DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 17 Jan 2019, 01:54

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 35 posts ]  Go to page Previous  1, 2, 3, 4
Author Message
PostPosted: 08 Jan 2019, 21:28 
Offline

Joined: 11 Nov 2013, 22:35
Posts: 14
First build question.
How big is too big when it comes to B+ voltage?

Along with the recommended GZ34 rectifier tube I also bought a 5U4GB and a 5Y3GT. I have never had a chance to try different rectifier tubes before and I wanted to try for myself. I measured voltages with the different rectifier tubes.

At the wall, I have 124Vac. Referencing the schematic, B+power voltage measured after the 3rd/4th cap in the PSU. B+ driver voltage measured after the 33uf cap.
With the GZ34, for B+ power tube= 374Vdc, 110mA, for B+driver = 313Vdc, 1.55mA
and for comparison
With the 5Y3GT, for B+power tube = 356Vdc, 104mA, for B+driver = 298Vdc, 1.5mA
With the 5U4GB, for B+ power tube = 346Vdc, 102mA, for B+driver = 289Vdc, 1.5mA

If I use the GZ34, should I get the B+power down to 360Vdc? Spec sheet for the 3663A list 400Vdc Max on the plate voltage. I think I know the answer, but I would like confirmation. I have been using the 5U4GB for now.
I think I am okay with the measured B+driver voltages because of the plate load resisters, but Iet me know if that should be a concern.

Feedback would be appreciated.

thanks,


Top
 Profile  
 
PostPosted: 09 Jan 2019, 20:01 
Offline

Joined: 23 Feb 2017, 02:02
Posts: 270
You can have a voltage dropping resistor then short it by a chassis mounted switch this way you can accomodate for multiple types of rectifiers.

If you want over kill absolute reliability solid operation then I would suggest 200v.
http://www.valvewizard.co.uk/se.html
"If you are wondering why it appears that the signal voltage can now swing higher than the HT voltage, it is because this is exactly what happens! Inductances abhor changes in current. When current through the transformer increases it stores energy, which is released when the current falls again, allowing up to twice the HT voltage to be developed."


Top
 Profile  
 
PostPosted: 09 Jan 2019, 20:50 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1520
Location: US Pacific Northwest
ToddD wrote:
If I use the GZ34, should I get the B+power down to 360Vdc? Spec sheet for the 3663A list 400Vdc Max on the plate voltage. I think I know the answer, but I would like confirmation. I have been using the 5U4GB for now. I think I am okay with the measured B+driver voltages because of the plate load resisters, but let me know if that should be a concern.
Ok, lets break this down logically.

First, don't worry about the driver stage. All these voltages are just fine.

Now, the design point for the power stage is Vp=261v and Ip=90mA. This puts plate dissipation at about 23.5W (i.e. acceptable). Since this is a self biased amp, the total B+ circuit (transformer, tube, and bias resistor) need to be taken into account for B+ determination. The transformer has a 94Ω primary resistance and therefore a voltage drop of 94Ω*0.090A≈8.5v. The bias voltage is 0.090A*910Ω≈82v. So the required B+ voltage is 261v+8.5V+82v=351.5v. This is about what is on the load line design sheet (1v different). Whatever you do to the component values, this approach remains the same.

Now let's look at your GZ34 measurements. B+=374v and Ip=110mA. You used an 820Ω bias resistor so the bias voltage should be 0.110A*820Ω=90v. The transformer drop is about 94Ω*0.110A=10.3v. This makes the plate voltage 374v-10.3v-90v=237v and the plate dissipation 237v*0.110A=30.1W. This is too hot to run the 6336. In my build, I ended up with about 200Ω of series dropping resistance in the power supply to get the B+ down to where I wanted it with the GZ34.

So you have two choices, stick with the 5U4GB and be happy, or add some series resistance to get the B+ down and use the GZ-34. In the end, it's really about the plate dissipation of the power tube. The design point runs the power stage at about 80% of the rated plate dissipation. Under these conditions the 6336A tubes should last a good long time. At 30.1W you'll burn (literally) through power tubes much more often. Since these are generally pretty expensive tubes, I recommend lowering the plate dissipation (by lowering the B+) if you want to use the GZ-34 rectifier.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


Top
 Profile  
 
PostPosted: 10 Jan 2019, 19:33 
Offline

Joined: 11 Nov 2013, 22:35
Posts: 14
Suncalc,
Thanks for the response and more importantly thanks for the math.

With the 5U4GB I have a plate dissipation of 25.8W. I will leave that in for now and order some dropping resistors. 470 ohms should get ~ 24.5W plate dissipation with the GZ34.

Follow-up thought,
I also have some 1000 ohm x 10watt resistors. I could increase my cathode resistor and change my bias some which I think will lower my current draw. In this case would there be a preferred direction between higher voltage and lower amps vs. lower voltage and higher amps? Just curious.


Top
 Profile  
 
PostPosted: 10 Jan 2019, 20:33 
Offline
Project Author
User avatar

Joined: 06 Apr 2009, 10:08
Posts: 1520
Location: US Pacific Northwest
ToddD wrote:
In this case would there be a preferred direction between higher voltage and lower amps vs. lower voltage and higher amps?
In terms of plate dissipation it's really about energy balance. The plates are heated up literally by the bombardment of electrons. The plates cool by means of black body radiation (electromagnetic emission). By design, the plates are manufactured to be able to radiate a certain amount of power at equilibrium. This is set so that long term power flow into and out of the plate structure keeps the plate temperature below an acceptable limit. So at the temperature limit, the plates can radiate a maximum amount of energy per unit time (i.e radiated power flow). If the plate dissipation is below this number you are good. The temperature limit is set based on the allowable outgassing rate of certain coatings; but this outgassing is not linear with temperature. The maximum temperature is set to keep below the point where the outgassing grows to an unacceptable level. Contrary to popular opinion, there is no benefit to the plates or the tubes in limiting the plate dissipation to some low value. Once plate dissipation is below about 90% to 95% of max, the plates will outlast all of us.

So it's really about where you are placing the operating point and load line. At a set load impedance, there is benefit in keeping the current higher at a fixed plate voltage thereby reducing distortion. On the other hand, by reducing the current (again at a fixed load and plate voltage) the bias increases. This allows you to get larger voltage swings in the output transformer primary and hence more power (provided that you don't drive the tube into cutoff). So there is a trade off between distortion and max power.

Power stage design is complicated. :D

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 35 posts ]  Go to page Previous  1, 2, 3, 4

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: Google Adsense [Bot] and 12 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy