Ampeg Jet Weird Design

[Helmholtz]

My explanation of the tremolo circuit:

The input tube V2 is wired as an inverting amp with voltage feedback from plate to grid. The first V1 is the LFO that drives the grids of second V1, which act(s) as a variable/controlled resistance which in turn modulates the gain/feedback of the input tube V2 by variable voltage dividing.

The 69 wiring of the 2 triodes of the second V1 makes sense as there is no DC current. When the plate signal of V2 goes positive, only the right side triode of the pair conducts while the left triode is blocked. When signal polarity reverses the left triode of the pair conducts and the right one blocks. So we get a modulated "bipolar" impedance.

[Justin Thomas]

That makes sense to me, as best as my limited understanding can make. I've been experimenting with what I see as voltahe dividers to try to make the trem deeper; changes to schematic are as listed above. I've mostly been trying to lower the laeger portions of what I see as voltage dividers. Any other suggestions? As it is now I get a few good pulses then it dies...

Overall the amp siunds good so I want to get it right; I'll keep mucking around as I think & document any further changes here. The only suggestion I'm not open to: new preamp tubes. Just go price me some 6C10s or 6BK11s & you'll see why!

Thanks Helmholtz, I'll apply your theory to make tweaks.

Justin

[pdf64]

Is that cathode bias voltage changing with the first few pulses?
It may be worth trying LED biasing (rather than cathode resistor and bypass cap), but need to find a suitable LED (or stack a series pair if more bias voltage is needed).
Is the F node HT decoupling ecap definitely good?
The lowish resistor values in the phase shift network may be loading the stage excessively, thereby reducing its gain. Instead of bigger value caps, try higher value resistors, especially that 1st 120k after the plate.

[Justin Thomas]

Thanks pdf64, I'll check out what I can later this afternoon. I think I'll put all the resistors back to stock & start again.

Justin

[Justin Thomas]

Okay, I've put everything back to stock except for the Fender-style input jacks but still using 47k input resistors & no 100k on the one jack.

I can get regular tremolo now, with the right tubes in the right position. The volume issues have sorted themselves after I threw a new can cap in it, and hum is greatly reduced.

In answer to pdf64's questions,

The oscillator is pulsing steadily. Cathode voltage reads between 1.3 & 1.65V to ground regularly. The AC on the filter cap node is .015V, though I don't know what you mean by "F." I just see A, B, C?

I don't have any LEDs handy but might be able to get some. If not I'll have to order. The trwm is now regular, but weak - really weak. More like "I left the ceiling fan on in the room" weak. But it's there. So if I could build some strength/depth, that'd be cool. I'll futz with the two 120k resistors in the oscillator meanwhile, & report back if anything changes.

Justin

[Helmholtz]

The forward voltage of LEDs depends on color, increasing from red to yellow to green to blue. Red ones are between 1.6V and 2.2V. The required 1.5V lies in the voltage range of infrared LEDs.

[Dave H]

I don't have any LEDs handy but might be able to get some. If not I'll have to order.

You could use a couple of ordinary diodes (not LEDs) in series.

[Justin Thomas]

It seems I'm getting the bias I need as is. The schematic shows 1.5V, and my average between extremes is 1.46 according to my meter. Are you saying I would want the average to go up? As in use 3 diodes for 1.8? More? I've git lots of regular diodes... They're all 1000V/3A, but, hey...

One other question I do have: with the tremolo switch actually clicked to "off," the voltage on the cathode steadily rises. I click it off & it goes to 10VDC & rises to 20. I removed my meter then. I've got a 25V cap as the bypass cap & it hasn't blown up yet, so that's good. I just wonder how high would it go?

Ugh, so much left to learn...

Jusrin

[Helmholtz]

One other question I do have: with the tremolo switch actually clicked to "off," the voltage on the cathode steadily rises. I click it off & it goes to 10VDC & rises to 20. I removed my meter then. I've got a 25V cap as the bypass cap & it hasn't blown up yet, so that's good. I just wonder how high would it go?

I don't think the voltage actually rises if you don't connect your meter. If you do, the tube uses your meter resistance as a large value cathode resistor causing a small cathode current which charges the cap until some equilibrium is reached.
To test switch off the tremolo, wait some time and then shortly connect your meter.

[Justin Thomas]

So I tried upping the two 120k resistors in the oscillator. No help there... I did one first, then added the other. I'll try them separately (one at a time) later tonight.

Justin

[Helmholtz]

The first thing I would have tried to increase tremolo intensity was to increase the cathode cap to say 220µ. The intent is to increase very low frequency gain. Ideally the cathode voltage should not vary with oscillation.
This might cause a little turn-on delay, though.

[Justin Thomas]

Thanks! Ill tray anything at this point. Not sure I've got a 220uF but I've dwf got bigger than that...

Justin

[Justin Thomas]

Returned the resistors back to 120K AND put in a 250uf/25V cap I had. No love. I may just start rebuilding the tremolo circuit one component at a time... All the resistors test well within spec & I've replaced nearly every cap in it. So I'm outta guesses.

Thanks to all, & if I stumble across the answer I'll post it here!

Justin

[Helmholtz]

2. One 470k resistor (near the input in the schematic) is now 220k.

Which resistor is this? If you increased one of the 47k grid stopper to 220k, this will reduce input gain by a factor 4 (and consequently reduce tremolo intensity). With this inverting amp wiring the grid stoppers are part of the feedback network/voltage divider.

Unfortunately I don't see a possibilty to increase input impedance without decreasing gain with this circuit.

[Justin Thomas]

Hey Helmholtz,

Iwas these two I tried lowering. I tried them separately. The one specifically that you are asking about is the circled one on the left; "near the input" is just a vague location because my schematic doesn't have designators. I'm also working from my phone here so am a bit limited & hafta get creative sometimes.

I haven't done anything to any 47k resistors. And currently everything except for the input jack wiring & trem oscillator bypass cap (now 250uF) is like the schematic. The grid stops are still 47k. Nothing I'm trying really seems to change much of anything.

Justin