18Watt.com

Hold on... I suggest that you first find out if it's the rectifier tube that's flakey or that the problem is somewhere else.
1. Pull out the rectifier tube and start the amp up
2. measure the voltage between pin 8 and 2 (should be +- 5V AC)
3. measure the voltage between pin 4 and 6 (should be +- 640V AC)

If this is all ok switch off the amp. Put the rectifier tube back in and pull the power tubes and preamp tubes.
1. measure the voltage between ground and pin 8 on the rectifier (should be around 400V DC)
2. if it's again around 355V DC then your rectifier tube is the suspect, if the voltage is rather up to spec then the problem is probably in the bias circuit or the power stage.

My guess would be the bias setting. OTOH, I think my Princetons have about 370V DC on the plates and the tubes draw about 22mA.

So all of this worked out as it should. I pulled the rectifier out and measured pins 8 and 2 were as they should be (600+. My DMM maxes out at 600v) and so were pins 4 and 6 (5v). So I did the next step, Put the rectifier tube back in and pulled out all the other tubes got like 420v @ pin 8

I made up a voltage chart. My B+ side is still way low and my control grid voltages @ V5P5 and V6P6 are -214.9 and -216.9 and I believe should only be -34.
I'm spending a significant amount of time checking a rechecking. Looking very closely for the problem to present itself.

I've never built one of these, but have you tried adjusting the bias setting?

I made up a voltage chart. My B+ side is still way low and my control grid voltages @ V5P5 and V6P6 are -214.9 and -216.9 and I believe should only be -34.
I'm spending a significant amount of time checking a rechecking. Looking very closely for the problem to present itself.

I think you're looking at the wrong point for the -34V. This is the voltage at the wiper of the intensity pot, leading to the 220k grid load resistors. Take the voltage there and report back

I think you're looking at the wrong point for the -34V. This is the voltage at the wiper of the intensity pot, leading to the 220k grid load resistors. Take the voltage there and report back

-229.4v is what I just got off the intensity pot "sweeper", (center lug)

I've never built one of these, but have you tried adjusting the bias setting?

That's kind of the rub here. I'm not getting any reading at pin 8 of either V5 or V6. So I plugged in my Biasmaster, turned it on and then powered up the amp. Immediately it registered 30+mA then steadily rolled down to 0mA as it warmed up. Both tubes.
I also checked the trem pot and it registers 0-22k ohms. Appears to be good working order.
Until I hear diff, I'm going to keep looking for something grounded or connected wrong. I'm sure that elevated negative reading off the intensity pot is related

In the bias circuit there are some things you could check. The 100k resistor..does it read 100k? Is the electrolitic cap working like it should? Is the diode ok?

In the bias circuit there are some things you could check. The 100k resistor..does it read 100k? Is the electrolitic cap working like it should? Is the diode ok?

100k resistor reads good.
22uF E cap reads 21.5uF. I'm not sure how to determine if it's working correctly.
As well, I'm not sure how to confirm if the diode is working ok.
They all appear to be installed correctly appears to be installed correctly

foreverstrung wrote: ↑

Wed 08/09/23 11:05 am

In the bias circuit there are some things you could check. The 100k resistor..does it read 100k? Is the electrolitic cap working like it should? Is the diode ok?

100k resistor reads good.
22uF E cap reads 21.5uF. I'm not sure how to determine if it's working correctly.
As well, I'm not sure how to confirm if the diode is working ok.
They all appear to be installed correctly appears to be installed correctly
Bias issue.png

A diode can be overheated really quick when soldering. I always use a heat sink when soldering one.

A diode can be overheated really quick when soldering. I always use a heat sink when soldering one.

I replaced the diode. Powered up the amp and the mA rose to approx 30 mA and then started rolling down to 0mA again
I'll continue checking and rechecking all my work for an error.

what values do you get when you turn the bias pot all the way down (CCW) ?

NOW... by looking at your pics again it is very unclear IF there is any ground wire running from your filter caps and bias supply. There should be a black wire going to the ground lugs alt the power transformer bolt (which is by the way a very common way of bad practice, ALL ground connections should have a dedicated bolt to the chassis and NEVER be held down by a bolt that serves another purpose)
If there is no ground connection for the power amp that would surely explain your issues) when you get that thing sorted out there are some things you should address on the lead dress:

NOW... by looking at your pics again it is very unclear IF there is any ground wire running from your filter caps and bias supply. There should be a black wire going to the ground lugs alt the power transformer bolt (which is by the way a very common way of bad practice, ALL ground connections should have a dedicated bolt to the chassis and NEVER be held down by a bolt that serves another purpose)
If there is no ground connection for the power amp that would surely explain your issues)

Yahtzee. Thx man. That missing ground was the culprit. Knucklehead move missing it. Still dialing in a few things now that my voltages are in much better ranges. I'll send a sound clip when I'm finished. I still haven't built the combo cabinet as it's hot as **** here in SE Texas and my wood shop is not air conditioned.
I have a reverb unit.A 9AB3C1B, Long Decay, 3-Spring. Is there a preferred reverb unit or is this one acceptable?

Glad it's solved!
About the tank... these are the things I always do when building Fullerton type amps:
- tank 9AB2C1B medium decay (long decay is waaaayyyy too much IMHO)
- swap the 1M grid load resistor in the input of the reverb (pin 2 to ground) with an A1M dwell pot to dial in the desired effect wash. It's a no brainer.. I think it's a necessary part of the reverb circuit. On a standard chassis you could remove the reverb footswitch jack (who needs that?) and move the tremolo jack to that hole + put the dwell pot in the hole where the tremolo jack was.
- lower the cathode bypass cap from 25uf to 10uf on the recovery stage (V3 pin3)
- delete the cathode bypass cap on the gain stage after the recovery stage (V3 pin8) for a less noisy reverb while maintaining a good amount of effect
All of this guarantees you that there is still plenty of over the top reverb, but you have the ability to dial in your reverb easier. On a F*nder there is way too much reverb already on 3, but with these mods you'd have to turn it to 8 or so to have the same amount of reverb. The result is that you have a more effective sweep on the mix knob, plus you can play with the dwell as well.

tank 9AB2C1B medium decay (long decay is waaaayyyy too much IMHO)

Is there a diff unit you'd recommend? Or do I make the changes you suggested and use this one?
TIA

foreverstrung wrote: ↑

Fri 08/11/23 5:50 am

tank 9AB2C1B medium decay (long decay is waaaayyyy too much IMHO)

Is there a diff unit you'd recommend? Or do I make the changes you suggested and use this one?
TIA

Just use this one, but in the future you better buy the one I use. It’s just nicer

- delete the cathode bypass cap on the gain stage after the recovery stage (V3 pin8) for a less noisy reverb while maintaining a good amount of effect

To do this can I just run a jumper around this cap and just leave it in place or would it be best to remove it completely and then bridge the connection?

foreverstrung wrote: ↑

Sat 08/12/23 3:11 pm

- delete the cathode bypass cap on the gain stage after the recovery stage (V3 pin8) for a less noisy reverb while maintaining a good amount of effect

To do this can I just run a jumper around this cap and just leave it in place or would it be best to remove it completely and then bridge the connection?

Noooooo … you would also be jumpering the 1.5k resistor

delete the cathode bypass cap on the gain stage after the recovery stage (V3 pin8) for a less noisy reverb while maintaining a good amount of effect


To do this can I just run a jumper around this cap and just leave it in place or would it be best to remove it completely and then bridge the connection?


Noooooo … you would also be jumpering the 1.5k resistor

Me not being clear again. I meant jump just pos/neg of that 1 E cap. If I just remove it, the circuit is incomplete, right? So can i leave it in place and just run a little jump from neg-pos or remove the E cap and just bridge it? Or something else?

You should really learn to understand schematics. When you look at the schematic you will see that the 25uf/25V electrolytic cap and the 1500 ohm resistor are both starting at the cathode of V3b and both end at 47 ohm resistor going to ground. That cap is called a cathode bypass cap. When you would jumper it you would also jumper the 1500 ohm bias resistor and almost short the cathode to ground. It wouldn’t be a real short but that tube would be biased reeeeeeeaal hot. If you were building it on a turretboard you would immediately see what I mean.

You should really learn to understand schematics. When you look at the schematic you will see that the 25uf/25V electrolytic cap and the 1500 ohm resistor are both starting at the cathode of V3b and both end at 47 ohm resistor going to ground. That cap is called a cathode bypass cap. When you would jumper it you would also jumper the 1500 ohm bias resistor and almost short the cathode to ground. It wouldn’t be a real short but that tube would be biased reeeeeeeaal hot. If you were building it on a turretboard you would immediately see what I mean.

Learning is a process, right? lol.

foreverstrung wrote: ↑

Sun 08/13/23 8:24 am

You should really learn to understand schematics. When you look at the schematic you will see that the 25uf/25V electrolytic cap and the 1500 ohm resistor are both starting at the cathode of V3b and both end at 47 ohm resistor going to ground. That cap is called a cathode bypass cap. When you would jumper it you would also jumper the 1500 ohm bias resistor and almost short the cathode to ground. It wouldn’t be a real short but that tube would be biased reeeeeeeaal hot. If you were building it on a turretboard you would immediately see what I mean.

Learning is a process, right? lol.

It certainly is and it never stops.. lol
I hope I didn’t offend you with my response. No pun intended

You should really learn to understand schematics. When you look at the schematic you will see that the 25uf/25V electrolytic cap and the 1500 ohm resistor are both starting at the cathode of V3b and both end at 47 ohm resistor going to ground. That cap is called a cathode bypass cap. When you would jumper it you would also jumper the 1500 ohm bias resistor and almost short the cathode to ground. It wouldn’t be a real short but that tube would be biased reeeeeeeaal hot. If you were building it on a turretboard you would immediately see what I mean.

Learning is a process, right? lol.

It certainly is and it never stops.. lol

I hope I didn’t offend you with my response. No pun intended

I was a little butt hurt after first reading your response, but it passed quickly. I know I'm not even close to mastery of reading schematics, but I'm much better than 6 months ago. Then I understood nothing. Now I can at least muddle my way through. To be honest, I did'nt even review the schematics before asking the question, so it's my fault for not even investigating and just trying to get an easy answer without working for it



Here's a video clip. the modifications you suggested worked out awesomely. Especially the dwell pot. Every Fender amp I've ever had with reverb I could never set more than 3. This dwell pot gives so much more control. Really happy with this.
Thx again.
Forgive the sound clip/video. It's long and harsh. Just playing with some sounds. So many more available with this amp too. tremolo is nice and clean as well, but not any on this sound clip