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Old 18th August 2011, 09:00 AM   #921
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Quote:
Originally Posted by tubelab.com View Post
If you are indeed talking about inductance, the only way to find out if the toroid in question has enough inductance is to hook it up and measure the low frequency response and test for saturation IN THE CIRCUIT you intend to use. For a guitar amp, you want full power down to 80 Hz. Generally get a transformer that has a VA rating just a bit bigger than the expected power output in watts. The bigger transformers often don't have enough inductance to handle heavy bass. I have found exceptions though. I got a 50VA transformer because my amp puts out over 25 watts at full crank.
Sorry if this topic is OT wrt the amp challenge, I am just curious to learn from you experienced builders on how one would start to guess at what may or may not work before buying the transformer

The math on how to calculate the impedance is pretty straight forward, since we can derive the turns ratio from the transformer's spec, but how do you figure out the inductances, thus frequency response if the manufacturers do not provide the data in their spec sheets?

Anyway, did you check out the Piltron 4002-1? It goes for $12 and its spec looks pretty good, but may be under-powered for your needs...

Jaz
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Old 18th August 2011, 12:10 PM   #922
Fenris is offline Fenris  United States
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Please tell us your source for these Plitron transformers! Their website has them for $365 (CAD). Unless the Canadian dollar has undergone a severe and unannounced devaluation, I'm not sure they'll meet the $100 price limit.

For this $100 challenge, one of the big issues is finding quality transformers that are affordable. It's easy to find proper transformers if the budget is unlimited, but that's not the goal here. So some experimentation and testing of tranformers not normally intended for OPT use is being done in order to find an inexpensive alternative. I'm using a smaller (10VA) Antek transformer in mine (see post #781), but I'm not going for the same power output as George. For a bedroom practice amp 3-4 watts is more than enough with a sensitive speaker.

Maybe in the next week I can test mine out. Circumstances (work and kids) have kept me from being able to do anything in the last two weeks, and the next few days aren't looking good either. For those of you with guitars handy, do you know your nominal signal voltages? I saw earlier in the thread that estimates were between 0.1v and 6v. I'm assuming 2v p-p, but I intentionally drop about 20dB that can be added back if I need it. I don't have an electric guitar anymore - a motorcycle accident 10 years ago took away enough flexibility in my left wrist to keep me from playing.
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Last edited by Fenris; 18th August 2011 at 12:20 PM.
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Old 18th August 2011, 12:38 PM   #923
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Quote:
connect a dpeaker direct to the tube output via a 500 or more ohm resistor?
Think about this for a minute. If you connect an 8 ohm speaker in series with a 500 ohm resistor and then connect the combination up to ANY amplifier where does most of the power go?

Hint: It is a voltage divider and about 1.5% of the power will make it to the speaker. So you are going to save $15 on the transformer but you will spend $$$$$ on the 64 watt amp you will need to get 1 watt to the speaker. The 63 watt resistor needed will cost as much as the transformer it replaced.
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Old 18th August 2011, 12:48 PM   #924
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I saw earlier in the thread that estimates were between 0.1v and 6v.
There is a large variation in pickups. I measured numbers years ago in my cheap Korean Squire Strat in the 100 to 400 mV range but these were RMS numbers. I have a few guitars to play with now but can't play well at all anymore due to nerve damage and arthritis symptoms. I have not put the Strat back together yet though.

If I get home in time to play tonight I can connect the scope across the amp input and see what is there.

Quote:
Please tell us your source for these Plitron transformers!
I didn't even look in the Plitron listings because I know what they cost. Check their Surplus Transformer pages often though. Several forum members got lucky a few years back. They posted some 400 Watt toroid OPT's for $135 USD. I got two. They rock!
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Old 18th August 2011, 05:17 PM   #925
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Whenever using paypal always give the wrong password for the first try! If it is accepted watch out!!! If it doesn't go through then use the right one.
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Old 18th August 2011, 05:26 PM   #926
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Whenever using paypal always give the wrong password for the first try! If it is accepted watch out!!! If it doesn't go through then use the right one.
Excellent idea, I never thought of that. It probably should be SOP these days. Especially with Ebay or banks.

Maybe we should all use the same vulgarities for the first try to send a message to the hackers. At first they would laugh. After the next 1000 they would get annoyed.
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Old 18th August 2011, 06:59 PM   #927
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Originally Posted by tubelab.com View Post
I sometimes use a junk email address to fill the inboxes of the dubious emails I get from banks that I have never used asking me to verify my account.
They are definitely not banks. Like, on this screenshot from some Asian site it is not IRS.

Click the image to open in full size.
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Old 18th August 2011, 10:05 PM   #928
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Originally Posted by costis_n View Post
So, I've made the final (GU50 single ended) stage of the prototype and hooked it up to the power supply. It looks like that.

Click the image to open in full size.

The issue is that since there is ~25V cathode bias, the G2 voltage is a little low, it may be 255 V from G2 to ground, but the cathode bias is stealing from that. So I get pretty low G2 current (~1.2 mA) and power output is low-ish, and also because of the G2 resistor being only 1 kilohm , I get very little compression.

So I am going to do this

Click the image to open in full size.

The G2 resistor is going to be carbon comp 1W, and will be adjusted on test. cold be anything from 56 to 120K. This will give me compression too.

So, does it look OK to you, guys? Maybe there is another way to do it better? (no sand please!). Maybe I should "split" the G2 feed resistor in 2 and have a capacitor in the middle, to have less compression?

Another question: My "customer" (read victim ) is asking about GU50 longevity in this amp. It is running around 85mA, 480V cathode to plate. What do I tell him?

...anyone?
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Old 19th August 2011, 12:45 AM   #929
Trout is offline Trout  United States
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Quote:
Originally Posted by jazbo8 View Post
A bunch of the posts were taken down wrt to Plitron transformers... Anyway, I apologize again for posting the link without checking it out fully... I also notified Plitron about the other site just in case.

Jaz
I wondered what happened lol, I thought I got banned or something.
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Old 19th August 2011, 01:45 AM   #930
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I wondered what happened lol, I thought I got banned or something.
Someone mentioned pictures of babes with guitar amps and a moderator decided to veto the path that this thread might have taken.

Well, back to our regular scheduled program.......

Quote:
For those of you with guitars handy, do you know your nominal signal voltages? I saw earlier in the thread that estimates were between 0.1v and 6v. I'm assuming 2v p-p...........If I get home in time to play tonight I can connect the scope across the amp input and see what is there.
For tonights experiment I plugged the guitar into the two gain stage version of AMP2.0 and wired the scope right across the guitar. The amp has a 100K ohm resistor across the guitar input and a 47K grid stopper into the triode grid. I tried two guitars with very different personalities but got similar results.

I used an old Guild S300A-D which is a good guitar for loud rock and metal. The -D means that it came from the factory with two high output DiMarzio hum bucking pickups. It is known as a "hot" guitar.

With the volume and tone controls on the guitar set for max output and the neck pickup only (highest output). I tried several single notes with the scope set to trigger and store on the note attack. The largest signal I saw was 1.416 V p-p. It took a pretty hard whack on the string to get there. Picture enclosed.

I then set the scope on a slower sweep and just cut loose with notes, chords, random noise and a few screaming feedback induced long sustained notes. I saw a few excursions to 2 volts P-P but most often the output was in the 800 to 1500 mV range using the neck pickup, or both pickups regardless of the phasing switch. Using the bridge pickup only yields 500 to 1000 mV. Oddly the episodes of feedback which were by far the loudest only produced average to below average voltages, 600 to 1000 mV.

There are two kinds of feedback, an acoustic coupling where the sound from the speaker excites the strings causing an infinite sustain of the currently playing note or chord. If playing a chord one of the multiple notes will eventually become dominant while the others die off. Moving, shaking the guitar, or rappidly flipping the pickup switch can change notes. Given enough gain you can continue to play note after note without picking.

Electrical feedback occurs when the magnetic field from the volce coil gets into the pickups causing a squeal. It sounds nasty and can usually be avoided by standing at an angle to the amp. The Guild is particularly prone to this type of feedback.

I switched guitars to an Epiphone Les Paul. Named for the inventor of the electric guitar it is used for rock, blues, but rarely metal. Think Carlos Santana. Mine has two "budget" single coil pickups. Epiphone is owned by Gibson. The good Les Pauls are Gibsons and you could spend $2000 on one. Mine is a mid level Epiphone. Sherri got it for me as a birthday present when I was jamming with my daughters friends about 13 years ago. It was $450 then.

Again single notes played hard or chords played at loudly but not too hard produced signals around 1.5 volts P-P with occasional excursions to 2 volts P-P. The bridge pickup could get to 1.2 volts with some rough playing. The Les Paul did not often exhibit the electrical squeal so I could get right next to the speaker and get some good feedback going. The voltage hit 616 mV max during a long multi note run without picking. I will do this again whenever I put the Strat back together or get my old Univox / Mosrite out of the closet and restring it.

While turning the room lights off to photograph the scope, I saw it. A dim red glow in one of the output tubes. I realized that I hadn't checked the bias since I got a bigger power transformer. I changed the cathode resistor to "get the red out."
Attached Images
File Type: jpg SingleNoteGuild_A.jpg (193.5 KB, 205 views)
File Type: jpg NoGlow_A.jpg (273.8 KB, 204 views)
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