help needed with 1 tube reverb

[ericlc]

Hi,

I am building an ODS clone and I can really use some help to resolve an issue with the reverb.

The idea was to add an 1-tube reverb (12ax7) to the ODS clone.

The amp is up and running but the guitar signal is not reverberated.
However, when I shake the reverb tank a little, a 'splash' comes out of the speaker as expected.

Below is a partial of the amp's schematic including the voltages and parts.

I have checked the wiring as well as components (tubes, reverb connectors, reverb transformer) a few times now but have not been able to resolve the problem.
So, ideas and feedback are most welcome.

Thanks in advance

Eric

[John]

I would try increasing R55 to 1 meg (or more, in increments). I'm thinking you're not getting enough signal to the reverb circuit. At least, trying that will give you something to do until someone who knows what they're talking about comes along.     If you have a resistor sub box, this is a great place to use it.

[tubeswell]

Try increasing R55 or decreasing R56

[2deaf]

The effects loop doesn't (or shouldn't) have enough signal to drive the reverb driver.

Get rid of R64 (56K).

Use a 12AT7.

If you fix these issues, the reverb should feedback because R55 (100K) is way too small.

[ericlc]

Thanks you all for the swift reply.

Now I have two options
1. Change mixer resistors (and reverb pot? :: seems to work in D'Lite Blue Monkey
2. Change tube

@2deaf
Regarding the 2nd. option

Regarding R55, what value do you recommend? 220K?

Or would a 6BM8/ECL82 be an even better alternative than using a 12AT7?
In that case you have a power pentode (7 watt) and 1/2 a 12AX7 for the recovery stage.

[2deaf]

One stage of a 12AT7 will drive that reverb fine.  It works for Fender and it works for me.  The general consensus of opinion seems to be that a 12AX7 will put out 1 watt and a 12AT7 will put out 2.5 watts under similar conditions.  That's why a 12AT7 would be a better choice in this application.

The classic Fender circuit uses a 3.3M for R55 but I've seen 2.2M a lot and even smaller ones.  Need to get the reverb drive going before you can try some trial and error on R55.   

[tubeswell]

Regarding R55, what value do you recommend? 220K?

You need to go real high like 1M or 3M3. the greater the voltage division on the dry channel, the wetter

[tubenit]

Every one tube reverb I have implemented has worked well for me.  For R55,  I have used a 100k to 220k range with NO problem at all and had plenty of reverb never turning the reverb pot higher than 3-4. And typically with dwell around 5-7.

Remember that you have a 220k resistor into the reverb pot wiper and a 2.2M reverb pot "working as a mixing resistor" around the 100k.

Fender uses a 100k pot and a 3.3M resistor.  So, I don't see much difference there, IMO.   

The advantage of the 100k into the LTPI is that less signal into the LTPI is attenuated which is a plus with an ODS build. 

Please do not take offense, but I am thinking it is still a wiring issue or perhaps something amiss with the reverb tank or the shielded wiring to the tank. Have you tried another tank?

With respect, Tubenit

[2deaf]

I built your driver circuit and it took 2Vp at the grid to get the Accutronics recommended 0.32Vp coming out of the driver transformer.  0.5Vp from your effects loop will drive the reverb plenty to get some reverb out of it, so I agree with tubenit about the wiring and tank issues.  Could be the driver transformer, also.

I didn't notice the 220K on top of the 2M pot when I commented about the 100K and feedback.

     

[ericlc]

@2deaf
You build the driver circuit and got it working. Wow.

@tubenit
No offense taken at all

I guess I am back at checking the wiring and checking components. I an way that is good news.

I do have a reverb tank in my Voc AC15 CC1 but the input impedance is 800 Ohm rather than 8 Ohm.
So I am not sure if that will work out

Indeed the transducer of the input of the reverb tank might be broken even though that would mean the MOD tank is DOA.
The DC resistance of the output measures 213 Ohm. And the input 1 Ohm.
From an accutronics chart this look OK for a 4AB3C1B tank.

I did check the reverb transformer.
At least The DC resistance of both coils are up to specification

Regarding the grouding/shielding I have a question.

* The RCA jacks to the reverb tank are not isolated.
* R63 (220K) is grounded to the chassis via a lug under the RCA jack for the output of the reverb tank
* The black wire of the (secondary) of the reverb transformer is grounded to chassis via a lug under the RCA jack for the input of the reverb tank
* There is continuity between the amp's chassis and the chassis / output ground of the reverb tank
* The DC resistance between the (isolated) input of the reverb tank and ground is 1,4 Ohm.

Does this look OK or is it required to run ground wires to the ground bus bar / triode ground to put the reverb to work?

Thanks in advance.
/Eric

[tubenit]

Indeed the transducer of the input of the reverb tank might be broken even though that would mean the MOD tank is DOA.

My son had that happen on a fairly new reverb tank, but was able to fix it.

I would also perhaps jumper across the passive FX loops and see if that changes anything.  Probably won't but wouldn't hurt to try it.

IF I am understanding the other issues like grounding that you are mentioning,  they sound OK to me.

And of course, you don't have the reverb tank plugged in backwards?  I have actually done that once when I was in a hurry.  Seems like I could still get the crash (?) but the sound was very low volume and perhaps distorted?  It was yrs ago so I can't remember for sure?

With respect, Tubenit

[ericlc]

It might indeed be an issue with the reverb tank

I have a MOD 4AB3C1B
From the accutronics / MOD chart the last digit indicates that
* input grounded
* output insulated

But my tank is wired the other way!
* (white, 1 Ohm DC resistance) input is insulated
* (red, 213 Ohm DC resistance) output is grounded.

And the black wire of the input looks like it never was soldered to the RCA connector side

/Eric

[ericlc]

 :
We have reverb!
It was the loose tranducer wire

Thanks you all very much for your feedback and suggestions

[tubenit]

Very cool!  You got it resolved!  Congrats!

With respect, Tubenit

[ericlc]

Thanks!
And respect to you guys

[Willabe]

Great!

Now how much verb do you have?



           Brad     

[tubeswell]

Great!

Now how much verb do you have?



           Brad     

 

[Willabe]

I meant is it more than you need?


                    Brad     

[ericlc]

The reverb already delivers what I need from a practical point of view.
But I would like the amp capable of deliverin a bit more reverb than what I have now.

Yesterday I have increased mixing resistor (R55) to 200K.
And I must do more fixes / cleaning up before I can tell you the end result.
The amp is operational again in a few days.
I will let you know.

/Eric

[ericlc]

Increasing R55 to 200K did the trick for me.

It is as explained by Tubenit and others
It won't 'Dick Dale' but it is good enough

In another build I might experiment with an 6BM8/ECL82 tube
But I am more than happy with what I have now.

/Eric

[tubenit]

Thank you for taking the time to review the 1-tube reverb!  I honestly appreciate hearing how others experience and view it.

with respect, Tubenit