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Sounds like you need to stop and take a break,, LOL.

With power Valves and PI Triode in these voltages will be much lower but I'm just using your 50VDC as guide.

If plates are say 505VDC
then screens should be just under 500VDC
Grid (measured on the grid pin) should be very close (within a volt or so) to the Negative bias voltage you have setup.

Cathode should be your mV reading on the DMM.

Meanwhile sit down and work out which pins are the right ones,,, then write them down and put them in front of you until it sinks in.

Don't panic if the Valves are new and you red plated them. As long as you powered off as soon as it happened you will have only taken out a few hours life span.
Tubes are pretty tough and will allow a few misshaps.
Phil.

phatt wrote:Sounds like you need to stop and take a break,, LOL.

With power Valves and PI Triode in these voltages will be much lower but I'm just using your 50VDC as guide.

If plates are say 505VDC
then screens should be just under 500VDC
Grid (measured on the grid pin) should be very close (within a volt or so) to the Negative bias voltage you have setup.

Cathode should be your mV reading on the DMM.

Meanwhile sit down and work out which pins are the right ones,,, then write them down and put them in front of you until it sinks in.

Don't panic if the Valves are new and you red plated them. As long as you powered off as soon as it happened you will have only taken out a few hours life span.
Tubes are pretty tough and will allow a few misshaps.
Phil.

Well, I haven't measured the power tube pins with the tubes in. I'll do that tomorrow. I have the datasheet pin diagram drawn out in front of me all the time but that still doesn't eliminate user mistake The power tubes aren't worryin me too bad. My main goal was to finish it by Friday when I'm visiting home so I could surprise my dad with it (he doesn't know I'm makin one ha) and I have a new set of power tubes at home. Plus, I just dont want all the other expenses to be for nothing if I can't get this workin right. But I'm sure I'll get it--just not sure if it will be by friday.

Using the Duncan claculator at 500V on the plates the ideal current draw per tube would be between 30 to 36 mA with an upper limit of 42 mA. Beyond that and you're looking for trouble.

You really should have a one ohm resistor between each cathode to ground; it's the only way you're going to know what each tube is doing. You can use a 10 ohm resistor but you'll need to multiply your mA readings by 10; I like the 1 ohm cause it a simple 1-1 conversion.

Keep in mind too when using the cathode resistor method you are reading ALL the current the tube is drawing; both plate and screen current. The screen current for a 6L6GC is around 5mA so you can subtract that from your readings. But even if you ignore the screen current the resistor method will get in the ball park.

TT

Well, I'm actually pretty confident in what the power tubes are doing and the bias except the volume pot problem. When I turn it past 1 or 2, it squeals horribly and the power tubes' current rockets up what would cause that to happen? It is completely mystifying me. Could it be related to the fact I took the EXT speaker out? I have the 820 FB resistor and the output transformer lead going to the one output jack. I didn't think that could be it but I didn't know. I figure if I fix the problem that is making it squeal, it will probably fix the hum.

Now when I turn the bias up around 20 (nowhere near where I need to have it) It has this aweful whining flutter sound. And also when I turn my standby back on, it sounds like a siren going up in pitch and dies out. Which is a similar sound I get when I turn the volume pot up above 2

Another new development. It only squeals and sends the power tubes soaring when the second preamp tube is in which is the half of the 12ax7 with the 3.3M and 10pf going into it. I've swapped different tubes in and out and tried all combinations and it only squeals when that second tube is in. so what does this mean?

Okay, the weird squeal is in fact in the power tubes. I realized today that by thinking it was the 2nd preamp tube, I was really just cutting off the flow to the power tubes eliminating the squeal. When I bias one power tube at 20, the other shows around 18. Is that a big enough difference to cause the squealing? And I cant tell but it almost sounds like the squeal is coming from the output transformer. Please tell me this is something I can easily fix

Well, it's actually the output transformer for sure. when I move the leads around with a stick, the frequency of the squeal changes and if my volume knob is not too high, I can make it go away by wiggling them. However, if I want to play the amp on 4 or 5, it will still squeal. Should I just try to route them the farthest from the filter caps as possible?

can anyone help before I have to unveil a $600 piece of junk to my dad tomorrow? Any ideas are appreciated.

Turnerguitar wrote:can anyone help before I have to unveil a $600 piece of junk to my dad tomorrow? Any ideas are appreciated.

Probably not chum!
If this is a first build???? Then it will at least teach you that there is a lot to understand before you can build Amps on the fly.
Winky winky,,, no it's just not that simple even though it might look that way when you stare at schematics,, Yuk yuk

Time to sit down and absorb a few things.

One of the best ideas I ever read was to turn your schemataic upside and start again by following the circuit right through.
70% of issues are ground problems. hint.

It's a good idea to read schematics from Node numbers rather than a picture on paper.
It may seem hard at first but it will save heaps of headaches in latter builds.
Phil.

Yeah, I'm definitely getting humbled and learning a lot. And thanks for getting me this far in the build. If the hum is only present when the power tubes are in, does that mean anything or does it just mean the power tubes are amplifying the hum?

Turnerguitar wrote:Yeah, I'm definitely getting humbled and learning a lot. And thanks for getting me this far in the build. If the hum is only present when the power tubes are in, does that mean anything or does it just mean the power tubes are amplifying the hum?

Well it tell's you something is working LOL..
Tube Amps are ***High Voltage, High Imp, low current devices*** as such They are very prone to hum.

You will learn to get around some of it but the old classic circuits do tend to hum a little to much.
That said in a live gig you never hear it,,,but for most bedroom bashers it can be quite disconcerting.
Phil.

phatt wrote:

Turnerguitar wrote:Yeah, I'm definitely getting humbled and learning a lot. And thanks for getting me this far in the build. If the hum is only present when the power tubes are in, does that mean anything or does it just mean the power tubes are amplifying the hum?

Well it tell's you something is working LOL..
Tube Amps are ***High Voltage, High Imp, low current devices*** as such They are very prone to hum.

You will learn to get around some of it but the old classic circuits do tend to hum a little to much.
That said in a live gig you never hear it,,,but for most bedroom bashers it can be quite disconcerting.
Phil.

Well, this hum is so loud that when I play it at volume 2 or 3, the hum is louder than my playing so I don't think this is normal hum. Could the routing cause this? If so, what should I look for: signal wires too close to the filter caps, signal too close to power wires, filament wires too close to signal?

Oh dear me?
Sounds like a layout issue,, or a grounding problem somewhere in the circuit.

Ok maybe time to rethink the circuit.

Draw out the schematic except this time Number the *NODES* this is just so important if you want trouble free circuitry.
Only other way is copy the wiring layout *Exactly* as the old fenders.
It really is important that you don't create ground loops. Trust me I've efed up many a circuit because I did not understand the ground path.

The ground path in most schematics is *assumed* As such it gets little attention.
For the novice this can turm into a nightmare which is the symptoms you are describing here.

Pay attention to each ground path,, Just as you would obviously pay close attention to the HT supply path.
Phil.

phatt wrote:Oh dear me?
Sounds like a layout issue,, or a grounding problem somewhere in the circuit.

Ok maybe time to rethink the circuit.

Draw out the schematic except this time Number the *NODES* this is just so important if you want trouble free circuitry.
Only other way is copy the wiring layout *Exactly* as the old fenders.
It really is important that you don't create ground loops. Trust me I've efed up many a circuit because I did not understand the ground path.

The ground path in most schematics is *assumed* As such it gets little attention.
For the novice this can turm into a nightmare which is the symptoms you are describing here.

Pay attention to each ground path,, Just as you would obviously pay close attention to the HT supply path.
Phil.

do you mean just make sure each ground path is actually going to ground? or does it matter which grounds are grounded together and where on the chassis they are grounded to (example: filter caps grounded at same chassis point as input jacks)

I think i'm gonna star ground the amp today. i'll let you know what it sounds like after that.

Hi turnerguitar, have a read of merlin's article about grounding in amps here: http://www.freewebs.com/valvewizard2/grounding.html

Turnerguitar wrote:I think i'm gonna star ground the amp today. i'll let you know what it sounds like after that.

Hold up, before you make a drastic change in the hope of accidentally fixing your problem, have a think about what you're doing first, and try to understand the reasoning for the various grounding schemes. I'm no expert by any means though (in fact I probably know relatively little) but a star ground scheme might not necessarily be what your amp needs. One thing you need to be aware of is to avoid crossing power ground with signal grounds - even though they might both be connected to the chassis, the position of the grounding points is important to avoid injecting power hum into your signal path.

How about posting a picture of your amp build, and maybe point out where your ground connections are? It might be easier for someone reading to see that and perhaps give some specific advice.

mysticwhiskey wrote:

Turnerguitar wrote:I think i'm gonna star ground the amp today. i'll let you know what it sounds like after that.

Hold up, before you make a drastic change in the hope of accidentally fixing your problem, have a think about what you're doing first, and try to understand the reasoning for the various grounding schemes. I'm no expert by any means though (in fact I probably know relatively little) but a star ground scheme might not necessarily be what your amp needs. One thing you need to be aware of is to avoid crossing power ground with signal grounds - even though they might both be connected to the chassis, the position of the grounding points is important to avoid injecting power hum into your signal path.

How about posting a picture of your amp build, and maybe point out where your ground connections are? It might be easier for someone reading to see that and perhaps give some specific advice.

thanks, i'll do that before I overhaul the whole ground layout

mysticwhiskey wrote:Hi turnerguitar, have a read of merlin's article about grounding in amps here: http://www.freewebs.com/valvewizard2/grounding.html

Good one chum Saves me trying to babble it out.
Phil.