DIY Audio Projects Forum

Hi ... to get more gain I'd like to try to use a 6N6P instead of ECC802S.
Anybody tried to do it before? Any suggestion about changes?
Thanks

Why do you think you'll have more gain? You'll have to rise current through a 6N6P to be linear and it'll work better to interconnect's capacitance, that is all.
I used the tube in SRPP version as a driver for 300B, but connect it with a CCS in the top cathode resistor position to increase gain.

I have been studying this circuit and am wondering what function the 3k9 resistor on the grid of the upper valve performs. I apologize if this has been answered, I may have missed it, but I have gone through the entire thread without success thus far.
Thanks for any insight regarding this.
Jazzzman

Hi, All grid stoppers have much the same function....preventing oscillation. Or to some extent in gain stages ringing and overshoot. The mechanism is the same. The resistor and internal capacitance of the tube form a filter. It is possible to leave them out (many folks do), but my scope shows better high performance with them in there. Without them the top end goes up into the lower RF ranges and sensitivity there is not wanted. By essentially reducing the high frequency response of both sections it may also make them track together closer. I don't know if I could prove that though.

The other question regarding more gain....not usually needed in a line stage preamp. Actually the gain is on the high side for one. However there is no reason the circuit can not be used with other tubes. With changes in the cathode resistors you can use 12AT7, 12AX7, 5751, 6/12SL7 and being careful with the B+ 6DJ8s all work. For 12AX7 at 200V 910R at 300V 1200R, 5751 at 200 820R at 300V 1200R, 6/12SL7 the same as 5751. I don't recall what values I used on the others. I would not use the 6DJ8 as an SRPP much above 200V as it is moderately easy to harm on or the other triode on start up. You naturally can use other values, and many will work, just these seem to be the sweet spots IMO. Remember that the Z out will be higher for most of them. You can use any of the ECC/EC clones of the tubes as well.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Hi all! This is my version of this project with rectifier tube and also choke. I powered it up yesterday and it works. No listening yet. Just checking voltage. I havent got my ECC802S tubes yet so I used a spare 12AU7 from another project.

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Tube interested and open for new projects always and too often...

Hi, what should the 5k trim pot be rated for on the LR8’s? I had a 1/2 watt there and it failed in about a month of listening. Not sure if it was under rated.

Thanks

Hi, Clearly something wrong with it. I use 1/4 watt ones. You can of course, measure it when you get the voltage right and then replace it with a fixed resistor.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

That’s what I ended up doing. Both channels came in between 213-214V.

Thanks

Hi again. I have now got my ECC802S and the sound is wonderful. No ground loops or 50Hz or multiples are detected. Just a sort of low "wite" noice. Its not going up with volume. I have tried with better/more capacitance at the B+ (B+ is around 212-214V) but nothing happens and also tried more capacitance at DC heaters but no result. (DC heaters measure 6,4VDC)
Is it possible to be noicy resistors? Or the outer grounding of the Alps Blue? The resistors are 0,6W metalfilms and the ones in first parts of powersupply is wirewounded..Maybe weak Catode bypass caps? I dont need them (extra 3dB)so is it better to get rid of them?
Owerall it works soo good (and drives my singleended KT88 with Lundal outputs so well) but you always want it to be perfect
Thanks Bruce for sharing all your knowlege and building skills.

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Tube interested and open for new projects always and too often...

Hi, Tubes and indeed electronic components will always generate some noise. First I would get rid of the cathode capacitor. It is only needed in those rare cases where extra gain is wanted. Most line stage preamps only have a gain of between 1 and 5 times. Without the capacitor the Forewatt has a gain of about 7. With it closer to 11. The hiss in the output is not affected by the controls. In any amplifying device there is a design choice in whether to control the gain at the input or at the output (you could do both). If you control it at the output then the noise level will seem to be lower at lower volume settings. The danger is that some signal sources will be large enough to over drive the gain stage or at least make it run in a less than best range and increase distortion. With the control at the input there is no possibility of over drive or non-linear operation, but the noise at the output will be constant. I went for the lower distortion vs slight decrease in noise floor. Since most power amps have a sensitivity of about 1-2 volts the residual noise level of the Forewatt is normally sufficiently low to not be an issue. Mine (and the kits) all measure about -90dbv. Very quiet. Some amps however, are much more sensitive and may amplify the residual noise so it is audible. For such amps I would be inclined to put trimmer pots on the output of the Forewatt, or perhaps better yet a some to be used as a "passive preamp" which is just a volume control in between the preamp and amps. Possibly a gain control of about 100K in the inputs of the power amps. I do that on the mini blocks as they have more gain than the bigger amps and need to be padded down.

BTW all the other things you were trying would not hurt, just will not have the desired effect. One thing that might work is to put a 47-50uf electrolytic capacitor from the heater circuit to the signal ground. It is un-predictable which side will be best as it seems to be related to the actual build and components. Try the hot and cold sides and see if it matters. I do this in the phono preamps where ever bit of noise is important. Sometimes it makes no difference, other times as much as 2-3 db.

Then too finally the tubes may quiet down as they break in. Some brands are just noisier than others as well. What ones are you using?

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project