Low Wattage Amp To Pre Amp Conversion NEED HELP

[MR.Moochy]

Hello, I'm new,
Just wanna test the waters here, hopefully there's not a million of these identical posts

I have a early 60s "hardware store-ish" amplifier
It has one knob, which acts as a on-off switch as well as volume 1-10
Recently the output transformer has died
When turned on, the output transformer puts out a spark

I'm wondering if anyone has an idea for turning this amplifier into a sort of pre amplifier
To run directly into a higher powered tube amplifier

This amplifiers tube layout, from the three input jacks, is 12AU6 - 50C5 - 35W4

Any help would be greatly appreciated

[PRR]

Welcome!

> 12AU6 - 50C5 - 35W4

This frightens me--- this is the lineup for a "Hot Chassis" amplifier where the signal is directly connected to one side of the power line. These can be deadly!

But... the image shows both an audio transformer and a power transformer. This would be one of the last of this class of chassis, after repeated re-regulations forced true hot-chassis amps off the market.

You can just buy a 2K:speaker 40mA transformer and fix it as good as new. "Good" being in the ear of the beholder. These amps are junk. The world is full of much finer designs. However guitarists still find use for "bad" amplifiers.

Hammond 125ASE or 125BSE will work. (The A lump is technically "too small" but is likely bigger/better than the original... these amps were cheap to the core.)

A Power Transformer like 120VAC:6VAC 5VA (about 1 pound weight) will also work. This would be far from optimum cost/performance except small power transformers turn-up cheap and free.

You really should change over to a 3-pin power cord. This will be a little safer and less buzzy than the 2-pin with capacitor to chassis.

[printer2]

How about a Marshall type preamp into a Class D amp? Use a bridge rectifier on the 120V transformer for the high voltage supply.

[vigilante397]

I agree with everything that's already been said (including the welcome to the forum ), but if all you're looking to do is turn it into a preamp you could always just try disconnecting everything after the volume pot and taking the middle lug of the volume as your output. It's hard to give better advice without a schematic, but that's always worth a try.

Side question though, how do you know the output transformer is shot? Such a thing can absolutely happen, but working as a tech I've seen a lot of people blame a bad OT for everything: no output, output that sounds bad, output is too quiet, fuses blowing, etc., and it is very rarely (not never, but very rarely) actually the OT.

[PRR]

> without a schematic

Well, you can almost read it from the picture. Aint many parts in there.

Here is a related plan for thinking or comparison:



(Do NOT! emulate the death-trap hot-chassis power wiring shown on this plan!! All internal power SHOULD come from a power transformer (which my Kent lacks). A half-azzed once-legal variant gets B+ from transformer but all heaters run right off the wall-plug-- this is "ok" IF the heater insulation does not break-down. Which it can.)

[Jeema]

To answer the question:

Yes, it could be turned into a pre-amplifier.

You could replace the broken output transformer with a power resistor and coupling capacitor and turn the last stage into a common cathode amplifier stage which could then feed into an amp (although the 50C5 is a beam power pentode that was really designed to drive an output transformer).

The resistor would have to have a high enough wattage rating to handle the B+ voltage.

This would only be recommended if the amplifier has a power transformer and is not the transformerless type like other people mentioned.  Otherwise it would be a safety risk to attempt this.

[anotherjim]

I think there are possibly 3 good things in that kind of antique.
The output transformer (OT).
A retro case.
A smooth sounding old speaker.

It sounds like your OT is bad if it's emitting sparks. It could be that the speaker has an intermittent connection (commonly a fracture in the braid at the cone end). The sparking could, if you're lucky, only be on crap that's become conductive - but more likely the transformer coil insulation is fried in which case it's junk to all except those who like winding transformers.

[pinkjimiphoton]

I agree with everything that's already been said (including the welcome to the forum ), but if all you're looking to do is turn it into a preamp you could always just try disconnecting everything after the volume pot and taking the middle lug of the volume as your output. It's hard to give better advice without a schematic, but that's always worth a try.

Side question though, how do you know the output transformer is shot? Such a thing can absolutely happen, but working as a tech I've seen a lot of people blame a bad OT for everything: no output, output that sounds bad, output is too quiet, fuses blowing, etc., and it is very rarely (not never, but very rarely) actually the OT.

if you do this tho, check to make sure there is no dc coming off the volume pot, as it will melt whatever is downline from it. i'd put a decent sized cap from the wiper of the volume as a new output just to be safe.

these amps are wicked dangerous as paul and others repeat... you should probably invest in an isolation transformer with these kind of crappy 6o's amps.. i gotta kay 5o3 widowmaker on my bench waiting for me to find the iso i ordered from it. the prob with these amps is the chassis connects to neutral in a wall outlet as ground, not ground... so if you inadvertantly reverse the power plug, your chassis becomes hot with line voltage. zzzaaapppppp cracklecrackle DEAD.
so please be careful with it.

me? i'd gut it, use a class d power amp off ebay for 5 bux and build my fav fuzz for a front end.