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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dumont (where theres smoke theres sound)

I swear,
I am a sucker for punishment sometimes, After setting aside the deathtrap Kay guitar amp I had posted about earlier this week.
I decided to try and get something a little more or less safer working.

OK, Here It Is,
Dumont Model RA506 Mono upright console record player.

So,
I take the time to remove the 2 prong cord and wire in a nicer 3 prong jack and cord setup.

Added a line fuse

I converted the aux line socket into a place to plug in a remotely mounted encased power switch. I am pretty sure I wired the mains correctly, There is a clear view of it in my photo.
I used the information I found Here Mains

Since this baby is I'm guessing late 40's to early 50's, I kind of knew what to expect. ( A Nightmare)

Tubes Tested perfect, I have 3 sets for this unit.
12AX7 1st & 2nd stage, 6C4 inverter, EZ81 rectifier, And 2 EL84's.

So Anyway I get everything attached, And Proceed to power up.

15 seconds and I start to hear a mild hum from the speaker, Turn on the old mono turntable, Neat Music. But not very loud .

About 20 seconds later I start seeing the first hint of Yep, You guessed it "SMOKE"

SeeChassis Underside Photo

I marked the referance items in red. Item "A" Componet that was smoking 56 ohm 5W Resistor

My First guess, Of Course, Bad Electrolytic Caps. So I replace the Electrolytic Caps & Resistor. (the cap got pretty warm during power on)

Ok, So now The Cap is new, And the resistor is new ( A & B in photo)

I proceed to power up again, And now the music is much much louder, But still has conciderable background hum. About 60 seconds lapse and the 22K 1/2w Resistor (C in photo) Begins to emit a fine trail of smoke.

I quick shut it down. And Thats where I am. Now this time the Electrolytic Cap wasnt even warm to the touch.

I could sure use some ideas here, As this actually seems like a fairly decent amp for its age.

Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
The ceramic caps and the pot cases say to me mid-late 1950s maybe through the early 1960s. By the mid-60s it would "have to be" Stereo. I very much doubt that type of pot case was produced in the 1940s. Also the 10% resistors were uncommon in consumer gear through the 1940s. (20% was common through the 1950s.) And 6BQ5(EL84) was not widely used in the US until the 1950s. I can't say I've ever seen an EZ81 in US gear; does it have a 6xx4 number?

Yes, this is a better than average record player. Three tone controls, push-pull.... this was a DeLuxe model. If you kept the original speaker and cabinet, it should make fine sound. DuMont was a name to be reckoned with: Allen B. DuMont put his name on the first commercial oscilloscopes, worked in TV, leveraged that into a major TV network.... and then it all fell apart by 1960. The company continued for a while, and the name a lot longer, but your recordplayer seems to be from the end of DuMont's golden years.

> seems like a fairly decent amp for its age.

An age when mundane amps were built better than anything you see today, sonny. Oh, sure, you have to patch some smoke-leaks, but most of us that age have some leaks and bald-spots.

> I converted the aux line socket into a place to plug in a remotely mounted encased power switch.

Clever, but I dislike it on grounds that this remote switch now fits every outlet in the house, and can dead-short them. Be careful, or be clever a different way.

As a guitar amp, it should be very nice, though power chords will soon destroy the shellac/paper speaker. A similar guitar speaker in the same cabinet might be very fine. You may need more gain than it has now, and the volume control at the input is not ideal for a high-gain guitar amp. What you need is one stage of 12AX7 before the volume control. Dunno where you put it; maybe punch out the phono input jack and mount a 7-pin socket, use 6AV6?

Any resistor that has smoked HAS to be replaced. Or cut-out so you can smoke-check and volt-check the rest of the amp, and then come back to the parts fed by the smoked resistor.

The 0.15 cap behind the treble control is blistered and surely not healthy.

It is a lovely photograph but I'm not going to trace-out the circuit for you tonight. Dunno what your smoking 22K does, or what makes it smoke.

What is that odd 5-socket connector with two large 4K7 resistors near the center?

Can you estimate the proper tube voltages (plate, grid, cathode) and are the tubes working at reasonable voltages?

Hard to know if it smokes faster than you can put a meter on. Look: in an amp like this, power current will probably go to zero with all the signal tubes removed. Take out 12AX7, 6C4, and EL34s. Power-up. Watch the main cap voltage. It may go too high, no-load, for the good of that cap, but it should hold for a minute and nothing should smoke.

Then put the tubes back one at a time. I really doubt the 12AX7 stages can make smoke, I'd try the 6C4 and the EL34s, one at a time. If they don't smoke, check the voltages and see if they look reasonable. Especially check grid voltages: a leaky coupling cap will pull a "zero bias" grid way positive, which makes the tube suck big current.

Don't run just one EL84 for more than a minute. With high B+ and a 2-tube cathode bias resistor under just one tube, the tube will overheat. But that should be enough time for a quick voltage check.
 
PRR said:
I can't say I've ever seen an EZ81 in US gear; does it have a 6xx4 number?

6CA4 Appears to be mullard


I converted the aux line socket into a place to plug in a remotely mounted encased power switch.
I replace the turntable in the cabinet, Rather than add power switch wiring to it, I remotely mounted the Amp Switch in the cabinet and made it easy to just unplug everything for chassis service.

Any resistor that has smoked HAS to be replaced. Or cut-out so you can smoke-check and volt-check the rest of the amp, and then come back to the parts fed by the smoked resistor.

Changed The 22K, and the 56 ohm, And Can Cap 40/40-450V was 40/40-350V

The 0.15 cap behind the treble control is blistered and surely not healthy.

I left that particular cap in place, It looks sad due to the messy wax encasing it, But, While the record played, All controls were working very effectivly, Bass, Treble, Presence & Volume


What is that odd 5-socket connector with two large 4K7 resistors near the center?

2 Prongs are for the power lamp,The lamp is located remotely in the lower section of the cabinet Acording to the chassis label on the rear of the cabinet, That is also where an electrostatic tweeter plugs in. There were no tweeters of that type in the cabinet, nor had there ever been. Its possible that chassis was also used in another model that had those tweeters.


Frank:


It looks like the 22k resistor goes to the EL84 screens.If the resistor is smoking, one or both screens are drawing too much current. Possibly an internal short or a gassy tube.Change out the EL84 tubes.

I Tried 3 different sets of know good EL84's, I also took the original tubes to a known working amp I own to see if my tester is giving me false readings.
The Tubes are confirmed good on both my tester and my other amp.

Heres rough start on the power supply schematic, Im going to try to continue drawing it out as I go.

Schematic

Oh, BTW, Heres a picture of the three pieces. The cabinets are in lovely condition. The 2 floor speakers are both 2way (12inch with 3 1/2 tweets) I am not sure if they are also from dumont, But they sure look like a match in style and finish.
Dumont Unit

Gene
 
Replace all signal caps and power supply caps. With the amp turned off and the caps discharged, measure the impendances of all transformers to check for shorts.

Also measure all resistor values; replace any 10%+ out of spec

Interestingly, The PS Cap discharges almost as fast as I shut the unit off. I had noticed this prior to changing it. Something upstream must be the cause, But I can Find no dead solid shorts yet. The Transformers seem to be fine.

I am suspect of a couple disc caps at this point, There really isnt much to test. I havent powered up again yet with or without tubes as Its not going to be fun nor quick switching out the remaining caps.

I suppose its possible that the 22K 1/2 w resistor is under rated, I have never seen an amp with wattage that low in that area of the PS.

I have built several nice project amps,(from schematics) but I am not designer.
I am by no means a newbie with a soldering iron or working around HV devices. Its just been many years since I had first played with tubes.

It cant be to bad off as it did emit sound for 30-45 seconds, And the smoke wasnt more than just a fine hint of a trail(almost not visible) Had the unit been in the cabinet it might have been overlooked until it was dust.
Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> It looks like the 22k resistor goes to the EL84 screens.

No (I think). Rectifier feeds through 56 ohms to lower-left main cap lug. That feeds both screens and the OPT CT. It also feeds the 22K to the top-right main cap lug, which runs left over an orangish wire to three resistors in the preamp/driver area: 100K to 6C4, 470K to right side of 12AX7, 100K? to left side of 12AX7. Perfectly reasonable values.

Pull the little tubes. In that condition there should be no voltage drop on the 22K. If there is, then cut the orangish wire. If the 22K still shows drop, the top-right unit of the main filter cap is leaking all to heck.

> where an electrostatic tweeter plugs in.

Ah. Obvious once 40 year old memory cells are rung/wrung. There was a line of really low-cost electrostats. They didn't work very well. Supply or reliability may have been problematic, leading to using dynamic tweeters to get boxes out the door.

> picture of the three pieces. The cabinets are in lovely condition.

Ah, surely late-1950s until mid-1960s. Also the bookends and even your wall paneling is of that era. I'm inclined to think the speakers were not made FOR that main console: difference in the corner trim and base molding. I suspect the speakers are newer (or newer-design) than the main box. But they may have been options suitable for several main-box models. Even a trick to move some older-production mono boxes onto a "Stereo" market, by adding two external speakers (on one amp: who will know?). If you say they have the same finish, likely they are from the same woodshop and same general contract. IAC, a real find.

Two paper 12-inchers may survive the output of this amp even in guitar duty. If they are 1-inch voice-coil, they sure should eat 5W-8W power chords all day long. For heavy-metal, you might want to discretely switch-off the tweeters so they don't smoke.
 
Frank Berry said:
2 Prongs are for the power lamp,The lamp is located remotely in the lower section of the cabinet Acording to the chassis label on the rear of the cabinet, That is also where an electrostatic tweeter plugs in. There were no tweeters of that type in the cabinet, nor had there ever been. Its possible that chassis was also used in another model that had those tweeters.


Gene

Gene, I just pulled two of these tweeters from a Zentih turntable last week, yours for stamps if you want them, hit me off list if you'rr interested. They're the rough US version of the gold Grundig electro tweets, copper. Interesting little creatures.


rgrds,
Brad
 
bradself said:


Gene, I just pulled two of these tweeters from a Zentih turntable last week, yours for stamps if you want them, hit me off list if you'rr interested. They're the rough US version of the gold Grundig electro tweets, copper. Interesting little creatures.


rgrds,
Brad

Wow, Thanks for the offer Brad,

I also have several sitting here from a Grundig that wasn't practical to restore.

Right Now, I am transferring the circuit off the chassis into a schematic/drawing or at least trying.
I am hoping to remove the electrostatic related components entirely from the unit if possible.

Worst case scenario I will just rewire it into an AX84 guitar amp.
I did learn one valuable lesson so far, " Fix the electronics BEFORE the cabinet" haha, I got this baby looking nearly show room and its empty!

I Pulled the backs of the speaker cabs, Very well built, Nicely padded inside. 1- 12" alnico looking beast and a tweet in each. The console itself only had a 12" standing alone.

I got this stuff from a recently silenced key in the Ottawa Illinois area.
I also aquired 2 monoblock el84 amps.
I am not able to identify them as of yet. If the relabeled tubes are a clue A large circle around a
V
M interlaced closer vertically of course. Made In GT Britain
Perhaps Voice Of Music?
gene
 
OK, Don't laugh to hard at my pieced together artwork here.

Its at least an accurate rough draft of the PS + components in the output area.

The 22K is indeed 1/2 w and honestly that has to be well under rated. I am actually surprised it lasted the past 40 odd years.

But then again, I guess the real question is why has it smoked now.
I can understand the 56 ohm frying due to the old old electrolytic cap, ( Both have been changed)

But Now, I have the 22K replaced. I Have not applied power however for 2 reasons, #1 I would like someone's second opinion on its wattage rating, & # 2 No sense burning a second resistor needlessly.

Also, I would like to remove as many of the components as possible from the circuit for the electrostatic tweeters as I have no plans to ever use such devices.

Here Is The Rough Draft Schematic

gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> remove as many of the components as possible from the circuit for the electrostatic tweeters

Leave them. The twin capacitors and two 2K7 resistors form a Zobel across the plate winding. It may not need it, but it sure won't hurt the music, and just might be part of the frequency compensation.

> Here Is The Rough Draft Schematic

Well, no, that can't be right. The output stage fed through the 22K ????

The attached is what it has to be.

Rectifier, 56 ohms, 40uFd, feeds 6BQ5 plates and screens.

22K from there to another 40uFd feeds all the little tubes.

I'm unsure how the 6C4 grid meets the second 12AT7 plate. I don't know what the 12AX7 cathode resistors are, or if they are bypassed. I do think the second 12AX7 cathode must get some feedback from the speaker winding.

If you really have the 22K feeding the 6BQ5 plates or screens, it is wrong. Maybe some previous repair was confused which main-cap lug was which.
 

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PRR said:

Well, no, that can't be right. The output stage fed through the 22K ????

The attached is what it has to be.

Rectifier, 56 ohms, 40uFd, feeds 6BQ5 plates and screens.

22K from there to another 40uFd feeds all the little tubes.


EXACTLY!!!

I just rewired it to do so. YES it now is dead silent, and [email protected] loud!!
Clearly this baby must have seen a service job that missed slightly, Its no wonder the power cord was cut off. Perhaps the previous owner had been planning on a repair but sadly never got to it.

No matter what, THANKS for your help, It is now fully functional!
Gene:wave:

Best of all, 20 minutes and Sniff Sniff " No More Smoke"
 
wa4htz said:
Hi Gene,

The tubes you are describing may be from M-O-Valve. Not quite as ledgendary as Mullar or Brimar but as I gather, quite respectable. Maybe some of the more knowledgable European tube gurus can shed some more light on them. Frank??


Not Exactly,
A large 0 surrounds a V hovering above an M.

Made In GT Britain around the bottom.

My gals got our ftp space full of old ebay pics again, So I'll post a pic of the tube later after I run the disk clean.
Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> dead silent

Interesting. I don't think I have ever seen THAT little supply filtering on a full amplifier. Somebody really sharpened a pencil on this amp. Interesting to hear that it works well.

> [email protected] loud!!

Should be. Twin-6V6 amps were King of nearly-high-end audio. While there were 6L6 amps in home and guitar duty, there were many more 6V6 amps. Gibson was mostly 6V6, didn't even offer 6L6 until around the time this amp was made (when the guitar moved from the back of a big-band to the front of the cowboy band). Yes, I know this is EL84/6BQ5, not 6V6: power is similar, just that the 6BQ5 is a lot easier to drive than 6V6. And the difference between 6V6/6BQ5 and 6L6 is only 3dB to 6dB of watts, not a huge difference.

Does it play loud with an electric guitar? It seems to be set up for a crystal phono cartridge, 0.1V to 1V nominal level, and 'lectric gitar amps are usually 0.01V to 0.1V sensitivity (the highest gains only used for effect). It it impossible (from here) to know if the equalization is suitable for electric guitar. Quite a few older guitar amps were near-flat, with passive +/- bass/treble controls like you surely have. Fenders often have a dip around 400Hz rising to a shelf at 2KC. If the Fender midrange control is full-up and the bass and treble are set nominal, the response is not too different from what you would use on an unequalized crystal cartridge to get RIAA equalization.

Is the 6C4 a 9-pin? If so, a radical plan would be to re-wire to the Fender 12AX7 preamp and 12AT7 driver stage plan, a real Classic. But I'd be inclined to leave it roughly as-is, with what minimal modification needed to give anough gain for guitar. The minimal-est "modification" is to just use some pedal-FX box that incidentally has gain.
 
Basically,
A 6C4 is a 7 pin 12AU7.
6C4 Data

Not exactly the best choice to drive the EL84's but from my ears point of view. Sounds very good on the original turntable.

I think at this point now that its running at what appears to be correctly. The Dead silence with no signal. I would have expected at least some hum with this particularly simple powersupply.

I am going to tinker with the first stages a bit this evening and see if I can get it more guitar ready basically.

More Pictures soon
Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> a 7 pin 12AU7 ....Not exactly the best choice to drive the EL84's

As a Cathodyne, unobjectionable due to huge feedback. Has good drive current. Really more driver than 6BQ5 need.

But being 7-pin limits clever re-design. Hmmmm: the canonical Fender push-pull driver is a common-cathode pair, so you could use one of the 6J6-pinout 7-pin twin-triodes. Even 6J6 itself: it isn't real linear but at 6BQ5 grid levels and working push-pull, it is more than linear enough for jazz.