Magnatone 425 hum problem

knuckleberryfinn

I bought an old 1961 Magnatone 425 bass amp which had been chopped and turned into a head last year and finally got round to trying to get it working again on Friday.  By some miracle or other - after rebuilding the whole of the power amp which was a complete mess, and replacing all the 220K plate resistors and a couple of pre-amp caps - it works!  I was planning on selling it but it sounds so bloody good I'm going to keep it - I don't think they're worth much anyway.  

The only remaining problem is that there is quite a lot of hum.  I'm in the middle of doing all the usual stuff to get rid off this (and it seems it's the grid lead going to V1b, or maybe the star grounding scheme), but I've noticed there's an anti-hum switch in the schematic.  This has been disconnected, so I'm wondering if it's unsafe by modern standards.  Is it safe?  Might it help?  Here's the schematic...

http://elektrotanya.com/PREVIEWS/63463243/23432455/magnatone/magnatone_425_sch.pdf_1.png

ICBM

The 'anti-hum' switch has been disconnected to make it *safe* - whatever you do don't reconnect it, at the very least not with the original cap. It is safe with a modern Class Y cap, but no other type.

It connects the chassis to the live or neutral wire to suppress noise in the absence of an earth wire, which was not available in the US in the old days. The connection goes via the cap - often called the 'death cap' for good reason, because if it shorts then the chassis (and the guitar, via the cable) can become directly connected to the live. A Class Y cap is certified to produce a guaranteed open circuit in the event of failure and is intended for live connections.

Better still is to fit a proper mains earth, which you need to do by replacing the cable with a three-wire one, and using a stepdown transformer with a through earth connection, which most do have.

Have the filter caps and all other electrolytics been replaced? If not, do so.

knuckleberryfinn

Ah okay, thanks.  I didn't realise this was the notorious death cap.  It did seem a bit risky to be attaching the chassis to live via a single .047 cap!  I won't be reconnecting it.  

The mains cable is three-way, and earthed through a step-down transformer.  I've replaced all the electrolytic caps.  So it's definitely safe.

I've been moving the shielded cables between V1A and V1B around (the chassis and control panel are separate on this amp, with the chassis mounted in the bottom of the cab - so V1B's shielded grid wire is a metre long!) and I think once it's mounted in a cab and all the cable runs snug against each other it won't be too bad.  I'm going to fit a master volume, which will make the hum more manageable too - the amp is so loud that the hum is completely insignificant when playing through it.  I was planning on fitting a LAR/MAR PPIMV but the bias resistors in this amp are 100K instead of the more familiar 220K.  Can I still use a dual 500K pot?  I've got a couple and would rather not have to buy a 250K one.  Could I adjust the values of the potential divider in the bias supply so that the 100K resistors to the grids should then be 220K?

ICBM

Yes, you can use a dual 500K. The best way is to parallel the upper and lower parts of both sides of the pot with fixed resistors - about 120K should work - that not only gives you an effective pot value of between 96-160K at any adjustment, it makes the bias feed proof against pot contact failure.

I would guess the 1 metre grid cable could have something to do with the hum .

knuckleberryfinn

I'm not sure I understand that.  Do you mean 120K across each pot track?  How would it work as a volume control like this?!

I used the one below for my Bassman and it works a treat, but was just wondering how to get the bias feed right.

http://tylergrund.com/images/tylers_PPIV.jpg

knuckleberryfinn

I've just realised that if I mount the master volume on the control panel I'm going to have another set of metre-long grid wires.  Damn.  Maybe I should put the master volume in the chassis?

knuckleberryfinn

ICBM said:

Yes, you can use a dual 500K. The best way is to parallel the upper and lower parts of both sides of the pot with fixed resistors - about 120K should work - that not only gives you an effective pot value of between 96-160K at any adjustment, it makes the bias feed proof against pot contact failure.

I would guess the 1 metre grid cable could have something to do with the hum .

Sorry @ICBM, I think I've got this all muddled up.  Am I right in thinking you are saying I should do the LAR MAR as above, but change the 2.2M resistors for 120K?  I've just realised that the LAR MAR one IS the 250K pot version!  

I built it up with 2.2M resistors at first and was getting a variation to the bias voltage on the grids from -25 down to -14.  So that alerted me to the fact that I had used the wrong pot.  I changed it to 1Meg, and the voltage variation was still large, but not as bad.  Using 120K I am still getting a variation between -25 and -23.  Does this matter?  Should I try and get it so it doesn't change at all, or not bother?    

ICBM

Yes, basically that circuit - but you can go even further and add another pair of resistors between the other two pot terminals, ie four resistors in total.

I do this on amps which are prone to poor PPIMV pot contacts - ie Marshall DSL201/401s especially! Since it prevents either bias failure or signal drop-out, and the worst that happens is an unbalanced waveform. I use 1M resistors normally, since the goal is just to ensure a continuous contact rather than reduce the value of the pot.

Yes, I would put the MV in the chassis and keep the wires as short as possible.

knuckleberryfinn

For some reason I just couldn't get this to work - I checked and checked the wiring and couldn't find anything wrong.  In fact, I seem to remember having trouble with it last time, but I couldn't be bothered to open up my Bassman and see how I'd done it.  So I just separated the master volume from the bias supply in the end, as below.  I used .1 caps instead of .022.  This works fine.  Thanks for the advice - especially about the death cap!

EDIT: oops, forgot the image...

http://www.ampmaker.co.uk/artiste/postpi.gif