Hi Guys
I'm in the process of designing a novel speaker/hi fi system. I'd happy to go into details of the speaker system, if anyone's interested. Meanwhile, as a speaker designer, my electronics knowledge is limited and I would be grateful for some help.
I have a requirement for a preamplifier with 6 inputs and 8 outputs (yup, you got that right). Actually, I would only use 5 of the outputs in Phase I but all of them are used at the same time. Everything is active, each of the outputs attaches to a separate (normally Type 'T') power amp.
Phase I
6 stereo outputs - all running together:
1. Main (full-range) speaker
2. Supertweeter
3. Bass speaker (subs)
4. NXT 'layered sound' panel - runs in parallel with main speakers (Google it; it works!)
5. Rear speakers to be connected in 'Hafler' mode via a delay processor
Phase II (if and when I can afford it)
The main stereo power amplifier (#1) will be substituted for 4 stereo SET amplifiers. (My full range speaker is based around 4x 2" drivers - each driver will be driven by an SET)
(Please continue to the next post where I explain where I need help)
I'm in the process of designing a novel speaker/hi fi system. I'd happy to go into details of the speaker system, if anyone's interested. Meanwhile, as a speaker designer, my electronics knowledge is limited and I would be grateful for some help.
I have a requirement for a preamplifier with 6 inputs and 8 outputs (yup, you got that right). Actually, I would only use 5 of the outputs in Phase I but all of them are used at the same time. Everything is active, each of the outputs attaches to a separate (normally Type 'T') power amp.
Phase I
6 stereo outputs - all running together:
1. Main (full-range) speaker
2. Supertweeter
3. Bass speaker (subs)
4. NXT 'layered sound' panel - runs in parallel with main speakers (Google it; it works!)
5. Rear speakers to be connected in 'Hafler' mode via a delay processor
Phase II (if and when I can afford it)
The main stereo power amplifier (#1) will be substituted for 4 stereo SET amplifiers. (My full range speaker is based around 4x 2" drivers - each driver will be driven by an SET)
(Please continue to the next post where I explain where I need help)
As my electronics knowledge and construction skills are relatively meagre, I thought that, as far as possible, I'd use the amplifiers that are available, ready assembled, on eBay. I suspect that with a little tube rolling, capacitor replacement, etc. they could sing nicely. My budget will extend to $US hundreds, not thousands.
Approach 1
My first thought was to use a nicely laid out passive preamplifier. I would use a stepped attenuator for volume (I'd prefer a transformer volume control but it is probably beyond my budget) and silver internal wiring.
The problem with this approach is that the output impedance wouldn't allow enough outputs running together. So I thought I would add a tube buffer stage (from eBay). If I go this way, should I connect the buffer before the preamp, after the preamp or before or after the volume control?
Approach 2
Here I would choose a tube preamplifier that I like the look of and add more inputs and output sockets. Do I just use splitters like 'Y' splitters for this or is it more complicated?
That's it! I'm happy to further explain anything that I haven't made clear and I look forward to reading your comments.
Regards
Steve
Approach 1
My first thought was to use a nicely laid out passive preamplifier. I would use a stepped attenuator for volume (I'd prefer a transformer volume control but it is probably beyond my budget) and silver internal wiring.
The problem with this approach is that the output impedance wouldn't allow enough outputs running together. So I thought I would add a tube buffer stage (from eBay). If I go this way, should I connect the buffer before the preamp, after the preamp or before or after the volume control?
Approach 2
Here I would choose a tube preamplifier that I like the look of and add more inputs and output sockets. Do I just use splitters like 'Y' splitters for this or is it more complicated?
That's it! I'm happy to further explain anything that I haven't made clear and I look forward to reading your comments.
Regards
Steve
> 8 outputs ....each of the outputs attaches to a separate (normally Type 'T') power amp.
There must be dozens of amps using the "Class T" trademark. The first one I found (Lepai LP-2020A+) has these specs:
Input impedance: 47k ohms
Input sensitivity: 200 mV
Eight 47K loads together is 5.8K.
ANY transistory preamp can drive 5K.
Especially to a mere 0.2 Volts.
This is Not A Problem.
A '741 or TL072 op-amp will easily do this job. A single transistor running 1mA will do it.
I used to drive two to fifteen recorders from one output hardly any beefier than common chip opamps.
> I like a little bit of 'tube' sound
A tube would not be my first thought when facing a 5K load. Also I wonder how much "tube sound" you can get at 0.2V signal level. A 12AU7 cathode-follower working from 100V-200V with a 250r-1K cathode resistor would be something to try, and simple. A hotter design would suffer less level-change as the number of connected amplifiers is changed.
There must be dozens of amps using the "Class T" trademark. The first one I found (Lepai LP-2020A+) has these specs:
Input impedance: 47k ohms
Input sensitivity: 200 mV
Eight 47K loads together is 5.8K.
ANY transistory preamp can drive 5K.
Especially to a mere 0.2 Volts.
This is Not A Problem.
A '741 or TL072 op-amp will easily do this job. A single transistor running 1mA will do it.
I used to drive two to fifteen recorders from one output hardly any beefier than common chip opamps.
> I like a little bit of 'tube' sound
A tube would not be my first thought when facing a 5K load. Also I wonder how much "tube sound" you can get at 0.2V signal level. A 12AU7 cathode-follower working from 100V-200V with a 250r-1K cathode resistor would be something to try, and simple. A hotter design would suffer less level-change as the number of connected amplifiers is changed.
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Thanks for that, PRR. It's good to know that my outputs are not a problem and any active stage will do.
Out of interest, my Class T amps are Topping brand. My main power amplifier, which drives the full-range speakers without filters or crossovers, is the Topping TP60 which I intend to modify by replacing the volume potentiometer with a fixed resistor and changing some capacitors.
As for the preamp, further thought has crystalised my thinking. I have an oldish valve (tube) based, modified Audcom AP-110 preamplifier that I rather like and it generates plenty of signal. I propose reboxing it as follows:
Separating the mains input and transformers and putting them in a separate enclosure.
The main enclosure will contain a means of controlling the volume - stepped attenuator or whatever - plus switches for the inputs.
Then the AP-110 preamplifier - plus outputs.
Is this basic topology correct?
Also, for the outputs, do I just use 'Y-type' splitters or are more components necessary?
With thanks
Steve
Out of interest, my Class T amps are Topping brand. My main power amplifier, which drives the full-range speakers without filters or crossovers, is the Topping TP60 which I intend to modify by replacing the volume potentiometer with a fixed resistor and changing some capacitors.
As for the preamp, further thought has crystalised my thinking. I have an oldish valve (tube) based, modified Audcom AP-110 preamplifier that I rather like and it generates plenty of signal. I propose reboxing it as follows:
Separating the mains input and transformers and putting them in a separate enclosure.
The main enclosure will contain a means of controlling the volume - stepped attenuator or whatever - plus switches for the inputs.
Then the AP-110 preamplifier - plus outputs.
Is this basic topology correct?
Also, for the outputs, do I just use 'Y-type' splitters or are more components necessary?
With thanks
Steve
Adding a Buffer to each output allows you to make connections to any output without affecting the performance of any other output.
Adding a Buffer also allows each output to drive an interconnect cable without having to think about lost treble.
Adding a Buffer also allows each output to drive an interconnect cable without having to think about lost treble.
Capacitance of cable to your loads is a real barrier to a vacuum tube output.
My PAS2 with 12AX7 output won't drive more than about six feet of RCA connector coax. More than that the highs get filtered off. And making sure my one solid state amp load has higher than 100k input resistance has been a real quest. 99.999% of the designs posted on solid state amp forum have 100k impedance or lower. My ST120 dynakit had it, but had major overheat problems plus soft crossover distortion.
Even with 600 ohm drive of an op amp, 12 feet of RCA cable caused my amp to pick up several radio stations simultaneously. I had to up the input filter of the amp from 22 pf to 68 pf to eliminate this RF interference.
Tube sound without the distortion of overdrive is, IMHO, a myth. And 12A*7 don't have the distortion characteristics of 6L6 or 6V6. I have managed though major modification to get a ST33078 op amp disco mixer to sound better than my 12AX7 equipped dynakit PAS2. I screwed up the PAS2 by replacing the GI wax sealed paper caps with polyester. It is now too bright and harsh. One paper cap, the tenth replaced, had the wax case burned off in 1961, causing major gain imbalence by 2009. Little details of design like the inductance of the caps affect sound IMHO more than the active element used.
Op amps are not created equal. The 4558 ones the disco mixer came with were hissy, whicn means IMHO the predecessor 741 was probably hissy. See Improving a "Disco mixer" to mid-fi performance - diyAudio
In the under $1 op amp category, an study using instruments made by a poster on this forum found 33078 the best, with NJM2068 a close second. I paid $.38 each for my ST33078 . See above thread for a couple of tricks to avoid oscillation with fast op amps (Ft>2 Mhz). I definitely had 1-2 mhz oscillation first pass of 4558 to 33078 swap.
In the $2 op amp category, LM4562 (NOT NJM) has many followers. Of course there are those $6 OPA things, which I haven't fooled with.
A LM4562 might drive 8 cables at 800-1000 pf per cable, but I think a separate lower drive current op amp for each would be more secure against one power amp cable interfering with another. Andrew just said that in post 9.
I find the $1.50 DIP package project boards from mcmeletronics.com makes the tiny packages suitable for point to point wiring with a 3mm wide chisel tip iron. This is the consumer division of farnell of UK. I don't know if the main UK warehouse carries these. If not, some place in France had some similar DIP project board. I find phosphor bronze contact DIP sockets less likely to block tiny input currents over a decade compared to tin or brass plate.
Have fun.
My PAS2 with 12AX7 output won't drive more than about six feet of RCA connector coax. More than that the highs get filtered off. And making sure my one solid state amp load has higher than 100k input resistance has been a real quest. 99.999% of the designs posted on solid state amp forum have 100k impedance or lower. My ST120 dynakit had it, but had major overheat problems plus soft crossover distortion.
Even with 600 ohm drive of an op amp, 12 feet of RCA cable caused my amp to pick up several radio stations simultaneously. I had to up the input filter of the amp from 22 pf to 68 pf to eliminate this RF interference.
Tube sound without the distortion of overdrive is, IMHO, a myth. And 12A*7 don't have the distortion characteristics of 6L6 or 6V6. I have managed though major modification to get a ST33078 op amp disco mixer to sound better than my 12AX7 equipped dynakit PAS2. I screwed up the PAS2 by replacing the GI wax sealed paper caps with polyester. It is now too bright and harsh. One paper cap, the tenth replaced, had the wax case burned off in 1961, causing major gain imbalence by 2009. Little details of design like the inductance of the caps affect sound IMHO more than the active element used.
Op amps are not created equal. The 4558 ones the disco mixer came with were hissy, whicn means IMHO the predecessor 741 was probably hissy. See Improving a "Disco mixer" to mid-fi performance - diyAudio
In the under $1 op amp category, an study using instruments made by a poster on this forum found 33078 the best, with NJM2068 a close second. I paid $.38 each for my ST33078 . See above thread for a couple of tricks to avoid oscillation with fast op amps (Ft>2 Mhz). I definitely had 1-2 mhz oscillation first pass of 4558 to 33078 swap.
In the $2 op amp category, LM4562 (NOT NJM) has many followers. Of course there are those $6 OPA things, which I haven't fooled with.
A LM4562 might drive 8 cables at 800-1000 pf per cable, but I think a separate lower drive current op amp for each would be more secure against one power amp cable interfering with another. Andrew just said that in post 9.
I find the $1.50 DIP package project boards from mcmeletronics.com makes the tiny packages suitable for point to point wiring with a 3mm wide chisel tip iron. This is the consumer division of farnell of UK. I don't know if the main UK warehouse carries these. If not, some place in France had some similar DIP project board. I find phosphor bronze contact DIP sockets less likely to block tiny input currents over a decade compared to tin or brass plate.
Have fun.
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On the subject of 5k = super easy load, well, I wouldn't say so. Of course an emitter follower at 1 mA could drive 1.5 V into that, though how comfortably is quite another matter. A quick sim gives me THD in the order of -60 dB, which is not exactly great. I would entrust such a follower (on e.g. +/-15 V) no less than about 20k. When you need 4 mA per channel to drive 5 kOhms at a measly 1.5 V well, that suddenly makes you appreciate op-amps a great deal... (A 5532 needs about that much current, but will drive 9 V into 600 ohms without major complaints.)
Guess what the PAS-3 used its extra tubes for? Cathode followers.
You might find the trick mentioned here useful:
http://www.lencoheaven.net/forum/index.php?topic=15150.msg221347#msg221347
470k load nominal? No surprise there.
Plan B, bodge in a source / emitter follower using some high-voltage MOSFET / BJT. Takes a good handful of components though, since the output already is ground referenced.
Here's a little video that might interest you, about identifying the outer foil end in modern-day film caps, a detail just about forgotten in the days of physically small components and low impedances.
https://www.youtube.com/watch?v=BnR_DLd1PDI
Note that the scope method generally requires a scope that's floating, i.e. not tied to protective earth.
It would be interesting to test whether a few hours/days in the oven (at non-wax-melting temperatures) would dry these buggers out again, but if it really is the paper going acidic that's the problem then it's probably hopeless. In any case, they're supposed to measure just like your average modern-day polypros when not bad for once - might as well use those.
For lower-level, lower-impedance circuitry, these days there are better options even in the jellybean class. NJM2068 has below 3 nV/sqrt(Hz), and NJM4580 is in the same vicinity and features pretty good output drive as well.
Looking at the PAS2 schematic, I'm not surprised that thing is a diva. It's a plate output plus a bunch of high-impedance tone control circuitry in the feedback. Feedback probably gets this down to a few kOhms, but there's a limit to everything.
Guess what the PAS-3 used its extra tubes for? Cathode followers.
You might find the trick mentioned here useful:
http://www.lencoheaven.net/forum/index.php?topic=15150.msg221347#msg221347
470k load nominal? No surprise there.
Plan B, bodge in a source / emitter follower using some high-voltage MOSFET / BJT. Takes a good handful of components though, since the output already is ground referenced.
Usually when that happens, the old caps were leaky (as these old paper caps generally are these days) and did funny things to DC operating conditions, like sending current into the grid and overbiasing the tube. Metallized polypro would be a better choice but still I don't really think it's the caps.
Here's a little video that might interest you, about identifying the outer foil end in modern-day film caps, a detail just about forgotten in the days of physically small components and low impedances.
https://www.youtube.com/watch?v=BnR_DLd1PDI
Note that the scope method generally requires a scope that's floating, i.e. not tied to protective earth.
Oh dear. The other ones may have been bad, but that one must have been really bad. If moisture was allowed to get in, leakage alone must have been astronomic.
It would be interesting to test whether a few hours/days in the oven (at non-wax-melting temperatures) would dry these buggers out again, but if it really is the paper going acidic that's the problem then it's probably hopeless. In any case, they're supposed to measure just like your average modern-day polypros when not bad for once - might as well use those.
Always depends on what your signal levels are! 4558s and 741s have around 8-10 nV/sqrt(Hz) of voltage noise density, back in the day that actually was "low noise". TL07x have more than that, LM358 a lot more (>40 if memory serves). Noise levels in that vicinity are entirely irrelevant in a line-level follower, they make for an OK if unspectacular MM phonopre or preamp input, but for a tape head preamp, MC prepre or dynamic mic pre you really want something else.
For lower-level, lower-impedance circuitry, these days there are better options even in the jellybean class. NJM2068 has below 3 nV/sqrt(Hz), and NJM4580 is in the same vicinity and features pretty good output drive as well.
My question relates specifically to my modified Audcom AP-110 valve preamp. I know it has plenty of gain but gather this may not be relevant to the question of outputs.
How can I determine whether or not my amp (as is) will happily drive these outputs? Is there a modification I could do that would enable this?
What is the output tube?
What does the tube datasheet say the drive impedance is? (12AX7 is 65000 ohms at 250 v, with 1.2 ma of plate current max)
What is impedance of your load? Figure out the length of each cable, either RCA/coax or phone plug/twisted pair. RCA/coax is ~110pf per foot. Parallel each with the impedance of the amp input, say 100k or 47kohms typical. Convert capacitance to impedance using the formula, then parallel them, unless you can do the vector algebra and parallel them the right way with complex arithmetic. For omega in the capacitance formula use 2*pi*20000 hz, since that is the highest frequency humans can hear.
Divide parallel impedance value into 2 v typical line level input voltage. That is the current required to drive all that.
Unless your preamp has a power tube on the output, suspect it will not drive all that. That is why we keep talking about op amps. 2000 ohm load ones start having trouble with about 12' cable and and input. This includes 4558, 5532 & 33078. I got radio interferance with 12' RCA cable driven by 33078 until impedance of amp end was lowered with a 68 pf ceramic cap. That cap has lower impedance in the megahertz range than it does in the audio range, due to the frequency in the cap impedance formula.
Lower output impedance op amps like NJM4580 and LM4562 listed above can drive more length than that. Peavey uses 4580 to drive 100 foot twisted pair from mixer in audience to on stage amps.
Have fun.
What does the tube datasheet say the drive impedance is? (12AX7 is 65000 ohms at 250 v, with 1.2 ma of plate current max)
What is impedance of your load? Figure out the length of each cable, either RCA/coax or phone plug/twisted pair. RCA/coax is ~110pf per foot. Parallel each with the impedance of the amp input, say 100k or 47kohms typical. Convert capacitance to impedance using the formula, then parallel them, unless you can do the vector algebra and parallel them the right way with complex arithmetic. For omega in the capacitance formula use 2*pi*20000 hz, since that is the highest frequency humans can hear.
Divide parallel impedance value into 2 v typical line level input voltage. That is the current required to drive all that.
Unless your preamp has a power tube on the output, suspect it will not drive all that. That is why we keep talking about op amps. 2000 ohm load ones start having trouble with about 12' cable and and input. This includes 4558, 5532 & 33078. I got radio interferance with 12' RCA cable driven by 33078 until impedance of amp end was lowered with a 68 pf ceramic cap. That cap has lower impedance in the megahertz range than it does in the audio range, due to the frequency in the cap impedance formula.
Lower output impedance op amps like NJM4580 and LM4562 listed above can drive more length than that. Peavey uses 4580 to drive 100 foot twisted pair from mixer in audience to on stage amps.
Have fun.
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Thanks indianajo.
I thought that the original tubes were 12AX7 and that they were rolled with 12AU7s (with some other component changes). However, the original amp spec says:
Model AP-110 Vacuum Tube Preamplifier (stereo) by Audcom Co. LTD.
This is a pure vacuum tube (high-end) preamplifier. It is especially designed for audiophiles, and is much improved over the previous AP-100 model for those who owned a high quality stereo power amplifier and are anxious to improve musical dynamics and sound space to experience a truly live concert. The AP-110 is tube based line stage with 18 dB of gain. It is a pure class A (single-end) with dual-triode featuring a pair of military grade 6N3PE tubes. Each dual-triode 6N3PE tube is used as two triode tubes for two stages of voltage and current amplifiers, respectively. With a little local feedback on the first stage it gives very good linearity and transient response, and without global feedback to keep tube tonality. It reaches 18 dB gain with a signal-to-noise ratio above 100 dB. It is magical! It uses high quality (audio grade) electrolytic and metallized polypropylene capacitors. Along with two top-quality transformers mounted far away from the circuit board and RCA input jacks, its filtering characteristic is very uniform across the frequency range and adds to the purity of the sound.
So, I'm not sure on the tubes and I will open it up later and check. Interconnects are generally of high quality and 1 to 2 feet long. Whatever the tubes or the extravagance of the above description, it is a sound that I like and will use if possible.
Meanwhile, does the description of the unmodified unit offer any help here?
With thanks
Steve
I thought that the original tubes were 12AX7 and that they were rolled with 12AU7s (with some other component changes). However, the original amp spec says:
Model AP-110 Vacuum Tube Preamplifier (stereo) by Audcom Co. LTD.
This is a pure vacuum tube (high-end) preamplifier. It is especially designed for audiophiles, and is much improved over the previous AP-100 model for those who owned a high quality stereo power amplifier and are anxious to improve musical dynamics and sound space to experience a truly live concert. The AP-110 is tube based line stage with 18 dB of gain. It is a pure class A (single-end) with dual-triode featuring a pair of military grade 6N3PE tubes. Each dual-triode 6N3PE tube is used as two triode tubes for two stages of voltage and current amplifiers, respectively. With a little local feedback on the first stage it gives very good linearity and transient response, and without global feedback to keep tube tonality. It reaches 18 dB gain with a signal-to-noise ratio above 100 dB. It is magical! It uses high quality (audio grade) electrolytic and metallized polypropylene capacitors. Along with two top-quality transformers mounted far away from the circuit board and RCA input jacks, its filtering characteristic is very uniform across the frequency range and adds to the purity of the sound.
So, I'm not sure on the tubes and I will open it up later and check. Interconnects are generally of high quality and 1 to 2 feet long. Whatever the tubes or the extravagance of the above description, it is a sound that I like and will use if possible.
Meanwhile, does the description of the unmodified unit offer any help here?
With thanks
Steve
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In any case that preamp will never straight drive 6 (even less 10) "Class T amps) for the very good reason that except for the full range duty one, all others will be driven through some kind of electronic crossover or buffer:
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Yes, Andrew, I noticed that 🙂
Anyway, it will have changed with the modifications. I'll check out the interior as soon as those nice Samsung people return my mobile from the repair shop. I can then take some photos when I open her up. I'll report back!
Anyway, it will have changed with the modifications. I'll check out the interior as soon as those nice Samsung people return my mobile from the repair shop. I can then take some photos when I open her up. I'll report back!
Good point. The actual equipment driven by the AP-110 preamp is:
5 stereo outputs - all running together:
1. Main (full-range) speaker - drives a Topping TP60 main amp directly. Later, it will drive 4 stereo KT120 SET tube amps as a Winter option!
2. Supertweeter - drives a smaller Topping TP22 directly. The supertweeter is then filtered passively (high pass) at 6dB/Octave.
3. Bass speaker (subs) - drives a DSPeaker Antimode 8033 S-II (room equaliser) and then to a BK Electronics BSBP 275 sub amp
4. NXT 'layered sound' panel - runs in parallel with main speakers - drives a Cambridge Audio A1 Mk 2 Special Edition (used this old integrated amp because it's an economical way to provide tone controls so I can cut the high treble and low bass)
5. Rear speakers to be connected in 'Hafler' mode via a delay processor - drives an Alesis Nanoverb digital effects processor (using the delay) and hence to another Topping TP22
I shouldn't have forgotten all that - DUMB! Also, it's 5 outputs not 6. Still, I hope that clarifies and possibly makes things easier.
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Okay, wikipedia has data on 6N3P, which is Americanization of 6H3Pi
Basic data
(per each triode)
Uf = 6.3 V, If = 350 mA, µ = 36, Ia = 7.7 mA, S = 4.9 mA/V, Pa = 1.5 W
The anode power limit is 33% higher than the TungSol 12AX7 at 1 W.
TungSol said you could get 1.3 ma out of the anode with 250 V plate voltage.
So even if your preamp has the output drawn by a capacitor off the plate, you've got about 2 ma to play with.
If you can actually make one foot long RCA plug coax cables, 6N3P might have a chance in **** of driving your 5 amps. It is only 600 pf maybe. I don't think you can buy one foot RCA plug cables.
It is worth a $25 experiment anyway. The schematic in post 5 looks plausible. Get the 5 resistors, get a 6 by 2 RCA jack panel, wire it up. Put it in a grounded steel box to keep the RF out. Box ground is safety ground, RCA rings are analog ground. Separate RCA rings from panel with O-rings. This avoids muddy mixed up ground paths. Preamp in one jack, out 5 other jacks, separated from each other by resistors.
With 4.7kohm resistors I don't think the five loads will damage your tube. 5 5 k in parallel, draws 2 ma @ 2 v signal. More resistance could get current down to 1.5 ma in tube spec. It might sound stupid, highs might disappear, but that is why we do experiments. If it sounds good, you win. If not, $25 down the drain.
Have fun.
Basic data
(per each triode)
Uf = 6.3 V, If = 350 mA, µ = 36, Ia = 7.7 mA, S = 4.9 mA/V, Pa = 1.5 W
The anode power limit is 33% higher than the TungSol 12AX7 at 1 W.
TungSol said you could get 1.3 ma out of the anode with 250 V plate voltage.
So even if your preamp has the output drawn by a capacitor off the plate, you've got about 2 ma to play with.
If you can actually make one foot long RCA plug coax cables, 6N3P might have a chance in **** of driving your 5 amps. It is only 600 pf maybe. I don't think you can buy one foot RCA plug cables.
It is worth a $25 experiment anyway. The schematic in post 5 looks plausible. Get the 5 resistors, get a 6 by 2 RCA jack panel, wire it up. Put it in a grounded steel box to keep the RF out. Box ground is safety ground, RCA rings are analog ground. Separate RCA rings from panel with O-rings. This avoids muddy mixed up ground paths. Preamp in one jack, out 5 other jacks, separated from each other by resistors.
With 4.7kohm resistors I don't think the five loads will damage your tube. 5 5 k in parallel, draws 2 ma @ 2 v signal. More resistance could get current down to 1.5 ma in tube spec. It might sound stupid, highs might disappear, but that is why we do experiments. If it sounds good, you win. If not, $25 down the drain.
Have fun.
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