Which Tubes do you run in VRD's & why ?

[henry4841]

8 hours ago, tigerwoodKhorns said:

@henry4841

 

My understanding is that the Prima Luna will allow you to plug in even mismatched tubes and will adjust itself to correct.  I saw a video where Kevin deal mentioned this stating you don't even need to match the quads and you can run from the EL34 up to the KT150. 

 

I really appreciate the info that you provided.  I took some electronic courses in undergrad 30 years ago when I was getting my engineering degree and really want to be knowledgeable on this. 

 

To cut to the chase, I can run the KT150s but will not get the full power output from them. That is fine, but I imagine that there is a range on those graphs where the tube is designed to work and if you get outside of that area in any direction it is not ideal. 

 

Here is a great video where he goes through the simple calculations to get full power form the tubes.  About 14 minutes in he shows that if you run at lower current than as designed, the curves are steeper and it is easier to clip moving down the load line.  I will not pretend to completely understand the chart, but I have seen many similar charts where you move out of the optimal range and things do not work correctly, safety factors decrease, etc. 

 

 

 

 

 

Here is an initial question.  This chart is from the video.  If the VRDs run at 1.6A heater current, is that too low as it is below spec?  I just noticed that he does mention this at the end of the video.  The other items are all stated as 'max' voltage current and dissipation so I assume that these are OK so long as everything is run in a balanced fashion. 

 

I'll ping Craig on this one. 

 

 

 

 

As the chart says possible, depends on your PS transformer and how it is rated. I am working on a 300B amplifier right now and the PS transformer is rated for 6 amps. You add the current all the tubes need to operate from the data sheet and as long as you do not exceed the PS transformer rating you are good. Craig would know the rating of the transformer used to be able to answer you question. When changing tubes, different type as in this example, one should dig into the data sheets of the tubes and the PS transformer. The tranny I am using right now is the Hammond 274BX. Check the data sheet for this tranny and it gives all the ratings. Those big 150's run a lot of current as well and one must check to see how much current you have from your transformer to see if able to change. For instance this trans is rated for 750V's at 201ma. You then check the tubes you plan on running to be sure you do not exceed this rating or you will have a hot buzzing transformer if not a destroyed one. https://www.mouser.com/datasheet/2/177/200-1390014.pdf    I am running my KT-88s at around 120ma a pair so my Hammond 274BX is more than capable at 201ma. 

 

I was not sure if you had an electronic background of which you do so this video is the best I have seen explaining the function of all the graphs and figures in data sheets for tubes. Technical and kinda dry but very from one of the best. https://www.youtube.com/watch?v=1pU3jNquDUo&t=229s  Roger does an excellent job for those that are interested in such things. 

 

Sounds like you would make a great diy'er. Jump into the water. 

[mark1101]

I think I still have the same exact tubes as when I posted here in 2018.  Still have Amperex Bugle boys in all 4 small signal positions, the Penta Labs KT-88s (sometimes use the Genelax KT-88s), and a Sovtech rectifier.  I use the VRDs regularly and they are very pleasing.  This is my 3rd pair.  I kept buying and selling them, trying other amps, until I realized the VRDs are some of the very best amps I've tried for the Lascalas I have.

 

My NBS preamp is holding up as well.

[tigerwoodKhorns]

1 hour ago, mark1101 said:

I think I still have the same exact tubes as when I posted here in 2018.  Still have Amperex Bugle boys in all 4 small signal positions, the Penta Labs KT-88s (sometimes use the Genelax KT-88s), and a Sovtech rectifier.  I use the VRDs regularly and they are very pleasing.  This is my 3rd pair.  I kept buying and selling them, trying other amps, until I realized the VRDs are some of the very best amps I've tried for the Lascalas I have.

 

My NBS preamp is holding up as well.

Mark,

 

Do you have a pdf of the owner's manual?  I emailed Craig but no reply.  I really need to learn to bias these things. 

 

I think that I have a few bad tubes too so I don't want to turn them on.  One of the 12 AU7XA tubes looks done, all of the silver at the top is missing.  Do these need to be matched?  I found a good deal on an untested single on ebay that might be worth rolling the dice if they don't need to be matched. 

 

I always wonder why people sell them.  I now have seen several member say that they have had them two or three times.  I will store these if I stop using them as I will likely get the itch later on. 

 

I don't know if it is outdated, (Probably) but Craig's site says there are only 28 pairs made. 

 

 

 

[mark1101]

Sorry, But I do not have an owner's manual for VRD's (or NBs).

 

I know there are different configurations out there and so slightly different bias settings.  It is not difficult to set the bias.  All of the VRDs I have had used the same bias system with the testpoints and pots on top of the chassis.

 

As far as tubes go you should be looking for 12AX7 tubes with matched sections and matched tubes.  I am less familiar with the 12AU7 but I think they only need to be matched tubes and not matched sections.  I could be wrong on that point.  Of course you want a matched quad of the power tubes.

[henry4841]

19 hours ago, tigerwoodKhorns said:

 

 

I think that I have a few bad tubes too so I don't want to turn them on.  One of the 12 AU7XA tubes looks done, all of the silver at the top is missing.  

 

 

 

Would those tubes be RCA clear tops? If so the getter, silver, is on the sides not the top as most tubes. Check the sides of those tubes. Very rarely does a pre tube fail. Most of the tubes that get tired are the power tubes. 

 

Correct me if I am wrong but is not the VRD a takeoff of the Dynaco ST-70? Most of these PP EL-34 family of tube amps have similarities to the Dynaco. 

[Audible Nectar]

You DO want the 12AU7s to have matched sections (phase splitter). But Mark identifies the same tube sets I use (save for the fact I use real Mulalrd 5AR4s cuz I have them) and I find them to be excellent.

 

This is what the amp is "designed center" for, it has other options but I don't recall how to set it up. Haven't relly needed to because the current setup with Amperex smalls and Penta KT88s have been sooooo good in that amp, and even as a cap roller (I now have all Teflons in my NOSValves gear) can vouch for the quality of that setup.

 

To get into the specifics of the other tubes you could use, and in what specific combinations, I would talk to Craig, as I don't think any of those options were laid out in the manual. If you ask me, an amp performs and sounds best with the tubes it was designed for, and having heard VRDs with GEC KT66 and a few others, the stuff that works "stock" actually performed better, because that's what the amp is designed for.

[henry4841]

A correctly designed amplifier is biased for a specific tube. This is not to say other tubes will work but not to top performance unless re-biased. Most of the adjustments of bias from factory is a narrow range for the specific tube that came with the amp. My experience is for an EL-34 amp a 6L6, KT-77, KT-88 and others of the EL-34 family will work but one has to be careful of glowing plates. Changing to a big KT-150 is another matter. Will not harm the amp or tubes but the 150 will be just run lets say half speed for an example and not to full potential. Could be wrong. There could be enough bias range built into the amp but unlikely. Doubtful the PS transformer is rated high enough for the added current those big tubes require if re-biased correctly.

 

Craig needs to respond for correct answers.

[Marvel]

2 hours ago, henry4841 said:

Correct me if I am wrong but is not the VRD a takeoff of the Dynaco ST-70? Most of these PP EL-34 family of tube amps have similarities to the Dynaco.

Yes, but he might want to argue that point. Mark Deneen also helped him in the beginning, another touchy point.

[Audible Nectar]

The correct answer (I think, he may differ) is that the big Dynacos (like the Dynaco III and similar tube rectified simple circuit amplifiers) were the "inspiration" for the idea of building the VRD, but it's not really "in common" with the Dynacos aside from being ultralinear (front ends are definitely different). At the time the VRDs were released there weren't really much for good NEW options for high efficiency horn people like us on tube amps - Craig himself built an entire biz on rebuilding the vintage gears MADE for horns because those pieces rebuilt really WERE the best options back then (and are still often very good to near best). So the VRDs were in effect a custom made solution for high efficiency horn people, especially 2 channel Heritage types, who wanted a "reference amp" built to do that job. See also the JuicyMusic gears (Mark) which had similar aims with differing approaches (and who effectively built "The Preamp" for many on this forum). JM>VRDs got to be like a broken record here on gear lists and for good reason. The rest, as they say, is history.

[tigerwoodKhorns]

11 hours ago, henry4841 said:

Would those tubes be RCA clear tops? If so the getter, silver, is on the sides not the top as most tubes. Check the sides of those tubes. Very rarely does a pre tube fail. Most of the tubes that get tired are the power tubes. 

 

Correct me if I am wrong but is not the VRD a takeoff of the Dynaco ST-70? Most of these PP EL-34 family of tube amps have similarities to the Dynaco. 

 

Nah, the 12AU7A is shot.  See the picture.  Looks like it got a leak in the vacuum.  I bought a nice looking but untested on off eBay.  Craig said that it doesn't need to be a close match in this position. 

 

 

 

10 hours ago, henry4841 said:

A correctly designed amplifier is biased for a specific tube. This is not to say other tubes will work but not to top performance unless re-biased. Most of the adjustments of bias from factory is a narrow range for the specific tube that came with the amp. My experience is for an EL-34 amp a 6L6, KT-77, KT-88 and others of the EL-34 family will work but one has to be careful of glowing plates. Changing to a big KT-150 is another matter. Will not harm the amp or tubes but the 150 will be just run lets say half speed for an example and not to full potential. Could be wrong. There could be enough bias range built into the amp but unlikely. Doubtful the PS transformer is rated high enough for the added current those big tubes require if re-biased correctly.

 

Craig needs to respond for correct answers.

 

Craig was very kind to respond and send a manual.  He is mostly retired and I appreciate his hospitality. 

 

Craig said that you can run the KT150 and bias it but it will pull the transformer to its ragged edge with the heater.  Translation, don't run the KT150s in VRDs.  Craig said the KT88, KT90 and KT120s are OK.

 

Looking at the cart below this makes sense. 

 

So I have a beautiful, just tested, matched and Cryoed quad of Upscale Audio Tung Sols for sale.  The boxes look new as well as the tubes.  I'll post them in the garage sale section some time soon, but I am interested in a swap for some similar condition KT88s if anyone is looking for KT150s.

 

My mistake will make someone very happy. 

 

 

 

 

[tigerwoodKhorns]

Oh man the learning curve is a little steep here and I am not feeling well today. 

 

Can I get some info on the rectifiers? I have a pair of JJ 5AR4s.  This is what Craig lists on his website. 

 

I found the attached info on the rectifiers. 

 

I also have a pair of CV593s which appear to be Mullards.  Craig recommends Mullars in his manual but I'm not sure if these will work as he lists the 5AR4s. 

 

The 5AR4 (GZ34) are a high current rectifier and listed as OEM for the Dynaco ST-70s

https://mullard.org/collections/audio-valves-gz34-5ar4-cv1377-rectifier

 

The CV593s are a medium power rectifier. 

https://mullard.org/collections/audio-valves-gz32-5aq4-cv593-52ku-5v4g-5z4g-gz30-rectifier

 

Can I use the CV593s or sell them and look for some 5AR4s? 

 

I really want to get this right and it would be a real shame to ruin these Mullards or worse yet, the amps. 

 

@henry4841@Audible Nectar

 

[henry4841]

Did some checking on data sheets and it appears to be a no go. Not a lot of info on the CV593 but I found it to be a GZ32. I have not looked that carefully at the charts but this is what I found glancing at the numbers. 

 

http://www.r-type.org/exhib/aaa1104.htm

http://www.r-type.org/pdfs/gz32.pdf

https://frank.pocnet.net/sheets/093/5/5AR4.pdf

 

Compare the numbers yourself. Data sheets tell the story in electronics. The VRD is pushing any rectifier pretty hard so you need a really good one. You should have no trouble getting rid of those tubes you cannot use. Tube prices have really increased of late. 

[tigerwoodKhorns]

7 hours ago, henry4841 said:

Did some checking on data sheets and it appears to be a no go. Not a lot of info on the CV593 but I found it to be a GZ32. I have not looked that carefully at the charts but this is what I found glancing at the numbers. 

 

http://www.r-type.org/exhib/aaa1104.htm

http://www.r-type.org/pdfs/gz32.pdf

https://frank.pocnet.net/sheets/093/5/5AR4.pdf

 

Compare the numbers yourself. Data sheets tell the story in electronics. The VRD is pushing any rectifier pretty hard so you need a really good one. You should have no trouble getting rid of those tubes you cannot use. Tube prices have really increased of late. 

Got it, thanks for the info.  I had a feeling when the sheet said the 5AR4 is a high power and the CV593 is medium it was a no go.  In the manual Craig also recommends a spare set of Sovteks or the Mullards if they are not too expensive. 

[Sprinter1104]

On 2/10/2023 at 10:03 AM, tigerwoodKhorns said:

Mark,

 

Do you have a pdf of the owner's manual?  I emailed Craig but no reply.  I really need to learn to bias these things. 

 

I think that I have a few bad tubes too so I don't want to turn them on.  One of the 12 AU7XA tubes looks done, all of the silver at the top is missing.  Do these need to be matched?  I found a good deal on an untested single on ebay that might be worth rolling the dice if they don't need to be matched. 

 

I always wonder why people sell them.  I now have seen several member say that they have had them two or three times.  I will store these if I stop using them as I will likely get the itch later on. 

 

I don't know if it is outdated, (Probably) but Craig's site says there are only 28 pairs made. 

 

 

 

I have the owners manual if you still need it. I can scan it in tomorrow if you do.

Rick

[tigerwoodKhorns]

3 minutes ago, Sprinter1104 said:

I have the owners manual if you still need it. I can scan it in tomorrow if you do.

Rick

 

I have an early pair and Craig sent me the manual.  If you have a different manual please send it.  I like to keep this stuff in my server and when people ask post it if I have time.  My way of giving back. 

 

I also got some good info from Craig if you are ever looking for this stuff: 

• The transformer can provide up to 4 amps to the rectifier so keep this in mind when searching for a replacement if you look at other than the 5AR4.  I got lucky with some vintage Mullards.
•  
• VRDs can handle the KT88, KT90 and KT120, but the KT150s will pull too much current on the heater (or at least put the transformers at their edge).
•  
• The 12AX7 tubes do not need to be a great / exact match to each other in this position.  This really helped me match a vintage Amperex with a cheap eBay tube. 

 

I hope this helps someone. 

 

 

[tigerwoodKhorns]

@henry4841

 

Quick question.  When installing tubes for the first time, should I turn the bias all the way down, or maybe most of the way down) when I first install them and then adjust? 

[the real Duke Spinner]

22 minutes ago, tigerwoodKhorns said:

@henry4841

 

Quick question.  When installing tubes for the first time, should I turn the bias all the way down, or maybe most of the way down) when I first install them and then adjust? 

Plug and Play

It can not be that far off

[henry4841]

2 hours ago, tigerwoodKhorns said:

@henry4841

 

Quick question.  When installing tubes for the first time, should I turn the bias all the way down, or maybe most of the way down) when I first install them and then adjust? 

No.