2nd Gen Build Started (that didn’t take long…)

[Raoul Duke]

Yes, I’ve been bitten by the bug!

As the title says, I’m about to start a new build. I’m fortunate to have moved a couple of guitars that weren’t being played as much which has given me some capital to work with; plus I have most of what I need already just through looking at Tony’s layout and adding a part or two to orders here and there over time. Sprung for one of Erwin’s chassis (which are fantastic quality and really capture the look!) and I think I’m ready. Here’s what I’m planning:

- 2nd gen circuit with bass pull pot to eliminate the .02uf from the tone stack (3rd gen tweak, we’ll see how it works)
- NOS brown drop di-film caps (El Menco/CDE from the ‘60s)
- Mr Manning’s PS board including the B+5 node, the new 2nd gen layout with PI and OD trimmers, and 12v relay board
- I’m already building the relay, bias, and FET boards from Tony’s layout because I have all the components (including a 2N3823)
- Got Hammond Bassman iron from one of the e-bay sellers blowing them out at half price (they shipped from Mouser?!)
- For the finish: planning to use pig or goat suede - that will be a first for me

This will be a bit different than my previous build where I acquired a fairly whole BOM in a trade - so much of the planning was already done. I learned a ton from that experience and I’m eager to apply it to a new project that I start from the very beginning. The idea here is to stay as true to the 2nd gen as possible, but incorporate changes that make practical sense (to me anyway, lol). All opinions and advice are very much welcomed and appreciated!

Thanks and I’ll keep you all posted

[Raoul Duke]

All right, first question:

I’m keeping Martin’s PS board just to keep all the filtering in one place. I’ll adjust the values to 2nd gen specs - so this is what I’ve come up with:

2 x 100uf caps
4 x 22uf caps for the B+3-5 nodes
Drop the balance resistors between the 100uf caps to 220k (from 270k)
B+4 and 5 stay at 2k2 and 22k respectively
My main question due to lack of understanding how the B+ flows is: am I adjusting the 2k7 5w for the overall or is that determining what flows to the B+3? I’m inclined to start around 3k and adjust once up and running unless someone can tell me why that’s a bad idea?

Thanks!
Marc

[dbharris]

Hi Marc,

Looks like we are going to be building sibling amps at the same time. I'll get my thread started soon.

The dropping string on the 2nd Gen is B+ to power tube plates -> B+2 with choke to power tube screens -> B+3 with 2k2 to phase inverter -> B+4 with 22k OD tube-> B+5 with 2k2 to Clean tube.

For #102 the first 2k2 is swapped for a 2k7. Good luck with your build.

-Dan

[Raoul Duke]

Thanks Dan!
Explaining it as you did - it makes more sense to me now - and I appreciate that!

It had occurred to me that the 2k7 should be dropped, but given my experience with my 50w 102 build - I went the other way. As I’ve said before: I’m still learning the basics of how electricity works (travels) and how that relates to what we’re doing here.

I think it’s great we’re building similar amps at the same time. Looking forward to your thread!

Thanks again, much appreciated!

[rootz]

All your tubes use electrical power. The cathodes are heated by the 6.3V filaments on a separate AC supply. The high voltage part in the preamp doesn't use as much power, but if you increase the resistance in the power supply to the cap buffering the energy, the voltage will get lower on that cap. Decrease the resistance in the power supply and the voltage will increase. Decrease the 2k7 to 2k2 and voltages in the phase inverter, od section and clean preamp will increase. Alter only the last 2k2 in line and only the clean preamp voltages and FET (if applicable) will change.

[Raoul Duke]

Ok, thanks rootz!

So that would be the 2k2 resistor in the upper right hand corner of Martin’s power supply board if I’m interpreting this correctly (B+5)?

[Raoul Duke]

Getting organized and separating parts into groups to try and be more methodical in my approach. Recent tasks:

1. Verifying the outer foil on caps. These came marked - but I checked them all anyway.

2. Prepping the chassis and ensuring I have all the hardware needed. Erwin’s chassis are very nice and need no alterations to work as designed - just verifying how everything will sit and trying to prevent any layout issues. For instance: relocated the relay board about an inch to the left (looking from the rear) to avoid crowding in the “3 switch area”. The location is now closer to Tony’s layout, but the chassis is drilled for the 4th gen relay board set-up. Just a preference choice really. Would work either way - just trying to buy myself more buffer space around things to avoid collateral damage as I perfect my soldering, lol.

3. Getting familiar with the iron to include bench testing the OPT. Not certain if this makes sense, but I measured resistance from the brown and blue wires to the red wire. They measured 45.5 and 50.2 respectively - which I think means brown goes to V5 (closest to the PT) and blue goes to V4 (less resistance = more voltage)?

4. Verifying board layouts before drilling. I didn’t do this last time and learned the hard way that an eyelet even just 1/8” off can be a potential delay/problem. I was intent on creating boards true to the layout - but opted for an adaptation of Martin’s FET board just because I think it flows better and I understand it. So now I’m up to 3 out of 5 of Martin’s boards just because they make more sense to my limited understanding of electronics (meant as a compliment, lol).

5. Trying to pre-assemble as much of the controls as possible before installing just so I’m not burning stuff (and myself).

Should have some pics soon.
One question: what’s everyone using for 2nd gen control knobs? Got some of the Uxcell from Amazon, but they required some work and I’m not sure they’ll look right.

One more question: I have an older-style (6v non-LED) pilot light like the photos in the 2nd gen file thread. What’s the preferred way to feed that light? Heaters from PT to V5, then back to the light? Vice-versa? PT to Terminal strip to distribute the 6.3v both ways? Something else?

Thanks in advance for any comments/advice/suggestions!

[rootz]

Ok, thanks rootz!

So that would be the 2k2 resistor in the upper right hand corner of Martin’s power supply board if I’m interpreting this correctly (B+5)?

Without looking: it should be between B+3 and B+4. That is between the PI tube and the OD tube filter caps. So you end up with: B+ --> choke --> 2k2 (instead of 2k7) --> 22k --> 2k2 --> FET.

[GAStan]

Raoul,

Only suggestion I have is check the holes in the chassis for correct size. On my build that I used Erwin's chassis for I found many holes just a bit too small and needed to "chase" them with with the appropriate drill bit. I used SAE hardware and pots etc... I think the issue is the metric/SAE thing compounded by the powdercoating. There were also some holes that I countersunk, on the sides and under the OT.

This is by no means a criticism of Erwin's excellent chassis of which I currently own 2...and saving my pennies for a SSS hopefully before he sells them all.

[Raoul Duke]

Thanks Glen!

I’ve found that the difference you speak of allows me to “self tap” 6-32 machine screws in place (which can be handy sometimes) but I definitely chase any 8-32. The 4-40s aren’t a problem though. I was thinking the same thing regarding the metric/SAE differences.

I agree with you that they’re great build platforms. Erwin really captures the aesthetic and the metal work is top notch for sure. His source(s) definitely know their craft(s).

[martin manning]

3. Getting familiar with the iron to include bench testing the OPT. Not certain if this makes sense, but I measured resistance from the brown and blue wires to the red wire. They measured 45.5 and 50.2 respectively - which I think means brown goes to V5 (closest to the PT) and blue goes to V4 (less resistance = more voltage)?

No, that won't do it. You need to do the same as on your 102 build:
Gound the primary CT, which in this case (OT sitting on the bench) means connecting it to the secondary common.
Apply low voltage AC across the secondary.
Measure voltage from each plate lead to the secondary hot lead.
The one with the higher voltage is the V5 plate lead, shown as Brown in the 2nd Gen layout.

I have an older-style (6v non-LED) pilot light like the photos in the 2nd gen file thread. What’s the preferred way to feed that light? Heaters from PT to V5, then back to the light? Vice-versa? PT to Terminal strip to distribute the 6.3v both ways? Something else?

Option 1.

[Raoul Duke]

Understood on both, thanks Martin!

[Raoul Duke]

More questions:

On the layout:

1. the bias supply is the blue wire from the PT and the ground from the bias board runs into the relay board ground?
2. is the bias voltage fed through the power tube screen grids to pin 4?
3. on the relay transformer: am I tying the primary wires together on the terminal strip or is there another purpose for the strip? I see the white wires there, but the F313X doesn’t have any white wires, so I’m not sure what’s going on there.

Really trying to understand the flow of things before I get going on this one.

Thanks!

[dbharris]

More questions:

On the layout:

1. the bias supply is the blue wire from the PT and the ground from the bias board runs into the relay board ground?
2. is the bias voltage fed through the power tube screen grids to pin 4?
3. on the relay transformer: am I tying the primary wires together on the terminal strip or is there another purpose for the strip? I see the white wires there, but the F313X doesn’t have any white wires, so I’m not sure what’s going on there.

Really trying to understand the flow of things before I get going on this one.

Thanks!

1. Yes, blue is bias. Bias board grounds at the star next to the PT with the main filters. Yes, on the layout the bias and relay grounds are tied together. There is space for a ground tab near the pilot lamp that would be a better place for the relay board ground.

2. Bias voltage is through the 2.2K grid resistors on the tube sockets to pin 5.

3. Depends how long your leads are. You can tie them to the terminal strip and then run a set of wires to the AC in or fly them straight there. I think in some of the 2nd Gens, HAD would tie the relay transformer there then run a pair of wires to the courtesy outlet. Which I assume you are omitting.

-Dan

[Raoul Duke]

Thanks Dan,
I see the yellow and green wires to pin 5 now. Funny how you lose sight of the obvious when you stare at something long enough, lol.

Yes, I’m omitting the courtesy outlet. The leads aren’t very long, so I’m thinking the terminal strip probably is the neatest way to organize the run to the AC in vs having “2 into 1” splices flying above the PT, but I’m still pondering that…

Interesting side note regarding the relay board: I was looking at some 2nd gen ODS pics and see that HAD sometimes fed the indicator light from that board vs the heaters. I see a lot of the same type of light as mine - but some are shorter.

My question about power tube pins 4 was also because I can’t really tell what’s going on with those terminal strips. It looks to me like the 470R goes from pin 4 to a terminal, and the red wire goes to the same terminal - then that’s repeated on the other socket, then backs up to the B+2 eyelet on the PS board. Am I close?

I appreciate the feedback