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Old 8th January 2012, 10:04 PM   #1201
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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
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Old 8th January 2012, 10:20 PM   #1202
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by CSlee View Post
T My boss said that sometimes the contact on silver switches goes bad if less than 50mA is passing through it. News to me.

Until the next drama
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
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Old 8th January 2012, 10:28 PM   #1203
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Welcome back Gingertube.......



Quote:
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.
Donor amps are the way to go. Unfortunately I haven't found many jap amps that have ultra linear taps.


Brgds Bill
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Old 4th February 2012, 05:44 PM   #1204
rongon is offline rongon  United States
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Default 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.

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Old 4th February 2012, 06:57 PM   #1205
Yvesm is offline Yvesm  France
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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.
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Old 4th February 2012, 08:12 PM   #1206
rongon is offline rongon  United States
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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...

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Old 4th February 2012, 09:04 PM   #1207
Yvesm is offline Yvesm  France
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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.
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Old 4th February 2012, 09:51 PM   #1208
rongon is offline rongon  United States
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Quote:
since the cathodes will always be positive why not simply a diode between cathode and ground ?
It will be "off" after warm up !
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?



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Last edited by rongon; 4th February 2012 at 10:04 PM. Reason: added
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Old 5th February 2012, 09:58 PM   #1209
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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
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Old 6th February 2012, 01:54 AM   #1210
rongon is offline rongon  United States
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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?

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Last edited by rongon; 6th February 2012 at 02:08 AM.
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