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6C33C SE Amp - Ideas on refinement?

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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:

HobkirkSE-Rev02j.jpg


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...
 
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?
 
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....
 
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
 


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
 
Hey,

Had some sparetime and spiced your circuit. With the componentvalues from the schematic, it will not work at all. This as Ua over V3 is almost nonexisting. Maybe something wrong with your drawing? R3 might be the first to adjust.


Used 300 and 400V for driver A+, together with Imax, 200mA, through the 1640 at 200V B+.

As Miles also pointed out, you can not have a levelcontrol inside the NFB loop.

Also the 12AX7 will be to hard loaded driving both a 100k Ra and a 50k load.

With three cascaded gainstages you will have far to much raw gain. So get rid of at least one stage.
 
revintage said:
Hey,

Had some sparetime and spiced your circuit. With the componentvalues from the schematic, it will not work at all.

I just had to start with this quote... IT DOES WORK! :cool: and it works pretty well indeed using 'em old trusty ears and good music.

There is something wrong with my schematic. The NFB is attached to the wrong tube! It should go to V2...! Ooops...
 
Jeb-D. said:
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?

The PSU topology is the usual "vintage" Cap-Choke-Cap affair. I use half-wave rectification with soft-recovery diodes on the B+ and a full bridge rectifier on the A+ also with soft-recovery diodes. On BIAS is use a standard 1A BR.

The buzz gets louder when the volume gets turned up.

Star ground all out! All ground leads go to a single point.
 
Alastair E said:
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....

Care to expand on this subject? Like I said... I am a "freshman" when it come to tube electronics. I still need to differentiate between my "cathode followers" and "whatsitnames".
 
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