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I just had my Manley amps repaired and I use 6CA7 tubes with 12ax7 instead of the stock configuration of EL34 with 12at7's
Now with the stock tubes EL34 is says bias should be .3MA. When I try and bias my amps after the repair 6 of the 8 get down to about .310 but 2 wont go lower than .416
Is that a problem? the manual says never go past .5 with the EL34s
What's your thoughts? should I bring these back to the person who worked on them or is the .416 Ok with the 6CA7's?
You got 6CA7s that are too hot (hot = higher current under a given set of operating conditions). Your amp does not have sufficient bias adjustment range to bring the current down to spec. There is nothing wrong with the tubes per se - they are just a misfit for the amp.
You need a set of 6CA7s that are mid range to cool biasing (cool = lower current under a given set of operating conditions). Then you'll be able to adjust the bias to the specified range.
Yes it IS a problem that the 2 tubes are at .416! That is roughly 30% over the spec for the amp.
And finally the .416 is NOT current (ma.) It is VOLTAGE!! It's the voltage developed across a 10 Ohm resistor in the circuit between the tube cathode and ground by the current flowing through the tube and on through the resistor. Using Ohm's Law you can compute the current. The current equals the voltage (.416 volts) divided by the resistance (10 Ohms) which equals .0416 or 41.6 milliamps (abbreviated ma.)
If you own a tube amp it behooves you to learn about bias and biasing so you don't get confused by it.
BTW - what brand 6CA7s are you using?
Sent a couple to grant fidelity because I was interested in the Treasures and they were willing to test the ones I had. And they said they tested over 125% strong....25% more than the max..I tried to contact Tube Depot they would not respond.
Thanks again for your reply.
Your a knollegable person. That's why I posted. Yes I mixed up ma and mv stupid mistake but I do know the difference. I replaced the old tubes that were used for repair and they were low but able to bias correctly at 300mv. I'll order new tubes with that in mind (cool) I was using the EH 6ca7s
Thanks again for your advice
The EH tubes out there these days tend to be hotter biasing. In fact it is difficult to get cool biasing EHs from the factory right now. So be careful that the tube vendor knows what he's selling!
That's probably why Manley wont sell the 6CA7 replacement tube set up any more. They used to offer both the EL34 or 6CA7 replacement tube packages for these amps, but no more and its probably because of this issue....
Thanks again. regards
fin, this is what I was told by Manley staff.
The Snappers were designed with EL34s and initial production shipped with those. That is why you'll see EL34 identified on the top plate. Then they began seeing problems with quality from their supplier so switched production to 6CA7s. After some period of time they found acceptable quality in EL34s again so changed back again. That is how they are produced today and what they recommend for re-tubing. Sorry, I can't identify timeframe for the changes.
Mine had 6CA7s when I bought them and since they were strong I didn't want the expense of changing back to EL34s.
"The piano ain't got no wrong notes." Thelonious Monk
They test very strong. Marshall's bias pot seems to have enough range to handle this. This tube lasts forever in road-bound amps. Really tough cookies!
I'm having a different biasing problem with mine - unstable bias. After a brief period the bias in one amp begins to vacillate between about 200 and 450 mV. Manley techs advised 280-320 is the desirable safe range.
I also have 6CA7 outputs. The 12AX7 is the recommended match. Those with EL34 can utilize 12AT7, 12AX7, or 5751.
I took my Snappers to the factory for repair (spending as much on tolls as gas!). They found two other small problems, which they repaired, but no bias issues. Of course as soon as I returned home the problem reappeared. Now I'm waiting for a friend who knows electronics to help with some measurements so I can get back to Manley.
In your case I'd talk with Manley but I'll guess your bias is too high.
"The piano ain't got no wrong notes." Thelonious Monk
I had a local guy fix them after I fell on one and poped some tubes and puff of smoke. I live in NJ so shipping to manley would be $100's just for shipping let alone the time in transit and repair time. The guy did a good job within 2 weeks no shipping and repaired them, I had some old tubes that I had him put in. Sounded great when I got the amps back I then ordered new tubes so I didn't check the bias before inserting the new tubes.
So tonight I'll switch back to the old ones he used to see if its the new tubes
12AX7 are not "drop-in" substitutes for the 12AT7 - they have different gains and other tube characteristics. Similar tubes to the 12AT7 are the 6201, ECC81. and 6679. Not any experience here but I believe they are very similar with just different nomenclature. BTW that is in the manual.
I married the perfect woman. The downside is everything that goes wrong is my fault.
The switch to 12ax7 is for the switch to 6CA7 as per Manley.. They run 12at7 with EL34's. Im sorry I don't know why technically why their circuit asks for that switch. As mentioned I'll check the 2 output tubes with some old ones I know work to see if its them.
Think you mean 300mv not 300ma. 300ma would burn up the tube pretty quickly.
Why the 12ax7's? you need the drive the 12at7"s can give. You may have a couple of bad output tubes, bad coupling caps, or a low bias supply voltage.
Good suggestions, First I'll make sure he put back the right 12AX7 and then I'll replace the 2 6CA7's that I cant bias with some old ones I have to see what happens.
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