Ibanez TSA15H poweramp in

[swever]

I know this is probably not the right place to discuss tube amps, but I have a feeling there are many competent people here, so I thought I shall ask a few questions.

I am planning to get this amplifier soon as my first tube head. I don't have an option to listen to it in person but I do like the demos a lot and it seems to me this amp has the tone I am looking for at the moment. They say it takes pedals well, which is obviously a priority for me And the price is good for a first tube amp

The only thing that concerns me a bit is that this amp does not have an actual fx loop. Instead, it has a loop that is located between the built-in tubescreamer and the rest of the amp. I am not really worried about using fx before the preamp. But it may be non-optimal in case I use external preamp. It may work fine, because apparently the whole amp has a lot of headroom, and I have to try it to tell if its a problem or not.

Because of that I wanted to ask if you guys can tell me how complicated would it be to to add direct poweramp input to this amp. Is it just a matter of adding an extra jack socket and a couple of wires or does that require to modify the circuit significantly?

Schematic, some gutshots

Any other ideas and opinions about this amp in general are welcome as well!

[swever]

I finally pulled the trigger and ordered this amp without listening in person. I was sure I was going to like it after reading so many glowing reviews. Which I kind of do - the clean tones are incredible. Even with my single-active-humbucker-in-bridge very metal guitar I can get absolutely beautiful cleans. I think its the best clean tones I've ever heard. It sounds soft and detailed when I pick lightly with my fingers, it sounds good when I use pick and play single notes or strum chords. I never though I would get any sort of "vibe" from this guitar on cleans, but with this amp I do.

BUT

It sounds bad with any sort of od/dist/fuzz. 
I first tried the built in tube screamer and all I can get is a honky dull mushy kind of sound. With either the amps treble or TS's tone set high it still sounds honky and mushy, but with added harshness and annoying high-mid presence. No matter what settings I tried, I could not get a nice tone with balanced timbre and dynamics - that flows and shines and lives.

Then I tried it with all my homebrew pedals and none gave me a satisfactory result.  The pedals with no tone controls or tonestacks sound harsh and raspy - even with amp's treble at 0. A rat sounds either to dull or too spiky and/or honky. Even dr. boogie which I believe is a good "preamp" type pedal does not sound right. Usable - maybe, but not beautiful.

I found this thread. In the end OP said he ended up using 8 ohm output tap with 16 ohm speaker and it sounded better. So I tried mismatching the impedance too - tried both 4 ohm and 16 ohm taps with my 8 omh c-rex loaded cab with no significant improvement.

I have to mention that I run it at really low bedroom "tv" volume. I am almost sure it's not the real reason though, I tried cranking it up a couple of times and it still had the same character. BTW that's another issue with this guy. The first few degrees of the volume pot rotation don't really give any difference in volume. With it in that area I hear very thin low level fizz - just the high freqs of the input signal. Then at some point it steeply goes into normal volume control.

Return is almost out of question due to too high return shipping costs. I think I will be able to sell it here for more than I payed for it anyway if I decide to. And then I really love the cleans, may keep it just for that. I want to see if I can non-destructively mod it. The first thing I'd love to do is to bypass the tonestack completely, which should be rather easy judging by the schematic. This should allow me to see if using an external preamp is going to help.

If that works, maybe at some point I'd could replace the TS circuit with something more useful for me - like another eq/tonestack.

If someone minds to have a look at the schematic in my first post - how do I go about bypassing the tonestack? I think I can just completetly remove the tonestack board (between cn3 and cn4) and replace it with just a volume pot. Is that correct? Should I use a 1M as well without the rest of the tonestack?

[thermionix]

The first few degrees of the volume pot rotation don't really give any difference in volume. With it in that area I hear very thin low level fizz - just the high freqs of the input signal. Then at some point it steeply goes into normal volume control.

That is normal for a tube amp.  They all seem to have that minimum volume "shelf," below which the amp doesn't really seem to be quite "on" yet.  My amp is 40 watts and at that minimum volume can still be too loud for late night apartment playing.  So in that situation I use a pedal set for below-unity volume.  Compressor if I want clean, otherwise distortion/OD/fuzz.

The rest I can't help you with as I am unfamiliar with the amp.

Edit:  Well, I guess it's worth asking, what kind of tubes?  How is the bias set?

[swever]

I haven't opened mine yet. The tubes are supposed to be two 12AX7B "CHINA" and two 6v6  TUNG-SOL. Please have a look at this link for some pics.
There seems to be no official info about bias from Ibanez. And there is some controversial information about it in the webs, some say it's fixed bias, some that it's cathode biased. The consensus seems to be that there's no need to adjust bias manually - its plug and play with 6v6's. Can't comment on any of that though as I have absolutely no experience with servicing/modding/building tube amps - just starting my way on this path. 

[thermionix]

If you pull the power tubes, and turn the amp on standby, you can check pin 5 (count pins counter-clockwise from the notch, looking at it from the outside).  If you get negative DC voltage, maybe around -40VDC measured to ground, that's fixed bias.  Or possibly a combination of fixed and cathode bias, but that's not very common.  If you get 0VDC, it's cathode bias, plug and play.

[thermionix]

Oh I just noticed you linked the schematic in the first post.  It's fixed bias.

[thermionix]

Oh hey man, I found one of your problems.  Locate and remove C10 (100pF).  It's like a Fender bright switch that's always on.  Adds annoying fizz, especially at low volumes.  Just snip it out and don't look back.

[swever]

Gotta try that! No jumper needed in it's place, right?

As for fixed bias... what does that mean in practice?

[thermionix]

Nope, no jumper needed.  Turn the amp off, unplug it, flip the standby switch to ON and wait about a minute.  That will drain the high voltages out of the filter caps.  Then you should be safe to work on it.  There is no DC or dangerous voltage on C10 even when the amp is running, but best to drain it out just in case you accidentally touch something else.

Fixed bias means it should be adjusted every time you put in new 6V6s (use matched pairs).  The adjustment is made at trimmer RV6.  No point trying that unless you have a way to measure idle current and you know what number you're going for.  This is done on a live amp, with potentially deadly voltages, so take it to someone qualified if you aren't 100% sure you can do it safely.

The tube portion of this amp is VERY much based on old Fender amplifiers from the 60s.  If you wanted to get a little more Fendery, you could change the treble cap (C1) from 470pF to 250pF.  That's not such a big deal, and not nearly as easy as clipping out C10.

Oh, and I just noticed that they use the component designations (like "C10" or "C1") more than once each.  I guess once per PCB.  Both of the caps I'm referring to should be on the same PCB as the volume and tone controls.

[swever]

Hell yes! Removing C10 did make a HUGE difference! Now it does sound right! Takes all my pedals well. Now I can clearly hear each pedal's own character really well - they all sound much more different from each other than I ever thought.

I am still not a huge fan of the tonestack. Particularly the treble control. I definitely will try the mod that you suggest! What kind of difference should I expect? It would be super neat if it would shift the frequencies adjusted by treble pot higher - id love to be able to control the very highs without touching higher mids that much.

And then since the amp is modular I think I can try some other tonestacks - I have no problem etching another pcb to put in place of the original tonestack. Would be fun to see if that shall work. Anything special to be aware of in that regard?

Btw, here are some gutshots of my unit. Looks a bit different. The tubes are all labeled "Ruby". Should probably mean that these are selected chinese tubes. Wich probably means I'm not in a hurry to replace them and have enough time to learn how to measure idle current and find out the number I should be going for 

thermionix, thank you very much for very useful information and help! What a great place this board is!

[Frank_NH]

Interesting thread!  I was looking at the tube screamer part of the circuit and it uses an op amp buffer at the input and an op amp boost stage (for optional boost) after the tube screamer.  This could be used by someone wanting a TS-9 pedal with a foot switchable boost.

[J0K3RX]

thermionix - good catch! R61/6k8 could be replaced with a 10k pot for a full Fender tone stack.. Although, you would have to be creative on the placement since there is not much room on the face plate for another knob..

[swever]

A mid control... that would be great. A stacked/concentric pot may be one possible solution for fitting it without drilling the chassis.

Which voltage rating is acceptable minimum for the c1 (250pF)? The original 471 is 1kV, does it really have to be that high?

And the original question is still actual. I'd greatly appreciate any advice or directions on how to implement direct poweramp input. As well as opinions if it makes any sense or if simply bypassing the tonestack is a better solution for pairing this amp with external preamps.

[thermionix]

It would be super neat if it would shift the frequencies adjusted by treble pot higher

Yep, that's what it would do.  A midrange control would be cool too, but 6.8K is a nice value if you want to leave it fixed.  Many old Deluxe Reverbs sound perfectly fine with that setup.

Which voltage rating is acceptable minimum for the c1 (250pF)? The original 471 is 1kV, does it really have to be that high?

I would stick with a minimum of 400V, though you're more likely to find one rated at 500V.  That cap wouldn't see more than 200V in normal operation, but if you ever had to power it up without tubes in, while diagnosing a problem maybe, it could see close to 400V.

thermionix, thank you very much for very useful information and help!

No problem, glad I was able to help!

[thermionix]

Noticed there's no NFB.  Pretty un-Fender, would have to hear the amp to form an opinion on that though.

The caps across the speaker jacks are a new one on me, never seen that before!

[swever]

I did some reading on negative feedback and it appears to be a measure to increase fidelity and decrease distortion of an amp. Not sure if my thinking is correct, but it seems to me this amp does not really need any improvement these areas. On the other hand, NFB is also supposed to decrease amp's gain, which could be beneficial, as it is still too loud for home use. How hard would it be to implement NFB (probably switchable) to this circuit? Any directions on where to find info about this topic are very welcome!

And yes, caps across speaker jacks seems to be a very uncommon thing. Do you have any guess why they did that? Would it make any sense to try to remove them?

Btw I did find one 220pF 1kV cap that I bought when I was just starting the whole diy electonics thing and that I had no use for. I guess its been sitting in the box and waiting for this day! =)

[thermionix]

And yes, caps across speaker jacks seems to be a very uncommon thing. Do you have any guess why they did that? Would it make any sense to try to remove them?

My guess is that they were trying to tame the high end a little.  Maybe high end that wasn't otherwise tamed with NFB.  Seems an odd way to go about it.  And the two caps (per jack) doesn't make any sense to me either.  Personally, I would want to see what it's like with them removed.  Nothing would blow up, that's for sure.  Maybe lift one leg each of the two caps that are across the jack you're currently using.  Easy to put back if you don't like the result.

NFB tames the high end and low end, lowers gain somewhat, and adds some compression.  Wouldn't have much effect on volume really.  If you don't think the amp needs the changes, I wouldn't bother with it.  It's very simple, but maybe not so easy to implement on a PCB.  And there's always the 50/50 chance you get to find out what positive feedback sounds like!

[swever]

I changed the treble cap in tonestack to 220pF yesterday. I'm not completely sure I like it more this way. It seems the amp has got slightly darker overall - which means I need to push the treble control higher. And after about halfway it starts to grab too much of that higher midrange and makes a pronounced spike somewhere around there and again makes everything sound tinny/harsh/annoying - just like it was with a 470 pF cap. I'm going to experiment with a lower value "mid" resistor or maybe even a pot and then see if I decide to put back the 470 pF cap for treble. Or maybe it's time to consider trying what negative feedback can do to this circuit 

I removed the output jack caps too. It seems it added just a touch of "air", but I'm not absolutely sure whether it's not just me wanting to hear that... I think I will leave it this way unless there are any safety/reliability concerns.

[thermionix]

Messing with the midrange resistor should be fun.  Increase for more mids, you can decrease it all the way to zero for extreme scooping, won't hurt anything but your tone.

You can also change the mid cap (C3), Fender used 22n, 33n, and 47n in that type of tone stack.  Yours is 47n stock, 22n would brighten up your mids.

You can also play with the slope resistor (R1).  Yours is the typical Fender 100K.  Marshalls usually used 33K or 56K, though that was in combination with different cap values, different pot values, cathode follower, in a different place in the circuit.  Still something you can tweak.

A buddy of mine has a Deluxe Reverb, and he has the Marshall tone stack values in his "Normal" channel; 500p, 22n, 22n, 56K.  Sounds really cool, quite different than the regular values in his "Vibrato" channel.  Two voices in one amp.

Have you played around with the Duncan Tone Stack Calculator?

http://www.duncanamps.com/tsc/

[swever]

Yea I did try the Duncan Tone Stack Calculator. Not sure how to choose the load and source impedance values, and it seems there isn't a simple method to tell that does not involve intense science that I am so bad at

Have yet to try messing around with mid cap and slope resistor.

I added a mid control and now I think I want it to stay. Also I am not really very much into the TS-boost section, so I think I am going to completely remove it to make space for the mid pot. Just need to figure out how to correctly terminate it. And what to to with the remaining two knobs/holes and two switches.

The plan is to move the high/mid/low controls where TS controls are now. What if I connect some pots (eg voume) with wires instead of directly to pcb? Can it cause any sort of problems?