1-Tube reverb 6BM8/ECL82

[adamG]

Hi Guys,

Could you assist me with 1-tube reverb with 6BM8 or ECL82.
1. Is 6BM8 more powerful than single 12AX7?
2. What prefefable reverb driver's primary impedance should be?
3. What is the tonal character of 6BM8 comparing with 12AX7 in reverb circuit?

I attached two basic schematics, which I'll be following.

Thank you in advance.

Regards,

Adam

[tubenit]

In ARCHIVES, there is information on 6BM8 "one tube reverbs"                      http://el34world.com/Forum/index.php?topic=7957.50

Reply #64   (reportedly a proven 6BM8 design)

Timbo's  Moody GA-40 6BM8 reverb              reply #36    http://el34world.com/Forum/index.php?topic=16984.msg169044#msg169044

6BM8 reverb YouTube       

And the 3 attached are reported as proven designs on TAG (The Amp Garage) for D-style amps.

With respect, Tubenit

[mwelch55]

Tubenit,

Looking at the bottom two schematics, wouldn't turning the reverb mix pot all the way down completely shut down the output of the preamp (since it would ground out the input of the third triode)?  I seems like there should be a resistor between the wiper of the pot and the input of the third triode.  I'm no expert, but that's what I am seeing.

Mike

[tubenit]

I typically insert a 330k between the reverb pot and the return insertion point.   I think you may have a valid point that with that schematic it could send signal to ground.

With respect, Tubenit

[92Volts]

All those have resistors in series with the plate and/or B+ to significantly reduce output. The Fender Vibro-King is an interesting example that uses a non-handicapped power pentode (EL84) to drive the reverb tank: http://el34world.com/charts/Schematics/files/fender/Fender_vibro_king_manual.pdf

Design is similar to a normal SE speaker output, and impedance can be similar (5k)... or not. Power is overkill anyways, closer to 5w than 0.5w-1w typical from a 12AT7 (12AX7s aren't really used for this). If you have a reverb tank that could take that without melting, you'd get a lot of signal out of it.

The triode has less gain than a 12AX7, closer to a 12AT7. Besides the 12AT7 being what's normally used, you might need less gain depending how much power you allow out of the driver stage.

[adamG]

Hi Guys,

Thank you very much!

If I am decided for the option with driver transformer, than which one shall I use and what its primary impedance should be?

Regards,

Adam

[adamG]

All those have resistors in series with the plate and/or B+ to significantly reduce output. The Fender Vibro-King is an interesting example that uses a non-handicapped power pentode (EL84) to drive the reverb tank: http://el34world.com/charts/Schematics/files/fender/Fender_vibro_king_manual.pdf

I like this idea of EL84 as the driving tube.
Did you build it? If so, could you share with us that experience?

Regards,

Adam

[TIMBO]

Hi Adam, Fender reverb drivers work with most tubes.
http://www.classictone.net/40-18034.html

Doug has them.

[adamG]

Hi Adam, Fender reverb drivers work with most tubes.
http://www.classictone.net/40-18034.html

Thank you Tim, very much

Kind regards,

Adam

[adamG]

Tim,

One question more, if I may?
Certainly, I could see "standard" 022921, that is 12k of primary. But, I've got a possibility to wind my own one, where I can provide different ,maybe more suistable, impedance.
So, could it be 5-6K, acc.to 6BM8 specification? Actually, I am not sure of it, because its data sheet specifies 5,6K at 200V (RCA).
Secondly, in GA-Moody you applied a single 6BM8, which might be my closer target.
FI Sonic needs 1,5 tubes. Which ones may be a better goal acc.to your taste?

Thank you in advance.

Regards,

Adam

[adamG]

Tim,

One question more, if I may?
Certainly, I could see "standard" 022921, that is 12k of primary. But, I've got a possibility to wind my own one, where I can provide different ,maybe more suistable, impedance.
So, could it be 5-6K, acc.to 6BM8 specification? Actually, I am not sure of it, because its data sheet specifies 5,6K at 200V (RCA).
Secondly, in GA-Moody you applied a single 6BM8, which might be my closer target.
FI Sonic needs 1,5 tubes. Which one may be a better goal acc.to your taste?

Thank you in advance.

Regards,

Adam

[TIMBO]

A couple of bits here..
http://www.classictone.net/40-18034.pdf
http://www.datasheetspdf.com/pdf/987020/RCA/6BM8/1

The driver data says 25000 ohms
The 6BM8 says 20000 ohms

CLOSE ENOUGH

[adamG]

The driver data says 25000 ohms
The 6BM8 says 20000 ohms

I can see in data 20k as plate resistance. But 5,6K is for load resistance.
I might be wrong. Please, correct me.

Regards,

Adam

[adamG]

Maybe these two are a good duet?
I mean 6BM8 and 1750B.

Regards,

Adam

[92Volts]

EDIT: missed the most recent post above mine. The 1750B would work and is similar to the one I recommended.

I like this idea of EL84 as the driving tube.
Did you build it? If so, could you share with us that experience?

I haven't built it. I know of it because I repaired another Fender (12w Champ) where the output transformer was the same as the reverb transformer in the Vibro-King!
This is similar to the transformer used in that design, and it's cheap: https://www.tubesandmore.com/products/transformer-output-8-w-single-ended

The pentode section of the 6BM8/ECL82 is lower-power, but basically similar to EL84. The 6BM8 must run at lower current (larger cathode resistor)... under those conditions a higher impedance closer to 10k would be "ideal". 7k or 8k transformers are likely the most affordable/available if you wanted a bit more power than 5k, because they're used in designs like the normal Fender Champ.

The actual "reverb driver" transformers that have been recommended will also work, but 22k+ impedance is "too high" and limits power output.

This might not be a problem as you likely don't need maximum power from the 6BM8. Keep in mind the DC current limit of smaller transformers like that if you do use them-- the 6BM8 could "safely" draw more current than the 12AT7s the transformers are usually used with.

[PRR]

> 6BM8 says 20000 ohms

Plate resistance.

Not Load resistance.

If we actually wanted the full 3.5 Watts possible, we'd lean toward the 5K load suggested.

But we don't want to SMOKE the poor spring. A part-watt is ample. Mis-match is good.

[tubeswell]

You could use something like a Champ 15W 5k-10k SE OT with a 6BM8 pentode - to pass more clean bass


Since your concept is experimental, the 4R tap on this one would mean you could increase the reflected load to 14k with an 8R input transducer on your pan - for different driver-side characteristics https://www.tedweber.com/w022905m. Would also make it easier to experiment with different circuit configurations using different triode-pentode types.


You could also try different tricks with the pentode setup, like trying it with un-bypassed screen dropper/resistor to get screen current feedback for reduced gm and gain = cleaner reverb pan driving signal etc.

[kagliostro]

BTW, looking for other I've just seen this schematic that uses an ECL86 (6GW8) and without transformer





Franco

[2deaf]

1. Is 6BM8 more powerful than single 12AX7?
2. What prefefable reverb driver's primary impedance should be?
3. What is the tonal character of 6BM8 comparing with 12AX7 in reverb circuit?

1.  Yes
2.  25K
3.  I don't know

The main consideration in choosing a primary impedance is what you want from the secondary.  The tank input transducer depends on the current through the coil to twist the springs, so we need to know how much current is required.  Accutronics published a value of 28mA_rms, so we can safely disregard that just like Mr. Fender did.  Ten times that would be a better design target which would be about 400mA_p.

The output transformer steps down voltage and steps up current in accordance with the turns ratio.  Once you know the turns ratio, you can calculate how much primary current you need to get 400mA_p out of the secondary.  From that you can select a tube that can swing the necessary primary current. 

Can a 6BM8 swing enough current to get 400mA_p with a 5482 to 8 Ohm transformer?  No.  (Well it might, but it would be grossly exceeding the maximum limits)  It would take a 6V6 or 6BQ5 type and even then 8000 to 8 Ohms would be a better choice.

Can a 6BM8 swing enough current for a 25K to 8 Ohm transformer?  Yes.  In fact, the strapped 12AT7 pair that Fender uses has the same 5W rating as the 6BM8.

[tubeswell]

BTW, looking for other I've just seen this schematic that uses an ECL86 (6GW8) and without transformer





Franco

TFT Franco - I note that in both these examples, the ECL86 is wired in triode-mode (with the screen tied to the plate). So it won't be delivering the output power that it would in pentode mode, but it also has lower output impedance.

[Toxophilite]

If one used the 6BM8 one tube reverb can one just insert the reverb signal back in just prior to the phase inverter?

[sluckey]

TFT Franco - I note that in both these examples, the ECL86 is wired in triode-mode

It may be worth noting that neither of those schematics use the 6BM8 triode for recovery, so it's not really being used as a one tube reverb circuit. Probably because the triode gain is only 70?

Maybe the rare 6GW8 would be make a better one tube reverb. Too expensive for me though. 

[kagliostro]

Of course I agree with Tubeswell and Steve

Franco

[TIMBO]

I agree mate.
I have used the 6GW8, 6BM8 and 6DX8 in one tube reverbs.
I have found these work great with capacitor coupled circuits.

So if you are wanting to use a transformer coupled reverb and the above tubes are too expensive, I'd be looking at the 12AX7 or 12DW7.

[kagliostro]

https://www.ebay.it/itm/ECL84-6DX8-TELAM-Rohre-tube-Test-100-NOS-NIB-4x-vorhanden/273144313897?hash=item3f98ab7829:g:O~QAAOSwISRakwYX


Franco

[adamG]

Thank you Guys for so many replies.

Best regards,

Adam