Well I have 2 chokes on the way from Coffeedj, and will give them a try. I may try sheilding the chokes to tackle the hum problem. Were you ever able to check out that Parallel / henry issue with the chokes? Also, can I email you for some help on a BOM for the revision 2 schematic? I'm married to that version, as I have OD3's in hand, and want to try the optional power suppy that uses the 5u4GB rectifier. I have decided to build the pre, and 2 mono blocks concurrently. That way when I get stuck on one project, I can move over to another one!
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You'll have to excuse my suprise now, but with two coils, wound so that attaching them in series yeilds 300 Hy, putting them in parallel will yeild just a bit more than 75 Hy. The two coils won't couple perfectly, so this 'bit more' will be small v. the L-series/4 value.
OTOH, if you've got this working some other way, do tell us in enough detail so that we can duplicate your results.
cheers,
Douglas
OK--I've remeasured the coils using a LCR bridge meter at 1KHz. Assuming that you have the dual lead coils with the Green wrap, parallel the Blue & Blk leads, and the Red & White leads to achieve 20ma of current capability. In series mode you connect Red and Black and the chokes is between Blue and White; and you have 10ma of current capacity.
Measurements are:
Parallel mode: 80H (Spec is 75H)
Serial mode: 347 H (Spec is 300H)
I've noted about 5% variation in values between different chokes. The other thing to note is that this measurement is at 1KHz. It drops to about 1/2 the measured value at 30Hz. Hence, the reason for using two chokes--you would like to have about 40H minimum at 20Hz.
My supplier is making some new chokes with a larger core that should help the situation. Hopefully only one choke will be required, however, in listening tests with various users everyone liked the double choke sound over the 'single' choke sound (just a little bit!). This was even when playing a record with no bass--like a violin solo.
Hope this helps. See my next post: 12B4 kit almost ready!
Measurements are:
Parallel mode: 80H (Spec is 75H)
Serial mode: 347 H (Spec is 300H)
I've noted about 5% variation in values between different chokes. The other thing to note is that this measurement is at 1KHz. It drops to about 1/2 the measured value at 30Hz. Hence, the reason for using two chokes--you would like to have about 40H minimum at 20Hz.
My supplier is making some new chokes with a larger core that should help the situation. Hopefully only one choke will be required, however, in listening tests with various users everyone liked the double choke sound over the 'single' choke sound (just a little bit!). This was even when playing a record with no bass--like a violin solo.
Hope this helps. See my next post: 12B4 kit almost ready!
FINALLY!
The 12B4A line stage kit is shipping (beta version).
The latest schematic and panel layouts can be found at: 12B4A
I have two DIY'rs working on the beta version--if you are interested send me an email directly. I hope to have the final kit with chassis, pictures, etc. by mid-summer.
The final kit will ship standard for right around $1000 and have options for headphone transformer output, silver tube sockets, and a few other things. There will be a discount if someone wants to make their own chassis--in fact if you are of that ilk, I can ship the kit now. Still working out the bumps in the anodizing process for the chassis components.
A write up will be on OldStockAudio.com very shortly.
The 12B4A line stage kit is shipping (beta version).
The latest schematic and panel layouts can be found at: 12B4A
I have two DIY'rs working on the beta version--if you are interested send me an email directly. I hope to have the final kit with chassis, pictures, etc. by mid-summer.
The final kit will ship standard for right around $1000 and have options for headphone transformer output, silver tube sockets, and a few other things. There will be a discount if someone wants to make their own chassis--in fact if you are of that ilk, I can ship the kit now. Still working out the bumps in the anodizing process for the chassis components.
A write up will be on OldStockAudio.com very shortly.
hi Damian,
not sure if you still frequent these pages... i finally built the preamp! it was on hold for a while, i got the parts you sent me, after waiting many months for my ex to let me back into the house to retrieve them!
Only thing with this preamp is the constant ringing in the back ground, its as if the tubes are so sensitive they are picking up my fridge and passing it through a high pass filter... have you ever experienced this before?
stuart
not sure if you still frequent these pages... i finally built the preamp! it was on hold for a while, i got the parts you sent me, after waiting many months for my ex to let me back into the house to retrieve them!
Only thing with this preamp is the constant ringing in the back ground, its as if the tubes are so sensitive they are picking up my fridge and passing it through a high pass filter... have you ever experienced this before?
stuart
12B4 Ringing tubes
Glad you got it done!
Check for microphonic tubes. The 12B4’s can be a problem there. I use two dampers placed at the 1/3 points on each tube for my 12B4’s. I have heard that ringing—and dampers has always fixed it for me.
PS: I've made a few changes to the 12B4 design and it sounds even better now--nearly indistinguishable from my phono/line stage. I think you have those changes incorporated into your kit.
Glad you got it done!
Check for microphonic tubes. The 12B4’s can be a problem there. I use two dampers placed at the 1/3 points on each tube for my 12B4’s. I have heard that ringing—and dampers has always fixed it for me.
PS: I've made a few changes to the 12B4 design and it sounds even better now--nearly indistinguishable from my phono/line stage. I think you have those changes incorporated into your kit.
Thanks Damian & Sgregory,
I will hunt down some rubber rings and try them, such a sensitive preamp! I'm also considering trying different tube models, I'm not sure but the 12b4 may just be too clean for me, no doubt it is one of the cleanest most natural pre's i've heard. But i'm using some 10 inch tannoy golds, a leak stereo 20, a home made dac and a home made mac to deliver my music. I'm sure vinyl will return to my life soon but in the meantime I may like to try a 'warmer' tube in the linesatge.
Any recommendations? - would be great to find one that would work with 150V (od3,s) and use the same anode chokes that damian sent me....
cheers Stuart
oh and how many caps should I have on the cathode? would a bypass cap make a difference?
I will hunt down some rubber rings and try them, such a sensitive preamp! I'm also considering trying different tube models, I'm not sure but the 12b4 may just be too clean for me, no doubt it is one of the cleanest most natural pre's i've heard. But i'm using some 10 inch tannoy golds, a leak stereo 20, a home made dac and a home made mac to deliver my music. I'm sure vinyl will return to my life soon but in the meantime I may like to try a 'warmer' tube in the linesatge.
Any recommendations? - would be great to find one that would work with 150V (od3,s) and use the same anode chokes that damian sent me....
cheers Stuart
oh and how many caps should I have on the cathode? would a bypass cap make a difference?
Damping & Isolation for tube amps and Turntables
On isolation feet I have a very effective and CHEAP solution that I use for my phono player--may work here as well. I keep forgetting to share it with the DIY community. I'd be interested to know if others have tried this technique.
I take four pieces of 1" to 1.5" diameter car radiator type hose (thin wall is best for light loads) and cut to a length of about 1-2"--depending on the total weight to support. Cut a longer length of 1/2" sound isolation foam and jam into the interior of the hose so that it is packed tight. Use a heavy base like stone and set the four dampers under the four corners. The constructed dampers should flex about 1/4 to 1/2 of the diameter. If not--use thinner wall tubing or a heavier base. Place your amp/turntable on top and you are done.
The key is that the radiator style hose is bi-material (rubber and cotton) and thus interrupts the transmission of sound as any bi-material substance would. The foam adds a third material and dampens interior vibrations. With this technique and a 15 lb piece of marble I was able to isolate my turntable so that even a fist thump on the stereo stand did not transmit through the needle. You can buy this hose by the bulk for pennies a foot. My total cost was less than $5.
PS: I don't know if this will solve the 12B4 problem, as I think the issue is internal to the tube. Let us know what you find.
On isolation feet I have a very effective and CHEAP solution that I use for my phono player--may work here as well. I keep forgetting to share it with the DIY community. I'd be interested to know if others have tried this technique.
I take four pieces of 1" to 1.5" diameter car radiator type hose (thin wall is best for light loads) and cut to a length of about 1-2"--depending on the total weight to support. Cut a longer length of 1/2" sound isolation foam and jam into the interior of the hose so that it is packed tight. Use a heavy base like stone and set the four dampers under the four corners. The constructed dampers should flex about 1/4 to 1/2 of the diameter. If not--use thinner wall tubing or a heavier base. Place your amp/turntable on top and you are done.
The key is that the radiator style hose is bi-material (rubber and cotton) and thus interrupts the transmission of sound as any bi-material substance would. The foam adds a third material and dampens interior vibrations. With this technique and a 15 lb piece of marble I was able to isolate my turntable so that even a fist thump on the stereo stand did not transmit through the needle. You can buy this hose by the bulk for pennies a foot. My total cost was less than $5.
PS: I don't know if this will solve the 12B4 problem, as I think the issue is internal to the tube. Let us know what you find.
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Cheap low mu tubes
I was looking at compactron power tubes the other day and it turns out there are a fair number of high mu triodes with decently low Rp. Not the same thing--but definitely a thing to remember for future designs.
High load resistance is a real source of noise in early low signal gain stages and in tone controls.
Check out the 6AK10 and 6AC10 for example. There is a complete phono stage and line stage in a single tube.
I was looking at compactron power tubes the other day and it turns out there are a fair number of high mu triodes with decently low Rp. Not the same thing--but definitely a thing to remember for future designs.
High load resistance is a real source of noise in early low signal gain stages and in tone controls.
Check out the 6AK10 and 6AC10 for example. There is a complete phono stage and line stage in a single tube.
I use two OD3`s, one each channel, each running close 30ma. Just found out, that one of them is bad, and then they both stop glowing. I think the one left, could not stand the curent alone for two channels. Swap the fault OD3 get me up and running again 
The PSU is common for both channels. But then parallel to the choke input (each channel) i have regulation: R (91ohm) - OD3 - ground (one each channel).
I just dont understand why the serial resistors before the channels/regulators get so hot. Is a bad OD3 to ground a short circuit?
When the PSU sneering and resistor almost smoke, there must be a lot of current running throw ......
(sorry for my poor english...)
Cheers Jan
The PSU is common for both channels. But then parallel to the choke input (each channel) i have regulation: R (91ohm) - OD3 - ground (one each channel).
I just dont understand why the serial resistors before the channels/regulators get so hot. Is a bad OD3 to ground a short circuit?
When the PSU sneering and resistor almost smoke
, there must be a lot of current running throw ......(sorry for my poor english...)
Cheers Jan
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I'm thinking that your line voltage may be a little higher than the design. 30ma is quite a bit--20ma or less is about where those 0D3's should be operating. Design was for 15ma. Measure current through the OD3 through the 91 ohm, not the 470 ohm dropper. Current through the 470 ohm is total current to OD3 plus the 12B4. Can you recheck and make sure? Are you using C100 the voltage boost capacitor? If so, remove and recheck. That is only needed if your line voltage is low.
When one tube goes bad, and pulls too much current--it will keep the other tube from firing into regulation mode because the strike voltage is too low. If resistors are getting hot--yes too much current is being drawn. Let's make sure your supply voltage is correct first.
When one tube goes bad, and pulls too much current--it will keep the other tube from firing into regulation mode because the strike voltage is too low. If resistors are getting hot--yes too much current is being drawn. Let's make sure your supply voltage is correct first.
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