EL34 or 6CA7... and bias question

[BlacknWhite]

Recently picked up a Traynor bassmaster mark II and it currently has EL34 tubes in it. I know originally they came with 6CA7 tubes. I have never had a tube amp that has run either of these so I was just wondering the difference if any. Also with the old bassmasters I have read multiple articles of people stating they did not worry about biasing the amps as they were built to pretty much be plug and play as long as matched tube sets are used. They do not have bias adjust pots so that's the only reason I'm asking. My other tube amps I can adjust myself.

 

[BassmanPaul]

The EL34 is not a direct equivalent to the 6CA7. The EL34 is a pentode, it has three grids, while the 6CA7 is a beam tetrode, it has two grids and beam forming plates.

Interestedly I just looked up the specs and the 6CA7 is now listed as a Beam Pentode. I guess my information is a bit dated!!

 

[beans-on-toast]

The amp is fixed bias. To change the bias a resistor in the bias circuit has to be changed. At the time, the 6CA7 tubes that Traynor bought had a tight tolerance and were pretty much plug and play. Tubes are more expensive today and people like to optimize the performance of the amp to get the most out of the set of tubes.

The bias circuit itself should be checked, the parts may not be original. Someone might have adjusted the bias. Whatever tube type you go with, you can have a tech check and set the bias.

Sound depends if you are using NOS or current manufacture tubes. Some say that the 6CA7 sounds better in the low end, the EL34 emphasizes the mids. How they will sound in your bass amp depends.

If you like the way that your amp sounds, enjoy it. Although it never hurts, well maybe your wallet a bit, to experiment.

 

[catalin gramada]

Hi. Not quite drop in replacement, but close. Just be sure pin1(suppressor) and pin8 (cathode) are tied together to ground. All 6ca7 I used shows extended reponse in bottom end (don't say better or not) relative to el34. As already was mentioned bias check in is compulsory demands when replace the power tubes, even you replace with the same product.You can do it easy adjustable to a point.A balanced option may help you to keep the noise to minimum level.

 

[BlacknWhite]

The amp is fixed bias. To change the bias a resistor in the bias circuit has to be changed. At the time, the 6CA7 tubes that Traynor bought had a tight tolerance and were pretty much plug and play. Tubes are more expensive today and people like to optimize the performance of the amp to get the most out of the set of tubes.

The bias circuit itself should be checked, the parts may not be original. Someone might have adjusted the bias. Whatever tube type you go with, you can have a tech check and set the bias.

Sound depends if you are using NOS or current manufacture tubes. Some say that the 6CA7 sounds better in the low end, the EL34 emphasizes the mids. How they will sound in your bass amp depends.

If you like the way that your amp sounds, enjoy it. Although it never hurts, well maybe your wallet a bit, to experiment.

Thanks for all the info!. One more question . And before I ask I never plan on changing out the resistor or doing any of the work myself. That always goes to a tech. This is just for curiosity and so I can check the bias. I'm more than comfortable checking with a multimeter.. plate current and bias test points but I have never owned a vintage tube amp like this before and do not know where to actually check the bias current. Could someone help me out on this. I will post a pic of the schematics and the guts.

 

[catalin gramada]

check in the bias of power tubes measuring the current through it at iddle operation. Most safely to not do it on plate but to cathode instead. It is more easy to instal some precision resistor 1-10 ohm from cathode to ground. This resistor is so small as it will not influence the operation of the tube. So you need to replace those short green and white wires and from those points solder from each tube one resistor to those chassis grounding point. You will not measure the current but the voltage over resistors instead. Use I = U/R formula and you will now what current is running through tube. Those resistors should be as precise as you afford (usually 1%) and not necessary bigger than 1/2w . For a proper biasing you will need an scope, otherwise you ears or sort of general rule 70% as max dissipation works as well. For instance if the plate voltage is 450v at iddle and the tube have 25w, biasing at 70% meant to divide the max power dissipation 25w at 70% meant 17,5w. As the power is product between voltage and current P=UxI and we know the voltage 450w for the max power at iddle 17,5w we can found I=P/U meant 17,5/450=0,0388 ( round 0,039 A) .So yo will have to ensure 39mA through the tube adjusting the bias voltage into the grid of the tube. This voltage can be anything between -35 v -50v depends by tube, the load of the tube, voltage, how used the tube is etc. You will have to ensure those negative voltage into the grid adjusting its value to ensure those 39mA iddle current. If you have installed say an 10 ohm resistor from cathode to ground you have to measure voltage over it. As I=U/R meant U=IxR meant U= 0.039×10=0,39V or 390mv. This is the voltage you have to ensure over a 10ohm cathode resistor for a 450v plate voltage to bias you tube at 70% dissipation.

 

[BlacknWhite]

One of the popular method to check the bias of power tubes is to measure the current through it at iddle operation. Most safely to not do it on anode but to cathode instead. It is more easy to instal some precision resistor 1-10 ohm from cathode to ground. This resistor is so small as it will not influence the operation of the tube. So you need to replace those short green and white wires and from those points solder from each tube one resistor to those chassis grounding point. You will not measure the current but the voltage over resistors instead. Use I = U/R formula and you will now what current is running through tube. Those resistors should be as precise as you afford (usually 1%) and not necessary bigger than 1/2w . For a proper biasing you will need an scope, otherwise you ears or sort of general rule 70% as max dissipation works as well. For instance if the plate voltage is 450v at iddle and the tube have 25w, biasing at 70% meant to divide the max power dissipation 25w at 70% meant 17,5w. As the power is product between voltage and current P=UxI and we know the voltage 450w for the max power at iddle 17,5w we can found I=P/U meant 17,5/450=0,0388 ( round 0,039 A) .So yo will have to ensure 39mA through the tube adjusting the bias voltage into the grid of the tube. This voltage can be anything between -35 v -50v depends by tube, the load of the tube, voltage, how used the tube is etc. You will have to ensure those negative voltage into the grid adjusting its value to ensure those 39mA iddle current. If you have installed say an 10 ohm resistor from cathode to ground you have to measure voltage over it. As I=U/R meant U=IxR meant U= 0.039×10=0,39V or 390mv. This is the voltage you have to ensure over a 10ohm cathode resistor for a 450v plate voltage to bias you tube at 70% dissipation.

Without changing anything the way it stands now is there a way to measure it? Or do I need to have a tech make those changes? Thanks for all the info though I appreciate it.

 

[catalin gramada]

Without changing anything the way it stands now is there a way to measure it? Or do I need to have a tech make those changes? Thanks for all the info though I appreciate it.

You can but you have to keep a DC mA meter between each cathode and the ground. From you picture you have tied first cathode to second cathode then with green wire to ground. You have to disconnect those white wire which link both cathodes and consider to run each cathode with own wire to the ground. In actual circumstance is not important if there are tied as there are , but you should be able to measure currents or voltages separate so you have to measure the current through each tubes , not both together.
My advice is to not try to measure currents over the plate, is too dangerous to mess with...

 

[Passinwind]

Without changing anything the way it stands now is there a way to measure it? Or do I need to have a tech make those changes? Thanks for all the info though I appreciate it.

There are several variants on this theme: Bias Testing

 

[catalin gramada]

But keep in mind, I just take a look at You schematic and I strongly suggest this operation to be done by a tech. And I explain why. You have 560v supply on the plate and almost as same 560v on the screen (huh...!). I don't know how much you power transformer will drop onto fully load. We can suppose it is a very good quality one and drop 10% around, meant full dimed the power tubes will be supplied at , say 500v. I don't know what primary impedance you output transformer have, but also can suppose you screen supply have to ensure 380v around when full dimmed to not exceed absolute max 10w screen dissipation. I don't know if those common 1k screen resistor will be big enough to drop 500-380=120v on it to ensure safe voltage in respect with screen current draw for the operation point you have. Highly recommended to go to a tech. A bench should be necessary to set you operation point to you amp to exploit in safe condition. But an adjustable bias voltage pot, with a balance option to adjust the offset will be simple to implement and is a useful tool, I.m pretty sure it will not charge too much.

 

[BlacknWhite]

Thanks for all the info guys. Going to see if my tech can make the changes and school me on how to check the bias afterwards

 

[BassmanPaul]

Hi. Not quite drop in replacement, but close. Just be sure pin1(suppressor) and pin8 (cathode) are tied together to ground. All 6ca7 I used shows extended reponse in bottom end (don't say better or not) relative to el34. As already was mentioned bias check in is compulsory demands when replace the power tubes, even you replace with the same product.You can do it easy adjustable to a point.A balanced option may help you to keep the noise to minimum level.

G3 in an EL34 is better tied to the negative bias supply rather than ground.

 

[beans-on-toast]

Very nice looking amp. It appears all original except for a recap. Buying a serviced amp is a good thing.

The bias circuit is outlined below, where to measure the DC voltage is also shown. A omission on many of their schematics is that they don't specify the the AC line voltage. Normally it was 115VAC. At the correct line voltage, this schematic indicates that with 537VDC on the plates, the bias voltage for the 6CA7 should be -46 VDC. If the line voltage is not 115VAC, the internal voltages will not be the same. All DC voltages are relative to a chassis ground. The proper way to set the bias, as mentioned, is to measure the cathode current at pin-8 of the power tubes. A safe way to do this is with a bias probe that is set up to either measure current of indirectly by measuring a voltage which will allow you to calculate the current. The procedure is described here in the TalkBass Portablex Wiki: How Do I Re-bias My Fixed Bias Amp?. In this case, the bias is set by adjusting R31. It is for a different amp but the idea is the same. You would have to use the tube data sheet to determine what the maximum plate dissipation current for whatever tube that you use.

It all seems very complicated but isn't really once you have successfully done it once. A teck will check out all of your amp and bring it up to spec. More work than checking the bias needs to be performed.

 

[agedhorse]

G3 in an EL34 is better tied to the negative bias supply rather than ground.

It depends on the impedance of the bias supply. Any current leakage to G3 will tend to pull the bias voltage more positive which can lead to a runaway condition. This is more likely if the return path has any resistance.

Sometimes this will work fine, depending on how the bias supply is generated.

 

[BassmanPaul]

It depends on the impedance of the bias supply. Any current leakage to G3 will tend to pull the bias voltage more positive which can lead to a runaway condition. This is more likely if the return path has any resistance.

Sometimes this will work fine, depending on how the bias supply is generated.

Yes indeed but every amp that I have built has worked flawlessly. By the neg supply I mean right off the rectifier/filter node.

 

[catalin gramada]

G3 in an EL34 is better tied to the negative bias supply rather than ground.

Dear Paul, the suppressor it was designed to control the secondary emission th, a lot of tubes wired the suppressor to cathode internally, by default. For fixed bias what you suggest is to tie the g3 to the control grid (the point where the negative bias voltage is applied) and in this way you create a second control grid between screens and the plate. I have a hard time to understand why?
For a cathode bias g3 will stay together with cathode at the bias potential as time the bias is developed over cathode resistor.
This amp have a fixed bias power stage. Anyhow its purpose it is not to stay at particular voltage as time all tube voltage operation is relative to cathode. It should be tied to cathode it is correct to say. When I referred to the ground I referred by fact the cathode is tied to the ground, and not referring to any "0" potential. In a fixed bias arrangement the cathode is tied to the ground and in consequence the suppressor is.

 

[BassmanPaul]

It’s not tied to the G1 in any way.

 

[catalin gramada]

It’s not tied to the G1 in any way.

Then you talk about a cathode biased power stage, not a fixed bias like this amp is.

 

[BassmanPaul]

Then you talk about a cathode bias power stage, not a fixed bias like this amp is.

You are misunderstanding me I think. I am only talking about Fixed Bias operation.

 

[iiipopes]

Somewhere between twenty-five and thirty years ago, I ran a Marshall 2203 as a guitar player, not having fully converted to bass at that point. As recommended, my local tech checked out all the guts to make sure nothing needed attention, and re-biased it from its original 6550's to EL34's. Well, the EL34's weren't quite what I was after. I needed a little bit more clean headroom. So I went with Electro-Harmonix "big bottle" 6CA7's. When I took the amp in to be re-biased, my tech told me 6CA7's essentially biased the same as EL34's, simply swap out and rock on. And that I did with that amp until I finally sold it twenty-some years later, with periodic checkups along the way.

I prefer the 6CA7's to any other power tube. They have the clean sparkle of a 6L6GC, the guts of a well driven EL34, but without the harshness that can come with 6550's, even though they have essentially the same plates. And after trying NOS, original Sylvania's and other brands, the Electro-Harmonix "big bottle" 6CA7's have the best tone of any I have used.

Pay attention to all the above with what it takes to re-bias the power section of an amplifier, as well as bring it up to date, including such things as making sure the filter capacitors aren't leaking and the controls have been de-oxed. One slip and the high voltages can kill a person. Unless you are specifically trained and comfortable working on amplifiers, and have a proper work bench with protected outlets, please leave it to the professionals. As Harry Callahan states, "A man has got to know his limitations."

One final thought: Traynors are Marshall copies, but there are some significant differences as set forth above. I remember my tech saying this means you have to pay even more attention to the details, especially the grounding, so nothing goes awry.