Troubleshooting request for 15 watt LP PCB build

[rock_mumbles]

If you look at rj's picture, it looks to me like the presence pot wiring is the wiring for the Express presence control with a 5k pot not the wiring for the Liverpool presence with 25k pot

[martin manning]

Indeed, and the presence pot seems to be 25k instead of 5k... Is that the case BP?

If so, you can tack the 4k7 across the outer legs of the pot. That should get your voltages in line and the amp should play.

[RJ Guitars]

If you look at rj's picture, it looks to me like the presence pot wiring is the wiring for the Express presence control with a 5k pot not the wiring for the Liverpool presence with 25k pot

Ah yes, that is sounding familiar to me. I liked the simplicity of that presence circuit but I think the bigger driver for using it in this build was that this circuit board was intended for dual use with an Express or Liverpool style build. I sorta recall discussing this on TAG back in the development days of the PCB board and I think Jana helped me sort through the differences in the presence circuits. In the end it seemed they were fundamentally equivalent and I kept the XP style... I had sorta already broke protocol for a Liverpool build at this point.

We'll see how this last one sounds once he gets it tweaked into submission. Mine is one of my best 2 sounding Wrecks but the crazy part is how loud and full it is with those little bitty transformers. It's a real lightweight to carry around too.

[martin manning]

IMO (and I'm not alone) the old-style (Bassman) presence sounds better, is more effective, and well worth the bit of noise when the pot is tuned.

[Blue Parrot]

RM, MM, and RJ – Yes, this is a 25K pot. OK, I think I am following this. It seems the consensus is to tack the 4.7K resistor across the outer legs of the presence pot - please correct me if this is wrong. Unless I hear otherwise, I will do this, measure and post the final voltages, and give the amp a try later this week to see how it sounds. I will report back – thanks!

Scott

[martin manning]

Yes. Even better would be to use a 5k6 resistor to make the pot look like 4k6 (25k in parallel with 5k6 = 4k6), or a 6k3 for 5k.

[rock_mumbles]

Tack a resistor across the outer lugs of the pot to get the presence/NFB and LTPI tail resistance correct ...

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I think I also prefer the bassman presence with 5k pot ... it has a nicer sweep ...

[Blue Parrot]

Sorry for the delay – I was waiting for a resistor order to arrive. I placed a 6.3K resistor (measured 6.09K) to the outer lugs of the presence pot as suggested. Resistance from pins 3&8 to ground now measures 44.6K.

Measured voltages are posted below. Although the PI cathode voltage dropped (from about 78 volts before placing the resistor down to 63 volts), it did not drop as far as expected. I suspect the 63 cathode voltage is probably still too high? Not sure where to go from here – suggestions?

Scott

[martin manning]

Check the value of the plate resistors R15 and R16 to make sure that they are 100k. Have you tried a different tube in the PI location?

[Blue Parrot]

Thanks Martin - I was looking at voltages posted by JoeCon for three full-sized LP builds in the Liverpool documents section (next to last post). In JoeCon’s post, the average PI pin 3/8 (cathode) voltage was 56.6V with an average B+3 of 262V. I am seeing 63 volts with a B+3 of 290V. Wondering if the 63 volts is that far off and also curious if this higher cathode voltage reflects the 10.7% higher B+3 in my build?

I used R16 = 100K (98.5K measured), and the R15 = 82K (80.9K measured)/C12 = 47pF combination as shown as an option in the schematic. From the PCB photos posted, these seem to be the values that were chosen. I am using a new balanced tube in V3 but have one other new tube I will try. I’ll report back on whether switching the V3 tube changes the cathode PI voltages.

[Blue Parrot]

The new tube did not make a huge difference. With a new Sovtek 12AX7WA in V3, the PI cathode voltage was just a shade lower at 59 volts compared to 63 volts before the tube swap. The rest of voltages remain essentially the same.

Scott

[martin manning]

I think you are in good shape. With your circuit values and a 290V supply a computer sim puts the plates at 229 and 224, grids (at junction of Rk and Rg) 62.5, and cathodes at 64.

I would replace the 25k presence pot with a 5k to get the better presence effect. The shunted 25k doesn't quite have the same response, and is more like that of the 4k7 tail with a 25k pot.

[Blue Parrot]

Martin – thanks for running the simulation. Since things seem in line I will close the chassis for the time being. I will order a 5k linear pot and install upon arrival, as you suggest. I’ll post back once I make the pot swap and familiarize myself with the amp.

Thanks RJ, Tillydog, Zippy, Rock Mumbles, and other TAG members for your time and trouble shooting help. I look forward learning from, and participating in, this forum in the future - you folks are a great resource!

[Clyde]

Trash the presence circuit and put in a cut control.

[Blue Parrot]

I wanted to post the final changes to the 15 watt Liverpool PCB amp. At Martin’s suggestion, I replaced the 25K presence pot (that had the shunt resistor) with a 5K linear pot to emulate the presence control of an Express. The presence control now works fine (44.4K ohms measured from pins 3&8 on V3 to ground). Also, I needed to replace V4 and V5 with new tubes since the originals had red plated. I used RI Mullard EL84s. While testing the other controls, I noticed the mid tone control was hooked up backwards. Easy fix - just changed the ground to the opposite outer pot lug. To make sure earlier photos don’t cause confusion for others, I have posted a photo of the final corrected pot wiring. In addition, to be complete, I also wanted to post the final voltages after all changes.

Initially, I didn’t care much for the tone. As others have noted, the amp it is very loud. I had a hard time dialing in a tone appropriate for the blues I play using my Les Paul. The treble was rather shrill and the amp had a nasty raspiness when overdriven. However, after playing for about 25 hrs, the tone has significantly mellowed as the amp burned in. The treble is no longer harsh and the raspy aspect disappeared although the amp still can be edgy when overdriven – which is good. Doesn’t seem to like my ODC overdrive although it tolerates a touch of overdrive from a Tube Screamer, and a touch is all it needs, if even that.

Bottom line is this amp is growing on me and I currently like it a lot. I bought some 5751s and even some 12AY7s tubes for V1 or V2 – I’ll try these, but I like the tone enough as is that I probably will leave the 12AX7s installed.

To complete this project, I plan to get a cabinet and, in keeping with tradition, the amp needs a name. I had a nice feminine name in mind for the amp but given all the problems I had getting this going (admittedly, mostly my fault) I had to come up with something different. Given the ghoulish and mischievous misbehavior I have seen out of this amp, my wife came up with a fitting name – the final name for this amp is Elvira.

Clyde, I will look into the cut control. I think Mark Fowler posted something that describes this a while back, I’ll check. The amp had a harsh treble initially but as it has settled in, the treble has become fairly nice (although it is still bright). Regardless, the cut control would be good to know about. Thanks.

Again folks thanks for all the help – much appreciated!

Scott