superfly with dm160 tube as an indicator only.

[Scruffie]

That R200 radio looks very nice as a small tube amplifier combo, good choice!

In fact I was interested in the schematic, It would be nice for a battery powered headphone amplifier?

My intention was to use the indicator tubes as the amplification stages, but I guess it won't work and I would need to use regular subminiature tubes with lower current requirements, like the tubes that were used in the old portable radios. I think a battery powered guitar amplifier could also be a nice build, but the lack of output power in lower current tubes is a problem. Even the murder one isn't loud enough with a small speaker, and the 5276 only draws 50mA.

Anyway, I'm interested in your final results and some pictures of the final product, I guess it will be an awesome build.
I have enough subminiature amplifiers sitting at home at the moment to start another build , but you never know...

Yeah it's perfect, the stock 4R 5-1/2" speaker has a decent sized magnet and coil and sounds really nice, definitely not tinny. Also the back opens on a hatch so open/closed back is easy and to top it off it has a little spinning mount. It's a sought after model but I lucked out on one that had been pre-destroyed so I didn't have to feel guilty about re-purposing it (plus I think in good condition they're worth a fair bit anyway).

I didn't draw a schematic for it, as I said it's just a fender champ. I don't think you'd get much use from a battery with it! The heaters alone take 450mA.

The DM160s tend to be microphonic from what Rick's said, the IV-15 might be usable for some small signal amplification though.

I agree a portable battery powered tube amplifier would be great but i've looked before and for available and usable tubes the 5672 is honestly the best bang for buck I could find... you could run a couple in series to up the available power?

[PRR]

> -f = 2.91V

I believe this should be closer to 1.5V. You are not going to have anywhere near 3V peak on this amplifier. I would say to decrease the to-ground resistor on the indicator filament, but now it looks like this shares with power tube filament??

Also move indicator tube grid from the voltage divider direct to the power tube grid. You don't have enough indicator drive, you don't need the divider.

[PRR]

> lower the 10k resistor to say... 8k2 that should suffice? Or would I be wiser to increase the 1M5 resistor.

Either trick *increases* the output voltage.

I figure 1.375Meg instead of 1.5Meg. If you find some 2.7Meg in your drawers, two parallel is near enough. Or 22meg||1.5Meg is close.

[Scruffie]

> -f = 2.91V

I believe this should be closer to 1.5V. You are not going to have anywhere near 3V peak on this amplifier. I would say to decrease the to-ground resistor on the indicator filament, but now it looks like this shares with power tube filament??

Also move indicator tube grid from the voltage divider direct to the power tube grid. You don't have enough indicator drive, you don't need the divider.

No the DM160 filament is on its own power from 12V with the 270R/100R that Merlin suggested (he stated that while the datasheet states 1.5V he found that for most DM-160 dark was actually closer to 2V).

I'll try hooking it straight to the power tubes grid then as you say (I wasn't sure if that would lead to the opposite issue of it constantly being on from the slightest input) I suspect that may be the solution.

[Scruffie]

> lower the 10k resistor to say... 8k2 that should suffice? Or would I be wiser to increase the 1M5 resistor.

Either trick *increases* the output voltage.

I figure 1.375Meg instead of 1.5Meg. If you find some 2.7Meg in your drawers, two parallel is near enough. Or 22meg||1.5Meg is close.

Ah okay, I thought the lower the feedback pins voltage the lower the voltage.

IIRC resistors in parallel combine wattage so I should be okay with a 1/4W there in which case I have some 2M7.

Thanks for the advice

[Scruffie]

Well for the DM160 removing the grid divider helped a lot, I now get some movement around 2 on the volume pot so I think as you said PRR i'll raise the filament up (or down depending on how you want to look at it) a teeny bit so it's not biased completely off to get a bit more light out of it and call that good.

I couldn't find the 2M7 resistors I had so I increased the 10k resistor in the SMPS which didn't go quite so well, at 11k and 12k it dropped the start up voltage but it was motor boating until it crept up on its own to 180V. Once I stuck 15k in I was able to dial in a steady 160V but now the motor boating doesn't die off so my layout might not be quite as good as I hoped, a hand near seems to change the pitch slightly and the voltage by about 1V... hoping that once it's shielded with a proper ground in a box though it might improve but I shall have to see. I also couldn't find a 250V 4u7 filtering cap with low ESR at my usual suppliers so that's probably not helping being how intolerant these things are.

[Scruffie]

A 180R in parallel with the 100R to ground on the DM160 gives the best balance of light and dark in this case in case anyone was interested, so about 62R (2V) is the magic number, anything more and it doesn't go to full dark as Merlin proposed.

[Scruffie]

Okay, all boxed up and shielded but still getting motorboating from the SMPS

If I tweak the trim so the voltage gets up to 185V, the problem goes away and everything runs silent but the 5902 is only rated to 165V max on the plate, while i'm sure it'll survive a while at that voltage as the sub-minis are fairly sturdy little things, i'd prefer to get as long as possible life out of it (cause I don't fancy having to try and change a tube in this any time soon!).

Is there a good reason changing the lower resistor in the voltage divider to 15k would have caused this over changing the 1M5?

Is it likely the 4u7 250V cap not being low ESR is causing this? The voltage is steady.

[thomasha]

Hi,
I had a lot of different noises from SMPS, but motorboating I only experienced in a charge pump in the Murder one design, when trying to reduce the resistor between output plate and preamp stages plates. And it only was noticeble at max volume.

Increasing the resistor between power and preamp solved the problem, but I guess it's a different case.

I used the MAX1771 SMPS in a lot of designs, from a single tube Ecl84, the obsession amp, superfly, a 3 tube jtm and a 4 tube jcm like amp with russian tubes. I always use 220k and 1k and a 1k trimpot for the voltage divider that sets the output voltage (I guess I copied that from Frequency Central, and since then always use them). All of the designs I built run on at least 200V, but in some designs I could go as low as 150v, I guess it was more associated with the current requirement of the amp, than the values of the divider itself. And normally it's noisier at lower voltages.

I use the capacitors I have available, and never experienced motorboating because of that, on the contrary, there are some nasty high frequency noises sometimes in higher gain settings.

When you increase the lower resistance you increase the voltage that goes to the chip, the variation could be higher, what could produce the motorboating.

(I'm just guessing and waiting for a scientific explanation from PRR, but I never had this kind of problem with the 220k, 1k pair)

Good luck! And if you can, post some pictures, I would love to see the final result

[Scruffie]

I think the culprit may be the output cap not being low ESR, I tacked on a 10uF cap across power and ground on the amp board and the motorboating changed to a tremolo, I think it's having trouble charging and discharging it at the frequency I want.

For now, I have an extra 470R/4u7 filter from the SMPS to the amp, it drops the voltage enough so that the transformer gets 175V and the plate has 164.8V on, justtt under max while the SMPS is at a happier voltage.

So I need to grab a 1k 1W resistor at some point and see if I can find a 4u7 250V low ESR cap so either that'll fix it or I can just drop the voltage enough to not have to worry about it.

I'm not much of a picture poster but i'll see if I can get a couple, it's nothing too special to look at but i'm happy with it (once it's working 100% anyway).

[PRR]

Someone who knows switchers should comment, but AFAIK....

Switcher dropping-out when set-voltage is low and load is small suggests you want a larger inductor.

[Scruffie]

The designer of this specific SMPS has posted on the forum in the past so i'll shoot him a PM to see if he has any insight.

The inductor i'm using should be beefy enough though I think, it's this 2.1A power inductor http://futurlec.com/Components_Others/PIND100pr.shtml says it's "Ideal for Switching Power Supplies" and the SMPS design says for currents over 50mA to just pick an inductor rated 2A+

I should also mention that when I put the multimeter probe near the voltage divider, the frequency changes a lot so it could be my layout and the feedback pin getting interference.

[rankot]

Is it possible to use some of those "magic eyes" (1M3/DM70 for example) as amplification triodes?

[thomasha]

if you overcome the heater difficulties it would maybe give a very low gain. There was a schematic around here.

[amptramp]

Yes, you can use a tuning eye tube as an amplifier but you will not get that great an indication from it.  If you are operating in the linear region, you will get an indication that gets fuzzier as the amplitude increases.  I happen to have a battery tube portable FM radio with a DM71 using 220K to ground on the grid and 220K to an 85 VDC plate source as the paraphrase inverter in a stage that drives push-pull DL96 outputs.  The DM71 plate runs at a nominal 62 volts.  This is about 100µA plate current and anything from 80 to 100 µA works well and gives a decently bright indication.  Since it is an inverter, it is operating at a nominal gain of -1 but this can be boosted.

[frequencycentral]

Is it possible to use some of those "magic eyes" (1M3/DM70 for example) as amplification triodes?

Yes it is. At one point I breadboarded two cascaded DM160 in a Valvecaster kinda way. This was probably 6 years ago, so I can't remember the details but I'm pretty sure I used a 12V supply with 100R voltage drop resistors to each heater. Looked kinda cool, the blue filament pulsing away as I ground out a heavy rhythm.

[thomasha]

Hi,
I was messing with some battery tubes thought, why not give this a try?

Here the picture of the rig:


and here a comparison between parallel single-ended operation vs. self-split push-pull

The fact that the self-split has a lower output is my fault, not as easy to do when the heaters are in series and the cathodes are at the same time the filaments. After the video I tested some changes that increased the output a little bit:
- Bypass the filament, so that the signal on f+ is the same as at f- for the last stage (coupled through the cathode/filament) where the stage grid was "grounded" at a previous filament (-2V grid bias), bypassed to ground through a cap.

My oscilloscope was showing 0.15V RMS, what gives about 3mW. The celestion greenback helped a little, but it is only loud with headphones.
By the way, this runs at 70V. I guess the max. plate voltage is 100V, and the schematic only presents the operation at 50v.

Another thing that may be changed is the fact that the first tube is always on, because the guitar signal is not enough to make it blink. I tried using a Klone and it kind of moved, but nothing fancy as with the following stages.

Was a fun test, but not something I would build. Even a 5672 in SE gives me more power, and I only would need one DM160 well biased, not 4 at different bias points that won't even blink.

Cheers!