replacing tubes, speakers C50

[Eminence_Front]

So if i have EL84's (6BQ5) by GrooveTubes, and I replace with a SovTek EL84 (6BQ5) is rebiasing necessary ?

Are the plate voltages the same for the same rating tubes ?



Also, speakers are currently wired in series, for 16 Ohm load. Any particular reason for them coming from the factory with 16 OHM configuration ?

[telecaster911]

EL84's should have a range of plate voltage as a guide point to setting the bias. I would at least check it to see where it's running. I biased mine right in the middle 'cause I couldn't tell the difference in tone, hot or cold.

Here are the instructions for biasing my amp. Do not use these settings for yours. There is no EL84 info in there, but it's a good reference. Also, don't dig around in there and get yourself killed. There are lethal voltages even unplugged due to the filter caps storage. The direction that you turn the pot in the reference below may be the complete opposite for yours. Mine has test points and the pot is accessible without taking the amp apart. I have no idea why all amp makers don't put test points on the back.

Prages may have some good information as he did a tube change on his C30.

BIASING INSTRUCTIONS (From owner's manual)

Disclaimer: Do not attempt this procedure unless you have complete understanding of these instructions, and realize that there are lethal voltages present inside the amp, even when turned off!

Biasing Instructions:
These instructions are here to help you properly bias your
power tubes when replacing them with new ones. To do this,
all you need is a multi-meter and a small flat head screwdriver.
This is a safe and quick method for biasing your tubes and
keeps your amplifier functioning properly. Your amplifier has
3 bias test points, 2 Red and 1 Black (ground connection), and
an adjustable bias potentiometer attached to the back of the
chassis. The Red test points will allow you to read the current
draw of each tube and the bias pot will let you adjust the bias
current when you change your power tubes.


1. Insert new power tubes; allow warming up on Stand-by for at
least 5 minutes.


2. Adjust the bias pot to the full clockwise position with a small
flat head screwdriver before the amp is taken off Stand-by. This
ensures that the tubes will not be running too hot before a bias
adjustment can be made.


3. Set your volt meter to the lowest DC volts settings, and insert
the negative probe of your volt meter into the Black ground connection.
Each power tube has its own Red colored probe connection,
allowing the user to check the bias of each tube. Insert the
positive probe of your volt meter into either Red jack.


4. For amps using 6L6 power tubes, adjust the bias pot to give a
reading between 25 to 35 mv for each tube. For amps using
6V6's adjust the bias between 18-27mv. For amps using EL34's
adjust the bias between 30-40mv. Lower settings will sound
cleaner with longer tube life. Higher settings will have an earlier
break-up and shorter tube life. A properly matched set of tubes
will generally be less than 5mv off from each other.


5. Leave the amp on for at least 20 minutes and re-check the
bias. Tubes will drift over time and checking and adjusting the
bias periodically will extend the life of your tubes.

hope this helps. wave0


Make sure you talk to Mark, he may be able to help you with this or at least point you in the direction of a good amp tech. AOK

[Eminence_Front]

This actually stems from a convo Mark and I had this morning during the great southern california smoke-out.



I took my amp in and he suggested i get it looked at for a potentially bad / microphonic tube. It intermittently gets a scratch / buzz on both channels when sitting idle.

if he smacked it, it would shut up for a few minutes.

After seeing it had all GT's in it, Mark suggested I may want to go with a little better quality tube. So, I looked online and found the SOVTEKs for a decent price ($40 matched set of EL84's)..


I was going to check the bias voltage, and maybe rearrange some tubes to see if I can pinpoint which one is the offender.



I just don't know if it's more likely a power tube or a pre-amp tube

[Mark Wein]

Make sure you talk to Mark, he may be able to help you with this or at least point you in the direction of a good amp tech. AOK


My advice is to find a good amp tech...unfortunately the one I use is in North Hollywood...I can't find anyone worth a shit in Orange County although I think Doug Roccoforte of Roccoforte Amplifiers will do amp service and he is in Brea.

[Mark Wein]

if he smacked it, it would shut up for a few minutes.



I use this technique with some of my students :weebz:

[Eminence_Front]

Well, I could no longer stand the curiosity so I broke out my DMM and my gator clips and pen-iron and went to work tonight.


I pulled the cover off and took a bias measurement on the GT-EL84 power tubes.


4-5 were measuring 56mA, 6-7 were measuring 52mA. So I swapped the inner pair of tubes and have gotten a measurement of 54.5 and 55.5, which is the closest I could get them to this point.


I did some reading and found 1-2mA is within tolerance, but 54.5mA seems hot to me. Based on my research it should be around 50mA or therabout. Although, I am sure OEM specs tend to be conservative so driving them a little hot may be necessary for performance reasons (and not so much for reliability / warranty.)


I also found an excellent doc for adding a Cermet Trim POT to make the bias adjustable since currently the C50 is fixed bias via resistors. (and I lost those math skills a long time ago.)



Anyway, I am getting closer to solving this little problem. Pics tomorrow on my work for anyone interested.

[telecaster911]

If a tube is microphonic it usually is a preamp tube. just give the 12ax7s a tap with a pencil till you find the offending tube. Dude00

[Mark Wein]

Well, I could no longer stand the curiosity so I broke out my DMM and my gator clips and pen-iron and went to work tonight.




I forgot...you actually know how to fix things...we await your pics! AOK

[Eminence_Front]

Ok, so I found out via some reading that Pre-1 12AX7 is engaged by itself on clean channel, and that the amp mixes the other 2 12AX7's in only on Lead / Drive channel.


So, seeing as how the noise is present on each channel, that kind of means that it has to be the 1st position pre-amp tube, if in fact it's a preamp tube causing it.

So, I swapped 1 for 3, and I am going to see if it has a noticeable impact on the problem. I also ordered a JJ-12AX7 and a Mullard NOS ECC83 pre-amp tube that I am going to swap in place of tube-1 to see how it affects things. Sounds like some pretty good results have been had by mixing different PreAmp Tubes in position 1, which by all accounts, has the most impact on the tone of the amp. (since if you play clean, it's the only tube you ever use.)


Here are some pictures, in case anyone is interested in taking Bias measurements on their Peavey Classic. NOTE: Once you place the Positive probe on the transformer primary side, you have full Current on the other end of your negative probe. DO NOT TOUCH THIS TO EARTHGROUND OR YOU'LL COOK YOUR T-former at best, and kill yourself at worst.

From what I have read, you want to see a reading of no more than 54mA - 55mA (I am slightly over the threshold.).....The EL84 Power tubes are supposedly rated to perform best between 22-27mA each. Since you're measuring current to the pair, you divide by 2 to get a rough estimate of per-tube Current.


http://www.chucklesmafia.com/C50-internal1.jpg

http://www.chucklesmafia.com/C50-internal2.jpg

[Prages]

I wouldn't completely rule out the problem being a power tube.

I had a bad power tube in my Classic 30 that caused a lot of popping and cracking.

FWIW, I've never bothered with biasing either my 50 or 30 when changing tubes, and never had any problems with either.

[Eminence_Front]

Probably because the fixed bias is set up such that you can't easily tweak it. The larger cross section of EL84's out there almost all run well at about 25mA.

(this is according to all the doco I am reading, I am NOT an expert.)


So stands to reason peavey set it up to accept almost any drop-in replacement tube. The factory spec is 50mA per side, or 25mA per tube.


Only those dedicated to the pursuit of the Tonez would take it in to have a set of NOS boutique tubes matched and biased into the power circuit for balance. Otherwise, a few mA either way makes zero impact.


For those of us who aren't necessarily on the short list for Grammy's, I am sure almost any tube combination is going to be relatively similar.



I am going to pull out the fixed value resistor and cap and replace it with a Cermet Pot though. Just makes it easier in the future to have work done on it. You can much easier tweak the bias cooler or hotter depending on desired outcome. Plus, it looks helluva easy and only costs $6.