Happy new year to all - I'm now "back on deck" after 5 weeks away on leave.
While on holidays a tour of some juck shops turned up a couple of old tube amps, one is a "Trio" with EL84 PP outputs. It will have its output stage rebuilt to Baby Huey scheme.
Kenev,
The SVP2 is VERY similar to the FVP schematic on the Vaccuum State website.
Cheers,
Ian
While on holidays a tour of some juck shops turned up a couple of old tube amps, one is a "Trio" with EL84 PP outputs. It will have its output stage rebuilt to Baby Huey scheme.
Kenev,
The SVP2 is VERY similar to the FVP schematic on the Vaccuum State website.
Cheers,
Ian
This is very common on signal source switching....That is why switches are rotary IE self cleaning...or the use of sealed relay contacts..Just for interest on PLC control systems contacts used are loaded with resistors to overcome the contact resistance and stop / reduce oxidisation.. And its not just on silver contacts...have a look at BT equivalent relay contacts and types of metals used<<<these are used for the same the reason..
All just for fun... 🙂
Regards
M. Gregg
Welcome back Gingertube.......🙂
Donor amps are the way to go. Unfortunately I haven't found many jap amps that have ultra linear taps.
Brgds Bill
Donor amps are the way to go. Unfortunately I haven't found many jap amps that have ultra linear taps.
Brgds Bill
Extra simple version
Hi all,
I'm trying to talk a friend of mine into building a Baby Huey. He was going to build a standard 6DJ8 split-load inverter RC to EL84 UL amp, along the lines of the one in Valve Amplifiers. I suggested he give Baby Huey a try.
However, when I showed him the schematic, he complained about all the complexity of the source followers and CCS's. (He's transistor-phobic, and a DHT fan.) So, in an effort to convince him, I tried to simplify the circuit as much as I possibly could.
The goal is not to eek the maximum performance out of this, it's to get an idea of what the Schade feedback sounds like, and to get a pleasant sounding amp with minimal effort (lazy).
Can you give me your thoughts on the circuit I drew up (attached)?
Much appreciated.
--
Hi all,
I'm trying to talk a friend of mine into building a Baby Huey. He was going to build a standard 6DJ8 split-load inverter RC to EL84 UL amp, along the lines of the one in Valve Amplifiers. I suggested he give Baby Huey a try.
However, when I showed him the schematic, he complained about all the complexity of the source followers and CCS's. (He's transistor-phobic, and a DHT fan.) So, in an effort to convince him, I tried to simplify the circuit as much as I possibly could.
The goal is not to eek the maximum performance out of this, it's to get an idea of what the Schade feedback sounds like, and to get a pleasant sounding amp with minimal effort (lazy).
Can you give me your thoughts on the circuit I drew up (attached)?
Much appreciated.
--
Attachments
Forget the potentiometer beetween the cathodes (the whole schemo is "auto balancing") and replace the CCS for a single resistor 🙂
The higher the negative supply, the better. The resistor value should be 1000 times the Uneg. Here 24K Ohms will give roughly 1mA in the 12AX7.
So far, so good 😎
Yves.
The higher the negative supply, the better. The resistor value should be 1000 times the Uneg. Here 24K Ohms will give roughly 1mA in the 12AX7.
So far, so good 😎
Yves.
Thanks Yves.
OK, I'll gladly get rid of the pot in the 12AX7 cathodes.
Regarding the tail resistor, isn't it better to have as high impedance a load as possible there? The problem I've had in the past is that if you go crazy and put say -270V there, with a 270k resistor, there will be problems exceeding the heater-to-cathode voltage at turn on (before the tube starts conducting, so the cathode will be down at like -250V while the heater is at 6.3VAC or whatever). Is that a problem? Maybe the Broskie diodes from grids to cathodes would help? It would be easy enough to cobble together a -270V supply...
--
OK, I'll gladly get rid of the pot in the 12AX7 cathodes.
Regarding the tail resistor, isn't it better to have as high impedance a load as possible there? The problem I've had in the past is that if you go crazy and put say -270V there, with a 270k resistor, there will be problems exceeding the heater-to-cathode voltage at turn on (before the tube starts conducting, so the cathode will be down at like -250V while the heater is at 6.3VAC or whatever). Is that a problem? Maybe the Broskie diodes from grids to cathodes would help? It would be easy enough to cobble together a -270V supply...
--
Good point, but since the cathodes will always be positive why not simply a diode between cathode and ground ?
It will be "off" after warm up !
And, probably, few hundred volts at 2mA will be easy obtainable from the same winding than the one used for the B+ 😉
Yves.
It will be "off" after warm up !
And, probably, few hundred volts at 2mA will be easy obtainable from the same winding than the one used for the B+ 😉
Yves.
OK, so the diode would need to be rated for high voltage, like a 1N4007.
I've been trying to understand how the diode would work.
The more I think about it, the more confused I am... Where should the diode anode go? Where should the diode cathode go?
--
Last edited:
If possible (depending on the preamps drive capability) reduce that 470K resistor on the grid of the 12AX7 to say 47K - to keep the Miller Cap roll off as high as possible.
For the proposed resistive diff amp tail then a single diode should go from 12AX7 cathodes (Cathode of diode) to 0V (anode of diode) should do the trick. That clamps the cathode voltage to not more than -0.6 Volts during power up. Once up and running the cthaodes will sit at around +1.5 to +2.0 volts and that diode will be reverse biased.
A LOW CAPACITANCE diode will work best - use a 1N914 (1N4148) or similar rather than a big power diode.
Cheers,
Ian
For the proposed resistive diff amp tail then a single diode should go from 12AX7 cathodes (Cathode of diode) to 0V (anode of diode) should do the trick. That clamps the cathode voltage to not more than -0.6 Volts during power up. Once up and running the cthaodes will sit at around +1.5 to +2.0 volts and that diode will be reverse biased.
A LOW CAPACITANCE diode will work best - use a 1N914 (1N4148) or similar rather than a big power diode.
Cheers,
Ian
Thanks Ian.
Should be no problem to reduce the 12AX7 grid leak to 47k. The line amp is likely to have a 2k to 3k output Z.
So the diode is put in the opposite way to "LED bias". Instead of reverse biasing the diode for its +0.6V drop, it sits below ground until the 12AX7 warms up and conducts current. I think...
OK, since there's a +320V supply, a -270V supply could easily be made. Then a whopping 270k resistor could be put in the tails of the 12AX7. Since this will dissipate only the 1.5mA or so drawn by the 12AX7, it should only be necessary for it to be a 2W type, right?
270V(.0015A) = 0.4W
Leave lots of "headroom" so a 2W should be OK.
But what about the diode? Will it need to be able to handle about 320V DC of reverse voltage? If so, that would destroy a 1N4148, correct? Would a 1N4004 be OK instead? Or does that have too high capacitance (I think I see the problem. A capacitance there will unbalance the LTP at higher frequencies. Right?)
Looking at the data sheets, 1N4148 capacitance is 4pF, while 1N4004 is 15pF. Do you think 15pF would create a problem?
--
Should be no problem to reduce the 12AX7 grid leak to 47k. The line amp is likely to have a 2k to 3k output Z.
So the diode is put in the opposite way to "LED bias". Instead of reverse biasing the diode for its +0.6V drop, it sits below ground until the 12AX7 warms up and conducts current. I think...
OK, since there's a +320V supply, a -270V supply could easily be made. Then a whopping 270k resistor could be put in the tails of the 12AX7. Since this will dissipate only the 1.5mA or so drawn by the 12AX7, it should only be necessary for it to be a 2W type, right?
270V(.0015A) = 0.4W
Leave lots of "headroom" so a 2W should be OK.
But what about the diode? Will it need to be able to handle about 320V DC of reverse voltage? If so, that would destroy a 1N4148, correct? Would a 1N4004 be OK instead? Or does that have too high capacitance (I think I see the problem. A capacitance there will unbalance the LTP at higher frequencies. Right?)
Looking at the data sheets, 1N4148 capacitance is 4pF, while 1N4004 is 15pF. Do you think 15pF would create a problem?
--
Last edited:
Rogon,
The diode is only there to protect the tube at power up, before it has heated and begins to conduct. During this time the cathode will try to go to the full negative rail. The diode will clamp this to one diode drop below zero. Once the 12AX7 is conducting current the cathode will sit a little above 0V and the diode will be reverse biased, effectively out of circuit, except as you stated for its capacitance which will tend to compromise balance of the diff amp at higher frequencies. I would definitely go for a small signal or low power diode rather than a 1N400X series.
Note that reverse voltage rating is NOT a problem. During startup, when its doing its protection of the 12AX7 heater/cathode job it is forward biased. After startup there will be only a couple of volts (reverse bias) across it.
Using resistor power dissipated = I squared x R, dissipation will be about 0.6W. A 2 Watt resistor will be ideal.
Cheers,
Ian
The diode is only there to protect the tube at power up, before it has heated and begins to conduct. During this time the cathode will try to go to the full negative rail. The diode will clamp this to one diode drop below zero. Once the 12AX7 is conducting current the cathode will sit a little above 0V and the diode will be reverse biased, effectively out of circuit, except as you stated for its capacitance which will tend to compromise balance of the diff amp at higher frequencies. I would definitely go for a small signal or low power diode rather than a 1N400X series.
Note that reverse voltage rating is NOT a problem. During startup, when its doing its protection of the 12AX7 heater/cathode job it is forward biased. After startup there will be only a couple of volts (reverse bias) across it.
Using resistor power dissipated = I squared x R, dissipation will be about 0.6W. A 2 Watt resistor will be ideal.
Cheers,
Ian
Thanks again!
OK, here's what I got. Grid leak resistor is now 47k, -270V supply to the LTP tail, diode to clamp the cathodes to near ground potential while warming up. I hope it's workable now...
--
PS - Should the grid leak resistors for the EL84's be increased to 1M? Or is that hard on the EL84's?
OK, here's what I got. Grid leak resistor is now 47k, -270V supply to the LTP tail, diode to clamp the cathodes to near ground potential while warming up. I hope it's workable now...
--
PS - Should the grid leak resistors for the EL84's be increased to 1M? Or is that hard on the EL84's?
Attachments
Last edited:
There's no reason to make the input resistor so low. It doesn't affect frequency response.
All good fortune,
Chris
All good fortune,
Chris
That looks like a "go'er".
Coupling caps could be 100nF (3.3Hz -3dB point) instead of 220nF (1.5Hz -3dB point). If you have 220nF use them but the smaller 100nF value would be fine.
Run without global feedback unless the speakers really need it (leave out the 12K to 22K resistor). I've generally found I like that local feedback set resistor (15K to 33K) at 15K to 18K but feel free to experiment - Set the local feedback first without global feedback connected. As you increase this (local feedback) resistor you put more demand upon the diffamp front end. You might like to get the diffamp current up marginally (to 1.4 to 1,5 mA) by using 180K tail resistor or used a -320V rail and a 220K.
All that can be tweeked once up and running.
I have to (respectfully) disagree with Chris - that input resistor is in parallel with the 12AX7 Miller Capacitance and WILL affect the top end response. The main top end frequency limit in this basic BH incarnation will be the 12AX7 driving the EL84 Miller Capacitance. Thats why the "full bore" incarnations use a MOSFET Source Follower buffer between the 12AX7 and the EL84s.
Cheers,
Ian
Coupling caps could be 100nF (3.3Hz -3dB point) instead of 220nF (1.5Hz -3dB point). If you have 220nF use them but the smaller 100nF value would be fine.
Run without global feedback unless the speakers really need it (leave out the 12K to 22K resistor). I've generally found I like that local feedback set resistor (15K to 33K) at 15K to 18K but feel free to experiment - Set the local feedback first without global feedback connected. As you increase this (local feedback) resistor you put more demand upon the diffamp front end. You might like to get the diffamp current up marginally (to 1.4 to 1,5 mA) by using 180K tail resistor or used a -320V rail and a 220K.
All that can be tweeked once up and running.
I have to (respectfully) disagree with Chris - that input resistor is in parallel with the 12AX7 Miller Capacitance and WILL affect the top end response. The main top end frequency limit in this basic BH incarnation will be the 12AX7 driving the EL84 Miller Capacitance. Thats why the "full bore" incarnations use a MOSFET Source Follower buffer between the 12AX7 and the EL84s.
Cheers,
Ian
The driving stage's output impedance also appears in parallel at the amp's input. This input resistor only needs to be large for good compatability with other high impedance stuff (vacuum valve stuff) but it's easy enough to do.
Thanks,
Chris
OK Chris is quite correct.
I was taking a short cut to short circuit those who will see that resistor and substitute a same value pot. In that case the upper arm resistance of the pot will add to the driving stages output resistance and that is when we can get into problems. If using a pot stick to 47K (50K) maximum. For a single resistor you can use the higher values.
So my appologies to Chris.
Cheers,
Ian
I was taking a short cut to short circuit those who will see that resistor and substitute a same value pot. In that case the upper arm resistance of the pot will add to the driving stages output resistance and that is when we can get into problems. If using a pot stick to 47K (50K) maximum. For a single resistor you can use the higher values.
So my appologies to Chris.
Cheers,
Ian
Hi Everybody!
Which is the better PP topology of EL84? (with only one preamp tube)
-a triode amplifier and a phase splitter
-a differential amplifier
Which has a better sound quality??
And what do I have to modify if I want to use the amplifier in class-A push-pull?
Thanks
VD
Which is the better PP topology of EL84? (with only one preamp tube)
-a triode amplifier and a phase splitter
-a differential amplifier
Which has a better sound quality??
And what do I have to modify if I want to use the amplifier in class-A push-pull?
Thanks
VD
It doesn't seem like the 12AX7 should need grid stoppers at all, certainly not on the feedback side that is already fed with resistors (just keep those near the tube). In the current configuration resistance at the amp input combined with some added capacitance to ground for r.f. filtering can improve immunity to nearby a.m. broadcast, ham, and c.b. radio signals. The cap just needs to be small enough to not degrade audio bandwidth. Don't count on the miller effect for r.f. filtering because lower stage gain at radio frequencies (due to loading of next stage etc) lowers the effective miller capacitance. If the input pot is made smaller, the filtering effects of capacitance won't be so unpredictable (source and setting dependent). The idea is to avoid r.f. at the grid since the poor frequency response (at r.f.) in later stages won't remove stray audio that resulted from the r.f. being rectified (detected) at the input grid.
If one was willing to go with much lower input impedance and run in inverting mode, performance would be superior due to the lack of a common mode signal being handled by the input stage. Also, then the feedback would bring down any hum or microphonics from the input triode the major source of them, something it currently cannot do at all. Connected this way, the stress (and distortion) in the input stage is LOWER than open loop instead of worse, since there's only the smaller error signal being handled.
I know that some want really high input impedance to support preamps with weak output sections, but wouldn't it be better to fix those instead (adding cathode/source followers etc)? With a typical RCA cable having several hundred pF of capacitance rolling off high end, that alone should be enough reason to abandon working at high impedances for interconnecting.
If one was willing to go with much lower input impedance and run in inverting mode, performance would be superior due to the lack of a common mode signal being handled by the input stage. Also, then the feedback would bring down any hum or microphonics from the input triode the major source of them, something it currently cannot do at all. Connected this way, the stress (and distortion) in the input stage is LOWER than open loop instead of worse, since there's only the smaller error signal being handled.
I know that some want really high input impedance to support preamps with weak output sections, but wouldn't it be better to fix those instead (adding cathode/source followers etc)? With a typical RCA cable having several hundred pF of capacitance rolling off high end, that alone should be enough reason to abandon working at high impedances for interconnecting.
Last edited:
Is 330V much for 6P14P ? (8k ohm)
I know, the maximum allowed is 300V, but more 30V cause problem, or early die for tubes?
VD
I know, the maximum allowed is 300V, but more 30V cause problem, or early die for tubes?
VD
