New Build Advice. Speaker Grill Buzz, Microphonic Tubes and More Headroom

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I completed this amp recently stuffed into an old car radio chassis. It is supposed to follow Triode 5C3 (cathode biased) on channel one and Pentode Gibson BR6F on channel two. Metal Pre amp and PI tubes. Schematic attached any comments on the design?

It sounds reasonable but has some issues....

1) There is not enough clean headroom. Thinking this is because the Power Transformer has only about 60ma, so I ordered a Hammond 270DX with 104ma as a fix. Waiting for this to arrive.

2) There is a lot of tube vibration on certain notes. I have tried several 6SC7 and 6SJ7 but all have a similar microphonic affect. Would there be any suggestions to try to reduce vibrations? Glass tubes? Military tubes? I tried some cork to dampen the vibration which helped, but still present.

[acheld]

Wow.  That is so impressive that you crammed all that together and the issue is NOT hum. 

You are suspecting that this issue is mechanical, given the proximity of the speaker to the tube chassis -- and you're probably right.  But it is worth testing -- easy enough to do by using an external speaker rather than the internal one.   

If that maneuver does away with microphonics, then you can start to work on damping either the chassis itself, or the tubes, or -- most likely -- both.  Just to be clear, I'm talking about the metal (chassis) upon which the tube sockets are mounted. 

Amazing build!

[mozz]

Preamp plates tied together? Are those voltages measured or calculated on all tubes?

[DummyLoad]

pentode preamp needs a grid leak R.


if you use both input on the triode preamp, you probably want a grid leak R there as well.

if the pentode channel headroom is a issue try using a smaller R for g2 to reduce gain - it's a 2.2M now, try a 1M, 820K, 680K, etc..


--pete 

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Wow.  That is so impressive that you crammed all that together and the issue is NOT hum. 

You are suspecting that this issue is mechanical, given the proximity of the speaker to the tube chassis -- and you're probably right.  But it is worth testing -- easy enough to do by using an external speaker rather than the internal one.   

If that maneuver does away with microphonics, then you can start to work on damping either the chassis itself, or the tubes, or -- most likely -- both.  Just to be clear, I'm talking about the metal (chassis) upon which the tube sockets are mounted. 

Amazing build!

Thanks for the kind words.
This forum provided some great advice to eliminate hum on heater wiring, transformer location, using shielded wire on the signal line.
The design has a ext speaker jack and the rattle is gone when using an ext speaker.
The cork gasket is on the top of the tubes held tight by the back plate. I also tried a few tube vibration rings.
I like your idea of isolating the tube mount chassis which is held in place with screws. Maybe some rubber washers. Might also try isolating the speaker with additional gasket.

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Preamp plates tied together? Are those voltages measured or calculated on all tubes?

Yes Preamp and PI plates from the same source. And pentode screen. Any issue with this? Same grounds as well.

The measured voltages are handwritten in the pic below
Is it odd to get 120v on PI plate 2 and 108v on PI plate 5?

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pentode preamp needs a grid leak R.


if you use both input on the triode preamp, you probably want a grid leak R there as well.

if the pentode channel headroom is a issue try using a smaller R for g2 to reduce gain - it's a 2.2M now, try a 1M, 820K, 680K, etc..

--pete

Thanks for the advice. Why would you use a grid leak on a cathode bias? What values should be used.

The reason I have two inputs for the triode is to patch between the triode and pentode to get both preamps.

I like the higher gain of the pentode, but will give your suggestion a try. It does match the BR6F design though. Both the triode and the pentode channels start overdrive at about 1/4 volume so its really too early for me. Thinking the PT just doesn't have enough current for the 6v6's

If anything I would like to match the gain of both channels. I do have a spare triode stage in Channel 1. Do you think using that would work or too much? BTW - this spare triode stage is currently not used but there is power on the plate. Any issue with that?

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More Headroom
upgraded the PT to the Hammond 270DX which provides 550vct @104ma and switched from 6x5 to 5y3.
The 270dx is almost the same size as what the lower current Webster PT it replaced.

Less Microphonics
upgraded preamp and PI from metal 6sc7 to glass 6sc7g
upgraded from metal 6sj7 to glass 6sj7wg (I learned W is mil spec);
I also tried 5693 which did not help the SJ channel. The extra gain of the pentode preamp is nice. I like it

All in all it seems pretty good at this point.

[PRR]

Will it still pull-in WLW-AM from four states away?

Very neat!

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I Need advice now on the buzzing that I am getting from the metal speaker grill
When playing certain notes, I get annoying buzzing from the grill
When I apply pressure to the grill (with my hand) the buzzing is gone.
Looking for clever idea for a permanent fix

[sluckey]

Make a gasket. Rubber or cork sheets are cheap. Or even consider foam pipe wrap such as...

     https://www.lowes.com/pd/Frost-King-0-125-in-Foam-Plumbing-Pipe-Wrap-Insulation/1069439

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What exactly do you mean by 'clean headroom'?
Guitarists might use that to refer to the SPL that their rig can generate, but also to the non-overdriven range of the amp's gain / volume control.
So max amp power, max system gain, or even just the gain control's taper.

[PRR]

....When playing certain notes, I get annoying buzzing from the grill....

There's no permanent fix. Our minivan is coming around 200,000 miles and buzzes EVERYwhere. I fixed one by jamming a pine branch in the exhaust manifold shield (my mechanic thought that was dangerous). Every rattle we foil, two more buzz up.

Learn to play one-handed (so you can damp with the other hand).

Don't play those notes. (We don't drive 35; 37 is quieter.)

What is the sound of one hand clapping? Right: outside of kōans, no sound. The grille has a partner it is buzzing against. Maybe the case, maybe a bracket. (In different circumstances, a cracked grill will buzz against itself; lots of that in our minivan.) Find the "other hand" and cushion or spacer it.

Hey, Little Minivan

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What exactly do you mean by 'clean headroom'?

the max volume before the amp begins to overdrive (distort) the tubes