Bluesmaster PI reprise

[Raoul Duke]

Well, it’s proven time and again that vacuum tubes ain’t what they used to be, lol.

Ok - results to report:

Installed the 115k/95k plate resistors on the PI and still couldn’t balance the PI. Not willing to quit - I decided to just keep temping-in different value combinations and landed on 120k/82k. Weird, right?

Well, the PI balanced with the EHs just to the 82k side of center and with the JJs just to the 120 side of center on the trimmer. A low volume test confirmed no negative effects on overall tone - but I’ll reserve my final opinion until tomorrow when I can make some real noise.

Now I’ll be the first to admit I don’t understand how the math works on this - but this odd combo seems to be working.

[martin manning]

the PI balanced with the EHs just to the 82k side of center and with the JJs just to the 120 side of center on the trimmer.

I admire your tenacity, but a 38k offset seems excessive to me. Are you hearing any difference in the sound?

Swapping the power tubes has little effect, so the weirdness might be in the PI. The PI tube sections are well matched, as confirmed by e-tracer per your post above. Are you sure you have the correct resistor values in the remainder of the PI?

Another possibility is that the measurement of balance is somehow faulty. That could be due to some extra resistance in the cathode grounds. Maybe a careful check and reflow of those connections would be a good idea.

Beyond that I would want to look at it with a scope to get a better idea of what is going on. Given the uncertainty, I would be inclined to stick with the layout plate resistor values and center the trimmer.

[Raoul Duke]

I thought it was a lot too - but it balanced, lol.
No negative effects on the sound; I just played it loud for 30 minutes or so after warming it up for 15. Sounds as good as it did before but the sweet harmonic is not as obvious (I think?) - but still there.

I re-flowed the cathode grounds and went through the balance process again and the results were the same as last night. Just for reference, I measured resistance and voltage at those plates after warm-up:

120k resistance: 125.2, 258.2 vdc
82k resistance: 86.5, 275.4 vdc

I don’t know if this tells us anything, but thought I’d mention it in case it matters.
I guess reverting to the stock values and adjusting by ear is good too; but kind of feels like giving up, lol.

[martin manning]

The voltages reflect the resistances, so nothing looks out of the ordinary there.

I guess reverting to the stock values and adjusting by ear is good too; but kind of feels like giving up, lol.

I hear you, but something seems to be off here.

[Raoul Duke]

Just measuring values on the board and my negative feedback resistor (marked as 100k) is measuring 4.5k. I’m going to try a new one and see if that helps.

Nope - didn’t help. Measured 100k when I got it out. Strange because every other resistor on the board measures at its marked value; even in circuit. I checked the jumper wires under the board and they match the layout. Now I’m trying to cross reference them with the board plans to see if I crossed something up.

[martin manning]

Just measuring values on the board and my negative feedback resistor (marked as 100k) is measuring 4.5k. I’m going to try a new one and see if that helps.

It's in parallel with the PI tail resistor via the OT secondary, so that is what you should see measuring it in-circuit.

[Raoul Duke]

I re-traced and re-measured everything again today and everything seems fine. Weird.

Is there any hint that the feedback side (lower value) seems to effect the trimmer more? I don’t know what to check next.

Also, after returning to the stock layout plate values I noticed that the amp is definitely more “muddy” with the 120/82k values. Got to say - even with the PI unbalanced; this amp really rips.

I would like to get it right though…

To that end - if a decent ‘scope is the way to figure this out; then I’ll start researching that. I keep putting it of but it seems a worthwhile investment even if I’m only doing a couple of amps a year. Plus - I get to learn more

[Raoul Duke]

After a couple hours of exhaustively searching and reading posts here, reading Amazon reviews, and watching YouTube videos I think I have a couple of possible candidate's for an oscilloscope:

- Fnirsi 1014D
- Hantek DSO2D10

They are both considered entry level, but have a definite price difference. The Hantek seems like it does everything I’d ever need and the Fnirsi seems like a more basic/less complicated machine. I’d like to only make this purchase once - so I’m looking for opinions as to which makes more sense for guitar amps and long term use.

Thanks all!

PS: if this should be a new thread in “Technical Discussion” I’d be happy to move it - just started it here in context with how it came up in this application.

[GAStan]

I've got a 1014D. For the price it's a good scope and has done what I've needed so far. It came with two 1x/10x switchable probes and one 100x probe. It has a bit of a learning curve but what doesn't these days? If you have any questions about it feel free to ask either here or via pm.

I have no experience with the Hantek so am unable to offer a comparative opinion.

One suggestion I will make-consider purchasing the most capable equipment within your budget. I have never complained that a piece of test equipment had too many features, but I have used equipment that I wished had more.

[Raoul Duke]

That makes a lot of sense Glenn, thanks!
It was your previous (months ago) mention of the 1014D that got me looking at that one to begin with - so thanks for the pointer there as well!

My sticking point is that I really don’t know what I need. I think two channels seems to be a required feature. Signal generator would be nice - but I have other ways to do that. Beyond that - I don’t know what to look for. Kind of showing my inexperience with these admissions, but we all start somewhere, right?

Thanks again, much appreciated!

[martin manning]

I think you're on the right track. 2 Ch at a minimum, and both of those options have typical specs for an entry level instrument at this point in time. IMO the built-in generator is a very desirable feature. More than adequate for audio work, and maybe you can get to the bottom of this power amp balance mystery.

[GAStan]

One thing to look at is max voltage. The 1014D has a max voltage of 40 volts. This equates to max of 400 volts with a 10x probe. Occasionally I need to use the 100x probe. Not a big deal but something to be aware of as you have to go into a configuration menu to let the scope know you have changed the probe. I'm not sure how the Hantek is in this regard.

Another difference I noticed between the two are the power cords. The 1014D requires a special USB adapter cord. If mine goes bad I'm not sure where to get another. It looks like the Hantek uses a standard power cord that is easily obtained.

[Raoul Duke]

I think you're on the right track. 2 Ch at a minimum, and both of those options have typical specs for an entry level instrument at this point in time. IMO the built-in generator is a very desirable feature. More than adequate for audio work, and maybe you can get to the bottom of this power amp balance mystery.

Understood and thanks Martin. Solving the balance issue has pushed me over the edge on this, lol.

[Raoul Duke]

One thing to look at is max voltage. The 1014D has a max voltage of 40 volts. This equates to max of 400 volts with a 10x probe. Occasionally I need to use the 100x probe. Not a big deal but something to be aware of as you have to go into a configuration menu to let the scope know you have changed the probe. I'm not sure how the Hantek is in this regard.

Another difference I noticed between the two are the power cords. The 1014D requires a special USB adapter cord. If mine goes bad I'm not sure where to get another. It looks like the Hantek uses a standard power cord that is easily obtained.

Great points Glenn. I would never have thought to consider the power cord thing - even though I’ve been stumped by similar issues with other devices in the past. I’ll definitely look into the voltage limits as well

Thanks!

[Raoul Duke]

All right guys, I ordered the Hantek after watching a bunch of YouTube videos and considering everything discussed here.

Once I’m set up with it - my first order of business is to tackle the mysterious PI in my Bluesmaster. Having never done this type of troubleshooting before - can anyone recommend some reading for me to get smart on how to go about this?

Any suggestions are welcome and appreciated! Thanks!