How to work out heater resistor value? (And a few other submini amp questions)

[Scruffie]

Okay i've gone for 2 x Pentode Preamp stages & 2 6088 Output Stages.

Would a schematic like this be a long the right lines for having this work? I'm not sure about the values really but hoping from what I read and simmilar designs i've seen they're rightish.

[Scruffie]

Okay... tried one of the tubes with a 390R heater resistor, only getting 0.4V on it?

[R.G.]

Okay... tried one of the tubes with a 390R heater resistor, only getting 0.4V on it?

Does that mean you're getting 8.6V across the 390R?

That's 22ma all by itself, which should have the filament at full voltage. It's possible that these filaments have a thermal "hump" to get over, where they have to have more current to start or they'll never get hot enough to self limit. Odd, but it might happen - two stable states in the temp-vs-voltage plot, prevented from getting to the second and hotter one by the limits of the resistor in series.

Another question is - are the filaments glowing orange?

[Scruffie]

Okay... tried one of the tubes with a 390R heater resistor, only getting 0.4V on it?

Does that mean you're getting 8.6V across the 390R?

That's 22ma all by itself, which should have the filament at full voltage. It's possible that these filaments have a thermal "hump" to get over, where they have to have more current to start or they'll never get hot enough to self limit. Odd, but it might happen - two stable states in the temp-vs-voltage plot, prevented from getting to the second and hotter one by the limits of the resistor in series.

Another question is - are the filaments glowing orange?

I've sorted it now, for some reason my supply which claims 200mA wasn't enjoying powering 4 of them plus a charge pump, my 300mA supply seems content (not sure if the other one is broken or not, it worked to supply 1 tube 1.5V though.

And the amp sounds! Need to try it with a few more Charge pump stages (only had 50V cap so got it to 43V) tweak it so there's less bass content as it just sounds like the tubes are sagging on the thicker strings right now.

Otherwise, on the high strings up the neck it's got quite a nice fuzzy/overdriven sound and is just audible enough to be useful can even hear a 'bit' of clean if I roll the guitar volume back.

Final question (for now) as the transformer is so very wrong (although I think i'm right in saying that as the speaker is 70ohm, yes 70, connected to an 8ohm speaker that it should help with the impeadance mismatch, that was my understanding of what I read) is there a suitable line transformer I can use, tried to research this but couldn't really get what wattage meant regards primary impeadance, of which I need 40k with 2 parallel tubes.

And yes the filaments are glowing orange, but it's hard to spot! About as thick as a piece of hair.

[iccaros]

these tubes were designed for hearing aids with 75K ohm and 80K ohm speakers. So yes, 40K is the math, but I do not think it matters at this low a wattage, I have used a line transformer to get 20K and I have not had a tube die..  but a simple 22K fender reverb transformer is cheep, and if you put 16ohms on the output, you would have 44K reflected, but I think 22K would work just as well in real life. may be louder.. for what that is worth.

[R.G.]

Ah! OK. Sagging main supply would make sense too. I've run into some things where there was a hump of some kind to get over before it operated; glad it's not that.

On the transformer (mis)match: The idea of transformer ratios really only applies in the transformer mid-band of frequencies, where neither the loss of bass due to too-low primary inductance nor loss of treble to too-high leakage inductance or too-high shunt capacitance cause a lot of divergence from ideal behavior. Using a transformer as you're using it could be expected to lose some bass, as the tubes "want" to see a 70K load, but the transformer primary inductance is designed for a 1K (right?) load and drive. So at mid band and higher, you get the expected operation, but the bass rolloff is a couple of octaves higher than it would be with the notionally correct transformer.

... as a guess... 

[Scruffie]

these tubes were designed for hearing aids with 75K ohm and 80K ohm speakers. So yes, 40K is the math, but I do not think it matters at this low a wattage, I have used a line transformer to get 20K and I have not had a tube die..  but a simple 22K fender reverb transformer is cheep, and if you put 16ohms on the output, you would have 44K reflected, but I think 22K would work just as well in real life. may be louder.. for what that is worth.

Line transformers are substantially cheaper and easier to obtain here, Fender ones can be about 3 times the cost, not that they're too expensive for an amp build but for a 21mW amp, expense that I would not find worth it. I am using a 1k miniature transformer here, fun that it works with the amp but sure something else would be better.

Oh I doubt it'll die or anything, just that from what PRR said i'll loose power and a little bit extra at this stage can't hurt.

Ah! OK. Sagging main supply would make sense too. I've run into some things where there was a hump of some kind to get over before it operated; glad it's not that.

On the transformer (mis)match: The idea of transformer ratios really only applies in the transformer mid-band of frequencies, where neither the loss of bass due to too-low primary inductance nor loss of treble to too-high leakage inductance or too-high shunt capacitance cause a lot of divergence from ideal behavior. Using a transformer as you're using it could be expected to lose some bass, as the tubes "want" to see a 70K load, but the transformer primary inductance is designed for a 1K (right?) load and drive. So at mid band and higher, you get the expected operation, but the bass rolloff is a couple of octaves higher than it would be with the notionally correct transformer.

... as a guess... 

Okay, interesting, I wouldn't expect much mids or bass anyway as this is just a cheap 60s celestion radio speaker i'm using, even missing the dust cap and with a few holes in it (I have one in good condition for later, at 30ohms).

Yup, it's a 1k cheapy radio shack transformer I had sitting around, if that's the case then, seeing as the bass & mmids sounds like crap anyway, doesn't seem like i'm missing out too much, quite happy with it to be trebley, what PRR said made me think i'd loose a fair bit of power.

Think now seeing as this actually sounds quite good with my EQ pedal boosting just the 1k6 & 3k2, the rest cut (other than poor decay, I can get some nice rock to hard rock tones at brilliantly low volumes, perfect for thin walls or as basically an alternative to earphones) I might buy a DM160 to work as a pre amp tube and give me some pretty lights and use one of the preamp 6088s for just a touch more output so I can lower it or raise it a bit if I wish to... 110mA of heater current plus whatever the chargepump and plates use then power it off 8 x AA cells for fun (still less power than a 386 amp at full blast if I recall?).

Thanks all for the help!

[iccaros]

I can send you two JAN6418's if you want to try those as preamps, and use all of your pentodes as power tubes.. just pm me..  I have a metric but tone, I am slowly... slowly working on using two to replace the opamps in a rebote.

[Scruffie]

I can send you two JAN6418's if you want to try those as preamps, and use all of your pentodes as power tubes.. just pm me..  I have a metric but tone.

Hmm... interesting, I did look at those tubes before I went with 6088s for higher mW output and higher max voltage but they could work nicely as preamps.
I just tried 3 tubes at the output with 1 in the preamp, things are getting better for gritty cleans so that could work nicely.

I'll see how experimenting goes and drop you a PM, thanks for the offer!

I am slowly... slowly working on using two to replace the opamps in a rebote.

Have a look at madbeans site, he was working on a PT2399 delay using a 6111 IIRC, might give you some ideas.



Dropping C3 & C4 on that schematic seems to have cured the flabby bottom end, i'm not sure on the math of those parts but I do understand the caps work as filters, no harm seems to be coming without them.
I also got 50V out the charge pump before a cap went and that really helped with the volume and tone so once i've ordered some higher rated caps and got it up to 63V instead of the current 41V i'm hoping this is pretty useable, especially with an enclosed speaker.

Edit: Using the right tap instead of the center tap brought the volume up a bit too 
Don't think i'll need any more output once I up the voltage 23volts, I just need to tune it now. But thanks for the offer on the tubes