1 channel Tremolux in PR Chassis questions

[owen]

How close is too close for a choke? Here’s a photo of the current PT home, and I thought it’d be cool to put the choke right where the PR filter cap can sits between the OT and PT.

Should I relocate this?

As for the tube proximity to the PT, if there’s a heat issue, I can easily add a small fan driven by a regulated PSU tapped off the mains.

[Latole]

I leave it there.

[owen]

I leave it there.

Thanks! wiring it all together today. Should be testing once I have the right OT. hopefully it goes much smoother than the super.

[owen]

single channel circuit changes question:

The only change I’ve made so far is deleting the first channel and tube, then changing the cathode bypass cap resistor before the tremolo section from 820r to 1.5k since the ‘normal’ channel is gone.

is there anything else that needs to be changed? specifically after where the two channels would sum?

schematic and layout attached.

[sluckey]

I would replace the 220K mixing resistor that's connected to point "Z" on the board with a straight wire jumper.

[owen]

I would replace the 220K mixing resistor that's connected to point "Z" on the board with a straight wire jumper.

ah yes. no need for 220k mix resistors. i get it now. thanks again!

[sluckey]

Probably too late for an underboard jumper but this would look good on top side. Just run the jumper wire under the components.

[owen]

well this one was easy. all wired and fired. Aside from tremolo thumping, it’s very strong and loud. there is some white noise/hiss with inputs grounded and volume cranked. amp is completely silent with preamp tube pulled.

voltages are 450 at the first filter cap and plates. bias current is 46.7mA on one tube, 45.1 on the other.

once the trem thump is sorted (will try lead dress then pot diode) I’ll revisit my Super.

 

Probably too late for an underboard jumper but this would look good on top side. Just run the jumper wire under the components.

thanks sluckey. I managed an under board jumper straight to that cap this morning. Glad to see you suggest over the top could have worked too.

[tubeswell]

once the trem thump is sorted ...

Try upping the size of the 220k bleed out resistor on the LFO Buffer output to the next size up (270k) or the next size after that (330k) etc etc until the 'thump' is gone

[owen]

I’m not totally in love with the tapped treb pot tone stack so far. less mids where I want, some where I don’t (on my 2x10 cabinet with goldfoils.)

Try upping the size of the 220k bleed out resistor on the LFO Buffer output to the next size up (270k) or the next size after that (330k) etc etc until the 'thump' is gone

will give it a shot, thanks! my 6g2 thump was cured with lead dress, this wasn’t.

I may do the LED mod as well.

[sluckey]

I may do the LED mod as well.

That will probably make the thump worse. How much negative bias voltage do you measure on the Intensity pot?

[sluckey]

Post the voltages for both 6L6s. Pins 3, 4, 5, and 8.

[owen]

Post the voltages for both 6L6s. Pins 3, 4, 5, and 8.

it’s biased hotter than i’ve done bias trem amps so far. wanted to see what it does at volume. I’ll bias it cooler again to check, but thought the thump was there around 32mA

-44.2 on intensity pot

“V6” 6L6 Next to rectifier:
3) 449
4) -43.9
5) 450
6) 47.6mV

“V5”
3) 449
4) -43.6
5) 450
6) 46.5mV

[sluckey]

That trem circuit has always been a bit finicky with 6L6s. The trem and bias interact with each other. Probably why Leo ditched that trem circuit in favor of the more reliable roach trem.

Look at Hoffman's Princeton Reverb schematic to see how to easily add a bias pot to your amp. Then usually you can twiddle the bias to get a suitable tremolo effect. There's usually a compromise between what you consider good bias and good tremolo.

EDIT... Do you realize your plate voltage is 84V higher than the Tremolux schematic?

[owen]

That trem circuit has always been a bit finicky with 6L6s. The trem and bias interact with each other. Probably why Leo ditched that trem circuit in favor of the more reliable roach trem.

Look at Hoffman's Princeton Reverb schematic to see how to easily add a bias pot to your amp. Then usually you can twiddle the bias to get a suitable tremolo effect. There's usually a compromise between what you consider good bias and good tremolo.

EDIT... Do you realize your plate voltage is 84V higher than the Tremolux schematic?

Ah I See. I’ve only done 6v6 bias trem so far. There’s a bias pot in there. 600v caps on the first b+ node. 500v after.

I know the voltages are higher by quite a bit. I went with 650v hammond PT and it’s hitting a new 5AR4 and pair of JJ 6L6GCs. my wall voltage today is 126.7v

I couldn’t find an easily affordable transformer with lower voltage. Mercury makes a 560v, but that’s a ton of money for a ‘spare parts’ amp.

Mojo’s 125P6A/68409 replacement is a whopping 660v @300mA

[sluckey]

First off, can you kill the thump by simply turning the INT down a bit? Still have decent tremolo depth?

Next try this... Monitor the mV on a 6L6 cathode. Slowly adjust the bias pot while listening to the tremolo thump, BUT KEEP AN EYE ON THE METER SO YOU DON'T ABUSE THE 6L6s. Is there a spot where the thump disappears and you still have acceptable bias current?

[owen]

First off, can you kill the thump by simply turning the INT down a bit? Still have decent tremolo depth?

Next try this... Monitor the mV on a 6L6 cathode. Slowly adjust the bias pot while listening to the tremolo thump, BUT KEEP AN EYE ON THE METER SO YOU DON'T ABUSE THE 6L6s. Is there a spot where the thump disappears and you still have acceptable bias current?

with tremolo on, thump is present from 1/3 intensity to max intensity even when biased colder to 25ma.

(bias adjustment attempted ranged from 25 to 47 via bias pot. consistent measurements read when tremolo was grounded via footswitch).

[sluckey]

when grounded via footswitch

Say what??? Grounding the footswitch will totally kill the trem oscillator. There should be ZERO thump, at least caused by the tremolo. Just remove the tremolo tube. Do you still have some thump?

[owen]

when grounded via footswitch

Say what??? Grounding the footswitch will totally kill the trem oscillator. There should be ZERO thump, at least caused by the tremolo. Just remove the tremolo tube. Do you still have some thump?

edited for clarity. i only switched off the tremolo to check the bias, making sure I didn’t go too hot on the way back up.

zero thump when tremolo tube is pulled or the footswitch engaged.

[tubeswell]

How did you get on with increasing the 220k? Any joy?

[owen]

How did you get on with increasing the 220k? Any joy?

clipped in a diode, got rid of the deeper frequencies of the thump, though there is a still a tiny bit present. however, now the trem is a hard on/off chop at half intensity which is pretty awesome. the bias during tremolo use is swinging less frantically and in a tighter range with the diode across the intensity pot.

will try that resistor tonight and see what i get without the diode.

cheers!

[owen]

How did you get on with increasing the 220k? Any joy?

270k still has thump from 1/2 pot up and a loss of intensity. 470k even further loss of depth (zero trem until 1/2 up the dial) but still thumping from 1/2 up to max.

the diode on the intensity pot kills the amp response a little bit, but seems to be the most acceptable choice for now.

[tubeswell]

How did you get on with increasing the 220k? Any joy?

270k still has thump from 1/2 pot up... 470k … still thumping from 1/2 up to max.

Okay then, the thump obviously isn’t coming from too much trem output. It’s more likely to be unwanted coupling of the LFO into the signal path (and if the LFO triode is in a separate bottle from any of the preamp signal triodes, then its most probably lead dress and/or LFO ground returns mixed up with signal ground returns.

[sluckey]

Tack solder a 22µF (or larger) between point A (on the schematic) and ground. This is the screen node which also supplies the tremolo circuit. If no joy, get B+ for the trem circuit from the PI B+ node. Any better?

[owen]

Tack solder a 22µF (or larger) between point A (on the schematic) and ground. This is the screen node which also supplies the tremolo circuit. If no joy, get B+ for the trem circuit from the PI B+ node. Any better?

no luck on adding a 22uF off of point A. moving the trem B+ lead over to the PI node didn’t fix it either, however the voltage is now in spec with the schematic at that point.

I tried also dropping to 117vAC with my variac and the PI B+ feeding the tremolo tube, maybe a slight reduction in the overall bass content of the thump, but still present 1/2 INT to max.

when idling the thump is most present, when playing it’s either gone or masked very well. i chilled the bias way down to roughly 45% with no appreciable change, and finally back up to ~65%

it’s totally livable since this is my own amp, but my obsessiveness persists.

[sluckey]

I'd put a Trem-O-Nator in it and never look back. 

[ac427v]

I'm still stuck on the high B+ issue. The 5R4GY-A-B rectifier can drop around 50-60 volts depending on load and transformer current capability.

[pdf64]

What mV is measured at each cathode with the volume down, trem intensity set to thump.

[owen]

I'm still stuck on the high B+ issue. The 5R4GY-A-B rectifier can drop around 50-60 volts depending on load and transformer current capability.

any issue using a directly heated rectifier in this amp? would I need some sort of b+ delay? my coupling caps are 600v+ rated, filters are 500v.

[pdf64]

Unless the HT gets >550 for the few seconds between the rectifier and the rest of the valve heaters warming up, I wouldn’t worry about it.
Decent ecaps have a momentary surge voltage rating typically 10% higher than their continuous working voltage rating.

[owen]

What mV is measured at each cathode with the volume down, trem intensity set to thump.

fluctuating between roughly 18mV and 60mV min/max. voltage from b+1 fluctuates up very high as well, close to 480v

bias voltage is steady -46v

[owen]

Unless the HT gets >550 for the few seconds between the rectifier and the rest of the valve heaters warming up, I wouldn’t worry about it.
Decent ecaps have a momentary surge voltage rating typically 10% higher than their continuous working voltage rating.

copy. thank you! I'll order a couple. I also don't mind trying to find a 275-0-275 transformer that can manage the current. I left the leads on this one long incase it didn't work out for this amp.

I'll take it in to my concert hall with a couple meters on it to see what happens cranked up with blissful 2.85 second natural reverb decay

[pdf64]

What mV is measured at each cathode with the volume down, trem intensity set to thump.

fluctuating between roughly 18mV and 60mV min/max. voltage from b+1 fluctuates up very high as well, close to 480v

bias voltage is steady -46v

How do the readings compare between the push pull sides?

[owen]

Update with 5R4GYAB rectifier. 399v plates with a colder bias, still the exact same magnitude of tremolo thump. no change with lower B+ and colder bias. knocked wall voltage down with the variac, negligible change.

Hammond’s power transformer replacement for ‘blond/brown’ tremolux is the same for the black panel tremoluxes. I assumed this was the case purchasing, but also found threads of real 6g9-B tremoluxes running hot, figured maybe that was the case with modern tubes and wall voltage.

found yet another thread with this exact same concern using modern PTs for fender part numbers…

for kicks, ordered a 275-0-275 edcor that might be here in two months…

the plate voltages and bias current swings at max trem intensity between both power tubes are extremely close, both tubes even having the exact same plate voltage.

edit: just did a min/max with the 5R4, the bias current reading across the 1R resistor swings from 100mA down to 0.

[tubeswell]

Is your LFO triode sharing a bottle with one of the preamp signal triodes?

[owen]

Is your LFO triode sharing a bottle with one of the preamp signal triodes?

the preamp channel is in one tube, tremolo alone in the second, phase inverter on the third.

i only removed the ‘normal’ channel and its preamp tube, increased the cathode bypass resistor from 820 to 1k5, everything else is built to fender style layout.