Tweed BluezMeister with 5879 OD

[10thTx]

Thought I'd share this. IMO, it has a very cranked overdriven tone with the OD engaged. It sustains extremely well with mid boost and OD engaged.

It some ways this has some similarities to Mat's great "86 Special" in it's topology but uses a 5879 in the OD instead of EF86 and adds an active FX loop which works excellent as a master volume. It also uses different value components in the topology than Mat's.

compare to: https://tubeamparchive.com/viewtopic.php?t=22195

SCH editable schematic and layout are here:
https://tubeamparchive.com/viewtopic.ph ... 475#281475

with respect, 10thtx

[David Root]

Interesting! Did you ever try a pentode/triode TV tube instead of the 5879/IRF820 source follower?

Like a 6U8 for example, with the triode as a CF replacing the MOSFET?

[10thTx]

The only pentodes I've tried in a preamp are 5879,5654 & 6AU6. All of those I like and were successful for me.

The 5879 is in particular a very nice tube, IMO & I have plenty of them.

With respect, 10thtx

[Structo]

I think you have talked about it before but what does the MOSFET do for you in these circuits?

[David Root]

I like the 5879 too, I have also used it as a driver, which it's not really designed for, but seems to work OK on a SE KT66.

I've not looked at the two 7-pinners before, thanx for mentioning them.

Nice thing about the 6U8 and similar TV tubes is they are really cheap considering you get a NOS pentode and triode for maybe $5 or so.

Tom, the MOSFET source follower does the same thing as a triode CF after the pentode, ensures passing lots of gain/voltage by providing a low impedance output from the pentode. As to MOSFET SF vs triode CF, I don't know but I bet 10thTX will have something to say on that, he's a very knowledgeable guy.

[10thTx]

I think you have talked about it before but what does the MOSFET do for you in these circuits?

My experience with the 5879 tube is that it greatly benefits from a CF when used in the OD position (not so in the V1 position like on the HoSo56).

I have used tube and mosfet cathode followers. I truly can not hear any difference between the two of them tone wise.

The mosfet CF behind the 5879 significantly smooths the 5879 overdrive tone. Whatever slight high end "hash" in the tone is eliminated with the CF behind the 5879 (in the OD position only).

I have done an A/B comparsion of the 5879 with and without the CF and come to this conclusion. It seems to add a measure of smooth compression also, IMO.

Reportedly, Carlton has a CF in his Dumblish overdrive. While it's not behind a 5879, maybe there was a reason that he liked it there instead of the tradition 2 gain stage overdrive? Just a thought.

With respect, 10thtx

[EtherealWidow]

I have used tube and mosfet cathode followers. I truly can not hear any difference between the two of them tone wise.

AC or DC coupled? In which circuits?

[10thTx]

AC or DC coupled?

Just like in the schematic in the first post. DC coupled when using a mosfet.

http://www.valvewizard.co.uk/dccf.html

vs

http://www.valvewizard.co.uk/accf.html

With respect, 10thtx

[EtherealWidow]

And you don't hear a difference between a DC Mosfet CF vs a tube one?

[10thTx]

And you don't hear a difference between a DC Mosfet CF vs a tube one?

That is correct. I can NOT hear a difference.

I don't know if you have read "Mosfet follies"?

http://www.geofex.com/Article_Folders/m ... tfolly.htm

With respect, 10thtx

[EtherealWidow]

That's exactly the article I had in mind.

Some amps use the triode cathode follower to generate distortion. If your amp is one of those, the MOSFET will NOT generate distortion like the triode did. The MOSFET will accurately reproduce whatever the amplifying tubes send it. In most amps, the fact that there is a MOSFET there instead of a triode will not be audible. I have personally done this mod, and I can detect no hint of harsh solid state sound; no one I've asked for the opinion can either.

I dunno. Although I wouldn't expect a harsh SS sound, I would at least expect some audible difference. I guess since they're typically not used until a couple stages in there's not that much of an audible effect.

[David Root]

I had not seen that RG Keen article before, good stuff.

Also, seeing that DCCF article of Merlin's again reminded me of the bootstrapping of a triode gain stage by a CF (Last para. in the article).
Has anyone tried that and is it worth a try with a pentode bootstrapped, and the pentode without a cathode bypass cap, in order to reduce/eliminate the high end hash that often happens in OD?

[Jack Hester]

Here's a concept drawing that I made of Jeff's circuit, using a triode as the CF. And rather than his PI, I used the second triode in the tube as a Cathodyne PI. Saved adding another tube. The PI was taken from Merlin's book.

Forgive the unconventional schematic. I draw literally like it might be a P2P build, using actual tube internal layout taken from data sheets.

Look to my V4 for the CF and PI.

There are a couple other differences, but the CF and PI are the major ones. Please comment. Thanks.

Jack

03Jan14 - Note: See rev-C below

[Jack Hester]

Here's an updated version of my concept drawing of 10thTx's. He redrew it to a more readable schematic, along with a layout. If his doesn't get posted here, it can be seen over on the Hoffman Amp Forum.

Jack

11Jan14 - Note: See rev-D below

[10thTx]

Here is Jack's versions.

More info here

With respect, 10thtx