6V6 line preamp

[Poindexter]

Hi,

I have a line amp that uses (or used, more in a minute) the 6AQ5, which is a miniature 7-pin 6V6, and it worked a treat:

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Except that I had a terrific problem with microphonicity in the 6AQ5s. They have cylindrical plates which ring like a bell, most of them. I now use a different tube, but I haven't noticed any significant microphonicity in 6V6s, which I use as power tubes, so you are the victim of a Good Idea. I have a more complete project description on my site:

The Project

If you have any questions, ask away.

Aloha,

Poinz

[Salas]

Hello Poinz

-Nice to have seen your gear favoring highly on 6moons. Congratulatios!

Story

-I had those RCAs and I was thinking of building your Musical Machine maybe with an E80CC first stage twist a long ago, but that idea fell victim of needing the power that my EL34 / KT88 amp with AN Trans 185s gives, I had developed it to a satisfying degree by then, when my speakers got 4 more woofers added. They are 94dB but 4-6 Ohm now.
Abandoned project's nice tubes got to play music through something else I thought, and not to idle in drawers.
Somehow I felt that a 6V6 triode strapped will be gutsy and musical as a line amp with 350V B+.
I simulated the rough circuits I posted in meantime (thanks again for the models Canada) and I saw that they seem mainly ok. Only change is I can use a 560R instead of a 500R to go slightly more linear. Thats fortunate because 560R is standard value and I will get 2W Kiwame for that. For the CC circuit 2x10K 5W Kiwame in parallel will do (3.7W idle dissipation needs 2-3X part rating not to become brown and fail soon if of 5W rating). Also the 180R can be 100R for slightly more gain (100R-220R totally ok basically). That gain will be about 7X cathode bypassed for CC and maybe 5.5 for SRPP. The tubes shall idle at about 5.5-6W which is conservative for life time but not skinny. Any 100uf to 1000uf 25V Ck cap will do. I like Silmic and BGN. I prefer 1000uf bcs the lower the corner the less the cap's THD. I dont like the sound of local feedback. I even find a little loop NFB better sounding than comparable amount of LNFB, seriously!
Will take me some time to build from scratch, but an idea is to get some noval to octal adapters and use my ECC82 preamp rewired for testing. It sports a 360V B+ PSU with 5R4GY RCA rectifier tube , 3x 20H chokes, 2x current source heater supplies (have to modify 2 resistors only for 0.45A per tube) has selectors, RCAs, pot, etc. Can I string 2x 6V6s per channel and feed 0.7A in the SRPP case? Don't tell me they have some allergy for series heaters please! I JUST HOPE the 6V6s will not turn out ringing in a line stage! Especially after I will probably have my preamp gutted for their charms!

Nosy questions

-Should I forgo the CC circuit? What do you think? I see you ended up SRPP. Is the CC veiled? Why you went so low on B+? I can see that nice linear niche to the left plate curves chart corner, but 2V auto bias isn't precarious with CD sources of (at least) 5.6V p-p output? Also the RCA manual gives cathode to heater 200V max and you have (175/2)-6.3=81.2V C to H (only) on upper tube, so you float your heaters for noise reasons mainly?

[Salas]

BudP answered my email very kindly.

He confirmed use of 6V6 as a current sourced CF (driving an SE to balanced output conversion transformer) after a 6G6 CC stage. He said it sounds much better than with smaller tubes. He uses 16V cathode op. point.
I chose 15V (I hope it proves there about in real test). So all lights are green.

[gianis]

I think that the big broblem for this project is the microphnicity, i have build a pre with 6v6 tubes 3-4 years ago.. and as i remember i rejected many 6v6s for this reason..only two Bendix 5992 [6v6] and two RCA black plate was enough antimicrophonic and suitable for this case..

[Salas]

I do have 4 RCA black plate smoked glass 6V6s, testing very strong. Lets cross fingers! I will test soon on a donor 6SN7 octal pre with tube regulated PSU. So no gutting of my pre yet...
How was the performance?

[gianis]

The perfomance was excellent,,big sound strong and also soft,,I think that the output power tubes as preamplifier have better sound than the normal small tubes [forgetting however the microphonicity and heater noises] .. I attach a pic from an extraordinary preamplifier with 5998 triode tube

[jane]

Not 6V6, but I am for the moment building a preamp based on EL84. The design goals are balanced output, low output impedance and ~10dB gain. I will use it together with a solid state power amp ("ICE power", class D mono blocks).

There will be two chassis, a separate power supply. The preamp will look something like this:

Jan E Veiset

[Salas]

Looks like you are building a winner there Jane! Good luck and please let us know the results after first tests.

[Salas]

Hello again! Rough test gave very good results. The regulated supply did not hold up to more than 260 VDC B+ so I used a 17mA ,11.2V cathode operating point. Wire wound sandcast plate 5W resistors, 680R 1W generic carbon rk and generic Matsushita ck. Basic white Mundorf (SCR) coupler. The common cathode circuit was tested.
Well...bigger 3D, no edge, more reverb and depth info, more musical detail than the 6SN7 cc&cf or SRPP, or grid leak cc&cf, or 12AU7 cc&cf that have been used previously in the system.
Piano sounds superb and big drums too. Lots of air and body but no flatness or edge nowhere. Voices more liquid and human, better defined with better front to back spatial resolution. Strings are velvety, basses huge and open with no pace loss. Very good drive, optimal gain. Treble stems out seamlessly from upper mid naturally, not like a distinct entity.
Microphonicity is confined to a glassy low level 'ding' sound when hitting the 6V6 glass with a fingernail or when turning the stepped attenuator located right next to it. No locking with the feedback from the speaker just 0.5m near. After half an hour some periodic 'hsst' sounding transients showed up accompanied with woofers travelling on right channel but I am suspicious of the small regulated supply 12B4 pass element getting taxed with heat. Maybe it was the right 6V6 heater aswell, but I doubt it bcs of the cone travel, looks like a DC shift. I will test with heavier supply in due time. Now its given away to be further auditioned and inspected in longer term in another system by a fellow DIYer. If it turns to behave well in the long run it will very probably get adopted as my premium pre with proper custom PSU, best parts and antimicrophonic box build (there is more performance to be expected given the compromised B+ and generic components first test). Will keep you posted.

[Salas]

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