Working through a Ampeg Super Echo Twin ET-2b

[twilightofthedogs]

Howdy folks, Lately I have been working my way through a well used Super Echo Twin and I just wanted to share my experiences, as these are fairly rare and there isn't a ton of info out there on them. It's a beast of an amp for a novice/intermediate such as myself. This is the version with solid state rectification and 7591 tubes. Here is audio and some images of the amp in question.

Ampeg Super Echo Twin - YouTube

My general impression of the way the amp works is that it is somewhat more complicated that the usual explanation of "two reverberockets in box." There is a lot of inter- connectivity in the different channels that this doesn't quite explain. After staring at this thing all weekend I am just beginning to wrap my head around it... sort of.

Here is one of the schematics I have been looking at:
Prowess Amplifiers - Ampeg - Schematics - ET2b Superechotwn

I have also been looking at the schematic that is on the back side of the back panel. There are small differences, primarily in the tremolo circuit. The one in the amp matches my amp better in that regard. The one in the amp also has voltages listed. I haven't found a good example of this one online, and will post a pic later.

I have been through the amp and replaced the two big cap cans (JJ 40/20/20/20 cans work out very well). I also replaced all the bypass caps and checked all the coupling caps for leakage (none found in the green caps, the .005 in reverb circuit was quite leaky). In general all the voltages seem to be about right.

I have everything up and running about how I think it should, except for the tremolo. I have spent days messing with the tremolo, trying to get it to work right. Basically, when the speed and depth are turned all the way down I still get a tremolo type effect, but it has weird funky rhythms. As I turn up the volume and depth it starts to behave about like it should, but starts to thump like crazy in time with the tremolo. When looking at voltages I can seen pulsating from the tremolo way back at the first gain stage, which doesn't seem right. The things that I have done that seem to help are changing the resistor at the cathode of the power tubes. Increasing this seems to help reign in the tremolo. The thing that seems to help the most is by increasing the capacitance at the cathode of the phase inverter V4. Swapping out the 100uf cap with a 220uf seems to get the tremolo to about where it is supposed to be. My gut tells me that if the amp was working like it should, it would work correctly with a 100uf cap.

If you are still paying attention, here are some discrepancies I have found between the amp and the schematics I have been looking at:
The schematic on the back panel of the amp lists the voltage dropped at the cathode of the power tubes as 23v (or 25v hard to read). For the life of me, I can't figure how this can be correct. I am getting 360 at the plates which is the correct voltage as listed. I am getting about 14 or so volts dropped at the cathode. If I want to get 23v I had to change the resistor from stock 140 to about 300, which pushes the plate voltage up higher than the 360 specified. My guess is that 15v is about right and the schematic was drawn for the 6V6 version and didn't get changed. I have seen other ampeg schematics (a reverberocket, I think) that show 15v at this location with 7591s and 140 ohm cathode resistor.

Also, my schematic shows 170 volts at the plate of both V5b and V6b. I get 190 volts at v5b and about 170 at v6b. The plate resistors are also swapped on my actual amp. I have a 120k at v5b and a 270k plate resistor at v6b, this is the opposite of all the schematics I have seen. The different voltages seem to be in line with the difference resistor values, so it seems like the voltages of 170 and 190 are actual about what they should be. I also have an original .1 coupling cap at the plate of v6b, compared to the .05 at the schematic.

Another difference is in the tone stack. The tone basically worked as another volume control when I first got it. It was also a very dark amp. Now, I have heard these are dark, or warm amps, but this seemed excessive. I discovered that instead of the .002uf cap at the tone control I actually had a .1uf cap. It was an old wima, and as far as I could tell, original. I swapped it out with a .002 and this seemed to fix the tone control and brighten the amp considerably.

Anyway, that's about it for now. Kind of a beast. I will post anything else I can think of.

[twilightofthedogs]

One more thing. I did find wall voltage on the chassis before swapping out the power cord and removing the death cap! So, be careful folks.

[twilightofthedogs]

And here is the ampeg ET-2b schematic from the back of my amp.

http://31.media.tumblr.com/73e3416ec...a16o2_1280.jpg

[Enzo]

There was originally a "death cap," a cap from chassis to one side of the mains. That results in mains voltage on the chassis depending upon the power plug. When you find mains voltage on the chassis, and the amp lacks a ground switch, then flip the wall plug over. That way the cap is no wired to the neutral side instead of hot. Or remove it and wire in a three-wire grounded cord. That is what a ground switch does, select which side of the mains gets the cap to chassis.

The two channels may be interconnected, but it is still pretty much two amps in a box. The left side speaker output is also used to drive the reverb spring unit. Not a new idea, Hammond had been doping that in organs for years. They returned the reverb signal to the other amp. And the trem is a bias wiggler on the left channel.

I* don't doubt your drawing matches the amp better. The one from Prowess is one of the infamous Joe Piazza re-draws, that are known for their mistakes and omissions. he also never included part numbers, so we can never refer to R12 and C19. I always look for a real Ampeg drawing before resigning my self to a Piazza.

So you get trem effects all the way down? Make sure it is actually the trem you hear. With the trem depth at zero, getting your effect anyway, change the speed setting. Does the effect sound also change? As to weird trem rhythm, scope the plate of the trem tube. What does that look like. It may take some fiddling with the scope to get a readable waveform on the screen at such high amplitude and low freq.

The trem runs off the screens node of that channel power amp B+. Is there any trem bumps on that node? Again, verify by changing speed control.

Oh, and does turning the trem off by the FS stop this effect?

As to thumping when turned up too high, look at what the trem is doing. If the trem signal is too hot, it can affect the power tubes, and if they draw slugs of current on each trem peak, then that will put corresponding dips in your B+, ehich will then affect everything downstream from that B+ point, including the preamps.

Listen to your gut. redesigning a circuit is rarely the proper approach to curing a symptom. Find the actual problem, don;t change all the part values.

23v across 140 ohms? I get about 164ma, or 82ma per tube. And at 360v plate, roughly 28 watts per tube. Does that sound right to you? Put 140 ohms in it, what do you get? Does that yield reasonable results? If so, leave it. What would you have done if those voltages were not on the drawing? My factory drawing looks like yours, but has no voltages. Treat the amp accordingly.

Note how Piazza has renumbered the tubes? Use the tube numbers from your real schematic. What he calls V5 is what they call V4. I see a 0.1uf at the plate of V4a. Your factory drawing also seems to have tube numbering issues, on the right side, they have the PI as V7 and the outputs as V8, V9. But there are only 8 tubes, and pretty sure the tube list at the bottom is correct and the numbers on the schematic itself wrong. SO let us make sure just which V5,6,7,8 we are talking about.

I sure don;t worry about a 20v difference in a B+ reading. remember, there are two power supplies here, and they may not be in agreement to start with, so before concerning over a 20v difference, check to see if the supplies are diferent, and that explains it all.

[twilightofthedogs]

Thanks for the awesome response enzo, I will check out the tremolo stuff and get back to you. I ordered another set of 100uf caps just in case. I had noticed about the tube numbering. Like I mentioned, my guess is the drew the schematic for a different version and didn't do a great job of updating everything. My guess is that V1 and v10 are the rectifiers that show up in the other models.

[Enzo]

You are very generous to assume they drew a different version. Unfortunately Piazza redrew all the AMpeg schematics, and people use his drawings instead of real ones, but they are full of errors. I never trust them.

[twilightofthedogs]

Went back and replaced the 330uf cap at the cathode of the phase inverter with stock 100uf cap and the freaky trem came back. When the tremolo is disengaged there is a heartbeat-like tremolo effect that comes out of the amp. It thumps, and really does sound a lot like a heartbeat, I can see the power tubes beating out of sinc with each other. I tried to scope the plate of the trem tube, but couldn't really get anything but gobbly gook. Horizontal lines beating in time with the "heartbeat" were about all I could get. With the tremolo engaged the "heartbeat" floor remains, and the trem works about right, only it seems far too intense, and causes a lot of thumping at the speakers. I do pickup the "heartbeat" at all the high voltage nodes as well. As I am typing this, it seems likely that maybe the trem isn't the problem, but something else is causing the irregular "heartbeat?"

[twilightofthedogs]

I would also say that this only happens on the channel with the trem.

[Enzo]

Could be something else motorboating. That is why I asked if turning the speed control affected the rate of your unwanted beat.


The LFO is really slow, at least compared to say 60Hz. So a scope has to have the sweep set really slow to resolve a waveform out of it. First set the vertical so the entire gobblygook fits on the screen. Now slow the sweep way down. If a single cycle wants to be five times the width of the screen, you will just see a curved line bouncing up and down. SLow down the sweep, and the image will be real blinky, but you ought to be able to see a waveform in the motion.

[twilightofthedogs]

OK, finally got this over the weekend. Turns out the 510k ohm resistor at one of the plates of the phase inverter was open. Through some trick of parallel resistors, it was giving a reading in circuit that was not far off from 510k. However, when I compared with the values on the other side of the amp, they were way different, so I was able to detect the problem. Also found that one of the 47k plate resistors at the phase inverter was reading about 120k. Replaced both of those and now everything seems to be in good shape. Thanks for your assistance Enzo!

[Mars Amp Repair]

Hello folks,
RIP for Enzo...sure appreciate his input here as well as the thousands of other posts that i have referred to over the years.
Anyway to the ET2B: The one I'm working on has been recapped by a prior tech & the job was done well. Other than obvious tube issues it had, it has a dominant Buzz (not a hum) in the echo channel only with the volume pot turned up. No buzz with it turned down.
I can hear the buzz on the grid of the preamp tube V6 pin3 even with nothing connected to it. If I short it to ground, the buzz also goes away.
I've tried all the standard fixes eg: Lead dress, changing grounding scheme by separating it between preamp & driver/output stages.
I added a virtual ground to the fil and tested with its own centertap connected or disconnected to no avail. You might think why even try it with the centertap connected...well in the world of filament virtual grounding, I've actually solved some noise issues like this...Don't ask why...I don't know why!
I've even disconnected the filament connection to the preamp tube and powering the fil w/external DC. Interestingly if I disconnect the ext fil connection as the tube is cooling down the buzz is gone. In my mind that kinda isolated the issue to the filament imposing the buzz in the stage.
I've watched TwililghtofTheDogs video & definitely hear no buzz at all. I'm about to give up & just call it a 'design' issue.
Any ideas?
BTW as for the idle wattage of the 7591's: I had to put a 62ohm resistor in series with the existing 150ohm cathode resistor91 to reduce the idle wattage to an acceptable value.
Another weird thing is that even with new JJ 7591 MP tubes, I get a loud 'POP" when I tap those tubes. Pretty much like the sound you get when power tubes are old a worn & ready to short. No arcing, tho and the tubes continue to function. I really can't explain that one unless 7591's for some weird reason are more susceptible to physical weakness.
Thanx all...glen

[Helmholtz]

V6 pin 3 should be a cathode. Do you mean pin 1?
Which schematic are you using?

Are all input jack switching contacts good?
It is essential that all inputs are shorted to ground with nothing connected.

[Mars Amp Repair]

V6 pin 3 should be a cathode. Do you mean pin 1?
Which schematic are you using?

Are all input jack switching contacts good?
It is essential that all inputs are shorted to ground with nothing connected.

My Bahd.
Yes, Pin 1 6SL7. Yes all jacks are well grounded to the chassis with non-rusted star washers and shunting lever switches burnished & contacting well on all input jacks. Thanx Hemholtz.

[Helmholtz]

Did you verify using an Ohmmeter that input are actually grounded?

[twilightofthedogs]

RIP Enzo for sure. He was a international treasure.

I still have this amp, but I gotta say, it's one that continues to plague me. I'll get one thing sorted and then another thing comes along. I can't remember what the current issue i'm having is, its been a while since i plugged it in. It's also definitely an amp that i have spend a day or two just thinking about before my head is in it deep enough to remember what all is going on! All that is to say, I don't think i'm going to be much help solving the problem. I will say that of all the weird stuff this amp has thrown at me, a buzz like you are describing isn't one of the issues.

I am going to ask this only because I didn't see you mention it as something you've tried, and because I've definitely wasted days and dollars myself before slapping myself on the forehead for skipping step 1, but I'm assuming you've swapped the preamp tubes around to verify that the issue stays with the socket, not the tube? Good luck!