Tiny Terror bias?

[Jacew]

I have been playing my new TT for some time now, and noticed that I like 7w setting much more. It has more compressed tone (perhaps a bit too compressed), and it has much better response to guitar setup (like pickup selection), than 15w. It seems to have more spark too.

In fact, I think casual listener might be unable to hear difference between pickups when playing in 15w mode...

Is this kind of difference common thing in amps with switchable power? And could it have something to do with different bias in each setting? Based of what I've read, TT is cathode bias, and is biased with a single 120 ohm resistor. Which makes power amp tubes run cold in 7w and too hot in 15w.

From here: http://music-electronics-forum.com/t1477/

180 ohms is a good choice at 15W power; I replaced the power select switch in mine with a DPDT (Mouser sells the same switch brand and family) and used the other pole to place a 100 ohm resistor in parallel with the 180 when I went down in power. Worked great.

Could this type of mod even out the difference between 7w and 15w?

It would certainly be good for tubes, and considering that I like cold running 7w more, I guess the 15w running optimal bias wouldn't sound too bad either...

[zzmoore]

Re: Tiny Terror bias?

It makes sense.
It is simply a matter of giving it a try and see how YOU Like It.
good luck

[beaubrummels]

Re: Tiny Terror bias?

Hotter tube bias always sounded cleaner with more sparkle and better to me. Shortens the life of the tubes, though.

My Orange AD50 sounds better to me at 30 watts. I believe because the lower power is Class A rather than Class A/B at 50. Not sure if their smaller Terror amps have that difference? 7 and 15 are both Class A?

[Jacew]

Re: Tiny Terror bias?

I think both are class A/B, as switching is done by simply dropping the voltage on tubes. I don't know if that can change the operating class though...

[Jacew]

Re: Tiny Terror bias?

Ok, so I did this mod.

Little background:
First of all I'm gonna change my opinion about the original 7w setting. Being really fizzy and crackly, it's really nowhere near as good as 15w. (I think my speakers weren't yet properly broken-in back then...)

So I changed the SPST 7w/15w switch to DPDT to accommodate cathode resistor for both levels. I positioned the SPST switch next to the Power Transformer and wired it to switch between other set of resistors to allow adjusting the amp voicing with cathode resistor in 15w setting.

So cathode resistor for 7w setting ended up being 76 Ohms. For 15 watts, I have this switch for 3 different "voicings"; first they were something like 134, 145 and 156 Ohms, which were useless except the lowest. Better than original 7w, but still very sterile, cold and fizzy. So switched resistors and settled to choice between 122 (120 ohms was the original 15w), 126 and 132 ohms.

Here's the verdict so far (after two days):

It was really surprising how big difference such a small change makes there. 122 is really the "standard" mode, I don't think there's noticeable difference there. Very round and blooming voice, that TT is known for. While it is great for pure "overdriven rock" tone it's ever-presentness gets problematic. Like I said earlier, you can't get much variety tone-wise from guitar controls.

132 on the other hand gets bit to the cold side. So with tubes I'm using (JJ EL84) I settled to 126. It does have some of the warmth, but doesn't get overpowering. I could describe that it adds a hair of that Vox-like clarity and bite to the tone.

7w setting, which that switch doesn't affect, has a very usable tone of it's own too now. Rounder and hotter. I can't say how much of that is because of the lower voltage and resulting lower headroom. It's pretty difficult to compare them head-to-head.

[Gearjoneser]

Re: Tiny Terror bias?

Good info. I never had a problem with the tone in 15W mode, but rarely like any amp in half power mode, including the TT.

I never think in terms of cathode resistor value, only tube mA. When optimal at 126, did you measure the mA of each EL-84?

Unfortunately, I don't have a bias tool that fits EL-84 sockets, so I never explored bias in these amps. Plus, EL-84 amps usually lack a bias pot.

[Jacew]

Re: Tiny Terror bias?

Good info. I never had a problem with the tone in 15W mode, but rarely like any amp in half power mode, including the TT.

I never think in terms of cathode resistor value, only tube mA. When optimal at 126, did you measure the mA of each EL-84?

Unfortunately, I don't have a bias tool that fits EL-84 sockets, so I never explored bias in these amps. Plus, EL-84 amps usually lack a bias pot.

I don't own a bias tool, so I didn't measure the mA. Just went by my ears, and information from that link in my original post.

It does lack the bias pot, I changed the resistor to higher value, and put that switch to add second resistor in parallel with it to set the resistance, plus resistor in series when set to 15w. It went a bit complicated, because I got short of right value components.

EDIT: And yes, lowering the cathode resistor value to 73 ohms made huge difference to 7w tone. I would say it fixed it..

[Gearjoneser]

Re: Tiny Terror bias?

But then, you've got to wonder if the tubes are running way hot in 15W mode. If you can't measure your experiment with a meter, at least keep the chassis exposed in a dark room so you can look at the power tubes. If they start looking too hot or turning red, you lowered the resistance too much. Over biased amps are past the point of sounding bold and compress too early.

I must say, I love the Orange Tiny Terror. It's a real tone machine with 3 knobs that do so much. I only wish it had an FX loop.

[Jacew]

Re: Tiny Terror bias?

I didn't go hotter than standard with 15w (120 ohm resistor), and haven't seen any red-plating with 7w either.

TT indeed is great machine. It's gain and volume give nicely wide range of tone for such a simple amp. I've never found that much use for it's tone control though. It always sits between 10 and 12 o'clock: If you go lower, it gets boomy with too little highs, and higher, it also accentuates higher mids, making tone really harsh.

I'm actually thought about adding FX-loop in it too. That is really the biggest drawback of it.