Grundig Tube Radio…need to replace cathode resistor, value unknown...

[TubeGeek]

Grundig SO 121 CA Tube Radio Console on the bench.


There is no audio output. Output tubes are 2 x el84.


Found the el84 pair has an open shared cathode resistor.  Unfortunately I cannot tell what value the resistor should be.  Measures 3MΩ on my DMM, that can't be correct.  All i can make out on the resistor is 80...


I have been looking for the schematic for about a week now online with no success.  There is a radio museum website that may have the schematic available but my customer will not pay $25 for the schematic.  Radio museum requires that I create an account and pay a $25 fee to see schematics.


The owner of this radio is a colleague so I am trying to do them a favour.


I did find another grundig radio schematic that uses one el84 output tube and it's cathode resistor was 170Ω.


What would a safe value of cathode resistor be? 80Ω? 250Ω?


The cathode bypass cap is 47µF 160V.

[sluckey]

A lot of p/p EL84 amps use 130 or 150Ω cathode resistors.

[kagliostro]

A pair of el84 in PP

the Vox AC15 uses a 130R 5W resistor

I think to be safe you can use a 200R 5W resistor

EDIT:

Well I give a look to some radio schematics (that are SE, so you can consider that the catode resistor is two times the value required in a PP)

http://www.stereo8.net/data/uploads/grundig-tk-32/grundig_tk_32_schematic.jpg

http://www.mbzponton.org/n2awa/radio_Grundig_2066PX_schema_English.jpg

http://www.tube-classics.de/TC/GermanTubeHifi/Poweramps/Grundig%20NF2/NF2schematic.jpg

http://www.stereo8.net/data/uploads/grundig-tk-32/grundig_tk_32_schematic.jpg

and as told by Sluckey 130R 150R is a good safe value 200R will be too cold

K


p.s.: I posted at the same time with Steve (Ciao Steve)

[TubeGeek]

If a SE el84 is using a 170Ω value, a PP should be double that value to be safe, correct?

[TIMBO]

Hey TubeGeek, I agree with the guys. I have a Hammond amp from an organ, it has a PP el84 with a 130r resistor.

[TubeGeek]

OK that's what I'll do.  Thanks again guys.

[eleventeen]

That Radio Museum place isn't my idea of a good time.


I once found a guy who had the garage of WW2 radio doom---and among tons of stuff, he had Audel's (Maybe it was another brand) "Perpetual" Radio Manuals which was SEVENTEEN FEET of shelf-space of 4" & 5" binders of schematics. It was huge. enormous, brands or radios that went away 50 years ago.


Anyway, I contacted Radio Museum and donated it all to them. A local guy came and picked it up.


So, do ya think I could get 1 stupid free schematic out of them a few months later?


Nope.

[kagliostro]

What I really don't like about Radiomuseum is this

A friend told me that Radiomuseum accept members that didn't pay the 25$ fee, you must give them some schematics that they don't have

and for a period of time you can consider as a member

What I don't like is that this friend explained me that Radiomuseum consider of their property the schematic you sent them and you are no more

allowed to give to other those schematics 

No, I can't accept that a schematic of an old radio or recorder or transmitter is property of someone that pretend to be payed for it even if they have nothing to do with the properties of the original brand

K

[HotBluePlates]

... Radiomuseum consider of their property the schematic you sent them and you are no more allowed to give to other those schematics 

Well I doubt Radiomuseum is going to kick down your door if you give a schematic to someone else, so tell them, "Andate tutti a 'fan__lo!"

[kagliostro]

"Andate tutti a 'fan__lo!"

Before Cyrillic, now the Italic language, HotBluePlates I had no idea that you were such a connoisseur of languages

 

Franco

[HotBluePlates]

I cheated and found a list of Italian curses...  I figured a direct translation of American curses wouldn't make sense to an Italian ear. 

[kagliostro]

found a list of Italian curses..

were you by any chance referring to the speeches of a renowned but recently arrived in politics, as Italian (not) parliamentarian

that is famous for that course

 

Franco

[PRR]

80 Ohms, 1 Watt

> cathode bypass cap is 47µF 160V.

Factory-schem value is 50uFd, no voltage given. 47u 160V is likely to be a modern replacement, and is of course all the same. Voltage is supposed to be 7V (any higher would melt the tubes). So you could go smaller, but what's in there is fine.

> to be safe you can use a 200R 5W resistor

Guitar amps run much higher plate and screen voltages than Grundig did. The 80 Ohm value is correct for Grundig's conditions.

[TubeGeek]

80 Ohms, 1 Watt

> cathode bypass cap is 47µF 160V.

Factory-schem value is 50uFd, no voltage given. 47V is likely to be a modern replacement, and is of course all the same. Voltage is supposed to be 7V (any higher would melt the tubes). So you could go smaller, but what's in there is fine.

> to be safe you can use a 200R 5W resistor

Guitar amps run much higher plate and screen voltages than Grundig did. The 80 Ohm value is correct for Grundig's conditions.

Thank you so much PRR.  I was thinking that half of the 170 ohm value I found online is what it actually was.

[Ritchie200]

TG

I know you got an answer but maybe this will help too.

Jim

www.sinocharter.com/SO121Schematic.jpg

[kagliostro]

Note that the final tubes of which TubeGeek is talking are el84, the final tubes of the schematic are el95

K

[TubeGeek]

While looking for the schematic, I found a forum where someone had posted pictures of this same console unit and in one of the pictures, there was a schematic.  I emailed the person and asked if they'd help a fella out and lone behold…  I received a perfect picture of the schematic today!  It was clear and it is 80Ω 1W.


Thanks for all the help.


update: replacing the cathode resistor brought the amplifier back to life. 

[Refugee]

Before replacing the resistor you need to find out what caused it to burn out.
It looks like you have been lucky and the resistor has let go before the output transformer.
The test you need to do is to pull out all the EL84s and power it up and then measure the grid one voltages. They should be zero.
If those coupling capacitors are letting anything through it will burn again. Perhaps replace them anyway even though German capacitors are usually reliable on sets of this age.
Check all four grid resistors with a meter with power switched off on the radio.
Once those are checked and replaced if faulty replace the 80 ohm resistor and bypass capacitor and power up with a volt meter connected across the cathode resistor and with the tubes fitted and note the voltage reading should be about 7 volts.
If it goes high pull one tube at a time to locate the faulty one. Then move the faulty one to the other channel to prove it.

[TubeGeek]

Before replacing the resistor you need to find out what caused it to burn out.
It looks like you have been lucky and the resistor has let go before the output transformer.
The test you need to do is to pull out all the EL84s and power it up and then measure the grid one voltages. They should be zero.
If those coupling capacitors are letting anything through it will burn again. Perhaps replace them anyway even though German capacitors are usually reliable on sets of this age.
Check all four grid resistors with a meter with power switched off on the radio.
Once those are checked and replaced if faulty replace the 80 ohm resistor and bypass capacitor and power up with a volt meter connected across the cathode resistor and with the tubes fitted and note the voltage reading should be about 7 volts.
If it goes high pull one tube at a time to locate the faulty one. Then move the faulty one to the other channel to prove it.

Thanks for the advice. 


I replaced the el84 power tubes, all electrolytics and the cathode resistor.  Couldn't see any further problems.  Voltage was around 7 volts on the cathode which is what you and PRR mentioned.

[Ed_Chambley]

I am a member of the site Radio Museum.  I have also shared much information there.  Attached is the Schematic.  There are specific reasons why the community doesn't openly share schematics, but if $25 is too much to have access to all the radio information as well as photos and contacts of people who may just have that original zenith knob you are looking for, you are not into radio.

Really, we only want members who are into radio.  The $25 can be avoided, but it is very cheap price if you have ever tried to restore a nice old radio.  The contacts selling parts who are members will save you way more than $25.

Anyway, I am a member in good standing and absolutely don't know where else I could get information and some radio parts if it were not for the Radio Museum.  Not to mention the cool photos.

[sluckey]

So now that you've teased us with the RF and IF section, what do we have to do to see the audio section? That's what TubeGeek really needs.

[Ed_Chambley]

Try this