Ibanez TSA5TVR (cute little 5w TV looking amp) with no filament voltage

[christarak]

My head is in overload. It looks like this should be a simple fix, but it has me stumped. I've spent way too much time on a cheap amp....it's now a matter of pride and principle.

This little Ibanez amp is not behaving with the filament voltage. . (Hopefully the power supply schematic was uploaded OK.)
Here are some clues. The voltage seem OK with the 2 tubes pulled out. When they are plugged in the filament voltage drops (greater drop with both tubes in place). This is supposed to look like a table, so please use your imagination :-)

---------------------Tubes out------Tubes In
D17 Cathode-------9.6 V-----------5.82V
6V6 pin 2----------9.57 V----------2.00 V
R54-R60 junction-9.58 V----------2.5 v

I have replaced Q8 and R60. I have installed my own 2 "test" tubes for 12AX7 and the 6V6 just in case the customers tubes are dragging down the filament.

(My apologies for not being able to post the complete schematic, but I gave my commitment to the Australian distributor that I wouldn't).

[Enzo]

So what is the AC voltage at P5, P6, and what does it do under load.

Do you get the same readings if you measure right at the bridge diodes?

Measure resistance from P5 to D17 anode. Measure from those points, not just across the fuse. I am doing it to test the fuse and holder, but we want to know if the whole path is good.

[Dave H]

Do as Enzo suggested first and if that is all OK measure the filament current. Set your meter to DC current and connect it across Q8 C to E. The raw supply only has a 1000u smoothing cap so there will be a lot of ripple which could be why it's sagging to 5.82V. Q8 is running 60mA of base current and it has a min gain of 25 (@ 1A) which implies more than 1.5A filament current. I don't think it has enough gain to drive the (cold?) filaments. The (hot) filament current for a 6V6 + 12AX7 is only 0.75A.

[christarak]

So what is the AC voltage at P5, P6, and what does it do under load.

Do you get the same readings if you measure right at the bridge diodes?

Measure resistance from P5 to D17 anode. Measure from those points, not just across the fuse. I am doing it to test the fuse and holder, but we want to know if the whole path is good.

Thanks again for your help Enzo. I've carried out your tests and Dave H's. To save repeating, I'll put findings of both recommendations under Daves. :-)

[christarak]

First I performed Enzo's tests. Then I measured Q8 BC&E (What is the purpose of Q8? Is it to shut off the filament supply if there is a short on the filament load?). Then I did Dave's tests with some surprising results

Test.................No Tubes............With tubes
P5-P6..............8.22VAC..............8.12VAC
D17-D18..........8.22VAC..............8.09VAC
R (P5-D17).......0.04 ohm (Fluke 87V zero'd first)

Q8C................9.67VDC....................5.82VDC
Q8B................9.52..........................2.46
Q8E................9.43..........................1.88

IC-E................1.17mA.....................230mA (6V6 only - the way I had the board precariously perched, I was unable to install the 12AX7)

Then I noticed something very interesting. Because my Fluke had shunted across the Collector - Emitter junction, the filament was now glowing and 6V6 Filament voltage was 5.26V (that's pretty close to 6.3V :-) ) AND I could hear a pleasing "shhh" out of my speaker. (no signal because of no preamp valve). When I removed my meter the Filament voltage dropped to 2.58v and all joy was lost. Perhaps I should just return the amp with my Fluke attached.

Should I be suspecting Q8 that I had already replaced? I would take a photo of my abomination of a transistor transplant, but I know I would be cast into the wilderness as being unworthy of sharing the same internet space as these two electrospiritual gurus.

I await your words of wisdom
Let it be...

Chris (the ham)

[Enzo]

For me you just verified that good AC was getting to the bridge.


It sure sounds like the transistor isn't working. But interesting to me that an E-C short there didn't put the full 9v onto the tube heater.

Are we sure that R54 is 56 ohms?

[christarak]

I see what you're saying, Enzo. I checked R54 and R60 again, and their values look fine. I haven't bridged any of the tracks. I might try replacing the TIP31C again just, in case I cooked that somehow. What would be the consequence of me permanently bridging Q8?

[Dave H]

It sure sounds like the transistor isn't working. But interesting to me that an E-C short there didn't put the full 9v onto the tube heater.

9V is the peak unloaded voltage. With both tubes in there could be something like 5V p-p ripple. The peak of the ripple will be at 9V and the trough at 4V so the average reading should be around 6.5V but it's reading even lower than that. I'd have to get the scope out to see what the ripple really is. I'd also check C49

[Enzo]

It has a thousand mike filter...

With his base at only a couple volts, his emitter will follow along down there.

[Dave H]

I might try replacing the TIP31C again just, in case I cooked that somehow. What would be the consequence of me permanently bridging Q8?

Yes, replace Q8. I think the circuit is marginal. It will take a high gain Q8 to make it work. Bridging Q8 will put a lot of ripple on the filament supply which may be audible in the speaker. If it is you could try increasing C49 until the filament voltage is 6.3V. My Valve Junior uses a 4,700u cap and it doesn't hum. Going back to the original circuit I think 1000u is too small for C49 try a 4,700u to see if the filament voltage increases. It says it should be 5V on the schematic. Why are they running the filaments at 5V?

[Dave H]

It has a thousand mike filter...

With his base at only a couple volts, his emitter will follow along down there.

I don't think 1000u is enough

Yes the emitter will follow but why is the base at only a couple of volts? It puts 3.4V across the 56ohm which is 60mA of base current. Either Q8 is low gain or C50 is leaky. I wonder if the spiky waveforms are causing confusing DC readings on the Fluke? Could you check with a scope christarak?

[christarak]

I don't think 1000u is enough

Yes the emitter will follow but why is the base at only a couple of volts? It puts 3.4V across the 56ohm which is 60mA of base current. Either Q8 is low gain or C50 is leaky. I wonder if the spiky waveforms are causing confusing DC readings on the Fluke? Could you check with a scope christarak?

Thanks Dave. I'll get the scope on it today and watch the ripples. I'll be back (after you get a sleep)

[christarak]

Well...there was way more ripple than I expected. I removed the 1000uf and it measured open circuit on the Fluke (I checked the Fluke with a spare 2200uF cap to be sure it can read this. No prob at 2240uF). Sadly this cap is too fat to fit, so its off to the parts store tomorrow morning. Just to be sure I changed Q8, R54 and R60. I might even change C45 just because its annoying me with its whimpy, "why am I even here" looks. What's that you say C50? "What about me?". OK, you're going too.
Thank you Dave and Enzo. Please check in tomorrow to see if your experience has helped a colleague from the Land of Oz.
Chris

[Dave H]

What is the purpose of Q8? Is it to shut off the filament supply if there is a short on the filament load?

It's not to shut off the supply. It's for ripple reduction. There will be a lot of ripple on the Collector but the ripple on the Base is smoothed by C50 and as Enzo said the Emitter (output) follows the Base.

[Enzo]

a colleague from the Land of Oz.

Oh, I forgot. For this circuit to work down under, you must turn it upside down.


(Yes, I never get tired of that joke)

Your 2200 cap may not fit, but you could tack it into the circuit to see if it makes the difference.