Submini Tube Reverb request

[PRR]

> 6N16b .... They're just not ballsy enough to drive a reverb tank, it would be like trying to use a 12AU7 in your amp.

It is not that far off from a 12AT7, which was the tank driver in many-many Fender reverb amps.

It can stand 350V. It can pass 4mA-8mA at 200V. Put it with the Fender reverb transformer, a likely op-point could be 200V B+, 22k load, -7V on the grid for 8mA both-halves.

As in the Fender, you need 1 or 2 12AX7-style stages in front to get from 20mV to ~~5V at the power tube grid.

As further evidence: 6N16B based 1W full tube guitar amp

[vigilante397]

> 6N16b .... They're just not ballsy enough to drive a reverb tank, it would be like trying to use a 12AU7 in your amp.

It is not that far off from a 12AT7, which was the tank driver in many-many Fender reverb amps.

It can stand 350V. It can pass 4mA-8mA at 200V. Put it with the Fender reverb transformer, a likely op-point could be 200V B+, 22k load, -7V on the grid for 8mA both-halves.

As in the Fender, you need 1 or 2 12AX7-style stages in front to get from 20mV to ~~5V at the power tube grid.

It's a fair point, a 6N16B might work as a replacement for the 12AT7, but you definitely won't be replacing all of the tubes with 6N16B. I'm still planning to try with all 6N21B.

[tubegeek]

> 6N16b .... They're just not ballsy enough to drive a reverb tank, it would be like trying to use a 12AU7 in your amp.

It is not that far off from a 12AT7, which was the tank driver in many-many Fender reverb amps.

It can stand 350V. It can pass 4mA-8mA at 200V. Put it with the Fender reverb transformer, a likely op-point could be 200V B+, 22k load, -7V on the grid for 8mA both-halves.

As in the Fender, you need 1 or 2 12AX7-style stages in front to get from 20mV to ~~5V at the power tube grid.

It's a fair point, a 6N16B might work as a replacement for the 12AT7, but you definitely won't be replacing all of the tubes with 6N16B. I'm still planning to try with all 6N21B.

How come? A circuit that uses the optimal tube for each job should be easier to design and sound better. The parameter that's most important for driving a transformer is low rp, the parameter that's most important for voltage amplification is high mu.

There are full-sized tubes with two dissimilar triodes that are ideal for something like this - I've made a low-wattage hifi amp with one 13FM7 per channel for example. They were developed for TV sets and the low rp (< 1 K) side will drive the hell out of a transformer. Mu of the voltage amp side is 66 if I remember tight, way plenty for this application. One GT sized bottle would be small enough for a project like this, no?

Better design than the Fender original. Choose your favorite high mu for the recovery and mix job, done-zo.

If I were forced at gunpoint to use the pencil tube, remember that a Fender used both halves of a 12AT7 in parallel to halve the rp going into the transformer.

[vigilante397]

How come? A circuit that uses the optimal tube for each job should be easier to design and sound better.

1) I already ordered boards designed around the 6N21B, they'll be here Tuesday.

2) 6N16B are microphonic as hell, and putting microphonic tubes in an enclosure with a stomp switch has never worked well for me.

3) While the 6N16B could work I'm still not convinced it's the optimal tube for the job. That's what prototyping is for

4) I can still try the 6N16B for the driver tube if I don't like how 6N21B does the job. They have a different pinout but it's not that hard to twist leads around to use different tubes.

[PRR]

> . The parameter that's most important for driving a transformer is low rp

True; but a reverb tank likes a high impedance. There's no simple perfect answer.

> 6N16B are microphonic as hell

So is the reverb tank, no?

I'm not saying it is sure to work. Or even likely enough to change your plans.

Yes, most Gain stages in a guitar amp should be more 12AX7 Mu.

[danielzink]

The two tanks I've my eyes on are:

https://www.amplifiedparts.com/products/reverb-tank-accutronics-8ab2a1b-medium-decay-3-spring

The tank is 9 1/4" long
Input at 10Ω
Output at 2575Ω

The most compact true spring tank:
https://www.tubesandmore.com/products/reverb-tank-accutronics-amc2bf-2-spring

Is 5" by 1 3/4"
Input at 150Ω
Output at 1500Ω

Not knowing *anything* about reverb circuits - will components need to be swapped on the PCB to accommodate differing impedances or are they pretty flexible ?

Thanks !

Dan

[vigilante397]

> 6N16B are microphonic as hell

So is the reverb tank, no?

Yes, but the reverb tank is going to be outside the enclosure (at least that was my intent) so I won't be stepping on/near it. I've only done one pedal with 6N16B and it sounded good, but everytime you turned it on it had a lovely glass "ping" sound.

https://www.amplifiedparts.com/products/reverb-tank-accutronics-8ab2a1b-medium-decay-3-spring

The tank is 9 1/4" long
Input at 10Ω
Output at 2575Ω

The Wave, the circuit I used for this, uses a 9AB3C1B tank, which is 10Ω input and 2575Ω output like that one, so that should be pretty much spot on. The length is different (the wave uses a 17" tank) but that just affects reverb time. I'm trying to buy as little as possible (generally how I do things ) so I'm planning to recycle the tank out of an old solid state amp I was getting ready to throw away. The main thing that would be inconvenient about using a different impedance tank would be picking a different transformer.

[danielzink]

> 6N16B are microphonic as hell

So is the reverb tank, no?

Yes, but the reverb tank is going to be outside the enclosure (at least that was my intent) so I won't be stepping on/near it. I've only done one pedal with 6N16B and it sounded good, but everytime you turned it on it had a lovely glass "ping" sound.

https://www.amplifiedparts.com/products/reverb-tank-accutronics-8ab2a1b-medium-decay-3-spring

The tank is 9 1/4" long
Input at 10Ω
Output at 2575Ω

The Wave, the circuit I used for this, uses a 9AB3C1B tank, which is 10Ω input and 2575Ω output like that one, so that should be pretty much spot on. The length is different (the wave uses a 17" tank) but that just affects reverb time. I'm trying to buy as little as possible (generally how I do things ) so I'm planning to recycle the tank out of an old solid state amp I was getting ready to throw away. The main thing that would be inconvenient about using a different impedance tank would be picking a different transformer.

*My* particular build is going to be styled after the G615 stand-alone reverb unit....only in a "submini" package.

I'm hoping to keep it as small as possible.

I've built the SFA (https://www.diystompboxes.com/smfforum/index.php?topic=111758.msg1029422#msg1029422) and I think it would be a hoot to set the SFA on top of a "kleenex" box size reverb unit.


It would be great to use the 4" reverb tank (for my purpose)...if you know which transformer would be better (or am I just looking for a reverb transformer that matches the impedances required ?).

My SFA:

[vigilante397]

I don't know which transformer you would need off-hand, but yes a transformer that matches the input impedance of the tank is what I would assume you need.

Quick update: I got the boards Tuesday but just got around to stuffing one tonight because I've been out of town all week. It also never occurred to me to check the impedance of my reverb tank, and it is way too high for the transformer I have (2Kohms vs 10ohms). So right now the circuit is basically behaving like a clean tube preamp with extra steps, but the good news is at the very least the preamp part works. I'm using a 12V power supply (makes heaters a lot easier) and my wall wart can supply 1.5A, haven't gotten around to measuring how much it's actually pulling yet. So I guess I'll have to order another tank, which means this may get pushed back a little more because I hate ordering things At the very least I'll get some pictures posted in the morning though.

[tubegeek]

I'm using a 12V power supply (makes heaters a lot easier) and my wall wart can supply 1.5A, haven't gotten around to measuring how much it's actually pulling yet.

If you're running a SMPS and the heaters off of the same 12V supply, definitely measure the heater voltage before too long - I've had 15V coming from a "12V" supply, the heater load isn't enough to drag a beefy wall wart down far enough, so you may need a series heater resistor to get the heaters down to 12V. I don't know how much 15V would shorten the life of the tubes - subminis are rated for long life, no one wanted to replace soldered-in tubes so they built them pretty tough. But it'd probably kill them eventually.

You can try a lower-rated 12V wall wart (800 mA for example) and probably get closer to the 12V spec across the heaters. The SMPS will still be getting enough current, you'll just probably have to re-tweak the adjustment pot to get you to your target B+.

[Ben N]

I've only done one pedal with 6N16B and it sounded good, but everytime you turned it on it had a lovely glass "ping" sound.

Bug or feature? (Hmm, , perhaps a set of 12 tuned microphonic tubes with piano hammers...)

[vigilante397]

So I just measured the current and it's pulling about 1.3A. With my 1.5A supply I'm reading 6.01V on the heaters (they don't have a parallel option like 12AX7 so I'm running the tubes in series pairs) and B+ is sitting happy at 200V. Not my first rodeo

So here's the promised pictures. The cap on the right is a little big because I used a 630V cap instead of 400V. Hooray for not ordering things





And since I found out the "nightscape mode" on my phone camera takes awesome tube shots I always have to have one of these:

[tubegeek]

Not my first rodeo

I know, I know. I was just being a yenta. 6.1 will just have to do I guess.

I like the night vision goggle view! Cool layout!

[PRR]

> ...a transformer that matches the input impedance of the tank is what I would assume...

Actually, we normally drive a tank from a high Z source. That gets better frequency response. (In Fenders, the secondary:tank may look near-match, but the tube rp is higher than winding nominal Z.)

> the impedance of my reverb tank, and it is way too high for the transformer I have (2Kohms vs 10ohms).

Given the above, you may not need a transformer. Resistance-coupled is lots less efficient but a lot of fine amps do it anyway. 2k is "low" for a tube so drive is hi-Z.

In this sim with the wrong tube, I also mis-noted your 2k nominal tank (which really rises 1k-5k across the reverb band). You will get more Volts than shown (which is about 2dB short of clipping). Your mu=30 tube will need different cathode resistor. 1K RP is about as low as you can go. Get Vp to 75%-50% of supply.

[vigilante397]

Given the above, you may not need a transformer. Resistance-coupled is lots less efficient but a lot of fine amps do it anyway. 2k is "low" for a tube so drive is hi-Z.

In this sim with the wrong tube, I also mis-noted your 2k nominal tank (which really rises 1k-5k across the reverb band). You will get more Volts than shown (which is about 2dB short of clipping). Your mu=30 tube will need different cathode resistor. 1K RP is about as low as you can go. Get Vp to 75%-50% of supply.

Duly noted, thanks Paul. I didn't even think about the fact that the input impedance of the tank was close to the primary impedance of the transformer. I'll pull the transformer off tonight and try going straight into the tank from the driver.

[danielzink]

Any luck ?

[vigilante397]

It's still on my mind, I promise. The board is still stuffed and waiting for me to try it without the transformer straight into the tank. I'm in the process of cleaning my garage so I have more room to work, but I should hopefully be able to get to it sometime this week.

[vigilante397]

So I still haven't tested the transformerless tank hookup. I keep bouncing between more important projects and being too worn out to work on things I should actually be able to do it tonight though.

[vigilante397]

Update: Finally got it wired up transformerless and tried it out, and alas it doesn't look like the 6N21B can drive either of the tanks I have. It works great as a clean preamp with the reverb disengaged, and with it engaged I can get the lovely washy sounds when I whack the reverb tank, but the 6N21B can't seem to wobble those springs by itself.

I still don't really want to spend money on a lower impedance reverb tank (my wife won't let me order things until I get rid of a few of the dozen pedals I have for sale ), so what should I try next? Swap the reverb driver for a 6N16B and see what happens?

[Ben N]

Maybe a 2-stage 6N21B driver, a voltage boost pushing a CF, or else both halves of either the 6N21B or the 6N16B in parallel to push some current through that tank. (Have a look at the reverb circuit in reverb-transformerless variant of the Traynor YSR-1.) Just thinking of ways to get some low impedance drive going.