New Build Blues Amp

[Pdavis68]

So I've got this amp build on my breadboard right now and it's got some issues. I think I actually have a bad 12AU7, so I'm going to do some tube rolling (I have about 45 12AU7s, surely I can find a good one). Unfortunately, my tube tester is behaving badly and I need to fix that as well... But I thought I'd go ahead and post the schematics of what I have so that if there's anything obvious I'm missing, maybe someone could point it out and save me some time.

There's a very basic schematic of the PS (after rectification. SS rectification) and a separate schematic of the amp as it is on the board. Also a photo of the board if anyone's interested.

Then I have shots of the oscilloscope (sorry they're blurry. They looked okay on the phone). The photos are taken at the grid of each of the 4 stages. The first 2 stages are measured at .2V/div and the latter 2 at 1V/div. As you can see, the first stage is amplifying cleanly and then the second stage has some cutoff clipping. What I can't figure out is what's happening in the third stage (the 12AU7). I chopsticked it and it started oscillating after several taps, so I think it might be a bad tube. I've got a total of six 6U8s and a bunch of 12AU7s, so I'm going to start rolling some tubes.

I realize that my setup is going to be prone to noise and oscillation regardless of all other considerations. This is only the second amp I've tried to breadboard on it. The first one after some shortening wires was pretty noise-free and completely oscillation free, so I think it's possible to have it work if my setup is good.

Things are a little crammed on the board. I've got some more barrier terminal blocks on the way to help me lay things out a little better (like getting the PS filtering further away from the PCB) in the future.

Also, as for the PS schematic, I have the voltages as I measured them during testing. It's on a variac and only running at about 90V right now. The very low values on the resistors is because I'm trying to maximize voltages (hence the huge caps to make up in the filtering). I may be asking too much of it, though and really, I plan on getting a higher voltage transformer for the final build. The current transformer is 230V secondary (but down about 20% because of the variac).

Please feel free comment on any of this. The design is a combination of a few different schematics and some of some shooting from the hip in a few areas. Some things, like the resistors in parallel with the pots is just because I'm testing different values.

Also, with the exception of the wire connecting pins 2&7 on the 12AU7 and the wire to the grid on the 6V6, all other grid wires are grounded coax.

Thanks!

[loudthud]

The 12AU7 is not biased properly. The plate Voltage is probably very low, around 75 Volts. It needs 5 or 6 Volts at the cathode to get the plate Voltage up to around 60% of the B+.

[Pdavis68]

My B+ is quite a bit lower, but the biasing of the 12AU7 comes from this.

On my setup, at full power the B+ is about 280V with 85V at the plate. Why does it need to get the plate voltage to 60% of B+?

[Pdavis68]

I followed your advice. Swapped the LEDs for 2.7K and 3.3K resistors and fully bypassed and it's MUCH better. I still need the gain knobs turned down quite a bit. It's not distorting nicely (yet), but this is a huge improvement. In fact, it was so quiet when I fired it up I didn't think it was working. Then strummed the guitar and music. It's amazing to me that this mess of wires can be so quiet.

[Pdavis68]

I'll post a new schematic soon, but I've changed R9 to 68K. As I said before, I have replaced D1 and D2 with 2.7K and 3.3K resistors and bypassed with 100uF caps. It's sounding a lot better now. It appears C2 (the only vintage cap I stuck in this because I didn't have a .1uF cap handy) is leaking like a sieve. I'll have to dig around and see what I can find to replace it. I think all I have are vintage caps in that value range. Surely I can find one that doesn't leak.

[loudthud]

I don't know what those AX84 guys were thinking. Maybe they wanted to make lots of distortion. With such a low plate Voltage there was probably lots of grid current causing scratchy pots. It might work better with a 12AX7 or 12AT7 but you'd need two or three LEDs in series to bias a 12AU7. Another factor is that a 6V6 needs about twice the grid drive of an EL84.

[Pdavis68]

I can't seem to get the 12AU7 right. Currently the way it's configured, I've got a B+ of 279V, and with no signal, 145V at the plate (22K plate resistor) and 5.9V at the cathode (2.7K cathode resistors)

I can get a pretty clean sine wave going in at 5V or less, but coming out the top half of the waveform appears to look okay, but the bottom half looks a lot like the bottom half of the waves in Stage4X5 photo in the first post. If I cut the signal going into the 12AU7 down to under 1V or so, it doesn't get too messed up, but it quickly goes bad after the signal goes over 1V. I've tried a few different 12AU7s and I get the same results.

Updated with the latest schematic.

[MarkusBass]

Is it just a schematic, or is it a LTSpice simulation (I see that you used LTSpice)? If it is a simulation, why don't you post it here to check it? BTW, on the first schematic one resistor (R10) is 1000k and on the last one it is 100k only. Is it a mistake, or is it on purpose? I'm also not sure why would you want two different cathode resistors for U2a and U2b since they are connected in parallel. What is the reason for this?

Mark

[Pdavis68]

Mark, I'm just using LT Spice for the schematic. I'm not doing any sims with it. I've got everything laid out on my breadboard, so all the numbers I'm posting are measured.

Yes, I cut R10 to 100K because I needed to seriously attenuate the signal. Because it's on the breadboard, it's subject to lots of changes, so the attached photo is the latest. I've cut back on the plate resistors on the 6U8. I can't figure out what's going on with the output of the 12AU7.

I've attached a few more scope photos.


Stage3_input shows the input into the 12AU7 @ 5V/div.

Stage4_input_5v_div_25pct shows the input to the 6V6 (between C8 and R17) with VR3 @ 25%
Stage4_input_5v_div_50pct shows the input to the 6V6 (between C8 and R17) with VR3 @ 50%

How do I fix that?

[eschertron]

LT's comments about distortion are right on. I built a small amp in which I use an AT7 and optionally a AU7 to drive a Hammond SE transformer. My load is much less that your 22k R11, and for the AU7 (paralleled, like yours) I'm biasing at about -11vdc to get the center bias I want I want. If your bias is warm - and I suspect it is - grid limiting may be responsible for the lopsided signal you get.

[Pdavis68]

LT's comments about distortion are right on. I built a small amp in which I use an AT7 and optionally a AU7 to drive a Hammond SE transformer. My load is much less that your 22k R11, and for the AU7 (paralleled, like yours) I'm biasing at about -11vdc to get the center bias I want I want. If your bias is warm - and I suspect it is - grid limiting may be responsible for the lopsided signal you get.

Yes, that's all starting to sink in. I don't think I'm going to be able to get the voltage swing i need out of a 12AU7. I've just been going through my tubes. I have a bunch of 6BQ7s (same pinout as 12A?7. Amplification factor of 38.) that I think I'll try. I also have a couple 12AT7s, so I can move up to that if necessary. Also, for the time being, I'm just going to go with half a tube (single triode) to simplify things and then I'll try paralleling them later, once I get the kinks out. The paralleling is just to get extra harmonic content to thicken up the tone a bit. Not sure how well it's actually going to work in practice.

[Pdavis68]

Going out of town for a few days, so I won't be able to make much progress. Loudthud, thanks for your help and your comments prodded me to go back and reread chapter 1 of Merlin's book which explained the 60% of B+. Thanks for that! As I'm actually doing this stuff for real instead of it being a mental exercise, more and more stuff is making sense to me in that book. Unfortunately there's still a ton of stuff I don't know. It's going to take a while to get good at this.

[loudthud]

This business of connecting two triodes in parallel and biasing them differently can give crazy results as the scope shows. If it were me, I'd bias them the same and tweak the bias for best result, then stick a trim pot in there to offset them to see if any improvement could be had. You might end up with a switch or another user accessible pot.

[Pdavis68]

I'm not even convinced you need to bias them differently. Most of the triodes I tried varied by 1/2 volt or more at the cathode, even with the same resistors on both sides, presumably because of a difference in current. I'm just g oing to test with a single triode there until I get the other issues solved. Then I'll try to add the other half.

[Pdavis68]

Finally got to work on the amp again today. I had a transformer on my breadboard that was just for the heaters (it was a big Thordarson with an HV tap as well). Before I left town I was working on it and when I pulled my scope probe to check something, it dragged a piece of wire over the primary on the Thordarson and toasted it, so that cost me a bit of time. So I can't run my variac as low now since the heaters are running on the same transformer as the HV now...

So I rebiased the triode on the 6U8. I completely redid the power supply and the voltages are now very different. But bottom line, I'm very close to where I want to be. My grid leak between the 12AT7 and 6V6 is too small, so I can't overdrive the 6V6 yet, but the tone is really sweet sounding right now. Everything maxed out, it's just on the verge of breakup and sounds really nice. But obviously I want to be able to push it harder.

I did some reading of Merlin Blencowe's chapter on small signal pentodes and I've decided I need to swap the triode and pentode in the 6U8 so that the triode comes first and the pentode comes second, so that's next on my list.

The current setup is:

6U8 Pentode -> 6U8 Triode -> Volume/Gain -> 12AT7 -> Volume -> 6V6

What I need to do is:

6U8 Triode -> Volume/Gain -> 6U8 Pentode -> 12AT7 -> Volume -> 6V6

I need the gain pot moved before the pentode so I can choose to overdrive the pentode or not (Merlin's argument was you can't overdrive the pentode straight from the guitar, so there's no point in having it in the first position, since it's that overdriven pentode sound you're going for. Makes sense.)

I'm enjoying building based on a function generator and oscope instead of doing the math on everything. I hate math. This might be slower, but I enjoy it more.