Advice needed...

[The4thWatcher13]

Apologies all, I forgot to post the schematic of the current working version last night. Here it is. I'll be swapping out the current jack for a "self grounding" one as well. I've also got a 47uF /160V electrolytic I can try for the 6AV6 too if need be.

Hamilton Electronics Tube Amp Iso Mod - Current 6V_H_6AV6.png

[Bieworm]

Great thinking there guys... this amp needs to get a shell that's equally out of the box than the amp itself. Keep thinking.. not too conservatively though!!!

[The4thWatcher13]

This is the original 7" speaker, mounted in the lid of the record player. The whole record player was built in a metal case! EIA code on the speaker says 270834. I think that means Quam-Nichols built. It doesn't sound bad at the moment but I can't help wondering what other speakers will sound like. I've got a Fender Blues Jr. cabinet next to me that I use for testing things and I'll give that a try next fire up. The sticker on the speaker says "Hamilton Electronics Chicago Ill.

Front.jpg

Back.jpg

[TriodeLuvr]

The speaker might be OK, but I wouldn't use a metal cabinet for this. Too bad, it's really cool looking! Something is wrong with C2, both in this schematic and in the original that you posted. The top of C2 should be a ground node. Both ends of this cap were grounded in the original, so I just ignored it.

Jack

[The4thWatcher13]

Ha, you beat me to it Jack! I was just writing:
I think I realized something wrong. C2's (.005) connection to ground would stop V1 (12AX7) cathode from having a voltage drop to ground.
I messed up drawing the most recent version of the schematic. I found this out by replacing the original bias cap/resistor pair for the 12AX7. When I put it back together the way it was drawn the cathode voltage shot way up (+70V) and nobody likes that! The schematic has been updated. I agree with you on the metal cabinet. The whole thing being metal with an ungrounded plug is insane! I just ordered some shielded cable which should be here tomorrow. Once that's in we'll see what's up the the hum/buzz. (Bah, hum buzz!) But I'm actually wondering if that's all there is to it. By the way, this thing sounds great through the Fender Blues Jr. cabinet. It's definitely got a little volume now. The "breaking up" or distortion I thought I was hearing I believe was an artifact of the 7" in speaker and not the circuit itself. We'll have to inject a little vitamin D, me thinks..

[JMPGuitars]

The whole thing being metal with an ungrounded plug is insane!

It would be insane for you not to add a grounded plug. Please ground your amp for your own sake.

Thanks,
Josh

[The4thWatcher13]

The whole thing being metal with an ungrounded plug is insane!

It would be insane for you not to add a grounded plug. Please ground your amp for your own sake. Thanks, Josh

The Iso-Trans, a fuse and, of course, a grounded plug were the first things done with this amp. Safety first bro.

[The4thWatcher13]

While I'm waiting for the shielded cable to arrive today I took Jacks advice about toying with R7 in the paraphase circuit to affect the tone. Where once was a 390K resistor there is now a 470K resistor. It's definitely sounding more like a guitar amp now. I'm tempted to "make it sound even more" like a guitar amp by putting a 560k in there but I really need to understand more about WHY it's sounding more like a guitar amp. I've been reading about paraphase inverter circuits online all day and my next move is to turn to Merlin's book to get a more granular take on things. Now if I can just get rid of this annoying hum I'll really be "cooking with gas"!

[TriodeLuvr]

Changing R7 creates an imbalance in the phase inverter. That produces an asymmetrical waveform, which by definition contains more even-order harmonic energy.

Do you have a scope capable of viewing the ripple at the 6AV6 anode resistor (the power supply side)? Maybe your DVM can do that with its graphing function?

Jack

[The4thWatcher13]

Changing R7 creates an imbalance in the phase inverter. That produces an asymmetrical waveform, which by definition contains more even-order harmonic energy.
Do you have a scope capable of viewing the ripple at the 6AV6 anode resistor (the power supply side)? Maybe your DVM can do that with its graphing function? Jack

From my readings earlier I kind of had the impression that a time constant of some kind was being affected between the inverted and non inverted signals. Merlin's book confirms what you've stated as well.
I do have a regular scope, and my scope-meter is capable of graphing as well. The graphs can be made to record both the "apparent" voltage, Min/Max/Average as well. The DC looks smooth to me. While running a trace (graph) the average DC is +117 for a couple minutes then drops down to +115 for about a minute then goes back up to +117. It does this over and over with varying times of decreased voltage. The image below shows the up and down trend over about 15 min after warm up. Should I assume that what would concern you more than R7 at the moment is the 6AV6's supply? I do remember you mentioning it. I planned to look into the relationship between PS node resistors and their respective cap sections. Is it possible that attaining distortion via the phase inverter in this way will also affect the preceding tube when they share the same supply cap segment?

6AV6 Plate Resistor (supply side).jpg

[The4thWatcher13]

Dude, you're not going to believe what I just did. When I replaced R7 I removed it's respective 6CU5 and when I took these measurements I had forgotten to put it back again! It's in now and therefore my trended voltage at the supply side of the 6AV6 plate R is steady at +114. There was a small dip at about 4 min but after that the voltage has been steady.

6AV6 Plate R (supply side).jpg

[TriodeLuvr]

I'm only trying to help you eliminate one possibility regarding the hum. R7 is a totally different thing, really just experimental and not related. Just so you know (in case anything weird happens down the road), operating the amp with only one output tube really stresses that tube. Essentially, it causes the single tube to attempt to flow roughly the same current that would have been shared by two.

The DVM graph as it is doesn't represent a sufficiently sensitive scale for what we need. We're looking for mV, not Volts. I'm not familiar with that model Fluke, but maybe there's a way to increase the effective vertical sensitivity. The total sweep time in order to see 120 Hz ripple across the screen won't be longer than 0.05 or 0.1 second.

Jack

[Bieworm]

You can always add filter caps on the rectified current. I remember doing this with my lamington reverb stand alone reverb unit... but sadly to no avail. Never found the source of the hum. But I ran it through the fx loop so the hum was minimal. You can't do this.. but no harm in adding 1 or 2 extra filter caps. Put those before you start extracting current for the circuitry. Don't forget separating them caps with 5 or 10k resistors of a high enough wattage.
Did you test if it's through the heaters? It's easily possible to check this ...

[JMPGuitars]

Inspecting/testing your heater wiring is a great idea. That is often a source of hum. I can't see your heater wiring from the low res photos, but it should be tightly twisted and not able to move at all. You may want to consider elevating the heaters and see if that helps with the hum.

Thanks,
Josh

[zaphod_phil]

+1 for the heater elevation.

In my experience, rectifying the heater supply generally introduces switching noise, which can be even harder to get rid of than AC hum!

Also in my experience, the number one cause of hum is poor grounding. I highly recommend referring to the article here on "modern grounding".

I have had really excellent results using a combination of modern grounding and elevated heaters, even with very high-gain amplifiers. No background noise at all!

[The4thWatcher13]

I'm only trying to help you eliminate one possibility regarding the hum. R7 is a totally different thing, really just experimental and not related. Just so you know (in case anything weird happens down the road), operating the amp with only one output tube really stresses that tube. Essentially, it causes the single tube to attempt to flow roughly the same current that would have been shared by two.
The DVM graph as it is doesn't represent a sufficiently sensitive scale for what we need. We're looking for mV, not Volts. I'm not familiar with that model Fluke, but maybe there's a way to increase the effective vertical sensitivity. The total sweep time in order to see 120 Hz ripple across the screen won't be longer than 0.05 or 0.1 second. Jack

Yeah, I knew it wasn't good for that to happen. I've been reading up on grounding schemes as well. Gonna try some things today. I'll switch to a real scope as well. Good scope info, thanks!

[The4thWatcher13]

You can always add filter caps on the rectified current. I remember doing this with my lamington reverb stand alone reverb unit... but sadly to no avail. Never found the source of the hum. But I ran it through the fx loop so the hum was minimal. You can't do this.. but no harm in adding 1 or 2 extra filter caps. Put those before you start extracting current for the circuitry. Don't forget separating them caps with 5 or 10k resistors of a high enough wattage.
Did you test if it's through the heaters? It's easily possible to check this ...

I'll be checking the heater wiring today. The heaters for the 6AV6 are just run straight to the breadboard and back again to the amp! Probably not good. In regard to additional filter caps room is definitely limited in this thing so I'm hoping to have to add as little as possible, but if you gotta, you gotta. How is checking for "heater hum" an easy task? Thanks for the info and input.

[The4thWatcher13]

Inspecting/testing your heater wiring is a great idea. That is often a source of hum. I can't see your heater wiring from the low res photos, but it should be tightly twisted and not able to move at all. You may want to consider elevating the heaters and see if that helps with the hum. Thanks, Josh

I'll try to increase resolution for further photo submissions. It's a real cram job in this little amp but I'll take things one step at a time to try to isolate the problem. If I've got to elevate, I'll elevate. Levitate, even!
Thanks Josh

[Bieworm]

You can always add filter caps on the rectified current. I remember doing this with my lamington reverb stand alone reverb unit... but sadly to no avail. Never found the source of the hum. But I ran it through the fx loop so the hum was minimal. You can't do this.. but no harm in adding 1 or 2 extra filter caps. Put those before you start extracting current for the circuitry. Don't forget separating them caps with 5 or 10k resistors of a high enough wattage.
Did you test if it's through the heaters? It's easily possible to check this ...

I'll be checking the heater wiring today. The heaters for the 6AV6 are just run straight to the breadboard and back again to the amp! Probably not good. In regard to additional filter caps room is definitely limited in this thing so I'm hoping to have to add as little as possible, but if you gotta, you gotta. How is checking for "heater hum" an easy task? Thanks for the info and input.

First off.. take Josh’s advice on tightly twisting the heater wiring. It should be twisted over as much of its length as possible. Avoid heater wiring being too close to any other wires or components as much as possible. Also avoid them being able to move around.
For the check if the hum is in the heater wiring:
Desolder the heater connections at the first tube they reach after the transformer. Then solder 2 short wires at that tube socket, with the other ends stripped some. Connect the heater wires to those stripped ends with insulated alligator clips. Put the amp on and normally there should be the same hum. Leave it on and disconnect the heater wires. It takes some seconds for the tubes to lose the light in their filaments, but if the hum stops immediately when disconnecting .. then you know it's heater hum... if the hum persists for a few seconds, the problem is likely somewhere else.

[TriodeLuvr]

I've been looking over the photos you posted, thinking about the additional tube socket. Even with a fair amount of metal working tools and options for bending and cutting sheet metal, it isn't evident to me how you'll add another tube and create an amp that can be bolted into a case. I think you'll eventually need to remove the output transformer and mount everything on a new chassis.

Just to present one option, Lloyd Pans sells square-sided aluminum cake pans that are 14 gauge (.063") aluminum with welded corners. This is heavier gauge than the thin stuff sold by Hammond and probably about the limit for working with hand tools. A 6" square pan in 2" or 3" depth is about $20. They have rectangular pans too, but they're probably much larger than what you need for an amp like this. Anyway, one of these pans would be an inexpensive option if you have tools like chassis punches and a step drill.

https://lloydpans.com/bakeware/cake-pan ... -pans.html