Lowering tube preamp gain?

[MKG]

I’m starting my journey into tube audio. Reading about design and theory when I have downtime but I haven’t gotten far. Built an ear834p clone that came out quite well. Latest project is another Chinese kit- a 6j4 + 6p6p tube pre. The plan was to build it and then slowly replace all of the guts with better components... but as of right now it’s gain is way too high, especially with digital sources. Other than putting some resistors inline with the input or output I’m not sure how to lower its gain- hoping y’all can guide me towards the best solution. If the lowest volume setting is in line with 7 o’clock, then I can’t turn it past 8:30 without significant crunchy clippy distortion. Schematic is below, I know it’s hard to read through all of my pencil marks. Are resistors the best way to go to lower gain? Is there alternate tubes I can swap in and if so, do I need to change any wiring to match the pin out? Is there another option I’m not aware of? Is it just a really crappy design?
I’m also getting some bleed over from other channels, but I think that may just be the super cheap selector switch.
Current tubes in it are nos:
GE 6x5gt (rectifier)
Russian 6v6gt
GE 6au6

Thanks for your help!

 

[peterh]

A few resistors at the input is what you need.
One in series (R1) followed by one to ground (R2)
If they are equal you will halve the signal.
Vout = Vin * ( R2 / R1+R2)

 

[BinaryMike]

Any chance of seeing a proper schematic?

A 6AU6 and a 6V6 per channel implies so much gain that it's almost certainly designed with feedback. Closed-loop gain could be adjusted downward by tweaking the feedback divider resistors, which could simultaneously reduce distortion and output impedance.

 

[MKG]

Any chance of seeing a proper schematic?

 

[BinaryMike]

As near as I can tell, both stages are triode-strapped and feedback puts overall gain at roughly 10X (20dB), which is about right or just slightly high for a line amp stage. Have you measured actual gain? Are the anode voltages reasonable?

 

[MKG]

I’m still in the baby learning stage. I have not measured actual gain and I don’t know what reasonable anode voltages would be, but I’ll do a bit more reading and researching and get back to you with an answer.

 

[Lavane]

The schematic shows 230v primary. I'm guessing yours is probably 110v? Several preamps ive built or bought from China use 110v transformers. I've found voltages can be too high coming off the secondary when plugged into a 120v outlet. As BinaryMike suggested, check your voltages. You may find the heaters are running hot also.

 

[MKG]

I’ll get in there tomorrow evening and carefully check my voltages. Thanks for the help!

 

[MKG]

Input- currently, CCA to schiit modi to it, or tt to ear834p clone to it
Amp- solid state. My amps only have rca in, and are not integrated.

I could very well be wrong, but my research showed that as far as tubes go;
6p6p = 6v6
6j4 = EF94 = 6au6 = ? 6s2p = ? ec98
(The two question marks mean I was unsure of those two)

I’m capable of following a schematic and know some BASIC electronic theory, but I’m still barely in a kiddie pool with arm floaters on when it comes to it, even more so with tubes, but I know enough to not kill myself.

 

[MKG]

Sorry, chromecast audio.
All line level inputs.

 

[MKG]

Yes, see above.

The kit pre has volume control, I think I may be misunderstanding you.

 

[gadget73]

6v6 is an odd choice in a preamp. Could be used as a very stout cathode follower though, zero gain and you should get plenty of output current to feed basically anything you could hook to it.

 

[Dr. Ears]

Input- currently, CCA to schiit modi to it, or tt to ear834p clone to it
Amp- solid state. My amps only have rca in, and are not integrated.

I could very well be wrong, but my research showed that as far as tubes go;
6p6p = 6v6
6j4 = EF94 = 6au6 = ? 6s2p = ? ec98
(The two question marks mean I was unsure of those two)

I’m capable of following a schematic and know some BASIC electronic theory, but I’m still barely in a kiddie pool with arm floaters on when it comes to it, even more so with tubes, but I know enough to not kill myself.

My guess is the 6s2p / em98 are they correct substitutes.
I avoid equipment, when I can't determine good substitutes for the Chinese or Russian tubes.

 

[Dr. Ears]

I’m starting my journey into tube audio. Reading about design and theory when I have downtime but I haven’t gotten far. Built an ear834p clone that came out quite well. Latest project is another Chinese kit- a 6j4 + 6p6p tube pre. The plan was to build it and then slowly replace all of the guts with better components... but as of right now it’s gain is way too high, especially with digital sources. Other than putting some resistors inline with the input or output I’m not sure how to lower its gain- hoping y’all can guide me towards the best solution. If the lowest volume setting is in line with 7 o’clock, then I can’t turn it past 8:30 without significant crunchy clippy distortion. Schematic is below, I know it’s hard to read through all of my pencil marks. Are resistors the best way to go to lower gain? Is there alternate tubes I can swap in and if so, do I need to change any wiring to match the pin out? Is there another option I’m not aware of? Is it just a really crappy design?
I’m also getting some bleed over from other channels, but I think that may just be the super cheap selector switch.
Current tubes in it are nos:
GE 6x5gt (rectifier)
Russian 6v6gt
GE 6au6

Thanks for your help!
View attachment 1385442 View attachment 1385443

You did a great job on the build!
Look at the Japanese kits modified and offered by Victor Kung of vkmusic.ca.

 

[MKG]

There is no spec sheet. A pile of parts and a schematic are all that came, any info on the web is poorly translated gibberish.

 

[MKG]

Currently an onkyo m-501. Sometimes a hafler dh-200 depending on my mood and which speakers I have on deck.

 

[MKG]

And those would go inline with the source (input of preamp), yes?

 

[MKG]

Okay...
So, I think I have two problems and potentially a third wiring issue.
Voltages below are with tubes out.
1: inputs are too hot. That’s been solved if I buy some inline signal attenuators.
2: transformer is a 110v 50hz. Can I wire in a bucking transformer to bring the voltage down or am I better off sourcing an appropriate 120v 60hz transformer? What should be 270 to the rectifier is 296v, what should be 6.3 to the rectifier heater is 7.10v.
3: I might have the 6p6p & 6j4 Heater’s wired incorrectly? Going from (according to the schematics) the 6p6p pin 2 OR 7 to ground or 6j4 pin 4 OR 3 to ground I get 3.53v. Going across 6p6p pin 2 to 7 or 6j4 pin 4 to 3 I get 7.10v
I found another schematic online (below) and none of my voltages match.
Ignore the wiring mess, the goal was to put it together and get it functional and then replace all the bits and route wires appropriately.

 

[Lavane]

I've used dropping resistors to get 6.3v for the heaters. The last one I did I think I used a 10 ohm 5 watt wire wound. Heaters were just above 7v. I've noticed more and more switching supplies being used lately instead of the 110v transformers. I had two Chinese preamps with high voltages due to the 110v primary.

 

[MKG]

Attenuators ordered, resistors to drop voltage I can definitely do.
So my last question is should the ac voltage on the 6v6 & 6au6 be 6.3v across pins (3.15 per pin) or 6.3 per pin (individual pin measured to ground)?