Tubes test OK, but noisy?

mhardy6647

may the force be with you!

Yeah, the 6V6 got used in radio, tv, and hifi (and industrially) - but it's a power amplifier tube -- not something typically found in a small-signal amplifier per se. It could be a voltage regulator in the P/S of this preamp, the more I think about it (the 6L6 was sometimes thus used in HV power supplies, e.g.).

The only other use I can think of in a preamp is as a space heater ;- )
(Actually, unless its dissipating power, it won't get too hot -- the heaters in tubes don't draw much current, so they don't make much heat at all, really)

What kind of preamp is it?

Nightfall

Dared MC-7P

mhardy6647

heh, who knew?

http://www.diyaudio.com/forums/tubes-valves/102352-6v6-line-preamp.html
http://basaudio.net/blog/salas-6v6-preamplifier-linestage/

I'd still be very interested to see a schematic for Nightfall's preamp -- seems awfully complicated to me...

mhardy6647

Thanks for the schematic -- wow, that thing is complicated for a line level preamp. From the "more is better" school of design.

But, yeah, the 6V6 is in the HV power supply; it's even pretty clearly marked on the P/S part of the schematic. Not in the signal path per se.

Must be regulation for the HV B+ (350 VDC) for the tubes' plates.

I now realize it has an MM preamp too, eh? D'oh. OK less complicated than I thought.
Just the P/S is complicated. Not necessarily a bad thing -- interesting to see.

Note the DC supplies for the small signal/high gain tubes' filaments. Looks like the only filament operating at AC are the rectifier & the 6V6.

mhardy6647

sorry, I edited the text in the previous post -- I am fighting THIS computer, too :-/

heiney9

Nightfall wrote: »

Are you talking about hiss that gets louder as you turn the volume up? I get that too, I thought it was just the nature of tubes?

Most of my tubes are very, very quiet even as I ramp up the volume. The 12AU7 is lower gain than a 12AX7. I'm talking tubes from the 50's and 60's, not new production.

heiney9

Nightfall wrote: »

I'm using a RCA 6V6GT and 5U4G, Holland made Amperex 12AX7's and 12AT7. I can also hear it from 7' away before 12'oclock on the dial. I have tried GE 12AT7WC, Sylvania 12AT7WC, Mullard 12AT7, a clear glass RCA 6V6GT, and nearly brand new Chinese tubes in all of the mentioned slots - all of them had the same amount of hiss.

How do the Tung Sol's sound?

That's not normal. Or I should say I have a lot of tube gear and it's all very quiet. I can put my ear right up to the tweeter and hear some minor hiss. If you are getting it 7' away with all different types of tubes, something else is going on.

H9

heiney9

Old Tung Sol's are a fantastic tube, very much unappreciated and underrated. Try a 5751's in place of 12AX7's.

mikeyb128

heiney9 wrote: »

Nightfall wrote: »

Are you talking about hiss that gets louder as you turn the volume up? I get that too, I thought it was just the nature of tubes?

Most of my tubes are very, very quiet even as I ramp up the volume. The 12AU7 is lower gain than a 12AX7. I'm talking tubes from the 50's and 60's, not new production.

My Cary is dead quiet with its 12au7's and RCA balck plate JRC from 1964.

mhardy6647

12AU7s, of course, contain two lower mu triodes than a 12AX7 or 6922 (e.g.).

Might be worth pointing out that another common source of noise (IN OLDER EQUIPMENT!!!) are old carbon composition resistors. They get noisy (and drift in value) after a few decades.

Modern film resistors (i.e., any modern preamplifier's resistors) should be pretty quiet.

ALSO, of course, it's worth mentioning that we don't usually think of noise in a vacuum (no pun intended, for a change)! The most relevant parameter is usually the ratio of the noise level to the output signal (actually, usually the reciprocal of that quantity, S/N).

The absolute level of hiss/hum/noise can become more and more of a factor when more gain stages are present, higher gain is used (i.e., S/N of a phono preamp's output will be lower than that of the line level analog outputs on a CD player) OR when higher sensitivity loudspeakers are used.

For example, 1 mV of hum might be quite audible on speakers with sensitivity of 100 dB per 2.83 VAC input at a distance of 1 meter (100 dB/1watt @ 1meter @ 8 ohms), but inaudible on loudspeakers with sensitvity of, say, 82 dB/watt @1 meter @ 8 ohms).

In other words, whether the level of noise (of any kind) is audible on one's system depends on numerous variables.

heiney9

The Dared Nightfall has the issues with belonged to Nspindel and it is modified (for the better was my understanding) so I would suggest touching base with him. This is not old vintage gear in need of a refresh. Is this a new issue or has it been noisy all along? All my experiences with Dared products has been stellar. I still own 3 tube pieces and they all exhibit low noise.

What speakers are you running as Mhardy points out, very sensitive speakers will exhibit a higher level of hiss because of their sensitivity. Also, is this only when using the phono section? Or all the time?

H9

heiney9

Have you swapped every single tube to eliminate a bad tube? All it takes is one. I would start at A) cleaning all tube pins B) replacing each tube (1 by 1) to eliminate tubes as the issue C) reach out to Neil and see if can help troubleshoot. D) start a thread on the DIY forum for ideas.

H9

P.s. Also the rectifier tube could be noisy to be sure to look at that too.

heiney9

It seems we have two people asking about tube noise. I just realized the OP is not Nightfall, but Muncybob.

All my suggestions still apply, but I was directing them to Nightfall since I thought he was the original OP.....................doh,,,,,,,sorry

H9

muncybob

It's all good, I'm still learning along the way...but, it does appear Night has a different problem than me since he has the hiss all the time and I don't. I'll be cleaning the pins once again on the TF's and give them another shot.

Since these tubes don't crackle or pop(or any other noise) is it correct to say they are noisy when the only issue is a hiss? Is it possible that they may perform better in another piece?

txcoastal1

I had a hiss noise on my Melody pre, discovered it was a PC plugged into my external DAC causing the issue

mhardy6647

hiss, crackle, pop -- now I'm gettin' hungry! Where's mah cereal bowl, Uncle Jed?

;- )

(and,BTW, yes, I realize the Dared preamp Nightfall's asking about is modern -- I just wanted to mention the specter of noisy passives in the context of the broader topic of "noisy hifi components)

Heh, I even realized, albeit dimly, that there're two different noise/hiss topics from two different folks multiplexed into this thread ;- ) Been trying to keep 'em straight.

Noise is an interesting topic -- in both the analog and the digital domains.

heiney9

mhardy6647 wrote: »

Noise is an interesting topic -- in both the analog and the digital domains.

Yep, and tracking it down usually involves a careful process of elimination in order to isolate the source. Sometimes it's something simple, other times it requires a trip to the shop. :-).

As we have all stated, many time the tubes are the source of the noise.

mhardy6647

An oscilloscope is a very handy tool for such adventures.

heiney9

If one knows how to use it. Afraid that's beyond my capabilities and I am probably guessing the OP's as well.

mhardy6647

understood - and fair enough.

Nightfall

So, some new information. When I have a line level input selected on the Dared I have to have my ear up to the tweeter to hear the hiss. Inaudible from listening position even when cranked up loud. Only when I'm on the phono input is the hiss audible from listening position when the volume is turned up.

mhardy6647

This is the extra gain of the phono preamp stage, required to boost the cartridge's output to line level.

It's endemic (heh -- even solid state, high gain preamps will evince some hiss).

Nightfall

So, completely normal?

@muncybob are you on a phono input as well?

mhardy6647

probably -- I'd have to hear it to be sure, though.
If everything's turned up enough, and the speakers are sensitive enough, any phono stage will have some hiss associated with it.

muncybob

Nightfall wrote: »

So, completely normal?

@muncybob are you on a phono input as well?

Yes, the phono stage is the only input that I hear hiss. But as stated, using the Tung Sol 12AX7 tubes I need to crank up the volume way past a level that I would normally use to hear any hiss and even then I need to be really close to the speakers. Inserting the Telefunken tubes the hiss is easily audible from the listening position at mid volume setting.

It may well be the phono stage of your preamp. Part of the reason I picked up the PAS4 preamp is that the phono stage just blew away the others that I auditioned including a Conrad Johnson at over twice the $$.

I plan to try cleaning the Tele's once again this weekend and see what happens. Other than the hiss during quiet passages or between tracks, I like the sound of these tubes but the hiss will def be a distraction to me.

heiney9

It's rare for a TFK to be noisy, but they are tubes, they are old and they do have a finite life. Of the hundreds of tubes I have only 3 or 4 have been noisy in this way (hiss).

It happens; and if the Tung Sol's are quiet then you know it's not the unit. BTW, Tung Sol tubes are probably the most underrated US tubes around. They are an excellent tube. They are among my favorite tube companies.

H9

muncybob

Well, I cleaned the pins again but this time with Deoxit...twice for good measure with 12 hours allowed to dry between cleanings(overkill I know). Popped the Telefunkens back in and MUCH to my surprise the hiss is all but gone. I get some audible hiss at around 2 o'clock which would be pretty loud and I think at that volume the vinyl surface noise would mask the tube hiss? Can't try that tonight though as the Mrs is sleeping before going to work .Makes me think maybe I need to re-clean the other tubes too!

I have found another situation though. I noticed tonight that while some vinyl is playing if I switch to any other source I can very faintly hear some bleed through of the vinyl in the left channel only. It will slightly increase as I crank up the volume. What gives now?

mhardy6647

Interesting, indeed. Congratulations!

No idea 'bout the bleed-through. Maybe poor isolation in the switch or more to the point wiring to the switch.