Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tone Junkie on June 20, 2010, 10:22:12 pm
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What do I need to do to change the circuit from 6l6,s to el34,s Im looking at two differant amps and the way the tube is wired seems differant . thanks Bill
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Pin 1 Grid3 -must- be connected to pin 8 Cathode.
Since pin 1 is not used on most 6L6, many 6L6 amps use the pin 1 tag as a handy tie-point for G1 or G2 resistors... these must be removed from pin 1 and a new home found.
Then check idle bias current. For happy idle, El34 requires similar G1 voltage to 6L6, but not always the same, so it must be checked. At the typical AB763's plate voltage of 460V(?), you aim at 45mA and need to stay below 65mA per tube.
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Do basically 2 things:
1. Make sure you have nothing wired to pin 1. It is often used as a tie-point for the 1.5k grid resistor running to pin 5. So if there is something on pin 1, remove it. Then add a jumper to connect pin 1 to pin 8.
In a 6L6, the is no suppressor grid, there are beam-forming plates which perform a similar function. These are internally connected to the cathode, so there is nothing connected from inside the tube to pin 1. But the EL34 is a true pentode, and has a suppressor grid, which has a connection brought out to pin 1.
2. Make sure you have an extra 1A of filament current available per pair of EL34's. A 6L6 draws 0.9A of filament current, while the EL34 draws 1.4A. If you pop them in without either reserve current available or with an added filament transformer to power the output tubes, you will do what I did when I first tried to convert an amp from 6L6 to EL34: instantly melt the insulation off the heater wires! Or burn up you power transfromer, which will be a bigger problem.
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thanks guys this is actually a new build I had an old music man amp. I bought for $20.00 needed an output transformer so i replaced it the thing sounded like shit, buzzy with a lot of hiss probably just needed a cap job but I got to listen to a good one at a gig. I really didnt like it much so I thought I would just build it into an ab763 with el34 because that was the output trannie I had. So i picked up a proper fender style PT. and made the board it was just As I was finishing my wiring I noticed the differance in the two pinouts.
That tying into pin 1 thing kind of threw me for a bit . I appriciate your help I had kind of muddled threw it before I got to read this and Im glad you confirmed my thoughts. on the differance between the two. Thank you Bill
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What has been said about heater current holds true, but... I disagree with tying pins 1 and 8. After having read on the 'Tales from the Tone Lounge' site about supplying positive voltage to pin 1, (he gives a very accurate description and diagram on how to do this in his JCM800 mods section), with the reason being that the tubes will last longer by supplying positive voltage to pin 1, so I tried it. Amp is still going strong after 50 hours or more of playing, and it hasn't had any effect to the tone.
I've since done the same thing to a Train Wreck Express based amp with EL34s and this amp also works perfectly although this amp has probably only seen about 20 playing hours at this stage.
I'd be very interested to read discussion for pros and cons on this mod, but as I said, I've done it with no problems at all at this stage.
www.tone-lizard.com/Marshall_Myths.htm about 2/3 down the page.
Tinkerer
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Tone-lizard does not suggest connecting pin 1 to a positive voltage. Instead, he says "We can further promote tube reliability by connecting the Suppressor Grid to the bias supply, as seen at point #2."
(http://www.tone-lizard.com/images/Improved_Suppresor_Grid.gif)
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I have tied pin 1 to pin 8 on an SE, cathode biased EL34 amp, which is, in effect, connecting it to a positive voltage.
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I have tied pin 1 to pin 8 on an SE, cathode biased EL34 amp, which is, in effect, connecting it to a positive voltage.
True, BUT... relative to the Cathode, it has the same potential (especially if done at the socket) and therefore should be OK. It is at 0V in relation to the cathode, and equal to the plate-cathode difference in relation to the plate. Everything is relative to the Plate and Cathode on a tube, hence Cathode biasing works.
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Okay one other issue Is the 470ohm before diode the resister that controls the bias rate in this circuit I have 29 and 31 MA at full bore on my bias pot and I need to change it to get to the rite Bias for my el34. Thanks Bill
Darnit if it was marshall id have this down already but the fenders are a little new to me my last just one worked fine no tweaking . I need to get in the 34 to 36Ma range Im thinking
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Increase the 470Ω to 1K. If still need to fine tune, adjust up or down to suit. A bigger resistor will result in increased cathode current. Same principle applies to Fenders and Marshalls.
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Thanks my Iron is heating up as I type i just wanted to get this up and running tonight. i have a 5e7 board crying my name and an empty chasis sitting in my other amp stand. I think Im addicted. :laugh:
Bill
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Slucky i did a 1k then a 1.5k and still no movement I will try larger then get back to you thanks Bill
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Sorry I was being a pussy about going to big When I put on a 2k and it only raised it about 3 points I got worried then I thought my 10k pot was only moving it 9 to 10 ma. so i went 10k perfect. sorry for being overly cautious. thanks Bill
Being a newbie sucks but this was another great lesson learned you kind of remember the hardway better