the grand return. 6CB5 triode PP
cleaning out the parts bin after about 8 years out of things. got a nice quad of 3D21A's in ST bottles that have been now through 3 moves or so and hopefully aren't Kaputt. same with a sleeve of 6bz5's that seem to be pretty hardy little 6DJ8 lookalikes. i had a few other tubes and options but these seem to be the best bet. i am in the states for vacation but am now based in Berlin so bringing larger amounts of transformers back in luggage or shipping them becomes a huge hassle. i have some no-name 6.6K PP OPT's sitting in Berlin, unfortunately no UL taps, but i figured this would be an OK use for them.
brushing off my load line skills and using some handy online calculators, it seems like i can get together a decent 2-stage amp that will more or less reach full volume with a standard 2 volt input. i'm a little fuzzy about how to draw load lines for CCS so i just drew the slope for the 6BZ7's based on the 33K load resistors, which is above the rule of thumb for 3 times the plate resistance that i remember from somewhere. same with the anode voltage. let me know if there are any glaring mistakes here.
my thinking is that if i build some compact monoblocks with a pair of nice potted 8H 200MA chokes i'll fly back over with, i can later drop in some more macho tubes if i start itching for more power. or go to pentode mode (no UL taps here unfortunately). but my speakers are some Klipsch RF-52 at 96dB efficiency so high power is not crucial for me.
http://images.klipsch.com/RF52IIcutsheet_635042119188100000.pdf
i guess my main questions are whether i'm completely wasting my time with these obscure tubes. they're pretty but for only 15 watts' worth of plate dissipation for a whopping 6.3V 1.7A heater, it's hardly much bang for the buck. 6.8 amps for power stage heaters is more than most single power transformers can handle, for example, so that's another reason to go monoblocks.
anyone have any opinions about linearity? seems like a fair amount of 2nd harmonic distortion, as with the driver, that will hopefully get cancelled out by the PP action.
power supply will be garden variety. 500VAC, choke input to 450, minus 20 or so with a 5AR4, then allowing say 20V of sag to reach a nice round 400V. and if it's running a tiny bit high i guess it won't be a crisis.
feels good to be back!
cleaning out the parts bin after about 8 years out of things. got a nice quad of 3D21A's in ST bottles that have been now through 3 moves or so and hopefully aren't Kaputt. same with a sleeve of 6bz5's that seem to be pretty hardy little 6DJ8 lookalikes. i had a few other tubes and options but these seem to be the best bet. i am in the states for vacation but am now based in Berlin so bringing larger amounts of transformers back in luggage or shipping them becomes a huge hassle. i have some no-name 6.6K PP OPT's sitting in Berlin, unfortunately no UL taps, but i figured this would be an OK use for them.
brushing off my load line skills and using some handy online calculators, it seems like i can get together a decent 2-stage amp that will more or less reach full volume with a standard 2 volt input. i'm a little fuzzy about how to draw load lines for CCS so i just drew the slope for the 6BZ7's based on the 33K load resistors, which is above the rule of thumb for 3 times the plate resistance that i remember from somewhere. same with the anode voltage. let me know if there are any glaring mistakes here.
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View image: 6bz7 triodemy thinking is that if i build some compact monoblocks with a pair of nice potted 8H 200MA chokes i'll fly back over with, i can later drop in some more macho tubes if i start itching for more power. or go to pentode mode (no UL taps here unfortunately). but my speakers are some Klipsch RF-52 at 96dB efficiency so high power is not crucial for me.
http://images.klipsch.com/RF52IIcutsheet_635042119188100000.pdf
i guess my main questions are whether i'm completely wasting my time with these obscure tubes. they're pretty but for only 15 watts' worth of plate dissipation for a whopping 6.3V 1.7A heater, it's hardly much bang for the buck. 6.8 amps for power stage heaters is more than most single power transformers can handle, for example, so that's another reason to go monoblocks.
anyone have any opinions about linearity? seems like a fair amount of 2nd harmonic distortion, as with the driver, that will hopefully get cancelled out by the PP action.
power supply will be garden variety. 500VAC, choke input to 450, minus 20 or so with a 5AR4, then allowing say 20V of sag to reach a nice round 400V. and if it's running a tiny bit high i guess it won't be a crisis.
feels good to be back!
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I have used both of these tubes, but not in the same amp.
You have the 3D21A. They have a 15 watt plate. I have the 3D21WB which have a 40 watt plate, but the internals are the same, so your tubes will make excellent triodes at a lower power level. I was getting about 75 watts from a pair in triode push pull. 15 to 20 watts should be no problem for the smaller version.
I had a bunch of those tubes, but all were used, and possibly abused pull outs from military scrap. Of the 6 or so that I had, only 2 actually worked. The others were gassy. Some would glow bright blue, while others would go into runaway at moderate power levels. The two good ones worked so well, that I bought 5 new ones for an amp that is still waiting to be built. It might be a good idea to bias up your tubes to see how well they work before building the entire amp.
Some info can be found in post # 331 of this thread.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-34.html
The 6BZ7 is a TV tuner tube. It is designed with a variable pitch grid so that the gain can be varied by changing the bias. It was also intended to be used in cascade circuits. It does not work well in common cathode gain circuits with large signal levels, but does quite well as an LTP input stage with a CCS in the tail.
Added some links in case the pictures aren't coming out big enough.
Tubelab, your posts on the WB's were a large inspiration for me in grabbing these tubes in the first place. They're prohibitively expensive in NOS condition, compared to the 20 GBP or so per piece with the A's, so I figured I'd try my luck. Really glad to see you giving a thumbs up though. Can't wait to bring my Unibits back to Germany and start cutting some aluminum!! I have these more or less locked in pure class A so I figure it might be a bit less than 15-20 watts but I have big hopes for the sound.
my friend Ali suggested putting a trimpot on the 10M45 gate instead of the 62 ohm resistor, seems like a reasonable idea.
Tubelab, your posts on the WB's were a large inspiration for me in grabbing these tubes in the first place. They're prohibitively expensive in NOS condition, compared to the 20 GBP or so per piece with the A's, so I figured I'd try my luck. Really glad to see you giving a thumbs up though. Can't wait to bring my Unibits back to Germany and start cutting some aluminum!! I have these more or less locked in pure class A so I figure it might be a bit less than 15-20 watts but I have big hopes for the sound.
my friend Ali suggested putting a trimpot on the 10M45 gate instead of the 62 ohm resistor, seems like a reasonable idea.
well, there are a couple of things to consider with the output tubes. i am using the 3d21 WB datasheet for my calculations. that is a much tougher tube built to take 3 times as much current, so the scale of the graph is off. i had to stretch it considerably in photoshop to try to get into the scrunched up area near the bottom.
i am prioritizing here as much clean swing as possible out of the amp. a -30v bias sits just under the plate dissipation maximum of 15 watts. another thing to keep in mind, as far as i know, is that the mismatch between positive and negative swing here is largely second harmonic distortion. this means that the imbalance here should be cancelled out by the opposite tube behaving just the same way.
i am absolutely biasing the 3d21A tubes in class A. taking a look at the operating points, you see that 30v is more or less in the middle of the possible swing here. it might dip a tiny bit more negative but we don't care about that.
i have individual constant current sources for each tube to make sure they are all pulling the same current. i would rather sacrifice a bit of voltage swing from mismatched tubes than have DC saturating the output transformer core. correct me if i'm wrong here, i'm a bit rusty with all of this, but the bypass capacitor "grounds" connected to each other and lifted from signal ground by a 1M resistor ensures that there is no net current flow into or out of the push-pull pair. the 72mA draw remains constant and simply alternates from one tube to the next.
i am prioritizing here as much clean swing as possible out of the amp. a -30v bias sits just under the plate dissipation maximum of 15 watts. another thing to keep in mind, as far as i know, is that the mismatch between positive and negative swing here is largely second harmonic distortion. this means that the imbalance here should be cancelled out by the opposite tube behaving just the same way.
i am absolutely biasing the 3d21A tubes in class A. taking a look at the operating points, you see that 30v is more or less in the middle of the possible swing here. it might dip a tiny bit more negative but we don't care about that.
i have individual constant current sources for each tube to make sure they are all pulling the same current. i would rather sacrifice a bit of voltage swing from mismatched tubes than have DC saturating the output transformer core. correct me if i'm wrong here, i'm a bit rusty with all of this, but the bypass capacitor "grounds" connected to each other and lifted from signal ground by a 1M resistor ensures that there is no net current flow into or out of the push-pull pair. the 72mA draw remains constant and simply alternates from one tube to the next.
I got mine for USD $20 each back when I was designing the driver board. That was a reasonable price, and just about my max for any tube. I grabbed all 5. They are obviously unused, but there are no brand marks or any other marks on the tube other than an etched # 7403 which is the Bendix number for the 3D21WB.
I have not tried the two CCS, bypass cap method of forced class A, but I have used a single CCS in the tail of a pair of output tubes in the same amp. Tweak the grid bias in one of the output tubes to achieve balance. Note that many OPT's themselves are not perfectly balanced, thus requiring a small offset in the tube balance to achieve minimum distortion.
I used a solid state CCS in the tail of a pair of 6L6GC's operating in class A triode. I got about 10 watts of some very nice sounding dynamic music. The amp used the same design that was developed in the 6L6GC AB2 thread. Three LTP's, a 6SN7 feeding another 6SN7, feeding the 6L6GC's wired as triodes. I never got around to trying the 7403's in class A before life events stalled most of my tube experiments for a few years.
I'm not sure exactly how you would do this, the gate needs to be slightly negative with respect to the source, and although not shown on the data sheet the 10M45 MUST have a stopper resistor in series with the gate or it WILL oscillate in the VHF range!
Yep thats exactly how it works and your doing it for exactly the right reason as well.
Don't scrimp on those caps, their effective value is halved and most people seem to choose woefully small values for this roll. i always use 1000uf and Allan Wright went to 8000uf in this position. They only need to be twice the voltage rating as the bias point so 64V caps should do OK.
Shoog
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Umm Soren, I mistakenly thought the 62R was the current set resistor but it is in fact the gate stopper, keep that. You need another resistor in series with the source in order to set the current in these devices. Check the datasheet to get a rough estimate of the current versus the resistance (62 ohms is actually a reasonable estimate) and then apply the trim treatment to taste. Thank you Tubelab for spotting the mistake!
Ah! Definitely a goof on my part then. Yeah 62 ohms was me shooting for around 36mA per device. so we'll definitely do a 1K stopper as well. I'll write up a clean version of the schematic with a basic PSU as soon as I'm done making some techno
I built two Mono blocks using 3D21B's Direct driven by 6BL7's with fixed bias.
I think the 3D21b can handle a lot more than 15 watts; I don't think the 3D21a would however. I still set the bias on the 3D21B's as if they can only handle 15 watts. I have not tested the amp for power output, but it can go awful loud on Two 15 inch RCF's per channel with JBl horns for the HF
I had no luck with 3D21WA or WB even brand new NOS tubes were gassed I have not tried the 7403's Yet.
Phil
I think the 3D21b can handle a lot more than 15 watts; I don't think the 3D21a would however. I still set the bias on the 3D21B's as if they can only handle 15 watts. I have not tested the amp for power output, but it can go awful loud on Two 15 inch RCF's per channel with JBl horns for the HF
I had no luck with 3D21WA or WB even brand new NOS tubes were gassed I have not tried the 7403's Yet.
Phil
as was famously said in Monty Python and the Holy Grail. "Not dead yet!"
Parts on the way for this sucker at last. monoblock construction using all my oddball parts. New manufacture power transformer from Ask Jan First (Z mount), US military potted choke, and some 60-watt-sized no-name eBay OPT's. Let's see what happens.
The nice thing about this is my power supplies are very overbuilt. Each PT is good for 200mA. By bridging a couple pins on my power tube sockets, all I need to switch over to 6L6/KT90 type tubes if I want more power is to rewire the heaters. Pulling an extra 10mA per tube sinks my supply a reasonable 10V.
PSUDII was a snap to get running in Wineskin.
Parts on the way for this sucker at last. monoblock construction using all my oddball parts. New manufacture power transformer from Ask Jan First (Z mount), US military potted choke, and some 60-watt-sized no-name eBay OPT's. Let's see what happens.
The nice thing about this is my power supplies are very overbuilt. Each PT is good for 200mA. By bridging a couple pins on my power tube sockets, all I need to switch over to 6L6/KT90 type tubes if I want more power is to rewire the heaters. Pulling an extra 10mA per tube sinks my supply a reasonable 10V.
PSUDII was a snap to get running in Wineskin.
Have you ever tried degaussing an OPT that exhibits this problem, to see if the DC balance setting for minimum distortion changes?
I have tried the age old method of hitting the primary with 240 volts AC, then slowly reducing the voltage to zero. Didn't make a difference. Of course I tried this with some budget OPT's, or a few vintage pieces of unknown quality.
I have a pair of 300 watt (at 20 Hz) Plitron toroidal OPT's that also want a slight imbalance (3 or 4 mA) to reduce saturation effects at real low freqs. My lowest test freq is 6 Hz from an old HP204C. Even if the OPT's were perfect, the tubes are not going to be matched in Gm over the entire range of current they will see in operation, so all we can do is minimize the effects.
I have a pair of 300 watt (at 20 Hz) Plitron toroidal OPT's that also want a slight imbalance (3 or 4 mA) to reduce saturation effects at real low freqs. My lowest test freq is 6 Hz from an old HP204C. Even if the OPT's were perfect, the tubes are not going to be matched in Gm over the entire range of current they will see in operation, so all we can do is minimize the effects.
For proper degaussing, I think it's necessary to drive the xfmr into hard saturation and then slowly back it down. 240VAC at 60Hz on the primary probably wouldn't achieve saturation with most OPTs, so this question might have to go unanswered for the moment. My concern arises from the possibility of permanent OPT core magnetization caused by output stage failures.
this might seem like a stupid question. but do i need a blocking capacitor between the bottom grid of the LTP and ground? i just realized my older amp schematic has one, and this example circuit here has one...
http://www.angelfire.com/electronic/funwithtubes/images/Amp-Long_Tail_Pair-1-A.gif
http://www.angelfire.com/electronic/funwithtubes/images/Amp-Long_Tail_Pair-1-A.gif
In that type of design, yes. The miller capacitance of the triode will result in a loss of high frequencies with a large value grid resistor. The circuit shown has a 470K ohm, so decoupling it to ground is needed. It runs the grids several volts above ground which is why you also have a blocking cap on the input.
If you have the LTP circuit where the cathodes are returned to a negative voltage through a resistor or CCS chip, and the input grid is ground referenced, then you tie the other grid to ground through a low valued stopper resistor.
If you have the LTP circuit where the cathodes are returned to a negative voltage through a resistor or CCS chip, and the input grid is ground referenced, then you tie the other grid to ground through a low valued stopper resistor.
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