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Old 10th April 2008, 10:03 PM   #1
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Default Recommend an input stage triode please

Attached schematic. Basic ST-70 iron, variant of El Cheapo, but I wanted a little additional gain, hence the input stage. Don't want gobs of feedback, just enough to give a decent damping factor. Amp will be 6dB HP filtered at about 60Hz.

So which tube is recommended for the gain stage? Tubes I have on hand are few: 6SN7, 12AT7, 6N6P, 7119. This amp is not worth the 7119, IMO, and the SN7 might be considered a waste of a good tube given the ST-70 iron, but they are easy to get. 6N6P is a nice little tube, but likes a lot of current/lower voltages. Tough to get from my 420V B+.

Any dual octal or 9 pin triodes are welcome. No AX7 or AU7 please. THX.
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Old 10th April 2008, 10:08 PM   #2
SY is offline SY  United States
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ECC81 or 6SL7. Go with the classics.
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Old 10th April 2008, 10:20 PM   #3
G is offline G  United States
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How about a 6C8G? Nah. Too rare. It seems like a 5670 would be a good candidate. Never mind. Not enough gain from either. Sy's suggestion sounds like the way to go.
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Old 11th April 2008, 12:09 AM   #4
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Default Re: Recommend an input stage triode please

Quote:
Originally posted by zigzagflux
Attached schematic. Basic ST-70 iron, variant of El Cheapo, but I wanted a little additional gain, hence the input stage. Don't want gobs of feedback, just enough to give a decent damping factor. Amp will be 6dB HP filtered at about 60Hz.

So which tube is recommended for the gain stage? Tubes I have on hand are few: 6SN7, 12AT7, 6N6P, 7119. This amp is not worth the 7119, IMO, and the SN7 might be considered a waste of a good tube given the ST-70 iron, but they are easy to get. 6N6P is a nice little tube, but likes a lot of current/lower voltages. Tough to get from my 420V B+.

Any dual octal or 9 pin triodes are welcome. No AX7 or AU7 please. THX.

Dude,

That's a Mullard style circuit in your drawing. It's "classic" and certainly is a reasonable option.

You can use "El Cheapo" topology, with the NFB connected to the non-inverting triode of the 'T7 splitter by "fronting" it with a common cathode 6SN7 section whose cathode resistor is unbypassed. Such a 6SN7 section is highly linear and is just fine, without loop NFB being applied. As the cap. coupling the voltage gain triode to the splitter is outside the NFB loop, a Soviet surplus PIO part could work out well.
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Old 11th April 2008, 02:38 AM   #5
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Eli:

Yes, I recall you mentioning something like that in a previous thread of mine. How would I compensate the amplifier for high frequency response? Would I keep the paralleled RC across the 6SN7 plate resistor?

Also, wouldn't this method require a negative supply?
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Old 11th April 2008, 03:59 AM   #6
Sheldon is offline Sheldon  United States
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I had a nice commercial EL34 UL amp with 12AT7 on the input and 12BH7 for the splitter/driver. That was before I started building amps, so I don't know how the feedback was applied, but the distortion profile was very good.

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Old 11th April 2008, 04:16 AM   #7
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Quote:
Originally posted by zigzagflux
Eli:

Yes, I recall you mentioning something like that in a previous thread of mine. How would I compensate the amplifier for high frequency response? Would I keep the paralleled RC across the 6SN7 plate resistor?

Also, wouldn't this method require a negative supply?

Yes, "El Cheapo" topology requires a negative rail. You can get that without any additional "iron" by SS rectifying the B+ and voltage multiplying the 5 VAC winding of the ST70 power trafo.

The "textbook" way to obtain phase compensation uses a cap. in parallel with the NFB resistor. An o'scope is used to tweak the value of the compensating cap., for good square wave performance.

My technique for phase compensation is brute force, but it works reasonably well. Use inductive WW resistors to load the phase splitter anodes. That peaks HF gain. Short the NFB loop out above 80 KHz. by connecting a mica cap. of approx. 150 pF. between the "hot" wire of the O/P trafo and ground. This method relies on the natural HF roll off of the O/P trafo to bring things into alignment.

High gm small signal types are indicated in an amp with loop NFB. High gm provides resistance to HF error correction signal induced slew limiting. Both the 6SN7 and the 12AT7 are reasonably high gm.
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Old 11th April 2008, 08:33 AM   #8
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Quote:
The "textbook" way to obtain phase compensation uses a cap. in parallel with the NFB resistor. An o'scope is used to tweak the value of the compensating cap., for good square wave performance.
At least it was until Crowhurst demonstrated why that might not be such a good approach.

"Understanding HiFi Circuits" has a clinical dissection of that means of compensation.
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Old 11th April 2008, 03:59 PM   #9
GordonW is offline GordonW  United States
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I was just looking around... and was checking out the 5842/417A for this.

Looks like it could be the "mack daddy" of input tubes. Will work great on low B+ (150V or less), which can work well with the direct-coupled Mullard style phase splitter.

Cgp is only .55pf! Cg is about 9pf... but that part DOES NOT MULTIPLY with gain! For a gain of say, 40, in the input, the 5842 would only have about 30pf effective input capacitance, while a ECC81 would be over 60pf, and the 6SL7 would be over 110pf! This means the 5842/417A would have TWICE the bandwidth of the ECC81, and close to FOUR TIMES the bandwidth of the 6SL7 at the same input driving impedance!

Also, with a plate resistance of only about 1.8K (!) and a transconductance of 24K micromhos (!!), it could drive the crap out of ANYTHING.

As long as a gain of no more than30-40 would work on the input stage, this could be a REAL contender. These tubes aren't cheap-cheap... but I note they can be had for only about $25 each, from some common places...

Regards,
Gordon.
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Old 11th April 2008, 05:05 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by GordonW
I was just looking around... and was checking out the 5842/417A for this.

Looks like it could be the "mack daddy" of input tubes. Will work great on low B+ (150V or less), which can work well with the direct-coupled Mullard style phase splitter.

Cgp is only .55pf! Cg is about 9pf... but that part DOES NOT MULTIPLY with gain! For a gain of say, 40, in the input, the 5842 would only have about 30pf effective input capacitance, while a ECC81 would be over 60pf, and the 6SL7 would be over 110pf! This means the 5842/417A would have TWICE the bandwidth of the ECC81, and close to FOUR TIMES the bandwidth of the 6SL7 at the same input driving impedance!

Also, with a plate resistance of only about 1.8K (!) and a transconductance of 24K micromhos (!!), it could drive the crap out of ANYTHING.

As long as a gain of no more than30-40 would work on the input stage, this could be a REAL contender. These tubes aren't cheap-cheap... but I note they can be had for only about $25 each, from some common places...

Regards,
Gordon.
Hi Gordon,
I like and use the 5842 a lot, so much so in fact that I now have pretty close to a life time supply in my stash. The one potential caveat is they like to operate at very high plate currents for the best linearity - I generally run them at 10mA and beyond which could be a problem in an ST-70 type design unless allowances are made for the current. I would say using the 5842 would call for the use of the 5687 as a splitter, again at relatively high plate currents.. Great bandwidth and drive capability - would drive quite a few EL34 in parallel with ease. Borrow the 5-20 topology as the basis for this design. Oops I guess I have drifted away from the original intent of the thread..
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