Granco FM 610U puzzle

[John M0GLN]

Originally Posted by usradcoll1

Did you see the tricky way they introduced audio feedback? They just connected the low end of the volume control to the output transformer secondary.

I noticed that wire when unsoldering the speaker from the output transformer but didn't realise what its function was.

John

[John M0GLN]

Originally Posted by usradcoll1

The resistor is installed very neatly, but it does look like an afterthought!

Another reason for thinking it could be an afterthought, I've just seen on another forum where a poster said that his Granco had a 50C5 instead of the listed 35C5, mine has the same, now the original line up of valves with a 35C5 plus series resistors ( 4 X 10Ω ) would give a volt drop of 110.4V, now when Granco made this 470Ω resistor modification the volt drop would reduce to 103.4V, a bit too low for US mains, the easiest way around this would be to change the 35C5 to a 50C5 to give 118.4V.

John

[Nuvistor]

Are you saying that without the mod the valve used will be 35C5, with the mod it will use a 50C5 valve?

Whichever way the 19T8 is going to be severely under run, always a possibility the stage had too much gain and as Lawrence suggested a quick bodge to keep the radios moving on the production line.

I have looked at service magazines from 1955-57 and can find no service info apart from the one posted in this thread.

[John M0GLN]

That's what seems to have happened, original schematic shows 35C5 without the 470Ω resistor and the 19T8 heater running at 19V, with the 470Ω resistor added the 19T8 heater will drop to 12V and the heater chain will require a mains supply of 103.4V, too low for the US mains, two sets I've seen ( one on the web and mine ) with the 470Ω resistor have had the 35C5 changed to a 50C5, presumably to suit the US mains voltage.

The 19T8 will be under run with 12V instead of 19V but that's what's been done.

John

[usradcoll1]

Originally Posted by Nuvistor

Are you saying that without the mod the valve used will be 35C5, with the mod it will use a 50C5 valve?

Whichever way the 19T8 is going to be severely under run, always a possibility the stage had too much gain and as Lawrence suggested a quick bodge to keep the radios moving on the production line.

I have looked at service magazines from 1955-57 and can find no service info apart from the one posted in this thread.

I just found a schematic for a year older AM-FM model Granco receiver that shows the 470 ohm, 1 watt resistor across the 19T8 heater. It also shows the 19T8 heater in the middle of the series chain, instead of the end.
Maybe the set in question, is possibly wired the same.

[John M0GLN]

That's interesting, so the under running of the heater seems to be a bit of a Granco peculiarity, I've just checked mine and the 19T8 is between the 50C5 output and the 12BA6 IF amplifier, not as shown on the Sams schematic, is the output a 50C5 or a 35C5? But the AM-FM model may have an extra "tube".

John

[usradcoll1]

The set I'm referring to is the model 750, AM-FM receiver shown in the 1957 Supreme manual, which are reprints of the original factory issued schematics. The set is considerably different than the FM only model and it does use a 35C5 output and a 12BE6 AM osc mixer.

[turretslug]

I wonder if this under-running was to keep hum low? One mechanism for hum production in valves is stray ends/loops of heater wire sticking out of the ends of the cathode cylinder that become emissive and inject a hum component into the anode current- as the cathode coating is deliberately formulated to be plentifully emissive at lower temperature than plain metal, backing off heater temperature can minimise emission from the actual heater without undue effect on the cathode's emission. Thermionic detectors of any sort are vulnerable to hum, and the hum level can vary greatly between makes and individual samples of valve depending how tidily the heater is contained within the cathode, so Granco might have been covering their backs by under-running.

[usradcoll1]

That was the impression I got, hum reduction!
I guess, I never noticed some of the steps taken to mininise hum or distortion in the first amp stage.

[Nuvistor]

It was suggested in an early post about him reduction, I had not heard of reducing the heater voltage to reduce hum before. If it’s been used in UK sets I have not seen it.

It does question the quality of some 19T8 valves that were in production at that time, perhaps running the heater at 60% rating also led to the valve being replaced more frequently, though not an intentional side effect.

I find it fascinating the way problems were overcome to keep performance high and costs low.

[ukcol]

My calculations suggest the power dissipated in the 19T8 is reduced to about 1.5 watts from 2.85 watts by the addition of the 470 ohm resistor. It occurs to me that perhaps the 19T8 heater was vulnerable to failure due to imbalance of thermal inertia in the heater chain and this is a cheap get round to improve reliability.

[Nuvistor]

That’s certainly another possibility Colin.

Edit
I have found this page from 1961, a 19T8A was introduced with controlled heater warm up time.

http://www.mif.pg.gda.pl/homepages/f.../12AT6Aetc.pdf

It’s list of valves introduced with controlled warm up, hence the 12AT6A number in the link.

[ukcol]

Originally Posted by Nuvistor

That’s certainly another possibility Colin.

Imbalance of thermal inertia in heater chains is a relatively new concept to me and is an example of the many gaps in my electronics knowledge that have been filled by this forum.

See GMB's post number 2 in this thread.

[John M0GLN]

Thanks for all the interesting posts, it looks like the Granco reason for adding the 470Ω resistor was an attempt to compensate for problems with the 19T8, problems that would not have shown up in the original design but if appearing later the resistor would have been an easy mod' to do both on existing sets and future production, unfortunately the set in my possession doesn't work very well, the sound is very faint so I can't try it with or without the resistor to see if there is any change.

John

[usradcoll1]

Are you using a transformer to power it, as it is designed for US 120 volt mains?
Does it still have the original selenium rectifier and is the HT (B+) the correct voltage? If it's low, the storage and smoothing caps might be suspect.
The schematic should be close enough, even though it's not the proper one.

[John M0GLN]

Yes I'm using a transformer which I've just measured and it's giving out 119V, the selenium rectifier has been replaced by a 1N4007 and R21 the 900Ω resistor by a 1000Ω resistor, these two mod's were done before I bought it, the other voltages including B+ are all about 5-10% higher than the schematic ones, it seems to have got worse in the last few weeks, it used to take about 20 minutes to warm up and then suddenly come loud but now it just stays very quiet, it could be a dry joint or something which changes with temperature, I'll have to have a good look at it, the only thing I've done is to replace C2 the 2µF electrolytic across the 19T8 ratio detector and C24 the 10000pF coupling capacitor for V6 which is now the 50C5.

John