1960's Barvic 15AM tube amp with Lenco phono

[James101]

New to site and to electronics...bought amp 2 years ago as prep for hobby, am now engaged in learning the ropes.

Can anyone direct me to info regarding the Barvic 15AM? what is it that I have, exactly? after some study this past month, and opening the system up, it seems to be a mono, PA system with a built in phono, there is a microphone jack, and two speaker jacks, one normal and one marked "echo".

My intention with project is to learn as much as I can while refurbishing the system, and I am open to using the parts to build a stereo tube amp for speakers that I am planning to build.

I am using Morgan Jones' "Building Tube Amplifiers" as reference & guide, though I work & learn best by "seeing & doing", so am taking the Barvic apart first, cleaning it up, putting it back together, and then planning a new "build" that will drive my speakers & phono.

I have tried to attach some photos, but may have messed up, but I will post as is and prey.

Thanks to you all, hope to be an asset to the site some time in the future.

[oc disorder]

Hi James and Welcome to the forum !

small suggestion.. try and make the photos a bit smaller , I found them to be almost 2 meg which took a while to load.

Can you see what type the tubes are ... eg ECC83- 12AX7 etc and give us a list.

Also you have to take a shot underneath of the components.. that's what we hang out here for (among other things)\

That shot could benefit from a larger resolution than the cabinetry.

I'm unaware of that brand in Australia but I did find this on AudioKarma

Barvic PP el84 | Audiokarma Home Audio Stereo Discussion Forums



It would be a major undertaking for you to create a stereo amp from this.

You would need another output transformer .. possibly difficult to source another identical to that one and
I doubt the power transformer could take the extra load of more tubes.

Does it work now ? I guess the cartridge for the phono may be a crystal type .. perhaps convert the pre-amp stage
to take a more modern better quality cartridge.

Of course this would all be mono.

Perhaps google RIAA tube phono preamp and see whats already out there.

It may be a sensible idea to make a solid state one which wouldn't involve any dangerous voltages initially.

That may mean a separate power supply for the pre-amp .

[g1]

A bit more info here, apparently they were made in Vancouver. Not sure if it's the same model, the pictures are missing:
http://music-electronics-forum.com/t26010/

[James101]

Well, this is exciting, thanks for the feed-back, and some of my excitement comes from what y'all write makes sense to me, I can "see" a direction now...will post pics as soon as I have digested what you both suggest and "make it happen" in the real world...I will post soon. Thanks

Oh, and yes it does work, yet bass is missing when I play a record, and knob does not have any effect, treble knob works fine--unit as a whole looks like it went to alot of parties in the 60's, cigaret burns and glass rings on cabinet, yet someone loved this things and keep it working...more later

[James101]

15AM inside & valve ID's



The tubes are in labeled sockets, while the tubes have a ID: I will list socket then tube:

U709--EZ81

729--EF86/Z729 (capped)

N709--EL84

N709--EL84

729--EF86/Z729 (capped)

12AU7--MN (capped)

12AX7--MR (capped)

Wow, the inside is hairy & intimidating...is this normal for gut config on tube amps?

[oc disorder]

"Wow, the inside is hairy & intimidating...is this normal for gut config on tube amps?"

Not really normal these days , before printed circuit boards when labour was cheap
wiring components directly to the next component "point to point" was how it was done.
Depending on planing and forethought some turned out better than others.

There is a school of thought that point to point is superior to printed circuit boards.
There probably were a number of pcb's made that were not designed very well that seemed to support this theory.

With pcb design it's crucial to understand that parallel tracks have some capacitance
particularly over some distance so the layout has to be well thought out.. there are other design points too that have
to be taken on board.

So some modern tube Hi-Fi's may boast that they are point to point and can look like a rats nest inside ,
I suppose their price should reflect that they saved heaps on a competent board designer and the pcb process
but it's usually the opposite!!

Mind you a well set up tube hi-fi sound , can really make you want to get one - except be prepared to mortgage the house !

Here's a random googled image of a Jadis tube amp wired point to point.
Notice the copper rails which components can connect from , the way they have tried
to bend all component leads at right angles and the lonely pcb at the back housing the sockets and the rotary selector switch connections.


Here's two links to a modern amp "Firebottle Inspire" similar in physical size to yours .. this doesn't have the complication of extra tubes for mic input etc.

New amp arrived -- gut shot inside | Audiokarma Home Audio Stereo Discussion Forums

Audio Asylum Thread Printer



Modern capacitors are a lot smaller than their previous incarnations!!!!

Speaking of which... maybe good time to decipher those elderly capacitors with coloured dots !

Color codes for Capacitors

Replacing Capacitors in Old Radios and TVs

I guess it's a good idea to try and sketch out the schematic as best you can.. break it into sections , here in my example
of the first stage of drawing I often just print out the sockets on a blank page and use pencil and coloured biro to get started.
Then I gradually refine it and straighten it up. Note: not really necessary to draw the heater (tube filament) wiring.
That's fairly obvious and besides clutters the drawing. BTW the heater wires are twisted together to minimise hum.
As they are ac at one instance of time they are at opposite potential thus their "radiation" for want of a better word
is cancelled or at best reduced when they are twisted together.

Here's a similar output stage to yours from a circuit posted by Ken Moon here

http://music-electronics-forum.com/t33726/

DaisyCutter by Randy Fay (Phaez Amps)


I noticed that your phase invertor (phase splitter) connects to the output tubes from the anode and cathode
which makes it a "cathodyne pi".

The Valve Wizard

The "Pi" role here is to create 2 x outputs 180deg out of phase to each other um two outputs of the same signal
but one inverted compared to the other i.e. reverse polarity.

This enables the output stage (push pull) to have the two o/p tubes pushing and pulling the signal in the output
transformer primary and thus by way of magnetic induction the secondary waxes and wanes in the same manner to the AC signal
but the DC HT going to the center tap to the primary doesn't connect to the secondary as they are separate windings.

The anodes or plates of the output tubes get their DC via the o/p trans. and vary their ac in the primary according to
the variation at their input - the grid which gets it's signal from the Pi. Clear as mud ? ... hopefully not quite so hairy
and intimidating.

The grid is kinda like a gate. It's not just open or shut it can vary from ajar to wide open with all the points in between.

Current flow in a tube from cathode to anode (anode is + positive and attracts electrons from cathode.)

If the grid is varied with a negative voltage it reduces the flow in the tube as it deflects (as neg.) or limits the electrons attracted to the anode.

Thus a small variation there a big variation in the transformer output.. hey presto an amplifier !!

Randell Smith the Mesa Boogie guitar amp designer/owner wrote a cute piece attempting to explain tubes in a humourous fashion
in most of his Mesa Boogie user manuals.

It's called "On Triodes, Pentodes & Irishmen:A look into the inner workings of a Vacuum Tube"

You may have to download a whole user manual to read it properly

https://www.manualshelf.com/manual/m...h/page-34.html

Hopefully the above is of help to you.Do you have or can borrow a multimeter?

Perhaps draw out the tone circuit so you can get that working.
It's possibly just a silly fault like one component leg is touching the chassis or the pot. is damaged.
Of course you have to work out the dotted capacitors value.......!

Another identical amp will give you stereo with one altered as a stereo preamp or an external S.S. preamp.

Of course the guys on the forum would be itching to turn it into a guitar amp like the Vox Night Train or similar.
Here's a start on the schematic and I've added the bases for the single triode and the tube rectifier.

[oc disorder]

Noticed on the "dotted capacitor code page" there are two different ways of reading the dots !
As both of yours seem to have a definite Gold in the tolerance position of the first way (Figure 3-21.)
I make the larger one 5000pF or 5nF or 0.005uF (.005mfd). and the smaller one 250pF or 0.25 nF or 0.00025 uF /mfd .
We'll stick to .005uF and 250pF as they are more manageable.
I guess these may be part of the tone circuit. Not sure what the voltage rating would be.. probably aprox. 250v.
Any more "dotty" ones?

[James101]

Thanks OC, I see some of what you are pointing out, and some of the knowledge from my Theory & Practice reading "lite-up" when reading your reply, giving the reading somthing to attach to in the real-world--again, this is giving me direction...

since my last post I tried to find a place in my city (Nanaimo, BC) to buy components, and found a place called "MakerSpace"--turn out it is a clone of spaces in Vancouver and other large cities, and it is a work space dedicated to electronics and invention, with full sets of tools, Printed circuit board etchers, 3d printing, full carpentry and metal shop; well, really everything & all people like us need to succeed--Space is three years old and has 50 members and costs 40$ a month for 24/7 access...

I am going to my first "Electronics Meeting" tonight, and joining MakerSpace on Saturday...

Till then I have stopped playing with the Barvic--on Saturday I will take it the MakerSpace and begin the process anew.

With MakerSpace to work in plus the support of MEF I can do this project, then move onto other projects...I will post next week, this will give me time to take your last post and explore what you found...Thanks again, OC.
James101

[oc disorder]

Wow , that place certainly sounds like a good idea - most local colleges here have dropped electronics.. wish there was one around here !!

I thought that I may have overloaded the subject but to do it properly there's more to it than "move that grey wire to terminal f ".

No one has corrected any of my posting so I guess it's probably ok.

Hope it goes well for you.

[James101]

Hello, Just a quick up-date...Am set up at MakerSpace now...am applying OC's recommendations from post #6...just working my way through the material...currently am making a diagram of paper caps so I can order replacements, then will replace...will draw out bass-tone circuit while waiting for part, though the rheostat itself (right word?) seems shot; when I turn the knob it feels like "nothing" (no resistance), while the other three knobs feel-like "something" (a resistance).

OC, your guidance through posts has set me up for success, thank you, and the combination of your experience & directions coupled with MakerSpace, well, both have changed my life for the better, as I just go there every-day and work through it all...in just tree weeks I have gone from sitting in a dark apartment with nothing to do, to sitting in a bright space surrounded by electronics and soft-ware programmers, and robotics builders and a "heaven" of tools AND I have my own project to play with and clear direction from MEF members...wow...very grateful...

will post progress soon,

[g1]

That's an amazing deal for the makerspace. I checked around and where I am it's a $150 sign up fee plus $150 per month. And it doesn't sound as well equipped as Nanaimo either.

As far as the pots/controls (potentiometers as opposed to rheostats), you seem to be referring to the physical resistance. This is determined by the amount of grease in the bushing and can vary greatly from pot to pot. From the electronics standpoint, it's the electrical resistance that matters to the circuit. This is something that would be measured with a multi-meter, or by listening to whether the control is doing it's job in the circuit.

[James101]

Thanks...i measured the resistance and you where right, all the pots work, so I will trace the circuit next and see...

a question: I have itemized the caps and went to BCRobotics and they can get me all the caps, but where concerned that I may not get the sound of the original amp, due to others varibles inherent in my old caps..

can I just put in the new technology? or do I need to find old-new stock to maintain the original sound (and intentions) of the engineering?

[g1]

The original (new) amp would have had new caps, so I think their concern is misplaced.
That argument is usually to state that "new caps will not sound like the old amp sounds now", where the old amp is in working condition, and the aged parts are supposedly giving a sound deemed better than the original sound (aka mojo etc.).
Of course they could also mean that the parts available at the time of manufacture had a looser tolerance so you may be getting a part of a different value than the original one. But this is impossible to determine at this point, so it's not really relevant.

[James101]

Does anyone know what this is?

Cylinder on top



On the top of the PA is a heavy round cylinder with "Barvic" on top, underneath it has 4 wires out, 2 going to the mic-in jack, 1 going to a valve and 1 more going to a strip...I have been unable to find anything on what this is, and I hesitate to take it apart blind.



Any ideas?

As an update on project, I am now waiting for 16 new caps for re-capping and I hope to have this step completed in ten days.

After this I am intending to make the PA into a guitar amp (thanks OC for the direction to Valve Wizard & Regis Coyne's page

Converting old tube PA amps to Guitar amps)

for those who are interested in my experience with the "Maker-Movement" (which I did not know existed 30 days ago) MakerSpace is amazing in a way that words can not do justice: while the tools & complete shop set up is stunning, the Experience contained within the membership is immense, retired electronics engineers, CNC business owner, alternative energy guru's, software engineers and developers, master carpenters, astronomy phd (currently helping 8 members make 6" mirror lens for telescopes) and a definite focus within the Space on 3D printing.

Tuesdays are electronics night, and two engineers are here to answer question, one who started out in the sixties and current today running his own business, and another from the 80"s via Silicon Valley who is the Raspberry Pi rep--last Tuesday they taught me how to hookup an oscilloscope to my PA and reading the waves, and while the PA works, the caps are definitely deteriorated, so onto re-capping.

The "Barvic" cylinder has them all stumped though, so hopefully MEF can clarify or give me direction, otherwise I will just bite the bullet and open it up.

Thanks
James101

[g1]

That should be an impedance matching transformer for the mic input.