Raoul Duke wrote: ↑Fri Jan 05, 2024 7:56 pm
I'll keep researching the generator input then.
Found a perfectly balanced 12AX7 GE long plate in my tube hoard. Thinking it might be a good PI candidate.
If it were me I would use that balanced long plate in the OD spot (as long as it's not too microphonic) and find a closely matched AX for the PI. You may have to roll through a few with a 10K pot
Congrats on the build
Tony
" The psychics on my bench is the same as Dumble'"
Raoul Duke wrote: ↑Fri Jan 05, 2024 11:28 pm
Searching Craigslist for used test equipment locally as well. Maybe I’ll get lucky and find something cheap that does everything…
I dug out my USB powered Bluetooth receiver, connected it to my phone, and ran a signal generator app into it. For a 400Hz sine, I measured output with my DMM at 1.5V RMS, and 400Hz, which should be sufficient for the purpose here. I also looked at the output on a scope, which confirmed the voltage and frequency measurements, and a good quality sine wave. With that level of input, I'd expect to see about 18W into a resistive load for a 50W ODS power amp.
Thanks Tony!
I actually have a couple of short-plate balanced ANOS AXs in my hoard, so I will take your advice for certain!
I appreciate the suggestion
Martin, thanks for doing that! What receiver do you have?
I’ll definitely grab one now that I know it works for what I need.
Been researching small ‘scopes and found a FNIRSI DSO-TC3 on the “A” site. I was googling it and researching what it does - but hadn’t figured out if it would be useful to a guy who might build just a couple of amps.
Raoul Duke wrote: ↑Mon Jan 08, 2024 1:10 amWhat receiver do you have?
I’ll definitely grab one now that I know it works for what I need.
Mine is a few years old now, and I don't see the exact one I bought on Amazon anymore. Look for the simplest Bluetooth Receiver/Adapter, something like this one: https://www.amazon.com/Bluetooth-Adapte ... gL4X_D_BwE
51ntiBNmJCL._AC_SL1500_.jpg
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Raoul Duke wrote: ↑Mon Jan 08, 2024 1:10 am
Been researching small ‘scopes and found a FNIRSI DSO-TC3 on the “A” site. I was googling it and researching what it does - but hadn’t figured out if it would be useful to a guy who might build just a couple of amps.
The scope you're looking at appears to be single channel. My preference is 2 channel minimum which is what I have now. I have a FNIRSI 1014D and I think the quality is pretty good.
The one you're looking at has some nice features. Built in LCR meter is nice. A single channel scope would be useful for starting out but IMHO limits ones learning.
Where a 2 channel is nice is that you can compare two signals such as the input and out of a gain stage. This allows you to clearly see not only voltage gain but also any distortion that may be present. As ones experience increases other things such as a slight phase change become apparent.
Both scopes have a 40 volt maximum signal it can look at. Using a 10:1 probe this increases to 400 volts. That being said I have not blown mine up yet. Just be aware of this limitation. 100:1 probes can be purchased to safely look at even higher voltages (my scope came with one).
I would humbly suggest you consider just how deep you wish to delve into the intricacies of an amplifier rather than only the number of amplifiers you may build a year. This can be a pretty deep rabbit hole...
All of that said the scope you are looking at I feel is a fair deal and would be useful as long as you paid attention to the 400 volt max.
I think that little scope is an amalgam of the ATMegga component tester and o-scope products that have been around for a while. Personally I'd go for something a little more capable. I agree 2 channels is useful, and a four-channel is not required very often . There are some so-called "Tablet" scopes that are an evolving niche, priced similarly to the low-end Chinese bench scopes. One such is this one, that has an x100 probe: https://www.amazon.com/FNIRSI-1013D-Plu ... f967d&th=1 Older analog scopes are nice too, but the price of digitals has come down so much that they are less attractive price-wise than they once were. I have an early 90's Tek 2246 that is a really nice piece of gear, and my son has this very handsome Eico: https://ampgarage.com/forum/viewtopic.p ... 91#p393891
Hey thanks guys!
I’ve been cramming info about scopes for the past few days and I’m on overload; so hearing some actual experienced opinions is very helpful.
I got into this hobby as a way to learn something new after retiring about a year ago. Been playing for 40+ years and never really picked up much about how amps work; but have always enjoyed sampling amps vs sampling guitars (weird?).
All that said, I can see me building a few more based on models I could never afford - like Dumbles, TWs, etc. I tend to jump into rabbit holes once I get going, so a scope is likely in my future given all they are useful for.
Already have a box of parts for my next build growing… 2nd Gen ODS. Got to get this one right first though, lol.
Should have that Bluetooth receiver Wednesday, so the big power amp test will be Wednesday or Thursday
Raoul Duke wrote: ↑Tue Jan 02, 2024 3:15 amOoops, 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
Went through all this and here’s what I got:
Flashes as per description
HTR: 3.2vac
PI plates: 465vdc
Bias: -65vdc
Caps drained as per description
Stopped here because the bias pot wasn’t sweeping voltage on either power tube as measured at pin 5 with red probe, black probe clamped to chassis, then tried black probe clamped to 1R resistor off pin 8.
Maybe some wiring error in the bias circuit? I would go through the bias circuit and check wiring against the eyelet board layout in the files section. I believe there are some differences re the schematic/layout. At least you have plenty of negative voltage.
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Last edited by martin manning on Wed Jan 10, 2024 8:47 pm, edited 1 time in total.
Ok, I think I see a couple of things:
1. In the picture of the pot, you show a grey jumper between lugs 3 and 2. I don’t have that.
2. I have the resistor grounded to the pot vs a chassis tab.
Everything else is as drawn.
I’ve checked and reflowed the bias supply board and pot connections. That all seems fine.
Sorry that jumper doesn't belong there. With that jumper removed, what is the voltage on lug 1 vs. lug 3? If you have a powder coated chassis it's possiblee that the pot case isn't getting grounded. Also the 15k shown might need adjustment.
