debugging adding reverb to deluxe

[Steve A.]

Also a good idea to shield PPIMV's.

Yes! Some leads should be shielded to prevent them from affecting other more sensitive stages. I really doubt that the PPIMV would ever pick up RF noise but it sure can contribute to the RF environment. (Yes, its not usually radio frequencies but just crap in what I call the "ether" of your environment sometimes transmitted by electrical wiring in your walls.)

BTW mikepumel mentioned not allowing wires to cross. Good luck with that! When they do have to cross have them do it at right angles. When my father was growing up on a chicken farm (ranch?) in Petaluma in the 20s and 30s they had ran a long wire parallel to the power lines and used that to energize light bulbs in the hen houses for heat. (They did not tap into the wires, just borrowed some power parasitically.)

Steve A.

[mikepukmel]

Yes! Some leads should be shielded to prevent them from affecting other more sensitive stages. I really doubt that the PPIMV would ever pick up RF noise but it sure can contribute to the RF environment. (Yes, its not usually radio frequencies but just crap in what I call the "ether" of your environment sometimes transmitted by electrical wiring in your walls.)

BTW mikepumel mentioned not allowing wires to cross. Good luck with that! When they do have to cross have them do it at right angles. When my father was growing up on a chicken farm (ranch?) in Petaluma in the 20s and 30s they had ran a long wire parallel to the power lines and used that to energize light bulbs in the hen houses for heat. (They did not tap into the wires, just borrowed some power parasitically.)

Steve A.

Oh man did I make a whole bunch of petaluma chicken house wiring, unintentionally of course. Working hard on a character flaw that lets me put more wiring in than needed "in case I have to fix something later" (duh). After the shortening, re-routing I got rid of a WHOLE BUNCH of wires crossing. It actually looks a lot neater in there as well.

I was very surprised that the oscillation was causing the output tubes to suck so much current. OK so learning a lot about lead dress. I swear the amp sounds better than before as well. But maybe it has to do with looking at the pile of little wire clippings in my garbage can


Sorry, what's PPIMV? something phase inverter something?

[mikepukmel]

If you practice good lead dress then you should only need to shield the most sensitive leads. The input lead and perhaps the second stage from pot to tube grid (if it's a cascade stage amp). For some reason very many cascade amps run straight from a pot on the front panel to a tube at the back of the amp across the preamp board.?. Also a good idea to shield PPIMV's. They seem to cause trouble for some, so just shield the leads and keep them away from the preamp. Everything anyone ever wanted to know about lead dress, transformer location/orientation, grounding schemes, etc. is here on this forum ready to be searched and viewed.

Our new friend with the "tubes in tubes" Bassman is having better than expected success in short order working out the issue. But he IS an engineer and was practicing some level of understanding when he bundled his leads.

You're issue seems to be working out too. Lesson learned and more to follow.

Re lead dress: Working on it Chuck! And I have shielded wire on the inputs from the input jacks to the preamp tubes and from the preamp tubes to the vol control. Didn't know where else it would be beneficial to put it.

Sorry to ask again, but what' s PPIMV?

[The Dude]

I have no doubt that it does sound better. Another side effect of poor lead dress is (or can be) phase cancellation- something you wouldn't even notice if you never changed lead dress and were able to make a comparison. It's slight, but it's there.

[The Dude]

Post-Phase-Inverter-Master-Volume

[mikepukmel]

It is all about circuits, not about parts. If the reverb control determines if it oscillates or not all it means is the reverb return stage is INVOLVED. It may be the the reverb tube is in fact oscillating on its own. But more likely it is forming a positive feedback loop with the power amp. That could come from power supply coupling - the B+ in one stage affecting that in another. That could come from sensitive grid wires coming too close to strong signals in other stages.

If your 1 ohm resistor was for one tube - as in a resistor for each - then the dissipation would be 19 watts, a bit steep for a 6V6. If the resistor is for the pair of tubes, then the 0.046v, which is 46 millivolts, is for two, meaning 23ma per tube. 410V x 0.23A = 9.4 watts.

Each tube has its own 1 ohm cathode resistor so .. . oh boy those poor tubes!

Enzo, this sounds like one of the most important lead dress items: "That could come from sensitive grid wires coming too close to strong signals in other stages."

[mikepukmel]

Post-Phase-Inverter-Master-Volume

Ah ok Thanks Dude. No MV on this amp, but definitely going to write this into my binder of important stuff. Its growing.

[Enzo]

Ever aim a PA mic at a speaker and the feedback just shrieks? The result is the map cranking as loud as it can. When your amp breaks into RF oscillation is is the exact same thing, but at a frequency you cannot hear and your speakers cannot make. RF doesn't mean it is picking up radio stations, it just means a frequency above audio.

So when it happens, you amp is cranking out shrieking you cannot hear to the most power it can. So it saps your power.

By the way, have we kicked this thing off the bulb yet? It sounds like you haven't. And that alone can screw up how it performs. It isn't blowing fuses, get rid of the bulb tester.

A scope will instantly tell you if it is oscillating at RF or not.

[The Dude]

And, if you don't have a scope, bring your dog into the room. He'll let you know if it's oscillating.

[Chuck H]

I have no doubt that it does sound better. Another side effect of poor lead dress is (or can be) phase cancellation- something you wouldn't even notice if you never changed lead dress and were able to make a comparison. It's slight, but it's there.

The opposite of the oscillation. Yep. People don't notice because nothing squeals But it's just as real and has a definite affect on tone.

[Steve A.]

When your amp breaks into RF oscillation it is the exact same thing, but at a frequency you cannot hear and your speakers cannot make. RF doesn't mean it is picking up radio stations, it just means a frequency above audio.

I understand that you are talking about frequencies above the audio range causing oscillation but what exactly would you call the crap picked up well within the audio range that for example can be heard plugging in a guitar that is not properly grounded or shielded? There may be 60/120hz hum but there can also higher frequency crap well within the audio range, often coming from the electrical wiring in my condo. (When neutrals and grounds are used interchangeably you can get a LOT of noise coming from the electrical lines, even the circuit that your amp is plugged into.)

From lack of the proper term I just lump all of that together as "noise from the ether"...

BTW my Jordan Boss Tone from the late 60s would pick up a radio station from Sacramento... you could almost make out what they were saying. What would you call THAT?

Thanks

Steve A.

By the way, have we kicked this thing off the bulb yet? It sounds like you haven't. And that alone can screw up how it performs. It isn't blowing fuses, get rid of the bulb tester.

I can't even bear to think of the possible damage to tubes caused by running them at very low filament voltages. EVH ran his original Marshall at around 80-85VAC using a variac so depending on the wattage of the light bulb what kind of voltages do you get with a guitar amp plugged into a light bulb limiter? (I read that his EL34s lasted about 15 or 20 hours but there is a lot of false information on the internet.)

For mikepukel: the light bulb limiter should be used only when turning on an amp for the first time, or when you have made changes that could change the amp draw or possibly blow a fuse. When the light gets brighter than usual (this will depend on bulb wattage as well as amp draw of device plugged in) it usually indicates excessive current draw and there is a problem that needs to be corrected.

I guess it would be okay to use it on initial startup after ANY changes have been made to the circuit, just to be safe. As long as the bulb does not glow brighter than it usually does it should be okay to connect your amp to full power.

Once an amp has been initially cleared using a light bulb limiter I will use a clamp-on digital ammeter to monitor current without lowering voltages whenever I think it appropriate.

Good luck!

[Enzo]

Frequencies do not CAUSE oscillation, feedback in a circuit causes oscillation. Any oscillation will happen at some frequency or other. Motorboating is a very low frequency, squealing is audio oscillation, and higher frequencies are RF. In the case of this thread we are concerned with possible oscillation. In his case, I suspect RF oscillation.

Not all noise is the result of oscillation. RFI - radio frequency interference - is radiated crap. Noisy brushes on your power drill interfering with your AM radio or TV for example. And your guitar picking up radio stations would be too. But those are not oscillation.

So our context is RF oscillation. Yes, picking up radio stations is RF, but that is another topic. Let me restate my original premise then. RF oscillation does not mean picking up radio stations. That is an unrelated RF interference. My shop was lass than half a mile from an AM radio transmitter, and I could easily pick up and listen to their signal whenever I wanted. What I intended was to avoid confusion because of th word radio in the term, but I see it had the opposite effect.

[mikepukmel]

Ever aim a PA mic at a speaker and the feedback just shrieks? The result is the map cranking as loud as it can. When your amp breaks into RF oscillation is is the exact same thing, but at a frequency you cannot hear and your speakers cannot make. RF doesn't mean it is picking up radio stations, it just means a frequency above audio.

So when it happens, you amp is cranking out shrieking you cannot hear to the most power it can. So it saps your power.

By the way, have we kicked this thing off the bulb yet? It sounds like you haven't. And that alone can screw up how it performs. It isn't blowing fuses, get rid of the bulb tester.

A scope will instantly tell you if it is oscillating at RF or not.

Enzo, thanks. Yeah, ditched the limiter bulb. All of the bad effects went away after I shortened and re-routed all of the wiring (current draw per 6V6 is back down to 10ma range, output tubes don't glow bright, volume is great, tone is great, limiter is not running so i don't know but expect it would not be glowing). Well, all of the wiring to 9 pin tubes. The output tube wiring looks pretty ok.

Definitely on my list of things to do is get a good used scope. Still looking. Find lots of them on craigslist, have to figure out how to determine if its good or not. The last time I used a scope was in physics lab in college, so many decades ago.

[mikepukmel]

Frequencies do not CAUSE oscillation, feedback in a circuit causes oscillation. Any oscillation will happen at some frequency or other. Motorboating is a very low frequency, squealing is audio oscillation, and higher frequencies are RF. In the case of this thread we are concerned with possible oscillation. In his case, I suspect RF oscillation.

Not all noise is the result of oscillation. RFI - radio frequency interference - is radiated crap. Noisy brushes on your power drill interfering with your AM radio or TV for example. And your guitar picking up radio stations would be too. But those are not oscillation.

So our context is RF oscillation. Yes, picking up radio stations is RF, but that is another topic. Let me restate my original premise then. RF oscillation does not mean picking up radio stations. That is an unrelated RF interference. My shop was lass than half a mile from an AM radio transmitter, and I could easily pick up and listen to their signal whenever I wanted. What I intended was to avoid confusion because of th word radio in the term, but I see it had the opposite effect.

Yep, understand that, about oscillation, thanks Enzo. Could be anywhere in the frequency spectrum (Within reason), anyway, sub audio, audio or above audio band. Im fairly sure it was RF oscillation as you suggested way up in this thread, since the tubes were, as you say, screaming, and the volume was very low.

[mikepukmel]

Put the amp through its paces last night, played an hour or so (without the fluorescent desk lamp on!). Volume, tone are good, reverb is pretty good for a new tank.

Got a hum that increases when I turn up the reverb control. Checked the wiring around the reverb tubes, to the reverb control. I twisted the wires to the reverb control, and those on top of the eyelet board. Ran the wires to and from the RCA connectors to/from the tank close to the chassis. Grounds look ok. Don't have a scope, so, this one is going to be tough for me to diagnose.

In another thread Leo mentioned that the tank should go into the amp in a certain way. Tried reversing the tank, didn't hear a huge improvement.

Enzo had outlined an audio probe in one of the MEF posts. Going to make one with some old computer speakers.

Maybe try shielded wiring on the connections to/from the RCA connectors inside the amp?

Any chance one of the new signal caps are bad from the factory? (maybe grabbing at straws here).