Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tubefiend on February 22, 2016, 12:47:26 pm
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Hello all,
Building a weird little single ended special, with two EL84's in a parallel SE configuration, and wondering the what-if's about pushing the plate voltage(s) to about 350 or so..The Fender blues junior runs them in PP at 347, but I'm not sure if the tube(s) can take a little extra "shove" in an SE fashion...My transformer no load secondary comes in at 311 RMS, and I was planning on using a 5AR4 rectifier, but I could always use a 5Y3GT if it gets too hairy I guess...
Thanks in advance,
Pete
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I'm definitely no expert in this matter.
But I could swear I have seen a number of circuits that push the EL84s with +380 B+,
and even a couple with +400 or slightly more. I could be mistaken, though. :dontknow:
I know that I made the Hoffman Stout on my BB with a listed +354 B+.
I think the EL84 can take +350 just fine.
As always there is that overall interaction between PT, rectifier, OT and output tube current to consider.
From my rudimentary understanding, with a properly biased output circuit, the higher B+ causes less current to be drawn.
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a little extra "shove"
Plate diss is your *main* consideration, within limits. Most all my SE builds run between 95 to 110% of max plate, downside, you will probably shorten the tube life. You probably won't arc-over at 350v, 450, I wouldn't take that bet.
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It turned out pretty interesting...With the 5Y3, I ended up with 301 Pv, 293/289 screen (before and after 1k stopper). I used a 220 ohm cathode resistor (separate cathode resistors and bypass caps for ea. EL84)for a cathode voltage of 9.3v. The 5AR4 took it into the danger zone with voltage and I didn't hear any benefits.
Measuring the actual current draw total through the OT, I got 61 mA. (since they are parallel EL84's that actually seems a tad light, but it sounds pretty good)
What made a real sound difference was the first preamp stage. I did the HiWatt parallel 12AX7A thing, with an 820 cathode resistor / 25 uF bypass cap, then into a Marshall copy tone stack, into one half of another 12AX7, then into the other half. On the last half, I skipped the bypass cap and introduced NFB through a 47k resistor. It sounds really good for crazy early Rory style "Laundromat" distortion and it cleans up ok, but I think next time I'll tone down the preamp stages a bit....All in all not too shabby. I like it when they fire up right out of the chute with no smoke or fire....
Thanks for the replies..
Pete
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I'll tone down the preamp stages a bit
easy testing for this is lower gain tubes, AY, AU
And ya, really nice when it just works as designed :icon_biggrin:
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This is a recurring topic. Tube specs say max voltage is 300. My Traynor Bassmate ran them in PP at 410V! Lots of builders run them in the mid 300's. Psychonoodler once pointed out the the real criteria is is plate dissipation = 12W. So, keep safely under that.
But typical SE operation is 250V. If the plate voltage is too high, it will be hard to impossible to cathode bias. E.g., a larger cathode resistor won't limit tube current, because it will "boost" plate voltage on the other side of the power circuit. The result is that the tube will keep re-balancing bias around the same, high, current draw.
A higher impedance rectifier may help. Also consider a dropping resistor in the B+ rail. I got my Bassmate down to 280V with a bucking tranny + dropping resistor.
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This is a recurring topic. Tube specs say max voltage is 300. My Traynor Bassmate ran them in PP at 410V! Lots of builders run them in the mid 300's.
Good to know I was recalling those high B+'s correctly for the EL84 circuits.
Since I'm a newbie, I always try to use one of the voltages posted in the data sheet. :dontknow:
Except when I'm following a known good design like the Hoffman Stout.
So far I don't seem to hear a big difference when the voltage gets turned up.
I also worry about prematurely burning up my old premium tubes with higher voltages.
I figure if they have already lasted over 50 years with 250 or 275 volts,
why not try to get another 50 years out of them with the lower voltages? :l2:
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This is a recurring topic. Tube specs say max voltage is 300. My Traynor Bassmate ran them in PP at 410V! Lots of builders run them in the mid 300's. Psychonoodler once pointed out the the real criteria is is plate dissipation = 12W. So, keep safely under that.
But typical SE operation is 250V. If the plate voltage is too high, it will be hard to impossible to cathode bias. E.g., a larger cathode resistor won't limit tube current, because it will "boost" plate voltage on the other side of the power circuit. The result is that the tube will keep re-balancing bias around the same, high, current draw.
I think I'm ok where I'm at with it, but definetly I wouldn't build it again this way exactly. My PT is rated 275-0-275 and at the 121 VAC I have, it runs 311 with no load. The 5Y3 was a n brainier as compared to the 5AR4 I originally wanted to use. Maybe next time I'll do a fixed bias with the bias tap on the XFMR.
A higher impedance rectifier may help. Also consider a dropping resistor in the B+ rail. I got my Bassmate down to 280V with a bucking tranny + dropping resistor.
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Wow, all that typing and I still managed to f up the post! All I was saying was I doubt I would build this again quite like this. My PT is rated 275-0-275 and on my supply, comes in at 311 with no load. The 5Y3 worked out pretty good. I'm a couple of volts high but the PD comes in at 11.8 watts so I'm going to leave it alone.
Next time I'll fixed bias the grid(s) and keep the PV closer to 250, or just PP it like so many others do, but I really prefer the SE sound. It jumps right out and rips a chunk out of your face with a Les Paul, and gets all Rory like with a Strat.
Thanks for the replies!!!