New Amplifier Design - "Murder One" - Submini Pentode, Low Voltage

[kalegood]

Ok, so after reading through this entire thread, I thought some other info might be useful.

Grounding: I have the input, output, and the black wire of OT grounded to the case (on one of the OT bolts).

The blue wire of the OT is getting 63 volts. (63 volts for pin 2 as well, rather than the above stated 70).

Not sure why I'm not getting any voltage at pin 1 (or out of the red OT wire).

[bluebunny]

If the "top" of your OT primary has volts, but the "bottom" doesn't, then perhaps the OT is shot?  It's a piece of wire after all, so there should be continuity from one end of the primary to the other (you could take it out and check).  BTW, I had my green wire go to ground on the secondary side.

BTW, welcome along, and good on you for reading all 26 pages! 

[kalegood]

Ok, looks like I fried the OT. This seems a bit odd to me because the amp was working intermittently for a while (if I wiggled the board around, I could occasionally get crackling). I thought that maybe the wires in the OT had gotten damaged, but wiggling around the wires while checking resistance gave the same results.

Any idea how I could have done this? I'm at about 63 volts on B+. The R3 resistor maxes out my analogue multimeter. Could the OT have fried if I had accidentally crossed the secondary wires for too long?

Why is it that my DIY projects are never cheaper than buying a pre-made item. Ah, well. Its a learning experience.

[bluebunny]

I only had sixty-something volts B+.  Too much/too many diode drops?  Perhaps Schottkys might have been better...      But anyway, works at the slightly "lower" voltage.  So far as the OT goes, an open secondary traditionally causes trouble.  More learned fellows will be along presently to explain why.    Not sure if a short is dangerous too...  Did you disconnect the OT to check the primary, btw?

[kalegood]

yeah, the OT was out of the circuit.

[thomasha]

Hi,
after using the output transformer in a 12au7 SE amp my murder one was resting in a box, but after some reading I tried to use a 220 to 5.5 V transformer as the output transformer and it worked!

The sound is still an issue... with my transformer the tone lost a lot of bass and there is a strange distortion when the volume is maxed.

Since this amp has such a low wattage, and this low voltage transformers are easier to find than the fender reverb transformer, at least in my country, would it be possible to adapt this transformer or design a smaller one?

I was thinking of starting with a lamination that has the air gap or cut an air gap in the original lamination.

Another approach would be change the lamination material, to obtain a better bass response from a smaller transformer, but this part is a little more difficult to understand and maybe someone could help explaining?

I read some audio transformer design articles, but the winding plays the important role, and the core size are never smaller than a core size required for a 5 W amp.

I know the winding plays an important role, but winding an output transformer without the required equipment is very exhausting. Change or adapt the lamination sounds easier to me.

The main goal is something as a low size and easy to find SE transformer. Or at least test some changes in the OT.

Cheers

[mcandmar]

Just registered to say thanks all for sharing the schematics and info, i couldn't resist building something so cute





At the moment i am using a basic speaker from a Fender Frontman 15g and it sounds surprisingly good.  I'm also impressed with the range of sounds it has from clean to distorted. Only change i made from the schematic was the 6111 heater resistor, it ran way too hot for comfort so i replaced it with an LM317 mounted under the circuit board using the chassis as a heatsink.

Cheers all!

[Fast Pistoleros]

Curious what the B+ connection is ? does that go to one ond of teh diodes and then the other two +12 go to the power supply input jack?  what is the data sheet specs on the bom specificaly the transformer? if i google search fender reverb transformer will that be the one to buy?  thanks

I think i got it .. the b+ is the -60-70 coming off the voltage inverter and the other two 12+ leads go to the +12 rail on the input supply , all get a common ground...is this where i can put my 1/0 switch with an LED indicator to toggle the circuit open and closed?

[substatica]

Test fitting my Murder One (temporary plastic protection bars). I saw somewhere someone used some rubber grommets for the tube holes so I did the same -- does that seem like a heat/vent problem to anyone? How hot do these little  tubes get? Currently they're snug against the grommets. Cheers.

[substatica]

Also, where would be the best place for the power switch be? I would think switch the 12 volts coming from the adapter and run the power LED off that with its resistor to ground. Is that how others are wiring it?

[Jdansti]

Also, where would be the best place for the power switch be? I would think switch the 12 volts coming from the adapter and run the power LED off that with its resistor to ground. Is that how others are wiring it?

That's how I did mine. The power switch is between the DC jack and the circuit. The LED and resistor are wired in parallel with the amp.

[substatica]

Have it all together but not getting any audio output I have to admit I forgot I put on a negative center power jack and had the polarity reversed when I first turned it on, would that have borked anything?

I measured 65 VDC coming out of the power section -- where should I look next?

[Jdansti]

Looks like you didn't damage the 1044. The reverse polarity shouldn't hurt the tube heaters and I don't think it would have damaged the tubes otherwise, but I don't know what output if any the voltage multiplier would have created when the polarity was reversed.

Just for thoroughness, if it were me, I'd check to make sure the guitar and cable work with another amp. Make sure your guitar's volume is turned up. Seriously, stupid little things like that can cost you a lot of troubleshooting time if you don't check. Also make sure your speaker and speaker cable work by testing with another signal.

If all of the above are fine, then use an audio probe and follow the signal starting at the input and work through the tubes. Use the schematic to see which tube pins to check for signal. I don't think you want to use the audio probe on the transformer output. You have a speaker to check that. Everything else in the signal path on the other side of the transformer is ok to check.

[substatica]

Looks like you didn't damage the 1044. The reverse polarity shouldn't hurt the tube heaters and I don't think it would have damaged the tubes otherwise, but I don't know what output if any the voltage multiplier would have created when the polarity was reversed.

Just for thoroughness, if it were me, I'd check to make sure the guitar and cable work with another amp. Make sure your guitar's volume is turned up. Seriously, stupid little things like that can cost you a lot of troubleshooting time if you don't check. Also make sure your speaker and speaker cable work by testing with another signal.

If all of the above are fine, then use an audio probe and follow the signal starting at the input and work through the tubes. Use the schematic to see which tube pins to check for signal. I don't think you want to use the audio probe on the transformer output. You have a speaker to check that. Everything else in the signal path on the other side of the transformer is ok to check.

I'll start with a signal probe. I see a dull glow form the Pentode, but no sound whatsoever, no signal, no noise.

[substatica]

If all of the above are fine, then use an audio probe and follow the signal starting at the input and work through the tubes. Use the schematic to see which tube pins to check for signal. I don't think you want to use the audio probe on the transformer output. You have a speaker to check that. Everything else in the signal path on the other side of the transformer is ok to check.

This is only my second tube amp, first was from a kit, so I'm not sure I'm following the signal through the tubes correctly. I'm not getting any glow off of the 6111 whereas the 5672's got a nice orange on the heater. The audio signal has tons of buzz and drops to almost inaudible once through the the 6111.

If I'm reading the spec correct the 6111 should have 6.3v at pin 3 and 6 for the heater yeah? I'm reading 12V at pin 3 and 0V at pin 6, could the heater filament be broken?

What other ways can I test the tube?

http://frank.pocnet.net/sheets/127/6/6111.pdf

http://frank.pocnet.net/sheets/127/5/5672.pdf

[substatica]

It's alive! Turns out I had the 6111 in reversed, I think that fried both tubes and eventually the IC, once I swapped all those out I've got sound! Quiet sound, but I think that's just the amp output power Now to take care of the motorboating when dimed!

[bluebunny]

There are mods in the thread to stop the motorboating.  (You'll have to read through to find them.)  Worked like a dream for me.

[substatica]

There are mods in the thread to stop the motorboating.  (You'll have to read through to find them.)  Worked like a dream for me.

Yup, already swapped R1, R2 for 100K and R12 for 470K. Motorboating gone -- does that mod drop the max output?

[bluebunny]

Not that I noticed.  I thought it was still pretty loud.  So did a mate of mine when we used it for some recording.  YMMV, of course...

[substatica]

Any idea why @frequencycentral used washers under the OT and roll bars? Just to give the OT a level base?