Ampeg Jet Weird Design

[Justin Thomas]

Hey All,

I am coming back to this thing again... I've realized that there is basically only one triode gain stage before the paraphase PI, which I guess does not really provide much gain in this circuit. My main complaint is low volume, though the tone is respectable for the most part. I just feel like it should be louder. But my question for today is,

What the heck is the purpose of the circled part of the attached schematic? I've never seen anything like this in any other amp. I see the tremolo oscillator, pretty standard, but this seems like a total waste of gain stages here? What's going on? Maybe if this is "useless" I could purloin a gain stage...

Anyway, thanks for any ideas!

Justin

[Randall]

It looks like part of the tremolo to me. It's grid is fed by the plate of the first oscillator triode, and it's output is fed back to the input, modulating it I think. But I don't understand why the plates and cathodes are tied together.

[Justin Thomas]

That's my question - why the opposite elements are tied together. Seems like an intentional waste.

I do know this design didn't last long; nor did the 6BK11s in it. But geez, an extra gain stage would be nice.

Justin

[eschertron]

First, how does that function? Is it modulating a load impedance on the first stage's anode? That's weird.
Second, since I don't know how it works, I shouldn't be offering any advice... but unless the oscillator really needs the extra current from two plates tied together, you could put it to use elsewhere.

[g1]

You can't free it up because it is forming "diodes" with the plates and cathodes cross-connected like that.
Haven't read through all of this, but it's discussed here:
https://www.thegearpage.net/board/in...ocket.2021824/

[Enzo]

The trem LFO feeds the grids of the two triodes. That causes currents through the two triodes to vary with the trem pulses. Changing current makes them for all the world a varying resistance. Note this back to back pair are connectd right to the amp input. The trem thus varies the resistance across the input...voila, trem affect on audio.

If you stole one of the triodes for a gain stage, then your trem would only affect signal on half the trem cycle, ie.either the positive or negative pulse.

[eschertron]

If you stole one of the triodes for a gain stage, then your trem would only affect signal on half the trem cycle, ie.either the positive or negative pulse.

I mis-read the schem. I see now that cathodes and plates are tied 69-style.

[dmeek]

Does the tremolo sound any good? I can't believe that circuit would work very well. Why not drive a fender style Neon/LDR unit with the LFO. Connect the LDR from V2-Pin7 to ground. and free up that dual triode?

[Enzo]

This way probably works well enough, I don;t recall Ampeg making stinkers. This method uses all parts they have on hand. Dual triode tubes and sockets, basic parts. Fender style means they'd have to make and stock photocell roaches and stuff.

We'd need to drive the opto. The trem signal coming out from the LFO won't drive the neon by itself, so one of those "extra" triodes would be needed.

[Leo_Gnardo]

Why not drive a fender style Neon/LDR unit with the LFO.

Patent. Ampeg would have to pony up some licensing loot. To Fender (oh no, the competition!) or whomever, doesn't matter. I use that as the explanation for the proliferation of vibrato & reverb designs in those days.

[Randall]

"Does the tremolo sound any good? I can't believe that circuit would work very well. "

My '64 Jet's tremolo sounds fantastic, very swampy.

[Justin Thomas]

Thanks for the explanations everyone. To answer whether it works or not, yes. It's not unpleasing, but I feel like I can't give it a fair shae because it's so quiet.

I've been comparing some old Ampeg schematics & found that MANY of the smaller amps only had one gain stage before the paraphase. There are some that use cathode-biased outputs, a paraphase, and a tremolo section that only uses a singe-stage LFO & maybe one more triode in a somewhat more "traditional" tremolo circuit. I think I'm going to try one of the simpler schemes and then that will free up 1 or 2 triodes for the preamp. I should be able to leave the PI & power anp the same, and there are a couple 2-stage Ampeg preamps out there.

I took a ton of pictures so I guess if I have to get it back to stock, well, pictures! I'm wondering if te amp is doing its job just fine & the low volume is a product of only that single front-end gain stage.

Justin

[pdf64]

"Does the tremolo sound any good? I can't believe that circuit would work very well. "

My '64 Jet's tremolo sounds fantastic, very swampy.

Is yours the ‘d’ version under discussion here? Earlier jets implement trem with a more common ‘power tube bias variation’ method, later ones use an optocoupler.
https://el34world.com/charts/Schemat...g_j12a_jet.pdf
https://el34world.com/charts/Schemat...eg_j12_jet.pdf

[Justin Thomas]

Coming back for more punishment...

I'm resigned to "it works." But it's very faint, or, I think so. Given the preamp tubes being $100/each, that's not an option! I do think one is stronger than the other because when I switch the 2 6C10s I get weak or none. So I left in the "weak" one.

So I see the trem oscillator output is shot into the input. I swapped the input jacks for Fender style though, and also increased the 5.6M resistor on the far left of the schem for a 10M, thinking it was part of a voltage divider? No effect. What do the .1 & 470K do, that are connected to the plate output of the preamp stage?

Other suggestions to get it pulsing better? I'm going to dink around with it in the meantime.
Thanks!

Justin

Edit: switched 470k to 220k. Sorta better, not much. Next!

[Justin Thomas]

Okay. Last night I swapped out the oscillator caps to slow down the speed. I did 2 of 3; the trem didn't work at all at low speed settings, but once it was a bit higher it would kick back in, but still not strong enough.

Today I went in & put one of the 2 caps I replaced yesterday back to stock so now there is only one .1 in the oscillator. I spent some time looking at the schematic & decided to try messing with (what seems to me) the voltage divider feeding the other 2 sections, the "diodes" that started this post in the first place. I just clipped in various values that ended up with the total being anywhere from 390k down to about 200k.

The stock resistor is now back, the 470k/10k network feeding the "diode" grid. I get only a few decemt-depth pulses of trem when I click the switch on before it dies out, so now it seems the oscillator does not oscillate enough. I replaced the cathode bypass cap on the oscillator as a possible, no change. So right now, the only changes are:

1. One .05 has been changed to .1.
2. One 470k resistor (near the input in the schematic) is now 220k.
3. The 5.6M near the input is 10M.
4. I wired in Standard Fender-style inputs, as neither of the originals were shorting-type.

One other change, I removed the wires for 6BK11s that grounded the internal shieldd of those tubes. The 6C10 says pin 8 is an internal connection, but I can verify that in MY 6C10s, there is absolutely nothing connected to pin 8, even though the data sheet specs "Internal Connection." Not on mine! I don't like having useless wiring lying around in my amp, especially sincexit was connected to ground with cathodes fronm other sections. The amp actually seems less hummy now as a result.

Please don't suggest new preamp tubes - the 2 6C10s I have are definitely different and if I swap them I get nothing for trem. Last night I had steady-yet-shallow. And no, I am not going to spend hundreds on new 6C10s!

Thanks for following on my mad adventure while my mucking about continues.

Justin