drtool
It might get loud In Houston
+ 3 on the LPS we had a shoot out with good old stock and nos. The LPS one hands down.
I know how you feel Ned.At this point in the game,unless I just ''happen upon'' some NOS in person,I won't waste my money.I firmly believe that much of the available NOS is all of the stuff that has been previously tried and rejected for various issues. Unless I can test a tube on my own equipment before buying,forget it.
Another excellent modern 12AX7 offering is the Sovtek 12AX7LPS. This and the JJ are my ''go-to'' tubes for this type,and they both seem to get a lot of love from other techs too.[/QUOTE]
I know how you feel Ned.At this point in the game,unless I just ''happen upon'' some NOS in person,I won't waste my money.I firmly believe that much of the available NOS is all of the stuff that has been previously tried and rejected for various issues. Unless I can test a tube on my own equipment before buying,forget it.
Another excellent modern 12AX7 offering is the Sovtek 12AX7LPS. This and the JJ are my ''go-to'' tubes for this type,and they both seem to get a lot of love from other techs too.[/QUOTE]
+4 on ARTS comments. The 12ax7LPS is a great tube in Phono sections. Pretty good in the rest too, but I'm not picky either. If it reads 75% or more on gm and the sides are matched to within a max of 5% I'll give it a try. And that's on MY tester. I get all my 12ax7's from Jim McShane, along with my output tubes and the matching has been absolutely great. 9 years of buying tubes from him, and not one failure, or an out of whack tube.
Larry
Larry
BinaryMike
Pelagic EE
One case that I recall vividly involved an FM tuner connected directly to a cable TV system, which had the cable shield tied to an earth ground at the point of entry into the building.
primosounds
Powered with pure tube sounds.
It is unfortunate that you have not been able to find good NOS 12ax7. I have found that even used OS ax7 and ecc83 are better in some of my vintage amps and integrateds than the new tube offerings. Like other "auction" buys you need to deal with trusted sellers that have a proven history. Then if you have an issue there is not a problem with resolving it.
Ned Sheats
New Member
Ned Sheats
New Member
A new wrinkle. This PAS2 does not like driving a transistor power amp. it looses 5 db overall and 9 db at 20 HZ. Its primary function will be driving the PPP 6as7 power amp for vinyl but since I like to sound off with the HES system now and then I wanted to also connect it to the yamaha 1050 . Any input I connect it to drops the output really bad so I thought of a single triode cathode follower to drive the HES amp external to and separate from the PAS@. Its poor little power transformer wont handle even one more filament. Any one have any experience or thoughts.
That's the right idea- an outboard buffer.
That's the main reason why I mod the PAS to have a single-gain-stage and cathode-follower output.
Though, with a cathode follower- you need to make sure you do NOT turn on the solid-state amp until the cathode follower is FULLY warmed up, and turn off the amp well before you turn off the cathode follower. This is because of the transient voltage on the output of a tube cathode follower will temporarily shift, as the output cap charges.
I am putting a "crowbar" output mute circuit into my latest PAS clone on my bench now- just so it will mute out the DC voltage shift transients at startup and shutdown. I'm going to use a little 555-based timer ($5 to $10 online from various sources) to control relays that short the outputs until the tubes warm up (about 20 seconds or so)...
Regards,
Gordon.
That's the main reason why I mod the PAS to have a single-gain-stage and cathode-follower output.
Though, with a cathode follower- you need to make sure you do NOT turn on the solid-state amp until the cathode follower is FULLY warmed up, and turn off the amp well before you turn off the cathode follower. This is because of the transient voltage on the output of a tube cathode follower will temporarily shift, as the output cap charges.
I am putting a "crowbar" output mute circuit into my latest PAS clone on my bench now- just so it will mute out the DC voltage shift transients at startup and shutdown. I'm going to use a little 555-based timer ($5 to $10 online from various sources) to control relays that short the outputs until the tubes warm up (about 20 seconds or so)...
Regards,
Gordon.
In stock form, no they don't like driving low impedance loads. The original design was set up expecting a 470K load to make the tone controls line up. You can pull a resistor off the output jacks to get it to drive a lower impedance load, but it still has a bottom limit.
External buffer is the way to go unless you want to beef up the power transformer, or eliminate the tube rectifier and switch to an LED pilot light. That *might* free up enough current for a cathode follower stage but it would still be borderline. Subminiature tubes could possibly be an answer for this, though I don't know if those would produce enough current off the cathode to drive a low impedance amp.
External buffer is the way to go unless you want to beef up the power transformer, or eliminate the tube rectifier and switch to an LED pilot light. That *might* free up enough current for a cathode follower stage but it would still be borderline. Subminiature tubes could possibly be an answer for this, though I don't know if those would produce enough current off the cathode to drive a low impedance amp.
BinaryMike
Pelagic EE
My preferred fix for this problem in PAS chassis is a built-in diamond buffer IC running open-loop and powered from the existing +/-11 VDC heater supply rails. For example, the BUF634 is still available in a conventional PDIP-8 package at this time: https://www.digikey.com/product-detail/en/texas-instruments/BUF634P/BUF634P-ND/251038
Ned Sheats
New Member
Thanks Mike.
Gordon I dont understand your point. Being a cathode follower if it comes on with the PAS2 (switched outlets) there wont be any voltage at the cathode till the tube warms up and it will be a gradual rise. THere will be a large cap isolating each output..probably..I havent done the numbers yet..somewhere around 1.0 ufd or more but it will also get charged gradually.Clearly I missed something somewhere.
Gordon I dont understand your point. Being a cathode follower if it comes on with the PAS2 (switched outlets) there wont be any voltage at the cathode till the tube warms up and it will be a gradual rise. THere will be a large cap isolating each output..probably..I havent done the numbers yet..somewhere around 1.0 ufd or more but it will also get charged gradually.Clearly I missed something somewhere.
With a 2.7uf cap on the output of the cathode follower, I have noted transient DC voltage of up to 15V for a second or two, with a "normal" solid-state amp impedance (about 40K) connected to the output of the preamp. Positive on startup, negative on shutdown.
That's no big deal if your driving a tube amp (because the tubes in the amp won't come up any faster than the tubes in the preamp)- but a SS amp (which comes on virtually instantly) will have issues, naturally. It makes my JVC bench test amp go into protect momentarily, if I don't wait about 15 seconds after turning on the preamp, before turning on the amp.
Regards,
Gordon.
maxhifi
Addicted Member
For a "quick fix", try putting a 0.25W 100k ohm 1% ohm resistor in series with the output, preferably right at the input to the power amp. This will form a voltage divided with the input resistor of the power amp, loading the preamp correctly at the expense of some gain.
If you put the resistor inside the RCA connector of your interconnect you can avoid modifying the amplifiers.
Usually old preamps have too much gain anyway, so this could be a viable solution, eapecially for occasional use, and is totally non invasive and almost free.
As for the transient issue during warm-up, just turn the preamplifier on a minute earlier than the power amplifier and this way the power amplifier Will never seeThe DC voltage
If you put the resistor inside the RCA connector of your interconnect you can avoid modifying the amplifiers.
Usually old preamps have too much gain anyway, so this could be a viable solution, eapecially for occasional use, and is totally non invasive and almost free.
As for the transient issue during warm-up, just turn the preamplifier on a minute earlier than the power amplifier and this way the power amplifier Will never seeThe DC voltage
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Ned Sheats
New Member
Your observation are great. I still cant picture how that 15 Volts would get to the cathode that fast and if it appears as the tube warms up it will be a gradual rise. The SS Amp is a Yamaha RXA1050 cap coupled inputs so as long as its a slow rise there shouldnt be any protection problem. Yes, if it were DC coupled and that 15 volts appeared it would probably end the speakers life since I am not sure any protection circuit would recognize that as a fault. It will be interesting when I test it on the old Technics I use on the bench..
Ned Sheats
New Member
The logic is to provide a flat frequency response into a low impedence solid state input from a high impedence tube amp down to between 10 and 20 Hz while still having enough voltage to drive the amplifier to full output. You might want to check the size of the coupling caps in any discrete bipolar transistor amplifier.
Ned Sheats
New Member
Thanks Max, all great ideas but a PAS2 preamp is great for freq response, noise, etc but it unfortunately, due to the lack of amy amplification after the tone controls, it doesnt have any gain to spare. I'm doing only the standard mods on the PAS2 such as removing the selenium rect and 12X4, replacing defective caps, and replacing the jack strips which are so worn they wont hold a standard RCA plug. When I get this tube ensemble finished I'll provide pictures..If I can figure out how to do it here.
maxhifi
Addicted Member
Hey, you sure about that? Manual says it has 20dB of gain on line inputs.. 20dB of voltage gain is a factor of x10 voltage gain.. Yes the tone controls do lose some gain, but there's a surplus of it before them, so the net result is still 20dB of gain you don't really need all of.
Considering a CD player puts out 2V and could on its own drive a power amplifier, it seems to me there's plenty of extra gain. Depending on your power amplifier, you may be able to lose 6 - 10 dB of gain without really noticing it.
With 65dB of gain for a moving magnet cartridge input, this should give you 7V RMS output with a cartridge capable of 4mV output. You could probably easily knock this down to. say. 1.5V out using a voltage divider and still be able to drive your power amplifier to clipping.
I think if the input resistor on the solid state power amplifier is in the 30k range, you could easily put 100k in series with the output of the PAS and still be able to get enough volts to drive your amplifier to clipping.
Another benefit to this method, is it substantially improves the noise floor of the system, which can really be a benefit if you use sensitive speakers.
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I don't think you need the corner frequency to be 1.8 hz. A 0.22 uF cap is more reasonable.