Tremolo Issue on 6G3

[bal704]

I'm building a Fender 6G3, and have a few questions about the Tremolo.

I'm building it per the Fender Schematic/Layout, but I'm using a SS rectifier rather than a tube. My B+ is 425V, vs 375V on the schematic, and in general my voltages are about 30V-40V higher than the schematic. I've built about 10 amps, but this is the first one with a Tremolo.

There is a lead that runs to the Tremolo pedal jack. As I'm in the middle of the build, I just have it taped off hanging in space connected to nothing. When I turn the amp on, I get a 'thumping' sound. I can hear the guitar signal underneath the thumping. When I turn the Speed knob, the thumping gets faster. When I turn the Intensity knob, the Thumping gets louder. From what I hear of the guitar signal, the Tremolo is affecting it, as it's 'Tremoloing' When I hook the lead running to the Tremolo jack to ground, the thumping goes away and the guitar signal is fine.

So far, when the Tremolo is grounded, the amp sounds great. It doesn't have as much breakup as I've heard on youtube clips, but I can't really turn it up past 9 o'clock or so.

What do I need to look for on the Tremolo circuit with regards to the thumping? The schematic shows -26V on the tremolo circuit just past the diode, and I'm reading -29V. The schematic doesn't call out a diode part number, so I used a 1N4007 I had laying around.

Any tips or suggestions?

[ToneMerc]

I'm building a Fender 6G3, and have a few questions about the Tremolo.

I'm building it per the Fender Schematic/Layout, but I'm using a SS rectifier rather than a tube. My B+ is 425V, vs 375V on the schematic, and in general my voltages are about 30V-40V higher than the schematic. I've built about 10 amps, but this is the first one with a Tremolo.

There is a lead that runs to the Tremolo pedal jack. As I'm in the middle of the build, I just have it taped off hanging in space connected to nothing. When I turn the amp on, I get a 'thumping' sound. I can hear the guitar signal underneath the thumping. When I turn the Speed knob, the thumping gets faster. When I turn the Intensity knob, the Thumping gets louder. From what I hear of the guitar signal, the Tremolo is affecting it, as it's 'Tremoloing' When I hook the lead running to the Tremolo jack to ground, the thumping goes away and the guitar signal is fine.

So far, when the Tremolo is grounded, the amp sounds great. It doesn't have as much breakup as I've heard on youtube clips, but I can't really turn it up past 9 o'clock or so.

What do I need to look for on the Tremolo circuit with regards to the thumping? The schematic shows -26V on the tremolo circuit just past the diode, and I'm reading -29V. The schematic doesn't call out a diode part number, so I used a 1N4007 I had laying around.

Any tips or suggestions?

Courtesy of Google "thumping tremolo"

http://vintageamps.com/plexiboard/viewt ... =8&t=88250

TM

[bal704]

Thanks for the link. My thumping occurs at any level above zero, otherwise I'd just keep the levels low and live with it. I don't really plan on using the tremolo much (if at all), but I'd like to get it working since it's part of the amp.


I'll triple check my values to see if I used a wrong value somewhere.

[jjman]

You could ac-ground the signal path at different points to determine where the thump is being injected. No roach so no "factory" fix here. If it's thumping only at the output tubes I would suspect badly matched tubes (for ac.)

[bal704]

What does AC ground mean? I have some different tubes, so maybe I'll try a different pair.

[tubeswell]

Just a few suggestions off the top of my head. You could try:
1) taming down the gain of the LFO stage with a smaller plate resistor, or
2)getting the LFP bias more centred (What do the idle voltages look like now?) or
3) attenuating the LFO swing by playing around with the voltage divider where it hits the output tube grids, or
4) biasing the output stage a tad warmer, or
5) sorting out your lead dress (if the problem is stemming from unwanted coupling).

[martin manning]

I wonder if the feedback isn't coming through the power supply. You could try moving the connection to B+ to the other side of the 1k resistor, to the screen node, but a better bet might be to branch off from the plate node with another resistor and filter to create a new node. To decouple this low frequency the R and C will have to be big, and you don't want to drop too much voltage. Fortunately the current draw is just one 12AX7 triode.

[VacuumVoodoo]

Just a thought: is the intensity pot 250k or perhaps a mistake has occurred and a 25k pot got installed?

[bal704]

I guess I'm not understanding which connection to B+. Are you saying the main B+ connection to the OT (marked with X in a box on the schematic)?

I've tapped off one of the two AC inputs (333 V AC on the schematic). Does it matter which one?


I've found some threads on the web regarding Trem Thumping, and they mentioned placing a diode across the Intensity pot to reduce the thumping. Tried that and didn't work.

I'm gonna check my layout, and reflow the connections. Maybe there's something simple wrong. I hope.

[Structo]

In reading about this problem, it seems the earlier tremolo amps that used bias modulation for the tremolo were plagued by poor lead dress which may lead to induction coupling or cross talk making the thumping noise.

Lots of people claim that is just part of the amp while others try to find a resolution.

I seem to remember something about the three ceramic caps .02, .01, .01 and a change there but memory fails me.

The later amps that used the opto bug usually present more of a ticking noise.

I would chop stick the lead dress to see if moving any leads change it.

Basically it is a low frequency oscillator.

[bal704]

Most of the forums I've read said the thumping happens when the intensity or speed is turned past noon or so. I could live with that, as I don't really plan on using the tremolo much. My problem is that it thumps even when speed and intensity are at zero. If the trem is engaged at all, it thumps.

I've used chopsticks on it to see if it makes a difference, but no luck so far.

Thanks for all the tips so far.

[martin manning]

I guess I'm not understanding which connection to B+. Are you saying the main B+ connection to the OT (marked with X in a box on the schematic)?

I've tapped off one of the two AC inputs (333 V AC on the schematic). Does it matter which one?

Yes that point X is the DC supply for the trem oscillator. I'm suggesting a new RC branch from there, or for a quick test move the lead from X to the screen supply node. If the behavior changes that is a clue that you're on to something.

I don't understand what you're talking about with tapping the AC.

[bal704]

I'll try moving the DC node.

On the intensity pot on the schematic, there is a -26V call out on the RH side of the pot. If you follow that back across an RC and diode, you arrive at the 333 V AC tap I'm talking about. If I'm not understanding that correctly, and thus have it hooked up wrong, maybe that's my problem.

[martin manning]

Ok I see, that's for the DC bias; you've got that right. Did you add a bias adjust trimmer? Bias-varying trem can be picky about the bias setting.

[bal704]

No trimmer, just the 22k pot on the schematic. Sounds like I may need to add one.