Adding a SS Reverb

[TIMBO]

Hi guys, Dug out an old project that is not going to beat me..............



Top schematic is a working circuit.
Second schematic is a working circuit.
Bottom schematic is NOT working.
By elimination I have got it working to a point.
With the reverb "out" connect to the master volume pot as in bottom schematic, there is plenty of bong when touching the springs.
BUT with the reverb "in" connected as per the schem it passes some signal but just barely.

I have tried many options with all having a similar effect.

Could this be a PHASE CANCELING problem as it gives a loud squeal when powering down.

[shooter]

Could this be a PHASE CANCELING problem

If I counted correct, the output of the Verb circuit is outta phase with the input to the verb.

[PRR]

What does "NOT working" mean?

No reverb?
No dry signal?
Lays on the couch collecting unemployment dole?

> Could this be a PHASE CANCELING

Reverb is random phase. Won't cancel.

[TIMBO]

Hey PRR, What I am meaning by "not working"is that when the reverb"IN" is connected to the circuit as in the bottom schematic it kills the signal to the point that the master and volume are maxed before it is audible and there is both wet and dry signals.
BUT when the reverb"IN" is disconnected the amp works fine and touching the springs the noise is heard.
So this is why I can't understand where the problem is.
 

[PRR]

Then something is not connected as shown.

In/Out reversed? 47r instead of 47K? Can't tell from here.

[TIMBO]

Hey PRR, Tried more combinations but with no joy.

I have added two 1k mixing resistors before the volume pot, this has fixed the problem of the reverb circuit cancelling the signal.

I'm just left with the power down squeal.

[PRR]

> two 1k mixing resistors

Not mixing, splitting.

If that is a "fix", then the input to the reverb driver is not what you drew. The plan shows roughly 10K looking into the driver input. That a 1K resistor makes a big difference means the input is acting like much less than 1K. Can't see the plan versus reality difference from here.

[TIMBO]

OK..........
PRR...........
PLEASE DON"T YELL AT ME
My thoughts were, there was something not right.
I did the original circuit board some time ago and shelved the project as I was unable to fix the squealing.
Another SS reverb circuit presented itself and it was also a no go.
Rather than try to salvage the board and components I got a new lot.
On close look at the layout of the old board I noticed that the 2N2222a were facing the wrong way for a NPN.(emitter at the top of the board for B+ connection)
So I had a close look at the MPF102 as they also was facing the wrong way. (source at the top of the board for B+ connection)
The MPF102 were totally connected the wrong way but were still working.

With a new board rewired, major success.
A little toooooooooo much success, boing to burn.
 

With the new wiring I'm getting a little fizz and buzz, this seems to be mainly coming from the tank pickups near the transformers.

[shooter]

but were still working.

I'd have to go re-read books, but I think a common collector is kinda sorta like a cathode follower for tubes.

[2deaf]

So I had a close look at the MPF102 as they also was facing the wrong way. (source at the top of the board for B+ connection)
The MPF102 were totally connected the wrong way but were still working.

Drain and source are interchangeable.

[PRR]

> wrong way for a NPN.(emitter at the top of the board for B+ connection)

This is why we take DC voltage checks. The wrong-connection would have been obvious. (If not from factory notes, or self-knowledge, by showing to others.)

Though in this case it might be hard to tell.

A BJT transistor "can" work backward. Just really bad. The "collector" won't go over 7V above the "emitter". And the current gain, typically 100, is more like 1.

The Darlington connection multiplies current gain. 1*1 is 1.

So the input impedance is roughly that 100r emitter resistor, times 1, or 100r.

This is too low for the tone stage to drive well. When you padded-out with 1K, now the tone stage can make some signal.

Note though that turning-down the Dwell should also have isolated your near-short reverb driver from the dry path. Was that tried?

> MPF102 were totally connected the wrong way but were still working

On a JFET, Source and Drain are (as 2deaf says) interchangeable for all practical purpose, until you get to specific offset FETs working above 10MHz.

[TIMBO]

Thanks guys,

Note though that turning-down the Dwell should also have isolated your near-short reverb driver from the dry path. Was that tried?
Not sure what you are asking here?

A couple of pics

[TIMBO]

Thanks mate.
Just a final tweak, I replaced the level pot with a voltage divider as I was finding that I had it on about "2" on the dial with heaps of reverb.
Anything above this I was also getting a small amount of fizz and buzz.
This seems to keep it at a good level and the "dwell" works much better for delivering the right amount of reverb.
I also like the arrangement of connecting the reverb output to the second half of the PI, it fattens the overall sound when the dwell is on "1-2" without any reverb.

Thanks to all for your 

[TIMBO]

Hi guys just another Question.
2 deaf has the tank as a 8EXXXXX which I have used a 8EB2C1b having an input DC resistance of 800ohms.
I tried a 8DB2C1D that has a DC resistance of 310ohms, I did not hear much difference in the amount of reverb.
Do the transistors care about the amount of the tank load ???

[2deaf]

The tank load is a capacitor (1.0uF) in series with a coil (127.3mH for 8E or 49.3mH for 8D).  At open string frequencies, the capacitor dominates the total impedance equation and the 8E presents about the same load as an 8D tank.  Since the load is about the same, the current through the tanks is about the same and the resulting drive is about the same.

At frequencies above 1KHz, the coil begins to dominate the equation and the 8D tank load becomes considerably less than the 8E tank load.  This means that an 8D tank load will have more drive at higher frequencies than an 8E tank load.

Some people feel that reverb sounds better if the lower and higher drive frequencies are attenuated.  The 8E tank and the 1.0uF capacitor are sort of tuned to attenuate these drive frequencies.   

[TIMBO]

Thanks mate, the circuit is awesome with of plenty reverb.
I ended up using the 8E tank as I felt it work well, I also was having some issues of the transformers causing some buzz/hum
This tank is a side mount, so it fitted nicely with very little buzz/hum


Thanks again for your help

[smackoj]

I have a couple questions Timbo; Are you using a separate PT for the SS part and the tube parts?  What, if any, is the main purpose for mating a SS preamp with a tube output section? I know that Leo Fender used this hybrid layout for his Music Man line of amps and Peavey joined the chorus with their own line of hybrid amps. Is there some secret mojo involved or did Leo and his copy cats simply want to build  a good sounding amp for less cost? Last question; the schemo you used for the SS reverb shows three MPF102s back to back after the reverb pan and feeding the  PI. I know these jfets are powerful little devices so it seems maybe overkill to me to use 3?  Was this necessary to get the proper impedence for the PI?

[TIMBO]

Hey mate, This is the project here. http://www.guitargear.net.au/discussion/index.php?topic=45138.0
It mainly started because a couple of amp builders way back when used the transistor preamp circuit.
I have one of these amps and found it had a very valve/tubular sound.
I really don't understand the whole workings of the transistors (well actually them and valves )
The PI and PA are a circuit that has been developed by a local guy that uses transformers that can be bought from the local electronics store. (low voltage)
I have to thank 2Deaf for the reverb circuit as I had tried another without success.
http://el34world.com/Forum/index.php?topic=19250.0
The idea of the "PROJECT AMP" was a low cost good sounding amp that you can parts at any electronics' store.