6p14p-EV TO 7868, now 7591 Power tubes

[AZJimC]

Okay, I'm STILL tweaking on my 18 W (EL84 design). What I've done so far regarding power tubes is replace the OP trans with a Deluxe type. It's one of those Classic replacement jobs, and said it's rated for 20+ Watts. I also swapped the EL84's for the Russian made 6P14P-ev due to it's increased capacity, and the fact that I have about 480+ B+ volts normally, and I could see it getting up to 500 or so if plugged into 120-125Vac.

Just had a screen melt down on one of those, and I'd been looking at the 7868/7591's on an old '67 Ampeg I got in to repair, and thought, (Somebody stop me) that tube could scream in this amp, offer more headroom, and idle while doing it.

So here I go! Are there any gotcha's I'm not seeing, other than the obvious rewiring?

I'm also going from 47mfd to 2.2 mfd on cathode bypass cap to help some flab in bass with the 15" speaker on neck pickup. The tranny has 6K6R primary, so it seems well suited for these tubes as long as I don't throw the whole 19w/tube P-P capacity at it. i'll take the cathode resistor to about 250R or a bit more if needed.

[eleventeen]

Price those 7868/7591 puppies out before you make the commitment.

If it were me, I would try to stick to 6L6s. They might be a tad oversize for the OT, and maybe you are concerned that the output  tubes would not be driven hard enough to get the kind of sound you're looking for, or that maybe the amp can't furnish enough drive. I can't answer those questions. But I *can* say you will be paying up for those 7868 or 7591 tubes.

[PRR]

> I have about 480+ B+ volts normally, and I could see it getting up to 500

That's your problem.

[AZJimC]

>Price those 7868/7591 puppies out before you make the commitment.

Ya, I had, and already got a couple on the way, but you're right. the electro harmonix are around 30 each everywhere i looked. they claim to be an accurate representation of the original.

The Voltage is an issue, I know, that's why I keep reaching for these high voltage tubes. I decided I'd like to try these, because they're 9 pin, the 7868, and Hi-Fi oriented, from what i read. hoping for more headroom on output when desired. the 7591's in the Ampeg do great cleans, so I'm anxious to see how they do. Not sure how I'll end up burning off or regulating the extra voltage, but I'll have to address it.

[HotBluePlates]

The Voltage is an issue, I know, that's why I keep reaching for these high voltage tubes. ... hoping for more headroom on output when desired. ...

To paraphrase myself from another thread:
"Which distorts first: Tube A with 18w of clean output or Tube B with 18w of clean output?"

Your amp's output power is determined by the power supply and output transformer, as long as you also use tube which support the rest of the plan (i.e., flow enough current at the voltage present).

A tube swap may give the illusion of headroom if the new tube has a larger bias voltage than the previous one, because now it takes a bigger input signal to get the same output from the new tube. If you're present for before/after, you might be fooled into thinking you have "more headroom" because the amp starts distorting at a higher volume knob setting. But the speaker will be outputting the same volume when that distortion occurs, so you didn't really gain anything.

As PRR pointed out, you're using a 300v tube in a 500v setting. So the real question is do you want 18w of power output, or do you want to use the power transformer you're using now? If you want 18w, you need a new PT. If you want to use your existing PT, get bigger tubes that are at home with 500v (and if you then have tubes capable of 50w, it is silly but maybe not "wrong" to have OT loading that chokes them down to 18-20w).

... I decided I'd like to try these, because they're ... Hi-Fi oriented, from what i read. ... the 7591's in the Ampeg do great cleans, so I'm anxious to see how they do.

The clean tone is gonna depend on your speaker and any limitations of your amp's circuit, not the output tube.

At one time or another, I've had a lot of classic amps. You know, if you set each amp's control for no distortion, then a '67 Princeton Reverb, '73 Marshall non-master 50w, Hiwatt DR504, Matchless Clubman, Vox AC-30, '55 Tremolux, '67 Super Reverb, and a Supro Thunderbolt all sound basically the same (within the limits of the speaker used). My experience was the individual character of the amps didn't show up until they were cranked up to distort. Given that's EL84, 6V6, 6L6 and EL34 represented, I think it's fair to say this shows the output tube alone isn't going to change your cleans if that's what you're looking for.

[AZJimC]

The power supply is suffering from modern voltages, the 6.3 is high. I've thought about a bucking transformer to drop it a bit overall. The tranny is, I assume, original to the Japanese amp I scavenged for this build. The rectifier is sand driven, though I do have a second 6v winding and have considered a tube rectifier, though I'd have to punch another hole.

Actually, a bit more power, than 18w, is what I'm after, and I just referred to it as more headroom. about 25w max, since I seldom ever run it wide open, shouldn't punish the OT at all. The 7868's also seem to have a nice crispness to them that I like, and a pair is comfortable at 30W, like in the Ampeg G-15. I have a PPIMV and I can get a lot of grind in the preamp if needed, and I can back it smoothly down to crispy clean when desired. I have a gain control on input that is a dual pot, half after first triode, and other half between the second and third stages. I have a BIG resistor on B+ that I can drop voltage for a power scaling type of output tube drive.

Reverb send is a 12at7, and recovery is a 6AV6 7 pin, as is the trem oscillator.

One concern, however, is that I intend to change the Trem to bias vary, and I have yet to see what kind of swing it will take to get a good trem on these tubes. The trem on input cathode will now give me a sort of clean to overdrive shift at certain settings.

FYI this is an amp that I experiment with and will likely end up never being actually finished. Space is the only limiting factor. Thanks for your input and ideas!

[PRR]

500V B+ on a 6.6K load is pretty-near a 50W-60W amp.

Pretending "it's only 20w?" and using miniature tubes is a path to dead bottles and even burnt transformers.

25W in 6.6K is more like a 340V B+. Get down in that zone, the amp can be happy.

Going from a nice 340V to 500V gives *double* (2.2) times the power. If the tubes don't die first.

[eleventeen]

" The reason I am even asking, because a lot of japanese market amps are 100V input.  I therefore believe a bucking transformer should be used.  "

True truth. You're not talking about 5% too high, you're talking about a rock solid 25% too high. You cannot ignore that.

[AZJimC]

Is this amp built for the japanese market, or is it an amp made in japan, built for export.  The reason I am even asking, because a lot of japanese market amps are 100V input.  I therefore believe a bucking transformer should be used.

I've seen that, and I have two of these, same mfg, though I forget the name. One is branded Crown, the other branded Takt. The Takt is a smaller 8" spkr amp with triode/pentode output tubes. It has a multitap trans with 100v and 117v taps. This bigger Crown has a 15" and no labeling on tranny taps, though the ones used have been used in the US for many years. The 6.3v is high, but not 25% high. I'm awaiting the bigger tubes so I can do more checking, but I'm leaning toward finding a very small transformer to drop about 5%. In normal operation with good tubes, they pull the b+ down to about 470. I only get higher when the wall volts is higher than most places. Various mods have been done with early parts to drop voltages. I found some leakage in cathode bypass cap that was pulling the cathode toward ground so that likely explains the sparker screen and redplating. Granted, it's pushing the edge at best, so I'm wanting a permanent solution. one possibility is to get a good 9 pin 6v rectifier as the Power trans has a winding for that. That could be a good solution as well.

Good call on the 100v, the smaller amp had been to lots of shops, and kept blowing tubes. when I got it I located the 100v tap still wired up while replacing the power cord. The change really was a good fix on that one.

The crown now has little to none of the original circuit, and I've added a tube, and much bigger OT, so maybe these bigger tubes will burn off a few more volts.

[AZJimC]

Hmm, I feel dumb now, or a little dumber than usual. I got the 7868's and learned the 9 pins are novar, not Mini-9 pin, so I'm gonna have to drill the chassis, no problem, and order in the novar sockets, OR, just pop in some 8 pin sockets that I already have, and order in a couple of 7591's. I think I'm going with the latter, since I also have a pair of used 7591's to pop in, plus I can stick el34 in there if I wish later. Not a lot of options with the 9 pin novar sockets. As I understand the 7868 and 7591 are the same tube with different basing. Either one should be okay with my voltage issues since they max at 550v on plates and 450v on screens. 450v plates and 350v screens are listed in run conditions, so I don't have to drop a lot to be in comfortable zone. Now the 18w amp will be more like 22w, or more if I want to swap the OP trans, but for now I'll try and run them lower than their capacity.

So, now I'll get the 8 pins installed, and I can get back to playing this amp and working on the customer's Ampeg. I'm anxious to get into it. I can always pop in a couple of novar sockets later if I wish to run these 7868 bottles. Looks like the chassis hole is about the same size. I read somewhere about using a couple of diodes to drop the heater volts a bit, since there's not a lot of room for a bucking transformer. Also seriously considering drilling out another hole for a mini 9 pin rectifier tube. I really want the compression, and that should help get the voltage back to reality as well.

[AZJimC]

OKAY! The big problem is solved. I've been thinking about how and which way I'd deal with the overall high voltage issue. I have thought about even adding a 6v rectifier tube, but that only drops the B+,and then I'd have to deal with heater voltage. I kept thinking about that extra heater winding. As I was rewiring my huge resistor/switch to drop only the plates rather than the whole amp for a lower power and more grind and sag, and trying to figure out a way to place a bucking transformer where there is no room, I thought, heck, why can't I use that extra heater winding for bucking. I thought of every reason why not, and there wasn't one I could come up with, so I rewired the incoming 123.3vac through the heater coil. Hooked one way would raise the effective voltage, the other way it would drop it. I got lucky the first time, and viola, the incoming power was now 115.8vac. Every voltage in the amp dropped perfectly down within comfortable specs for what I'm working with.

I did get the octal sockets installed, and popped in a set of 7591's, and the whole amp is much happier. Now, rather than a 500+vdc at higher wall voltages, I'm now maxing at 469 under load. I also rewired the stepping power switch (down - Off, middle - stdby, up - on) that I'm using for power drop so that up is full on, middle inserts an extra 10K resistor in screen supply, and down puts in the 2.5k side of the BIG resistor, dropping the B+ only to plates. This provides two very transparent output drops from full power. I now never drop power to the preamp, which was leaving a messed up tone at lowest settings. Now, one day, I can draw out the final schematic, since it's not so fluid once the major bugs are handled.

Bottom line, amp's happy, Jimmy's happy!