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BJF Reverb

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Atomanth
(@atomanth)
Active Member
Blev medlem: 1 månad sedan
Inlägg: 8
 

@bjf

thanks for responding. The voltages at T8 are:

pin 1 - 5.86

pin 2 - 1.49

pin 3 - 0

After adding the jumper as described above, I have good signal on the recovery side all the way to output. When I trace the signal from the guitar input jack, I have signal all the way through to both reverb outputs. The signal seems really weak though. I am not sure how much the signal should be amplified, but I’m getting roughly the same signal voltage at the transformer output that I am putting in from the signal generator. This is with the dwell control at max and the mini pot at minimum, which gave me the most signal. On the scope, the signal looks very distorted at these settings. 

is it possible that I have a bad pcb? If there was that open connection on the recovery side, I’m wondering if there is something printed incorrectly in other spots as well.


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Björn
(@bjf)
Medlem Moderator
Blev medlem: 2 år sedan
Inlägg: 16
 

HI there,

 
Then T8 is working just fine. This transistor has a trimmer to set gain through effect-this is to make up for the loss at the passive mixing point at the gate of T8.
You should be able to increase levle by adjusting this trimmer?
 
At your service
BJ

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Atomanth
(@atomanth)
Active Member
Blev medlem: 1 månad sedan
Inlägg: 8
 

That trimmer seems to work to increase the make up gain after the reverb effect. My problem is that I have a very small signal driving the reverb before the transformer. If I input a signal from a generator at 100mv 400hz, at the transformer output I get 70mv at max drive settings. Do I possibly have a bad transformer? The signal is very strong at the primary, but weak on the secondary side. I get a resistance of 1.5ohms between the secondary and ground. I think that seems ok or an 8ohm transformer, but I’m not sure. 


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Björn
(@bjf)
Medlem Moderator
Blev medlem: 2 år sedan
Inlägg: 16
 

Hi There

 
 
Voltage at Primary should be a whole lot larger than voltage at secondary ( something like 1/75 outof primary voltage but the secundary can provide a whole lot more current which is needed to drive the 8 Ohm load.
In fact signal voltage at secondary should be very small - now can the transformer drive  an 8 Ohm tank?
 
This output is really only useable when driving an 8 Ohm load
 
So it would difficult to say anythingelse than above before you have tried if the transformer can drive an 8 Ohm input tank
 
The Capacitor output is a whole different story btw
 
At your service
BJ

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Atomanth
(@atomanth)
Active Member
Blev medlem: 1 månad sedan
Inlägg: 8
 

@bjf

thanks for the reply. I’m lost then. I seem to be meeting all the right conditions to have this thing working. I have signal on the input side all the way through the transformer out. I have lots of signal voltage at the transformer primary as you described. I have tried an 8ohm tank, a 10 ohm tank, an 800ohm tank at the capacitor output, and I’ve tried multiple sets of cables. I can’t seem to drive any tank enough to get signal back into the recovery side. If I plug a signal generator directly into the recovery side, I get signal all the way to the output, just as I would expect. Is there any suggestion that I may not be looking at? Grounding issue with the tank or cables? 

Sorry to keep bothering with this. Everything seems just fine and I’ve troubleshooted everything I can think of. 


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Björn
(@bjf)
Medlem Moderator
Blev medlem: 2 år sedan
Inlägg: 16
 

Hi there,

 
This is a fairly simple circuit so there are limits to what can go wrong. I will try to help as best I can- so hang on in there
 
There could be a number of things 
 
Ground needs to consistent through system since signal passes relative ground
 
I must ask also, because I have had one or two broken tanks that can you verify the tanks are OK and working?
 
One tank I had there was a short between the wires that went to input transformer to strings-this was caused by a little metal flange that was supposed to hold the wires in place.
 
Shorts are difficult to measure in such low impedance systems though 😉
 
However I’ll ask Albin for the version pcb you have and have a look at that, just because
 
However from what you tell me:
 
Recovery amp has considerable gain because the output from tanks are weak
 
Can you tell me the voltages at T6 and T7 respectively?
 
If you’d run a signal generator into recovery stage the output would be heavily distorted if all works as it should.
 
Anything learnt on this will apply to discrete transistor stages in general, so there is an additional benefit
 
At your service
BJ

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Atomanth
(@atomanth)
Active Member
Blev medlem: 1 månad sedan
Inlägg: 8
 

@bjf

here’s what I have at those transistors:

T6

E - 1.007v

B - 1.603v

C - 3.810v

 

T7

E - 3.2

B - 3.81

C - 5.47

The tanks are good. I’ve tested them with current amps and did so again before I replied to this. With a 25mv signal directly to the recovery input, I only get distortion with the trim pot all the way up. I can clean it up by turning it back 10% of the way. 

thanks again for spending time on this with me. 


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Björn
(@bjf)
Medlem Moderator
Blev medlem: 2 år sedan
Inlägg: 16
 

Hi There,

 
 
Thank you,
 
Let’s see how this can be figured out
 
In faultfinding it is always best not to assume just methodically test
 
OK so recovery stage is working just fine
 
With the types of tanks you have the transformer output is the way to go. The capacitor output is meant for driving tanks with 600Ohms to 2,5K IZ
 
So looking at transformer output that is powered by a push/pull amplifier TR1 sets the bias  for the push/pull output pair. For max swing start with TR1 fully off and then increase and you can watch the output swing at the primary while doing this and you’ll see.
 
You said earlier you have a weak signal driving the the reverb before the transformer so what kind of input do you have driving the Universial Reverb driver?
 
If the circuit is inserted at a point of low signal you could build a preamp similar to T8 circuit and power that from +A-in case you have a J113 lying about. This would at max gain amplify by a factor 9
 
Further the output from the tank should be about 5mV at so the recovery amplifier needs quite a bit of gain 
 
At your service
BJ

 


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Atomanth
(@atomanth)
Active Member
Blev medlem: 1 månad sedan
Inlägg: 8
 

@bjf

Well, I finally got frustrated and tore the whole thing apart again...and found a little sliver of wire bridging the connection from the power inlet ground and the pad that connects to A+.  I’m sure you figured it was something stupid like that, but humored me anyways. Now I have plenty of power to push the tank. It’s really great. I actually housed it in a box big enough to fit a short tank internally, then I put a switch in place to move the signal to the jacks and it works great. So now I can use it for a nice splashy ambience with the internal tank, or I can hook up a long tank for some serious drip. 

Thanks again for the great support. Sorry it was something so simple. 


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renascent27
(@renascent27)
New Member
Blev medlem: 1 månad sedan
Inlägg: 2
 

@atomanth

Happy to hear it's up and running. If you can still access the board would you mind posting the voltages at T1, T2, T6, T7 & T8? I'd like to compare them to my build once it's complete. And I'll post my voltages as well for future reference. Thanks.


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bakelitetele
(@bakelitetele)
New Member
Blev medlem: 3 månader sedan
Inlägg: 3
 

I'm very close to moving forward with this reverb but haven't been able to find any demo/comparison of this circuit. Is it similar to a stand alone reverb unit, an amp reverb circuit, or is it's own thing entirely? Thanks for any help describing what this would sound like. 


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