Basic tube amp question(s)

Rolbista · Post by **Rolbista** » 14 Jan 2012, 11:01

thanks. just got back from the store, I bit the bullet and bought all resistors 1W and caps fo 400V and 3kV(that's what they had). The amp is a 20watt el84 marshall, max voltage is around 350V.

kleuck · Post by **kleuck** » 14 Jan 2012, 11:20

Around 385...

phatt · Post by **phatt** » 14 Jan 2012, 11:46

kleuck wrote:Around 385...

And replace power valves every 12 months,,,

At 300ish they might last for 10 years. wink.

Take your pick?

Phil.

kleuck · Post by **kleuck** » 14 Jan 2012, 13:09

You can just replace the cathode bias resistor for a higher one, 150 ohms is the value of choice most of the time (did myself in an amp wit 335 volts on the plates)
Perhaps 180 ohms for a 2061.
See 18 watts.com for more informations.

Rolbista · Post by **Rolbista** » 14 Jan 2012, 15:43

I gladly would, but 18watt has been down for half a year now (by this I mean that I couldn't register, I did, but confirmation link never came, no contact to admins on the site either).

kleuck · Post by **kleuck** » 14 Jan 2012, 21:28

Well it's all about bias, a 120 ohms cathode resistor gives a hot bias and makes the EL84 dissipate a lot, and the more plate voltage, the more dissipation.
So tubes don't last, back in the days they perhaps did, but modern tubes are not that sturdy.
So most 18 watts clone use now a 150 ohms resistor, when B+ is around 310-330 volts, some (rare) use 180 ohms, btu it's not out of question for a 2061.
Depending on you B+ you'll have to test the bias and the sound to choose.

printer2 · Post by **printer2** » 14 Jan 2012, 23:53

If the design calls for 250 ohm I generally use a 330 ohm resistor for the cathode (for an example). This biases the output cold, low mA, and then you can put a parallel higher ohm resistor (normally lower wattage, I have never had to use more than a 1/2 watt) to adjust the output to where you want it.

The voltage the tubes are run at effects the sound of the amp. Take the EL84's in an 18 Watt. They would last forever at 250v but you would not get the same sound out of them as you would at 330v. It is also not the voltage that is the major contributor to the shorting of a tube's life but how close to their maximum dissipation rating you run them. Same thing goes though, run them at 50% of their power and they will last forever, but you will not get the 'squeezing the last drop of life out of them' sound.

diagrammatiks · Post by **diagrammatiks** » 15 Jan 2012, 01:24

i'm betting you hit 350 or less under load.

phatt · Post by **phatt** » 15 Jan 2012, 13:08

I believe If you keep the HT below about 240VDC then a pair of EL84's running K bias will be Push Pull Class A.
Just a thought?
Phil.

diagrammatiks · Post by **diagrammatiks** » 16 Jan 2012, 01:20

not too sure. but you will burn out a pair of tubes much faster in pushpull A then in push pull a/b.

printer2 · Post by **printer2** » 16 Jan 2012, 03:36

diagrammatiks wrote:not too sure. but you will burn out a pair of tubes much faster in pushpull A then in push pull a/b.

Again depends upon the dissipation. Running at a low voltage in Class A will not stress a tube if they are dissipating well under their rated wattage.

Also you can bias a pair to operate in Class AB at a lower voltage also.

phatt · Post by **phatt** » 16 Jan 2012, 05:41

printer2 wrote:
diagrammatiks wrote:not too sure. but you will burn out a pair of tubes much faster in pushpull A then in push pull a/b.
Again depends upon the dissipation. Running at a low voltage in Class A will not stress a tube if they are dissipating well under their rated wattage.

Also you can bias a pair to operate in Class AB at a lower voltage also.

Spot on

.
Frankly if you are chasing tone mojo,,, Definitly look at lowering the HT.
The London power Amp implement some clever HT ideas and the rather low HT means you can run nearlly any Power Valve your heart desires.

Sadly most Amps sold in shops often claim 20 Watts from a pair of EL84.
Yes you can up the HT and claim 20 watts instead of 14watts if you want but the SPL difference is so small it's just wasting Valve life span on something that will only add harshness and (in a live Venue) will be no louder.
Phil.

phatt · Post by **phatt** » 16 Jan 2012, 06:09

From what I've learned by reading some fine print,, (you know those boring books with lots of complex maths, winky)
Most comments about power Valves gives the novice the impression that just a quick bias adjustment can fix anything that ails your feel good tone mojo.
I'd err on the side of caution when some Ex-spurt says 385 VDC plate voltage on EL84's is fine just adjust the bias to suit,,, durh even with half a brain cell,, common sense should tell you that at some point no bias voltage will fix the runaway issue.

Valves are quite voltage tolerant but in guitar amps where they get belted you want to have a reasonable safety margin.
Of course if you have access to a warehouse full of Valves then run EL84 at 400 Volts plus and just replace valves every second week. lol

If you care to read,,, The clasiic EL34 Power tube should really be running the screen grid at *Half the HT Voltage*.

I'm yet to find an EL34 powered Guitar Amp that shows evidence of that understanding.

Ever noticed how many EL34 Amps suffer premature failures?
As in all power Valves the screen is the weak link and should be protected at all costs.

In the words of King tut "The tonal difference between these things is often more to do with how much money has been spent on fancy modifications that actual measureable tone difference." But by human design failure to grasp the intricate most players think that everything has to be on the bleeding edge before tone is forthcoming, which is worlds away from the truth.

The *More is always better* mindset is hard to overcome.

A real story;
A local player had spent quite a substantial amount of money on a small EL84 Amp.
Now with a draw full of mojo Valves at his disposal he still could not attain the same mojo as my home made crapwatt amp.
Finally after my constant ear bashing that his HT was insanely high and will just add harshness no matter how well matched they maybe.

Short end,,
A Voltage drop and some simple mods has transformed it into something useful.
Needles to say his jaw dropped and he no longer reads stupid guitar magazines.

Also a respected teck I know well opened an old Sound City Amp a while back and noted thus;
With the *Conservative Voltages* used in this rig the Amp will likely run for another 30 years before it needs major work.

IME,, there is far more tone mojo in resistors and capacitors than you will ever find by purchasing bottled magic smoke with gold printing.

At the novice I would say there is so much BS in the Valve world that you really have to go back to basics and learn a lot of it yourself. Question everything,, including my comments.

What may work in one amp circuit maybe a disaster in another,, you really need to develop a wealth of understanding before you will make inroads.

just my views on it,,, Phil.

diagrammatiks · Post by **diagrammatiks** » 16 Jan 2012, 06:47

if you don't blow a full set of tubes every 12 minutes its just no good.

but in all seriousness, true class A is 100 percent idle dissipation. it doesn't matter what the ht is.

kleuck · Post by **kleuck** » 16 Jan 2012, 07:48

Well it matters indeed, cause most of the time, as pointed before, screen voltage is about B+, gives a good power musical distortion.
If you go for a high B+, you HAVE to limit screen current and voltage, the amp sounds differently, not only louder, a lower idle current -required to tame the dissipation- gives another sound too.
So yes, different voltages mean different sounding amps.
I run only 6P14P-EV when voltage is above 300 volts.

printer2 · Post by **printer2** » 17 Jan 2012, 00:45

diagrammatiks wrote:if you don't blow a full set of tubes every 12 minutes its just no good.

but in all seriousness, true class A is 100 percent idle dissipation. it doesn't matter what the ht is.

No, true Class A is when both tubes remain on all the time. You can run the tubes at 100v and have both tubes always conducting without getting close to the rated dissipation. On the other hand you could be running at a high voltage (say 400v with EL84's) or low impedance and be in Class AB while exceeding the the tubes dissipation limit.

printer2 · Post by **printer2** » 17 Jan 2012, 00:50

kleuck wrote:Well it matters indeed, cause most of the time, as pointed before, screen voltage is about B+, gives a good power musical distortion.
If you go for a high B+, you HAVE to limit screen current and voltage, the amp sounds differently, not only louder, a lower idle current -required to tame the dissipation- gives another sound too.
So yes, different voltages mean different sounding amps.
I run only 6P14P-EV when voltage is above 300 volts.

Have to agree. Different tubes have a different HV sweet spot for guitar due to the limitations (cheap simple circuits) that operate them in a particular fashion.

kleuck · Post by **kleuck** » 17 Jan 2012, 02:09

Indeed !
I recently built a single tube amp with an ECL82, rated for 7 watts of dissipation, obviously it's a SE, class A.
I run it around 6 watts (and 260 volts), so under max dissipation, but most schematics you can find -lower voltages and different OT impedance- are more conservative, and run the tube under 5 watts (and 200 volts)
Both class A, and both under 100% of the max dissipation.

diagrammatiks · Post by **diagrammatiks** » 17 Jan 2012, 03:04

printer2 wrote:
diagrammatiks wrote:if you don't blow a full set of tubes every 12 minutes its just no good.

but in all seriousness, true class A is 100 percent idle dissipation. it doesn't matter what the ht is.
No, true Class A is when both tubes remain on all the time. You can run the tubes at 100v and have both tubes always conducting without getting close to the rated dissipation. On the other hand you could be running at a high voltage (say 400v with EL84's) or low impedance and be in Class AB while exceeding the the tubes dissipation limit.

"True class A operation does not have to be above any particular current rating or dissipation. It depends on the tube type, the power supply voltage, the reflected impedance, and the required operating point. However, in general, when a class A power amplifier is designed, the bias point is chosen to correspond with the spot on the plate curves at the intersection of the load line, the plate voltage, and the maximum dissipation curve that gives maximum symmetrical swing in both directions before clipping. This means that the tube is biased right at maximum plate dissipation, which is okay, because the dissipation is maximum at idle in a class A amplifier, and does not increase with applied signal, as it does in a class AB or class B amplifier (it actually decreases to a minimum at full power). This is not to say that that is the only current and voltage that will work. If you lower the plate voltage by 100V, you will find another "optimum" spot where these lines intersect. If you change the reflected load impedance, you will find yet another optimum spot."

from aiken.

ppluis0 · Post by **ppluis0** » 17 Jan 2012, 03:10

kleuck wrote:Indeed !
I recently built a single tube amp with an ECL82, rated for 7 watts of dissipation, obviously it's a SE, class A.
I run it around 6 watts (and 260 volts)

Hi kleuck,

Can you post the schematic of that amp ?

I have a couple of ECL82 to try an small SE class A, but I find several diagrams on the web, and none of them are well siuted for guitar amplification, I think.

Thanks in advance,
Jose