Minimum cab size?

[cfg66]

I just finished a 18w TMB (Ceriatone) and wondering if I ****** myself by making the cab too small. Sounds fantastic but the pressure wave coming off the 12" Veteran 30 is pretty wild. Not to mention the heat of the EL84s. I always read that this circuit was loud but it has to be experienced to be believed. Cabinet ID is 22.5" w x 16" h x 8" d.

Any potential problems from having the tubes so close to the speaker magnet? There's about 1.5" clearance between the magnet and tubes.

Speaking of heat, I'm getting like 120C from these tubes (Russian Mullards), even after swapping the 120 ohm cathode resistor out for 180 ohms. Is that normal? Temp doesn't seem to fluctuate much between idle and load, but man these tubes get hot fast. Plate dissipation is 100% @ 12 watts. Thought about going to 200 ohms but the temp didn't decrease at all with the first resistor bump. I had to put some aluminum roll flashing on the inside of mount to keep the Tolex from melting.

Anyway, great forum! Fantastic resource.

Thanks,
Colin

[JMPGuitars]

Hi, welcome to the site.

That's a handsome cab.

Target dissipation is 85% for the power tubes. That should be your target too. If you want more advice than that, I'd suggest taking a look at one of my voltage charts in the Downloads section and share your voltages (there are TMB variants that should be close enough).

Thanks,
Josh

[cfg66]

Thanks! That would be great, really appreciate that.

Not sure about the right point to measure these values: for HV 1 & 2 values, I measured this at the HV PT output before the diodes at the rectifier tube, and may have gotten the order backwards. I measured A & B from each half of the can cap to ground.

Not sure where to take the C measurement. Is that plate to cathode? I'm getting 343 for that.

[Edited to correct the resistor value] Plate dissipation is slightly higher today, 343 plate-to-cathode today with a 13.4 voltage drop across the 180 ohm cathode, which is 12.8 watts/100.8% dissipation according to Rob Robinette's bias calculator.

[JMPGuitars]

Based on your voltages, you will probably need to increase the power tube cathode resistor.

Before you do that though, use some Zener diodes to drop the B+ voltage. Look at the link in my signature for more info if you need it.

I made this to show you what those letters mean in my voltage chart. Each letter is a B+ node, as you can see in this modified Ceriatone layout:

18wattTMBceriatone.png

Under normal circumstances, "A" will be the converted DC voltage coming from your rectifier.

Hopefully, that will help other people too.

Thanks,
Josh

[cfg66]

That's a huge help, thanks! I just added the missing measurements. Based on that, I'm 22/17v high at the can cap A/B test points- is that the same amount I need to bring the voltage down? If so, would a 5w 20v diode do the job?

I assume I should wait and remeasure the voltages after the diode is installed before sizing the new cathode resistor. But I was thinking of getting a couple of values when I order the diode, say 200 & 220 ohm. According to the bias calculator right now I need 210 to get to 85%, but not sure if the lower B+ might impact that.

Also kind of curious about the low rectifier plate voltages- any cause for concern there?

Thanks again!

[JMPGuitars]

The rectifier plates are normally a direct connection to the HV on the PT. You measured ~294VAC at the HV points, which is normal (slightly elevated, likely due to modern wall voltage).

The layout shows some diodes and a cap on the rectifier, so that could affect the voltage. If it's not supposed to, then verify the components are installed correctly. Photos might help.

Order values from 180Ω to 240Ω 5W minimum for the cathode resistor.

A 5W 20V could do the job, or 2 5W 10V might be a little safer. the thread.

Your assumption is correct; wait until the B+ voltage is corrected, then remeasure all voltages.

Thanks,
Josh

[cfg66]

Oh ok, that makes sense. I double checked the diode orientation and it's the same as the wiring layout. I included a photo of the rectifier wiring but it's tricky to show due to the chassis layout.

Sorry for all the questions, but had another one about your diode mod. Your layout shows a 220k resistor between the A side lug and ground lug of the can cap that the Ceriatone doesn't have, and also the resistor connecting the legs of the can cap is a higher value than what the Ceriatone called for. Should I include this additional resistor and the value change when I do this mod? Or maybe these are circuit specific- it looks like your (screen grid?) resistor is a much higher value as well? The Ceriatone has only a 100 R at the screen grid of the output tubes. (My apologies for any incorrect terms, still pretty new to amp circuits.)

[JMPGuitars]

You do want to add the 220k 2W resistor to ground at A. That's to discharge the caps, and potentially save your life.

The resistor between A and B doesn't matter. That drops the voltage between those two points, and what matters is your voltage is where it needs to be (which doesn't matter until after you add the Zeners).

Yes, I recommend using higher value screen resistors as per my schematics.

Please also review the soldering technique thread in my signature.

Thanks,
Josh

[cfg66]

Awesome, thanks. I've been discharging manually, good to know this will empty the caps.
Yeah, my soldering needs work for sure, appreciate the link!

[JMPGuitars]

I like to go back and review those videos myself every couple years. It's always good to brush up on, and improve our skills.

[cfg66]

That turret video is especially appreciated, this was my first turret board and the turret connections are all pretty awful. Great to have a solid reference. I need to go back and de/re-solder those connections entirely.

[cfg66]

Just installed the diodes and fired it up, and I'm down to 85.6% dissipation with a 190 ohm cathode resistor. Tubes are cooler too. Thanks Josh, I really appreciate your help with this!

Cheers,
Colin

[JMPGuitars]

You're welcome! I'm glad you got it sorted.

But, how does it sound??

[cfg66]

Everything I hoped for and more! Really glad I live in the sticks though lol, what a beast.

Hey, I sent you a PM about an alternate way to contribute to the forum (I don't have a Paypal account). Let me know if you didn't get it and I'll try again.

[JMPGuitars]

Post a demo video! A nice lookin' amp like that needs to be seen and heard.

Well, I could use a new car.