Ibanez TSA 5~15 Combo

[rldunc]

Purchased new. Replaced stock tubes. Do I need to bias? Have read that these do NOT need biased, have been told that YES they do. Really like this little thing. Wanting to sweeten this up a little. Also will be replacing the stock speaker.
Thanks. ✌️

[shooter]

Have read

the schematic holds the answer

[tubenit]

Here is the schematic. 

This shows it to be "fixed" biased which has adjustments.    The 6V6 cathodes are grounded not cathode biased.

This looks to be your first post?   Do you know how to work on amps safely?   Do you know how to bias an amp safely?

[shooter]

Thanks Tubenit, I figured if he could produce the schematic, we might be able to "walk" through it 

[Latole]

Yes amp need bias check and adjust if needed.

The bias should always be checked on any amp even on cathode bias amps often called class A or automatic / self bias.

If you ask about bias, you may not know how to ?
Best is to put same brand of tubes if bias was good before.

[shooter]

Best is to put same brand of tubes if bias was good before

I've found "best practice" is to verify bias, no matter what it did before.

[2deaf]

Here is the schematic. 

A fuse on the bias circuit.  Now there's something you don't see everyday.  I think I'll pass.

[2deaf]

Yes amp need bias check and adjust if needed.

The bias should always be checked on any amp even on cathode bias amps often called class A or automatic / self bias.

If you ask about bias, you may not know how to ?
Best is to put same brand of tubes if bias was good before.

The only time a cathode biased amp is called "Class A" is when it is, indeed, Class A.  If a cathode biased amp operates in Class AB, it is not referred to as "Class A" simply because it is cathode biased.

The brand of tubes has nothing to do with how they will bias up.  I have 32 Sovtek EL34's here and there ain't no two of them that bias the same.  In fact, the range is huge.   

[PRR]

> If a cathode biased amp operates in Class AB

It can go some into class AB. If it goes a lot into AB the rise of current screws-up the bias and it sounds janky.

[DummyLoad]

Here is the schematic. 

A fuse on the bias circuit.  Now there's something you don't see everyday.  I think I'll pass.

UL, VDE, and other safety agency directives - all taps of a power transformer must be fused. if the bias fuse blows, the power tubes go into runaway, blowing the B+ supply fuse, usually after much destruction: but hey, no fire, right? then again, why would a bias fuse blow? just my 2cents. 


--pete

[2deaf]

UL, VDE, and other safety agency directives - all taps of a power transformer must be fused. if the bias fuse blows, the power tubes go into runaway, blowing the B+ supply fuse, usually after much destruction: but hey, no fire, right? then again, why would a bias fuse blow? just my 2cents. 

Do those labs say anything about the fuses being realistic?  I'm having my doubts that a winding that is driving a load of 170uA can muster 500mA.  But just in case it can, the fuse is slow-blow making it all the more likely that the bias winding will burn up before the fuse blows.

[shooter]

all the more likely that the bias winding will burn up before the fuse blows.

after 38yrs, I've found many a good fuse in broke sheit 

[DummyLoad]

UL, VDE, and other safety agency directives - all taps of a power transformer must be fused. if the bias fuse blows, the power tubes go into runaway, blowing the B+ supply fuse, usually after much destruction: but hey, no fire, right? then again, why would a bias fuse blow? just my 2cents. 

Do those labs say anything about the fuses being realistic?  I'm having my doubts that a winding that is driving a load of 170uA can muster 500mA.  But just in case it can, the fuse is slow-blow making it all the more likely that the bias winding will burn up before the fuse blows.

agree - allow me to be clear: i too believe that the 500mA bias ckt. fuse is a joke: even if a diode junction fails short. perhaps what is more likely, is that the manufacturer retaliated against a compliance directive and installed a fuse that doesn't?

those "safety parts" are there to simply skirt a directive to sell in a particular market and a potential lawsuit. transformer thermal cut-out switches are much more useful.

murphy's law of TV repair:  a 100$ transformer will blow to save a 10 cent fuse. 


--pete