Accutronics Reverb 4BB3C1D .. Valve Driven stage without transformer

[GazCal67]

I am in the process of drawing up a schematic for a re-build of my old DSL401.
I have redone everything I intend to apart from the reverb.
The original circuit for the reverb was solid state, but as I only intend to re-build the clean channel with a few mods, I have room to add a valve driven reverb, using a 12DW7, with the 12AU7 section as the driver, 12AX7 as recovery.
No reverb transformer.

The reverb tank for that is as follows:
4BB3C1D.
Input Impedance 150Ω
Output Impedance 2,250Ω
AC Drive 6.5mA
DC Res 26Ω
Inductance 23 mH

I am happy with the recovery side of the reverb as the 2,250Ω output is pretty standard and a fender style recovery would do.
But I am struggling with the driver and 150Ω input impedance.
I have been doing a fair bit of research and trying to learn as much as I can but my theory is very limited when it comes to valves and at my age new tricks don't come easy.

On this site I have found a thread from 2016
https://el34world.com/Forum/index.php?topic=20975.msg222566#google_vignette

within that thread there is a solution provided by 2deaf, albeit for 4FBxxxx type tanks with 1475Ω input impedance.
What adjustments would be required to adapt to 150Ω input impedance ?

Attached images of the schematic provided by 2deaf

Thank in advance for any help.

[pullshocks]

within that thread there is a solution provided by 2deaf, albeit for 4FBxxxx type tanks with 1475Ω input impedance.
What adjustments would be required to adapt to 150Ω input impedance ?

2 deaf is no longer active on this forum.  Try posting your question on Amp Garage or TDPRI.


He helped me with a circuit that works well with an EB tank 600 ohms input impedence.  Seee "B" on the attached document.   Both the driver and recovery are different than the version you posted above.  You might want to read this thread covering how this circuit evolved.  https://el34world.com/Forum/index.php?topic=31634.msg348366#msg348366


 I dont know if it will work with a BB tank.   In all my research I never saw a tube driven circuit that used that low input imepedence.  My guess is you are better off buying a new tank or sticking to the SS driver.

[HotBluePlates]

... re-build of my old DSL401.
... the reverb.
The original circuit for the reverb was solid state ...

The reverb tank for that is as follows:
4BB3C1D.

... there is a solution provided by 2deaf, albeit for 4FBxxxx type tanks with 1475Ω input impedance.
What adjustments would be required to adapt to 150Ω input impedance ? ...

The required adjustment would be "use a solid-state driver" as Marshall did.

You can certainly do a no-transformer reverb circuit (I've got one in my 1962 Ampeg Reverberocket), but you'll need to use the 4FBxxxx tank as 2Deaf demonstrated.

The $30 you spend on the 4FBxxxx, 8FBxxxx or 9FBxxxxx reverb tank will be the smartest purchase you made for your amp.

[pullshocks]

I started out with the 2Deaf reverb driver in the OP of this thread, and an FB tank.  I found the circuit actually worked better with an EB tank.  It could be that my FB tank was in poor condition, I never tried it with a fresh FB tank.


2Deaf updated the driver plate resistor to 15K, cathode resistor to 750R, and the resistor going into the tank from 2.7K to 1.5K.  I can say this works well with the EB tank.



HBP, have you ever tried an EB tank in your Ampeg?  No reason you would have, but I'd be curious.

[HotBluePlates]

HBP, have you ever tried an EB tank in your Ampeg?  No reason you would have, but I'd be curious.

No, that particular amp is a literal museum-piece, and I'm not inclined to touch anything on it.

[GazCal67]

Hi, Thank you for the replies and info.

The required adjustment would be "use a solid-state driver" as Marshall did.     


The $30 you spend on the 4FBxxxx, 8FBxxxx or 9FBxxxxx reverb tank will be the smartest purchase you made for your amp.

Thanks HotBluesPlates,   I'm thinking you are right, especially as I'm not sure if the 4BB tank I have is still working 100%, to go with a known tank that works with a transformer less driver is the safer bet.
New 4EB*** .. AUS$70

I started out with the 2Deaf reverb driver in the OP of this thread, and an FB tank.  I found the circuit actually worked better with an EB tank

Thanks Pullshocks, it was your original post from 2016 that came up in my search and which I found very helpful, and lead to this post.
Good to know that it works well, and thanks for the link to the update on the progression to an 4EB tank 600Ω.

I'm thinking 4FB tank seems the best option. Theory out lined on the ValveWizzard site suggests the higher impedance the better .
https://www.valvewizard.co.uk/reverbdriver.html

The Ampeg GU-12 driver uses a 6U10 medium triode stage that has the same plate resistance as the 12AU7, 7700Ω, and a little less plate current of 9.6mA. So the 12AU7 should show similar performance     (Ref: Robert Megantz)

My project has a way to go, the guts of the DSL haven't even been removed yet and I'm still just tweaking the schematic
and wondering how I'm going to design the layout within the chassis while getting around the present placement of valve holes.. without adding more.. , turret board & tag strip will be the go.
I might get to posting on that as it progresses.


Thanks again.

(Edited)

[pullshocks]

As I recall, the Ampeg GU12 sounds good, but it doesnt have reverb.  It has "Dimension."


One other thing about EB vs FB--FB has fewer choices as to size, # of springs, and decay time.


Keep us posted

[GazCal67]

Thanks Pullshocks.

It does have "Dimension" !   If I go with that circuit and tank I might just add it to the panel if and when I get that far 

Still undecided on 4EB or 4FB.  The Megantz little green book does state the GU-12 set up results in a "Less-than-perfect coupling of drive amp to the reverb tank"..   
The Drive circuit for the 4EB you provided might yet be the better of the two, So I'll keep researching & Learning...  A long way to go.

[pullshocks]

You've got my curiosity up.  If I can find a 0.47uF poly cap in my drawer, I'll change my breadboard reverb driver to the GU12 reverb driver circuit.  Return circuit is already biased like the GU12

[GazCal67]

Just looking at a GU-12 schematic, the one I have shows the tank as "Echo Unit 4C" others show as "C unit".
Therefore I thinking the tank of the GU-12 might be 200Ω input impedance, not 1475Ω, or 4FB as I have read elsewhere.
Not as yet found any other info to back that up.

[pullshocks]

From what I have heard, "C" is an old code or designation that does not line up with more current reverb tank codes.  And that "C" is equivalent to FB

[Jonas]

I realize tone is subjective; what brand of replacement reverb tank is recommended for high quality, minimal mechanical noise?

(I need to get a FB or EB tank ordered)

[GazCal67]

From what I have heard, "C" is an old code or designation that does not line up with more current reverb tank codes.  And that "C" is equivalent to FB

Thanks for the update

[Merlin]

From what I have heard, "C" is an old code or designation that does not line up with more current reverb tank codes.  And that "C" is equivalent to FB