Testing Output on Peavey Classic 50

[awdman]

Ok, I am back


I built a 16 ohm Dummy Load (came out nice) and using my Basic Stamp Programmer made a 1khz signal and put everything on. I could only come out with 30Watts where Peavey is saying it is 50W RMS using my multi meter, dummyload, and signal using equation (22*22)/16. This is the max I could get out tweaking all of the knobs. My question is: Does this sound right, do I need to use a true signal generator or should the signal I am using work. If this does not sound right should what should I do from here? I do not have access to an oscilloscope. Here is the link to the amp specs http://www.peavey.com/assets/literature/manuals/Classic50_2005.pdf

Once again thanks for all of your help

Anthony

[phatt]

Hi Ant,
Once again, no one even notices the outright lies that some Big names clain an no one ever bothers to read the Valve charts.

Generally speaking,, 4xEL84's will deliver approx 28 Watts. (the classic Cathode biased Vox circuit)
Your amp maybe Fixed bias which will add a few extra Herbs but NO your amp does not put out 50 watts.

Between 14/17 watts per pair in PP mode.
And before you ask that Amp is not class A.
You need to run them at around 250 VHT for that magic stuff.

As with most of these tubes running on lower HT produces the better sounds but all the amp makers know that if the Amp says 50watts then most will buy the BIGGER amp,,, regardless of tone.
Phil.

[awdman]

That's good, right around target.  Second question is what is VHT short for. That is kicking my butt.

[phatt]

That's good, right around target.  Second question is what is VHT short for. That is kicking my butt.

Sorry mate,, wrote it in a hurry.
VHT,, Volts High Tention. Probably not the right way to state it but even the experts tend to shorthand a lot of words.
Phil.

[J M Fahey]

I guess it's Voltage High Tension , or Very High Tension, *Australian* for +B , the power supply voltage.
Notes:
1) *Australian*: language vaguely related to English, yet spoken by a few million people. 
2) Being of Irish descent, and growing in Spanish speaking Argentina, my father wanted me to practice English with a native speaker, so he sent me regularly to "Miss Palomba's" house, from the tender age of 8 or 9.
What was an Australian fine lady doing in an agricultural town in the middle of the Pampas is beyond me.
Later on, when I took regular English classes at a British Council approved school, they were *amazed* both with my fluency .... and my (to them) "terrible" pronunciation.
No kidding, it took me about one year to "un-learn" Australian and then catch up with the "owh deah !! bweooteehfooh! wondehfooh!" English they wanted.
Oh dear, the good old times !! 
3) There's no way to pull *clean* 50W from 4xEL84 tubes.
You *might* exceed rated voltage by 20% or 30% (at your own risk) and bias them real cold , say, 5mA each, but the sound will be dry, cold, "nails on a fingerboard", and any sustained distorted chord will probably turn them cherry red.
The fine sounding Classic 50 definitely does not behave like that, so ..... 50W (RMS) = 
Juan Manuel Fahey

[phatt]

Thankz fur pointin out dat I'z carn't even speeel in me own language. :lmao:
Yes, it's  High *Tension* with an S, not a T.  :grr
I'll have to visit "Miss Palomba's" house won't I?
Phil.

[J M Fahey]

Well, if you are around 8 to 12 Y.O. and appreciate freshly baked cookies, you'll love the experience.
A working Time Machine with a "-50" years capacity would help a lot.

[phatt]

Hi Anthony,
           King TUT's book has some good clear explainations about the common issues of tube amps. I liked his simple approach to valve life expectancy.
When it comes to power tubes in guitar amps,,The higher the voltage the shorter the life span.
Guitar amps (old and new) generally run the screen volts very close to plate volts.
This gives more power but leaves less margin for safety.
Some older tubes handled it but often the older amps used Valve rectifiers and also ran lower HT or B+.

Nowdays it's not uncommon to find a little EL84 amp with a HT of 370VDC or more ,,,now add SS diode rectification (less sag). So the tubes are coping a belting and something has to give.
Again King TUT; "Most common cause of tube failure, *Screen grid* over dissapation".

The plate is huge and can cope with some abuse but the Screen grids are delicate little wires.
Now add it all up; An overly high B+,,, plus screen volatge very close to plate voltage,,,,add a tight SS Rect.
Well those poor little screen wires are going to start rattling around inside (where you can't see) then start arcing out and before long you are shopping for new tubes.
*The Screen is the weak link*

It is actually possible that under heavy load the *Plate voltage* can drop a few volts and be at a lower positive potential than the screens,
A momentary situation but if it keeps on happening,,$$ ouch.
When that happens the screens actually become the most positive element in the Valve and hence cop a belting.
Remember that negitively charged electrons coming from the cathode will be attracted to the most positive element, they can't tell the difference between the plate or screen. (No safety margin)

TUT again; "If you run high B+ ***Drop screen voltages*** Be kind to your screen grids and Your Valves will reward you with a long and tonefull life span.
Your schematic shows R58 being 400R 5watt. Now again I'm guesstamating your B+ is well over 300VDC (The -14 bias volts hints there is a high B+) So I would just up R58 to 1K,, even 1k5.

This will do 2 things it will help prolong tube life and also add a bit more
compression to the output section.
Not only does it help the tube and give a better safty margin it makes the amp much more responsive. (BTW this is an old blues trick, I guess it depends on style of playing)
Most folks I've helped out with these simple changes like the wider dynamics you get from the powerstage.
I doubt you will even notice the slightly less output wattage.

I may well have missed something you need to understand, so go find the *Valve wizard site* good valve talk.
Also *Tone Lizard Amps*,, checkout his *Tone Lounge* pages good simple explainations there also.
One other option is to hunt down the *Dana VVR Kits* (A High voltage regulator for amps under 40 watts). that will allow you to set the Max HT or B+ . I know little about them but they do exist.
Or look at King TUT's ideas on regulators, also sells Reg kits .
His Site is called *London Power*
Or look at the old brute force approach, a big 10 watt drop resistor in the B+ path.
Phil.

[Enzo]

It is not voltage that kills tubes, it is dissipation.

If you ran them at 400v, you could bias them to 5ma, but that would only be 2 watts of dissippation.  And I suspect they would indeed last a long time at that level and sound crappy.

[phatt]

Remember it's not the bias it's the *screen*
Go track down some data.  you will find that working with *High B+* needs the screens *LOW*.  Often at HALF B+  If you want them to live.
Yes your kind of right in that Vavles are by and large *overvoltage tolerant* but often little consideration is made for *Screens*.
You can't just bias colder if you keep uping the B+ you have to back off the voltage on the screens as well as rebias.

Phil.