Anyone by chance have or know where to find 12at7 poweramp specs. I have a 2 watt universal OP tranny and several 12at7s that need connecting. A schematic could be cool but bias mA, drive V and load R for both SE & PP would be gold. Anything extra on the idea would be nice as well. Please note I have built a fair # of amps and just want a small simple one that will not bleed an inverter battery so I can play guitar in the mountains without the need for SS😉
Thanks ahead.
Thanks ahead.
Hmmm
Well actually you probably are kinda sorta wasting your time with 12AT7's
now a 12AU7 or a 12BH7 is more of a tube and it will give you up to 5-7 watts Push pull respectively.Right now I'm working on a nice littel Guitar amp that shoud fit the bill...do you have a Single ended Transformer though?
~Ric Carpenter
Well actually you probably are kinda sorta wasting your time with 12AT7's
now a 12AU7 or a 12BH7 is more of a tube and it will give you up to 5-7 watts Push pull respectively.Right now I'm working on a nice littel Guitar amp that shoud fit the bill...do you have a Single ended Transformer though?
~Ric Carpenter
Here's a URL for the 12AT7 spec. sheet (http://www.pmillett.addr.com/tubedata/HB-3/Receiving Tubes Part 2/12AT7.PDF).
Power amp or voltage amp, you still have to respect the tube's limits and characteristics. Rp is fairly high at 11 KOhms. IMO, 2 sections PSE or 2 "bottles" PPP will make setting the O/P trafo up easier.
Hope this helps the cause.
Power amp or voltage amp, you still have to respect the tube's limits and characteristics. Rp is fairly high at 11 KOhms. IMO, 2 sections PSE or 2 "bottles" PPP will make setting the O/P trafo up easier.
Hope this helps the cause.
Re: Hmmm
Hi Ric, Thanks for that! A 12AU7 would be fine, have a few of those as well. I have a universal OP that is advertised as both SE or PP. It is the smallest Hammond but still made as atube OP(125A) Rated 2watts. I do not want alot of power, prefer one tube watt
I intend to play it in a very small area and own lots of big guns. I would either like to use both sides of one tube as the PP amp or run both sides of one tube in parallel.
Thanks again.
Adam Perry
DrDiode said:
Hi Ric, Thanks for that! A 12AU7 would be fine, have a few of those as well. I have a universal OP that is advertised as both SE or PP. It is the smallest Hammond but still made as atube OP(125A) Rated 2watts. I do not want alot of power, prefer one tube watt
I intend to play it in a very small area and own lots of big guns. I would either like to use both sides of one tube as the PP amp or run both sides of one tube in parallel.Thanks again.
Adam Perry
Eli Duttman said:
Thanks for that Eli 🙂
I figered on the 12AT7 cause I know it works great in reverb and FX loops. I have lots of ATs, AUs and AXs. I know none of them are great OP tubes but I don't want great. I need low power consumption, prefer just 2 dual triodes including preamp in Mono.
So how do I change plate R to Load R. I have a few manuals but none of them list poweramp specs for preamp tubes.
I just dug up my OPT. It is 3 watts, 25ma & @ 8ohms has 9 taps from 2500 to 22800, 11600 being the closest match to 11000.
Thanks again, much appreciated. Never thought of writting PPP before, sure is easier than PP Parallel😉
Adam\
PS by "big guns" I hope everyone knows I meant a 4x12 and 100watts to play with.
The "rule" is that the load should be 3X Rp. IMO, you will do better with PP than SE with your OPT. Use a single 'T7 and wire the traffo for the max. load it can provide.
If the sections of the 'T7 are TIGHTLY matched, you could use cathode bias via a shared cathode resistor/bypass cap. combo. A way to deal with poorly matched 'T7 sections is to employ Dennis Boyle's combination bias. Each 'T7 cathode "stands" on an individual, unbypassed, 100 Ohm resistor. Separate 10 KOhm bias adjustment "pots" allow you to set each triode to exactly the same idle current. The 100 Ohm cathode resistor value is not "cast in concrete", but it makes the idle current math easy. 220 Ohm parts give you more cathode bias. You don't want the cathode resistor value to get large, as the O/P impedance rises along with the degeneration. Constructing the bias supply is not difficult. A low current 12.6 V. filament trafo feeding a full wave voltage doubler made from 2X UF4007s will do the job. 100 muF. 25 WVDC 'lytics in the doubler "stack" are PLENTY good.
If the sections of the 'T7 are TIGHTLY matched, you could use cathode bias via a shared cathode resistor/bypass cap. combo. A way to deal with poorly matched 'T7 sections is to employ Dennis Boyle's combination bias. Each 'T7 cathode "stands" on an individual, unbypassed, 100 Ohm resistor. Separate 10 KOhm bias adjustment "pots" allow you to set each triode to exactly the same idle current. The 100 Ohm cathode resistor value is not "cast in concrete", but it makes the idle current math easy. 220 Ohm parts give you more cathode bias. You don't want the cathode resistor value to get large, as the O/P impedance rises along with the degeneration. Constructing the bias supply is not difficult. A low current 12.6 V. filament trafo feeding a full wave voltage doubler made from 2X UF4007s will do the job. 100 muF. 25 WVDC 'lytics in the doubler "stack" are PLENTY good.
Eli Duttman said:
Right on Man and good stuff! I love to learn new rules but only to do with tubes😀
So if I go for the combination bias what would the idle current be across the 100 ohm at idle and even more important how do you determine this from the info at hand*
Please kind Sir, I want some more 😱
Oh and the reason I am leaning towards parallel connection is that I would not need a PI. At very most I plan to use 3 tubes but would like to try 2 first(again battery powered inverter dictated). Maybe even run strait single SE so I have 3 stages of gain with just 2 tubes. One 12ax7 feeding either 12at or au?
*Given the link that you generiously provided I would love to learn how to convert standard curves & specs to Poweramp specs. ie, now I know load=3xRp, thanks. How do I figer ideal idle current and drive V for AB1???
This is kinna important to me as I also have a box of old 6BG6s and again do not have Poweramp specs for this tube. Sorry for being a Manual and practical directed builder so far but I hope to learn some math here if you please🙂
adamamp said:
6BG6 is a 6L6 modded for horizontal output (i.e. rebased + plate cap). Tons of circuits abound. 🙂
Tim
Sch3mat1c said:
Nice circuit but I need something small. A couple of ?s for you about it if you don't mind. I have only seen the series plate Resistor in PPP amps. Do the 470 ohm Rs somehow make up for the 12AU7s match for OP tube use? To me, and I only have practical exp., they would help balance a PPP amp but just eat power in a PP amp? Why go for the 12AU in the preamp? I am not sayin it aint so just asking please🙂
But really I am leaning towards just SE so I can have 3 gain stages before the OP.
I would avoid the interstage transformer* cause it is not something I have in the bin, so to speak😉
*VOX style pigtail has become my standard for guitar. FWIW I have always used a Williamson driver PI among other things in my HiFi stuff.
adamamp,
Your Hammond O/P traffo does not have an air gap. That's why I think PP is better than PSE.
For PP, a 2 mA. idle current in each triode causes operation to be Class "A" until power O/P gets fairly "high", then the stage operates in Class "AB". Ohm's Law NEVER fails; adjust the bias pot. for a 0.2 V. drop across the 100 Ohm cathode resistor.
A Schmidt (differential) phase splitter has gain and makes the application of NFB very easy. A single twin triode with "matched" sections is what you use. Connect the P/S cathodes to the "legs" of a 100 Ohm pot. Connect the wiper of the pot. to a stack of red LEDs for bias (2 V./LED). Connect the LED stack to a CCS made from a Supertex DN3545 depletion mode MOSFET. Between the CCS and the 100 Ohm pot., setting the P/S to have "exact" balance is easy. Assuming 100 KOhm grid leak resistors in the P/S, connect a 1 MOhm resistor between the "hot" lead of the OPT secondary and the non-inverting triode's grid. Ground the "cold" lead of the OPT's secondary. That arrangement will provide a tiny amount of NFB.
Let's see, you said 3 "bottles". 1 twin triode for the phase splitter and 1 twin triode for the "finals" leaves a "bottle" available. A 3rd twin triode could be both a gain block and a tone stack. I do believe you have the makings of a "flea" power guitar amp.
Your Hammond O/P traffo does not have an air gap. That's why I think PP is better than PSE.
For PP, a 2 mA. idle current in each triode causes operation to be Class "A" until power O/P gets fairly "high", then the stage operates in Class "AB". Ohm's Law NEVER fails; adjust the bias pot. for a 0.2 V. drop across the 100 Ohm cathode resistor.
A Schmidt (differential) phase splitter has gain and makes the application of NFB very easy. A single twin triode with "matched" sections is what you use. Connect the P/S cathodes to the "legs" of a 100 Ohm pot. Connect the wiper of the pot. to a stack of red LEDs for bias (2 V./LED). Connect the LED stack to a CCS made from a Supertex DN3545 depletion mode MOSFET. Between the CCS and the 100 Ohm pot., setting the P/S to have "exact" balance is easy. Assuming 100 KOhm grid leak resistors in the P/S, connect a 1 MOhm resistor between the "hot" lead of the OPT secondary and the non-inverting triode's grid. Ground the "cold" lead of the OPT's secondary. That arrangement will provide a tiny amount of NFB.
Let's see, you said 3 "bottles". 1 twin triode for the phase splitter and 1 twin triode for the "finals" leaves a "bottle" available. A 3rd twin triode could be both a gain block and a tone stack. I do believe you have the makings of a "flea" power guitar amp.
LOLEli Duttman said:
Good stuff Eli OK on the PP. That air gap, is it needed because of the cornea(SP?) affect?
If I used a well balanced tube and a common cathode R, what would the value of a common R be? Sorry but I would just like to keep constuction simple and really like the sound of open loop cathode biased guitar amps. If I do not use a cathode C this way I think it stays pure class A?!?
I may have to use a Concertina Splitter again for ease of constuction but also cause I would like to have 3 stages of preamp. Two stages for gain and a cathode follower fed tone stack. The tone stack is just IME more responcive when fed by the follower and is well worth the stage I feel😉
Thanks for all the input really 😎
SE OPTs need to be air gapped because the standing DC current in the primary will saturate the core without the gap. The air gap is why SE OPTs are much larger for a given wattage than PP trafos are.
If you use cathode bias via a common resistor, you need to calculate the value from the idle current of both triodes passing through the resistor and the bias voltage needed (from the spec sheet) via Ohm's Law.
Omitting a bypass cap. across a cathode bias resistor has no effect on the class of operation. Unbypassed cathode resistors introduce local current NFB that raises both the I/P and the O/P impedances, as well as lowering distortion.
The class of operation of a stage is governed by how the tubes are biased. A Class "A" stage is biased 1/2 way to cutoff. A Class "B" stage is biased to cutoff. A Class "AB" stage is biased > 1/2 way to cutoff, but not to cutoff.
If you use cathode bias via a common resistor, you need to calculate the value from the idle current of both triodes passing through the resistor and the bias voltage needed (from the spec sheet) via Ohm's Law.
Omitting a bypass cap. across a cathode bias resistor has no effect on the class of operation. Unbypassed cathode resistors introduce local current NFB that raises both the I/P and the O/P impedances, as well as lowering distortion.
The class of operation of a stage is governed by how the tubes are biased. A Class "A" stage is biased 1/2 way to cutoff. A Class "B" stage is biased to cutoff. A Class "AB" stage is biased > 1/2 way to cutoff, but not to cutoff.
adamamp said:
They are to balance current in the superparallel arrangement, but actually are superfulous. I didn't know it at the time. The cathode resistors in fact provide 10 times more current-spreading action despite being half the value.
You mean my preamp which I've posted elsewhere? Or the preamp in this circuit? (In which case I assume you meant "why not go for...")
Personally I'd avoid the IST for technical reasons. The PP balance in the above circuit happens to go circular...erm... 90 degrees apart when you have one set of grids on horizontal and the other on vertical, on the scope. Anyway, around 18kHz I think, I observed such phasing.
Other things like overall phase and frequency response make them tricky devices at best when enclosed in a loop, and even when not.For SE, come to think of it, a circuit like this may be more suitable. (Doh, on editing it got saved in transparent format.) In fact 6SN7 is close enough to 'AU that you shouldn't have to change any values. 🙂
Tim
Attachments
Eli Duttman said:
Right on and thanks sincerely for all your help,
now I will try 🙂
idle A =2mA x 2 sides of PP = 4ma total
bias V for 2ma at 300V from plate curves = -6v
R=V/I
R=6/.04
R=150 ohm.
Is that it?
If so for future refence how did you determine the idle A?
Am I correct that max drive V = 2xbias V so up to 12?
Real😎Thanks again Eli!
Sch3mat1c said:
They are to balance current in the superparallel arrangement, but actually are superfulous. I didn't know it at the time. The cathode resistors in fact provide 10 times more current-spreading action despite being half the value.
You mean my preamp which I've posted elsewhere? Or the preamp in this circuit? (In which case I assume you meant "why not go for...")
Personally I'd avoid the IST for technical reasons. The PP balance in the above circuit happens to go circular...erm... 90 degrees apart when you have one set of grids on horizontal and the other on vertical, on the scope. Anyway, around 18kHz I think, I observed such phasing.
For SE, come to think of it, a circuit like this may be more suitable. (Doh, on editing it got saved in transparent format.) In fact 6SN7 is close enough to 'AU that you shouldn't have to change any values. 🙂
Tim
Yes the cathode bias/balance arrangement looks very simular to the one I used cloning a Williamson Musicians Amp and it was very effective.
Silly me, I ment poweramp. Why use a 12au7?
I would prolly reserve IST for class B Hi Power/ LowFi 😉
That is another nice circuit and thanks for the input but for now I need to go with PP and just one OP tube. I really dig the idea of a 1 tube watt amp.
Even got a name for it, the Flea Flicker
PS I looked over your Site. Is there a circuit diagram for the SS Tesla Coil?
1. I like the AU 🙂 2. I have a lot of them 3. The OPT shown just happened to be the right impedance for around 7 of the things 4. It was new and weird!
The problem with a 1-tube amp is to get that output, you'll need a really high Z primary. Alternately you can try class AB2, but the AU's grids are very high perveance (as grids go). Good for power rectification, bad news for signal driving.
Tim
The problem with a 1-tube amp is to get that output, you'll need a really high Z primary. Alternately you can try class AB2, but the AU's grids are very high perveance (as grids go). Good for power rectification, bad news for signal driving.
Tim
Sch3mat1c said:
Thanks again Tim,
I like new and weird! Thats why I dig your SS Tesla coil. One day I want to build one using a long bank of 807s!
Ok on the AU/SN. I dig them too! My first DIY poweramp used 2x5U4, 4x807 and 3x 6SN7. Basically a Williamson clone but with regulated HV(OP screens on down) and DC filaments. Even had switchable Pentode/Troide that only blew up once
I checked the specs on 6SN7/12AU7+6SL7/12AX7 a while back researching and cloning old Ampeg amps. I wanted to get away from octal based preamp tubes for cost, size and availabilty reasons. Still have lots of SNs but ran out of SLs🙁
As for my max OPTr Zpr,
@4ohms sec/27000ohms prim
@8ohms sec/22600...
Thats pretty high, I think😕😱😕
So is the idle mA and load Z the same for the AU as the AT?
So if 7 PPP AUs =6600 primary Z does that mean the ideal Z for one PP would be 46200 and with my OPTr I should really go for the 8ohm/22600 tap with 2x OPTu for almost perfect match for PPP hook up? How much and what kind of a differience does the ideal Z make?
?s?s who is this guy🙂
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