12AU7 -> small PT as SE OT -> spring reverb?

[Bergheim]

I have a 150 ohm tank on the shelf, and I want to have go at building a (good) reverb circuit.
The popular advice for serious reverb is to push 1 watt into the tank, and right now I have a little surplus of 12AU7's in the drawer, and a small power transformer that can hopefully substitute as an OT for the tank.
The PT is a 3-5 watt 230V input and 24V output, which gives a voltage ratio of 9.6:1 and an impedance ratio of 92:1. With a 150 ohm tank, the reflected primary impedance is about 13.8k, which looks pretty decent for a 12AU7.
1 watt into 150 ohms = 12.2Vac @ 81mA. I believe the 12.2Vac is RMS? If so, the peak to peak voltage is 12.2 * 1.4 = 17.1.
To get 17.1 volts on the secondary, the voltage swing over the primary has to be 17.1 * 9.6 = 164Vac

Uten navn.png

Biasing the 12AU7 to 5mA @ 300V, it seems it can easily develop the neccessary 164 volts @12mA across the load for 1W, but the output power calculation in the loadline calculator says the output at the specified output headroom (82 volts each way from the quiescent point) is only 0.24W.

Are my maths wrong?
And maybe the thing I should've asked first, will the transformer run into trouble with 5mA DC through the primary?

[Stevem]

If you want to use a push pull OT for a class A SE I believe it's going to need to much bigger then you might like it to be.

[Lauri]

You only need about 6.5mA RMS at 1Khz to drive a 150 Ohm reverb tank. Since your transformer probably can't handle DC current on the primary, you can use a 10k plate resistor on the 12AU7 and a capacitor to drive the transformer. I did a quick simulation of the circuit and it works. Of course there's a possibility it doesn't work with the transformer you have since inductances and capacitances of the transformer could be anything.

Screenshot From 2025-03-27 18-43-58.png

[TUBEDUDE]

Verb xfmr's are not necessary. They add expense, inefficiency, require space, risk potential magnetic field induced hum, etc. I prefer one of the current drive schemes. I just made an amp for grandson using an EF-80 for the transformerless
reverb driver.

[maxkracht]

https://valvewizard.co.uk/reverbdriver.html

Heres an article with a couple transformerless driver circuits. I tried the bottom example a couple years ago, worked great.

[Bergheim]

Really appreciating your inputs!

The reason I'm asking is because I want to make the most of what I have on hand already, as far as possible at least.

I also have a 600 ohm tank, so if anything else fail I'll probably try a cap driven setup. I've looked a bit at merlin's SRPP, so that's also an option. The amp at hand is a bit limited on B+ current as well, so I can't afford much more than 10mA DC for the reverb. Any circuit details on the EF80 reverb, TUBEDUDE?

I am not very well educated on transformers so I'm happy to be corrected.
Anyways, I measured the laminations of the transformer, seems like it's an EI48 with a stack/depth of 24mm (~1"), so looks like 10VA. Considering I'm only gonna pull 1 watt from it, it makes me wonder if the core might be big enough to handle the DC current. With 1W into the reverb tank, it's only about 10% of the VA rating. The primary DCR is 450 ohms, and with 5mA DC the voltage drop is 2.25V, which dissipates another 10mW.
I've tried to find formulas to calculate the DC current needed to saturate a certain size core, but for the most part the stuff I found is just "DC offset will saturate the transformer much easier, especially at high signal levels" and no details or examples of this voltage and that current etc.. So a bit vague if I may say.
I also found this thread about using a 40W PP OT in a Champ: https://ampgarage.com/forum/viewtopic.php?t=28731, and from the replies it didn't seem that unlikely to make that one work.
Being a reverb circuit, the signal into the transformer is mostly mids and highs, which, as far as I understand, requires less flux to drive than bass. Unsure to what extent though.

Am I chasing a lost case or is it worth trying? Are the signs of core saturation easy to detect, or will I release the magic smoke all of a sudden?

[TUBEDUDE]

Here's what I did for my grandson amp.

20250327_175725.jpg

You could use a Pentode/triode tube also. There are many, 6AW8, 7199, 12BH7, 6AN8, 6AU8 etc. The pentode drives the tank and the included triode is the recovery amp.
Also, look up Lamington Reverb for good info.

[Bergheim]

Alright, alright, maybe I'll ditch the transformer driven thing.
I read about the different transformerless approaches earlier, both the SRPP and the pentode driver at the channelroadamps site, but the numbers didn't add up with the advice of minimum 1 watt into the tank for serious reverb, so I ditched that idea right until now.
Found this video on YT, and was blown away that it seems to easily match the Fender reverb unit: https://www.youtube.com/watch?v=HWlHYcYR6J0
Transformerless it is then:)

[Lauri]

but the numbers didn't add up with the advice of minimum 1 watt into the tank for serious reverb, so I ditched that idea right until now.

Of course the numbers don't add up because the advice about needing to drive the tank with 1W is complete nonsense. For 150 ohm tank the recommended current to drive it is 6.5mA.

[jabguit]

https://valvewizard.co.uk/reverbdriver.html

Heres an article with a couple transformerless driver circuits. I tried the bottom example a couple years ago, worked great.

+1 on the transformerless design. I copied one from John Polstra's Channel Road Amplification page utilizing a 6GH8A as driver/recovery for use in my 807 amp. To my ears this is the best sounding reverb I've ever heard. Notes are not swamped or muddied by reverb but rather the reverb follows the notes and decay. Sounds a lot more like a good room than typical spring reverb and I used the Mod 3-spring medium decay short tank 8EB2C1B. This circuit is not nearly as gainy as a trad transformer type design, but it's also less prone to noise. Highly recommended...

cheers,

[Bergheim]

Of course the numbers don't add up because the advice about needing to drive the tank with 1W is complete nonsense. For 150 ohm tank the recommended current to drive it is 6.5mA.

Yeah, maybe it is nonsense. I figured there was something to it after seeing several reputable people, including a quote of Rod Elliot on Merlin's site stating that the reverb tank can be driven with up to 10 times the rated current with excellent results. That, in addition to the legendary Fender 6G15 which uses a power tube and a transformer to drive the tank, led me to conclude that surf-level reverb did indeed require a lot of power.

As mentioned above, after listening to the Lamington reverb, it seems the transformer coupled reverb may be kind of a nostalgic hoax, like so many other things in the tube amp world..

[lonote]

This is all great information, very timely with the small amp projects I am contemplating.

+1 on the transformerless design. I copied one from John Polstra's Channel Road Amplification page utilizing a 6GH8A as driver/recovery for use in my 807 amp. To my ears this is the best sounding reverb I've ever heard. Notes are not swamped or muddied by reverb but rather the reverb follows the notes and decay. Sounds a lot more like a good room than typical spring reverb and I used the Mod 3-spring medium decay short tank 8EB2C1B. This circuit is not nearly as gainy as a trad transformer type design, but it's also less prone to noise. Highly recommended...

cheers,

The Channel Road site seems to be not functioning for me, would you have a schematic of the 6GH8A circuit mentioned?

[didit]

Try this —

https://web.archive.org/web/20180921032 ... rb_driver/

Fully agree this circuit provides exemplary reverb. The only downside is sourcing suitable NOS tubes. Get a sleeve of five of whatever you choose so spares are available twenty years from now.


..

[Lauri]

quote of Rod Elliot on Merlin's site stating that the reverb tank can be driven with up to 10 times the rated current with excellent results.

Yes, Rod Elliot says it can be driven up to 10 times the rated current but suggest driving the tank up to only 2 times the rated current because after that it usually starts distorting. In a Fender style reverb circuit the transformer should be rated at least 1W but there's nowhere near that amount of power going in to the reverb tank coil.

[jabguit]

Try this —

https://web.archive.org/web/20180921032 ... rb_driver/

Fully agree this circuit provides exemplary reverb. The only downside is sourcing suitable NOS tubes. Get a sleeve of five of whatever you choose so spares are available twenty years from now.


..

Yep - that's it.
As for 6GH8As - they're readily available as NOS or good use from many sources, such as this one:
https://www.cedist.com/products/vacuum- ... de-pentode
I bought a mixed sleeve of 5 on eB for under $20, IIRC. In fact I think you'll find good used or NOS 6GH8As for less than comp quality 12AT7s all day long.....YMMV


cheers,