New IIC, Need an Expert!

[glawk]

So I picked up an old Mesa IIC yesterday from the original owner. The amp is in really good shape, sounds great. But...there are a few things I have questions about.

What does the 35 mean?

What is this weird knob? You can see in these pics what its connected to...what does it do?

 

[zodiac272]

nice! you got one! looks like it's pot attached to the power supply caps. have you noticed a difference when changing it's position? If you got it from the original owner... the old owner should know. looks like an aftermarket mod


scott

 

[glawk]

I tried moving it...doesn't seem to do anything? hmm...I guess the good thing is that it was installed in a screw hole so it should be totally reversible.

 

[glawk]

I think maybe it is a bias mod...i was playing with it and it do anything. After playing a while there was a power surge and the amp shut off. Now it doesn't turn back on...maybe the fuse is blown? Or the power tubes...they were just groove tubes but still :evil: :evil: :evil:

 

[glawk]

Ok, so I bought a new fuse...it powers on now. One of the power tubes does not come on. Just need to find out if it blew by chance...or because i was fiddling with that knob...

 

[zodiac272]

a bad tube probably took out the fuse and a grid resistor so that tube won't light up.

scott

 

[glawk]

a bad tube probably took out the fuse and a grid resistor so that tube won't light up.

scott

So I guess I have to take it in for service...still wondering what that weird knob does.

 

[zodiac272]

it would be a good time to send it to mesa for the "+" upgrade.


scott

 

[glawk]

Yeah, you are right. Although I did check that tube socket with another tube and it lit up. Took out the bad tube...the glass was loose from the base...guess it just blew up.

 

[dodger916]

nice! you got one! looks like it's pot attached to the power supply caps. have you noticed a difference when changing it's position? If you got it from the original owner... the old owner should know. looks like an aftermarket mod


scott

+1. My C+ was an upgraded IIA. It had a small bias trim pot on the board right where those wires are connected. My guess is it's a bias pot.

 

[Blueracer]

The control is almost certainly a bias adjust for your power tubes. If the bias is set too low (less negative) the tube will draw too much current and if you are lucky the fuse will blow. If you are not lucky the output transformer, power tubes, traces on the circuit board, screen resistor are all subject to failure. This is as dumb a mod as I have seen for a while. My suggestion is get it to a tech, preferably one qualified by Mesa, and get that thing yanked. Further suggestion. Do not turn your amp on until you have reversed this mod. The workmanship on this is iffy at best (I have seen worse) but the ultimate in dumbness is having the control externally accessable. Count yourself lucky if the only thing you damaged is the fuse.

 

[bancika]

I agree it's stupid to leave it exposed, but it's not stupid to have it since the chassis is already drilled because it will make it easier to swap tubes with other brands. I'd get it to someone to replace it with a locking pot that cannot be turned by mistake and make the wiring a bit tidier.

 

[glawk]

Tested quickly...the good tubes still light up...all the sockets seem to work. Going to go have that mod removed and retubed.

 

[Blueracer]

I agree it's stupid to leave it exposed, but it's not stupid to have it since the chassis is already drilled because it will make it easier to swap tubes with other brands. I'd get it to someone to replace it with a locking pot that cannot be turned by mistake and make the wiring a bit tidier.

The "dumbness" aspect of this mod from my point of view is that it is externally accessable and looks like some sort of volume or tone control. If I saw that on an amp my inclination would have been exactly what glawk did. Give it a turn and see what it does. I believe that glawk said that the pot was put in through an existing screw hole so it should be reversable. I don't have a problem with bias adjust mods and in general I don't agree with Mesa's "we will pick tubes that are good to go in the amp and fix the bias" philosophy. There is one aspect to be aware of though. If the amp has simulclass then the inner and outer power tubes are biased differently. Further there is only one bias voltage feeding the power tubes hence the inner and outer tube biases are coupled. This is problematic for an adjustable bias scheme unless more extensive modifications are made to provide separate bias controls. For the 60/100 watt switchable amps this is not an issue as all the power tubes are biased the same and a single control is sufficient.

So to summarize.

1. Get rid of the control on the outside. This is just an accident waiting to happen.
2. If the amp is 60/100 watt then there is a choice for a bias control or revert to stock.
3. If you do put in a bias control have the pot inside the amp.
4. If the amp is simulclass completely remove the mod and go to stock

Hope this is helpful and that you enjoy your amp.

Edit : Noticed that the amp is DRG meaning it is simulclass. As such I would remove the bias control altogether and revert to stock.

 

[dodger916]

There is one aspect to be aware of though. If the amp has simulclass then the inner and outer power tubes are biased differently. Further there is only one bias voltage feeding the power tubes hence the inner and outer tube biases are coupled. This is problematic for an adjustable bias scheme unless more extensive modifications are made to provide separate bias controls. For the 60/100 watt switchable amps this is not an issue as all the power tubes are biased the same and a single control is sufficient.

Good point. I should have caught that. The "D" in the "DRG" designation on the chassis indicates Simulclass.

 

[Mongo1]

Blueracer,

I'm curious why a Simulclass amp would not get the same benefit of an adjustable bias? I can understand the difference in the way the inner and outer tubes are configured. Would it not still be beneficial to have the capability, knowing it would be to dial in the collective bias of both pairs?

I ask this because I've been considering whether my MKIVa would sound better configured properly with some NOS 6L6's.

 

[zodiac272]

my C+'s are stock, one is simul and one is 100/60 watter....


With the simul, I can use STR 415's for all 6 output tubes, OR, I can plug 2 STR 416's in the middle, and 4 STR 415's in the outer pairs without swapping any bias resistors. I have all mesa branded STR 415 and STR 416. With this setup, I don't need a bias pot, but I believe a bias pot would work....

scott

 

[glawk]

Took it to the shop today, it's going back to stock.

 

[dodger916]

Blueracer,

I'm curious why a Simulclass amp would not get the same benefit of an adjustable bias? I can understand the difference in the way the inner and outer tubes are configured. Would it not still be beneficial to have the capability, knowing it would be to dial in the collective bias of both pairs?

I ask this because I've been considering whether my MKIVa would sound better configured properly with some NOS 6L6's.

The inner and outer pairs run different bias currents, so you can't get both pairs biased "right" using a single bias pot. You'd probably need to have bias pots for each pair of tubes.

For the Mark IVa, get a quad of STR 420s. I replaced =C=s with 420s and never looked back.

 

[mule#1]

Blueracer,

I'm curious why a Simulclass amp would not get the same benefit of an adjustable bias? I can understand the difference in the way the inner and outer tubes are configured. Would it not still be beneficial to have the capability, knowing it would be to dial in the collective bias of both pairs?

I ask this because I've been considering whether my MKIVa would sound better configured properly with some NOS 6L6's.

The inner and outer pairs run different bias currents, so you can't get both pairs biased "right" using a single bias pot. You'd probably need to have bias pots for each pair of tubes.

For the Mark IVa, get a quad of STR 420s. I replaced =C=s with 420s and never looked back.

Where did you get the 420's? Were they Mesa Labeled or just Chinese tubes?