Vintage tube radio project

[jay_back_in_chicago]

So, while working on my 18W project,friend brought me old 50's tube radio (Ex Yu made) to take a look at, and just left it as a gift


It's working, but plastic parts are sadly broke, and condition of it its not that good.
 4 tubes, 2 of them I identified as radio tubes, but 2 have no marks on them.

I attached some pictures bellow.

Output transformer is 4 ohm I think, and output tube is noval with PIN 7 going to OT

I will for sure scrape this for parts (resistors and caps, there is cool 50+50uF vintage looking MM cap), but is there any small tube guitar amp project that is suitable to this spec build?

[kagliostro]

Seems a nice plan and feasible

No Brand and model ?

Identify the Volume pot and find the input to it, then if you disconnect the potentiometer from the rest of the circuit you can try to inject a signal to test the power section

Franco

[frankenxtein]

Never heard of ex yu but , I'm not an expert. Finding a schematic for it would be the first thing to do. That would tell you a lot of what you have there. There are people that collect these things and it may be an item. Aside from transformers , chassis , sockets , tubes , jacks tag strips , .... I don't use old resistors or caps. How many leads are coming off the PT?

[kagliostro]

Verify if the PT is a Trasformer or an Autotransformer

Many old radios (fortunately, not all) used an Autotransformer instead of a Transformer

an Autotransformer isn't safe because there is no galvanic separation from the line

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Looking to the photo seems that the radio was a 1960 - 1970 unit, not a 1950 unit, but this isn't important for the reuse

Franco

[jay_back_in_chicago]

Never heard of ex yu but , I'm not an expert. Finding a schematic for it would be the first thing to do. That would tell you a lot of what you have there. There are people that collect these things and it may be an item. Aside from transformers , chassis , sockets , tubes , jacks tag strips , .... I don't use old resistors or caps. How many leads are coming off the PT?

my fault, Ex Yu means former Yugoslavia, it was made there

PT has all together 6 wires, 2 from 1 side and 4 from the other
OT has only 4 ohm tap, and 3 wires on the other side

Seems a nice plan and feasible

No Brand and model ?

Identify the Volume pot and find the input to it, then if you disconnect the potentiometer from the rest of the circuit you can try to inject a signal to test the power section

Franco

Brand is Iskra from Kranj, Slovenia
Model is Sava UKW

Using that I found that only output tube is el84

[kagliostro]

Measure with the tester if there is continuity between the wires on primary and secondary of the PT, if there is continuity it is an Autotransformer

A & B are Transformers C is an Autotransformer




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The 3 wires on the primary on the OT are common in SE radio, often there is a part of the winding that is the primary and on the third wire is connected G2 pf the power tube




--

I'm not able to find schematic, but I find other info (tubes are 2 x ECH81 - 1 x EM84 - 1 x EL84)

so you can know the current available on the heater winding

http://www.radiomuseum.org/r/iskra_sava_ukv.html

Franco

[HotBluePlates]

... friend brought me old 50's tube radio ...

Output transformer is 4 ohm I think, and output tube is noval with PIN 7 going to OT

I will for sure scrape this for parts (resistors and caps, there is cool 50+50uF vintage looking MM cap), but is there any small tube guitar amp project that is suitable to this spec build?

When I started out, my biggest mistake was taking something like this and thinking I'd strip the chassis and build something completely new in it.  And that I'd have "tube amp transformers" for something else.  What happened to me is I took the stuff apart, found out it wasn't suited to making some other guitar amp circuit I'd chosen, and then the parts sat unused for (now 25) years.

From a design standpoint, the power transformer, power supply (and rectifier type), output tube (type and circuit), and output transformer (as well as phase inverter and feedback, if present) are all sized to suit each other in the original application.

For that reason, the best advice I can give you is keep the power supply, output transformer, output tube(s) and speaker load 100% intact from the original product.  Preamps can be tinkered as far as space, sockets, your imagination and practical reality allow.  If you replace that radio frequency (and intermediate frequency) section of the radio with a standard guitar preamp circuit, you'll almost certainly wind up with a usable small amp.

I also believe you'll get the best value out of your salvaged item by adhering to the above.  I know I destroyed some perfectly-good items and wasted what parts I "recovered" from them because I didn't understand enough about amp building & design...

[PRR]

> the power transformer, power supply (and rectifier type), output tube (type and circuit), and output transformer (as well as phase inverter and feedback, if present) are all sized to suit each other in the original application.

AND the audio section IS an audio amplifier super-similar to a guitar amplifier.

On a simpler rig, you grab the top of the Volume pot, put one stage of gain in front, it IS a guitar amplifier.

Iskra seems to have made a lot of lab equipment. Only a few radios. The earlier Opatija deluxe seems to be a very related model. I would assume the audio section is 95% the same.

EL84 power output, worked near 10W Pdiss so 3W-4W output.

EABC80 in front, which is a hi-Mu (70) triode plus a mess of diodes to ignore. Think of half a 12AX7, 12AT7, or 6SL7.

There is an interesting tone control between the EABC and the EL. I would try to leave all this dead-stock.

The EABC grid runs to the Volume control. There is NFB from speaker back to the bottom of the tone control. This will work. In fact I am not sure it does much. (Some of this looks like anti-Capitalist engineering, more parts is more work for Workers, and good politiks.)

There is a ton of switching ahead of the Volume control for the various radio bands. Shielded wire to top of Vol pot, break here.

You can drive this directly with a guitar, but you will need a very strong arm to get near all 3 Watts out. You can put a booster pedal in front, beat it, see how you like it. The full monty would be to clear most of the EF89 socket, rewire as triode, use that as preamp. (Treat as-if it was 12AU7.)

[frankenxtein]

Iskra  Very interesting , military radios , Marshall ,  http://www.astronautix.com/i/iskra.html

 I was there when it was Yugoslavia , chasing those evil Soviets ahhhh the good old days  

[kagliostro]

Ciao PRR

I don't think there are EABC80 tubes on that radio

on the Radiomuseum link I posted there isn't a schematic, but the guy who posted the data indicated

"ECH81 ECH81 EL84 EM84"

http://www.radiomuseum.org/r/iskra_sava_ukv.html


The schematic (unfortunately not much readable) seems to be this

(EDIT: no, I don't think, I've read there are different chassis models, so they have different tubes)




Franco

[PRR]

ACCKKK!

If this SAVA is wired on that plan, it is HOT CHASSIS!!

Note that audio common "ground" is tied directly to the power line!!

There "is" a transformer but only for the heaters.

This can not be made into a guitar amp without much more investment than it is worth.

Harvest the EL84 and OT.

[kagliostro]

If those on the schematic is really the circuit of your radio

you can recovery also the PT but only as a heater transformer, as PRR say, no B+ from it

Franco