First D-style (102) build opinions solicited & build thread:

[Raoul Duke]

Just used a 9v wall wart and some clips and leads. My readings are:

V5 - Blue wire: 82 vac
V4 - Brown wire: 63 vac

I think it works out correctly. Sound right?

[martin manning]

It looks like the opposite of what you want, which is higher voltage from the primary lead to secondary hot on the Brown side. I believe you'll have to reverse the OT primary leads at pin 3's or the grid leads at the PI coupling caps, whichever is easiest.

The measurements you quoted are from the OT Blue and Brown plate leads to the secondary tap where you applied the 9 VAC, correct?
Your load resistor and the 9 VAC were both on the same secondary tap? Which one?

[Raoul Duke]

Ooops, typed wrong.

The 82 was on the brown (V5) and 63 on the blue (V4).
Answering your questions: yes and yes - the main speaker jack, switch set to 8R.

I was so thrilled I got it to work that I mis-typed. Luckily I wrote it all down as I did it.

Sorry for the rookie mistake, lol.

[Raoul Duke]

Your lead dress looks nice and tidy but I would try and separate the grid, cathode and plate wires a little on the PI. HAD also tended to run the VxA cathode wire along the VxB plate wire. Take a look at the pic of #0183 and you'll see what I mean. I normally start out with something close to this and then use a chopstick and move the wires while listening.

Ian,
I studied these 183 pics and then went back and looked at your High Plate Classic build (very nice build and interesting thread) and see the differences regarding the patterns around the tube sockets. Definitely helpful and appreciated advice, thanks again!

One question: what are the main and PS boards made of? Looks different than what I’ve seen and while 1/8” G10 is definitely durable - I’m hoping to try something a little less “toxic” in my next build. I could taste the G10 for hours after I drilled the boards and I did it outside wearing a dust mask.

Thanks!

[martin manning]

Ooops, typed wrong.

Ok, cool. Your NFB loop should work as intended.

When you get the heater power connected up to the PI, here's the procedure for setting up the power amp:

Center the PI trimmer pot

With no tubes installed and bulb limiter in the AC line, switch power on, leave standby on standby (Note only heaters and bias supply are on, but reservoir and hot side of standby have high voltage present).
- Bulb limiter should flash then dim
- Check for heater voltage, about 3.4 VAC to ground
- Check for bias voltage on both power tube pin 5's
- Ensure that the bias pot sweeps the bias voltage, where it should be something like -50 to -60 VDC to ground at its most negative. Leave the pot set for most negative voltage.

Switch Standby to Play
- Bulb limiter should flash then dim
- Check for B+3 on PI plate leads
Note it's safest to probe eyelets on the board rather than on tube socket pins where a slip can cause a short.

Switch main power off (leave standby on Play), and allow filter caps to drain. Monitor B+1 to confirm.
Switch standby to Standby

Install power and PI tubes
Connect a load to the speaker jack
Short PI input (Power Amp In) to ground with a clip lead

Switch main power on
- Bulb limiter should flash then dim
- Check for heater glow in tubes.

Switch Standby to Play
- Bulb limiter should flash then dim
- Check voltage on PI plate and cathode leads

Bypass bulb limiter (or switch power off, remove bulb limiter, and switch power on)
Measure B+1 voltage and calculate estimated bias current: 0.65 (Pa max)/(V B+1)
- For example, 6L6GC Pa max is 30W. If B+1 is 450V, estimated bias current is 0.65 30 /450 = 40.4 mA

Connect DMM from a power tube cathode (pin 8) to ground (across 1R current sense resistor).
Increase bias voltage until power tube cathode voltage rises to estimated bias (in mV DC on DMM)
- Limiter bulb will brighten
- Watch for smells or smoke
- Listen for whining from the OT indicating oscillation
- Measure voltage on the other power tube cathode and note which one is highest

Remeasure B+1 voltage and recalculate bias current
- Reconnect DMM at power tube cathode with highest measured voltage
- Readjust bias to target value

If all went well, the power amp is fully operational.

[martin manning]

what are the main and PS boards made of?

HAD used different materials at different times, including commonly available phenolic and fiberglass, and in some cases patterned Formica, which is what you are seeing there. Those who have been inside other Dumbles may have more info.

[ijedouglas]

Your lead dress looks nice and tidy but I would try and separate the grid, cathode and plate wires a little on the PI. HAD also tended to run the VxA cathode wire along the VxB plate wire. Take a look at the pic of #0183 and you'll see what I mean. I normally start out with something close to this and then use a chopstick and move the wires while listening.

Ian,
I studied these 183 pics and then went back and looked at your High Plate Classic build (very nice build and interesting thread) and see the differences regarding the patterns around the tube sockets. Definitely helpful and appreciated advice, thanks again!

One question: what are the main and PS boards made of? Looks different than what I’ve seen and while 1/8” G10 is definitely durable - I’m hoping to try something a little less “toxic” in my next build. I could taste the G10 for hours after I drilled the boards and I did it outside wearing a dust mask.

Thanks!

As Martin correctly responded, HAD used various materials. He mostly used formica for the preamp board and in #0183s case he used copper clad board for the PS boards and FET.

I have tested (as have others) the formica board and it is conductive so be careful if wanting to go down this rabbit hole. In my builds I use Garrolite. It is easy to work with (formica is brittle and cracks easily) and sounds great.

[Raoul Duke]

Thank you both!
I thought it looked like Formica and/or the fake wood grain (more plastic-like, burns in pretty colors but really stinks - ask me how I know, lol) used on Unicor furniture for the last 50-ish years.

I’ll just stick with the Garolite, lol.

Martin, I’ll set up for that testing and post results soonest. One quick question: if I see/smell smoke or the limiter starts increasing in intensity; have I already done expensive damage or can I prevent catastrophic failure if I shut down quickly? I know a lot depends how well I paid attention to what I’ve done so far and you can’t predict every outcome - just trying to mentally prepare for contingencies.

Thanks!

[martin manning]

if I see/smell smoke or the limiter starts increasing in intensity; have I already done expensive damage or can I prevent catastrophic failure if I shut down quickly? I know a lot depends how well I paid attention to what I’ve done so far and you can’t predict every outcome - just trying to mentally prepare for contingencies.

I think you are pretty safe now, having brought all of the power supplies up with no issues. The difference this time is the power tubes and PI will be active, and the amp could start to oscillate. Since you have verified the OT phase, at least that won't be an issue. The limiter brightness is your guide, and usually it will save you from doing any damage. Just watch closely and keep your eyes open for red plating power tubes and your nose open for electronics-getting-hot odor, which usually precedes the smoke. There may be a very faint odor as things come up to normal working temperature, and a 40W bulb will be pretty bright with everything working.

[GAStan]

+1 on the bright 40w with all tubes in. Also the brighter the bulb the less voltage passed to the amp.

One check I did with my new amp is have it powered in a dark room with all lights out. This makes it easy to see any arcing that may be present hopefully before it becomes a serious problem such as "tracking", HV following a conductive, but resistive, path. Over time Carbon builds along the path allowing more current until it adversely affects the circuit.This comes from my much younger days working on Radar with 15KV but I feel can't hurt here, too. I've seen some pretty cool things doing this on Radars...

[Raoul Duke]

Would I be better off using a 60w bulb? Got one ready if so.

I noticed with the previous power-up that with shop lights on the 40w was very faint (except for the flashes). I know this time the amp will be drawing more (increasing bulb intensity as you stated) so I figured I’d ask the 60w question.

Thanks for the suggestion GAStan, I’ll definitely try the low-light arc detecting during the test. Makes a lot of sense.

Thanks again guys!

[martin manning]

I like to use bulbs with clear glass envelopes so the filament can be observed directly. Incandescents are getting scarce, but clear 40W oven bulbs are pretty easy to find. For larger amps, paralleling a couple of them for 80W is an option.

[GAStan]

Here's a good short discussion on the use of various bulbs

https://ampgarage.com/forum/viewtopic.p ... 28#p285528

[Raoul Duke]

Also - I’m getting tripped up by the foot switch. Here’s the connections on the chassis side looking at the DIN Jack from the rear with the square tab at 12 o’clock:

Pin 1 - PAB power in from power board
Pin 2 - PAB out to manual switch (with relay connected to center terminals)
Pin 3 - ground from 12v VR with connection to manual switches across bottom terminals
Pin 4 - OD out to manual switch (with relay connected to center terminals)
Pin 5 - OD power in from power board

Apologies for switching gears Martin, but I remembered (today) that I hadn’t built the foot switch yet and meant to follow-up your explanation from days ago with a (crude) visual aid as my guide. If I might ask, could you please tell me if I mapped out my foot switch wiring correctly based on the connections I’ve made inside the chassis as stated above.

IMG_0166.jpeg

Also, it’s my understanding that if I have the .1uf caps on the chassis switches - I won’t need them inside the foot switch. Is this correct?

Thanks again (and again, and again…)

[martin manning]

This snip from icracer's 124 schematic shows the rear panel and footswitch wiring clearly. I believe you have both rear panel switches wired like the OD switch, but that's ok. Note that there are no LED current limiting resistors in the footswitch since they are on the footswitch power supply board. It shows 0.1u caps in the amp and in the footswitch, but when the rear panel switch is set to "Pedal," the 0.1u in the amp is across the footswitch switch, albeit at the other end of the cable, which will have some inductance. It's probably best to have a cap on both ends, but probably not strictly necessary.