SE EL34 Plexi-verb

[tubeswell]

Hi Phil, interesting concept for the reverb,

Could you please upload the .jsch file? I'd like to manipulate some of the symbols

[Phil_S]

For the record, the site doesn't permit posting of jsch files. I sent these by email.

New frustration day. I reinstalled the 5/15v board. There is good news and bad news. There is not enough good news but first here's the good.

The voltage multiplier is working. I can find 7V, 14V, and 21V on the board. This is perfect math for a 5VAC supply. The 15V supply output is at 19.7VDC. I am disappointed. Cheap parts? The probably isn't a problem, but am concerned about the 5V supply being too high when I get that working.

The bad news. The 5V supply isn't putting out anything even though it is receiving 7V input. I'll have to pull the board yet again and see where the 5V supply is getting lost. I did not rebuild the whole board, so I may have used something that was spent when it last smoked.

The worse news. The amp was seriously LOUD on the pop test, so loud I had to turn it way down. Today, The pop test give nothing with the 5/15v board installed, I've got B+ flowing, but no voltage drop to speak of after the screen node. It holds steady down the line at around 303V, so it is not drawing current. Tubes are installed. There is no cathode voltage on the 3 preamp tubes. The power tube has nearly 16v at the cathode, so I believe it is operating OK.

I am having difficulty imagining that installing the 5/15V board caused this. I'll be pulling the board as soon as time permits, but this seems strange.

[JazzGuitarGimp]

With 19.7V at the output of the 7815, I would suspect one of three things.
- The ground pin is not grounded.
- The regulator is toast (you _may_ be able to discern this by measuring DC resistance from input to output without power applied - if you've got near zero ohms, the device is fried). But this could also mean:
- There is an input to output short circuit somewhere on the board.

No +5V out of the 5V regulator suggests a +5V to GND short - you should be able to verify this with an ohmeter.

[Phil_S]

Thank you for your thoughts, Lou. As noted earlier, I am new to solid state things and am trying to sort this out and your comments were very helpful. I decided to strip the board for parts and start over on a different type of prefab board -- more jumpers, not trace to cut. I can really see where having what it takes to make a PCB is a plus.

Both the 7805 and 7815 read about 5MΩ between pins 1 and 3. Brand new ones are about 2.5M and 3M. On that basis, I'm going to replace them. LOL, I had to buy 10 of each anyway and they are cheaper than dirt!

I don't have what's needed to test 2200uF caps, however, I put them on the ohm meter. The continuity beeper went crazy on 2 of them, so those are out and perhaps were part of the problem. (I'm using 2x 2200 in parallel instead of 4700uF on the 5V side.) The other caps all metered for uF within reason compared to what they should be and I'm going to reuse them because I don't have replacements. I don't have enough spare parts on-hand or I would simply start over with all new.

While I've got that board out of the amp, I fired it up to check voltages again. The amp is back to normal, and loud. It has 120Hz hum, which I'll trouble shoot later. I succumbed to the camp that says to ground to the chassis near the input jack. I'm not sure I've got the ground scheme as it should be. In any case, there are tube shields to install and RF plate for the bottom before I leap to any conclusions.

What I'm really confused about now is why the amp works without the 5/15V installed and why it behaved so oddly with the board installed. It is clear the problem is related to the board. The amp was OK before I installed it, bad when the board was installed, and the amp returned to normal when I removed the board.

The 5/15V board connects to the reverb board. The Reverb board is essentially an effects pedal taking and sending signal from a pair of 470K resistors separated by a 220K resistor, and then the signal goes into the 6AQ7 recovery stage.

The only explanation I can come up with is that I'm tying the grounds together for the 2 boards and then running a ground wire back to the main turret board. I'm not seeing how a ground wire is responsible.

[Phil_S]

I am pretty sure I've got the 5/15V problem sorted out. I tested it out of the amp. The 7805 output is 4.7V. The 7815 output is 15.3V. Maybe I'm a step closer?

[JazzGuitarGimp]

What I'm really confused about now is why the amp works without the 5/15V installed and why it behaved so oddly with the board installed. It is clear the problem is related to the board. The amp was OK before I installed it, bad when the board was installed, and the amp returned to normal when I removed the board.

Without seeing the schematic, I'm just making an educated guess: Perhaps the reverb board was shunting the audio signal to ground?

I am pretty sure I've got the 5/15V problem sorted out. I tested it out of the amp. The 7805 output is 4.7V. The 7815 output is 15.3V. Maybe I'm a step closer?

That looks promising, though the +5V output looks a tad low - what is the tolerance of your regulator?

Edit: Also, what is the voltage at the input of the regulator?

[Phil_S]

Lou, see the full amp schematic posted in this thread on March 7.

I retested the 5/15V supply board. I'm not sure what happened earlier, but here is the result, a good one.

[JazzGuitarGimp]

Lou, see the full amp schematic posted in this thread on March 7.

I retested the 5/15V supply board. I'm not sure what happened earlier, but here is the result, a good one.

Ah, that's looking good. Curious, what is meant by "NC" as the voltage at the head of the 5V regulator?

Edit: And after having looked at the full schematic (thanks, I missed it), I don't think that any kind of a miswire on the reverb board would have caused a total loss of volume in the amp, unless you somehow had +15V coming out of the output of the reverb board.. So not sure what that was about. I'll be anxious to see how it goes this time. Hopefully it will work as planned.

[Phil_S]

NC: a reminder that wires cross without making a connection; "no connection." For someone like me, it is simple self-help.

Maybe with a working board, the problem will not recur.

[JazzGuitarGimp]

Only one way to find out. Let us know how it goes...

[Phil_S]

Hope isn't a problem resolution strategy. After hooking up the 5/15V board to the BDTR-3 board, the amp isn't working right. I did not install the BDTR-3 brick to keep it safe. I get a pop at the plate of the 6AQ7, so I think it is safe to conclude the amp is passing signal from that point to the speaker and the problem has got to relate to the BTR-3 board. There is no cathode voltage at the 6AQ7. The two 6EU7's (subbed for 12AX7's) were working before, but now there is no voltage drop from the B+ ladder to the plates, no cathode voltage, no pop at the speaker when probing.

Looking at the schematic for the board, if the brick is not installed, then the problem has got to have something to do with the stuff to the right of the brick. The active part of that is the 15V input to the 10K and the TL072 op amp.

I have a question about the op amp. Did I understand correctly that the unlabled pin at the bottom is pin 4, and that it is grounded? IOW, is the pair of 10K not right somehow?

And what is the output on the TL072 supposed to be on pin 7?

Am I barking up the wrong tree? I'd be very appreciative of some help!

Thanks.

Phil

[xtian]

I'm backseat driving, here, but I'm looking at this suggested build circuit: http://www.tubeampdoctor.com/images/Fil ... MODULE.pdf

and I'm wondering A) why do you have 100% NFB around your first gain stage, and why are you feeding DC into the second stage input?

Sorry, I don't remember where you got your circuit from.

[Phil_S]

I don't think it's back seat driving. I'm glad someone is willing, as there's obviously a problem. I have gone over the build and AFAIK it conforms to the schematic. (I could easily be wrong about conforming.)

One of our members, frankleslie, posted this as a working circuit. http://ampgarage.com/forum/files/belton_digrvb_644.pdf in this thread, http://ampgarage.com/forum/viewtopic.php?t=28834 (January 17, 2016)

Since the BTDR-3 is a stereo module, I improvised a bit to convert it to mono. I'm not sure I did what was necessary or did it right. I also decided to implement the send/return with a pair of 470K and a 220K between them.

Regarding the NFB, I freely acknowledge that I wouldn't know if NFB bit me on the behind. This is open for target practice. Do what you will. Maybe we can make it work?

To be clear, this is my first foray into solid state stuff. I am somewhat clueless about what the TL-072 actually does.

[xtian]

I don't know op amps either! Except that they amplify the difference between inputs. So generally, you hold one leg at audio ground (common), and one for the signal.

But look at the AccuBell schemo. R2 (22K) is the NFB path. Yours has a direct connection from the output back to common. That strikes me as a problem.

[xtian]

Also, was just browsing the TL072 datasheet, and see that it will operate at 5v. Is there a reason you chose 15v?