Marshall 2061x suggestions needed.

[FuzzFaceBuilder]

Hey all,

I’ve just come into a 2061x that breaks up too early, even with a replaced OT. (I received it with a replaced Classictone 18 watt OT.)

Hereʻs a quick list of all of the changes Iʻve made thus far:
1. Changed the Rp/Rk of V1 to 220k/1K5 (shared cathode),
2. Retuned the Bass channel cap values
3. Raised voltages just a hair via dropping string
$. Retubed JJ EL84ʻs/long plate Mullard AX7 in the PI/Yugo AT7’s in V1.

All of that has helped “audio congestion” and are steps in the right direction.

What else would you all recommend to get further in that direction?

[pompeiisneaks]

If you're trying to increase headroom, you could try some form of LNFB, there doesn't seem to be any GNFB on that amp, if I'm looking at the right schematic.

Also often EL84's are biased a bit hot(I don't know this amp but vox also often used a 130ohm cathode on the AC15, and it was biased quite hot), that has a 120ohm cathode resistor, I think I went up a bit higher on my AC30 to bias a bit cooler. Have you got a scope to see where the breakup is happening? Is it preamp or power amp?

~Phil

[goldenGeek]

The bright cap on The lead channel is part of the early breakup on that channel, and mainly why I love the amp. You could try another cap there, maybe 200-500pf-ish?

[FuzzFaceBuilder]

Phil,

Thanks for the heads up on adding GNFB. It wasn't quite on my radar as fine tuning goes, but I can absolutely see how this amp would benefit from GNFB.

Iʻve been brushing up on it, but Iʻm not nearly familiar with the cathodyne PI as I am with long tail. Would I just set up a feedback network off of one of the secondaries into the cathode tail like I would on a LTPI?

Thanks again,
Hale

If you're trying to increase headroom, you could try some form of LNFB, there doesn't seem to be any GNFB on that amp, if I'm looking at the right schematic.

Also often EL84's are biased a bit hot(I don't know this amp but vox also often used a 130ohm cathode on the AC15, and it was biased quite hot), that has a 120ohm cathode resistor, I think I went up a bit higher on my AC30 to bias a bit cooler. Have you got a scope to see where the breakup is happening? Is it preamp or power amp?

~Phil

[pompeiisneaks]

I'd think reading up on the valve wizard's site on the different kinds of PI's is a good way to make some sense of it. I bought his books and love them.

http://valvewizard.co.uk/

Go to the section on the left under Phase inverters and read up on LTPI. He discusses the DC and AC versions and the what/how etc.

~Phil

[sluckey]

The NFB would usually be applied to the cathode of the gain stage just prior to the cathodyne PI. Look at the Princeton 6G2 and the Princeton Reverb for two very similar but slightly different NFB circuits.

Since the cathodyne only has a gain of slightly less than one, many people like to consider that previous gain stage as part of the PI.

BTW, the 2061X uses a LTP not a cathodyne.

[M Fowler]

Not a lot more you can do to the 2061 the preamp tube is already at around 205v so increasing voltage there somewhat more might help decrease 10k dropping resistor to 8k2.

The V1 330uf bypass cap could be changed out so it's not so muddy.

Mark

[FuzzFaceBuilder]

Aloha Phil,

I guess I didn't recognize the layout of the PI in the amp. More reading to do. I appreciate it.

I'd think reading up on the valve wizard's site on the different kinds of PI's is a good way to make some sense of it. I bought his books and love them.

http://valvewizard.co.uk/

Go to the section on the left under Phase inverters and read up on LTPI. He discusses the DC and AC versions and the what/how etc.

~Phil

Aloha Mark,

Thanks for the suggestions! I have two 8K2 resistors in the dropping string, and a 15uf (that reads closer to 22uf) in the cathode position of V1.

Not a lot more you can do to the 2061 the preamp tube is already at around 205v so increasing voltage there somewhat more might help decrease 10k dropping resistor to 8k2.

The V1 330uf bypass cap could be changed out so it's not so muddy.

Mark

Hereʻs the amp as it sits now.

2061x current state.jpg