50 watt home build doesn't like 120v!

For those building and repairing amps on their own. Learna and share ideas.

Re: 50 watt home build doesn't like 120v!

Postby pdf64 on Tue Apr 12, 2016 10:43 am

Well done on getting a bias assessment method sorted out!

Now that looks to be ok, from the OP, I wonder if your amp is oscillating?
The most likely cause of that in a new build is that the feedback loop around the power amp is positive rather than negative.
Try swapping over the opposing polarity signals at any one point in the balanced part of the power amp, eg wires to power tube control grids, OT primary wires to power tube plates.
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Re: 50 watt home build doesn't like 120v!

Postby Dr Tony Balls on Tue Apr 12, 2016 1:37 pm

pdf64 wrote:Well done on getting a bias assessment method sorted out!

Now that looks to be ok, from the OP, I wonder if your amp is oscillating?
The most likely cause of that in a new build is that the feedback loop around the power amp is positive rather than negative.
Try swapping over the opposing polarity signals at any one point in the balanced part of the power amp, eg wires to power tube control grids, OT primary wires to power tube plates.


This is what I was gonna say as well. Try either swapping the OT primaries or just disconnecting the negative feedback for a bit and see if that solves anything.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Wed Apr 13, 2016 11:51 pm

Unit_1 wrote:btw, it's a nice looking build

I appreciate the compliment! That means a lot. I was really trying to make a functional, sturdy amp that looks good..lol. With it being my first real build, though, there are naturally some things I will do different next go-round. I'm excited to try again. I eventually want to build up to doing microphone preamps, signal processors, even microphones.

Dr Tony Balls wrote:Try either swapping the OT primaries or just disconnecting the negative feedback for a bit and see if that solves anything.


I tried disconnecting the negative feedback and it did not fix the problem. I also tried swapping the control grid wires. There is still a loud hum the second the amp is switched over from standby and a loud crack when I turn it off.

pdf64 wrote:I wonder if your amp is oscillating?


I too wonder if the amp is oscillating. I have a Sencore SC61 oscilloscope if that can help track down the source of the oscillations. I will do some research on where to begin with that. Any tips?

To Tracy Norton-
I will begin working on the build schematic tonight and pm it to you when it's complete. In the meantime I have a question for you. In the attached pic, what is the function of the resistor marked 10k(22k)? I used a 10k resistor but I'm curious why there is a second value.

Image


As always thanks again for the replies fellas. My post history shows how long I've been working on this, I'd love to finish her up! I appreciate the support.
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Re: 50 watt home build doesn't like 120v!

Postby pdf64 on Thu Apr 14, 2016 11:25 am

grasshopper wrote:Ok. I picked up the 1 ohm 1% resistors today and put them in place between the cathode and ground. Also, to make adjustments I installed a trim pot on the bias supply resistor. Initially the amp was biased a bit cold, about 31mA. I would guess this to be within range but I went ahead and adjusted it up to 36mA. I'm still having issues at anything over ~75v. :crybaby:

So is the 36mA cathode current with the mains at 75V or the full 120V?

grasshopper wrote:what is the function of the resistor marked 10k(22k)?

It's a HT dropper to the cathodyne phase splitter; a lower value would allow a larger max signal voltage output from it to the power tubes (and likely increase the pre-amp HT voltages too, as it seem to be a serial string type HT distribution system).
May be different values were used according to which power tube type the amp was intended for?
Also note that the power tube's shared cathode resistor value seems a bit vague, ie 200-250.
I think 200 at such a high HT would run the power tubes very hot, especially given the lack of screen grid resistors.
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Thu Apr 14, 2016 7:49 pm

the 10k (22k) dropping res is as pdf64 says. The values are derived from two Zodiac Twin 30 schematics out there on the interwebs. My '63 has a 22k. A sch I found noted as a '64, has 10k. This would effectively make more clean headroom at the PI.

pdf64: my 63 has 250ohm 10w....Mullard EL34s running at 25W (actually, a little above...like 25.7W or something like that). The '63 sch shows a 2.2K! But mine came with original 250ohm. My neighbor has an original '63. His has 250ohm as well.

grasshopper, you have to start trying what people suggest or why bother asking for help.
get that build schematic done, add the voltages from your spreadsheet in appr places, and post it so we can 'see' what the #ell you are up to.....DO A NEAT AND TIDY JOB. USE GOOD PENMANSHIP AND DRAFTING PRACTICES.

Some of the $hit I've see people draw up makes me want to 8itch slap them silly......
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Thu Apr 14, 2016 8:17 pm

I was looking at the pics.....
The PT and OT coils are in the exact same orientation and close together. I'm betting a significant amount of hum comes from that alone.
You have ALL the preamp tubes bunched together, with all the wires leading away to the passive components....none of the twisted, shielded, or 'dressed'....ripe for pos feedback (oscillation).

It's a pretty box. Tear it down and start over.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Thu Apr 14, 2016 10:39 pm

tracynorton wrote:grasshopper, you have to start trying what people suggest or why bother asking for help.


Flawless logic here. I agree with you, except for how I haven't been trying what people have suggested. Unless I missed something, the only thing I haven't done so far that was recommended, is draw the build schematic. In my last post I stated I was working on that. I don't draw schematics and make PCB's for a living so that's gonna take more than an evening during the workweek. Sorry, not sorry.

tracynorton wrote:It's a pretty box. Tear it down and start over.


So, against the spirit of my previous sentences I'm going to decline this recommendation. There are a GREAT MANY things I feel I need to do before scrapping this build is justified. I think that would be a stupid thing to do at THIS point. Honestly I think it's a reckless, irresponsible thing to suggest. But hey, it's your opinion, I respect that. I just disagree with you. Being that this is my first build, the problems presented here are chances to learn. To scrap the thing and start over would effectively squander these opportunities. I appreciate that you sent me the schematic and helped with picking out a couple components, Tracy, but I won't be upset if your attention to my build ends with your last post. If that's how this goes, thanks for everything up til now.

To whoever may still be paying attention, I will be working on the build schematic with voltages listed. I have also considered various diagnostic tests to confirm or rule out sources of the problem. I will post in detail when I have worked through those.
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Re: 50 watt home build doesn't like 120v!

Postby Unit_1 on Thu Apr 14, 2016 11:26 pm

Well tracynorton is right about one thing (I believe the tear it down sentence was tongue in cheek humor)

Your lead dressing on the preamp section is completely wrong, not trying to insult, but this is a learning process.

If you take a look at the preamp sections of fenders and especially hiwatts you'll see that none of the wires are parallel. Parallel wires on preamp tubes is like begging for feedback oscillation (looks like squiggles on the oscilloscope).

They should be as far apart as possible and when they cross, they should cross at 90 degrees or as close to 90 as possible. Vertical separation is also effective.

Also keeping the PT and OT as far apart as possible (see Hiwatts again) and oriented 90 degrees helps to prevent induction humming.

The blue wire that is curled up is "the ringer" on 5e3's and this was the arrangement that I found sounded the best (I've made a squatload of these)
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I threw this pic in to show it from a different angle, plus I just really loved this amp. Traded it for a guitar. Those Astron paper in oil caps are very hard to find and cost an arm and a leg but are worth it. These are the quietest caps I've ever used, so quiet you can't even tell which is the foil end on a audible cap tester I built. I bought 110 of these for a buck each on eBay from some guy that didn't know what he had, lol..

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This is one of the Hiwatt DR504's I built. Kind of a poor pic but if you zoom in you can see the preamp wire dressing is very sharp right angles to avoid feedback. This was before I switched to solid core teflon wire and I switched because I realized I couldn't get those perfect right angles that Hiwatt wiring is known for.
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And this is the Plexi clone I made with a clear polycarbonate circuit board. The angles are in large looping formations for the lead dress and cross perpendicular as much as possible. I also used shielded wire from the jacks to the inputs on V1. A very good practice to get into, btw.
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And this is a nice pic of a cat at sunset, because life is more than amps, lol.....

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And here I am toasting to your future success with a land shark beer!!! (Pic was taken years ago before I lost 50 pounds)

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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Thu Apr 14, 2016 11:28 pm

pdf64: The cathode current I actually measured is ~21.5mA at 75v, which should be ~34mA at 120v unless I'm mistaken.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Thu Apr 14, 2016 11:37 pm

Unit_1, man those are some beautiful amplifiers! I aspire to that level of skill and functional beauty. Are the ends of those Astron caps threaded? Interesting. I also noticed above your plexi clone..it looks like there are a couple 1 1/2 inch mini's mounted to a box? Being an (apprentice) electrician I can't help but comment on those.

I don't know where Tracy got the idea that I didn't use any shielded wire. All the input jacks are shielded going to the board, and from the board to the tube sockets. I may not have used it everywhere I should have but it can't be said that nothing is shielded. Somewhere along the way I started to keep notes on all this amp business, and I have recorded your tips and others on lead dress. When I rebuild I will be taking these tips into account, as well as the transformer configuration. I'm not about to give up on this one though.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Fri Apr 15, 2016 12:23 am

Image

One of those simple tests I mentioned earlier is to rotate the output transformer by 90­­­­° such as this Orange amp. Before I try to make this next point in defense of my build, I'm going to come right out and say that I'm aware Orange has their shit together (for the most part) and doesn't have the kind of lead dress problems I have in my amp. Now, see how close the preamp tubes are to each other? This is why I felt this was acceptable, or at least why I felt I could get away with it.
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Re: 50 watt home build doesn't like 120v!

Postby pdf64 on Fri Apr 15, 2016 10:18 am

grasshopper wrote:pdf64: The cathode current I actually measured is ~21.5mA at 75v, which should be ~34mA at 120v unless I'm mistaken.

I don't think that it's reasonable to assume a linear relationship between plate / cathode current and mains voltage.
Tubes don't behave like resistors.
Apart from the axis, there's not many straight lines on pentode charts!
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Fri Apr 15, 2016 3:59 pm

Yeah, I'll admit there was a voice in the back of my head saying "Is that really how it works?" FWIW though I turned it up to 120v and biased it according to the actual numbers also, with no effect on the problems. I tried swapping the primaries from the output transformer yesterday, and didn't get any sound out of the speakers. The amp itself started making a high pitched ringing sound. I switched them back and the amp is still making the same sound when I turn it on. :worry:

It's a beautiful day in Ohio and I'm gonna get out to play some tennis. When I get back I'm going to continue work on the schematic. I finished the power supply, output tubes, and phase inverter parts of it last night, and will try to complete channels 1, 2, and the tremelo section tonight. Once the schematic is complete I'm going to try to get the amp back to it's semi-functional state so I can get voltage measurements.
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Re: 50 watt home build doesn't like 120v!

Postby pdf64 on Fri Apr 15, 2016 5:55 pm

grasshopper wrote:...I'm aware Orange has their shit together (for the most part) and doesn't have the kind of lead dress problems I have in my amp. Now, see how close the preamp tubes are to each other? This is why I felt this was acceptable, or at least why I felt I could get away with it.

Consider that any amp is a combination of its circuit design, its layout (which inevitably adds an overlay of parasitic coupling between all parts of the circuit) and the precise spec / characteristics of parts used.

May be Orange get away with it because their design was tweaked to accommodate such a layout, and because they have a lot of experience in these matters.
Whereas you are attempting to implement a pre-existing circuit with a layout that may be hard pressed to accommodate it, with little previous experience.

grasshopper wrote:the amp is still making the same sound when I turn it on

Have you got a recording of this noise?

Have you checked it out on the scope, eg frequency, waveform, amplitude?

Is the sound from the amp in the previous clips via a free standing speaker chassis? If so, have you got a speaker mounted in a cab to use instead, for the recording at least?

Does the noise stop if you remove any pre-amp tubes other than the phase splitter? If not, how about if the phase splitter is removed?
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Fri Apr 15, 2016 6:14 pm

Unit_1 wrote:Well tracynorton is right about one thing (I believe the tear it down sentence was tongue in cheek humor)

Yes...I don't take much or anyone too seriously these daze. I talk like this to #ucking Kilgore every time he brings something over for me to look at. But he's my friend. I really have to hold my tongue over at TAG.

grasshopper wrote:So, against the spirit of my previous sentences I'm going to decline this recommendation.

Trust me....in the end, you will rebuild it.

grasshopper wrote:Being that this is my first build, the problems presented here are chances to learn.

Well you certainly picked the the most finicky circuit as a first build....the Zodiac is prob one of the most troublesome circuits in the first place.

grasshopper wrote:One of those simple tests I mentioned earlier is to rotate the output transformer by 90­­­­° such as this Orange amp. Before I try to make this next point in defense of my build, I'm going to come right out and say that I'm aware Orange has their shit together (for the most part) and doesn't have the kind of lead dress problems I have in my amp. Now, see how close the preamp tubes are to each other? This is why I felt this was acceptable, or at least why I felt I could get away with it.


re: tranny rotation....It is not enough just to 'rotate' your PT or OT as they sit. The coil windings can't be in same plane. One of the transformers has to be rotated 90deg in one axis, then 90deg in 2nd axis. But you have to be careful you don't align coil orientation w/ the choke, either. If you look at that Orange, it shows the transformers rotated in two direction. The coils aren't oriented in same plane.

re: preamp tube placement: You can't make a sound placement judgement on one pic. Do you have the pic of that Orange from the inside? You might find that all the passive components are very close to the tubes and wired so that they don't have same phase parallel conductors. Yes, you may have some shielded wire, but your pics aren't detailed enough to see this clearly (I took them into photoshop, bumped up the resolution, and zoomed best I could...). You need to take at least three closeup (300dpi or better) across whole chassis. It is clearly evident that some of the wiring runs all the way across chassis to get to associated passives.

I want you to have a successful build. But you don't jump on a Harley before you can ride a bicycle.
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