6BM8 one tube reverb

[tubenit]

I've found some different examples of 6BM8 reverbs that use "non-traditional fenderish" topologies or values.  One of the 6BM8 reverbs did not even have a reverb transformer.  Another 6BM8 reverb had numerous differences and no reference for reverb trannie or reverb tank. See attached.

I have several Fender replacement reverb transformers and tanks.  So, I'd like to use those and will not purchase something new.

I am wondering if I am using a Fender reverb transformer and tank, IF this reverb set up would work?   See Tweed BluezMeister TremVerb

Thoughts?

With respect, Tubenit

[kagliostro]

Hi Tubenit

I haven't an answer for your question, but I've this reference for possible use of a 6bm8 tube

if you didn't see it before, may be it is of some interest

https://www.tube-town.net/info/sed-technotes/docs/appnoteNO.10.html

https://www.tube-town.net/info/sed-technotes/pic/No10.fig1.gif

https://www.tube-town.net/info/sed-technotes/pic/No10fig3.gif

https://www.tube-town.net/info/sed-technotes/pic/No10fig4.gif



Franco

[sluckey]

Looks basically good on paper. I'd put a coupling cap on V1B-6. You may or may not need an additional gain stage just prior to the PI.

[jjasilli]

The Fender VibroKing has uses an EL84 to drive the reverb tank through a tranny.  I would use that schematic.  (The 6BM8 pentode section is much like an EL84.)


Transformerless reverb is not very lush.  I do use it for convenience sake, but "nothing beats real Fender reverb".  Here's some info with a sound clip:  http://www.channelroadamps.com/articles/reverb_driver/


I think the 6BM8 is a great idea for one-tube reverb. 

[tubenit]

Kagliostro,  thank you for the information.  I was familiar with only some of that.

Sluckey,  I appreciate you weighing in on this one and was hoping you would.  The lack of a coupling cap is noted and was a mistake.   

It will not need an additional gain stage prior to the PI, as I have done the one tube reverb in numerous amps (including the one my son plays currently) and it has plenty of reverb and amp gain given the low value of the mixing resistor.  Typically, the dwell is at 5-7 and reverb at 3.  So, I feel like it's a somewhat "proven circuit" in that regard.

Thanks, gentlemen!  I appreciate the input.

With respect, Tubenit

[tubenit]

The Fender VibroKing has uses an EL84 to drive the reverb tank through a tranny.  I would use that schematic

Thanks!  I'll take a look at that.

With respect, Tubenit

[tubenit]

OK, what I am purposing is pretty close in topology to the VibroKing reverb. Component values are different and the VibroKing adds a tone pot to the reverb which I don't need.

It does list the same reverb pan as the one I have per the VibroKing manual.

However, it lists a reverb trannie that is 024038 which has a 5.48k primary impedence and a 8 ohm secondary. This is to match the EL84 or 6K6/6V6, I guess?

The trannie that I have on hand and want to use is the 022921 which shows either a 22.k or 15k primary and an 8 ohm secondary.  I want to use that with the 6BM8 instead of buying one.

Is it going to be problematic using the 022921 that I already have?   IF so, then I'll go back to the one tube 12AX7 design.  However, I was hoping to try out the 6BM8 out of curiousity.

With respect, Tubenit

[Fresh_Start]

I know that I've seen a schematic using a 6BM8 for reverb taking the input straight from the input jack and re-injecting the reverb signal much later in the circuit.  Unfortunately, I can't find it right now.  Might be on paper instead of electrons.

The first schematic TubeNit posted confuses me.  It looks like there is a direct connection between the reverb send and return.  I thought that you always needed an isolation resistor between the two.  Funny because I have the same drawing saved from 4 or 5 years ago and didn't notice it before.

I've attached a drawing from my cache.  No idea where it came from.  It is a no-transformer reverb circuit with the pentode section of the 6BM8 wired as a triode (if I'm not mistaken).  The recovery triode is driven pretty hard with a 220K plate resistor and only 1.5K on the cathode.

Cheers,

Chip

[tubenit]

The first schematic TubeNit posted confuses me.  It looks like there is a direct connection between the reverb send and return.  I thought that you always needed an isolation resistor between the two

There is an isolation resistor of 150k. And note that there is a 330k resistor between the reverb return/insertion and the 1M pot. And note the 1M pot is only turned up to about "3" which means there is even more isolating resistance in the circuit.   This works!  I've done it many times with no issues.

The other thing that I like about the 150k isolation resistor being so small is that there is NOT a big loss in gain from previous gain stages as one would find on a Fender amp using 3.3M.

With respect, Tubenit

[jjasilli]

Re Tube : Tranny impedance matching: the 6BM8 has a plate impedance of 5600 @ 200 plate volts. http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6BM8.pdf Probably different @ higher voltages.  E.g., my Bogen PA amp runs them @ over 400 plate volts.  It might be worth experimenting with plate voltage. 


5600 plate : 15,000 primary impedance is somewhere near a 50% mismatch & probably close enough.  Some loss of power may result, but should not be an issue.  Some loss of frequency response may also result at the top & bottom, but this may be a benefit in the reverb section.  You might want to A/B the tranny's different primary impedances for tonal preferences.

[Heinz]

Jeff,

I built a transformerless reverb with a single 6GW8 a long time ago (in a previous life before the kids were born  ). It was part of my modular amp experiment. I chose the 6GW8 because the drive signal was somewhat weak and I didn't want to add another gain stage. You can replace this tube with a 6BM8 without modifying the component values. B+ in my amp was 300V but as you can see there is another filter stage with a 2k2 resistor so the actual voltage for the circuit should somewhere around 230-250V.

Eventually,  I replaced the pentode's plate resistor with the primary winding of a small power transformer which acts as an inductance and increases power output to the reverb tank.

Here's the link: http://dhost.info/jschem/t7/#modules_reverb

EDIT: Re-read your initial post. My circuit requires a 4FXX high impedance reverb tank which I assume you don't have... So this may not be what you are looking for. Anyways, it may be interesting for others.

[tubenit]

jjasilli,  THANK you!  Useful information.  I am not worried about high end loss in the reverb as I prefer the mid tones being reverbed more.  That sounds warmer & richer to my ears.  I understanding that using the trannie I have is probably a reasonable experiment.

Heinz,  Great hearing from you!  Been a while and I've missed your posting.  Your 6GW8 design is in the ARCHIVES as I thought it was an innovative and great design.   Yes, I'd like to try and use the typical Fender trannie and tank if possible.   Hope life is going well for you.

With respect, Tubenit

[kagliostro]

Tubenit

about pentode reverb did you read this ?

http://www.valvewizard.co.uk/reverbdriver.html

Franco

[tubenit]

Tubenit

about pentode reverb did you read this ?

http://www.valvewizard.co.uk/reverbdriver.html

Franco

That's useful info & for the most part where I am headed with 6BM8 pentode but using a dwell pot instead. And I plan to use a transformer also.

With respect, Tubenit

[jjasilli]

Tubenit

about pentode reverb did you read this ?

http://www.valvewizard.co.uk/reverbdriver.html

Franco

Great stuff; thanks Franco!  I'll have to read it a few more times ; hadn't noticed this section of Merlin's page before.  Great info on frequency response & drive power issues.  However, I'm still not a big fan of transformerless tube reverb.  Merlin speaks of functionality issues but not tone.  Both my Ampeg V4 and a Traynor Guitarmate I rebuilt use a triode or two, capacitor coupled to the reverb tank.  The reverb tone is adequate but unremarkable. 

[PRR]

> 6BM8 has a plate impedance of 5600 @ 200 plate volts.

Internal plate resistance is roughly 20K (see sheet page 2).

Suggested External load resistance is, indeed, 5600. We *always* mis-match pentode power amps.

Actually the optimum external load is *nearly* V/I, where V is the supply voltage and I is the plate current. For 200V supply the suggestion is 35mA. 200V/0.035A = 5714, which is very-near 5600 (and probably works the same).

The transformer driving a reverb is *not* 22K, 15K, 5K, or whatever the nominal numbers are. The reverb tank input coil is highly inductive. An "8 Ohm" winding is maybe 2 Ohms at 100Hz, near 8 at 800Hz, near 40 ohms at 4KHz. When transformed, same trend: 22K only near 800Hz, lower at 100Hz, higher at 4KHz.

Nevertheless the tank will probably work well if you get near the 800hz ratings.

Remember V/I ~~= R.

If you have a 300V supply, and say 15K transformer, then the current "should" be 300V/15K= 20mA. Check: 300V*0.020A= 6 Watts, the BM8 is rated 7 Watts, it won't melt.

The 22K transformer might be fine at 300V and a mere 14mA.

300V and 5K implies 60mA, but 300V*0.060A is 18 Watts plate dissipation which is way too much for 'BM8. Which is why for ~~5K load they suggest 200V supply and fudge the "5K" up a bit higher.

> did not even have a reverb transformer

Reverb tank coils come in several impedances, notably 10 Ohms and 1K Ohms.

A tube is a several K-Ohm thing.

The K-Ohm tube loaded with 10 Ohms would sag horribly.

The K-Ohm tube loaded with 1K-Ohms would sag somewhat.

10 Ohms (4, 8) tanks *need* a transformer.

1K tanks can use a transformer or not. But there is no readily available 5K:1K transformer, certainly not the cheap-junk that we prefer for reverb (instead of $90 5K:600 studio iron). A 5K 10 Watt resistor is readily available and cheap, and will pass the tube's DC current yet shunt much of the signal current to a ~~1K tank.

You do have to ask if you need Big Power Tube reverb. A bazillion Fenders got away with a twin 12AT7 driving 15K-22K with just 5mA or 10mA current. You could run the 'BM8 at 10mA, just increase the cathode resistor a lot. 300V 10mA or 3 Watts in a 7 Watt tube does no harm. It seems like a waste of capacity, but there isn't a cheaper total-parts solution to get a small power tube and a gain-tube in one bottle.

[TIMBO]

Hi guys, I haven't had time to read through the whole post, so i'll give you what I got.
The old Fi Sonic has a 6BM8 reverb with transformer but not markings to what it is. Primary side measures with MM is 135ohms. I am unable to get to the secondary side but the tank has a 1475ohm input and if this http://www.amplifiedparts.com/tech_corner/spring_reverb_tanks_explained_and_compared can give an approx. of what the secondary might be. Hope this helps

[tubenit]

I appreciate all the information and help. Thanks to all of you! 

Ironically, the amp I was going to mod is now going to my son "as is".  (It's a Tweed BluezMeister 6v6 & FX). So, the project/mod got
scrapped.

It appears that perhaps a more suitable reverb trannie for this project would be this Hammond:
http://www.hammondmfg.com/pdf/EDB1750B.pdf

This trannie is used by the VibroKing with a EL84 & appears to be suitable for the 6BM8 also.

With respect, Tubenit

[sluckey]

I was just thinking about your preferences for reverb this morning and it occurred to me... Since you like a more subtle reverb, why not consider building a cap coupled driver? There are plenty of good sounding circuits to model. Ampeg and Magnatone come to mind immediately. I think you would be pleased with the sound of either of those.

[echuta13]

I've been meaning to do a capacitor coupled reverb at some point.... I've just never gotten around to it.  I was thinking of using a 12DW7.  Here's what I came up with, but I'm pretty sure there would still be quite a bit of tuning to do on it to get it right.  I'd probably place a dwell control in there were the 220k grid leak resistor is on the 12au7 (500k?)...

[PRR]

We need to cut bass so the spring does not get muddy.

The 500pFd 220K input cuts-off not just bass but much of the midrange.

The 220K is a significant load on many g-amp stages, and isn't needed (grid resistor can be 1Meg).

500pFd 1meg is I think very traditional.

"Dwell" pot can be that 1Meg resistor.

Tank load can be as low as 22K with little effect (bit less top-end). Because this is a very sensitive point, the lower 22K *may* make less trouble with layout squeals when Reverb pot is turned up. If somehow you have two crates of 47K resistors, this is the place to lose 1 or 2.

I also wonder if the cathode caps need to be the 10uFd value that works good in full-bass stages. This may be a matter of taste. Or bad-taste: if you *want* the MegaThunder when you bump the amp, at least the recovery stage needs good bass.

Do note that the 12AU7 has low voltage gain, and may not get driven HARD when tapped out of some preamps (as always, it depends what it's connected to). You might end up with Dwell set at "10" and wishing it went to "11". (Which is why Leo dedicated a whole 12AT7 just to spring-drive.)

[echuta13]

Thanks for the feedback PRR!  The cathode bypass cap values I chose were a little arbitrary...  I will probably go with a 10uf on the makeup stage as a baseline and adjust to taste.  The driver I've seen with anything from no cap on up past 10uf in a circuit like this.  I have a cap decade box so I'll start small here and see how it goes.   

I was planning on running this in an 5f2a style amp.  For an insertion point, I was thinking that there may be two ways to go about it.  One is referenced at the end of this thread : http://el34world.com/Forum/index.php?topic=630.0 which it sounds like Psychonoodler had some success with, or something along the lines of what I've seen in a couple of Gibsons that tap the signal right after the first stage coupling cap, and return it right before the second stage.  The only problem I see with this is that I suspect that it would cause some interactivity with the volume control  (but it may give me the most signal gain).  Hrm...

[MakerDP]

Hi everyone! I joined this forum just so I could ask about this post. I am VERY interested in this 6BM8 one-tube reverb for a "California Dreamer" build I am just about ready to start. I also would like to explore using this in a stand-alone design, but I think I would need another 12ax7 for a gain/cathode-isolator stage and recovery stage so it would need to be a two-tube design, 12AX7+6BM8. I guess I could use the free triode as a dedicated cathode isolation circuit for the dry signal instead of having the one triode pull double-duty?

Has anyone had any luck with either of these ideas with a 6BM8, or any reverb with it in general? Not looking for "boingy" just a nice sounding useable reverb that sounds natural and doesn't draw attention to itself.

I have a Hammond 125A pp output transformer and a 4AB3C1C tank (8-ohm input) that I can put to use. That OT is rated for 3W and it's technically for push-pull . Could I still use it by just taping-off the center tap or do I need a true SE OT with a proper air gap?

I've found some different examples of 6BM8 reverbs that use "non-traditional fenderish" topologies or values.  One of the 6BM8 reverbs did not even have a reverb transformer.  Another 6BM8 reverb had numerous differences and no reference for reverb trannie or reverb tank. See attached.

I have several Fender replacement reverb transformers and tanks.  So, I'd like to use those and will not purchase something new.

I am wondering if I am using a Fender reverb transformer and tank, IF this reverb set up would work?   See Tweed BluezMeister TremVerb

Thoughts?

With respect, Tubenit

[rzenc]

Hi T,


Even though I don't recall seem it before, I believe you could make a transformer-less reverb driver with ECC88 set as cathode follower and with a virtual earth mixing stage, so that you could combine dry and wet signals as desired.


ECC88 Zout could be as low as ~80ohms and would be ''pur-fect'' to drive 1K Zin tanks (low losses). You can adjust bass reduction thru CX to avoid getting mud tones to the spring and CY to adjust overall bass response on recovery/mix stage.


Also, since it's a concertina phase inverter, CZ ought to be used to adjust  dry signal bass response.


Hope this helps,


With Respect,


Best Regards,


R.

[jbefumo]

Old topic, but  ... Here's a stand-alone reverb unit I just built using whatever was on hand -- putting it in a Stanley Lunch Box.  Tested with an old reverb tank from a Baldwin Organ and it sounded quite good.  Have a new MOD unit arriving tomorrow.  Will post some pics and recording as soon as I get it all tidied up ...

[tubenit]

THANKS for sharing your success with this!  I appreciate it very much.

If at some point, you make sound clips, I'd really like to hear some.

With respect, Tubenit

[jbefumo]

Plan to, directly. Was built as an experiment so not anything ready to show -- just ordered fresh wire and tube socket from Doug Hoffman to spruce it up a bit before taking pics and making a recording.  Planning on building another with a long tank in an old metal tool box that belonged to my grandfather -- that one will be a keeper for sure!

THANKS for sharing your success with this!  I appreciate it very much.

If at some point, you make sound clips, I'd really like to hear some.

With respect, Tubenit

[jbefumo]

I am totally enamored with that 6BM8 -- only became aware of them as a result of cannibalizing an old signal generator that used one.  When I discovered they were still (or again) in production, I was delighted.  So far I have built (and dismantled) a single-tube practice amp and a push-pull circuit with a pair comprising the power section and a LTP phase inverter, with just one more 12AX7 making for a complete amp.  It's just SO versatile.

THANKS for sharing your success with this!  I appreciate it very much.

If at some point, you make sound clips, I'd really like to hear some.

With respect, Tubenit

[2deaf]

Old topic, but  ... Here's a stand-alone reverb unit I just built using whatever was on hand . . .

Is it okay to run the screen grid at a way higher voltage than the plate?  Looking at the data sheet for the 6BM8, it looks to me as though the screen voltage will be higher than the plate.

The components on the 6BM8 pentode look to me as though they would be better suited to a lower current pentode.

I've never tried a 6BM8, but after looking at a typical characteristics table I would try the following:

R7:  5.6K
R8:  390
R9:  30K
C3:  10uf
C4:  10uf
C5:  10uf
R10:  Delete
Add a 4.7K resistor between C3 and the 8E tank

   

[jbefumo]

Thanks!  I'll double check all the components before I run it again.


Joe

Old topic, but  ... Here's a stand-alone reverb unit I just built using whatever was on hand . . .

Is it okay to run the screen grid at a way higher voltage than the plate?  Looking at the data sheet for the 6BM8, it looks to me as though the screen voltage will be higher than the plate.

The components on the 6BM8 pentode look to me as though they would be better suited to a lower current pentode.

I've never tried a 6BM8, but after looking at a typical characteristics table I would try the following:

R7:  5.6K
R8:  390
R9:  30K
C3:  10uf
C4:  10uf
C5:  10uf
R10:  Delete
Add a 4.7K resistor between C3 and the 8E tank

 

[jbefumo]

LOVE THIS FORUM!

Thanks!  I'll double check all the components before I run it again.


Joe

Old topic, but  ... Here's a stand-alone reverb unit I just built using whatever was on hand . . .

Is it okay to run the screen grid at a way higher voltage than the plate?  Looking at the data sheet for the 6BM8, it looks to me as though the screen voltage will be higher than the plate.

The components on the 6BM8 pentode look to me as though they would be better suited to a lower current pentode.

I've never tried a 6BM8, but after looking at a typical characteristics table I would try the following:

R7:  5.6K
R8:  390
R9:  30K
C3:  10uf
C4:  10uf
C5:  10uf
R10:  Delete
Add a 4.7K resistor between C3 and the 8E tank

 

[Paul1453]

I have a few of these vintage 6BM8s, not more than 3-4.

I've also got quite a few 6CX8s that are also a triode/pentode mixed tube.
I don't know if you can do that same thing with the 6CX8 or not.
Since you have a concept that seems to be working for you,
you might check your tube stock for other triode/pentode tubes like the 6CX8 that may also work.
No need to buy new tubes if you have others in stock that will also do the job. 

[2deaf]

I got myself an electro-harmonix 6BM8 and gave it a try at driving an 8E reverb tank.  After a couple of adjustments, I come up with the attached driver.  It may not be optimal and it has a 24ma appetite, but it sho nuff drives the pants off of that tank.

[jbefumo]

I got myself an electro-harmonix 6BM8 and gave it a try at driving an 8E reverb tank.  After a couple of adjustments, I come up with the attached driver.  It may not be optimal and it has a 24ma appetite, but it sho nuff drives the pants off of that tank.

Thanks, and good timing -- just finished drawing up a clean layout and getting started building a new board for it.  Will post it as soon as I square up all the lines .... I did change to the values suggested by 2deaf on his 5/28 post and they work well.  Will update my schematics when I'm done and post those as well.  On another subject - started playing the Jazzbass my brother gave me for xmas, so guess it's time to start thinking about bass amp designs ....

[moonbird]

jbefumo -


Did you ever get the chance to built / document this reverb unit? I would appreciate seeing you final schematic. Thanks.


BTW - I was able to order a Hammond 1750B OT from Digi-key for about $40 shipped. I want to try the cap coupled unit to and see which one I like better.