benadrian wrote: Fri Jan 14, 2022 1:17 pm Another thing to add is that the classic Vox amps have no negative feedback. That definitely has a sound. I believe a lot of people conflate the "no negative feedback" sound and playing feel with a "Class A" sound and feel, since they have been told that Voxes are Class A.

I think this statement hits the nail on the head.

benadrian wrote: Fri Jan 14, 2022 1:17 pm

Another thing to add is that the classic Vox amps have no negative feedback. That definitely has a sound. I believe a lot of people conflate the "no negative feedback" sound and playing feel with a "Class A" sound and feel, since they have been told that Voxes are Class A.

When you say "classic" what time frame are we looking at. Was negative feedback introduced in later models?

Mickey242 wrote: Fri Jan 14, 2022 3:50 pm

benadrian wrote: Fri Jan 14, 2022 1:17 pm

Another thing to add is that the classic Vox amps have no negative feedback. That definitely has a sound. I believe a lot of people conflate the "no negative feedback" sound and playing feel with a "Class A" sound and feel, since they have been told that Voxes are Class A.

When you say "classic" what time frame are we looking at. Was negative feedback introduced in later models?

If it's old, it probably does not have negative feedback. I beliece the AC30CC2 models also do not have negative feedback.

The amps like the AC50CP2 or the Tony Bruno designed amps; they MAY have it.
http://www.voxshowroom.com/uk/amp/ac50cp2_hood.html
I'm pretty sure I've seen it, but I can't find a modern schematic to prove it for me.

benadrian wrote: Fri Jan 14, 2022 1:17 pm
As for Mesa, I just had to look at what a SimulClass amps was doing. I dug up the original patent
https://patents.google.com/patent/US4532476A/en
It looks like it runs a set of power tubes in regular A/B, fixed bias, pentode (or tetrode) operation, and then another set of power tubes in a triode configuration with some fixed bias as well as some grid leaking going on.

This reminds me of another popular misconception because I've heard it from multiple Mesa Boogie enthusiasts (I'm sure others have done it):

"My Dual Rec is fixed bias so I change the power tubes myself! No tech needed!"

losthighway wrote: Fri Jan 14, 2022 5:24 pm This reminds me of another popular misconception because I've heard it from multiple Mesa Boogie enthusiasts (I'm sure others have done it):

"My Dual Rec is fixed bias so I change the power tubes myself! No tech needed!"

This is not entirely wrong... so long as they use Mesa tubes.
Mesa kind of worked backwards. They buy, measure, and rebrand power tubes to sell. They sell ones that test into a certain range of transconductance. Then, when designing power amps, they set the bias to work with power tubes in that narrow range. The bias is fixed, meaning that's it's a fixed negative voltage applied to the grids of the tubes. However, the bias is also not adjustable (without changing resistors). Many users confuse "fixed bias" with "non-adjustable bias".

So, these Mesa enthusiasts are both right and wrong at the same time Also, I believe Mesa tends to bias on the safe side, so if you toss in a decent set of matched tubes it probably won't be too hot/dangerous. It may be too cold, which is safe but can sometimes expose crossover distortion.

I've added a pot to at least one mesa amp to get adjustable fixed bias.

^ Cool. I did not know that. Good thing I didn't call them dummies!

benadrian wrote: Fri Jan 14, 2022 5:08 pm

Mickey242 wrote: Fri Jan 14, 2022 3:50 pm

benadrian wrote: Fri Jan 14, 2022 1:17 pm

Another thing to add is that the classic Vox amps have no negative feedback. That definitely has a sound. I believe a lot of people conflate the "no negative feedback" sound and playing feel with a "Class A" sound and feel, since they have been told that Voxes are Class A.

When you say "classic" what time frame are we looking at. Was negative feedback introduced in later models?

If it's old, it probably does not have negative feedback. I beliece the AC30CC2 models also do not have negative feedback.

The amps like the AC50CP2 or the Tony Bruno designed amps; they MAY have it.
http://www.voxshowroom.com/uk/amp/ac50cp2_hood.html
I'm pretty sure I've seen it, but I can't find a modern schematic to prove it for me.

^ cool, thanks for the info.

Deak Harp of Clarksdale is selling his tweed blues junior as I write this if anyone wants one

Still haven't got a Blues Jr. but a friend just gave me a Pro Jr. and I'm kind of floored by how much I like it. He had his amp guy do a couple upgrades but I don't know what they are. I'd still like a Blues Jr. for the shop I guess but this Pro Jr. is going to do just fine.

elisha wiesner wrote: Sun Feb 20, 2022 9:55 am Still haven't got a Blues Jr. but a friend just gave me a Pro Jr. and I'm kind of floored by how much I like it. He had his amp guy do a couple upgrades but I don't know what they are. I'd still like a Blues Jr. for the shop I guess but this Pro Jr. is going to do just fine.

Pro Jr. > Blues Jr.