Casey,
A high gm is needed to avoid slew limiting, particularly when loop NFB error correction signals are present. The gm of a single 'T7 section is larger than that of paralleled 'X7 sections.
The 5965 is a computer variant of the 12AV7. The mu is about 44. 12AT7 mu is about 70.
Bottom line, use 12AT7s as the Schmitt splitter/drivers.
A high gm is needed to avoid slew limiting, particularly when loop NFB error correction signals are present. The gm of a single 'T7 section is larger than that of paralleled 'X7 sections.
The 5965 is a computer variant of the 12AV7. The mu is about 44. 12AT7 mu is about 70.
Bottom line, use 12AT7s as the Schmitt splitter/drivers.
Anatech,
Generically speaking, this is good advice, but in my case..
1) I don't have anything to listen to it with right now, I still need to finish my table , and re-work my speakers.
2) I have other fish to fry (see 1), and I wan't this project to be completed first..I have a good bead on what vintage Fishers sound like, I've owned 2 of them in the past, so I should be able to tell (in general terms) if it is better or worse.
Eli,
Egg-suh-lent !
Ok,ok..I Shan’t utter 12AX7 and Schmidt in the same breath again
Generically speaking, this is good advice, but in my case..
1) I don't have anything to listen to it with right now, I still need to finish my table , and re-work my speakers.
2) I have other fish to fry (see 1), and I wan't this project to be completed first..I have a good bead on what vintage Fishers sound like, I've owned 2 of them in the past, so I should be able to tell (in general terms) if it is better or worse.
Eli,
Egg-suh-lent !
Ok,ok..I Shan’t utter 12AX7 and Schmidt in the same breath again
Nothing wrong with a doubler, properly designed.. . the only difference from a full-wave bridge is that the transformer is half the voltage and there's a bit more ripple current in the first caps. Same ripple, same regulation. Two diode "half-bridge" as used with tube rectifiers (and a few misguided solid state designs...) needs a transformer winding with a 40% higher rating to do the same.
Eli,
Ok, I read the thread and looked at the prints,
the first one...
http://www.diyaudio.com/forums/attachment.php?s=&postid=418336&stamp=1087396416
...requires 2 more tubes than are available, so that ones out. This one though...
http://www.diyaudio.com/forums/attachment.php?s=&postid=419692&stamp=1087569533
...raised my eyebrows. I have 1 bottle available per channel for the tone control. How do you think a paralleled 12AX7 would work here, or do you think this should be a 12AT7 as well? I thinkThorsten's circuit should be able to drive it.
Casey
Ok, I read the thread and looked at the prints,
the first one...
http://www.diyaudio.com/forums/attachment.php?s=&postid=418336&stamp=1087396416
...requires 2 more tubes than are available, so that ones out. This one though...
http://www.diyaudio.com/forums/attachment.php?s=&postid=419692&stamp=1087569533
...raised my eyebrows. I have 1 bottle available per channel for the tone control. How do you think a paralleled 12AX7 would work here, or do you think this should be a 12AT7 as well? I thinkThorsten's circuit should be able to drive it.
Casey
valveitude said:
Casey,
You give up too easily.
Notice that the circuit with 4 triodes per channel uses 2 of those triodes as cathode followers. Read "MOSFET Follies". Use ZVN0545A MOSFETs as the voltage followers. IMO, the FETs are a small improvement, as DC coupling is used between the gain triodes and the buffers.Casey,
FWIW, I'd like to see separate L/R 100 KOhm precision stepped attenuators in the 'T7 I/P grid leak positions used for volume and balance control. That eliminates a set of wipers in the signal path. It also allows reliable setting of a high pass filter, which protects the O/P trafos against core saturation due to an excessive loop NFB low freq. error correction signal. Cap. couple the selected source to the attenuators via 100 nF. parts. The turn over freq. is approx. 15.9 Hz. That's (IMO) a reasonable compromise between protection and bandwidth. If Noble or some other brand of pot. could be sourced with the requisite 1% tolerance, a cost saving might be realized without ruining filter reliability. 20% tolerance Alpha brand Carbon pots. will NOT do.
FWIW, I'd like to see separate L/R 100 KOhm precision stepped attenuators in the 'T7 I/P grid leak positions used for volume and balance control. That eliminates a set of wipers in the signal path. It also allows reliable setting of a high pass filter, which protects the O/P trafos against core saturation due to an excessive loop NFB low freq. error correction signal. Cap. couple the selected source to the attenuators via 100 nF. parts. The turn over freq. is approx. 15.9 Hz. That's (IMO) a reasonable compromise between protection and bandwidth. If Noble or some other brand of pot. could be sourced with the requisite 1% tolerance, a cost saving might be realized without ruining filter reliability. 20% tolerance Alpha brand Carbon pots. will NOT do.
So it is..my bad.
I really don't have the desire to implement a sophisticated tone control. It's going to be switched out the vast majority of the time anyway. Thanxs for the suggestion , but I'll either go with stock, or the simple anode follower circuit.
You and me both brother
...which brings up a question I would like to pose... Are reed relays ok for the audio path ? If not, why ? I got a whole slew of them somewhere (need to dig them out) and I was thinking the could be used in some attenuator/switching duties.Casey
Hey Eli,
In my quest for a reasonably priced 6KX8 (as elusive as the Unicorn) I came across this Rooski tube that may..I said may..be the same thing in a "Hammer & Sicle" wrapping. From the MachMat web site...
...sounds eerily familiar..no?
They go for $2.90 ea., and were made in the Voskod factory. So far a datasheet search has come up goose-eggs, but I will keep looking.
Thoughts ?
Casey
In my quest for a reasonably priced 6KX8 (as elusive as the Unicorn) I came across this Rooski tube that may..I said may..be the same thing in a "Hammer & Sicle" wrapping. From the MachMat web site...
...sounds eerily familiar..no?
They go for $2.90 ea., and were made in the Voskod factory. So far a datasheet search has come up goose-eggs, but I will keep looking.
Thoughts ?
Casey
Casey,
The triodes in the 6n2p are near equivalents of those in the 12AX7. IIRC, mu is 97 instead of 100 (BFD). The 6n2p pins out exactly the same as the 6922. There are open questions about section to section balance and noise factor.
This much I can say from direct experience, the Voshkhod made 6n1p is nice. No way is a 6n2p going to be as nice as a TFK or Mullard ECC808, but the Russian tube could easily be satisfactory.
On the affordable 6922 front, there are 2 reasonable choices. The JJ E88CC is voiced fairly close to W. European NOS. The EH 6922 is good from a noise factor and microphonics perspective.
The triodes in the 6n2p are near equivalents of those in the 12AX7. IIRC, mu is 97 instead of 100 (BFD). The 6n2p pins out exactly the same as the 6922. There are open questions about section to section balance and noise factor.
This much I can say from direct experience, the Voshkhod made 6n1p is nice. No way is a 6n2p going to be as nice as a TFK or Mullard ECC808, but the Russian tube could easily be satisfactory.
On the affordable 6922 front, there are 2 reasonable choices. The JJ E88CC is voiced fairly close to W. European NOS. The EH 6922 is good from a noise factor and microphonics perspective.
Eli,
Ya, looks like someone was trying to hype their wares. On another note..it seems you have access to some data on this tube, feel like sharing..I've still got bumpkis, inspite of my best search efforts.
Do you mean to say that this is an unkown, or are you saying that the unit to unit and noise specs are confirmed to be all over the map ?
That seems to be good news..from what I have read, it seems that the factory the tube comes from is critical .
Well, that would be asking to much. On the other hand, if its better than a TFK (which I have) or Mullard ECC88, and resonably close to the 808, then were talking. At the asking price of 3 bucks, I can buy 20 of this tube for the price of a single 808. I will probably be living with this unit for a while, and I like the fact that I can afford "backup" bottles if I go with the 6H2N.
Casey
Ya, looks like someone was trying to hype their wares. On another note..it seems you have access to some data on this tube, feel like sharing..I've still got bumpkis, inspite of my best search efforts.
Do you mean to say that this is an unkown, or are you saying that the unit to unit and noise specs are confirmed to be all over the map ?
That seems to be good news..from what I have read, it seems that the factory the tube comes from is critical .
Well, that would be asking to much. On the other hand, if its better than a TFK (which I have) or Mullard ECC88, and resonably close to the 808, then were talking. At the asking price of 3 bucks, I can buy 20 of this tube for the price of a single 808. I will probably be living with this unit for a while, and I like the fact that I can afford "backup" bottles if I go with the 6H2N.
Casey
So much has been covered in this thread up to this point, I thought I would post a summary of where the design sits right now…
Power supply:
*Wire a proper 3 wire power cord.
*Inrush limiters on HV winding.
*Replace exsisting diodes with Schottky devices.
*Up the main doubler caps to 330uf.
*Replace dial/tuning lamps with led’s (free’s up approx. 1 amp on the 6V filament string).
*Add a voltage doubler ( Schottky again) to the filament string for reg. 12V filament supply for phono section.
*Place low value resistor to filament string(s) to drop voltage back down to nominal (supply runs 9% higher due to 120V vs. 110V)
Power Amp:
*Regulate screen and bias on output tubes.
*Add small value resistor to cathodes for fusing and bias metering points.
*Replace OEM VA and phase splitter with a Schmitt spltter circuit.
Control Circuitry:
*Re-allocate a filter switch to perform tone bypassing.
*Replace stock tone control circuit with a anode follower Baxandall. (simpler and frees up a socket)
*Replace Volume/Balance controls with stepped attenuators.
Phono Circuit:
*Replace OEM circuit with Thorsten’s ECC83/ECC88 circuit, running TFK smooth plate in front followed by a UCC88 or similar 12V ECC88 variant.
*Run filaments from added reg. Supply.
*Tube reg. using available socket provided by tone control mod.
Whew…puts it in perspective listed like this.
If I missed anything discussed so far, please let me know, and if anybody can think of anything else I’d appreciate hearing it.
I would like to do some modeling of the power amp..has anybody made a spice model of a 7591? I can stumble my way through LTSpice, but I don’t trust myself to make a model.
Thanx,
Casey
Power supply:
*Wire a proper 3 wire power cord.
*Inrush limiters on HV winding.
*Replace exsisting diodes with Schottky devices.
*Up the main doubler caps to 330uf.
*Replace dial/tuning lamps with led’s (free’s up approx. 1 amp on the 6V filament string).
*Add a voltage doubler ( Schottky again) to the filament string for reg. 12V filament supply for phono section.
*Place low value resistor to filament string(s) to drop voltage back down to nominal (supply runs 9% higher due to 120V vs. 110V)
Power Amp:
*Regulate screen and bias on output tubes.
*Add small value resistor to cathodes for fusing and bias metering points.
*Replace OEM VA and phase splitter with a Schmitt spltter circuit.
Control Circuitry:
*Re-allocate a filter switch to perform tone bypassing.
*Replace stock tone control circuit with a anode follower Baxandall. (simpler and frees up a socket)
*Replace Volume/Balance controls with stepped attenuators.
Phono Circuit:
*Replace OEM circuit with Thorsten’s ECC83/ECC88 circuit, running TFK smooth plate in front followed by a UCC88 or similar 12V ECC88 variant.
*Run filaments from added reg. Supply.
*Tube reg. using available socket provided by tone control mod.
Whew…puts it in perspective listed like this.
If I missed anything discussed so far, please let me know, and if anybody can think of anything else I’d appreciate hearing it.
I would like to do some modeling of the power amp..has anybody made a spice model of a 7591? I can stumble my way through LTSpice, but I don’t trust myself to make a model.
Thanx,
Casey
I use inrush limiters at the AC input to the whole amp. That way the tube filaments are not abused as much at turn on.
I would suggest getting the amp running as it was designed, then spending some listening time before making modifications. When you are ready to begin modifications do one (or a few) at a time. This way you will know that you are starting with a working amp, and all of your components are good. Then you can assess your modifications as you make them. If you modify an unknown amp and it doesn't work, it will be hard to troubleshoot. If you make all of the modifications at once and there is a problem, you won't know what caused the problem. If the sound is not what you expected, then it will be very hard to figure out which mod is not doing what you expected.
As with any experiment try to minimize the number of variables.
I would suggest getting the amp running as it was designed, then spending some listening time before making modifications. When you are ready to begin modifications do one (or a few) at a time. This way you will know that you are starting with a working amp, and all of your components are good. Then you can assess your modifications as you make them. If you modify an unknown amp and it doesn't work, it will be hard to troubleshoot. If you make all of the modifications at once and there is a problem, you won't know what caused the problem. If the sound is not what you expected, then it will be very hard to figure out which mod is not doing what you expected.
As with any experiment try to minimize the number of variables.
Hello tubelab,
Makes sense...will do.
anatech,
...ookay..I give..sorta. Listing out the proposed mods in the post above, has given me pause
. The whole point was to get it up and going as quickly as possible.
Going through this exersise HAS laid out a roadmap for a nice not-Fisher amp that I can sneak up on later if I'm not happy with what I have.
tubeman,
How operational it is will determine how far I dive in. This is the Christmas present I gave myself...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5842719440
...You can see that , at a minimum, I need a new volume control. If its as advertised (always an "If") I will have a solid foundation. There are SOME things that will be done right out of the gate.
1) Shore up the power supply...soft recovery diodes, DC supply for all the filaments, inrush limiter, and possibly new caps(SDSLabs designed a capacitor board that fits under-chassis, and shares the power x-frmr hardware..I downloaded the plans). I'm still debating the screen reg...I'll probably do it.
2)NEW PHONO SECTION. I have no desire to live with a Fisher phono section, even for a little while..been there, done that..twice.This was one area that Fisher never..that I heard..got right. I will build the phono section descibed above. Its simple enough, I can probably do it in a day.
3)Recap the audio path. I've got an order on eBay with a dealer in Lithuania for a snout full of Russian military surplus PIO and silver mica caps on the way.
Now my question for this post...
I am having a hard time finding the Cree SiC Schottky's that Eli recommended, but Digi-Key sells HEXFREDS. I downloaded the data sheets, and they look good to me.
Can I go for the HEXFREDS in the HV supply (I'll still go Schottky where I can) and not loose any sleep ?
Casey
Makes sense...will do.
anatech,
...ookay..I give..sorta. Listing out the proposed mods in the post above, has given me pause
. The whole point was to get it up and going as quickly as possible. Going through this exersise HAS laid out a roadmap for a nice not-Fisher amp that I can sneak up on later if I'm not happy with what I have.
tubeman,
How operational it is will determine how far I dive in. This is the Christmas present I gave myself...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5842719440
...You can see that , at a minimum, I need a new volume control. If its as advertised (always an "If") I will have a solid foundation. There are SOME things that will be done right out of the gate.
1) Shore up the power supply...soft recovery diodes, DC supply for all the filaments, inrush limiter, and possibly new caps(SDSLabs designed a capacitor board that fits under-chassis, and shares the power x-frmr hardware..I downloaded the plans). I'm still debating the screen reg...I'll probably do it.
2)NEW PHONO SECTION. I have no desire to live with a Fisher phono section, even for a little while..been there, done that..twice.This was one area that Fisher never..that I heard..got right. I will build the phono section descibed above. Its simple enough, I can probably do it in a day.
3)Recap the audio path. I've got an order on eBay with a dealer in Lithuania for a snout full of Russian military surplus PIO and silver mica caps on the way.
Now my question for this post...
I am having a hard time finding the Cree SiC Schottky's that Eli recommended, but Digi-Key sells HEXFREDS. I downloaded the data sheets, and they look good to me.
Can I go for the HEXFREDS in the HV supply (I'll still go Schottky where I can) and not loose any sleep ?
Casey
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