Hi guys well I seem to be addicted to these old valve radios….and now found myself with a Mickey!

It’s a green 5 valve version - and works!

As usual though I stripped it down to check & it’s been mostly recapped already with just the 16μF electro remaining.

I cut the + terminal off under the chassis & hooked up a new one there.

It sounds pretty good - but with no tone control it’s a bit tinny for me…it’s had a new output transformer too I think as it looks newish.

Now I’m looking to add a little more bass & just wondering where to start….I upped the capacitance on the coupling caps on others to achieve this but I’m intrigued with the “bass boost” of these Astor’s….seems they have a centre tapped volume potentiometer which runs through a 15k resistor & .01 cap in series to ground which presumably grounds out the high frequencies at low volume.

Rather than up the coupling caps & add bass everywhere I’m just thinking of changing the .01μF c6 to .02μF.

But of course wanted to run it by the guru’s first! Do you guys think this is the way to go or have some better ideas?

This old astor sure looks so good at night - the green lit up Bakelite!

Thanks again!

This model is one of my favourites and normally have a nice rich and mellow sound without modification. The bass boost increases as the volume decreases below the tapped part of the control. Is the output transformer a correct type with and air gap core? You would have to be careful adding bass with a small set like this as it will overload it quickly. Bigger coupling caps can also introduce other issues like motor boating etc.

That’s good to know they normally sound so good! This is alright….but quite “trebbly”…

The output transformer looks newish - I suspect it’s been replaced. It beats the code e2 7000 on it - not sure if that’s a modern replacement?

I couldn’t help myself and increased the volume tap capacitor from .01 to .0122 - not much of a difference though!
Not sure how much I can go with it? Or maybe I should try & find a genuine output transformer instead?
Does anyone have a pic of the original to compare? I’m not sure how to upload a pic…I’ll see if I can…

The capacitor change will just alter the corner frequency. You could try and reduce the 15k series resistor to add more treble attenuation, being mindful that this only comes into play at volumes below the tapped point. Is the tapped connection still connected ok within the pot? This will cause no treble attenuation or “bass boost”. If the radio is weak, then the volume control could be adjusted too high for the boosting to be effective at normal listening levels.

Thanks Dave - wish I read this first! I increased coupling cap to output valve to .03μF which has helped a bit more….but would rather have just tinkered with bass boost section first. I may try reducing the resistor next…I guess it’s a bit of trial and error.

It’s interesting to contemplate where the ‘bass boost’ reduces when adjusting volume…I assume I’m around bass boost area as volume is only up 1/4 turn but I’d have to get the multi out to know. The volume switch seems to have some issues as volume is jerky & has static…& it’s tricky to get much contact cleaner in it.

With the increase in coupling cap & quite small increase in bass boost cap it’s sounding pretty nice now though!

I noticed when I pulled it down earlier the transformer was quite hot though - is a hot transformer normal on these?

Thanks guys.

I’ve never noticed the transformers in these getting overly hot. Power consumption should be around 40W so I would test for that. The little 6BV7 output runs pretty hot in these, but they can handle it without any problems. If the electrolytics are originals, I’d change them out, they’ll be shot and possibly consuming power.

Hmm I can’t easily check the power consumption…I’m a bit short on equipment. Was hoping to visit melb owner radiofest and check what equipment was for sale but couldn’t get down there. I’ve recapped the radio so caps are (hopefully!) all good.
I may check a few voltages and go from there - it was on for a few hours so perhaps that’s why as it doesn’t seem to get hot quickly.
I was surprised how small the output valve was - compared to the 6m5 in my hmv it’s about 1/2 the size!
It’s still a bit trebbly - think I’ll get an adjustable resistor to replace the 15k with so I can tinker easily with it…will basically be like an internal tone control (well at low volume only!).

You could also try say 25μF across the back bias resistor 125 Ohm. Abt 40V positive to chassis.

Hi Marc,

I’m bit of an amateur but can you (in simple terms lol) tell me the logic of how that works?

I can see that resistor - r25 so can easily run a parallel cap there.

Might be a lot easier than modifying ‘bass boost’ or adding more coupling capacitance….already upped that from .02
to .035….don’t know how far I can push it!

If you push to far you get distortion. The cap will at to short the AC signal and it will have an affect on the negative feedback and tube gain. It may attenuate the bass rather than help it so just clip it across rather than wire it in.

Marc

Well I’ve done all these ideas - added a vari resistor to the bass boost and also a back bias resistor cap.
The vari resistor really works like a tone control - although once volume cranked up about 1/2 way it stops!
But I think over 1/2 way is really out of normal listening range.
I didn’t notice a difference with a back bias resistor cap - but left it in (I actually just soldered it in the end - was too risky to clip!).

Overall sounding quite nice now!! They do have a certain magic these mickeys!

Pimp My Radio! You now have a hotted up Mickey

Haha! Sure sounds as good as it looks now!

I was mistakenly focusing on that bass boost & cap not the resistor….sounds so good with the extra bass now (well just less treble technically haha).

I tinkered with the bass boost cap just for fun with the radio on & as you described it just changed the cut off frequency…I ended up going back to the factory .01μF as that kept most of the sound intact but for the upmost ‘tinny’ frequencies….higher capacitance there just stripped more of the sound out.

I think my old Kriesler 11-97 is still my favourite to listen to as it’s hard to beat the 6 speakers but the Mickey is just so retro special with the bright green case & rich sound

Thanks again for your help guys!

Hi Marc was just interested to know as I’m trying to learn but the old HMV I was working on was a cathode resistor biased which I understood.
Just wondering with this Mickey does the back bias resistor mean this is fixed bias so that adding the cap accross the bias resistor kind of try to replicate a cathode bypass cap?

Update I’ve done some reading & learned that I had the cap the wrong way around! If it’s back biased then the tap will be -ve compared to chassis….so needed + attached to the chassis. I didn’t quite understand that…so I’ve corrected it now & I do think that has made a further sound improvement. So much to learn….


Thanks!

Photos uploaded.