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6L6 to EL34 Conversion

6/22/1999 4:17 PM
John K.	6L6 to EL34 Conversion I've heard about converting output stages from 6L6 to EL34. In looking over the RCA Tube Manual, I see the typical operating grid and plate voltages for two EL34's in push-pull is about 450v, while the same for 6L6's is 250v. As a baby in tube amp electronics, it would seem to me it might take major mods on the power supply (different transformer?). Can someone give a brief beginner's overview? Thanks.

6/22/1999 4:17 PM

John K.

6L6 to EL34 Conversion

I've heard about converting output stages from 6L6 to EL34. In looking over the RCA Tube Manual, I see the typical operating grid and plate voltages for two EL34's in push-pull is about 450v, while the same for 6L6's is 250v. As a baby in tube amp electronics, it would seem to me it might take major mods on the power supply (different transformer?). Can someone give a brief beginner's overview? Thanks.

6/23/1999 0:42 AM
anonymous	John, How major the mod is depends on what your final goal is when converting. The main reason that one would convert from 6L6 to EL34 (besides tone preference) is to increase total output power. But to increase total output power you need to raise the B+ ( I.E plate voltage) to maximize the use of an EL34. Since most modern EL 34's can handle more plate voltage and larger dissipation rating(thermal watts) than a 6L6-GC, you can effectively increase you power output by raising the B+ and rebiasing the tubes to a higher setting. The easiest way to increase B+ is to change your Tube rectifier(if the amp has one!) to a SS type for less sag in the power supply section. The next would be to find a higher voltage PT. Some questions to ask yourself before performing this type of mod are: Can my existing PT and OT handle the increase in bias current being drawn from the EL34's? Can the existing componants(I.E. power supply caps/ resistors, ect.) Handle the increase in voltage that is needed to raise the B+? Once you raise the B+ you have effectively changed the power supply design and have caused other voltages to be increased........what changes do I need to make to obtain the original voltages to the preamp plates?(if desired) Is the amp fixed biased or cathode biased? Do I have the correct parts/ circuit in place to correctly bias the EL34's to a suitable dissipation setting. I don't know if this answers your question(s).....but it's a little more insight about this type of mod. I hope it helps. Benjamin.

6/23/1999 0:42 AM

anonymous

John,

How major the mod is depends on what your final goal is when converting.

The main reason that one would convert from 6L6 to EL34 (besides tone preference) is to

increase total output power. But to increase total output power you need to raise the B+ ( I.E plate voltage) to maximize the use of an EL34. Since most modern EL

34's can handle more plate voltage and larger dissipation rating(thermal watts) than a 6L6-GC, you can effectively increase you power output by raising the B+ and

rebiasing the tubes to a higher setting.

The easiest way to increase B+ is to change your Tube rectifier(if the amp has one!) to a SS type for less sag in the power supply section. The next would be to find

a higher voltage PT.

Some questions to ask yourself before performing this type of mod are:

Can my existing PT and OT handle the increase in bias current being drawn from the EL34's?

Can the existing componants(I.E. power supply caps/ resistors, ect.) Handle the increase in voltage that is needed to raise the B+?

Once you raise the B+ you have effectively changed the power supply design and have caused other voltages to be increased........what changes do I need to make

to obtain the original voltages to the preamp plates?(if desired)

Is the amp fixed biased or cathode biased? Do I have the correct parts/ circuit in place to correctly bias the EL34's to a suitable dissipation setting.

I don't know if this answers your question(s).....but it's a little more insight about this type of mod.

I hope it helps.

Benjamin.

6/23/1999 1:56 AM
jason	Benjamin pretty much covered it all. Don't forget to tie pin 1 on the pin socket to ground. The suppressor grid in a 6L6 is internally connected to the cathode but not in an EL34 so you will need to tie pin 1 of your tube socket to ground to run EL34's. Heater current for a 6L6 is .9A and 1.5 for an EL34 so a pair of EL34's in pushpull operation will draw 1.2A more heater current than a pair of 6L6's. Need to be sure your power transformer can handle the extra current draw. Of course a 6V6 draws .45A heater current and people shove 6L6's(.9A) in deluxe reverbs all day long without a problem so who the hell knows. Food for thought. jason

6/23/1999 1:56 AM

jason

Benjamin pretty much covered it all.

Don't forget to tie pin 1 on the pin socket to ground. The suppressor grid in a 6L6 is internally connected to the cathode but not in an EL34 so you will need to tie pin 1 of your tube socket to ground to run EL34's. Heater current for a 6L6 is .9A and 1.5 for an EL34 so a pair of EL34's in pushpull operation will draw 1.2A more heater current than a pair of 6L6's. Need to be sure your power transformer can handle the extra current draw.

Of course a 6V6 draws .45A heater current and people shove 6L6's(.9A) in deluxe reverbs all day long without a problem so who the hell knows.

Food for thought.

jason

6/23/1999 2:15 AM
John K.	Thanks for the info - One more thing please? Very helpful and encouraging since it verified what I suspected after studying these circuits for a week. I don't understand about the output power, though. I thought EL34's and 6L6's were about the same in that arena, and the significant difference was tone and harmonic distortion. One more thing? I have an old Eico HF-20 that I now have the schematic for. Since it's only 20 watts I'm guessing it's not push-pull class AB. What do I look for? It's got great output distortion. If I can clean up the power supply, rebuild the preamp... If you could give me a clue as to how to analyze the power stage, I'd appreciate it. Thanks again.

6/23/1999 2:15 AM

John K.

Thanks for the info - One more thing please?

Very helpful and encouraging since it verified what I suspected after studying these circuits for a week. I don't understand about the output power, though. I thought EL34's and 6L6's were about the same in that arena, and the significant difference was tone and harmonic distortion.

One more thing? I have an old Eico HF-20 that I now have the schematic for. Since it's only 20 watts I'm guessing it's not push-pull class AB. What do I look for? It's got great output distortion. If I can clean up the power supply, rebuild the preamp... If you could give me a clue as to how to analyze the power stage, I'd appreciate it. Thanks again.

6/23/1999 3:04 AM
jason	I just found the schematic over on duncans site. The power amp is cathode biased. You can tell because of the 390 ohm resistor and 30湩 bypass cap on the power tube cathodes and there is no negative voltage beinng applied to the control grids(might be 350 ohm and 50湩, schematic is hard to read).The schematic shows 40v on the power tube cathodes so ohms law tells us the current. 40v divided by 390 ohms=102mA of current. Since the power tubes are sharing that cathode resistor the current splits through both tubes so each tube sees 102mA divided by 2=51 mA of current per tube. Keep in mind that you'll loose a few mA of current to the screen grids so you should have close to 48mA or so plate current per tube. If you now multiply the plate current times the voltage from cathode to plate you can get your plate power dissipation. The schematic shows 440v on the plate and 40v on the cathode so the voltage from cathode to plate is 400v. 400v times 48mA = 19.2 watt. Now as far as the actual power out of the output transformer. I'm not sure how you go about calculating that. I believe that the number of turns in the windings and Z have alot to do with it. Hope that helps some. good luck! jason

6/23/1999 3:04 AM

jason

I just found the schematic over on duncans site.

The power amp is cathode biased. You can tell because of the 390 ohm resistor and 30湩 bypass cap on the power tube cathodes and there is no negative voltage beinng applied to the control grids(might be 350 ohm and 50湩, schematic is hard to read).The schematic shows 40v on the power tube cathodes so ohms law tells us the current. 40v divided by 390 ohms=102mA of current. Since the power tubes are sharing that cathode resistor the current splits through both tubes so each tube sees 102mA divided by 2=51 mA of current per tube. Keep in mind that you'll loose a few mA of current to the screen grids so you should have close to 48mA or so plate current per tube. If you now multiply the plate current times the voltage from cathode to plate you can get your plate power dissipation. The schematic shows 440v on the plate and 40v on the cathode so the voltage from cathode to plate is 400v. 400v times 48mA = 19.2 watt. Now as far as the actual power out of the output transformer. I'm not sure how you go about calculating that. I believe that the number of turns in the windings and Z have alot to do with it.

Hope that helps some.

good luck!

jason

6/23/1999 3:26 AM
Benjamin Fargen	John, Sounds like you have the perfect makings for a Matchless "Clubman" Clone. The schematic for this amp is posted on the Ampage schematic database. I built one and it sounds great. I did end up rebiasing mine to class AB due to the abusive bias current that is needed to run this amp in class A.It was starting to heat up the trannies above my comfort level. My suggestion for the preamp would be to omit the 6SH7 due to the fact that this tube is exteamly microphonic and metal cased examples of this tube are even worse. Try the same circuit with an EF 86 substitute(thats what Matchless had to do!) or maybe even try putting a vox AC-50 or AC-30 top boost circuit in front of it. This could turn out to be a very cool sounding project! Have fun, Benjamin.

6/23/1999 3:26 AM

Benjamin Fargen

John,

Sounds like you have the perfect makings for a Matchless "Clubman" Clone. The

schematic for this amp is posted on the Ampage schematic database. I built one and it

sounds great. I did end up rebiasing mine to class AB due to the abusive bias current

that is needed to run this amp in class A.It was starting to heat up the trannies above my

comfort level.

My suggestion for the preamp would be to omit the 6SH7 due to the fact that this tube

is exteamly microphonic and metal cased examples of this tube are even worse. Try the

same circuit with an EF 86 substitute(thats what Matchless had to do!) or maybe even

try putting a vox AC-50 or AC-30 top boost circuit in front of it.

This could turn out to be a very cool sounding project!

Have fun,

Benjamin.

6/23/1999 3:38 AM
John K.	Thanks, Benjamin. Unfortunately, my acrobat reader can't seem to "read" anything, and I always get an error. Is the Clubman an EL84 amp?

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