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beginner forum: for novice and developing blues harp players > My Kriesler radio, reinvented as harp amp
My Kriesler radio, reinvented as harp amp
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SuperBee
3675 posts
May 01, 2016
7:32 AM
Photo
MindTheGap
1552 posts
May 01, 2016
7:41 AM
Looks very much like you just thrusted the mic jack, with force, into the front of the unit! I guess you've put a socket behind there! :)

When I describe the 'small amp tube sound' to people of a certain age, I sometimes say '1940's Bakelite radio sound'. They know what I mean.

I suppose a clip is out of the question? :)

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Killa_Hertz
1244 posts
May 01, 2016
7:42 AM
Very cool man. I have a few tube radios ive picked up. I ve thought about making one of these.

Give us a sound sample. What did you have to do to convert it to an amp?
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 photo 1461480733176-3_zpsi8pqqu3q.jpg
MindTheGap
1554 posts
May 01, 2016
7:44 AM
Ha, you see simultaneous requests for a sound clip. You cannot refuse...
MindTheGap
1555 posts
May 01, 2016
7:53 AM
You see, simultaneous requests for a clip. You cannot refuse us...

For Mr Spam Filter:
I got a hundred dollar car
Worth a thousand dollar loan
If that collection lady calls
Tell her I ain't home
Overdue bills
Stacked a mile thick
If you got a plan
Well you better talk quick

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Last Edited by MindTheGap on May 01, 2016 7:54 AM
SuperBee
3676 posts
May 01, 2016
8:00 AM
Usually I like to use the embed widget but doesn't seem to be an option with iOS so I hope the link is ok.
I have some more photos but I don't want to scare people!
That is the cab of a 1964 Kriesler model 11-81A mantle radio receiver. For a non-transistor radio it's quite compact.
It still has the original transformers and speaker, a 6"x4" oval. B+ voltage at the first filter cap is 300vdc which is about right for an EL84 amp.
The output transformer is tiny but does the job
All the receiver has been gutted but incredibly the amp is still using electrolytic caps recycled from the era it was built. Until recently the pots were original but I had to replace the tone pot as it had developed an internal short. It's a bit tricky to find a 25k pot with long shaft and power switch...ended up with a 50k for $10 plus $17 postage...but also have my amp back and working properly!
Amp is a single ended class A about 4 watts, 12 ax7 preamp, el84 power tube. The preamp stages are cathode biased without bypass for fairly moderate 20-25x gain. Quite useable for a harp mic. Standard arrangement with volume and tone circuit between the preamp stages...or as Rick Davis likes to call it, a 'tone recovery stage'...
I really like this amp. Turned up past noon is a bit much for the internal speaker so there is a cutoff Jack to allow use of an external speaker cab. Sounds great with an old 12"
The wiring is alarming. It's 'old school' messy point to point.
I have in mind a future project to lay out this circuit in a turret board and build it as a new amp. I think the tone control is very interesting but not sure what I'll learn when I analyse it...the build is making use of parts the builder had scavenged so perhaps a design based on the circuit could achieve similar results without the eccentricity.
I posted elsewhere that I don't much care what an amp looks like, but I do enjoy the looks of this one in its old Bakelite cab with illuminated tuner dial showing the call signs of the old Australian am radio stations...
SuperBee
3677 posts
May 01, 2016
8:07 AM
Ah,well I'll try recordings. It'll have to wait a while as I'm away from home for slightly indeterminate time.

Here's another photo, some of the circuit
tone circuit
SuperBee
3678 posts
May 01, 2016
8:20 AM
I am going to tidy up that input Jack. It's been done very roughly.
Rectifier is 6x4 btw. The conversion is not too tricky I suppose. This was built by Mark Carter in Belgrave, Victoria. He has gutted the chassis which originally held 5 tubes I think. I suppose he started with the power supply. Really it's a matter of ditching the receiver and keeping the amplifier. So he kept the tone and volume, removed the tuner and installed an input Jack in the space. Power supply filter caps were probably fairly appropriate as they were...but that matrix of caps, pots, resistors and earth points...that's gonna take some study.
SuperBee
3679 posts
May 01, 2016
8:30 AM
more photo

Those Elna caps are very old I think. The values are certainly specific to the original radio.

Last Edited by
SuperBee on May 01, 2016 8:32 AM
SuperBee
3680 posts
May 01, 2016
8:34 AM
This is the problem when you post links...spam alert!!! Someone's posting html...
MindTheGap
1556 posts
May 01, 2016
8:43 AM
Yikes. I see what you mean.

That's proper electronics!

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