Radio amp fuzz

[Digger1770]

Hi all, I have asked this question on my other favourite forum AGGH but thought it would be good to ask here also, so forgive me if anyone sees both.
I scraped a smashed Philips Philadelphia radio the other day. They have separate tuner and amp PCB's and was going to mod the amp for a small practice amp for a friend.  During testing of the circuit I reduced the voltage on my bench power supply and got a great fuzz sound below 7v right down to a broken splatter fuzz at 2v    . I have now built a variable power supply to go with the amp board to get this effect from the circuit. I would like to set this up as a pedal that can be switched back to a practice amp with external speaker (original Hi Flux from the radio that I have reconditioned). Any opinions on how to tame the speaker output to a amp input level? I cannot find the original schematic online but was looking at pot on the output for volume ( say 10-20k) and a switchable resistor in series to reduce the level when used as a pedal.
Does this sound ok? Should I use a small transformer to reduce the level?
Cheers Dave

[PRR]

> Philips Philadelphia radio

Unlike many modern transistor amplifiers, this old design *needs* a DC load. A 100 Ohm resistor will do.

There is no protection against shorted load, so you really want 100 Ohms in series between the amp and whatever (non-speaker) stuff you hang on it.

Stock power supply is 14V, so the RMS output is 5V.

You only need 1V for a Line Input, a fraction of that for Instrument Amp Input.

> below 7v right down .... 2v

At these points you get 1V or a fraction of a Volt maximum output.

Pot value should be above 100 Ohms but probably not over 10K if you might drive a long line (to sound booth). In short-cable (on-stage) work 100K is fine.

There's a wide range of loss which may be needed. Use a "Audio taper" pot.

EDIT-- there will be a small DC voltage on the output. You need a blocking cap.

[Digger1770]

Pot value should be above 100 Ohms but probably not over 10K if you might drive a long line (to sound booth). In short-cable (on-stage) work 100K is fine.

There's a wide range of loss which may be needed. Use a "Audio taper" pot.

Thanks PRR thats great, and makes good sense.


The voltage reg is on the vero.
The amp also has a bias trim pot on the board, would it be useful to make this an external pot?
Cheers Dave

[PRR]

> a bias trim pot on the board, would it be useful to make this an external pot?

NO!

One way sounds awful, the other way burns-up the transistors. Leave the bias alone.

[Digger1770]

NO!

Cool thanks that's one less thing to do anyway. Thanks PRR.
Cheers Dave

[PRR]

Actually....

You could rig a switch to *short* the pot.

Try it first with a clip-lead (and be careful to get it right).

What this will do is mangle the lowest-level sounds. As the signal fades the "tizz" comes up. In a hi-fi, it is a super-annoying flaw. However such a thing is done in a few distortion/fuzz effects.

[Digger1770]

PRR if the trim pot is shorted wont it then be at to high a voltage ?

the other way burns-up the transistors.

The pot will no longer divide the voltage source. I like the idea of this effect though
Could I use an external pot and have suitable resistors in parallel with the pot ie. lugs 1-2 and lugs 2-3 to limit the range of change in the bias
Cheers Dave

[Digger1770]

I had a hour this afternoon and knocked up a box for the speaker, I still need to sand and glue it but it looks nice and retro with the grill from the old radio.
I will try and do some circuit mods tonight hopefully.



Cheers Dave

[duck_arse]

is that jarrah floorboard? I always ask you about wood.

[Digger1770]

Th timber is Tassie oak wit a nice fiddle/flame in the grain, it came from a kitchen reno we did.  I have a fair bit but only some has this nice grain.

Dave

[PRR]

> if the trim pot is shorted wont it then be at too high a voltage?

No. This will cause a LOW voltage base-to-base on the output stage.

The instruction is to adjust bias for 3.0mA through "metering points", but it isn't that fussy and you probably should not mess with that. This gets the transistors off the dead-zero ("dead") point to where they conduct some and will be active for small signals.

By shorting the bias pot you cause *zero* voltage base-to-base on the output stage, thus zero current in the output stage at idle. Very small signals (under 0.1V) will not come out, larger signals will, but with a "dead band" in the middle of the wave. Annoying, nasty, bad for fidelity, "good" for messing-up a signal and standing out in a band.

The effect is rather abrupt and I don't think it is wise to be user-adjustable. Give the user the stock (biased) plan, plus the zero-bias (shorted pot) effect.



Re: that DC current through the speaker (or fake load): TR102 collector current flows through bias network, R111, and then to OUTPUT. TR102 can't do anything unless there is a DC path from OUTPUT. The loudspeaker normally does this. With no loudspeaker, you need a DC path such as a resistor (less than 1K but probably more than 10r, thus 100r). And the 6mA flow through R111 will cause a part-Volt of DC in this dummy resistor, which is too much DC to pass on to further processing or amplification. Thus the coupling-cap I showed.

Nice cabinet. Reflects the style of the radio.

[Digger1770]

Thanks for the reply. The schematic is great I didn't have one.
So the switch will bridge all three lugs? so TPST should do that.
I have some other ideas and questions but I will post when i have more time.
Thanks again.
Dave.

[PRR]

> bridge all three lugs? so TPST should do that

Two outer lugs! (Note that two lugs are already tied.) Or: to short the NTC part R107.

SPST.

[PRR]

> adjust bias for 3.0mA through "metering points"

In this day of battery-power meters that read milliVolts well, you would put voltmeter across a 1 Ohm resistor (R110) and trim for 3mV. Don't worry that it jumps from 2mV to 4mV, or that it may go to 5mV if you get a hot lamp near it.... it's not fussy. You only want more than zero but less than a lot (say 10mV).

And then with bias shorted it should go to zero mV, no idle current in output transistors, nasty break-up on very small sounds.

[FUZZZZzzzz]

maybe a little bit off topic.. i really dont know. I got some really cool results making amps out of these cassette decks. I've made two and they make really cool amps. Just run the guitar through the mic input and you'll get lofi distortion going to reall fuzzy when opening the volume on the deck.