SE EL34 Plexi-verb

[Phil_S]

Aha, so I seem to have at least a bit of brain function working in my favor!

[xtian]

Quick question: The Accu-Bell schematic does not explicitly show a bipolar +/- 5v supply. And the DIY guitar pedal schematics I've found feature a +5v supply only. I understand that the TL072 will work fine with a 5v differential (versus a 10v differential if powered by a bi-polar supply), but is the point here that there won't be enough voltage swing at the reverb output to fill the function we're building for the guitar amp?

[xtian]

Phil, what is the pin spacing on the BTDR-3? Also, what are you using as its socket?

[Phil_S]

Pin spacing is .1" and I bought 10 sockets! I think they were $1 with postage. Ebay.

Is this cap 39pF?

[xtian]

Is this cap 39pF?

Yes. And 3.5mm, thankyouverymuch.

[JazzGuitarGimp]

Lou: You are very fortunate. She leaves you alone for a while AND then she returns. It is the best of both worlds! Thank you very much for being so thoughtful and generous with your time, to create a circuit for me to use. I can see now just how lost in the weeds I am with this. My assessment is that it would take me months to do enough reading to get up to speed on what's actually required. So, I guess you must know how much I appreciate that you threw me a fish.

On the depth pots (not being argumentative, just exploring):
I understand "depth" to be the decay. This is a fixed interval on the earlier BTDR-1 and BTDR-2. Choices are 2s, 2.5s, and 2.85s. There is room in the early designs only for a Level pot. BTDR-1 is mono only. BTDR-2 adds stereo. BTDR-3, the most recent in the line, allows for adjustable delay. So, this is supposed to be the equivalent of short, medium, and long spring tanks in one package. On this basis, I got the idea that a panel controlled pot is intended, further suggested by Belton indicating a dual pot. Besides, I've already got a hole in the front panel. I suppose I can leave a dummy of some sort in there if I don't use it.

On the "level" control, I understand this to be the thing that controls the amount of reverb (for lack of a better expression). I can better understand your design here, which is to simply turn it all the way up (if that works out sonically) or find the sweet spot and leave it there. Then level control is managed by the A100K pot. In that regard, can I just skip the B10K pots, and just use the resistors as a fixed mixer? I suppose the problem there is that I need something in place for tweaking?

I'm not sure what all the questions are on depth and level ;-}

Martin: On the parallel network:
Do I remove R15 and R16, and simply send and return to each side of R14?

Or do I remove all three resistors, break the circuit continuity at that point, take the output from the Treble pot for send and simply return to C10, which is input to the recovery stage?

What are the implications if I keep the network in place?

My thinking on the parallel network (I knew what it was, just didn't know what it's called ;-} ) was that the BTDR-3 has no OFF switch. I figured if I turn it all the way down so there is little or no output, the network serves as a bypass of sorts. Are you saying this isn't needed? I have some concerns about how to pass the signal through when I don't want reverb. I don't understand/don't see the parallel path on the schematic drawn by Lou. All I can see is that the signal has to go through the brick.

Do I want to install some sort of switch to kill the reverb and direct the signal around it? Or am I just barking up a tree here?

Oh, and all this is really terrific. I'm learning a great deal. Thanks.

Phil

Hi Phil,

LOL, neither one of us like being separated, and it doesn't happen very often. She drove down to southern CA because her daughter had her second baby a few weeks ago, and she wanted to help with looking after the 5yo, cooking and cleaning, and jsut lending moral support. She was actually supposed to stay until Friday or Saturday, but she was having a hard time listening to the constant bickering between her daughter and hubby (kids - sheesh). When she told me she was leaving early, I told her I was sorry, and that I knew she had been looking forward to visiting her daughter and her THREE children (the hubby being the third...)

Anyway, when I was writing my explanation last night, it did occur to me that the "depth" control might be something more than just a level control, but the data sheet I found made no mention of this, so I am glad you bring this up. I have a feeling the manufacturer's reasoning for specifying a dual gang pot is probably based on using the device in a consumer stereo amp for music playback, in which case, it makes sense to not disturb the stereo "field" by having one decay time in one channel and another decay time in the other channel. And, if your intention was to have a WDW rig, this would probably make sense too. But, since you are using the device in a monaural guitar amp, I think using separate controls may present a unique opportunity to fatten the reverb signal by using different decays. Just a thought. But I can also see the desire to bring this feature out to the front panel, which could be done with either a dual gang pot, or a dual concentric shaft pot.

After thinking about it some more, I don't think the output-level-to-the-mixer trimpots are required. Again, if you choose to use two different decal times, then having separate control over the level of each output into the mixer makes sense. And this could actually be accomplished with one trimpot for balance. If you're interested in this approach, let me know and I will modify the schematic.

Correct, I did not include a "dry path" through the reverb circuit, as I thought your mix approach looked fine as is, so adding a dry path through the reverb circuit would be redundant. I also think that a reverb switch is unnecessary (unless you want to install a footswitch jack) because turning the A100K pot all the way down really should kill all of the reverb.

I think I covered all your questions. Let me know if I missed something. Oh, yes - that does indeed look like a 39pF cap

Cheers,
Lou

[JazzGuitarGimp]

Is this cap 39pF?

Yes. And 3.5mm, thankyouverymuch.

That's darn near 10pF per mm!

[JazzGuitarGimp]

Quick question: The Accu-Bell schematic does not explicitly show a bipolar +/- 5v supply. And the DIY guitar pedal schematics I've found feature a +5v supply only. I understand that the TL072 will work fine with a 5v differential (versus a 10v differential if powered by a bi-polar supply), but is the point here that there won't be enough voltage swing at the reverb output to fill the function we're building for the guitar amp?

There may possibly be some confusion about analog power vs. digital power. The brick is a digital device, and therefore needs a regulated +5V supply. There is no -5V requirement on the digital side. On the analog side, being able to get enough voltage swing out of the TL072 will require something more than a single ended 5V supply if we want to have enough signal level to interface to the amp's higher signal voltages without having to include a recovery amp for the reverb. While I think a single +15V supply will be enough, I would probably opt for a 15V bipolar supply (+/-15V) for a few reasons: it makes the audio path opamp design less complicated; it provides for much more headroom or larger output signal voltage swing; and, I just like running opamps on +/-15V My guess is, in a pedal setting, the opamp is run from the raw 9V battery voltage, single ended with a virtual ground at +1/2Vbatt.

[xtian]

Thanks for that, Lou. I'm going to order parts and BREADBOARD the thing, not solder (hey, Phil!).

BTW, the Fender HRD schematic has TL072s powering the reverb tank, and shows the +/- 15v supply for the op amps:

http://support.fender.com/schematics/gu ... ematic.pdf

[xtian]

I've been searching for a pre-built solution for power. Check this out:

http://www.ebay.com/itm/1-Power-Supply- ... 1640525750

What do you think? Doesn't appear to be SMPS, though not sure if I should be wary of a switch-mode PSU making too much noise inside an amp chassis? Anyway, seems like good specs and cheap.

[JazzGuitarGimp]

I've been searching for a pre-built solution for power. Check this out:

http://www.ebay.com/itm/1-Power-Supply- ... 1640525750

What do you think? Doesn't appear to be SMPS, though not sure if I should be wary of a switch-mode PSU making too much noise inside an amp chassis? Anyway, seems like good specs and cheap.

I'm thinking since it has a universal, wide mouth input, it's probably a switcher. Also the transformer looks mighty small for a 40W p/s, which implies it's running at some frequency well above 60Hz. So I'm pretty sure it's a switcher. Without ever having tried putting a switcher in a guitar amp, I can't really answer the noise question. My assumption is it's probably not ideal, depending on how gainy the amp is. But hey, it's only 18 bucks, so maybe worth taking a hit for the team and letting us know how you get on with it!

[Phil_S]

Is this cap 39pF?

Yes. And 3.5mm, thankyouverymuch.

I'm trying to avoid yet another parts order. This is scavenged off a board that came out of an old VCR, a very old VCR. Breadboard, eh? Gettin' fancy aren't we?

It looks like Lou responded just in time to my Depth pot question, so I guess I'll be placing an order because the only dual pot I seem to have is 100KA and this calls for 10KB.

I was wondering about losing the trim pots on the level out and looking at the Belton BTDR-2 for that. The mix network consists of a pair of 5K + 50KA pot + 39K. I make that to be something in the range of 41.5K to 89.5K. I guess the signal might need a bit of attenuation between the brick and the opamp? So, I might do something along those lines. I think I can make the trim pot accessible from the rear of the chassis so that I can dial it in without so much hassle. i will have to see what can be done. First, I need to do a layout.

[martin manning]

Lou, I believe the connection to TL072 pin 8 in your schematic should be +15V, not VGND.

[JazzGuitarGimp]

Lou, I believe the connection to TL072 pin 8 in your schematic should be +15V, not VGND.

Good catch, Martin! Thank you. I will correct and repost.

[JazzGuitarGimp]

Hi Guys,

I'm sorry it took so long to get the update posted - I went to open the schematic file this morning and it was corrupted, so I had to re-draft it. But there were a few more pressing issues to tend to first. I have replaced the original schematic on page 5. In addition to the correcting the error Martin spotted, I have removed the individual output level trim pots.

Thanks again, Martin, for spotting my error. If anyone else sees something suspicious, please let me know.

Kind regards,
Lou