Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on September 08, 2014, 12:40:10 pm
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I did a bit of searching here, and on The Net.
But I am still not sure what to do.
It SEEMS like most methods to drop B+ from the plates are in reference to a PT with a CT on the HT secondary (Jesus...that sounds like a line from Get Smart)
Anyway...I am sorry if I have missed it...I Know this question gets asked A LOT.....but I need to drop 50 Volts from the plates of this amp.
The plates are currently 520 Volts...even with Neg 50 Volts (max available) at the grid, I am close to 70%. dissipation
Is there a "good" way for me to ditch 40-50 Volts from this amp.?
I have nothing in place of The Brimistor ...and I Am Not using the 18k to ground from the screen supply cap.
Thank You
http://www.drtube.com/schematics/vox/ac100jen.gif (http://www.drtube.com/schematics/vox/ac100jen.gif)
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...and I Am Not using the 18k to ground from the screen supply cap.
Well, put it in. The extra current drain on the PT may just pull the B+ down to where you want it.
Hi current bleeders are a valid method to drop B+.
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...and I Am Not using the 18k to ground from the screen supply cap.
Well, put it in. The extra current drain on the PT may just pull the B+ down to where you want it.
Hi current bleeders are a valid method to drop B+.
1. "High Current Bleeder" ...is that what the 18k is.....or are you talking about another method.?
2. I had to order a new resistor...I am waiting for a 20k 25 Watt to get here from Mouser.....should be just another day or two. Will it drop that much Plate Voltage.?
Thank You
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Well...speak of the devil...my power resistor got here.
I did not install it, I just jumpered it in .....It knocks 8-10 volts off the screens, but there is no reduction in plate voltage.
So I am still going to need a way to get rid of some of this.
Last night, at Midnight, the plates were 520. Today they are 513. I guess that will be the wall voltage... I assume it Is/Can Be higher when there is less demand...late night-early morning.
I guess I could increase the grid voltage, but that seems like the wrong solution to 510-520 plate voltage.
Thanks
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What is your heater voltage?
Does it leave you enough room to knock some Vs off of the input, and drop the entire supply....ala vintage voltage adapter:
http://www.geofex.com/article_folders/vintvolt/vintvolt.htm (http://www.geofex.com/article_folders/vintvolt/vintvolt.htm)
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well the EL34's can handle that plate voltage. if you can get the bias right your amp is happy with the B+ it gets.
i know you have your mind set on lowering the plate voltage. so i'm just saying.
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1. I will check the Heater and Preamp Volts and get back to you.
2. I guess in my head I have the idea that there is some tipping point with the bias. Where does it become a bad plan.....1kV and -93 Volts on the grids.? The tubes will almost be turned off at some point. :smiley:
Thanks
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It may not be enough, but if you remove the first electrolytic from the input (rectifier) side of the choke and move it to the other side, you'll have a choke input filter vs a "C-L-C" "Pi" type filter. That could knock off maybe 20 volts +/- (complete guess) and a fat resistor in series (before the choke) might give you some more drop. The good thing about changing to choke input is---it's free.
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spec says 480V. you have 520V, i'd leave it be. keep the bias at 70% and you're good to go. did you use the 35V zener stack in the bias supply?
--pete
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It may not be enough, but if you remove the first electrolytic from the input (rectifier) side of the choke and move it to the other side, you'll have a choke input filter vs a "C-L-C" "Pi" type filter. That could knock off maybe 20 volts +/- (complete guess) and a fat resistor in series (before the choke) might give you some more drop. The good thing about changing to choke input is---it's free.
I am not sure how this stuff works.
But...there is about 40mA of current through each EL34 x4.
Can the choke take that much more.?
Thanks
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Looks like you would have 360v off the PT
SS Rectified is x1.41 -507v
Choke input is .08-.9 should be Approx 460v
I think this is correct, but the choke may need to be changed to one that can handle the current.
It actually provides better ripple reduction, but not common in guitar amps. I would put the 18K bleed resistor in and I simply increase the rail resistors to the preamp to get that closer to schematic values. That is usually what I do when older amps run higher voltage.
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spec says 480V. you have 520V, i'd leave it be. keep the bias at 70% and you're good to go. did you use the 35V zener stack in the bias supply?
--pete
I still need to check some other numbers, but.....the bias pots are both maxed out now. That never seems like a good idea.
What happens if the guy installs new tubes and they draw a bunch more current.?
I am using 2x 25 Volt 5 watt Zener.
Thanks
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do you have a schematic of what is built? i don't see any pots in the schematic in the link in the opening post.
if you have 50V stack of zeners installed, then 50V max. is all you're going to get. period.
pull the power tubes and lift the zener stack off ground, then power up and measure. with the pots both full up and both down. if you have a variac, run the amp up on the variac until you hit 520V B+, then measure.
--pete
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The schem called for 35 Volts worth of Zener.
I have 50 Volts and a bias pot for each pair of tubes.
I basically cut into where the Zener supply -35 to the grids, and used a bias pot for each pair.
50k pot...47k to ground off of pot out...220k across pot in and wiper.
RG Keen supplied the bias drawing for me.
Looks like I have 6.8 VAC on the heat.
About 220 VDC on V1 and 450 VDC on the PI...I can get that down with a resistor change I guess.
No...I do not have a Variac.
But I can pull the power tubes, lift the Zener, and measure B+
Will the pots effect anything at that point.?
Thank You
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The schem called for 35 Volts worth of Zener.
I have 50 Volts and a bias pot for each pair of tubes.
I basically cut into where the Zener supply -35 to the grids, and used a bias pot for each pair.
50k pot...47k to ground off of pot out...220k across pot in and wiper.
RG Keen supplied the bias drawing for me.
Looks like I have 6.8 VAC on the heat.
About 220 VDC on V1 and 450 VDC on the PI...I can get that down with a resistor change I guess.
No...I do not have a Variac.
But I can pull the power tubes, lift the Zener, and measure B+
Will the pots effect anything at that point.?
Thank You
if they're wired in correct, then yes.
--pete
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The Zener is stupid. What happens if wall-voltage changes? B+ goes up, Vg1 stays the same, *both* plate voltage and plate current go up, dissipation rises as *square* of wall-voltage.
Yank the big bottles. Yank the Zeners. See what bias voltage you get.
With 50K trims you *may* get the 60V or so you need to for-sure trim-down an over-enthusiastic EL34.
> Where does it become a bad plan.....1kV and -93 Volts on the grids.?
That could be a fine amp (though 1,000V is a bit much for EL34).
There's a 100W/pair setup with 775V on Plates, but just 400V on screens, and 40V on G1. Getting screen voltage down would be wise, but too expensive.
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OK...I will go ahead and pull the power tubes, and see if my bias pots still play.
Will get back as soon as.....
Thanks Again
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OK...I am at a loss.
I just, kind of, paint by numbers.
I Do Not have the knowledge most of you guys do...Why am I using those Zener.?
They are on the original schem...so I thought they were necessary.
I pulled the EL34, lifted the Zener ground and get:
1. 540 VDC on the power tube plates.
2. The bias pots swing from -25.0 to -65.0 (round numbers)
3. At the risk of repeating myself...what is the purpose of the Zener, if we can get useable Neg Bias without them.?
4. Then, another thing I have never seen is that...current sensing/current limiting capacitor in the bias circuit. What happens if THAT is removed. I guess if I need more more, for a bigger voltage drop from the boas circuit, I can make that cap bigger, and that would give me more Neg Bias Available.?
5. So where do I go from here.?
I assume my bias circuit will work fine without the Zener.
Is there some kind of Voltage divider I can make to knock down some of this B+, while also not restricting a lot of current and inducing a lot of "Sag".?
I understand it is screen Current/Voltage that will kill the EL34...but, is it difficult to get rid of B+ from a PT with no CT.?
Thank You
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3. At the risk of repeating myself...what is the purpose of the Zener, if we can get useable Neg Bias without them.?
yes, yes you can - use the pots. they (zeners) set the voltage at 50VDC in your case. as long as the bias supply voltage is > 50V then they conduct. the 35V zeners in the original were to fix bias at 35V no mater what the variance in wall power applied. in the day, tubes were much better built with greater consistency, and that probably was OK then, but as PRR points out it's dubious in today's world of tungsolski and deawoo toobz..
to clarify, let's say that the tubes hummed along happy at home @ 115VAC wall voltage that makes B+ 480VDC. now up the wall voltage to 125VAC (at a gig), we see a rise in B+ (8.6%). the -bias needs to rise too (more negative by -3V) but can't because the zeners have fixed the bias at -35V.
leave them out. use the pot to make the adjustment. if you must use them, then decide where you want the bias and build a stack for that and you won't need the pots unless you want LESS then the zener stack voltage.
>>>> best plan is no zeners.<<<<
--pete
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OK...Thanks Pete...now I see.
Man...no wonder those amps had a reputation for burning up.
Maybe old tubes were made better...but still...-35 Volts must have ran the life out of a lot of EL34...even in the Golden Years of tubes.
Thanks for that explanation.
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Howdy!
I had a similar issue, an older 110-115v transformer, and no room to add a bucking transformer, so I decided to try using the 6v (in this case) unused PT winding for rectifier heater. Since it is isolated, I surmised that running one leg of the 120v through this winding then to PT primary winding would buck or boost voltages, including heater, by a few percent, depending on polarity. I wired it up, and heaters came down to better levels, and the B+ dropped from about 520 to around 480 at idle.
I don't know if there is some reasoning I haven't thought of that would make this is ill advised, but I can say it works nicely for me. If you do have an extra heater winding unused, you might give this a try. If I'm totally out in left field, I accept correction, and disregard this idea.
Jim
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Thanks AZ -
Nothing wrong with your suggestion at all. As a matter of fact, I gave it a try. Not with an unused heater winding...but with a 6 Volt "bucking" transformer.
It did a nice jog of bringing the B+ into a more "normal" range...but it took the heater voltage to about 5.2 Volts. So I had to let go of that idea.
Looks like I will just have to live with 520 VDC on the power tube plates, and make sure I get the screens and preamp plates into order.
I think I have about 440 VDC on the PI.
So I will just need to drop that stuff down into normal.
Thanks For Your Suggestion :thumbsup:
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Question... Are you talking about a real AC100 or is this a clone that you are building?
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This is a real AC100.
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> probably was OK then, but as PRR points out it's dubious in today's world of tungsolski and deawoo toobz..
IMHO, it was a bad idea Back Then. But they were not so close to the edge that they got trouble right away.
G1 bias MUST track G2 voltage. And preferably more, if wall-voltage variation is more than "small".
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Answering what appears to be a related post here.
My original knee-jerk was to say plug in the output tubes and if they don't melt the screen voltage is fine. But I seem to recall your first post on the AC100 including some kind of malfunction... But I don't know what that malfunction was, and I'm probably too lazy to comb through all the other threads to find it. :icon_biggrin:
Short answer: if it's a vintage AC100, leave it alone and find EL34's that will work in it. You'll probably do more harm than good trying to reduce the screen voltage, at least to the amp's value.
Long answer: If it's a new home-brew, the builder probably picked a wrong power transformer. You can try various schemes to knock the screen voltage down, but it'll probably also not make 100w anymore and if you use a resistor there will beat LOTs of waste-heat in the chassis. The best way to reduce power supply voltage is to have a power transformer which outputs less voltage. Most all other methods tend to introduce new problems while they're solving your high-voltage issue.
EDIT now that I found this thread: Since this is a real AC100, leave it alone. Unless your wall voltage is obscenely high, whatever voltages are present have been there the life of the amp. These are rare beasts in the U.S.
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As I was saying .....depending on time of day, the screens are about 500-520 Volts.
When I look at tube data sheets, they are usually from one of the old books...for NOS tubes. THAT spec seems to be... G2 Max is 500. So this amp is running the screens at Maximum.
I guess I will give it a try ...like you say. My G2 resistors are 470. I will up those to 1k5 and see what happens.
How do you even calculate what the screens really draw (current) when the amp is being played...especially for the 34 and 84 type power tubes.
Well...sense I do not know, I will use 3-5 watt resistors on the screens. I should have some 1k5 in that kind of power...or I will order some.
Sorry for all the posts.
Because this IS a real Vox, I have proceeded with caution, which has translated into more questions than I wanted to flood the Forum with... Sorry about that.
The PI plates were at 450 VDC the last time I checked. But that should just be a matter of changing a board mounted resistor to squelch some of that.
Thank You
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You're doing all this the hard way... :icon_biggrin:
As I was saying .....depending on time of day, the screens are about 500-520 Volts. ... Because this IS a real Vox ...
The AC100 schematic (http://el34world.com/charts/Schematics/files/Vox/Vox_ac100.pdf) shows a voltage select switch (really an output and plug). One of the setting is marked 160v. Why not just use that instead of 115v?
Instead of changing resistor values, are using the new added screen resistors, the change PT primary tap will lower voltages throughout the amp. Try it; maybe it will lower them too much for your liking. If that's the case, and the problem is your wall voltage is well above 115v, then the best answer is to wire up a transformer to go in between the AC100's power cord and the wall to buck the voltage down to ~115vac.
Vintage Voltage Adapter (http://www.geofex.com/article_folders/vintvolt/vintvolt.htm)
Vintage Voltage Build (http://www.instructables.com/id/VIntage-Voltage-for-old-Equipment/)
When I look at tube data sheets, they are usually from one of the old books...for NOS tubes. THAT spec seems to be... G2 Max is 500. So this amp is running the screens at Maximum.
... My G2 resistors are 470. ... I will use 3-5 watt resistors on the screens. ... The PI plates were at 450 VDC the last time I checked. But that should just be a matter of changing a board mounted resistor to squelch some of that.
I think what we were missing this whole time is a holistic understanding of your situation. Bottom-line appears to be a wall voltage higher than 115v while using the 115v tap. Everything else you've been asking about in the other threads seems to come back to this, meaning you're patching symptoms instead of a problem.
AC100 (http://el34world.com/Forum/index.php?topic=17509.0)
Power Supply Drain (http://el34world.com/Forum/index.php?topic=17495.0)
Choke Question (http://el34world.com/Forum/index.php?topic=17513.0)
The vintage voltage adapter project solves all the symptoms you're chasing in one fell swoop, and requires no internal changes to the amp.
FWIW, EVERY amp from the 60's has internal voltages today higher than shown on the schematic. As a rule, none of them need fixing unless some part of the amp is dying. Was there ever a problem with this AC100 that caused you to start tinkering? If no, and EL34's don't melt immediately on power-on, I'd say yank out any changes, put it back to stock and call it a day. It's lived almost 50 years as-is, and will likely continue only needing electrolytic cap replacement.
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If you get rid of the 18K bleeder, put 1K resistors on the screens, and put a standard bias circuit in this amp, then you will have the same amp that everybody has been playing through for about five decades (except the PI and it wouldn't take much to make that standard, too).
I use 10W 1K screen resistors on my amps because 5W burns up when I overdrive EL34's. If you are not going to overdrive the output tubes, then put 6550's in your amp. 6550's are beam pentodes and beam pentodes don't have anywhere near the screen current that regular pentodes like the EL34 have, especially in an overdrive situation. It took me 750V to fry a set of 6550's, so 520V is a cakewalk for these tubes. Despite what everybody says, you can't tell the difference in sound between output tubes until you overdrive them (as long as you are using a fair amount of negative feedback).
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How do you even calculate what the screens really draw (current) when the amp is being played...especially for the 34 and 84 type power tubes.
Well...sense I do not know, I will use 3-5 watt resistors on the screens.
If your AC100 was left stock (no screen resistors), you'd unsolder the wire from pin 4 and place a milliammeter in between the wire and pin 4. Now you'd be directly measuring the screen current. Note the meter is in series (not in parallel, like measuring voltage across a resistor) because the current has to flow through the meter to be measured.
If you have a screen resistor installed, measure the voltage drop across the resistor. Now turn the amp off and measure the actual resistance of the resistor. Voltage / Resistance = Current.
For both of these, if the amp is idling, you're only measuring the idle screen current. Screen current increases when the tube's plate current increases, so to know what it does at full power output you will need to test while the amp is making full power output. You'll need a signal generator applying a steady tone to drive the amp, because if you just play guitar through it you'll get brief large spikes but the average signal level will be low. You'll probably also need a dummy load, because full power sine waves from a 100w amp will make you want to stab an icepick in your ears.
If you get rid of the 18K bleeder, put 1K resistors on the screens, and put a standard bias circuit in this amp, then you will have the same amp that everybody has been playing through for about five decades (except the PI and it wouldn't take much to make that standard, too).
Funny, but there's really nothing wrong with the stock AC100. If you really want to make it into something else, it's easier to just sell the AC100 and get the "something else".
I use 10W 1K screen resistors on my amps because 5W burns up when I overdrive EL34's.
You're probably doing something bad to the amp if that's the case (1kΩ 5w screen resistors burning).
Power = Current2*Resistance, so Current = √(Power/Resistance).
√(5w/1kΩ) = ~71mA
And you'd have to be puling much more over a sustained period of time to not just scorch a 5w resistor but actually make it burn. Your choke won't like that much current...
Also, 71mA * 1kΩ = 71v dropped across the screen resistor. The reduction in screen voltage due to this drop will reduce tube plate current (which also tends to reduce screen current as a byproduct), which equals reduced power output. Vox appeared to be trying to get every last watt, which is why there are no individual screen resistors.
If you don't mind the slight loss of power output (or like the added compression at/near full output power) you could just increase the resistance of the screen resistors. If you raise the screen resistor values, you won't need high-wattage resistor because the screen voltage will drop more when there is increased screen current, reducing plate/screen current and also keeping the dissipation of the screen and resistor lower.
All that aside, J Rindt hasn't actually said there was a problem with how the amp was performing originally... I suspect he was learning about the amp itself, read some things which were interpreted as "mandatory" or "upgrade" mods, and is tinkering stuff that doesn't need tinkering. I was guilty of doing the same when I was learning about amps.
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When I used the 160 selector, the Plate Voltage fell to about 395.
But the heaters also fell to 5 Volts. I was worried that was way too low to run the heaters properly.
The same thing happened when I used my 6 Volt "wall dropper"...the heater Voltage fell to 4.8-5.0 Volts...depending on time of day.
My wall Voltage is typically 122-127 Volts.
This IS my first experience with this amp. (and yes...I still have a lot to learn about ALL amps).
If I have ever heard anything about Vox amps in general, and the AC100 in particular...it is that they had a reputation for "Burning Up". So I am trying to get the amp to run at what most people would call more "normal" B+.
I do not know if this amp had any problems, but it had all kinds of crappy work done to it (I should post some pics)...real bad stuff. Wires with resistor hanging all over the place...resistors piled all over the PI...horrible globs of solder...and an attempt at a bias circuit that looked like school kids worked on it...it was just a mess. So I am trying to do a respectable job of getting this to run in 2014.
If I remove that 18k resistor, it will add about 12 Volts to the already High Screen Voltage.
I am actually using a 20k 25 watt resistor. At idle it dissipates about 12 Watts.
So at this point, I am keeping that 18k (20k 25 Watt) resistor, and I am installing 5 Watt 1k5 screen resistors...and I will see how it plays.
Well...after I deal with the screens, I will work on lowering the PI B+.
I have not even heard the amp yet. If I can get this sorted out, and there are no noise problems, I will get a friend over to play this for me.
Thanks For All The Help
I hopefully addressed all of the Concerns/Questions.
Speaking of that Voltage Selector...I do not trust that thing at all. I notice, in a lot of pictures, that guys have just by-passed that thing for their native wall voltage...I may do the same.
Thanks Again
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[/qYou're probably doing something bad to the amp if that's the case (1kΩ 5w screen resistors burning).
uote]
They don't actually go up in flames and I don't think they would flame even if you used a torch. They will get real hot and turn brown, however, after only a couple of tunes.
It is not only the choke that is oversized on these amps, they also use IN5408 diodes dumping into two 470uf cap.'s. I realize that most folks aren't using their 100W amps for outdoor arena metal gigs, but a lot of them will pop-in a pair of earplugs some day and turn them up to ten. It seems prudent to me to build one that will play at ten indefinitely, tubes willing.
The AC100 in question appears to have the opposite philosophy. It almost looks as though it was designed to fail at full bore, which is probably why you don't see them in pictures of your favorite rock bands. I noticed that Vox started using 1K screen resistors on their later AC100's as well as eliminating the 18K bleeder and redesigning the bias. This amp will be much more reliable if you do the same, especially at high levels.
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When I used the 160 selector, the Plate Voltage fell to about 395.
Well, then that's too much of a good thing.
The same thing happened when I used my 6 Volt "wall dropper"...the heater Voltage fell to 4.8-5.0 Volts...depending on time of day.
My wall Voltage is typically 122-127 Volts.
Believe it or not, some old equipment purposely ran tubes at slightly reduced heater voltage for longer life and less noise. So running the heaters at 5.something volts is fine. You may have reduced performance from some tubes, especially as you get down to 4.xx volts.
But the math doesn't add up if the ~40v reduction by using the 160v tap causes the same effect as a 6v reduction. I'd still advise you to se a bucking transformer to knock wall voltage down 12v before feeding to the amp, and it will solve all problems.
If I have ever heard anything about Vox amps in general, and the AC100 in particular...it is that they had a reputation for "Burning Up".
Mostly, that was the AC15 and AC30, because all tubes were inside a box formed by the L-shaped chassis and the cabinet, and only tiny vents were provided. So that's all about normal tube heat being confined without sufficient airflow, which then heated the chassis and cooked the internal components.
The AC50 and AC100 had MUCH more substantial vents. Even if they didn't, you wouldn't fix the ventilation problem without redesigning the chassis and cabinet. So In My Opinion, all this worry about the voltages is much ado about nothing.
The AC100 in question appears to have the opposite philosophy. It almost looks as though it was designed to fail at full bore ...
For all the worry over screen resistors, I have never seen a tube with a melted screen in the last 20+ years of doing this stuff. I say that while also having a 6V6 that redplated to such an extreme there is a hole melted in the plate. FWIW, the tube still functions and sounds pretty good! The hole in the plate happened before I got it, and I just keep the tube as a novelty.
The AC100 wasn't "designed to fail," it was designed to assure the full 100w output at full power. If you read the old tube books, tube manufacturers recommended no series screen resistance for class AB amps, because it reduces the maximum power output for the reasons I explained before. Hence, no screen resistors in the AC100. Or in most tube hi-fi's, like the McIntosh amps I have which were intended to run 24/7 for years.
The AC100 in question appears to have the opposite philosophy. It almost looks as though it was designed to fail at full bore, which is probably why you don't see them in pictures of your favorite rock bands.
I think you don't see them because of market forces, Vox's business model and poor business deals.
Vox didn't build its own amps, they were subcontracted out to other shops to produce while Marshall made its own amps. An English band could probably get 100w (or bigger) amps quicker from Marshall. American bands pretty much couldn't get them at all, because of Vox's distribution deal with Thomas Organ, who was selling solid-state amps using Vox's name in the U.S. and pretty much cut off Vox's biggest potential market. And Vox spread itself thin trying to make too many different products (guitars, pedals, amps, PA's, most of which flopped eventually).
Add to that that the circuit design is clean-only where a Marshall 100w could rock, and you've got the death-knell for big Vox amps.
So I'm still at the position that this amp would be fine stock if the wall voltage is reduced. Screen resistors weren't added until the modern Vox amps, and most of that design is "me-too" rather than engineering-driven (you find this is true if you do a search for "Vox AC100 history" as there's a site with all the 60's AC100 variants and their schematics & internal photos).
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Let me try my bucking tranny again.
I may be confusing my 12v vs. 6v readings. Wish I had it on a switch, but I rewired it from 12 to 6.
So you would not "worry" about 5 VAC on the heaters as too little to properly heat the cats.?
I will hook up my wall dropper again and let you know what the numbers are. This will be dropping 6 Volts, not 12.
Thanks
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OK...Yeah...I screwed up somewhere...got my numbers crossed.
With dropping 6 VAC off the mains I get (round numbers):
Plates 480
Screens 465
Heat 6.2
When I ditch the wall dropper and use the 160 Mains Selector, the same items are about:
375
360
4.6
So it would seem the best thing would be to use the 6 Volt Transformer.
This amplifier has a 3 Amp fuse.
The bucking transformer is rated at 3 Amps.
What numbers do I use to calculate the Current through the 6 Volt Tranny.?
I can put my meter in series with the 3 amp fuse, but I am wondering how to make a reasonable calculation.
Thank You
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For all the worry over screen resistors, I have never seen a tube with a melted screen in the last 20+ years of doing this stuff. I say that while also having a 6V6 that redplated to such an extreme there is a hole melted in the plate. FWIW, the tube still functions and sounds pretty good! The hole in the plate happened before I got it, and I just keep the tube as a novelty.
I have never seen a melted screen in 40+ years, but I have seen a lot of heat damaged screen resistors. I'm not disagreeing with you, in fact I would do exactly what you said if this were one of my amps and sell the thing. If I were going to play through it, I would do what I said.
The only difference is that you are talking about playing it at two and I am talking ten. The only reason for me to play it at ten is to get the output tube distortion that is so characteristic of rock since the sixties. High fidelity would be counter-productive for this use. I use 100W amps because they produce a sound that is different from a 50W amp and I record with EL34's because they have the sound that I like when overdriven. Both of these create all kinds of problems, but it is the only way you are going to get that sound.
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OK...Yeah...I screwed up somewhere...got my numbers crossed.
With dropping 6 VAC off the mains I get (round numbers):
Plates 480
Screens 465
Heat 6.2
Perfect. You're done.
This amplifier has a 3 Amp fuse.
The bucking transformer is rated at 3 Amps.
Perfect. You're done. :icon_biggrin:
What numbers do I use to calculate the Current through the 6 Volt Tranny.?
I can put my meter in series with the 3 amp fuse, but I am wondering how to make a reasonable calculation.
Thank You
You wouldn't calculate, unless you have complete data for the power transformer.
You could estimate by multiplying Volts * Amps for all secondary windings, adding all VA together and getting a total. Then divide that VA number by your applied wall voltage (after the bucking transformer).
But you have a 3A bucking transformer and a 3A fuse on the amp. You don't need to calculate anything because if the amp draws more than 3A from the bucking transformer, the fuse on the amp will pop and no current will be drawn from the bucking transformer. So you can call it a day.
The only difference is that you are talking about playing it at two and I am talking ten.
I think we must be talking past each other; maybe we'll sync up later.
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Sounds good to me...Thanks.
Just need to check the other plate voltage numbers, and then see how this amp sounds.....finally.
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I took some decent gut shots...but cannot, as of yet, figure out how to get them onto this new laptop.
The amp plays OK...you can hear a guitar when you strum the strings...no "weird" noises.
But there are 2 hum problems.
1. It is ties into the Volume Pot, and the frequency of the noise increases with volume. I would like to search for this issue later.
2. With no signal into the amp...there is a low, steady hum. I put my meter across the speaker jack, and it says 60Hz. This hum is effected by power tube bias...it seems to get louder as you bias the tubes hotter ...or at least the hum starts as the plate current starts to increase. These were new power tubes, so I had the Neg Bias Voltage Max. It behaved the same with the old unmatched tubes. As the bias voltage starts to get more positive...the hum starts to come on, and get worse.
Right now the tubes are at about 50%.
480 VDC and 27 mA.
This is the first amp I have ever worked on that had 2 heater windings...and I wanted to ask about that.
Right now, I am using the CT for each heater supply, and they are grounded to separate lugs.
I am thinking to try the 100 Ohm "Virtual CT"...and see if that helps.
Is there a Procedure/Theory when working with dual heater supplies.?
Should they both be grounded to the same place.? .....one set of !00 Ohm resistors for each winding, and grounded to the same spot, or can they go where it is convenient...or does it matter.?
Right now they are grounded to separate B+ ground lugs. That is how the factory had them grounded as well.
Thank You
From my old computer.....
http://i.imgur.com/AGx2htl.jpg (http://i.imgur.com/AGx2htl.jpg)
http://i.imgur.com/jnky8Xe.jpg (http://i.imgur.com/jnky8Xe.jpg)
http://i.imgur.com/eAaZjDh.jpg (http://i.imgur.com/eAaZjDh.jpg)
http://i.imgur.com/YzczZ9i.jpg (http://i.imgur.com/YzczZ9i.jpg)
http://i.imgur.com/5LJalcG.jpg (http://i.imgur.com/5LJalcG.jpg)
http://i.imgur.com/nCkvjU6.jpg (http://i.imgur.com/nCkvjU6.jpg)
http://i.imgur.com/idf2YtY.jpg (http://i.imgur.com/idf2YtY.jpg)