18Watt.com

Balls, ok, they're now in 3 and six, I thought I wanted one in each side.

Straight after standby going on all looks ok
B plus 318

Power tubes
Pin 3 11.4v
Pin 7 350v
Pin 9 333v

But then after maybe ten seconds in starts getting warm, b plus going up to 350, pin 7 coming down, pin 3 going up, then i panic and switch off. Will sit down and do more with the highlighter later.

Amp voltages appear to be stable with the bulb limiter.

chaliapin wrote: ↑

Mon 01/16/23 9:27 am

Balls, ok, they're now in 3 and six, I thought I wanted one in each side.

Straight after standby going on all looks ok
B plus 318

Power tubes
Pin 3 11.4v
Pin 7 350v
Pin 9 333v

But then after maybe ten seconds in starts getting warm, b plus going up to 350, pin 7 coming down, pin 3 going up, then i panic and switch off. Will sit down and do more with the highlighter later.

Amp voltages appear to be stable with the bulb limiter.

See my previous reply. Neither way will work with what you're trying to do.

Can I add a lower wattage value resistor in series with the big 150r ones?

Sorry I wrote my previous post before I read your last

JMPGuitars wrote: ↑

Mon 01/16/23 9:52 am

chaliapin wrote: ↑

Mon 01/16/23 9:27 am

Balls, ok, they're now in 3 and six, I thought I wanted one in each side.

Straight after standby going on all looks ok
B plus 318

Power tubes
Pin 3 11.4v
Pin 7 350v
Pin 9 333v

But then after maybe ten seconds in starts getting warm, b plus going up to 350, pin 7 coming down, pin 3 going up, then i panic and switch off. Will sit down and do more with the highlighter later.

Amp voltages appear to be stable with the bulb limiter.

See my previous reply. Neither way will work with what you're trying to do.

When on 2 tubes , shouldn’t he also switch the speaker impedance switch to virtually match the load?

chaliapin wrote: ↑

Mon 01/16/23 10:07 am

Can I add a lower wattage value resistor in series with the big 150r ones?

Sorry I wrote my previous post before I read your last

Define lower…

Err, yeah, good question, I only have 100r quarter watt. So if I rewire 3 and five to share the single cathode resistor do I basically have an 18watter for now?

chaliapin wrote: ↑

Mon 01/16/23 11:00 am

Err, yeah, good question, I only have 100r quarter watt. So if I rewire 3 and five to share the single cathode resistor do I basically have an 18watter for now?

Quarter watt? LOL !!! I think those will ignite instantaneously
Just rewire 3 and 5. Daisychain their cathodes (pins 3)to one of those 150R resistors. Then you have an 18 watt. Also set the imprdance switch to 4 ohms on a 8 ohm speaker, or 8 ohms on a 16 ohm speaker to match the OT primary with the secondary.

Ok, so v3 and v5 are now sharing the cathode connection. They don't share the resistor connecting pin 9 to point b, there's currently one for v3 and 4 one for v5 and 6 do I need to change that so it's the same as the 18w schematic as well?

chaliapin wrote: ↑

Mon 01/16/23 12:47 pm

Ok, so v3 and v5 are now sharing the cathode connection. They don't share the resistor connecting pin 9 to point b, there's currently one for v3 and 4 one for v5 and 6 do I need to change that so it's the same as the 18w schematic as well?

When you don’t have any other EL84’s installed it should resemble the 18 watt doing it that way. Flip it on first with the bulb, and if ok.. test without bulb

Well I really thought that was going to sort it out but around 20 seconds after switching on the standby the bulb starts getting brighter. Without the bulb limiter the b plus starts to rise and the humming begins. I'll get back to the painstaking checking but any ideas what I might be looking for would be great.

Another highlighter test done. No lightbulb moments. Haha. Sob.

Things maybe worth mentioning.

The resistor from a-b is not 1k but 1.5k.

V2 is grounded with the pre amp and inputs, not the power amp.

The screen from the mains side of the PT is grounded through a fuse to the star ground. The ct for 275v supply to the rectifier is also grounded there.

I honestly can't think of anything else im unsure of or haven't checked. I know I know, there must BE something...

Thanks for all your patience, mine is wearing a little thin with myself...

chaliapin wrote: ↑

Mon 01/16/23 2:53 pm

Without the bulb limiter the b plus starts to rise and the humming begins.

What does this mean? You need to be specific so we can help you. The B+ rises? To what? Does it stabilize? Or does it count to infinity (and beyond)?

Ignore the hum for a moment and get useful information.

Sorry, I get nervous when it starts humming and heating like it's going to blow up in my face...

Weirdly, it's behaving a little different this morning.

With standby off I have 230vac and 310vdc rectified at the standby switch.

Standby on

B+ 354
P3 11.6
P7 350
P9 325

Settling to

B+ 352
P3 12.2
P7 345
P9 322

That's after about twenty seconds when it all feels quite hot and hummy so I switch off. Volume is at 0. It could be normal hot but I don't know.

Sorry if this is very tedious for you guys, I really appreciate the help.

Those numbers mean your power tubes are biased at 106.7%. That's not a disaster, but that is also why we generally put a 180R cathode resistor in there. That will drop the bias. We aim for 85%, give or take... 100% is ok, but 85 is better for tube life. It's a cathode biased amp and it's generally accepted that 100% is no problem.
can you make a recording of the process from silent to excessive hum? The hum will decrease when it's biased to 80-90% for sure, but the hum you have may have another cause...making you chase your tail at the power section.

Thanks bieworm, I'll try to get something and post it, will the numbers look better when I have four tubes?

Can I ask which Ukrainian tube guy you used and I'll order a bunch. There's actually a guy in Ireland selling NOS reflektor military grade but 4 is going to be over a hundred euro and then I'd probably blow one up...

Also I only have am 8 ohm speaker and no 4 ohm output option until I finish the cab the 8 ohm speaker is in the 8 ohm output, is that affecting things/ voltages? I could hook up two loose 8 ohm speakers to give the correct impedance but would they get damaged just lying on the table?

chaliapin wrote: ↑

Tue 01/17/23 6:34 am

Thanks bieworm, I'll try to get something and post it, will the numbers look better when I have four tubes?

Can I ask which Ukrainian tube guy you used and I'll order a bunch. There's actually a guy in Ireland selling NOS reflektor military grade but 4 is going to be over a hundred euro and then I'd probably blow one up...

I doubt the numbers will change significantly with 4 tubes, but they will to some degree.
I bought mine at Etsy (Radiotubeshop)

Regarding hum inducers... I notice you have the B+ (DC) in parallel with a bunch of twisted AC wires (heaters, PT primary,..)
Somebody correct me if I'm wrong, but I avoid AC and DC wires in parallel. I don't know if that is a problem, since this is not a signal wire...

chaliapin wrote: ↑

Tue 01/17/23 6:49 am

Also I only have am 8 ohm speaker and no 4 ohm output option until I finish the cab the 8 ohm speaker is in the 8 ohm output, is that affecting things/ voltages? I could hook up two loose 8 ohm speakers to give the correct impedance but would they get damaged just lying on the table?

no damaging there, but it will sound like crap when not in a cab

So wall voltage up 6v to 236vac this afternoon. Here's the latest with two 8 ohm speakers in series into the 8 ohm socket. Hum seemed to come later and quieter, unaffected by turning volume up full with no input. Voltages now

B plus 357
P3 12.35
P7 354
P9 323

I'll try to get the cab finished to I can get the speakers into it.

chaliapin wrote: ↑

Tue 01/17/23 8:12 am

So wall voltage up 6v to 236vac this afternoon. Here's the latest with two 8 ohm speakers in series into the 8 ohm socket. Hum seemed to come later and quieter, unaffected by turning volume up full with no input. Voltages now

B plus 357
P3 12.35
P7 354
P9 323

I'll try to get the cab finished to I can get the speakers into it.

That means you have the bias at 110.8%
You will have to get some extra resistors for the cathode biasing. 180R, 220R,... prepare for that. The amp will get more quiet when the bias is set properly. But I think that regards more to the hiss instead of the hum.
You should also get some 5V 5W zener diodes to be able to knock down the B+ to 345V. But I would do these adjustments when you have the 4 power tubes in there.