Try the Rhopoint squaristor or econistor, from what i've read they are even better as i/v resistor.
+1
I have a set of econistors here in 20 ohm...will try them next...
The example schematic used for 12AX7 as an I/V converter shows 34ohm resistor used with the LM334 and also the anode resistors are changed from 24k to 120k.
Broskie *generally* recommends low mu tubes for the input. Isn't a 12AX7 considered a high mu? or would this be seen as an advantage in an I/V application?
I built an octal version and am currently using 6SN7s (equivalent to 6CG7) for both input and output, I/V resistors are 27ohm and I get just about enough gain into an F5 amp and I'm very happy with the sound. On days when I really want to crank it I could do with a little more gain so thinking about changing the resistor values or swapping in some 12AX7/5751 equivalents (12SL7?) in the input. Anyone have experience of 12AX7 in this position, or what are the downsides likely to be?
Thanks Blitz. So it sounds like the 6N23P is the happy medium?
I haven't yet tried any exotic I/V resistors. What about precision metal foils types?
This roll your own idea sounds interesting! I found a lacquered verson on-line but you have to buy 100s meters. This guy sells in small quantities and has a few different values 5Mtr. - Manganin wire Ø=0.06mm ; 155?/m | eBay
I asked about the lacquer: 0.06 to 0.17mm is lacquered, the 0.025 to 0.05mm is fabric coated.
I haven't yet tried any exotic I/V resistors. What about precision metal foils types?
This roll your own idea sounds interesting! I found a lacquered verson on-line but you have to buy 100s meters. This guy sells in small quantities and has a few different values 5Mtr. - Manganin wire Ø=0.06mm ; 155?/m | eBay
I asked about the lacquer: 0.06 to 0.17mm is lacquered, the 0.025 to 0.05mm is fabric coated.
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Hi Bud,
The change of the anode resistors from 24k to 120k for the 12ax7 is because the 12ax7 has a much higher internal resistance (70k).
The 12AX7 is certainly a high mu tube, but the voltage of the es9018 after i/v is very low(depends on the i/v resistor) something like 0.1v,
to get that to 2v we can use a high mu tube or change the value of the i/v resistor,
but the es9018 is much more happier with a low i/v resistor.
So that leaves us to high mu tubes.
High gain tubes are not all noisier than low gain, good examples are the 7025 and the 6n2p-ev, Here's a review of the 6n2p-ev (halfway, search for 6H2P)
Bad plate resistors can also be a source of noise.
Since you have an octal version, you could try the 6SL7/12SL7, they will give you more gain.
You'll also need to change the plate resistors to something like 82k.
To get an idea of the gain you get with different tubes, see the last pages in the aikido manual: octal, 9-pin
The change of the anode resistors from 24k to 120k for the 12ax7 is because the 12ax7 has a much higher internal resistance (70k).
The 12AX7 is certainly a high mu tube, but the voltage of the es9018 after i/v is very low(depends on the i/v resistor) something like 0.1v,
to get that to 2v we can use a high mu tube or change the value of the i/v resistor,
but the es9018 is much more happier with a low i/v resistor.
So that leaves us to high mu tubes.
High gain tubes are not all noisier than low gain, good examples are the 7025 and the 6n2p-ev, Here's a review of the 6n2p-ev (halfway, search for 6H2P)
Bad plate resistors can also be a source of noise.
Since you have an octal version, you could try the 6SL7/12SL7, they will give you more gain.
You'll also need to change the plate resistors to something like 82k.
To get an idea of the gain you get with different tubes, see the last pages in the aikido manual: octal, 9-pin
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Thanks for your schematic. You ended up with a very similar circuit to the Broskie I/V 12AXY, other than the CCS section where you use a different arrangement with diodes.
Also I note you paralleled up 100ohm I/V resistors. Is there an advantage in that over a single resistor on each line?
Danny, thanks for your reply and links. So the SL7 is the only next step up from SN7.
I am using dual mono PCM1794, data sheet says about 7.8mA so using in mono should be around 15mA and giving around 0.4V with my 27ohm I/Vs.
Like the ES9018, I read that the 1794 likes a low value of I/V too.
Your link suggest that I may be better of with a 9 pin input tube. I'd be happy to try it if it's worth it and otherwise I'd like to try some 12SL7s but, as this is my first project, I'm nervous about what other values need to be changed in the circuit. You suggest 82k plate resistors, are you able to advise what else should be adjusted to accommodate this please, or point me in the direction for how to calculate it? My B+ is 240V.
I just looked at the data sheet for 12SL7 and plate current is 2.3mA.
Any advice greatly received.
I am using dual mono PCM1794, data sheet says about 7.8mA so using in mono should be around 15mA and giving around 0.4V with my 27ohm I/Vs.
Like the ES9018, I read that the 1794 likes a low value of I/V too.
Your link suggest that I may be better of with a 9 pin input tube. I'd be happy to try it if it's worth it and otherwise I'd like to try some 12SL7s but, as this is my first project, I'm nervous about what other values need to be changed in the circuit. You suggest 82k plate resistors, are you able to advise what else should be adjusted to accommodate this please, or point me in the direction for how to calculate it? My B+ is 240V.
I just looked at the data sheet for 12SL7 and plate current is 2.3mA.
Any advice greatly received.
I did some more testing of the tubes - my dac's output is about 120mV using 100R resistors, so theoretically the 12AX7 should get it to around the 2vrms, however before I fiddle around with the 6.8R resistor controlling the LM334, I thought I'd test it with a 12AT7, because at least the 6.8R doesn't need to be de-soldered and the mu of 60 should be good enough.
As for the sound:
GE gray play 12AT7 - very low noisefloor, nice detail in the treble, bass a little muddy/bloated
Raytheon 12AT7WC (black plate, US made) - slightly noisier, though not annoyingly loud, excellent mids, better bass than the GE
The gain levels are fine for my purposes (speakers are 90dB efficiency, power amp is 3.5v sensitivity), so I won't bother changing to the 12AX7 set up as the 12AX7 is far more expensive and the gain will probably be too much if I change to a more sensitive power amp
I've left the Tung Sol 6CG7 as the output tubes, they're neutral, sound like 6SN7 and the transconductance is reasonable esp. as I'm using very short cables anyway
As for the sound:
GE gray play 12AT7 - very low noisefloor, nice detail in the treble, bass a little muddy/bloated
Raytheon 12AT7WC (black plate, US made) - slightly noisier, though not annoyingly loud, excellent mids, better bass than the GE
The gain levels are fine for my purposes (speakers are 90dB efficiency, power amp is 3.5v sensitivity), so I won't bother changing to the 12AX7 set up as the 12AX7 is far more expensive and the gain will probably be too much if I change to a more sensitive power amp
I've left the Tung Sol 6CG7 as the output tubes, they're neutral, sound like 6SN7 and the transconductance is reasonable esp. as I'm using very short cables anyway
Hi Bud,
I did a quick search for the optimal operating points for the 6SL7/12SL7 and according to kevinkr it is best at 1.5-2ma.
Here's another interesting 6SL7/12SL7 thread.
Let's take 1.8ma with a lowish 240v B+, to get 1.8ma you need to adjust the resistor of the LM334.
The 82K for the plate is a guess: Broskie uses 120K for the 12AX7, the 6SL7 has 2/3 of the internal resistance of a 12AX7, so 2/3 of 120k is about 82k.
The best way for finding the resistor values for the operating points is to simulate it with a Spice program like LtSpice, install the spice models, start from an existing spice tube circuit and adjust it to the Broskie Unbalancer.
That's how I started
I did a quick search for the optimal operating points for the 6SL7/12SL7 and according to kevinkr it is best at 1.5-2ma.
Here's another interesting 6SL7/12SL7 thread.
Let's take 1.8ma with a lowish 240v B+, to get 1.8ma you need to adjust the resistor of the LM334.
The 82K for the plate is a guess: Broskie uses 120K for the 12AX7, the 6SL7 has 2/3 of the internal resistance of a 12AX7, so 2/3 of 120k is about 82k.
The best way for finding the resistor values for the operating points is to simulate it with a Spice program like LtSpice, install the spice models, start from an existing spice tube circuit and adjust it to the Broskie Unbalancer.
That's how I started
Is anyone finding that V1/V2 and V3/V4 are getting slightly different filament voltages?
eg. on V1 and V2 (both 6n23p) I read 6.12v on V1 and 6.42v on V2. The plate/cathode readings also differ: 132.3v on V1 and 134.8 on V2.
I'd have thought each tube should be identical?
I have similar problems on V3 and V4.
On another note, normally to look at plate curves we can measure the voltage across the cathode resistor, then also take the plate voltage. This is ok for V3 and V4 (eg. I get 120v on the plates and 2.5v across the cathode resistors R13 and R14), however how do we take these measurements for V1 and V2 given there is a LM334 controlling the situation?
eg. on V1 and V2 (both 6n23p) I read 6.12v on V1 and 6.42v on V2. The plate/cathode readings also differ: 132.3v on V1 and 134.8 on V2.
I'd have thought each tube should be identical?
I have similar problems on V3 and V4.
On another note, normally to look at plate curves we can measure the voltage across the cathode resistor, then also take the plate voltage. This is ok for V3 and V4 (eg. I get 120v on the plates and 2.5v across the cathode resistors R13 and R14), however how do we take these measurements for V1 and V2 given there is a LM334 controlling the situation?
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I have neohm resistors in the four I/V positions. Do you think it would be worthwhile to add to other resistor positions? If so, which ones (please add the unbalancer position number).
Also, is there any need to replace the 0.22 uF wima caps in the C2 positions with higher quality film/foil capacitors?
Thanks
Kris
Also, is there any need to replace the 0.22 uF wima caps in the C2 positions with higher quality film/foil capacitors?
Thanks
Kris
Wow - long time since this thread was updated.
I am using neohm iv resistors, and recently changed R3,R4 and R13,R14 to Caddock MK132 which seemed to 'lift' the sound in terms of clarity and definition.
I settled on 6N23p-EV as input tubes and tungsram E88CC as output tubes. Hope this helps.
I am using neohm iv resistors, and recently changed R3,R4 and R13,R14 to Caddock MK132 which seemed to 'lift' the sound in terms of clarity and definition.
I settled on 6N23p-EV as input tubes and tungsram E88CC as output tubes. Hope this helps.
I am using the Janus regulators power supply. I think it delivers around 270 volts.
I used R3 = R4 = R13 = R14 = 300R
It works well because with 300R you can use any of the tubes in broskie's manual!
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