New build, having tremolo issues...

[balcorn80]

Hey all.

I belong to a tube amp club here in Oregon, (www.vacuumtubecollective.com) and I'm engineering a build/designing a layout for the guys in the club here and have run into a little bit of a stumper for me...

First, a basic of the amp's topography.
The phase inverter and power section of the amp are based on a Supro Thunderbolt, and the preamp/tremolo section is based on a Fender VibroChamp. I chose the VC because it had the Fender-style preamp I wanted, and it also had a tremolo that acted on one of the preamp stages, and since I was using basically the whole VC circuit up until where it goes into the power tube(mine goes into the PI here instead) it made it easy to pirate the layout design to put on a turret board. ;-) 
  The amp sounds sweet. clean, it sounds nice and fender-y. Once you get up into the volume though, that Supro-styled PI and power amp really sing and crunch nicely.
  My issues, though, are in the tremolo part of the amp. It's still hooked to the cathode of the second stage(minus the NFB and 47r resistor the cathode/nfb components'stand' on), and the only change I made compared to the fender circuit is I used a 50kRA pot for the intensity instead of a 25kRA pot because I could find a 50kRA...
  And it seems to work, until the intensity is over about 10 oclock. Then it still works, but overall amp volume start to drop, and continues to drop to almost nothing when the intensity is maxed out.
  When I look at the schematics for the VC, the AA764 and AB764 both show a little different hookup to the speed pot, and when I searched this forum for some info, I had seen a few different people used the VC circuit and didn't seem to have any problems like what I'm having. I noticed in a thread that Tubenit and Panhead had a 5e3 with 'verb and tremolo that they seemed to be working on together(or at least giving eachother feedback on) and their speed pot was wired up differently yet.
  My speed pot is wired like the AA764 shows, with the middle leg being tied to the outer leg that has the 100k resistor going to ground with the wire from the circuit coming to the LH tab(the AB764 shows the middle leg being tied to nothing, and to me that can't work) but in the Tubenit/Panhead diagrams, it looked like the middle leg of the pot had the wire from the circuit coming to it(as opposed to the LH one in Fender's designs) and nothing going to the left hand tab, while the right hand tab had the 100k-to-ground resistor on it. I haven't tried this yet, but was hoping for some feedback from you guys.(my RH/LH designations may be reversed, but I am using the Fender-style wiring/orientation)

Thanks in advance!

[Fresh_Start]

Do you have a schematic of the amp as built?

Respectfully,

Chip

[tubeswell]

What Fresh_Start said. A schematic of your as-built amp would enable others to help pin-point issues faster.

Also might be useful to take a look at the 5E9A trem (as it uses a paraphase inverter). Note that this type of trem doesn't work property if you try to implement GFB in the amp.

[balcorn80]

I don't have one in electronic form... I just downloaded the two programs that are talked about in the 'how to' section of the forum to make .sch files and was planning on doing that up this evening. I'll get right on it. Thanks guys.

[Fresh_Start]

I don't have one in electronic form... I just downloaded the two programs that are talked about in the 'how to' section of the forum to make .sch files and was planning on doing that up this evening. I'll get right on it. Thanks guys.

Don't start a schematic from scratch!  Tubenit has created an .SCH file for so many different amps and ideas that you're bound to find something close in here:

http://www.el34world.com/Forum/index.php?board=12.0

Alternatively, you can always scan a paper schematic into a PDF file and post that.

Cheers,

Chip

[sluckey]

There's a very high dc voltage on the cathode of the trem CF. That high voltage splits across a voltage divider made up of a 68K and the 25K INT pot. Using a 50K pot will allow a lot more of that dc voltage to be coupled to the 2nd preamp cathode, enough to probably cut off the 2nd preamp. Put a 47K resistor across the INT pot outer lugs, or increase the 68K to 120K should fix it.

[balcorn80]

Don't start a schematic from scratch!  Tubenit has created an .SCH file for so many different amps and ideas that you're bound to find something close in here:

Found one and jiggered it around. You'll be able to tell what parts I did. 

I'll try boosting that 68k to 120k when I get off work today. 

[balcorn80]

increase the 68K to 120K should fix it.

Nailed it, Sluckey. Thanks, man!

[sluckey]

Give it a try with the 47K across the INT pot. That would make the INT pot look like a 25K pot. Might change the taper though.

EDIT... Mouser has a 25K RA pot made by Alpha for $1.64.

[balcorn80]

I tried the 47k across the pot, and while it did work, it totally threw the taper of the pot out the window. The 120k in place of the 68k really sounds great. The trem is REAL similar to my SF Bronco's tremolo, which is good, because the first 2 tubes' worth of circuitry in this amp was lifted almost directly from a Bronco circuit(I just made it go into a PI and P-P power amp instead of a single-ended power amp) and because I really dig the Bronco/VibroChamp's general sound.

Thanks again for all the help.   

[balcorn80]

So the tremolo may not be as valiant as I had thought. There is a fairly significant gain drop when you turn the intensity up. It doesn't fully kill it like it used to, but it is pretty significant and grows as you turn it up, though the intensity doesn't seem to grow as much as the gain drops(if that makes sense...) So I'm gonna keep at it... I will probably end up ordering a 25kRA pot, though I hate to do that since the only ones I have found are Alpha brand, and the rest of the pots in this build are CTS. But if it fixes it, well, I guess I can't really ask for more, right?

[sluckey]

Here's an idea that may or may not work. Rather than connect the INT pot directly to the cathode of V1B, put a 0.1uF cap between the pot and the cathode. Easy to do and easy to undo. I don't have a clue how the cap will affect the strength of the trem, but it will prevent the dc voltage from affecting the gain of V1B.

[balcorn80]

Strength/Intensity of the trem is GREATLY diminshed with the cap in there... I'm gonna keep experimenting...

[balcorn80]

Well, its 1 a.m. and I gotta be at work tomorrow so I'm done monkeying with it tonight... But I took the .01 cap back out, replaced the 68k(the one I had upped to 150k then changed back to 68k) resistor going from the cathode to the pot with a 180k one. Seems to do the trick. Then I replaced the .01 cap connected to the grid of the LFO(12ax7) with a .02 cap for some slower trem action. Which worked great. But the trem's high speed setting is really fast. like unuseable stuttery fast if the intensity is maxed...

[tubeswell]

So the tremolo may not be as valiant as I had thought. There is a fairly significant gain drop when you turn the intensity up. It doesn't fully kill it like it used to, but it is pretty significant and grows as you turn it up, though the intensity doesn't seem to grow as much as the gain drops(if that makes sense...) So I'm gonna keep at it... I will probably end up ordering a 25kRA pot, though I hate to do that since the only ones I have found are Alpha brand, and the rest of the pots in this build are CTS. But if it fixes it, well, I guess I can't really ask for more, right?

The way to use a resistor to alter the taper of the pot depends on whether you put the resistor between the pot wiper and the input (antilog taper), or the pot wiper and the ground (log taper), or a resistor on each side of the wiper (log and antilog taper). If you use one or the other, then putting a largish cap across the wiper to the other lug will bypass the 'untapered' end of the pot rotation. But regardless of all that, I tend to use linear taper on intensity controls - YMMV. You might just be better going to a smaller pot if the big pot is OTT.

But if you want to keep the 50k pot, then another thing you could try, is setting up your CF output and intensity control differently. Have the cathode resistor from the CF going straight to ground.  Depending on the tube type you may want to increase or decrease the value of the cathode resistor (a 12AX7 would work better with 100k (or more) load; a 12AU7 would work better with a 10-20k load), and then use a coupling cap from the cathode of the CF to the intensity pot's input lug, and put the pot wiper to the cathode of the pre-amp tube in the signal path, and the other lug of the intensity pot to ground (i.e.: a more conventional voltage divider). This way you can add a (largish -say 47k to 470k) series resistor between the coupling cap from the CF and the intensity pot's input lug, in order to limit the overall strength of the intensity control as desired.

[balcorn80]

So I would want it to look something like this?

[tubeswell]

That's what I'm saying.  And if you want to decrease the overall intensity, add some series resistance between the coupling cap and the pot's input lug. Play around with the value of the series resistance until you are satisfied.

[balcorn80]

Yeah I forgot to show that resistor in my drawing. Thanks for the help!

Also, I can't believe i have been trying to draw up my schematics by hand for the past couple of years... I'm sure there are more complicated programs out there for doing this, but this jschem program has really helped me out. Along with the cats in this forum. Much love and respect for all those who have such awesome knowledge and their willingess to share it.

I now have to wait until 5 o'clock tonight to get home and try out this idea!