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BJF Reverb

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PeterB
(@peterb)
Active Member
Blev medlem: 10 månader sedan
Inlägg: 6
 
Postad av: @bjf

Hi Peter,

 

The transformer output is made to drive 8 Ohm tanks so yes it might be too low for 100 Ohm tanks.

Thank you for the response! I am driving a 10 ohm tank (sorry, “10ohm” looks a little like 100 ohm). Is there anything that I should look at in the driving amplifier to confirm it is working correctly? Thanks again!

PeterB


   
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Björn
(@bjf)
Medlem
Blev medlem: 4 år sedan
Inlägg: 29
 

Hi there Peter,

With Oscilliscope you can measure output at secondary of transformer under load and if you have a sine generator you'll see where max undistorted power is while adjusting the bias trimmer to the push-pull pair

 

At your service


   
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Björn
(@bjf)
Medlem
Blev medlem: 4 år sedan
Inlägg: 29
 
Postad av: @peterb
Postad av: @bjf

Hi Peter,

 

The transformer output is made to drive 8 Ohm tanks so yes it might be too low for 100 Ohm tanks.

Thank you for the response! I am driving a 10 ohm tank (sorry, “10ohm” looks a little like 100 ohm). Is there anything that I should look at in the driving amplifier to confirm it is working correctly? Thanks again!

PeterB

Hi there Pter

 

Ah yes my misstake then and as I said above measure output of transfomer under load and you will see...

 

Are you sure tank is working correctly at input? This will show up with oscilliscope

 

At your service

BJ


   
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PeterB
(@peterb)
Active Member
Blev medlem: 10 månader sedan
Inlägg: 6
 
Postad av: @bjf
Postad av: @peterb
Postad av: @bjf

Hi Peter,

 

The transformer output is made to drive 8 Ohm tanks so yes it might be too low for 100 Ohm tanks.

Thank you for the response! I am driving a 10 ohm tank (sorry, “10ohm” looks a little like 100 ohm). Is there anything that I should look at in the driving amplifier to confirm it is working correctly? Thanks again!

PeterB

Hi there Pter

 

Ah yes my misstake then and as I said above measure output of transfomer under load and you will see...

 

Are you sure tank is working correctly at input? This will show up with oscilliscope

 

At your service

BJ

Thank you again for the help. With the tank connected, if I look at the transformer primary (i.e. the "tone" pad, which I believe is at C9), I see strong signal. If I look at the 8 ohm out though, I see very weak signal - with 1Vpp at the primary, I see less than 0.0025Vpp on the secondary. If I disconnect the tank, I see 2.5Vpp on the primary and 0.5Vpp on the secondary - so I know the transformer is not broken.

I measure a small (1.5 Ohm) resistance at the input of the tank, so the input winding seems to be intact.

Everything else is working - I tested the C out with another tank, and that works fine. The recovery also works fine.

One other question though - if I wanted to modify the sweep of the mix knob to be able to get more "wet," would you recommend changing R26 or R27? I know there may be a volume drop, but that's ok with me.

Thank you!

Peter 


   
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Björn
(@bjf)
Medlem
Blev medlem: 4 år sedan
Inlägg: 29
 
Postad av: @peterb
Postad av: @bjf
Postad av: @peterb
Postad av: @bjf

Hi Peter,

 

The transformer output is made to drive 8 Ohm tanks so yes it might be too low for 100 Ohm tanks.

Thank you for the response! I am driving a 10 ohm tank (sorry, “10ohm” looks a little like 100 ohm). Is there anything that I should look at in the driving amplifier to confirm it is working correctly? Thanks again!

PeterB

Hi there Pter

 

Ah yes my misstake then and as I said above measure output of transfomer under load and you will see...

 

Are you sure tank is working correctly at input? This will show up with oscilliscope

 

At your service

BJ

Thank you again for the help. With the tank connected, if I look at the transformer primary (i.e. the "tone" pad, which I believe is at C9), I see strong signal. If I look at the 8 ohm out though, I see very weak signal - with 1Vpp at the primary, I see less than 0.0025Vpp on the secondary. If I disconnect the tank, I see 2.5Vpp on the primary and 0.5Vpp on the secondary - so I know the transformer is not broken.

I measure a small (1.5 Ohm) resistance at the input of the tank, so the input winding seems to be intact.

Everything else is working - I tested the C out with another tank, and that works fine. The recovery also works fine.

One other question though - if I wanted to modify the sweep of the mix knob to be able to get more "wet," would you recommend changing R26 or R27? I know there may be a volume drop, but that's ok with me.

Thank you!

Peter 

Hi There Peter 

 

It does seem the input transformer in reverbtank could be shorted. I have had one tank do that and it was caused by a metal flange suppoesd to hold the wires to inputtransformer inside tank shorting the two leads -this was several decades ago so I don't recall the measurements but short to ground and no short to ground where not far off 😉

Yes there is an easy way to make a panpot out of reverb level and I sent Albin instructions and schematic on that that Albin might be able to post

 

At your service

BJ


   
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PeterB
(@peterb)
Active Member
Blev medlem: 10 månader sedan
Inlägg: 6
 

@bjf

Thank you again. I think you must be right about the shorted input. I'll try to get another 8 or 10 ohm tank to test.

In the meantime, it would be great to have more information about changing the mix knob to a panpot style!

Peter


   
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PeterB
(@peterb)
Active Member
Blev medlem: 10 månader sedan
Inlägg: 6
 
Postad av: @bjf

Yes there is an easy way to make a panpot out of reverb level and I sent Albin instructions and schematic on that that Albin might be able to post

Since Albin hasn't responded, I'll try to figure this out myself. Looking at the board and the schematic, I think I could remove R26, and connect a pot between "mix a" which is at C13, and "(pan1)" which is at C14, and connect the wiper to "mix b" which should be at R27. But maybe I should remove R27 and replace it with a jumper? Any guidance would be appreciated! Thanks.


   
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cmchaney06
(@cmchaney06)
New Member
Blev medlem: 12 månader sedan
Inlägg: 2
 

@bjf

Hi there. Is R35 the LED resistor?

thanks,

chris


   
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Mbshaikh
(@mbshaikh)
New Member
Blev medlem: 8 månader sedan
Inlägg: 2
 

Hi Bjorn,

 

I bought and just finished assembling the Universal Reverb Kit. Super easy to follow instructions and everything was great! 

I am I think running into a similar problem as some of the others on here. I have a Mod Electronics 8ohm tank. When I plug into the 8ohm jack, I get virtually no reverb regardless of what the knobs are set at. However, when I use the regular return, the reverb sounds quite fine to me. 

I have tried changing the trimmers and have not yet been able to get usable output from the 8ohm jack. I have noticed that unless TR1 is turned nearly all the way counter clockwise, I get lots of distortion on both the 8ohm and regular jack. Note that I can tell that the reverb tank is actually working using either jack, since if I tap it I will get a loud crash sound (though much quieter using the 8ohm jack). 

Unfortunately I do not have an oscilloscope (I am fairly new at this), but have a multimeter and so can test any voltages. I also do not currently have another reverb tank, though I guess I could buy one. If I can’t figure this out I will probably buy an 800 ohm to see what happens, though I’m a little nervous I won’t get much output from it.

Any suggestions? 

This post was modified 8 månader sedan by Mbshaikh

   
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rasmuspersson
(@rasmuspersson)
New Member
Blev medlem: 6 månader sedan
Inlägg: 1
 

Hello @bjf!

First of all, thank you for this great kit!

I'm also experiencing the same problem as @mbshaikh describes above

and I haven't managed to find a suggested solution reading through this thread that fixed the problem.

Transistor output works fine and the reverb recovery input stage also performs as expected.

The transformer output is however silent and I've tested my 8ohm reverb tank in a guitar amp where it works fine

no shorts in the input transformer or between input leads and the chassi.

I'm able to trace a 1khz sine signal all the way to the inputs of T4 and T5 but then it disappears.

This is maybe expected, I have to little knowledge to make a conclusion my self here.

Sockets have been cleaned and seems to make good contact with the transistor legs when probed.

Do you have any suggestions on where I should troubleshoot next?

 

Have a great day!

// Rasmus 


   
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zarobodo
(@zarobodo)
New Member
Blev medlem: 6 månader sedan
Inlägg: 1
 

I also want to thank you for your help! You have a constructive forum. I almost immediately found the answer to my question.


   
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Björn
(@bjf)
Medlem
Blev medlem: 4 år sedan
Inlägg: 29
 
Postad av: @rasmuspersson

Hello @bjf!

First of all, thank you for this great kit!

I'm also experiencing the same problem as @mbshaikh describes above

and I haven't managed to find a suggested solution reading through this thread that fixed the problem.

Transistor output works fine and the reverb recovery input stage also performs as expected.

The transformer output is however silent and I've tested my 8ohm reverb tank in a guitar amp where it works fine

no shorts in the input transformer or between input leads and the chassi.

I'm able to trace a 1khz sine signal all the way to the inputs of T4 and T5 but then it disappears.

This is maybe expected, I have to little knowledge to make a conclusion my self here.

Sockets have been cleaned and seems to make good contact with the transistor legs when probed.

Do you have any suggestions on where I should troubleshoot next?

 

Have a great day!

// Rasmus 

Hi there Rasmus,

 

The transformer output is a push-pull power amp and T4,T5 are powertransistors driving the transformer. Trimmer TR1 sets the resting current for this output pair. If TR1 is set fully off the transistors are turned off. Tirning up TR1 provides a voltage to the input of the transistors and resting current through the transistors can be set. When powering up leave TR! fully off and then bring up TR1 until signal at the collectors is visiable with scope or reverb sound heard at recovery amplifier output. If resting current is set at the beginning of current draw voltage swing will be the largest and push-pull stage then works in class B-bias range allows setting hotter class but that means less power.So I suggest first trying to adjust TR1 to get the T4 and T5 to conduct and if you have a scope you should see the sine at the collectors.

It is possible that the transistors do not work and so you may want to test those in a transistor tester.

So if you don't have a transistor tester you can use other DVM funcions

Since these transistors are NPN they are eqvivalent to two back to back series diodes you can measure with diode tester and collector to base should read like a diode with anode to collector side and likewise emitter to base should read like a diode with anode to emitter.

You can also use highest resistans range to measure possible leakage. A good transistor should read open circuit but a damaged with leakage can measure a resistans. If there is a measured resistans the transistor is not reliable and can suddenly duck at applied voltage.

Hopefully these suggestions may be helpful to you

 

Have fun

BJ


   
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