Traynor YBA- Custom Sp.- convert 6ca7 (EL34) to 6550

[pbman1953]

If I'm right it's the  2 brown wires feeding the diodes


If yes-




On the left one 425




On the right one-   462


In standby on AC volts setting

[Willabe]

Yes the PT B+ secondary wires going to the B+ diodes.

In stand by they both should be 0 acv's.

The stand by switch in that amp, looking at the schemo, is wired up to the B+ center tap (CT). It disconnects the CT from ground so there should be no acv voltage when the CT is disconnected from ground.

(Unless for some reason lifting the CT allows the B+ to float up where it want's but will allow no current?) 

[Willabe]

Measure for DCV's in the red circle (after the B+ diodes, junction of B+ diodes and the 1st B+ filter cap) in stand by mode. It should be 0 dcv's.

[pbman1953]

Let me know if this is the spot. If so  71 volts DC scale

[Willabe]

Yes I think that's it.

See if you can find C26. That cap's positive end goes to where the B+ diodes come together.

[Willabe]

You enlarged the pic in reply 53 too much.

[sluckey]

OK, I had overlooked the fact that the STBY switch breaks the CT of the HT winding. That explains the high negative bias voltage when in standby mode. The voltages you have are normal.

[Willabe]

OK, I had overlooked the fact that the STBY switch breaks the CT of the HT winding. That explains the high negative bias voltage when in standby mode. The voltages you have are normal.

Ok,

Then if you still can't get the current draw down in the power tubes you need to change a resistor or 2 as explained below;

Leave the power tubes out until you are sure you have -dcv on all the power tube grid pins and have enough -dcv.

Take the power tubes out and measure the -dcv at each grid on the power tube sockets with the -bias set full up.

Post what you get.

If you have -dcv then;

Here's a hi-lighted drawing of you amps -bias. Blue circle shows 10K adjust pot with a 47K R below it. (You might have to make that R lager, maybe 68K or a 100K.)

The red circle shows a series R of 15K, you might need to make that 10K or a little less.

You might need to play with both a little.

R37 and R36 are the resistors to change.

I'd also change the 2 bias filter caps if there original. In fact I'd change ALL the electrolytic caps if there original. 

[Willabe]

Here's 2 links to our host Doug's library of tube amp information on biasing an amp;

http://el34world.com/charts/fenderservice5.htm

http://el34world.com/charts/Biascircuits.htm

[sluckey]

I don't think there is anything wrong with the bias supply. There is -74v at the output of the diode. That is applied to a voltage divider (15K, 0-10K pot, and 47K). The pot will swing the bias voltage from about -56v to -58.5v. Not much swing but it should make the 6CA7s run cool.

I suspect the tubes that were in the amp while the bias voltage was shorted to ground are probably now damaged.

You say your idle current is about 70mA. Is that for one tube? Two tubes? All four tubes? What is the procedure you used to measure idle current?

[pbman1953]

 

You say your idle current is about 70mA. Is that for one tube? Two tubes? All four tubes? What is the procedure you used to measure idle current?





Actual is about 68ma,


2 tubes, I'm in the hunt for a pair unless both of these are damaged


I measured using my Bias Rite

[sluckey]

Put your EL34s back in it. Do they work properly?

[pbman1953]

seem to

[pbman1953]

I decided to bias the El's and in order to get close to match i need the bias control is swung the complete opposite direction as the 6550's. full turn clockwise




But all tube fall in a good range between 60%- 70%


Does that mean anything? Because of the full turn

[Willabe]

Does that mean anything? Because of the full turn

It means, like Sluckey said, there's not much 'swing', ie, change of adjustment with the bias pot, you only have about 2.5dcv's to play with. And that to get the EL34's and 6550's to bias up at about 60% to 70% of max dissipation you have to turn the bias full up or full down.

And that's just with those 2 pairs of tubes. Different EL34's and 6550's might draw more current or they might draw less current even if their the same brand as you have now.

IMO, it would be best to get some more swing in the bias pot. It would be nice to get at least 20dcv to 25dcv more swing centered around what you have now so your covered with different tubes in the future.

R37 and/or R36 (see reply #57) need to be changed to a different value to do this.

Sluckey (and others who can do the voltage divider math) will know what value(s) to use.     

[Willabe]

I think you have to lower the value of R37, maybe by ~half to get more swing, BUT that will bleed some voltage off to ground so then to make up for that you have to lower the value of R36 to, maybe 10K?

Wait for someone else to reply to be sure.

                        

[pbman1953]

Looks like I need more on the upper to use the small draw tubes and less at the other way for high draw tubes




Thanks for all the help!!!

[Willabe]

Looks like I need more on the upper to use the small draw tubes and less at the other way for high draw tubes

Yes, I think you got it now. That's the 'swing'.         (Tubes that draw LESS current need less -bias dcv's to tame/cool them off, tubes that draw MORE need more -dcv to cool them down.)   

(After you get the -bias sorted out, I'd still change ALL those e-caps if their original.)

[sluckey]

Replace the 10K pot with a 50K pot.

[Willabe]

Replace the 10K pot with a 50K pot.

That's cheatin', that's the easy way.   

Can't you also do it by changing R36/R37?

(Fender BF AB763 amp's used a 10K bias pot and they had enough swing.)

[sluckey]

Can't you also do it by changing R36/R37?

(Fender BF AB763 amp's used a 10K bias pot and they had enough swing.)

The value of the pot needs to be equal to or greater than that 47K resistor in order to get a wide swing. You could try making the other resistors 10K and see what swing that gets you. My preference would be a 50K pot.

Fender didn't need a huge swing because he never intended to use different tubes that require such a different bias voltage.

[Willabe]

The value of the pot needs to be equal to or greater than that 47K resistor in order to get a wide swing.

OK, thanks.   

[jazbo8]

I don't think there is anything wrong with the bias supply. There is -74v at the output of the diode.

May be I missed it, where did the -74V figure came from? The schematic above says -66V. Assuming a plate voltage of 540V and a screen voltage of 515V, then the bias needs to be about -66V for 70% dissipation, so I hope the -74V figure is correct. Also, were the above screen voltages measured with the 1k/5W resistors already installed?

[Willabe]

Look at reply #46.   

[jazbo8]

Look at reply #46.   

Got it, so it's good and bad - if the bias voltage got higher, it would also mean that rest of the voltage did too, so what is the plate voltage now, and again, were the screen voltages measured with the larger screen resistors installed and hopefully close to the proper bias applied?

[pbman1953]

all measurements were done after the resistor install

[pbman1953]

I decided to take my tubes out of my Sunn 2000.  They seem to be fine. I should of mentioned before that I have a heat beat type buzz that I had with both sets. It goes up and down with volume.

Picture the amp, service side up on my desk. If I come close to the amp , like inches away the noise quiet down some. If I waver my hand mainly around the V3 preamp tube it all but goes away. I have a florescent above me and I turned it off but had no effect. Bias has no effect. If I remove V3 it goes away. I changed out V3 and there was no change

[pbman1953]

Heart beat not Heat beat

[pbman1953]

I'm here at the parts store all they have is a 20k or 100k.

Can I get by with a 20k?

[pbman1953]

Ended up using the 100k, I know there's a lot of turns but much better adjustments and dialed right in

[TubeGeek]

I have done this mod to my personal yba-3 custom back in 2012.  It has been rock solid since.  It can be done.

[sluckey]

Ended up using the 100k, I know there's a lot of turns but much better adjustments and dialed right in

What range of voltage do you have at each end of the pot? 100K may be a little touchier to adjust than a 50K pot but otherwise should be OK.

What do you mean by "I know there's a lot of turns"? Is this a multi-turn pot?

[pbman1953]

Not sure about the range but I could really dial both tubes


Turn wise it wasn't so bad, it's listed as a 28 turn pot


Any ideas on that crazy heart beat buzz I wrote about?


Plus my amp socket was fine. The amp didn't like one of the Bias Rite Meter sockets. I have to get with Weber anyway due to that crazy 1000 volt reading. The Bias Rite has been a great performer, over all. If I use it once a year that's a lot. I like it because it's safe

[sluckey]

Not sure about the range but I could really dial both tubes

That's what's ultimately important. It would be nice to have a note on the schematic that states what the range of negative voltage is with the pot set to zero ohms and 100KΩ. Might come in handy in the future during times of mild stress.

Turn wise it wasn't so bad, it's listed as a 28 turn pot

Well that should allow you easy and precise control of the bias.

Any ideas on that crazy heart beat buzz I wrote about?

None. Was it there before the mod? Does it do it with either tube types?

Has this mod been worth it?

[pbman1953]

None. Was it there before the mod? Does it do it with either tube types?Has this mod been worth it?




No, the buzz is a new effect but if I hover my hand mainly over the v3 preamp tube it subsides.






Worth  it ?  Yes, 6559's are my kind of sound. The El's and even the 6KG6  , from my YBA-3A super are grungy. I need the smoother larger sound of the 6550. The amp is scary sensitive in terms of dynamic range

Figure it this way and to be honest I'm a home hobbyist  that has always loved electronics. I never was schooled in it maybe I should of followed that into a career. I was in high end car audio for many years and changed out all kinds of parts. Improved EQ and CD player with better IC's alla Analog Devices.Made hundreds of custom length rca's. Helped Jim Fosgate with his Dolby Prologic 2 for the car. Had some fun and with great sound systems.

[Willabe]

Any ideas on that crazy heart beat buzz I wrote about?

           

[pbman1953]

Forget about the buzz, I moved the amp away from my desk and it's super quiet




Can't thank you guys enough!!


Case closed, for now

[pbman1953]

HI Guys


This is probably the best upgrade ,besides new caps, I've ever done. Best of both world installing the 6550's. On one hand I get the fullnest I wanted in a Sunn type sound. Then I get the pre-amp driven sound of a Traynor, focused and strong.


For other YBA owners with a Master and Volume it's real simple- Actaully many of you may know this but for those who want to experiment-






Sunn-   Volume control on 3-4, then use the master  as the main volume control


Pre-amp driven-  Volume control on 8-9, then use master volume as main volume




Real big-  Master on 10, then use volume control as main




Over driven-  volume on 10, then master as main




No worries, Traynor/ Yorkville documented this




Thanks to all you guys!!!