395 in Class A mode . . .

[phaboo]

Question for the tube experts out there:

I'm using a 395 in Class A mode--the tone is just nicer for what I'm doing. The 395 just drives some floor cabs with guitar speakers in them (think "guitar monitors")--I'm resister-tapping off the speaker outs to drive PA heads to drive main stacks behind me so that I get the nice Class A tone with a bazillion watts behind me. So, I'm really just using the 395 for tone--I don't need all the power.

Since I'm always in Class A, I'm only using half the tubes in the 395, right? Can I pull half the tubes, then, or do I need to load all eight tubes even though only four are doing anything? Does anyone know which pairs of tubes are Class A and which are Class AB? I'm trying to save a couple of bucks on tube changes.

Also, does anyone know what half-drive actually does? I know it lowers the wattage, but HOW does it do this? Is it reducing gain through the 12ax7s (for more headroom, driving the power tubes less) or is it actually changing something with the power tube configuration?

Yes, I am a ridiculous gear hog and I have way too much crap and my setup is entirely too complicated. It DOES sound amazing, though.

 

[Blueracer]

Typically Mesa has a switch on the cathods of the inside pair of tubes on the output to switch between simulclass and class A. When the switch is off it kills all current to that pair of tubes. The only thing operational for these tubes is the filaments. It is as if the tubes were not there. Given this situation it is fine to pull the tubes when in class A. Further, switching between class A and simulclass without this tube pair present is also fine. I have tried this with my iic+ and it works fine. That being said I don't have a schematic for the 395 so I am not sure that this is how it is done for this amplifier. I would call Mesa and request a schematic. If you get the schematic I can help you with this issue. I have Google'd for the schematics and did not find one on the net.

The second question you had concerns the half power switch. Again I don't have a schematic for the 395 but the 50/50 amplifier has a similar switch. What happens for the 50/50 amplifier is that the half power setting lowers the voltage to the phase inverter tubes just before the power amp. Basically it lower the head room for the phase inverter thus limiting how much signal the power stage sees. There is no change to the power section for this switching.

So why am I telling you about the iic+ output stage and the 50/50 half power switching. It is because Mesa tends to use the same designs over and over again. When you do get the schematics what I have suggested above are things to look for. I would not try anything with my equipment until I did get a schematic to confirm what the circuit is.

Hope this is helpful.

Gary

 

[phaboo]

Blueracer: Thanks bunches! I'd heard of people pulling tubes out, but I never knew if that was something that could damage things or not.

I've never found a 395 schematic online either. If anyone ever spots one, I'd love a copy!

 

[LUCRETIA]

i do have the 395 "schematic"...PM your email if u want..

 

[phaboo]

i do have the 395 "schematic"...PM your email if u want..

You have PM!

 

[ursinus]

PM sent too....
This what the guts look like:

Beautiful, aren't they?

 

[LUCRETIA]

replied..

 

[Blueracer]

The amp porn looks great. The 395 looks like a well built amp. The interesting thing in the picture is that it would be hard to tell which are the inside tubes since they are arranged in a 2x2 configuration. One way to tell what tubes are the AB verses the A is to first switch to simulclass. All the power tubes usually have a blue glow when in simulclass mode (or full power mode for non-simulclass amplifiers). Now switch to class A. The tubes that are being disabled will loose the blue glow. This method usually works but I can't say it always works. Anyway it is something that can be tried to identify which tubes are which.

Hope this is helpful.

Gary

 

[Blueracer]

Ok one more post. I looked at some gut shots of my iic+ which has simulclass. The wires coming from the transformer that are striped in color connect to the pair of tubes that are class A and the pair of wires from the transformer that are solid color are for the pair of tubes that are AB. In the picture of the amp the tubes that are on the inside (tubes farthest from the left and right edge of the amp) would be the class AB and should be the tubes that are disabled when in class A mode. Double check this with the blue/not blue color of the tubes as suggested in the previous post. Again hope this helpful.

Gary

 

[phaboo]

My tubes don't glow blue at all, but I'm not using Boogie tubes (sacrilege, I know).

Got the schematics that the above poster was kind enough to send. The class A tibes are the outside pairs. Pulled the insiide pairs and it ran just ducky on the Class A only.

Interesting thing from the schematics--apparently the 395 is the 500 less the 6550's. Now I want to find a 500 to play with. I used to have a Sundown that ran 6550's and the tone was amazing.

 

[Blueracer]

Glad to hear that it all worked out for you. Sometimes the tubes don't have the blue glow in which case that test is somewhat moot. I would appreciate a copy of the schematic if you don't mind. I have sent you a pm with my email address.

Cheers,
Gary