Oops. A little trouble with my Hoffman SR

[bnwitt]

  Well, I'm having a little trouble with my Hoffman AB763/6G16 build.  The normal channel sounds fine but the tremolo channel has low power and no tremolo.  The reverb is working.  I've got the tubes idling at 70% with 458VDC and 36ma  Attached is a voltage chart of what I am getting.  Anyone see anything wacky?

[sluckey]

You didn't list a voltage for V4 pin 3??? There is no tremolo channel on this amp. Tremolo signal is injected at the power tube grids and should have the same effect for the normal or reverb channel.

Post a bigger pic of the amp.

[bnwitt]

There is no tremolo channel on this amp. Tremolo signal is injected at the power tube grids and should have the same effect for the normal or reverb channel.

Now that is a good point.  I guess I should call that the Reverb Channel.  Voltage chart in first post has been updated.  V4 pin 3 = 2.0 VDC

[sluckey]

I'd say you have two issues that should be unrelated. Low volume on the reverb channel, yet the reverb is working. And the trem is not working.

Divide and conquer...

Looks like you forgot the jumper between V5 pins 1 and 7. Gotta have that jumper for trem to work. Oops. Looking at the wrong socket.

[bnwitt]

better photos at the dropbox link below:

https://www.dropbox.com/sh/xyo0sl3c4tpd3ye/AAA4yaUOSWHgQ41ngT3GYm39a?dl=0

[sluckey]

Recheck the voltages on V6. Something wrong there.

[bnwitt]

Ok, I found a bad solder joint on Pin 8 of the wire connecting V6 terminals 8 and 3.  The wire cloth prevented the solder from taking.  Updated voltage chart attached to this post

Now the low power problem is gone but still no tremolo.  I am going to try a different 12AX7 in V5 after I pull the tube and verify the continuity is good on the tube side of the socket in case there is a bad pin.

[sluckey]

One down, one to go. The trem circuit wants a strong tube. When the oscillator is working V5 pin 1 will be swinging big voltage at the trem speed. If using a digital meter, this will just look like the voltage is making erratic jumps. If using an analog meter you can see the needle smoothly swinging at the trem speed. (NOTE... You can actually measure the AC RMS voltage of the trem signal using a true rms meter such as most Flukes.) If V5 pin 1 is steady the oscillator is not working.

You do not need anything plugged into the trem footswitch jack to make the oscillator work. So, if you have a footswitch or a shorting plug plugged in, remove it.

[bnwitt]

Thanks for those details and the help Steve.  I've tried another tube after checking the V5 socket pins and still no trem.  I'm going to update the voltage chart, drain the amp and take some resistance readings to look for any bad connections or components.

[sluckey]

V5 pin 2... Follow the yellow wire back to the board. It's connected to the .1µF cap. It should be connected to the .01µF right next to the .1µF. Fix that and I bet the trem will start working.

[bnwitt]

And you would win that bet!  Yessir that was the problem.  Everything sounds great now.  I was just getting ready to dissect the oscillator section when I saw your post.  Thank you so much Steve.  As Maxwell smart would say "Missed it by that much."

[sluckey]

Max, you were "that close" to finding it too. A good layout, sharp hi rez pics, and good workmanship help a lot with this stuff.

Is the trem strong enough for your 6L6s?

[bnwitt]

Is the trem strong enough for your 6L6s?

Well it could be a little bit stronger.  I've got a 220k in place of the 470k resistor and a 0.47uf in place of the 0.1uf capacitor.  Maybe I should go to a 150K instead.  Also I think it could be a bit slower in the mid control position.  Maybe I'll play with that cap too.

[sluckey]

You know that the bias adjustment will affect trem strength, right?

This could be a good opportunity to try out the LED mod. Red or yellow work best. Makes a big difference. Once you try it you'll probably be hooked. Maybe even want to mount the LED in a nice bezel on the front panel. 

[bnwitt]

I'll give it a try.  Any old red LED?

[sluckey]

Well, you don't want one the size of a Tootsie Pop! 5mm size is very common. The red LED typically has a voltage drop of 1.7 to 2V and this is perfect for the trem tube cathode. Cathode of the LED must go to ground. If the LED doesn't flash you probably connected it backwards and it's no longer any good. Better grab another fresh one.

[bnwitt]

Got it

[bnwitt]

Ok, LED is in and working.  It's too early to crank up the amp but I'll check out the new intensity once the neighborhood  is awake.  One question, will this LED last as long as the original RC components?

[Glenn]

Boy we're Luckey to have a guy like Steve Luckey out there.... happy holidays everyone

[sluckey]

One question, will this LED last as long as the original RC components?

It will last forever. Or at least longer than you will be around. I've worker around thousands of 5mm LEDs ever since they began to show up in equipment back in the '70s. Never seen one fail. Now I have killed a few by installing them backwards or not providing proper current limit. 

[bnwitt]

It will last forever. Or at least longer than you will be around.

Gee.  Thanks for reminding me of my mortality.

[sluckey]

Did your trem effect get stronger?

[bnwitt]

Did your trem effect get stronger?

Yes it did.  Sounds great.  Had to give the amp to the Xmas gift buyers for Christmas morning reveal but it's coming back for final tuning in a couple of weeks so I'll get a bit more time with it then.  Thanks for all the help Steve.  Merry Christmas friend.

[bnwitt]

Boy we're Luckey to have a guy like Steve Luckey out there.... happy holidays everyone

That is the truth.  Steve has pulled my bacon out of the fire more times than I can remember.  Happy holidays to you too Glenn and all the Hoffman forum gang.

[PRR]

Not only do LEDs last decades at 20mA.... here it is working near 1mA, so life is longer.

I have not yet seen a worn-out LED. I expect I will: room-light LED bulbs run the LEDs *way* hot, so light declines in years not decades. Also the driver circuits are the cheapest junk and will die (that was the main death in CFLs: a diode or cap in the driver would stink and die.) But this trem LED is nowhere near that zone.

[shooter]

the LEDs *way* hot

the day after the filament demise, my Boss sent me for like 20 led screw ins, the early fail rate was like 30%
Always the PWM, some of the "defective" one's got pitched to my barn, re-wired to a couple wallwarts from then.  they have been running 24/7 every since. 6mths ago I tested a 7-segment from the dinosaur days, it's now labeled a 6 segment, but hey!!

[purpletele]

Did your trem effect get stronger?

Yes it did.  Sounds great.  Had to give the amp to the Xmas gift buyers for Christmas morning reveal but it's coming back for final tuning in a couple of weeks so I'll get a bit more time with it then.  Thanks for all the help Steve.  Merry Christmas friend.

That was entertaining!

I'm anxious to build mine, I have design details to complete and should be ready to start in a few days.

[bnwitt]

Well  hopefully you won't make as many mistakes as I did. I do believe starting and stopping so many times due to having so much other work on my plate was the cause.  What version are you building Purpletele?

[bnwitt]

Ok, I thought I would share the final voltage chart and annotated Sluckey schematic of the Hoffman Super Reverb I just completed for anyone who is building one.  This chart and schematic are specific to the Super Reverb version so all components on the schematic are the ones for that version.  I'll upload the layout I'm working on when it is done.  I usually finish my layouts before I start a build but this amp had to be ready for Christmas so the layout had to wait.
Barry