I messed up the bridge rectifier which is on 300b board and no sound. Also that 300 would not light up. I messed up by soldering the ground to the positive side where the 25 watt resisort connects. Had a pop but didn't fail right away. It gave out while sitting not powered on. Now I have voltage difference on both sides.

Hi dcminnick, re Isolation Transformer..........

An Isolation transformer will not not stop you from getting an electric shock it will limit the amount of current that passes through you , for example if the transformer is rated at 1 Amp them the Max that can flow is 1 Amp causing damage (still enough to Kill you ). If you did this across the mains supply depending on what circuit breaker / fuse you have in your fuse box eg. 100Amp 's could possibly flow ensuring a crispy outcome.

Also it isolates you from the house/building earth limiting max current flow.

The best analogy I have come across to explain this is "Horse and stable analogy".

Basically each horse wants to return to its own stable same as the electrons always return to there source from where they came from , so the electrons want to return to the transformer and not you as the lowest point of resistance resistance.

Do an Internet search for a better explanation.

So at the cost of 100-200$ for your safety is small price to pay . You will at some point have a shock , it happens to the best of us. Save yourself the embarrassment of being hooked up to a Heart Monitor for 24Hours in a paper gown with your "Ass" hanging out in hospital. Don't want to repeat that experience again.

At some point friends / family will ask you to look at some old piece of valve equipment they have so its best to use an Isolation Transformer to test as there is "The Death Cap" syndrome and you will find Live and Neutral reversed on some of the older equipment.

Also another piece of test equipment is the "Dim Bulb Tester" (Internet Search) . The Bulb glows "Bright" if there is a fault. Use these two bits of equipment when you don't know the condition of old equipment and first switch on of new equipment (May prevent a change of underwear in extreme circumstances).

Get the two , then you have always got them.

Chances are this won't be the only Valve Amp you will build ( ooh what does a Push/Pull O/P sound like) if it sound good other people will want one.

Stay Safe, Never Surrender

Will

@Audio Valve Nut (and anyone else that cares to weigh in).. re Isolation Transformer. I stumbled across Stephe's mention of using that in one of her videos. I'm kinda scratching my head because as I read on-line about these things, the comments and opinions, and prices, etc. are all over the place. (I suppose that's case for a lot of things these days.) Can you or anyone kinda boil down using an isolation transformer for the purposes of building vacuum tube amplifiers along the lines of the Skunkie designs work? I understand the purpose in a general sense, but have gotten a big confused by all the on-line chatter I've read. I'm about to embark on a build (I have nearly all the parts now) of an integrated amp (not a Skunkie design, but similar), from scratch, and I'm relatively experienced with electrical engineering and assembly / troubleshooting / repair, but not as much with vacuum tubes.

I have a variac, DMM, will be picking up a scope and signal generator (I'm familiar with use of all of these). But, my investment in starting build amps is getting pretty costly, and just wondering if I need to drop more coin on an isolation transformer.

What I'm looking for is (for the isolation transformer):

a simplified statement of purpose (relevant to this work)

how critical is it to have one for this kind of work?

what are the features that we want and don't want (like I get some have isolated grounds and some don't)?

any recommendations on which one(s) to buy?

can these be done as a DIY to save $? or does it end up costing as much as a purchased unit

Thanks!!

Don

Great New's , Good One

Will

A 50 with mods now working thanks to recommendations of Audio Valve Nut.

Will,

Thank you so much for your time, advice, and encouragement. You closed with "Never Give In, Never Surrender" and "Stay Safe." I have a framed poster of Winston Churchill overlaid with "Never, Never, Never, Give Up" on a wall in my combination home office and listening room. Despite that, I was getting close to giving up until you replied. BTY, I left out mentioning that I did make the mods all at once. After my failure, I watched all the videos again and carefully wrote out the step-by-step guide above. Then I stripped it down, ordered all new resistors, etc. (except capacitors) and re-did it --with the same result. The orange wire I burnt has continuity, but I will replace it and follow your recommendations. It will take awhile because I have not yet retired even though I'm 65.

Again, I can't thank you enough!

David

Hi Mr Wilder, Looks like you have got yourself in to a bit of bother.

Firstly did you do all the mods at the same time ? if so looks like you have several cans of worm's to sort out. Its time to clear your mind and start again. I had a similar problem a while back with a Pre Amp kit I built , I had an NPN transistor in were it should have been a PNP and my carrier is in electronics (Duh), it happens to the best of us.

A circuit diagram shoes how it is all connected up and you don't have to understand it unless you are the designer but some understanding will help during fault finding.You need to be-able to look at the diagram and find things on the circuit board to test. If your not able to do that I suggest you find some body who can help you out with that. If somebody came to me with your situation I would probably tell them its quicker to strip

it down and start again if multiple modifications have been added. Only do one mod at a time then test it before moving to the next one . This reminds me of my collage day's were we had to diagnose a fault with just a diagram and some voltages . I'll give you some pointer's to start with that may help.

You say in your post that you took out all the tubes out and got no voltages anywhere ,

if you take out the rectifier tube then there will be no voltages supplied to the circuit.

I am not being pretentious as how many of us have been testing for a power fault and one has not switched it on in the first place ( Put your hand up in the air if you have done this , there will be quite a few hand's in the air for that one)

We will start with the basic's

Safety first - you should be using an Isolation transformer to start with, if you are not then one hand in your pocket before you even start to test voltages, one voltage measurement at a time, switch off and let charge decay ( should be doing this any way don't care how experienced you are ).

Make sure you are capable of using a multimeter buy selecting the correct Volt's, Amp's Ohm's for the measurement you are taking.

PSU

We will start with the Power Supply. All voltages supplied from the PSU must be checked even if things are working (your post-" The lights on the meters came on" probably ?? getting supplied from F1, F2 Mains Transformer )

as this can create Red Herring's and lead to confusion. I would replace the chard wire you talk of as you may have to change later on as it has been stressed.

The way I would go about it is A,B,C,D as in diagram below , with Rectifier tube inserted. Disconnected were shown, Hand in pocket before Voltage Test's switch off allow to discharge for every measurement. As the PSU is effectively unloaded this should be done quickly but safely, watch that the voltage does not go above the reservoir capacitor max. voltage.

If you check all the voltages from the PSU and they are correct I would then connect B2 then B+ . If all is correct I start would measure the voltages on the diagram starting with the pre amp section. If all the voltages are incorrect (don't now if diagram is before modification or after modifications) then I would put it back to its original working state before any thing else.

This may be a lengthy process but that's what I recommend.

Never Give In , Never Surrender

Stay Safe

Will