Traynor YBA-3 overhaul...Phase inverter differences???

[TubeGeek]

I am working on a Traynor YBA-3 bass amplifier right now and have a few questions.  I've been putting a lot of thought into this amp cause I love the way it sounds for bass guitar.  I have two Ampeg 8x10 cabinets in the shop for testing purposes.  I bought one of these cabs to keep around for my bass rig.

First off I am thinking about replacing the tubes.  Currently it is loaded with 6CA7's.  One pair of Philips and a pair of Tesla's.  They are well matched and test good on my tube testers. I am considering new reissue tung sol 6550's or the SED 6550's or even the tung-sol kt120's.  Not sure yet.

I have a feeling there is some theory behind why 6ca7/EL34's are being used with a plate voltage of 547 VDC.  I notice that pin 1 (suppressor grid) is tied to the negative bias voltage supply, giving -41VDC on pin 1 of all power tubes.  This is different from most amps I have worked on.  An EL34's max plate voltage rating, according to the data sheet, is 450 VDC.  This got me thinking...hmmm how is this possible that the tubes aren't dead after all these years of use at that plate voltage.  I read in the RCA manual that it has to do with the tubes being run in pentode mode and this allows for higher plate voltages to be used.  Interesting.  Can someone elaborate on this and hopefully the light bulb will come on inside my head  Does adding the g3 connection add stability inside the tubes by controlling the stray electrons and assuring they are attracted to the plate?  If this is a smart way to do it, then why haven't I seen it before in Fender, Marshall's etc?
This thought has been resolved.


I have corrected a few things in the amp so far:

- found one of the PH204 diodes orientation was backwards in the rectifier.  Corrected that.
- Replaced all filter caps, will place an order to replace the preamp cathode bypass caps asap.  I don't have the 250uF 16V in stock.
- Installed 1 ohm cathode to ground resistors on all power tube sockets to measure bias current.
- Replaced bias caps, they were bubbling.

Biasing:

Initial bias was pretty high at 52mA on each tube and 67mA on one of the tubes.  After poking around and trying out different bias resistor values I settled on replacing the stock 47k with a 56k and a 10k pot for adjustability.  The bias is now set to 30mA per tube.

Traynor recommends biasing the amp by measuring across the 1k 20 Watt screen resistor until there is 8 VDC across it.  If I follow this method I end up with around 15-20mA per tube.  This still sounds good to me but I feel better with 30mA so that's where I have left it for now.  I do notice some crackling in the power tubes if I have a higher bias setting.  I'm betting the tubes are about to die when I hear the crackling as I can watch the bias swing up around 140mA when I'm playing a bass with hard attack.  I do notice red plating at the higher bias current too.

The transformers are HUGE in this amp and I have no doubt they can handle any extra draw from bigger bottles.

If I do order a matched quad of 6550's or KT88 tubes, I will have to remove the voltage on pin 1 and just tie pins 1 and 8 to ground, correct?

I will have more questions about this amp I'm sure. I just want to start a dialogue and hopefully learn a bunch in the process.  In fact I have already learned some cool things by digging into this amp and it's inspiring me to keep digging.

[Willabe]

What are those 8x10 cabs wired for? 16 ohms or 32 ohms, I forget?


           
                    Brad     

[TubeGeek]

What are those 8x10 cabs wired for? 16 ohms or 32 ohms, I forget?

SVT cabs are wired for 4 ohms mono or 8 ohms if you wire them in a dual 4x10 configuration.

I am using one cab at a time at 4 ohms. I guess I could connect both 8x10's and have an 8 ohm load.

The traynor schematic indicates an OT secondary impedance of 8 ohms. It's not exactly clear to me but I figure I am not the first to use an svt cab with this amp.  Traynor's 8x10 cab is also 4 ohms.

[Willabe]

Now I think I remember, SVT cab speakers are 32 ohm?


              Brad     

[TubeGeek]

Now I think I remember, SVT cab speakers are 32 ohm?


              Brad     

Yes.

[phsyconoodler]

Dude.  EL34's are rated to 800v.The screen voltage is limited to 450v in most data sheets.American 6CA7's can run for decades at those voltages in your amp and not even breathe hard.
 You cannot have the cathodes tied to pin 1 if you are getting bias voltage at pin 1.The bias voltage needs to be at pin 5.Unless someone has set them up like a Fender with a grid stopper resistor from pin 1 to pin 5,in which case it is incorrect for the tubes you have.
This is a no-no with EL34's,as they need the cathode an pin one connected together.
  Look again or better yet,take a photo and share it with us.

[alerich]

+1 on the 800VDC plates. Mine run at 640VDC in my Sovtek MIG60 and don't even break a sweat. As long as you manage the screen voltage well (MIG60 has a separate 350VDC screen supply) you can hammer the plates.

However, according to the schematics I saw online TubeGeek is correct about Pin 1 being tied to the bias supply on each output tube.

Edited to add: Well, shoot - he posted a schematic and I didn't even see it. Doh!

[TubeGeek]

I stand corrected on the Va max voltage.  

Phsyco...I do have bias voltage on pin 1 and pin 5.  I was referring to changing this connection if I install different power tubes.

Is there a benefit to having the negative bias voiltage on pin 1?  

Maybe I am not quite understanding things here. I don't have my notes with me right now but I seem to recall reading that doing this was a good thing to do for efficiency and would provide long life from the tubes.

Clarification on whether I should leave pin 1 with the bias voltage on it or if I should have the cathode and pin1's tie together and then to ground would be great as I am a little confused.

[Willabe]

Is there a benefit to having the negative bias voiltage on pin 1?

KOC talked about that wiring in some Traynor amps.

I'll look it up and read it again, be right back.


               Brad     

[TubeGeek]

Here's an interesting read, found it on another forum...it has some good info on this:

Re: Voltage B+ and grid EL34John, as usual - "It all Depends..."  

First off, when biasing a tube there IS NO best setting! There is a suggested maximum of 70% of the plate disipation rating for the tube. Some guys get away with even higher idle current but they probably have a relative in a tube store somewhere who can supply them for free! Anything less is perfectly safe but will result in less power or "warmth".

Traynor spec'd the bias on these old amps as a function of screen current. That's what you're really measuring when you check the voltage drop across the screen resistor. You described it as the B+ series resistor but I'm assuming it's R23, which feeds the screen dropping resistors only.

They call for an 8 v drop. What they're really measuring John is the combined screen current at idle. Old Pete Traynor figured that when the combined plate idle current was about right then the total screen current would cause an 8 vdc drop across R23.

Now we've got a couple of assumptions in this design factor. First off, who says the output tubes will always perfectly track the same amount of screen current to a given amount of plate current? Second, just how much idling plate current does this 8 volt reading represent and what percentage of the Plate disipation figure will that be anyway?

So what's the answer? First off, it's not that critical. A few ma is not going to make any noticeable difference to the human ear. Now an idle current of less than 50% may be noticeably less "warm" than the accepted 70% but most guys would never notice, especially if they never were given the chance to test the amp at 40%, 50% or whatever.

So Pete would have just shot for 50% or maybe even lower so that no matter what differences the above factors made - who cares? The amp would still be more than loud enough, the tubes would not burn out prematurely and the user would still be happy with his tone.

Now, having said all that I've still not really given you any advice on actually WHAT to do!  My taste may not be your taste (I happen to love listening to George Thorogood on 11) but here's what I would do:

First off, I'd forget about measuring across R23. As I had said, tubes can vary in the amount of screen current from published specs. Especially today with modern tubes not as consistent as those made in the Golden Years. You want to put in some 1 ohm 1 watt cathode resistors on each 6CA7 to be able to measure the cathode current through each output tube. Cathode current is the sum of both plate and screen currents. For practical purposes I ignore the screen current and just treat the reading as plate current. 30 mv across 1 ohm means 30 ma. The 1-2 ma extra of screen current means only a percent or two in the "70%" or whatever calculation. I defy anyone to notice and it means you'll be setting the idle current a tiny bit lower than your reading, which is on the safe side anyway.

Notice that Pete did not tie pins 8 and 1 together like in most EL34/6CA7 circuits. Pin 8 is the cathode and that will be grounded (you'll have to lift that to put in your 1 ohm resistor). Pin 1 is the suppressor grid. The other guys just grounded this because it was easier and "everybody else does it!". Pete understood his tube theory better and tied the suppressor grid to the negative bias voltage supply!

This has some deep theory behind it but to you and me it means 2 things - the tubes work more efficiently and they last longer! That's why many of Pete's amps still run the original tubes!  

Lastly, you want to know what you should set as the idle current figure. That's some easy math!

Pd=Vp x Ip or Plate dissipation = Plate volts x Plate current.

Now an EL34/6CA7 has a suggested maximum figure of 25 watts Pd. If we divide by the plate voltage we get the book maximum plate current. Lets use your 451 vdc figure and we get 25/451 = 55 ma or so. In a push-pull AB1 amp the accepted max is 70% of this figure so 70% of 55 ma is 39 ma.


This means we can bias the tube(s) to a maximum of 39 ma, FOR EACH TUBE! This means that you will read 39 mv across each 1 ohm cathode resistor. Anything less will not hurt anything. Do the math yourself for lower percentage settings - if you play a lot of trash metal you may actually like lower settings better! If you're a rockin' bluesman you'll almost certainly use the 39 ma figure.

Of course your plate voltage will go up or down depending on how much idle current is being drawn by all 4 tubes. So you set to 39 ma and recheck your plate voltage. If it's dropped more than a trivial amount then recalculate the desired idle current for the new plate voltage! The tubes only care about the final voltage as to what's the preferred idle current.

[Willabe]

OK, TUT3 says Traynor and Garnet both sometimes put a (negative) bias voltage on pin 1 , suppressor grid, if the tube had a separate pin out for the suppressor.

He says Traynors "reading and discussions with actual audio engineers led him to take advantage of the separately pinned-out suppressor grid of this (EL34, 6CA7) tube. Tying the suppressor to other voltages other than cathode potential has special effects."

Then "A negtive voltage reduces the gain of the tube and makes it operate more linearly."

He also says most guys rewire the suppressor conection to get a warmer sound. He also likes to use a 1K 5w screen grid stopper on each tube. Traynors screen circuit choice came from (at the time) popular British hifi design.



                     Brad      

[TubeGeek]

Thank you for that info Brad.  Appreciate it.

I want to spend a little time listening to the amp with the negative bias voltage applied to the suppressor grid and then off and tied to the cathode and report back on what I could hear

[Willabe]

 

[TubeGeek]

I have been working on this amp here and there when I have had time.

Since my last post I have:

Replaced 95% of the carbon film resistors for metal film, no technical reason, just wanted to do it. I notice the noise floor dropped lower since doing this.

Removed the ground switch and rewired power on/off.

Replaced all coupling caps with orange drops.

Replaced all ceramic disc caps with silver mica's.

Replaced all PH204 diodes with 1N5408's.

Replaced 22k and 100k preamp stage plate resistors with 2W metal oxide's.

Along the way I had one of the Philips EL34's develop a funky noise, the clip was quickly recorded with an iphone before it exploded, it didn't really explode yet.  http://soundcloud.com/glacier/philips-el34-labelled-as

I thought it was in the circuit but it was a tube.  I now have a quad of sovtek test tubes in the amp until I receive an order of Tung-sol 6550's. I will experiment with differences between the el34's and 6550's taking into consideration of the suppressor grid wiring.

Rewired the screen grid circuit.  Each power tube now has a 1k5 5W resistor to pin 4. Disconnected the 1k 20W string of resistors in the amp before and removed the 47 ohm resistors.

Added two missing 1k5 resistors to two of the power tube sockets control grids (pin 5)

I have a few more parts to replace, will get to it when they arrive on my doorstep.

[TubeGeek]

The phase inverter circuit is throwing me for a loop.  There are differences between the traynor schematic and what is actually in my amp.

I have drawn them up on my whiteboard for studying.  Easy to compare this way.  Also attached a pic of my amp for reference.

The differences are in the 100k resistors in the schematic vs. the poteniometer in my amp that is wired to the grids of the phase inverter.

I'd like to know which wiring scheme is more correct and what is going on here.

fyi...my amp does not have the master volume added.  I have attached a cropped image of the traynor schemtaic for reference too.  Trying to give as much info as possible.

[TubeGeek]

Before and after gut shots thus far...

[Colas LeGrippa]

el84 and el34 are true pentodes , I always thought supressor grids and cathodes had to be connected together to gnd ( fixed bias )

[TubeGeek]

el84 and el34 are true pentodes , I always thought supressor grids and cathodes had to be connected together to gnd ( fixed bias )

I guess not all of the time.  That is what I am learning by picking this circuit apart.  

"Tying the suppressor to other voltages other than cathode potential has special effects"....this statement has me intrigued.

Something that has crossed my mind while deciding what new power tubes to install was if there are any brands that have the cathode and suppressor internally connected, thus wouldn't be recommended for use with this amp???

[Willabe]

It seems that USA made 6AC7 (EL34) had beam forming plates and were not true pentodes.

And unlike 6L6, 6V6, etc, EL34/6AC7 did not have their suppressor grid tied internally to their cathode.

6AC7/EL34 had the suppressor grid on it's own separate pin.


                    Brad    
 
  

[TubeGeek]

It seems that USA made 6AC7 (EL34) had beam forming plates and were not true pentodes.

And unlike 6L6, 6V6, etc, EL34/6AC7 did not have their suppressor grid tied internally to their cathode.

6AC7/EL34 had the suppressor grid on it's own separate pin.


                    Brad    
 
  

Do you know if that is true for all makes this day?

[Willabe]

Sorry I don't. I think it would depend on if they are copies of USA or Euro types.

Hard to tell nowadays because they slap any name they choose on a tube.

I guess you could use a meter and do a continuity test on the cathode and suppressor pins?

I believe 6550 has beam forming plates with it's suppressor tied internally to it's cathode.
 
(I'm moving right now and have all my tube manuals at the new house.)
                   

                      Brad      

[TubeGeek]

I tested all my el34's for continuity between the cathode and suppressor grid, there is no continuity on any of them.

I rewired the cathode tied to pin 1, suppressor grid and played the amp.

In comparison I can hear a little more top end edge.  It's almost a little grittier in tone.  Not brighter just has an edge that wasn't there before.  The amp was more hifi sounding with use of the suppressor grid.  I like both tone's at the moment.  I am going to leave it like this for now or until I can audition the new 6550's on their way to me.

In a nutshell I believe what I am hearing is less linearity in the tubes, which is precisely what K'OC described. Or maybe I just think that cause I read it. LOL

[TubeGeek]

It is my understanding that Traynor like Fender, used different PI circuits.  Hoffman's schematics show only one yba-3, while another website Prowess shows several variants of the yba-3.  

The PI circuit you have looks a lot like a long tail.  Those with more experience will correct me. (I do welcome corrections by the way)

I have printed out three schematics of the yba-3. The first one is for a version that uses 7027 output tubes.  The second schematic uses el34 with no master vol.  The third one also uses el34's and has a master volume.  

I have no master volume in mine so I am referencing the second schematic.

I agree is is definitely a LTPI circuit.

I can see what is going on all except for the one 100k resistor connected to pin 7.  It is not included in the LTPI circuit that I am referencing in Merlin's book and also Kuhenel's book.  I am reading up on things while I do this for better understanding/education.

What is that 100k's purpose?

I am trying to figure out if I should rewire this circuit.

I did confirm the pot in my PI circuit is 1M.

Slowly but surely I'll divide and conquer, appreciate the input along the way guys.

[HotBluePlates]

The phase inverter circuit is throwing me for a loop.  There are differences between the traynor schematic and what is actually in my amp.

I have drawn them up on my whiteboard for studying.  Easy to compare this way.  Also attached a pic of my amp for reference.

The differences are in the 100k resistors in the schematic vs. the poteniometer in my amp that is wired to the grids of the phase inverter.

Are you sure the 1M pot is not connected to V3b's grid?

If it was, you'd have 2 circuits that are effectively identical. Pretend the two 100k resistors that connect to the 470Ω resistor are in series and replaced by a 200k resistor. Now, make that 200k resistor a pot, and connect its wiper to the 470Ω resistor, where the 100k's used to go.

Those 100k resistors are the grid reference resistors for each half of the phase inverter; they define the connection to the bottom of the 470Ω resistor, setting the bias of the inverter. If you have a pot instead, you can sweep the resistance and move the "ground reference" away from the mid-point of resistance.

I'm gonna guess this has little impact on d.c. bias conditions; try sweeping the pot and seeing if plate voltage of either section changes.

If for some reason it does change (which would be a surprise), it might allow some balancing of d.c. between sections. Much more likely is that it will alter the a.c. drive levels to each half of the inverter, probably with the goal of getting exactly balanced a.c. plate outputs for maximum output power from the amp.

You'd test that by applying some smallish test signal, connecting an a.c. meter from plate to plate of the inverter (or from grid-to-grid on the output section) and adjusting for 0v. That would be the same as saying each output has equal and opposite output signals.

You might not be able to get exactly 0v if, say, there were distortion components in the phase inverter that were different between halves. But you'd probably need an awful big test signal to run into that issue.

Further support to a possible wrong location for the 1M end closest to V3b: if you turn the pot too far, you short out the 6.8k tail resistor, definitely killing phase inverter balance.

[TubeGeek]

Oops...V3b's grid is connected to the 1M pot.  I made a mistake in drawing that out. Thanks.

What was causing me to think more into it was that in Traynor's schematic they used 100k's.  My amp has a 1M pot.  Wouldn't the difference in resistance have an effect?

I plugged a bass in and played while adjusting the pot...it does indeed help balance to acheive greatest power output!  When I have the pot fully to either clockwise or counter-clockwise, the power of the signal is attenuated.  It's pretty much all power at any other location of the pot's rotation.

I don't see a swing of voltage on the plates as I rotate the pot.

Things seem in order so maybe I'll leave it alone and not worry about digging any further.

[HotBluePlates]

What was causing me to think more into it was that in Traynor's schematic they used 100k's.  My amp has a 1M pot.  Wouldn't the difference in resistance have an effect?

You'd think so, right? 

Since these are effectively grid reference resistors, what would happen if you made such a resistor in a different preamp stage too small? Normally, that would be a heavier load on the preceeding stage, dragging that earlier stage's gain down.

But the long-tail's grid reference resistors don't connect to ground; they're "bootstrapped". Because of where the "ground end" is connected, a small amount of in-phase output signal appears at the bottom of the grid reference. That signal is because the tail resistor is not bypassed. The interaction of the input signal and this in-phase out at each end of this resistor gives the apparent effect of a much larger resistance than is actually there.

So with the 100k's, the effective input impedance is not 100k, but probably couple-megohms. It will be higher, but on the same order-of-magnitude with 500k (half the 1M pot).

There may be an aditional good reason for 1M, but that eludes me. It would seem to be more important to have some kind of build-out resistors so you didn't have 0Ω from wiper to an end when you turn the pot all the way.

I don't see a swing of voltage on the plates as I rotate the pot.

Makes sense. Ordinarily, those grid reference resistors don't flow any d.c., because there shouldn't be grid current. A.C. balance made more sense, but the way you had it drawn made me wonder about d.c. balance. Now we know!

[TubeGeek]

Thanks for that description HBP.  It is helping me learn more.

I'm almost finished with this amp, a few more components on the way and then I'll be able to button this one up.