Newcomer, Melbourne, Australia

[phatt]

The Transformer ACV is low (15-0-15 I think)
So yep all I needed was rectification and a pair of reg chips, As per image 3 :tu:

The halfrack case was a dud Alto Feedback buster gizmo I picked up cheap and so why make life hard
Once I gutted the inside I was left with everything I needed for a quick reverb build that also looked half decent.

I had about 4 different small signal OT's in my junk pile and as long as you go from low to high they all work to some extent.
As Jaycar had a part number with Z labeled on it I used that because it helps others who may want to try it out.
So just go find a small signal Tr with ~ 8/1k Z.
Obviously a 1/1 ratio is going to do nothing.
The Jaycar part number is MM2532 which is a 350mW output transformer, sell for around $5

The circuit *Without Tr* works like most other dead pan boring reverbs of this type mounted in many Amps But the moment you introduce the TR,,, A Whole new world of reverb awaits you.

The final result will obviously depend on the match between the OT and the drive transducer.
That kind of maths I'll have to leave to better minds to Decipher/ Ezplain xP

My dear old Laney KD linebacker has a very similar Reverb circuit and after building reverbs again it dawned on me that All I needed to do to dramatically improve the reverb was insert another line TR at the drive end ,, sure enough same result.


The SS Laney Amp also has a short tank so I've not had the chance to try this out on a long tank but One day if I'm lucky enough to stumble onto a long tank I'll bet it will come close to slapback reverb.
Longer springs does help the delay a little bit better,, getting that ding-gading-ga slapback effect is not so evident with short springs.
My Original MaxiVerb had the long tank and it *Definitely* had the slap effect when playing fast staccato notes.

These little Tr's are low quality and you loose some bottom end which is *Exactly what you want to have happen* as excess bass destroys the definition effect of reverberation which is the whole magic of reverb for guitar.
From my experiments the magic freq seems to be between ~ 1kHz and 3kHz.
The more I've studied Amp circuits the more I've come to see that often little attention is paid to the freq response of reverb circuits and yet it is critical to how well it works.

I recently fixed a SS Magnum Combo and as the board was upside down and the rev circuit was obvious,,all I did was kill the bass going to the driver and it made a world of difference.

Total System EQ is by far the most important aspect of guitar sound, even in the response of the reverb.

Re the heathkit circuit,,, Unlikely you will be happy. :duh
BTDT,, Well not that one exactly but I've breadboard tested a lot of Reverb circuits,,,,, now Did I mention I'm nuts?  :lmao:
The Maxiverb took me ten ish years to perfect,,, so call me mad methodical or even slow to catch on at times but I did get there in the end.
I think I learned more about amplifiers and guitar audio in that time than if I had gone off and just built a hundred fuzz boxes.
Phil. 

[Roly]

Lightglobe!

I think your observation about putting in a driving tranny can be explained by one of those easy to overlook details.  A valve output stage, particularly without NFB as reverb drivers are, acts much like a constant current source, meaning that the power delivered to the line driver coil tends to track its impedance.  This is more significant than driving a speaker because these tend to have a characteristic that rises steadily with frequency.



Solid state driving circuits are constant voltage sources, and you see in better designs a fairly large supply voltage and a largish resistor in series with the driving coil, and this again results in something close to current drive rather direct connection which would result in voltage drive which would over drive the bass and under drive the upper register.

[nodz]

@phatt/Phil,
I was reading the reverb information on the Elliot Sound Products (ESP) and from the information that he presents, I assume that the phabb-tverb circuit that you present is for low impedance coils only.
If a standard coil, like the Accutronics 4FB3A1C is used then discrete componentry is required as the op amps can't provide the necessary drive?
If this is the case, is the drive transformer not required either, as per the ESP page and the schematic of the Orange stand alone reverb unit below?
If you really wanted to, would if be possible to include a reverb transformer into the below circuit and rather than take the output off the emitter of the BFY51 transistor, include a transformer between the A+ voltage line and the collector of the BFY51.  I assume that this would need to be something like a 1:1 transformer as impedance of both transistor and tank is high?

[phatt]

Hi Nodz,
Ohh?
I have not seen that one before?

But a fair bet it will fair no better than others from that era.

Roly or Mr Fahey might be better qualified to answer this one. 

Up to you if you want to try things out but I've done my bit and it works and I'm not really qualified to comment on stuff I don't fully understand let alone explain. xP

All I know is my Reverbs *WORK as claimed* with the Tanks I have been able to accrue over time.
Heck I'm a pensioner mate and this is a hobby for me,, so I can't go out and test everything that sounds like a good idea. I just do not have the funds.

There are many reverb circuits around but I found that few actually worked when breadboarded.
What really scared me was finding the Accutronics site which has a page of suggested circuits for their products and I thought I had found the answer,, Surely they would have the ideal circuit for reverbs??

Well what a complete and utter load of crappus. @##$$%^&*&* Don't even bother with them.

As to Your post??
Your observation/explanation is a little confusing but this might help.

A Low impedance 8 Ohm transducer will measure 1 to 4 ish Ohms DC.
A High impedance 100 Ohm transducer will be more like 20~30 Ohms DC.

The DC reading will always be less than the Z Ohms.

There is a reference page somewhere showing how to decipher the number on Accutronics tanks but if you have a tank then from the above rule tiss not hard to work out if the drive is low or high.

There is no exactly right with these things as this is Spring Reverb and the bandwidth is shocking and you are not shooting for .001 distortion figures to impress hifi idiots.

At the end of the day you do have to suck it and see,,,I've given hours experimenting and just sharing my findings so I'd hazza guess that anything that reads above 40 Ohms DC should work with my setups.
If you have a very low Z drive transducer then my idea may not work.
Note; Rod Elliot mentions the *SOT* (select on test series resistor),, well that is Exactly what it means.
*You do have to tweak the driver with the Tank you have for best results*.

I did have a very low Z tank for a short time and had no luck driving it with my circuits.
Those circuits where likely for Valve units and may not be worth the hassle.
Unless I'm sadly mistaken, Rods Circuits are mainly Aimed at the medium to high Z units.

As most tanks I come across are medium to high Z that is what I've worked with so other ideas
I'll leave to those who wish to experiment.

The other hidden thing to bear in mind is the Tank design and quality itself, this can vary but Accutronics seem to be the best bet although I've heard good reports about *Mod* brand tanks being as good as the others.

Keep in mind the Tank design, Magnet quality, spring mountings, rubber dampers all come into the final result.

Heck I've got a nasty cheap one here that wobbles for a good 5~6 seconds as they forgot to insert the dampers inside the mount tubes which makes for some Wild reverberberberbebrations. lol.   
Completely useless for guitar.

Sounds to me like you just need to go breadboard some ideas and with the info you have already it won't take very long to work out what works and what is rot.
Phil.

[phatt]

Lightglobe!

I think your observation about putting in a driving tranny can be explained by one of those easy to overlook details.  A valve output stage, particularly without NFB as reverb drivers are, acts much like a constant current source, meaning that the power delivered to the line driver coil tends to track its impedance.  This is more significant than driving a speaker because these tend to have a characteristic that rises steadily with frequency.



Solid state driving circuits are constant voltage sources, and you see in better designs a fairly large supply voltage and a largish resistor in series with the driving coil, and this again results in something close to current drive rather direct connection which would result in voltage drive which would over drive the bass and under drive the upper register.

Yes Yes,,, I get it now. :dbtu: :dbtu:
Once again You have put into words what would take me a month of Sundays to grasp and then another year to explain to others. :lmao:
Thanks heaps,, I think I now have a much better grasp of this tricky subject.

I've actually grabbed that and filed under my Reverb folder,, cause I'll have to read it ten times for it to sink in xP
Phil.

[phatt]

Here is the mods I did for my (pick up and go play a gig) Laney Amp.

6 alterations and an OT.
What used to be,, Is the reverb on or off?
Is now,, Opps I better back off the reverb too 3 for normal playing <3)

I've noted the DC reading of both the OT and the transducer in the tank.
This is a nothing special short tank with no names as found in a lot of these low budget SS combo Amps.
Phil.

[phatt]

Try this for help in defining Tanks specs;

http://www.amplifiedparts.com/tech_corner/spring_reverb_tanks_explained_and_compared

So in the case of tank 4FB3A1C,
Then;

4 = long tank (2 spring rows)
F = 1,500 input Z
B = 2,200 output Z
3 = long decay time
A = both input and output grounded to case  (potential problem)
1 = no locking device
C = mounting side on

The DC reading will be around 200 Ohms on both ends.
Both ends grounded to tank can cause a ground loop so be warned.
Phil.

[phatt]

Sorry Nodz I did not reply fully, so now rush hour is over I've had more time to ponder an intelligent answer.

@phatt/Phil,
I was reading the reverb information on the Elliot Sound Products (ESP) and from the information that he presents, I assume that the phabb-tverb circuit that you present is for low impedance coils only.

No the opposite.

If a standard coil, like the Accutronics 4FB3A1C is used then discrete componentry is required as the op amps can't provide the necessary drive?

Yes they can with the aid of the small OT

If this is the case, is the drive transformer not required either, as per the ESP page

Either or seems to be the case.
The beauty of the Tverb OT is that it really simplifies everything and allows the whole thing to run on low voltage.

If you really wanted to, would if be possible to include a reverb transformer into the below circuit and rather than take the output off the emitter of the BFY51 transistor, include a transformer between the A+ voltage line and the collector of the BFY51.  I assume that this would need to be something like a 1:1 transformer as impedance of both transistor and tank is high?

Yes I did try all that but found the booster circuit to be the better option.
The booster circuit is common but few run the booster pairs from a much higher voltage.
My MaxiVerb unit runs the booster pair from +/- 35 VDC and that was the key.

The original article that ESP had did not have much in the way of better Drive setup and
If truth be known it was likely my constant complaints about lack of drive that caused those booster circuits which he added later.
All Credit to Rod as I've never purchased any of his kits but He helped me out a lot while I wasted a couple of breadboards designing many many Reverb ideas.

The Maxiverb is a little complex and requires extra voltages and I only know of 3 people who have built that unit but the Tverb idea is very close to the same result and a lot less work to get up and running so understandably I'm very impressed as even with limited electronic skills this can be made DIY.

I'd say if you have a 4FB3A1C tank then go with the Tverb :tu:
just watch the potential ground loop.
Cheers, Phil.

[nodz]

Thanks for your input Phil, very much appreciated.

I'm just trying to get my head around the use of the op amps and the small OT to provide the drive required  :cheesy:

I must admit that I am reticent to use the op amp type circuit.  This is purely my issue.  I have always struggled to get the op-amp type circuits working and have had much better success with discrete components.  This is not to criticise yours or Rod/ESP's work, I just don't have enough experience with them.  For instance many of the reverb recovery circuits I've seen on the net include an LM386 chip.  When I was in amateur radio, oscillation of these chips in audio circuits was the bane of my existence 

Many of the problems that I fear, could probably be overcome by purchasing the ESP pre-etched circuit boards and I may not have the issues that I am concerned about.

Another thing is the type of music that I listen to/play.  I play punk/hardcore (noise the older generation would call it :lmao: ) so I do not require huge amounts of reverb, just some to add depth to the guitar sound.  So I probably only require short tank/two spring/medium delay that is not driven particularly hard.  Having said that, designing/building for maximum amount of reverb possible means that I can turn it off if I want to

I don't have any tanks yet, that was just the one that the Orange circuit used, that's why I picked that one as as example.

While writing this response, I was searching around the net and found an old Gibson reverb schematic (see below), that uses that OT connected between the HT (17.5V compared with the +/-35 that the Tverb uses) and the collector of the reverb tank driver transistor, just as I suggested between the HT and the BFY51 in the Orange circuit.  The tank used is RV4C which is a 4BB2A1C equivalent in the Accutronics range.

4 = long tank
B = 150 input Z
B = 2,250 output Z
2 = medium decay time
A = both input and output grounded
1 = no locking device
C = mounting side on

Unfortunately, I can't find any info on the TF-1000R reverb transformer, but obviously got to match the o/p impedance of the BFY51 to 150 ohms.

These discussions on this thread have opened up a can of worms.  So much to try/experiment/do.  Might be another ten years of reinventing the wheel.   :duh 

Cheers Nodz

[Roly]

oscillation of these chips in audio circuits

The most common cause of this is supply bypassing, or lack of it.  You can get away with having an electro on each supply, +v & -ve, in the bench supply with long leads trailing across the bench, but only if you have a high frequency bypass from +ve to -ve (not ground), right at the op-amp.  Naturally it's better to have low frequency bypassing close to the amp, but high frequency bypassing is not optional.  Some op-amps like the LM833 have such high bandwidth that you need to think of dealing with them "VHF-style", but it's not a bad idea generally.

Trying to get gains higher than about x100 from a single op-amp, or across a dual or quad, tends to get problematic.  By the time you get up to needing gains of the order of x1000, say in a distortion meter, then spaced singles, tinplate shields, and other "RF-like" techniques help a lot.

Rod's reverb circuit is pretty conservative in this respect, so take care of your supply bypassing and you should be okay.

The reason that reverb drivers tend to have high supply voltages is mainly because they tend to act as constant current sources and need a fair bit of voltage headroom, particularly for "high-Z" springline drivers.

[phatt]

I have actually heard the GA-3RV but only on the net.

Double bum because the local pawn shop had one but it was the 110V import and he had no way of running it to hear it in the flesh,,Since then it has been sold. 

Yes the idea is valid but hardly worth the fuss when other stuff here will be easier to source parts for and perform better.

I note you mention the style of music you wish to play,, So i doubt you need a dripping wet surf sound reverb? Maybe you are over thinking the whole idea.
Phil.

[nodz]

@Phil/Phatt, yeah probably am over thinking it, the type of music means, that the original single pentode driver on the spring tank will do nicely...lol

[nodz]

Well now we probably are going over the top  :loco

[Roly]

Yep - that looks suitably OTT.

Just some observations;

- the power supply filtering points A-C not shown.  More importantly you can't feed three successive stage from the same HT point "D" because the first and third will be in phase; the reverb drive/recovery (and overdrive) stages may have a similar problem - stages that are/may be in phase must be decoupled from each other or you risk instability.

- unless you are expecting signal sources of several volts (e.g. CD players) I'd personally go for highest possible input impedance over front end attenuation ability ("Input Drive" control).

- the Overdrive "Output Level" control strikes me as being rather curious, more a "amount of tone" control - shouldn't it go between tone wiper to ground?

- the tremolo "Speed" and "Depth" controls are swapped over, and I'm a bit dubious that you are going to get sufficient depth with that method of coupling into the anode; the cathode is more common, and because of DC pumping the most common method these days is a neon in the LFO anode coupled to a LDR either across the signal path or in series with a cathode bypass cap.

- rather than have the output socket present a dead short to the OPT I'd be more inclined to hard wire a 47 ohm 5 watt resistor across the OPT secondary.

HTH

[nodz]

Hi Roly,
Thanks for the input.

Re: PSU

The PSU shown is for the preamp only, due to the fact that it has powered by a 16VAC @ 1.25A wall wart.
I was going to build modular, so the other units will each have their own PSU.  May be a little OTT for each stage to have its own PSU but as you said:

• Avoids the in phase/decoupling issues
• I can take my time to build
• I can spread the cost of the build over a longer period of time
• Makes debugging easier
• Makes refinements/alterations to circuits easier
• Makes repair easier, should anything go wrong

I was also going to simplify the PSU.  Originally when it was first designed, it used back-to-back transformers and it did have filtering points A, B, C and D.  Now that I am using a wall wart for the preamp only, I can dispense with the other filtering points.  I just haven't got around to modifying/simplifying the PSU circuit yet.

Also the fact that we are discussing AVA106, the 450V hextupler off-line for the quad of 6L6GC, negates the use of the PSU shown for the PI/PA.


Re: Input Impedance

I'll probably just go with a 1M resistor rather than the input drive control. As you mentioned previously only really duplicating the effects of the volume control on the guitar and as you also said, still probably not as good as the 5M/FET combination that you have been developing.
However, having said that, with the input control and the unit being built as stand alone, there is the option to be able to use it with other devices such as a cd player.  Originally I was considering duplicating the preamp for stereo channels for hi-fi listening. That's why the heaters of the 12AX7 are running 6.3V and elevated, as I am sure you are aware, common practice in the audiophile world.  However, I haven't decided whether I am going to do this yet.  In all likelihood I won't and therefore will change the input drive as suggested.


Re: Overdrive

I can't for the life of me find the circuit diagram that I used for this part of the circuit.  I know that it came from the EL34World/Hoffman amp site and was labelled as something like a Blues Amp.  I am trying to find the circuit to determine if I've transposed it incorrectly.  Ah, I've just found it Tweed Overdrive Special or Carolina Overdrive and yes you are quite right. I have got it wrong.


Re: Tremelo

Oops my mistake on the labelling.  When I moved the diagram around, I reattached the labels incorrectly.

The circuit was taken from the article:
http://www.preservationsound.com/wp-content/uploads/2012/07/Vibrato.pdf

I know that is says that it can be capacitively coupled to the first audio stage, but I inserted another stage prior to the effects loop and PI because it and the oscillator will be a stand alone unit.
I take your point about the possibility of obtaining sufficient depth and that the LFO/LDR is commonplace these days but I did find the uniqueness of this circuit appealing.  However, uniqueness doesn't necessarily means that it works...hahaha.


Re: OPT
You've mentioned this mod previously and I thought I had included it.  I must have missed it in the subsequent diagram revisions.

Updated the diagram and attached

Cheers,
Chris