Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on November 29, 2010, 01:27:31 pm
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Hi guys, I don't know if i saw this (or it got posted but did not work)but is it possible to parallel different power tubes together? The reason i ask is i found a schem of a 18w TMB that was setup to be able to change between EL84s and 6V6s with out having to change the cathode Bias. That got me thinking about this schem.Please correct me if i am way off the mark. Thanks
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EL84s and 6V6s don't bias the same. Typically, you'll see approx. 130-150 ohms under a pair of EL84s and a 250 ohm under a pair of 6V6s. Are you sure the schematic you found did not have separate cathode resistors?
The same would apply for fixed bias. One likes approx 12-15v and the other likes approx 30-35v.
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I remember Dockery Amps (no longer around) had a layout for a Matchless Lightning where you could you could run either 6v6 or el84 tubes. If I remember correctly it used a resistor that set a generic bias that worked for either set of tubes, but was ideal for neither.
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I've seen an Egnator amp that had a pot to blend from 6V6 to EL84. Always wondered how that worked. My guess (and this is only a guess) would be that if it were cathode biased, the 6V6 cathode resistor would go to lug 1 of a pot and the EL84 resistor would go to lug 3. If you grounded lug 2 at full rotation one set of tubes would have correct bias while the others cathode resistor would be the correct value of its cathode resistor plus the value of the pot. Turning from one side to another would increase the bias on one set of tubes while decreasing it on the other.
Just a guess?????? Don't quote me on this one. If anyone knows how it really works or has a schematic I'd like to see it.
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Since you must have two sets of sockets, just use two resistors also. Then use a switch to select which pair, or even simpler, only plug in the pair you want to use. Never plug in both pairs unless using a switch!
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Good Idea. I'm still curious to see how Egnator blends the two.
How do yellow jackets work? Are they just socket adaptors(plate to plate, cath to cath etc) or do they rebias for 6V6 or EL84.
I was thinking if you made a special adaptor that connects the 9 pin socket to the 8 pin plug plate to plate, screen to screen, grid to grid, heater to heater, but routed the cathode to one of the unused pins of an 8 pin socket you could run it to a seperate cathode resistor and not have to rebias(Cathode bias setup of course). You'd have to make sure you're not using that pin for a screen or grid resistor tie point but it'd work. .
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I can't open that file. What program do I need to download to view it?
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I can't open that file. What program do I need to download to view it?
SCH files can be opened and edited by ExpressSCH and JSchem. Both are free. The latter is lighter and easier for casual users.
ExpressSCH, Windows only, http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm
JSchem, all platforms supporting Java, http://dhost.info/jschem/
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Hi guys, Yes it is something i found on the Dockery amps 18w TMB schem. Their idea was to have the cathode set up to be able to change tubes ( two sets of sockets) without changing the cathode.My idea was to be able to, via a switch, switch not only between tube types but run them parallel with each other.IS THIS POSSIBLE? To have separate Fixed or Cathode bias is not a problem.
AS PER THEIR SCHEM
EL84 - 470uf,150 ohm
6V6 - 470uf,250 ohm
6v6 & EL84 - 470uf,250 ohm
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I'm still curious to see how Egnator blends the two.
I believe they use a ganged pot to vary the amount of signal going to each of the grids. Wired such that one is turned up as the other is turned down.
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Merlin, I found the Egnater Rebel 20 has this settup, but can't find a schem.
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My idea was to be able to, via a switch, switch not only between tube types but run them parallel with each other.IS THIS POSSIBLE?
As said, since the EL84 and 6V6 will each need their own socket, supply a cathode resistor for each like-pair of tubes; you already have noted examples of typical values.
If you have all 4 tubes running, but only 1 pair gets an input signal, then you will only hear output from that pair, right?
If you operate all output tubes cathode biased, then you could simply close a switch contact from grid to ground to "turn off" one set of tubes. Alternately, you could have ganged-pots wired as volume controls to each output tube grid (1 like-pair on 1 ganged volume pot), so that you can get any amount of each tube type.
You should only need a coupling cap before the "input" of the volume pot (the non-grounded outer lug), since there is no negative voltage applied to the grids in cathode bias.
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Thanks HBP, The mesa boogie blue angle has a three position switch that gives you EL84s by themselves, 6V6 by themselves and both together. The egnater 20 i think has the ability to blend the two together, which sounds like a great idea. would it be possible to post a schem just to confirm these ideas. Thanks
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Give a look to the attached schematic
Kagliostro
p.s.: the cathode bypass capacitors are connected in a wrong way
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Thanks K, Good kickup :huh:
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Thanks for the help guys, i think i've got it sussed. The plate and screen values might change when i get the get the power supply worked out. Thanks