KT66 SE amp, grid current or not? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th October 2006, 11:45 AM   #1
jhaible is offline jhaible  Germany
diyAudio Member
 
Join Date: Oct 2006
Default KT66 SE amp, grid current or not?

Hi,

I'm new here.

I'd like to build a SE amp with KT66 in triode mode, plate voltage around 400V, using a 5.2k primary impedance transformer.

This roughly fits a data sheet application, where a 4.5k impedance is recommended, giving 5.8W output power.

With the slight mismatch of my transformer (bought a pair of these because I wanted to use EL84 at first), I simulate approx. 3 ... 4 Watts of output power.

Now I wonder about extenting the range to allow grid current on occasional input level peaks. I don't intend to run the Tube at full power permanently (it's HiFi, not Guitar application).

(1) Is it save to do that, or will it considerably shorten the life of the KT66?

(2) I guess for this, I need a cathode follower driver stage.
I have a 100V seocondary winding on my mains transformer, which is intended for fixed bias of output tubes, and only provides 50mA (AC). Is it ok to use this to make a negative rail for the source follower driver stage, with only a few mA of idle current, i.e. not loading the negative rail too much, but providing _more_ for the case of KT66 grid current?

From theory I know that's just what a cathode follower is good at, providing a lot more current for th epositive half wave than the negative - I just wanted to check if there are any drawbacks in practice.

As I said, I'm new here. If this has been covered before, please be patient with me and just send me to the old thread.

Thanks in advance,

JH.
  Reply With Quote
Old 17th October 2006, 12:48 AM   #2
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
1) Momentary grid current on signal peaks will not adversely affect the life of most tubes since the grid current should only flow on occasional signal peaks.

2) The extra winding on your transformer is rated for 50 mA. This should be capable of supplying more than enough current for your uses. I would use SS rectification to be sure that the negative voltage is present before the positive voltage sources come up.

I have no experience with a KT66, however I use a 5K load with a 6L6GC. I run about 465 volts B+ with about 425 across the tube(cathode bias and OPT loss). I get 5 to 6 watts depending on which OPT I use (some are lossier than others). Using a 5K load will give up some power, but improve the distortion and damping factor.

I have tried using a mosfet and a negative voltage source to allow for occasional grid current. This did not offer much additional power output on a 6L6GC, but did improve the transient response. Since I was designing a simple amplifier, I chose to use cathode bias to eliminate the negative voltage requirement. None of the power transformers that I wanted to use have an extra winding or bias tap.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 17th October 2006, 07:26 AM   #3
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Sounds like a good idea to me. What you should achieved is some useful headroom and more graceful overload behaviour. This might make your amp seem louder than it really is.
  Reply With Quote
Old 17th October 2006, 09:36 AM   #4
jhaible is offline jhaible  Germany
diyAudio Member
 
Join Date: Oct 2006
Quote:
Originally posted by tubelab.com
1) Momentary grid current on signal peaks will not adversely affect the life of most tubes since the grid current should only flow on occasional signal peaks.
Thanks for reassuring me! I think I'll really do this then.
(I would feel even saver if someone confirmed that this applies to the KT66 in particular (you wrote "most tubes"). It looks big and sturdy, but it's my first time to build something >1W in tubes, so I'm better asking one stupid question too much ...)

Quote:
2) The extra winding on your transformer is rated for 50 mA. This should be capable of supplying more than enough current for your uses. I would use SS rectification to be sure that the negative voltage is present before the positive voltage sources come up.
That's my biggest concern, to blow the tube from loosing bias. I simply don't have enough HT voltage to be able to afford the almost 40V accross a cathode resistor.
In the power supply, it's SS rectification, too, for the same reason, so I'll have to take extra precautions for the negative supply to come faster.
I also read it would be important not to apply the HT before the tubes are heated up (cathode stripping?). Is this really important here? (I never thought about this in the small signal tubes circuits I'd build until now.)
In that case, my plan would be:

Both (positve) HT and negative supply will have a MOSFET source follower, with a chain of RC filters at its gate for smoothing and/or a voltage stabilisation. That way, I can control the turn-on of each supply with a rather high impedance circuit, just controlling the Gates of the MOSFETs. Different time constants in the RC filter chains should provide controlled ramp-up time, independend of the load. So there would be control of the negative voltage coming faster.

I wonder if I should use trimpots to divide-down the negative supply to set the bias then (before the cathode follower driver even - is this a good idea, or are there hidden dangers in this?), or build some servo loop. 1 Ohm cathode resistor and a diff amp + integrator servo loop looks tempting - my greatest concerns are how to build this fail-safe. The more complex the circuit, the more _unexpected_ ways it could fail ...

Quote:
I have no experience with a KT66, however I use a 5K load with a 6L6GC. I run about 465 volts B+ with about 425 across the tube(cathode bias and OPT loss). I get 5 to 6 watts depending on which OPT I use (some are lossier than others). Using a 5K load will give up some power, but improve the distortion and damping factor.
Are you running the 6L6 as triode or as pentode?


Quote:
I have tried using a mosfet and a negative voltage source to allow for occasional grid current.
A MOSFET for the drive stage (source follower), or as a current source for a tube driver / cathode follower?

JH.
  Reply With Quote
Old 17th October 2006, 09:36 AM   #5
jhaible is offline jhaible  Germany
diyAudio Member
 
Join Date: Oct 2006
Quote:
Originally posted by ray_moth
Sounds like a good idea to me. What you should achieved is some useful headroom and more graceful overload behaviour. This might make your amp seem louder than it really is.
Yes, that was the idea. Graceful overload is what I really want, when using tubes.

So I was kind of shocked how the waveform clipped (in simulation) with a high impedance driver. Ok, maybe the tube model isn't good enough to really examine the effects of grid current.

Having seen this, now I'm puzzled as to how so many tube amps do gracefully overload without a low impedance driver. Like, all the guitar amps?

JH.
  Reply With Quote
Old 17th October 2006, 10:46 AM   #6
diyAudio Member
 
Tubes4e4's Avatar
 
Join Date: Jul 2005
Location: Leverkusen
Hi,

Quote:
Originally posted by jhaible
Having seen this, now I'm puzzled as to how so many tube amps do gracefully overload without a low impedance driver. Like, all the guitar amps?
JH.
They just donīt overload "gracefully". Except you consider THD% numbers very deep into the two digits range still to be "gracefully"

Tom
__________________
If in doubt, just measure.
  Reply With Quote
Old 17th October 2006, 01:10 PM   #7
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
They just donīt overload "gracefully".
Many guitar amps are terrible in the overload recovery depertment, and they often see overload. There is a phenomenon known as blocking distortion (guitar players call it farting out) where the grid goes positive enough to allow the charge to be drained off of the coupling cap, causing bias upset. In severe cases the output tube can be cutoff as soon as the overload is removed. The charge must be restored through the grid leak resistor which is large. This can cause audible distortion for a second or longer.

I have no KT66's to test, but I have tortured several (similar) 6L6GC tubes. I doubt that you could pull enough current through the grid of a 6L6 to cause any damage by using a cathode follower.

Cathode stripping is a real possibility. The severity of this effect has been debated on this forum before, without a clear cut answer. I have adopted this strategy. I ignore it on any amplifier built using cheap tubes (under $10USD each), and use delay circuitry or a standby switch where pricier tubes are used.

I would use all of the available voltage for the negative supply. Your supply must be negative enough to keep current flowing through the cathode resistor on the most negative signal excursion (signal and bias voltage).

Quote:
A MOSFET for the drive stage (source follower),
The circuit that I use is here:

http://www.tubelab.com/powerdrive.htm

This circuit was designed for an 845 output tube which needs nearly -300 volts to achieve cutoff with an 1100 volt supply.

I currently have the 6L6 tube set up with a switch so I can operate in triode or ultralinear mode. I tried pentode mode, but I didn't like the sound. I am operating the tube near maximum ratings, which you may not want to do if you use good tubes. I got some Chinese 6L6GC's off of Ebay for $3 USD each, so I don't mind running them at their limits.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 17th October 2006, 01:19 PM   #8
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
They just donīt overload "gracefully".
Compared with SS amps, they do. Ask any guitar player who's compared the two. When an SS amp overloads, it clips vey sharply. This is largely due to the heavy negative feedback used. The attendant distortion is very harsh and unmusical. That's why some people say tube watts sound louder than SS watts, because if overdriven an SS amp sounds dreadful while a tube amp behaves better and sounds a lot better - more "graceful", if you like. Tubes clip too, of course, but with a rather more rounded waveform. There is not the same harshness and so the effect of transients causing momentary overload is less unpleasant.

What can make a tube amp sound nasty is if overloading causes grid current in the OP stage but it is capacitively coupled to the driver. When grid current flows, the coupling cap is discharged and then takes some time to charge up again when the transient is past. This means there is a finite recovery time, when the coupling cannot do its proper job and during that time there is severe distortion, which is exacerbated if there is significant negative feedback. Having a direct-coupled driver eliminates this problem because there is no coupling cap to become discharged and then recharged. Overdrive conditions are handled by relatively gentle clipping and recovery is fast.
  Reply With Quote
Old 17th October 2006, 04:14 PM   #9
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
One thing to keep in mind about going into A2.....
You need to change the plate load to do this....
If you look at your 5K plate load right now and plot it at your B+, you will see where it Y-intersects the current axis, which is is not much above the 0-bias curve.... if you want to reach up into the positive curves, you will need to make the load line smaller....such as 3K ...ect...depending on where you want to reach to... Then re-bias to be well centered in the full AC swing....
A follower is the ticket for your situation as a driver.... A 6V6 makes a great follower for pushing into A2 in your application...
Keep 6V6 in tetrode mode for follower...


Chris
  Reply With Quote
Old 18th October 2006, 02:26 AM   #10
vax9000 is offline vax9000  United States
diyAudio Member
 
Join Date: May 2006
Location: Cleveland
How is a 6V6 follower compared with tubelab's power drive? I guess power drive is much better. The only draw back is the big heat sink.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
2A3 Max Grid Current Rating? rdf Tubes / Valves 0 7th July 2009 02:10 AM
KT88s and grid current SpreadSpectrum Tubes / Valves 3 21st March 2008 07:33 PM
Grid stopper resistor required (KT66)? porkchop61 Tubes / Valves 24 18th March 2008 09:18 PM
6AS7G with grid current? Wavebourn Tubes / Valves 1 16th November 2006 11:22 PM
Current limit of EL84 grid? G Tubes / Valves 10 22nd February 2003 07:08 PM


New To Site? Need Help?

All times are GMT. The time now is 11:47 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2