Next Build Coming Up - 183 50w

[Raoul Duke]

Thanks Martin!
I planned to have the latest edition of the board plans in front of me when I ventured into “the relay vortex”; but unwittingly I just opened the schem and layout and went to town. Later on after stepping away for awhile - I wanted to check my work just to be sure and opened up the board plans and got that sinking feeling that I had wandered off course. So I opened up the schematic and layout and to my surprise - I had followed them correctly, lol.

I know there’s often a few paths to the same spot and was hoping this would be the case with this. I appreciate the vector check!

[martin manning]

I know there’s often a few paths to the same spot and was hoping this would be the case with this. I appreciate the vector check!

This function is simpler than the R/J, basically a DPST switch. There are different arrangements for R/J, and some Dumbles have reversed mode vs. relay state (as noted in the board .pdf).

[Raoul Duke]

Understood Martin - I saw the new, more descriptive page regarding the relays and it really helped me understand the differences much more. As always, your documentation is top notch and very educational for a beginner like me. Very much appreciated!

So I had so much fun in the “relay vortex” yesterday that I decided to visit the “co-ax vortex” today.

I should clarify my use of the word “vortex” as “a sub task within the build process where you become completely absorbed by the level of focus required to accomplish it - leading to complete disregard for external distraction and stimuli to include all sense of time and situational awareness.”

Got all the co-ax made-up and installed with the exception of the board connections. I used the RG316 that Sluckey turned me on to. I love the fact that the core doesn’t melt, it’s cheap, and only takes a day to get here off Amazon.

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Trying to get as much accomplished without transformers as possible. May figure out the tube socket leads coming off the board tomorrow; or - if the weatherman is right - it’ll be in the low 50s tomorrow so I may start on the head cab. We’ll see.

[Raoul Duke]

The weather wasn’t what I expected so…

Worked on the board today. Regarding Dan’s question about resistors the other day - I had forgotten that I followed Tony’s layout as to the material of the resistors and their locations. So I have CF and MF where they are shown in the drawing for the most part.

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I used TE H4Ps vs Dale or Mepco on the V1 and 2 plates. They are 1W/1% MFs, so same intent. Also had some older Mallory 4u7 electrolytic caps on hand - so after checking them on the LC75 - I used those as well. The “+” sign on them isn’t very conspicuous but I found it. Everything else is stuff I had laying around - to include the honkin’ big .022 I put on the PI “A” grid (1200v).

[Raoul Duke]

Well today I actually put the PT in with some angle grinding on some fender washers.
I knew there’d be some “pluses and minuses” using a stand-up transformer, so I tried to take the time to explore every possible way to manage the lead dress. I think I have a pretty good plan going forward. Got the choke hung as well.

Also - since the power section smoke test isn’t far off - I’m debating whether to use 18AWG heater wiring or some mil-spec 20AWG. I’ve researched this quite a bit and it seems to be a pretty even split AFA what to use where. I’ve been using the 20 for everything in a lot of my lower power builds; but I want to do what’s best for this 50-ish watt build. Maybe 18 for the power tubes and 20 for the pre-amp?

Here’s today’s pic:

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[dbharris]

Looking good Marc! I think your parts composition is leaning a littler more Bluesmaster or even 102 than 183. Regardless of that, I am sure your amp will sound great.

Somebody like Martin will know the math for the real answer to your heater gauge question. If it were my build, I would go with 18awg since the EL-34s will draw about twice as much current as 6L6s would. I have been following some advice Taylor posted years ago to use 18 for the power tube heaters and 20 for the novals.

-Dan

[martin manning]

Only one pair of leads is carrying the total heater current, i.e. the PT leads. The current in the next pair is reduced by the current flowing through the first power tube.

So, for EL34 ODS the first pair of heater wires carries

100W 3(1.5) + 3(0.3)= 5.4 A
50W 1(1.5) + 3(0.3) = 2.4 A

If you look up wire gauge (AWG) current ratings you can find charts that list the "ampacity" for chassis wiring:
AWG - Amperes
22 - 7
20 - 11
18 - 16

From this you can see that AWG 22 would be fine for a 100W EL34 amp with a safety margin of 7/5.4 = 1.30. AWG 20 would increase that to 2.04. Also note these are short runs, so voltage drop isn't an issue. IMO, anything larger than AWG 22 is difficult to work with on the preamp tube sockets.

[Raoul Duke]

Thanks Dan. My BM/HRM is more Marshall sounding than my 102 and 2G - only without the “zing” type of quality that I hadn’t really experienced before the 1987 I stumbled into a few months ago. If the EL34s, different iron, and different component values push it further that way than the BM - I’ll be satisfied. Even if not, I’m sure it’ll be something interesting and different.

I appreciate the current calculations information Martin - the hard numbers seem pretty conclusive. I also agree with you that AWG18 all the way down the line is really tricky to work with. I don’t have enough decent 22, so it’ll be 20 - which I’ve gotten pretty accustomed to.

Just did the polarity check on the OT so that’s all set.

Thanks again guys!

[Raoul Duke]

Ok, not a lot of progress today, but I’m at the point where I tend to slow down because I’m concerned I’ll forget something - so I really take my time and try to think a few steps ahead.

Aside from confirming the OT polarity, I got the heaters, HT, and power switch wired up. Seemed like the right time to mount the OT - so I did. I think I’m just about ready for the PS board…

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[Raoul Duke]

Got everything needed for the initial power section start-up connected, then double checked it all, then read my notes from the past few times I’ve done this test, then connected to the limiter. Threw the mains switch and got the flash to barely any glow, threw standby and got the faint flash to barely any glow. Then started taking readings.

1. HT to rectifier apx. 170vAC per side
2. HT to res. cap, through stand by to choke and B+ filters (~480vDC)
3. Raw bias voltage at “bias out” on bias board -41v with the 2w 100k dropper
4. 12v out of pin 3 on the VR and at the relay boards and pedal socket
5. Last, got 6.8vAC on the heater pins of V5 and the LED lights

I think I’m right where I need to be with unloaded voltages to include the bias, but would certainly take any suggestions. Pretty happy that I got this all right on the first try. I must be learning something, lol.

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[martin manning]

The only thing I see is that the bias voltage may need to be a bit more negative.

[Raoul Duke]

Thanks Martin!
I left the 100k temped in just because the bias is the thing I’m always fiddling around with and wanted a second opinion on. I’ll try a smaller dropper next. Should I be looking for -50 give or take, or more?

[martin manning]

If the plate voltage is ~450, then the bias probably wants to be around -41, so you probably want to add some wiggle room.

[martin manning]

Just for fun I used Spice to generate a derivative for plate dissipation vs. bias voltage at 450V Va and Vs. A one volt change in Vbias changes plate dissipation by ~10%.

[Raoul Duke]

Temped in an 82k and got -51.6 straight into the wall at 122vAC (no current limiter). I read that -30 to -50 is where I should be, and I think with a 10k pot wired “fail safe” and 27k resistor to ground I’m close?

I know “center” on an adjustment pot is a moving target and I’ve been reading up on bias circuits because I never get them right the first time - but the more I read, the more turned around I get. Maybe bias is just “get close enough” and a certain amount of trial and error is the fine tuning?