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VRD Tube Biasing

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Is it normal to need to need to turn the bias screw A LOT when changing rectifier and output tubes? My KT88s were right at .55. After swapping things out my EL34 were reading .02 to .08. I probably had to turn the pots clockwise (increasing) 10 or 12 revolutions to get them to .40. Figuring I'll start out on the low side. Initial impression is that my HF sections of the 2-way set up are way overpowering the bass bins which makes me think I've actually dialed them up a lot. I can knock the HF back or increase the LF to balance them out but since I turned the bias pots up so much It makes me wonder if I'm doing something wrong. In case it is not obvious I have separate amps for HF and LF. The LF system as a whole did not changes in any way.

 

59 minutes ago, Schu said:

Manual says in the .55mV range.

I've never run mine in the .75mV range... I'm perfectly happy where she's at right now and don't really miss any of the higher output power tubes because most of my listening is done in the 85db range.

 

So increasing the value say .55 to .65 actually increases the output in a KT88 set up? I agree I don't care about output as I typically listen on the quiet side. I want them to be "right" whatever right is.

 

It seems odd to me that I would have gone from a higher output tube (KT88) to a lower output tube (EL34) and the bias went from .55 to .40 and the amp made my HF section a lot louder. I had to make a 7dB change to get the HF/LF somewhat balanced back out. I'll do some actual measurements next. Does this pass the sniff test, am I in the ball park of doing things correctly? Am I overthinking this and I should just turn the pots as necessary to get the readings I want and move on?

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You sound like you are in the right ball park. Craig used pots with tiny incremental adjustments (I've forgotten the actual ratio), so it can take a few complete turns to get to the right setting when you change out power tubes and the rectifier.  As long as it's in that 40mV range, you are good to go. Give those new tubes time to burn in and let me know if it suits your listening. 

 

I really am dying to try a set of 350b's, but that is an expensive proposition. 

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Rich,

 

 Not ignoring you I did reply but promptly received a kick back notice of a permanent fatal delivery error. I believe your mail server has my mail server blocked. Here is what I* replied. I also tried to send you a current manual which should/could be the same as the one you have. 

 

 

Rich,

 

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX first line X out because it was personal in nature. 

 

        Glad you wrote me about bias confusion. Always refer to the manual that came with your amps. The folks mentioning .70 bias setting have some of the amps manufactured in the early to mid-2000’s and have not had them updated. I believe your manual should give you bias setting for EL34’s. If not attached you find one that does.

 

I will caution you that if using modern production EL34’s error to the cool side first than check the large plate structure inside the tube for red glow. Many modern production EL34’s are pretty weak. Also realize I have never had someone report back they preferred EL34’s in a VRD which makes total sense since it runs them too darn hard. You could help things out a little by running them with a 5U4 rectifier tubes which will lower the voltages in the amp a little but in the end the amps are still primarily designed to run the KT88 family of tubes.

 

Thanks Craig Ostby

 

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20 hours ago, rplace said:

Is it normal to need to need to turn the bias screw A LOT when changing rectifier and output tubes? My KT88s were right at .55. After swapping things out my EL34 were reading .02 to .08. I probably had to turn the pots clockwise (increasing) 10 or 12 revolutions to get them to .40. Figuring I'll start out on the low side. Initial impression is that my HF sections of the 2-way set up are way overpowering the bass bins which makes me think I've actually dialed them up a lot. I can knock the HF back or increase the LF to balance them out but since I turned the bias pots up so much It makes me wonder if I'm doing something wrong. In case it is not obvious I have separate amps for HF and LF. The LF system as a whole did not changes in any way.

 

 

So increasing the value say .55 to .65 actually increases the output in a KT88 set up? I agree I don't care about output as I typically listen on the quiet side. I want them to be "right" whatever right is.

 

It seems odd to me that I would have gone from a higher output tube (KT88) to a lower output tube (EL34) and the bias went from .55 to .40 and the amp made my HF section a lot louder. I had to make a 7dB change to get the HF/LF somewhat balanced back out. I'll do some actual measurements next. Does this pass the sniff test, am I in the ball park of doing things correctly? Am I overthinking this and I should just turn the pots as necessary to get the readings I want and move on?

 

 

    Rich,

 

           What you describe going from KT88 to EL34 is totally normal. Now when/if switch back realize the reverse is going to be true and you should adjust the controls about 8 full turns counter clockwise before you fire the amps back up with  KT88 type tube again. Each output tube type can take a hugely different applied negative voltage applied to the tube to achieve the proper bias current. Realize when you adjust the bias screw your not adjusting the actual voltage you read at the test point you are adjusting a voltage applied to the control grid of the tube that causes the test point voltage to change. 

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51 minutes ago, NOSValves said:

 

 

    Rich,

 

           What you describe going from KT88 to EL34 is totally normal. Now when/if switch back realize the reverse is going to be true and you should adjust the controls about 8 full turns counter clockwise before you fire the amps back up with  KT88 type tube again. Each output tube type can take a hugely different applied negative voltage applied to the tube to achieve the proper bias current. Realize when you adjust the bias screw your not adjusting the actual voltage you read at the test point you are adjusting a voltage applied to the control grid of the tube that causes the test point voltage to change. 

 

I was thinking that exact same things that when I put the KT88s back in I better turn them the other way BEFORE doing anything. Thanks!

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On 12/9/2019 at 2:20 PM, rplace said:

 

 

 

Brining this old thread back to life as it has lots of info for my question. I found this while searching for an answer I can't yet find. I do have an email out to Craig at @NOSValves but figured with so many VRD owners here you all might be able to help.

 

I've got some EL34s that I want to try just because I have them on hand and never run anything but KT88s in my VRDs. I've acquired some vintage GE 5U4GB rectifiers so I'm good there. I have my original manual from when I bought the VRDs new. In it with respect to biasing it says bias them at .50-.65 and Craig said he ran his at .55. I've always kept mine at .55 since I bought them. This thread seems to suggest .70-.80 is the range.

 

  1. So what is the proper bias for KT88s and if EL34s need to be run lower what is a good number for them?
  2. Was there a few different version of the VRD thus my manual says .50-.65 the one quoted above says .70-.80?
  3. If there are two different types of VRDs and mine are the .50-.65 variety AND EL34s need to be run lower what would be a good number? 
  4. I've got some Russian military 6P3S-E (GL6GC equivalent ) inbound, what should I run them at based on the above answers?

 

This thread has my curiosity as well. I'm running Quicksilver Silver Mono's and my PAIR of KT88's COMBINED bias is around .80 VDC. I think I'm at .78. I was told a KT88 alone should be around .40 so .80 made sense and this is what Quicksilver told me as well. Just curious if the above advice was for a PAIR of tubes. 

 

Also - Golden Lions were red plating on me at this bias so I switched to JJ's on Quicksilver's recommendation and no issues since. They DO NOT recommend Golden Lions. 

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On 12/10/2019 at 5:11 PM, NOSValves said:

Rich,

 

 Not ignoring you I did reply but promptly received a kick back notice of a permanent fatal delivery error. I believe your mail server has my mail server blocked. Here is what I* replied. I also tried to send you a current manual which should/could be the same as the one you have. 

 

 

Rich,

 

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX first line X out because it was personal in nature. 

 

        Glad you wrote me about bias confusion. Always refer to the manual that came with your amps. The folks mentioning .70 bias setting have some of the amps manufactured in the early to mid-2000’s and have not had them updated. I believe your manual should give you bias setting for EL34’s. If not attached you find one that does.

 

I will caution you that if using modern production EL34’s error to the cool side first than check the large plate structure inside the tube for red glow. Many modern production EL34’s are pretty weak. Also realize I have never had someone report back they preferred EL34’s in a VRD which makes total sense since it runs them too darn hard. You could help things out a little by running them with a 5U4 rectifier tubes which will lower the voltages in the amp a little but in the end the amps are still primarily designed to run the KT88 family of tubes.

 

Thanks Craig Ostby

 

 

 

As usual thanks for everything, Craig. I'll drop you a line from a gmail address and see if we have better luck with emails. Strange yours does not get through. I've not hear of problems from others in 3+ years.

 

Turns out I have a bad memory. They were not EL-34s there were new production Gold Lion KT77s. From memory (always a potential problem) the amp I had that did not last long in my system was an EL34 based amp that could also run the KT-88 family (flipping a switch). I found the stock EL34s noisy as I swapped out all the signal tubes with know good ones I had. So in a last ditch effort I bought the Gold Lion KT77s because Upscale Audio said the KT77 was a drop in replacement for the EL34. I ditched the amp shortly there after and have had the tubes sitting around unused for a while.

 

If you have something handy can you post which power tubes should use what rectifier tube? I've never been quite clear on the ins and out of all that. I would guess anything that starts with "KT" would use the 5AR4 but since I was told the Gold Lion KT77 was a drop in replacement for the EL-34 I don't know what to think. This is the first time ever I've put anything beyond the stock Valve Art KT-88s in my VRDs. I've had a few different 12AU7/12AX7 through them but no changes to the power tubes.

 

Now that I look at your VRD site I don't even see the KT77 listed:

Quote

The amps can run EL34, 6L6GC, 5881, 350B, KT66, KT88, 6550, KT90 and more (some tube choices require a 5U4GB rectifier tube)

 

Also, I did buy some vintage 5U4 rectifiers. Looking at my particular VRD manual there is no mention of bias for specific tubes. I'll attach a picture of my bias info.

 

It would be helpful to the novice like me if any of you have a chart listing Power Tube, proper rectifier and bias. Maybe I'm just over thinking it. Several posts up it sounds like  you can't hurt the amps only the tubes. Since mine look like day one, brand new, I don't want to start down a road that is going to create any problems.

 

rps20191212_155538.thumb.jpg.39c07aff054f48d216744dacd242fce8.jpg

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28 minutes ago, Schu said:

KT77 is a rare EL34 variant

 

I gather that. Any input on which rectifier tube or bias?

 

Thanks!

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I guess I meant to say, same as... it's drop in. Craig asked me to keep my el34 low... and it's been fine and balanced sounding at .40mv... the reason I use them currently over the 88's are that the presentation is MORE balanced across the entire range in my set up... 88s seem to push the mids WAY forward and over power the bass bin and tweeter imho. My sound stage is extremely dimensional and holographic and the speakers just disappear... of course much of that is other components and room treatment. 

 

Have you tried using the other output taps (4ohm, 8ohm, 16ohm)?

 

This is a quote from Brent jesse:

This tube is capable of MAXIMUM plate voltage of 800 volts and peak power at 25 watts! Of course, these are absolute maximum readings, at which stage nuclear meltdown occurs! In typical class A operation, I have seen suggested specs of 250 volts on the plate and screen, giving a conservative power output of 11 watts. The vintage versions of this tube have no problem running at 440 volts on plate and screen at about -38 to -42 bias volts, and this is what is often found in vintage amps. 

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2 hours ago, Schu said:

Have you tried using the other output taps (4ohm, 8ohm, 16ohm)?

 

Long ago when I first got them. I don't recall hearing any difference, but I also was a bit worried about not using 8ohm because my Khorns said 8ohms. I've since come to realize that does not really pose a problem. I should probably revisit that. Thanks for the reminder.

 

2 hours ago, Schu said:

MAXIMUM plate voltage of 800

2 hours ago, Schu said:

I have seen suggested specs of 250 volts on the plate and screen

2 hours ago, Schu said:

440 volts on plate and screen at about -38 to -42 bias volts

 

This is my one beef with tube equipment and tube guys in general, not most people here. I get the feeling they delight in knowing something others do not and very few seem to want to explain things for the novice in a basic way.

 

I bought a multi meter for the sole purpose of biasing my VRDs. I have no idea what the rest of the settings are for. Electricity is a scary thing when you are ignorant to most of it. I google a lot of things but I have zero background in electronics. I seem to have missed that semester in middle and high school taking wood shop, auto body repair, photography, graphic arts and engineering drawing classes along with Algebra and pre-calc. Ohms, Watts, Capacitors, Resistors, impedance I'm pretty much in the dark. I am however pretty good at following directions. The last thing I want to do is hurt my equipment, so I am more likely to do nothing over doing something I'm not sure of. Probably why the KT88s have been in there for 10+ years.
 

It must be my lack of understanding how all these things relate, but I would expect it to be as simple as:

If you are running an amp that is designed to use Power Tube X then you need Rectifier Y and set the bias between A and B. It must not be, because I've look and looked online and see things like this:

 

Quote

Biasing your amp doesn’t have to be difficult. As long as you know (or can test) the plate voltage in your amp, and have a way to measure the idle current, you can determine where to bias it, using our calculator. Select your tube type and amp class (Class A, Class AB Low, Class AB, or Class AB High) and enter your Plate Voltage or B+. The calculator will give you the bias current for each tube. Use this as a guide for setting your bias

 

Where do I come up with plate voltage and idle current, what is B+? What class is my amp, I'd guess A but what if I'm wrong? If I need to measure plate voltage or idle current is that done with the tubes already installed? If so will that change based on the rectifier I use because I don't know what rectifier goes with what tubes. See my frustration? It must be the case that I'm making this way more complicated than necessary. I have no problem putting a 100w light bulb in an socket marked 60w max. I figure the light bulb will burn out or the circuit breaker will trip, most likely nothing will go wrong. I don't have that level of ease with tube gear....maybe it is as simple as of the tube fits in the socket give it a try and see how it sounds....but I kind of doubt it.

 

 

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Rplace, if you want to learn some basics about power supplies and amps buy this:

 

https://www.ebay.com/itm/Practical-Radio-Servicing-2nd-Ed-Marcus-Levy-CDROM-Radio-Repair-KE3GK/111101526144?hash=item19de2ac080:g:~yoAAOxy--NRwZt0

 

It explains a great deal in relatively easy to understand language, and has served well as a teaching tool for tube electronics for decades.  Unfortunately, the book itself is ridiculously priced.

 

 

Maynard

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I hate getting shocked and seeing arches of plasma dancing around my face.

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1 hour ago, Schu said:

I hate getting shocked and seeing arches of plasma dancing around my face.


} Hell, isn’t that the allure of tubes? Otherwise, better stick it out with SS — 😏

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