2nd Gen Build

[martin manning]

While waiting on some parts to arrive I "balanced" the PI. Scare quotes because with the 5K trimmer I installed and the tubes in the amp, I could get close using the ampeg method but not all the way to a 0v offset. It settled around 1.7. I do have another 12AT7 and may try that to see if it gets me closer. Since my v3 plate resistors are the same nominal resistance, I added the trimmer mainly to achieve the difference there HAD designed into the circuit.

I'm calling that good enough for Rock and Roll in a 2nd Gen. My assumption being that HAD did not yet implement the trimmer in his design and was probably hand selecting tubes to get closer to a balanced power amp than a typical guitar amp of the day.

If you are still 1.7 mV DC off at the cathodes then I don't think you're close. If HAD was paying attention to this, he would have selected the plate resistor values to balance the PI. Then maybe he got tired of doing that and added the trimmer. I'd go with offset plate resistors. A 5k trimmer might be enough to cover variation between tube sections since the absolute resistor values are about half of those used for a 12AX7.

[dbharris]

While waiting on some parts to arrive I "balanced" the PI. Scare quotes because with the 5K trimmer I installed and the tubes in the amp, I could get close using the ampeg method but not all the way to a 0v offset. It settled around 1.7. I do have another 12AT7 and may try that to see if it gets me closer. Since my v3 plate resistors are the same nominal resistance, I added the trimmer mainly to achieve the difference there HAD designed into the circuit.

I'm calling that good enough for Rock and Roll in a 2nd Gen. My assumption being that HAD did not yet implement the trimmer in his design and was probably hand selecting tubes to get closer to a balanced power amp than a typical guitar amp of the day.

If you are still 1.7 mV DC off at the cathodes then I don't think you're close. If HAD was paying attention to this, he would have selected the plate resistor values to balance the PI. Then maybe he got tired of doing that and added the trimmer. I'd go with offset plate resistors. A 5k trimmer might be enough to cover variation between tube sections since the absolute resistor values are about half of those used for a 12AX7.

The 12AT7 tubes are definitely not balanced to begin with. I will take voltage measurements again to be sure that isn't my mistake. I'll try my other NOS tube. If that doesn't work, I'll pull one from my JM Sig build and see. If all else fails, I do have a set of proper resistance plate resistors I can swap in.

[martin manning]

I think with 51k/47k plate loads and 22k tail as shown in the 2nd Gen layout, a perfectly matched 12AT7 would have very nearly balanced outputs. Any particular tube might be off in either direction, so keeping the original offset in place with a +/- capability via the trimmer would be the way to go.

[dbharris]

I think with 51k/47k plate loads and 22k tail as shown in the 2nd Gen layout, a perfectly matched 12AT7 would have very nearly balanced outputs. Any particular tube might be off in either direction, so keeping the original offset in place with a +/- capability via the trimmer would be the way to go.

So I understand correctly, this also assumes a perfectly balanced OT and power tubes, correct? I was able to close the gap in the offset measuring across cathodes by swapping the power tubes between the sockets. That lead me to recall some prior discussions here that you are not really balancing the PI as much as the whole power amp. Is that a fair statement or am I misunderstanding?

Thanks!

-Dan

[martin manning]

That lead me to recall some prior discussions here that you are not really balancing the PI as much as the whole power amp. Is that a fair statement or am I misunderstanding?

You are correct. The best approach is to arrange the power tubes to get the best DC balance, and then use the PI trimmer to dial in the AC balance. Separate bias trimmers for left/right sides would help with the DC balance.

[dbharris]

That lead me to recall some prior discussions here that you are not really balancing the PI as much as the whole power amp. Is that a fair statement or am I misunderstanding?

You are correct. The best approach is to arrange the power tubes to get the best DC balance, and then use the PI trimmer to dial in the AC balance. Separate bias trimmers for left/right sides would help with the DC balance.

Thanks, Martin. I tried the 3 12AT7 tubes I have today. I did not note wall or B+3 voltage. But here is what I measured. In all instances, I needed the trimmer to be fully CW. Meaning my plate resistor connected to pin 1 has about 55K resistance while the other one has about 50K.

RCA black plate is the least balanced lowest offset at the power tube cathodes is 1.4mv and plate voltage is 292v (pin 1) and 278.6v (pin 6).

RCA grey plate, offset was 1.2mv and plate voltage is 274 (pin 1) and 270.7 (pin 6).

TAD with a few hundred hours of use, offset was 0.8 mv and plate voltage is 285.9 (pin 1) and 292.2 (pin 6).

Leaving the TAD in for now.

-Dan

[martin manning]

IMO it would be better if you could get it to cross zero.

[dbharris]

Thanks, Martin. I do plan to swap the plate resistors.

[dbharris]

Dale plate resistors removed and some Mk5 roederstein now in place with values matching the layout. Best I can get is a 0.2mv offset measured at the power tube cathodes.

PXL_20240524_132641240.jpg

I do think that if I had reversed the resistors as to the layout (51K followed by 47K) that the trimmer would have an effective range, at least with tubes I have on hand. But I did not want to deviate so much from the original design.

I have ordered a couple more 12AT7 tubes just to have some backups on hand. Hopefully they are matched better. Still waiting on an order to arrive to add the ratio pot.

-Dan

[bepone]

1mV AC balance on cathodes? this is negligible... also 2mV is negligible (impossible to hear that), i would leave this like it is

[dbharris]

1mV AC balance on cathodes? this is negligible... also 2mV is negligible (impossible to hear that), i would leave this like it is

Thanks. I am leaving the circuit as is. I will see if the new tubes are matched better. If not I will throw one of those NOS tubes in the JM amp.

-Dan

[dbharris]

Seemed like I had a better balance between the channels too but that required maxing out the volume and keeping overdrive around 50%. Was a low dB test though and may not be reflective of normal playing levels.
More fun later tonight after work....

I ran a simulation including source impedance for OD2, 10n coupling cap, and load impedance for the PI (2M). I found the best match is a 250kB Ratio pot with a 560k across the element. With the pot at noon, the load, frequency response, and level are essentially the same as for the fixed divider. The level/ratio can then be adjusted up or down as intended. The load changes slightly with ratio, but only +/-5K with +/- 20% rotation of the pot from center. This will be useful for anyone wanting to build a 2nd Gen in a 4th/5th Gen chassis having two OD controls, or for adding a ratio trimmer or control to a 2nd Gen build. It's in a very accessible spot on the preamp board, easy to add and to experiment with values

I made this modification. I'll post pictures later. This provides a useable ratio control similar to how a 4th/5th Gen "feels". Thanks again Martin!

[dbharris]

Here is modded with the single speaker jack and ratio control on the back. I'll order a 560K piher when I order more for an SSS.

PXL_20240527_185919859.jpg

PXL_20240527_185928956.jpg

PXL_20240527_190050234.jpg

I also swapped the choke leads today. I could hear a noticeable difference between the two. The reverse of how I built the amp had better clarity overall, more note separation, and felt more lively to play. I recorded clips on my phone of noodling the same riff in clean and overdrive (both normal channel). The recordings were 20 minutes apart. I can hear in those recordings what I heard in the room, but it was more apparent in the room.

I only tack soldered the leads, so I am going to connect them more permanently now in that orientation. After that, I'm going to lift a lead of the ceramic cap on the bass pot to see which I prefer. Getting close to finished here.

My impressions so far is that this amp does not get as saturated as #102. But it has a more raw sound that I definitely prefer with a strat. This amp is insanely touch sensitive too. Not sure if it is the lack of LNFB at V1b or low plates or a combination of things. Pick attack alone at high gain settings gives a wide palette of tones.

Just my 2 pFs.

-Dan

[Raoul Duke]

Looking good Dan!

[Dr d]

Hey Dan, the amp is definitely not as gainy as a #102. I found that reducing the 1m2 bypass resistor in the funky entrance to 1m helped in this regard (plus plugging into the FET!). I also found that reducing this value too far makes the OD too dark. Its definately a lively amp. I love mine