6C33C SE Amp - Ideas on refinement?

[GlidingDutchman]

Valued tube gurus of this here DIY Audio Forums.

I have mashed together some ideas I had and adapted a schematic to come to this design:



Cathode follower tube changed to a 12AX7 and driver tubes are JAN5814A. 6C33C NOS Russian Military triodes barely run in.

At the moment it is in a functioning but 'spaghetti' state on my workbench. I am quite pleased with the sound I am getting but... always a but...

After propping up chokes and all sorts of gizmos on the PSU I still get a darned "vintage guitar amp" buzz on the speakers. It is low-level but annoying. There is even active regulation on the HT of the pre/driver stage...

All ground points hook up to a central point (a la Star Ground!)

When the buzz is sorted I will go to the next step - cabinet making...

[azazello]

You can try to put non elektr. capacitor 1 uf parallel to choke/s/.

[Jeb-D.]

Nice to see another 6c33c SE (I'm in the process of building one too, which is in the terminal strip on plywood with spaghetti state as well).

Do you have a power supply schematic?

There could be some electro-magnetic coupling going on with that 'spaghetti' if it weren't done with any thought.

Does the buzz vary with the volume control setting?

Are you using cheapo rectifier diodes? Some types will give a hash that seems incurable when used in an SE amp.

When you say star ground, are all ground points going 1 spot? Or do you have local star grounds that go to a main star ground?

I would drop R10 down to 100k if your driver can handle it. 6c33c tends to be more stable and perform better with lower grid leak values. Especially with fixed bias.

You may want to consider adding a cap from P2's wiper to ground. That way the driver's load won't change with bias setting and it will add some extra filtering to the bias supply.

Where do you have your audio ground tied to earth ground?

[Alastair E]

Try elevating the Heaters to around 40V with respect to ground. A simple voltage-divider from HT will be OK, decoupled by a 0.22uF cap...

Works wonders with heater induced hum....

[Alastair E]

How about changing the 6C33C-B Driver stage into a Proper, Cathode-Follower, Its cathode to the minus rail via summit like a 22K and the set bias controlled by the driver's grid voltage...

This will get rid of the coupling-cap and present a nice low driving impedance to the '33. This in some cases can improve the stability of these big ole bottles, and improve bandwidth a little....
--Thats the way I did a SE with this tube, and is the way I use them in an OTL....

[Miles Prower]

Originally posted by GlidingDutchman
Valued tube gurus of this here DIY Audio Forums.

I have mashed together some ideas I had and adapted a schematic to come to this design

Get that volume control out of the gNFB loop! All you're doing here is fighting the gNFB. Turn it down, and the NFB loop will try to increase gain, and vice versa. It belongs outside the gNFB loop.

[tomchr]

Pardon my ignorance, but I don't see any cathode followers. What am I missing?

V1 appears to be a common cathode with a gain of roughly 100 (100k/1k - assuming infinite anode impedance - in reality probably more like 50).

V2 same as V1. About the same gain.

V3 common cathode again, this time with frequency dependent gain.

And another common cathode for the output stage in V4.

It looks like the dominant LF pole is set at 3.2 Hz by C1, P1. It looks to me like the HF pole is set by device characteristics. I'd probably change that. One could add a cap across the global feedback resistor. That's not the best way as this only limits the HF gain to 1 - not a -20 dB/dec slope I'd like. But that's one option. I would also cap couple the input to avoid having a DC potential there.

With the schematic as is (I still don't see that pesky cathode follower), one could achieve higher gain in V2 by using C2 as a boot-strap. In that case, the negative plate on C2 should connect to V3 cathode. But of course, this would affect stability so that needs to be checked.

Thanks,

Tom

[Jeb-D.]

Pardon my ignorance, but I don't see any cathode followers. What am I missing?

There is no cathode follower. Alastair, was suggesting to change V3 into a cathode follower.

[revintage]

About the lousy 5814/ECC82, I would say it is not good enough to drive the 6C33C. 5687 or 12B4 would be the way to go. Or why not a trioded 6E5P.

[tomchr]

There is no cathode follower. Alastair, was suggesting to change V3 into a cathode follower.

Thanks Jeb. For a minute there I thought my brain had blown a fuse. Changing V3 to a cathode follower would make sense if you want a low output impedance driving the output stage. Though, in that case, I'd try to reduce R13. Zout of the existing V3 circuit should be on the order of Ra || 10k as it is...

~ Tom