave it alone and enjoy it. If in the future it dies then I'd try to fix it then. And how I would do that is "only replace the part that died".

These old tube amps has some really neat parts in them that isn't available anymore. Mods/upgrades/fixes does change them. Like the tubes you replaced. As for the tubes - I'd try those old tubes again and only put in a new ones where it was needed. Those original tubes has to be pretty much priceless (at my income at least )

I think I'd just enjoy it "as it is" for as long as it will work properly. And you said "in fine working order" so its good IMO.

Good luck

[custom10]

Thanks to all for the responce. Trust me I have gotten the message about those voltages and will take extreme caution if I am ever inside the amp. I would first build the discharge lead as desribed and proceed with caution. I have always been interested in electronics/electrical workings so I have an understanding of electrical circuits and schematics but not tube amps. I am an industrial instrument technologist so mostly I work with electrical panel wiring, PLC's and all kinds of sensors, switches and relays etc. I have a small little shop at home whcih I use as a hobby spot (slot car fanatic) and I have started to aquire some equipment including an old oscilliscope. I just got the console stereo about a year ago as an inheritance and it has really impressed me with its natural warm sound...who'd of thunk it, my brother in law was going to toss it out when his father passed away, also got a 1938 GE cabinet style radio in excellent shape that still works! I was picking up Cuba the other night on one of the SW bands and I live in Saskatchewan!. Anyhow these old tube systems are my new found passion and I plan on finding out more about tubes, there circuitry and amp building/service.

I think I will leave well enough alone for now and not mess with the stereo untill it starts giving me trouble however I would like to find the schematic for the amp so I am ready to trouble shoot it when and if it is nessesary. Were can I look for schematics? I can't seem to find any numbers on the amp chassis and have searched the web for anything fleetwood related. Please advise

Thanks Terry

[Structo]

Well, there are a couple things that should be done to make it safe to plug in.
First is a three prong cord with a ground.

At the time of this maintenance you should make sure the fuse is on the hot lead first, then the power switch then it should go to the power transformer.

The next is all the electrolytic caps in the amp should be replaced.
They only have a finite life span and most would agree that around 12-15 years is the average life span of them.

I think the newer ones are better and believe it or not, E caps last longer if the amp is used regularly.
It's when they sit the electrolyte starts to break down and dry out.

These are usually deemed as mandatory safety mods and is acceptable to everyone except the hardcore collector who doesn't want anything changed.
But then it isn't safe to operate.

I always like to show this picture when an opportunity arises.

This is what can happen to old caps if something goes wrong.

[IMG:640:480]http://i23.photobucket.com/albums/b392/ ... C01892.jpg[/img]

[custom10]

I have contacted radiodaze for info on schematics etc. if anyone else can advise on were to find info please do tell. Thanks all

[Structo]

That is a beautiful old console!

Man, remember when they made them like that?
They were a piece of furniture.
A lot of times they would include a schematic on the rear cover or in a pouch inside the unit.

Generally, you only have to be concerned with the bias if you replace the power tubes.
The EL84's in this case.

I hope you didn't throw away those preamp tubes.
Many times those are still good, as they were made much better back in the day.
A preamp tube can last years and years in an audio amp like that.

I have a bunch of 12ax7 Baldwin Organ pulls that appear to be Raytheon long blackplate tubes.
All tested good in an amplifier but I don't own a tester.

I remember when I was a kid, we had a nice Zenith console TV set.
Black and white of course.
If it started acting up my dad would pull all the vacuum tubes and put them in a brown paper bag and off we would go to the neighborhood grocery store where there was a tube tester.

Back in those days the testers were basically a good/ bad test and possibly a leak test.

Then once you found one that was bad, you would have the store owner open the cabinet of the tester and he would pull out a new tube.
Then we would go home and plug them all back in and with luck we would be back in business.

The picture tube went out on that Zenith one time and my dad being a do it your selfer, ordered a new tube.
He brought it home one day and as he was walking through the family room door way, he bumped the neck on the door jam and broke it.
Man, I don't think I ever get more mad than that ever!

[MBD115]

Sorry. I didn't think about the 2-wire cord being in there. That does need upgraded indeed.

And I must agree about the electrolytic caps also.

(I guess I've been reading/thinking about that multi-page "Help recap my amp" thread too much lately. )

+1 Structo. Excellent advice and good reply man!!!!!

[custom10]

Thanks for the coments, OK so if I get around to changing the power cord I will use a grounded plug (three prong) and attach the green to the amp chassis correct?

[Structo]

Well depending on how your amp is built it could be a big challenge or if it is on a turret board or eyelet board it could be simple if you have any electronics experience.
The only way to find out is take a peek inside the chassis.

If you have soldering experience this could be done in an afternoon.

But if you don't feel comfortable around 400+v then either read up on safety measures concerning tube amps or take it to a tech.
And if you do take it to a tech, make sure he is used to working on tube amps.
When I was in college 1977-79 the instructor said we wouldn't spend hardly any time on tube theory since tubes are obsolete......
30 years later, they are still around and most guitarists prefer a tube amp over a solid state one.

[custom10]

Oh and Structo I kept all the original tubes, can't recall what make they were when I pulled them out i think they were RCA and also Fleetwood tubes. I can open the unit and check as I stored them inside with the boxes from the new ones. The guy (Joe) that owned this system was a big time ham radio user back in the day. He also repaired TV's and home radio sets. When he passed away a couple of years ago my brother in law may have trown out most of his stock of tubes and other goods. It was not untill I got ahold of this system (and the 1939 radio cabinet) that I began to appreiciate the glory of tubes. Who knows what kind of treasures ended up in the dump, word has it that he had hundreds of tubes that were just tossed, along side of his actual passing this is a sad sorry so I will leave it at that.

[MBD115]

I just did a little quick search for RCA 12AX7 1965 USED. You have 5 of them?

[custom10]

I have found the schematic for my fleetwood console amp. Not sure how to read it yet so was wondering if there is in fact a need/way to BIAS the EL84's

[martin manning]

The power amps are cathode biased and not adjustable. R325, the 180-ohm, 5W resistor between the two EL84's in the lower-right corner of the schematic, sets the bias. As long as it and it's mate on the left channel haven't drifted in value you are fine. You would have to lift them from the circuit to check, or take some voltage measurements with the amp operating to get an accurate assessment.

MPM