# Swap tubes in YCV40 to EL34



## venn (Mar 11, 2015)

I have been reading about switching tubes in my traynor to EL34's. I read where you have to change the bias resistance by swapping R78. However, the manual states that 
"The circuitry has been designed to accommodate any type of EL34/6CA7 as well as any type of 6L6/5881 output tubes (as long as the four, or two, output tubes are of a matching type). The amplifier has also been equipped with bias sensing points for each of the output tubes as well as a recessed bias adjustment trim pot. This makes tube replacement quick and easy." 

So do I not need to change the resistor? I emailed yorkville and am awaiting a response. Thought I would pick your brains as well. Also, where are the bias test points? Don't see them in the PCB layout online.


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## jb welder (Sep 14, 2010)

You missed the line above that in the manual where it says the EL34's are in the YCV50B. They use the same manual for both amps so it's a little confused.
So they are not saying either of the models are set up to use either type.
You must make the changes as required.
Bias will be measured across R91 & R92.
R78 is 18K with 6L6, 15K with EL34.
R106 is 10K with 6L6, 15K with EL34 (on schematic lower right area, sheet 4of6).

Now look at schematic sheet 3of6. At the top there is a whole area of parts that are different for the 2 models.
You need to make the changes as shown.


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## Budda (May 29, 2007)

Is it worth modding vs finding a 50blue?


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## venn (Mar 11, 2015)

Thank you very much!

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Budda said:


> Is it worth modding vs finding a 50blue?


I can't sell the damn thing, tried but no serious takers for what its worth. Don't feel like giving it away, and can't justify having two large amplifiers for a hobbyist.


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## jb welder (Sep 14, 2010)

Let us know what yorkville's response is. I'm not sure about the extra's on pg.3 of schematics, they may or may not be critical. I will try to take a better look later. It could be about "voicing" the amp different, maybe for the different speakers?
The R78 may give a big enough bias sweep as is, depending on the actual tube set.


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## Budda (May 29, 2007)

venn said:


> Thank you very much!
> 
> - - - Updated - - -
> 
> ...


Honestly just keep it in the classifieds and leave it stock. It will sell eventually. Also list it on reverb.com


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## venn (Mar 11, 2015)

jb welder said:


> Let us know what yorkville's response is. I'm not sure about the extra's on pg.3 of schematics, they may or may not be critical. I will try to take a better look later. It could be about "voicing" the amp different, maybe for the different speakers?
> The R78 may give a big enough bias sweep as is, depending on the actual tube set.


I was able to get through on the phone to their service tech. He said it is probably just a one resistor swap, but couldn't say for sure without the serial number. I'll have to get back to him tomorrow on that. I'll let you know what he says. Couldn't have asked for nicer people there at yorkville/traynor!


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## mrmatt1972 (Apr 3, 2008)

venn said:


> ?.. and can't justify having two large amplifiers for a hobbyist.


Yes you can! Lots of us do that and worse.


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## 5732 (Jul 30, 2009)

mrmatt1972:610582 said:


> venn said:
> 
> 
> > ?.. and can't justify having two large amplifiers for a hobbyist.
> ...


Much worse aarrgh!


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## venn (Mar 11, 2015)

Alright, so I just got off the phone with an extremely helpful service tech for yorkvill/traynor named Guy. From my serial number, I have the very latest edition of the YCV40 and should not require any change other than adjusting the trim pot to bring it to read 75-80mV. He did say that if for some reason the tube set that I get doesn't allow the trim pot to adjust within that range, it would probably require a change to R78. He said just to call back and he would figure that out for me. I have to say, everyone that I contacted at yorkville/traynor was by far the most helpful bunch of folks I have ever encountered. Nothing like a company, more like a local mom and pop business. I'm impressed.


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## jb welder (Sep 14, 2010)

Excellent. I hoped this would be the case. You will only need to change R78 if you can not get the idle current high enough.
For example, if you can only get the bias up to 60mV.
If this turns out to be the case, you can just add another resistor on to R78 in parallel. That way you will not have to remove the circuit board or the original R78. A 100K resistor in parallel with the existing 18K will bring the R78 value down to approx. 15K.


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