germanium ss guitar amp head horror wiring

[diggy fresh]

Hi folks, sorry , i know this isn't a guitar amp specific forum but the knowledge here is awesome! 

I found a small vintage Sears guitar amp head for very cheap and i'd like to mod it a little to lessen the noise.

It has vibrato, spring reverb, tone and is 100% discrete with transistors. Probably low wattage.

I see the potential in it but it probably lacks a little low end and is now extremely noisy. A loud hum.

I opened it up and was surprised the wiring is totally against everything i learned. (The mains wires making a big loop and very close to the spring reverb smh..).

So i'm thinking or re-routing the wires neater and maybe upgrading the psu to lessen hum as much as possible.

The amp is all germanium pnp so the power supply is a negative one, a very simple Full wave rectifier with two diodes (trafo with center tap) and a 1000uf cap.

I don't have much experience with psus and guitar amps,  could i make a "full wave bridge" rectifier to replace the full wave psu?  And add more filtering?

It will benefit i'm sure from rewiring. Twisting wires, twisting and shielding the spring reverb wires etc.

 

[diggy fresh]

Here attached is a schematic traced from the psu.

By the way, it is a two prong power cord ungrounded, i want to mod it for 3 prong grounded also.

Is adding a bridge rectifier and regulator a good idea? To reduce hum etc.

Thanks

 

[hymentoptera]

Can't help much, really.

But while you're in there you might consider replacing the 2-wire mains cable with a modern 3-wire cable. If the chassis is metal (looks like it) then mechanically secure the ground wire from the new 3-wire mains cable to chassis for protection.

 

[diggy fresh]

Thanks! Yes that was part of the plan.

This amp seems to be wired by a  8 years old japanese kid.

Tons of hum with volume turned all the way down.. (No guitar).

If anybody knows a better forum for this kind of question (amp heads?) Let me know.

Thanks

 

[joaquins]

A bridge rectifier is not suitable for this setup and there is no advantage on using one, in either case you have a full rectified wave.

Adding a regulator may be a good idea, you don't need a negative one since you have only one supply and it doesn't cares which end goes to were, it just puts the regulated voltage at it pins. Then you have to use the output of the regulator as ground and the reference ground of the regulator as your negative voltage.

Measuring with a DMM AC voltage across the PS would give you an idea of the ripple and you will know if that is really a problem or not. Then there is the Q of what is more important, if the PS or the wiring. If the measured voltage is really low probably the wiring, if is not so low upgrading the PSU first may be a good idea. How much is low enough, probably a few mV could be fine, depending on the circuit and implementation.

JS

 

[diggy fresh]

Thanks Joaquins! Very helpfull tips. Cleared a few things.

I plan to redo the psu anyways because it is so simple and badly located..

The main fuse is on the board right next to the amp section,  the two rectifier diodes there too.

I'm not sure about adding a regulator because i'd rather keep this transformer and i don't realy want to lower the available voltage too much.

I could try to make a mini psu board far from the spring and audio section.

There is only a 1000uf after the rectifier, maybe if i boosted the capacitance? Or some more filtering?




Thanks for any info, i'm not realy familiar with guitar amp circuits.

 

[tubestation]

Hello,
The wiring and Trafo placement won t  win a design Price, i agree.
But I don t think the heavy Hum  comes from this. But it s very dangerous and not a good Idea to have the metall Chassis not grounded from the Main Side, this is the first Thing I would change !  The  Voltage is probably -12 V ? You could disconect the whole Powersupply and plug a (Car) Battery or maybee You have a adjustable  Powersupply  from a Laptop or Monitor or another 12v 3-5 Ampere  Source instead and have a listen. The filtering Capitor was replaced already over the Years but this could be the first Part to look at.  I personaly would increase the Value to at least 4700 uF  ( minimum 1000uF/2200 uF - Ampere ) Did You  inspect the PCB ? Are there missing Parts ? A recap  of the Electrolytics or Tantals won t hurt .
Good Luck,
Greatings
Lothar

 

[joaquins]

Thanks Joaquins! Very helpfull tips. Cleared a few things.

I plan to redo the psu anyways because it is so simple and badly located..

The main fuse is on the board right next to the amp section,  the two rectifier diodes there too.

I'm not sure about adding a regulator because i'd rather keep this transformer and i don't realy want to lower the available voltage too much.

I could try to make a mini psu board far from the spring and audio section.

There is only a 1000uf after the rectifier, maybe if i boosted the capacitance? Or some more filtering?




Thanks for any info, i'm not realy familiar with guitar amp circuits.

You could get a LDO regulator to get lower noise and rather low voltage loss, that's what they are designed for.

JS

 

[diggy fresh]

You could get a LDO regulator to get lower noise and rather low voltage loss, that's what they are designed for.

JS

Very good idea Joaquins, thanks, i will check these out, i'm only familiar with the normal lm317, lm337 .  If i can find a decent one not smd, i will try that.

I'll try first to measure the rectified voltage after the trafo when i get a chance. Also the ripple voltage if i can with my simple dmm.

Eventualy, i'll try to do everything i can to reduce the hum below the hiss. Even if it means a completely new external psu.  I'd like to use this small amp for recording.

 

[joaquins]

Don't be afraid of SMD, even less with a regulator which will probably be quite big, and as you are going for a LDO at a pretty low current you don't need much to cool it down, probably with a decent PCB surface you will be good to go. The clearance to solder an SMD D2PAK package may be bigger than a TO92 transistor depending on the footprint. You'll be aiming for less than 3V dropout and pretty low current, since you could connect the output stage to the caps directly and use the regulator just for the previous low power circuits which are much more sensitive, let's say 100mA, about 60ºC/W junction to ambient in worst case, more if you add some area on the PCB to act as heatsink, that's about 20ºC elevation, not even to think about it, you could go twice as much without thinking on it. Of course if you get a TO220 part you could just screw it to the chasis with a silpad and don't worry about any heating problems, but please don't be afraid of SMD, they don't bite!

JS