The 1ohm Resistor Method for Biasing

[desertmouth90]

This is how I’ve done it

[pdf64]

This is how I’ve done it

Where's the bias!!! ie the negative voltage between control grid and cathode.
I doubt there's supposed to be a 470k grid leak resistor to the chassis?
Please provide the schematic and the rest of the layout.

Never fit output valves until sufficient bias voltage has been verified at their socket lugs!

[desertmouth90]

I haven’t done that either, that’s a virtual centre tap and I don’t need to as I have a centre tap on my Power Transformer.

[pdf64]

I haven’t done that either, that’s a virtual centre tap and I don’t need to as I have a centre tap on my Power Transformer.

It's not clear what 'that' is?
What virtual centre tap are you referring to?

The layouts in your last 2 posts are different.
Please provide an accurate layout to the amp you built, ie as you built it, ie the rest of https://ampgarage.com/forum/download/fi ... &mode=view
A layout that you subsequently modified is not valid.

[cdemike]

This is how I’ve done it

Check your amp against schematics of fixed-bias amps, and pay particular attention to how the bias circuit is set up. Note there is no connection between the grid and ground in those schematics, as contrasted against the 470K resistors you've shown here. Those will make you lose your bias potential and initiate power tubes' thermal runaway, since you're essentially eliminating or greatly reducing the bias voltage. In a circuit without a unique master volume circuit, a Vox high-cut, or similar complication in this part of the circuit, the grid bias reference resistors should be the only component connecting to the point in the circuit where the coupling capacitors coming out of the phase inverter connect to the grid stoppers. From what I can tell from the layout, there isn't any such addition to the circuit, so this applies.

In other words, delete the 470K resistors. Don't fire the amp back up until you can confirm all is well with your output transformer (checking the resistances against those listed in the datasheet is generally a good starting place, as is checking for any aberrant continuity readings). If the OT check out and you're not blowing fuses, I'd strongly recommend starting your amp on a lightbulb limiter first if you're not already using one. Depending on the tolerance margin between your filter capacitors' maximum voltage ratings and your B+, it may make sense to start the amp with only the preamp tubes installed at this point. Check to make sure you're getting appropriate bias voltage. At that point, it would be okay to set the bias voltage to maximum and install your output tubes (again using the bulb limiter if possible). If this doesn't cause any issues, then try starting the amp connected directly to wall power and bias the amp.

I suspect you may have torched those 6L6's based on what I've read so far, but if they and your OT have survived, this will be the safest way to proceed without committing you to an expensive parts and power tubes order.

[chaccmgr]

It’s a fair point, I used awful solder on this project. But however I will be rebuilding it at some stage. The idea is to get it working first

With all due respect: Working sloppy and expecting to get it working before you correct your mistakes is begging for trouble and repels those who are willing to help. Get your solder joints fixed now!
My recommendation: Have a correct schematic first (get it verified before you build if you are not sure, that you understand everything in your schematic) and follow the schematic strictly when you build the actual amplifier. Do only one thing at a time.

[desertmouth90]

It’s a fair point, I used awful solder on this project. But however I will be rebuilding it at some stage. The idea is to get it working first

With all due respect: Working sloppy and expecting to get it working before you correct your mistakes is begging for trouble and repels those who are willing to help. Get your solder joints fixed now!
My recommendation: Have a correct schematic first (get it verified before you build if you are not sure, that you understand everything in your schematic) and follow the schematic strictly when you build the actual amplifier. Do only one thing at a time.

Okay I will buy some Belton Sockets and redo them.

Also all I did was add 1ohm resistors to the grounds on pin 8 for each valve, otherwise its as per the layout and schematic above. The Bias pot was wired wrong as I assumed the bias goes into the input (Guess I read the schematic wrong).

Valves are visibly fine and I still get a signal.

[maxkracht]

Also all I did was add 1ohm resistors to the grounds on pin 8 for each valve, otherwise its as per the layout and schematic above.

As cdemike pointed out, the 470k resistors from grid to ground are not in your schematic and shouldn't be there.

You likely don't need new tube sockets, just need to make sure all of the connections to your tube sockets are good. The camera can make things look worse than they are, but if it doesn't look good, it probably isn't.

[desertmouth90]

Also all I did was add 1ohm resistors to the grounds on pin 8 for each valve, otherwise its as per the layout and schematic above.

As cdemike pointed out, the 470k resistors from grid to ground are not in your schematic and shouldn't be there.

You likely don't need new tube sockets, just need to make sure all of the connections to your tube sockets are good. The camera can make things look worse than they are, but if it doesn't look good, it probably isn't.

I apologise for the confusion, I wasnt using these resistors, it was more about the 1ohm Ground Resistors for Biasing. I am going to revise my Grounding scheme.

Yes I have checked the connections and they all work. I did have this up and running the other day despite me wiring the bias pot wrong. I still cant believe I did this, but you live and learn! The 6L6GCs still visably fine, I dont think I toasted them.

My plan is to get this working and then rebuild it as I started building it well over a year ago and I made some mistakes that not possible to reverse (Ie drilling out the chassis).

The 50w build I have in the work is going to be much neater and aesthetically pleasing.

[TUBEDUDE]

Awful solder?? Did you use plumbing solder, with the acid core? If so, suck it out and resolder with electronic solder, 63/37. Troubleshooting is easier with good joints.

[desertmouth90]

Awful solder?? Did you use plumbing solder, with the acid core? If so, suck it out and resolder with electronic solder, 63/37. Troubleshooting is easier with good joints.

No it was cheap stuff from amazon haha, okay but theres alot of it!

[maxkracht]

No it was cheap stuff from amazon haha, okay but theres alot of it!

Assuming it is the right type (63/37 or 60/40) and they weren't lying about metal content, your problem could just be not enough flux in the core. Easy to remedy by adding more flux.