Wind a Fender 6G15 reverb transformer...

[dougsta]

I've built a point to point 6G15 '63 reverb in my "dev" chassis but didn't have a 6K6 tube, to get it up and running I used a Soviet 6P6S (6V6 is close) and a small 6V6 SE OT from and old '50s radio. All working but I find the 6P6S/SE OT can overdrive the spring unit when cranking the dwell knob (does sound good on guitar tho) but I'm sure there is a risk of blowing the winds on the transducer.

So... why not wind a low watt (2W should be enough) custom OT for matching the various old tubes I have in my stash I have no use for:
NOS 6005 made in USA = mil spec 6AQ5 but close to 6V6
NOS Mullard EL91
Mullard EL42
Mullard EF80 (might be good for 1W)
Mullard ECL80 (don't bother with the shared cathode triode section)
Mullard PCL83 (12.6v heater - also has a low µ triode)
Mullard ECL82 (bonus - includes 70 µ triode)

I have some iron to used from old SE OTs (radio pulls) and some small modern cores from 100v line audio transformers.

Will need to match to 8Ω for the tank input

See photo.

Doug

 

[Tubetec]

I use a variable attenuator in the form of a wire wound pot to adjust the final drive level to the tank , now you can easily adjust your output tube drive and how much of that hits the spring . I never had any issues with smaller single ended amps for reverb drive , one time I did connect a tank directly across the speaker output of a Hiwatt 100 , it didnt take long before the transducer smoked .

 

[dougsta]

In addition to the 8Ω / 2250Ω tank (4AB3C1B) I tested the unit with I also have an Accutronics USA tank form the '80s (4BB3A1B) with 150Ω input Z.
Could I just add a secondary tap option to the planned OT so both 8Ω and 150Ω tanks could be used???

Attached is the wind numbers for a simple SE OT with 8 and 150Ω secondaries

Doug

 

[dougie.tamson]

I've knocked up a candidate OT today but went a bit further than the initial plan, I wound the primary x2 to give a CT so I can try it out push-pull on some small signal pentodes for a couple of watts and might make it hum less than a SE OT.

Kept the same gap and butt stacked lams.

I reused an old Grundig TK8 reel 2 reel OT core and bobbin.
I checked it on a low cost component tester, 24H and 723Ω for push pull use.

Also found some more candidate tubes, EL95, EF91 (push pull pair) and a pair of EF93's. For push pull I could shrink the OT size too.

 

[dougie.tamson]

Tested the custom wound OT with the existing 6V6'ski' tube and 8Ω tap, I wired the 2 equal primaries in parallel, sounds plenty drippy

Now the bonus part, I tried a 150Ω input tank found in solid state reverbs on the 150Ω tap and was plenty drippy too

I could even drive 2 tanks from the 2 taps, need to add a 2nd recovery triode (here's looking at you ECL82 for the extra triode) and mix in 2 different tanks, I have a 3 spring medium decay tank to try on the 150Ω tap and the 2 spring long decay on the 8Ω tap for spring reverb heaven...

I'll check the original 6V6 OT on the scope and compare with the custom OT next...

Doug

 

[Tubetec]

I've found a single triode connected EL84 into 8 ohms was plenty to drive any impedence of tank transducer , if fact if you want to drive the output stage into distortion you will still need to to attenuate a bit .
You only need a fraction of a watt to drive the tank ,anything above that and the drive transducer will saturate ,
the splattering driping sounds is the magnets making contact with the pole pieces , its fine if you want the surf guitar sound but it a bit unsubtle and inappropriate on other sources . Attenuating the drive gets you a much smoother sound but of course you have to try harder on the recovery side to keep noise out .

A push pull drive stage will have less PSU hum for a given amount of smoothing , but usually hum from the recovery side of the tank will dominate .

If you have a power transformer within a few feet of a reverb tank you can count on more hum at the output .
Putting two tanks together in a clam shell fashion is a good way to get better magnetic screening , but orienatation of the tank is important for them to work properly , if its not mounted in the correct plane the little magnetic slugs tend to touch the pole pieces and it sounds fizzly . Position of the mains transformer and its core orientatation in relation to the pickup transducer will also have a huge effect on the amount of induced noise you get, your typical guitar amp head with reverb will have appreciable amounts of hum due to magnetic induction . To get better performance you probably would need to either locate the tanks remotely from the amp chassis or alternatively locate the mains transformer at least a few feet away in its own metal box .

Ive generally found higher Z recovery from the tank to be best and as short a cable as possible . A 10k output transducer, an active DI with several Mohms at the input and short patch cable will do the job nicely .
Very much like a guitar pickup the transducer doesnt sound very good when the output is loaded down , it becomes gritty and the top end and looses clarity .
500-600 ohm output tanks do work well enough wired balanced and floating from ground into normal mic preamps with around 3k but

At the end of the day if your returning the reverb to a DAW why not decicate a small USB audio interface with a pair of mic/line/instrument inputs close to the tanks , a headphone or monitor out could be used to drive the tanks depending on impedence .

I use a 220 ohm 3w wirewound pot as an L attenuator across the tank drive transducer(150 ohm impedence is fine) , its good for direct connection to the speaker output any tube amp from 1-100w , of course you still need a speaker or dummy load connected especially for higher powered amps .

A lightbulb based limiter is an easy way to get a gated reverb effect from a spring . The Grampian Reverb is a good example of it but its easy to do with a with any small tube guitar amp .

 

[dougie.tamson]

I've found a single triode connected EL84 into 8 ohms was plenty to drive any impedence of tank transducer , if fact if you want to drive the output stage into distortion you will still need to to attenuate a bit .
You only need a fraction of a watt to drive the tank ,anything above that and the drive transducer will saturate ,
the splattering driping sounds is the magnets making contact with the pole pieces , its fine if you want the surf guitar sound but it a bit unsubtle and inappropriate on other sources . Attenuating the drive gets you a much smoother sound but of course you have to try harder on the recovery side to keep noise out .

A push pull drive stage will have less PSU hum for a given amount of smoothing , but usually hum from the recovery side of the tank will dominate .

If you have a power transformer within a few feet of a reverb tank you can count on more hum at the output .
Putting two tanks together in a clam shell fashion is a good way to get better magnetic screening , but orienatation of the tank is important for them to work properly , if its not mounted in the correct plane the little magnetic slugs tend to touch the pole pieces and it sounds fizzly . Position of the mains transformer and its core orientatation in relation to the pickup transducer will also have a huge effect on the amount of induced noise you get, your typical guitar amp head with reverb will have appreciable amounts of hum due to magnetic induction . To get better performance you probably would need to either locate the tanks remotely from the amp chassis or alternatively locate the mains transformer at least a few feet away in its own metal box .

Ive generally found higher Z recovery from the tank to be best and as short a cable as possible . A 10k output transducer, an active DI with several Mohms at the input and short patch cable will do the job nicely .
Very much like a guitar pickup the transducer doesnt sound very good when the output is loaded down , it becomes gritty and the top end and looses clarity .
500-600 ohm output tanks do work well enough wired balanced and floating from ground into normal mic preamps with around 3k but

At the end of the day if your returning the reverb to a DAW why not decicate a small USB audio interface with a pair of mic/line/instrument inputs close to the tanks , a headphone or monitor out could be used to drive the tanks depending on impedence .

I use a 220 ohm 3w wirewound pot as an L attenuator across the tank drive transducer(150 ohm impedence is fine) , its good for direct connection to the speaker output any tube amp from 1-100w , of course you still need a speaker or dummy load connected especially for higher powered amps .

A lightbulb based limiter is an easy way to get a gated reverb effect from a spring . The Grampian Reverb is a good example of it but its easy to do with a with any small tube guitar amp .

Thanks for the great info

While looking up the spec for my short 3 spring tank (accutronics 1980s from my 1st proper amp Fender M80) I noticed I'd written the input Z as 600Ω
then checked my long 2 spring 1980s accutronics and it is also 600Ω so my test with the 150Ω tap wasn't quite right though it did make reverb, i'll need to rewind it (lucky I didn't pot it!)
Also been testing the many small SE OTs in my junk box, all of them drive hard at max dwell.
I tried a colder bias resistor on the 6V6 - 1k changed to 2k and that tamed it but still lots of drive available.
The plan is still to replace the 6V6 with one from my junk box tubes, EL42 1st should be good for 2.5 watts..
Couple of photos from the bias res change.
6V6 with 1k, top trace is a 440Hz sine at the 5V6 grid, 5vpp lower trace is across the secondary of the OT
2nd pic with the 2k resistor
Doug

 

[Tubetec]

You dont need to match so maximum power is transfered to the load in this case , you have ample power in a watt or two even if your dumping most of it . Aiming a bit lower than the tank drive impedence is your best bet , then if you want to add more tanks the amp still has the drive capability .
I see the performance in the test is noticably worse with 2k load , try lowering the value to
2x1 k resistors in parrallel and see how it looks

 

[dougie.tamson]

You dont need to match so maximum power is transfered to the load in this case , you have ample power in a watt or two even if your dumping most of it . Aiming a bit lower than the tank drive impedence is your best bet , then if you want to add more tanks the amp still has the drive capability .
I see the performance in the test is noticably worse with 2k load , try lowering the value to
2x1 k resistors in parrallel and see how it looks

I tried at 500Ω and output increased as expected, even on the small Hammond 1750A, low end would have dropped a bit but we don't want too much low end in a spring.

Seems we can throw any old transformer (within reason) at the 6G16. I anyone wants to build a low cost unit from junk box bits the small 6v and 9v 230 mains PTs worked well

Here is some testing I did with the 6V6 and 2kΩ cathode resistor @ 1KHz sine, mini scope is on the 2nd channel to show Vpp and Vrms from the OT secondary:

1) Hammond 1750A reverb transformer for most Fender reverb combos (Deluxe, twin, vibroverb etc)
2) The DIY OT I wound
3) 6v 0.6va mains (230v) power transformer
4) 9v 0.6va mains (230v) power transformer
5) 24v 0.3va mains (230v) power transformer

All sounded ok too, bit more distortion from the power transformers (not gapped/butt stacked)

Next up is to try various junk box tubes and maybe record some samples up against the 6V6.

Doug

 

[andYz00m]

Send audio!!

 

[rock soderstrom]

Seems we can throw any old transformer (within reason) at the 6G16. I anyone wants to build a low cost unit from junk box bits the small 6v and 9v 230 mains PTs worked well

Next up is to try various junk box tubes

I played a DIY tube reverb for a while that was based on a PCL86 SE driver tube and an ungapped(!) 100V output transformer. I would not have heard any difference in a blind test to a 6G15 with an identical reverb pan.

 

[dougie.tamson]

I added 2 extra bases to the test chassis, 9pin for an EF80 and a B8A rim lock for the EL42.

1st up is a small signal pentode, the EF80, I've used it before in a low watt pp amp.
I used a 500Ω cathode resistor and dropped the B+ down to 265vdc from 365v used for the 6V6.

Here's a sound clip, lower dwell setting so not so drippy

 

[dougie.tamson]

Here is a retro synth riff going thru the junk box Mullard EF80 Aug 1955 smaill signal pentode 6G15 reverb clone.
Clip has moderate reverb added 1st half then dry. Arp Pro Soloist VST



Next is to push maybe 2 1/2 watts from an EL42 from Nov1955.

 

[dougie.tamson]

Here's an audio clip of the EL42 drive tube, clean guitar, dwell and mix is adjusted as the clip plays, surf's up at the end.
The EL42 was designed for use in car radios hence the size and rim lock. The EL85 is electrically similar but in a 9 pin bottle.



Synth riff again but with EL42




Doug

 

[dougie.tamson]

Next out of the junk box is an old Mullard ECL80.
This is a triode and pentode in the same bottle but the triode and pentode share the same cathode.
Only the pentode is used, I tied pin 3 cathode and pin 7 suppressor grid, same OT and 500Ω cathode restistor as before.

Guitar




Doug

 

[andYz00m]

Ohhhhhh this is awesome. Yo my ears the ECL80 would be my choice. Very clear and present verb.

they all have a really quiet and lovely tone though. Nice work! What a cool project!

 

[dougie.tamson]

Ohhhhhh this is awesome. Yo my ears the ECL80 would be my choice. Very clear and present verb.

they all have a really quiet and lovely tone though. Nice work! What a cool project!

Thanks for the +veFB
More old 1850s Mullards dusted down: an ECL82 and EL84 coming up next

Still to try various OTs too, small iron adds quite a bit of distortion, largest SE iron I have is for a 10Watter.

Doug

 

[musipol]

These all sound quite good, and s/n seems entirely usable based on these recordings. I guess you're just enjoying the experimentation..........or are you still looking for some other elusive sound quality?

 

[dougsta]

These all sound quite good, and s/n seems entirely usable based on these recordings. I guess you're just enjoying the experimentation..........or are you still looking for some other elusive sound quality?

Yes, the hum level is quite good, didn't even need a 50Hz notch filter in Logic Pro X.
I'm quite keen on finding which junk box tube gets the job done.

 

[rock soderstrom]

Yes, the hum level is quite good, didn't even need a 50Hz notch filter in Logic Pro X.
I'm quite keen on finding which junk box tube gets the job done.

EL84 and ECL82 or 86 will do a good job. The latter two tend to oscillate a bit, a clean setup and gridstopper will help.