2 Qs... 1961 Motorola Stereo EL84

[spiggy]

Hey folks, first post... I'm a guitarnuts fan - just shielded and star grounded my crappy Squire II Strat! Amazing.

OK, first some details... I got a 1961 motorola tube amp, stereo, a screaming 5 watts per channel, using one EL84 per side. All point to point wiring. I got this to use as a power amp for my Rocktron Voodu Valve preamp, which is one of those hybrid digital jobs, that uses a 12AX7 preamp tube for warmth and some buzz, but does the rest via DSP (a motorola chip actually). I've been pretty happy with the results so far, but have some questions. (I apologize in advance, I'm a fairly tube-newbie and the only electronics experience I've had is swapping out pickups, etc.)

1: The Motorola power amp (just a naked chassis) has one EL84 and one preamp tube per channel. I was told by the person I bought it from that he had disconnected the preamp from the hi-fi unit, and that now it was only a power amp. He said that the preamp tubes were used for voltage regulation only. But then I talked to someone else on the phone (whos actually an amp tech) and he said that I am undoubtedly getting preamp distortion from the preamp tubes as well as any power tube distortion. He wasn't working on my amp at the time and hasn't seen it, we were just chatting. But this was kind of disheartening to me. I'm fairly new to tubes, a couple of years, but I know the difference in sound between preamp tubes and power tubes. Preamp tubes when saturated make a buzzing sound like mosquitos, and power tubes have a smooth harmonically rich distortion, and compression. Now with the voodu valve I can dial in as much preamp buzz, as well as any processed distortion, etc that I might want (or not), but as far as the power amp, I don't want any preamp tube distortion, I just want to saturate the power tubes with the signal from my Voodu Valve, be it clean, buzzy, whatever. So, whos right? Is it possible the preamp tubes are just being used for voltage regulation and do not affect the tone of the signal? If they are distorting and mixing in with the signal, is there any easy way to get them out of the picture?

2: I bought the Motorola to use as a power amp, because it was very low wattage and also stereo to boot. After some reading on GuitarNuts and Amptone I realized that low wattage was the way to go for my home studio. I wanted to be able to get that overblown saturated tube sound at moderate volumes. But, after receiving my Motorola I was simply amazed to discover how loud 10 tube watts can be. I'd prefer to use both channels of course, as my preamp makes a lot of use of the stereo spectrum, but even one channel at 5 watts is pretty damn loud when maxed. I looked some at attenuaters, and the Weber Mass were looking good, but I'm poor and can't hardly justify buying two pricey attenuators for a measly 5 watts per channel. Mr. Weber used to have instructions on his site for making an L-Pad but removed them. I looked online and found a variable (potted) L-Pad that handled 15 watts, for like $7. I emailed Mr. Weber and explained my situation and asked if the $7 L-Pad would work as a low budget solution. He said that it wouldn't because tube amps use variable resistance. Now, the last thing I want to do is fry the ancient transformers on this amp, but is he telling the truth or just trying to promote another sale? I have a feeling if I saw inside a Weber Mass I wouldn't be amazed at all of the gadgetry. Probably the speaker driver, couple of pots, another resister or two? He said that at such low wattage I probably wouldn't notice a difference in tone between the Mass unit and a more traditional type of power dump, but even they were expensive for what I want to do here. Is there any cheap way to rig a couple of variable attenuators up with it the amp? I'm hoping to rack mount the motorola, attaching it to a rack-able computer/networking shelf, adding a wooden face, plexiglass window, and it would be cool to mount a power switch and two attenuator pots on the face as well. So anyway, as far as the attenuation goes... any thoughts on this?

Sheesh sorry for the looong first post. I'll hold off on any additional questions till next time. Lol. Any help would be greatly appreciated. Nice to have found this forum here. Thanks!

[UnklMickey]

spiggy said:

1: ... So, whos right? Is it possible the preamp tubes are just being used for voltage regulation and do not affect the tone of the signal? If they are distorting and mixing in with the signal, is there any easy way to get them out of the picture?



2:...Is there any cheap way to rig a couple of variable attenuators up with it the amp?...

hi Spiggy,

Welcome to Guitarnuts 2.


1 - since the EL84s require somewhere between several volts and tens of volts signal on the control grids (input) to drive them to full output, it is not likely that they designed the power amp to require that much input.

the "pre-amp tubes" (maybe 12AX7 or 12AU7 or 12AT7, etc) are most likely twin triodes. at least 1/2 if not both halves of those tubes are being used as drivers.

it is possible (although i doubt it) that the other half of the tubes are being used as voltage regulators. if that were the case you would almost certainly also have some gas filled tubes ( perhaps 0A2s or similar) as voltage references.

to insure that your output tubes saturate long before the triodes do,

you could decrease the supply voltage and bias them "hot"

(idling closer to saturation instead of midway between saturation and cut-off)

you would also want to bias the triodes so that they would reach saturation before cut-off.

since it will be likely that the grids will go positive and you will have some grid current, using "grid snubber" resistors would be prudent.

you will probably end up with a better situation if you move in this direction, and re-design the amp for about 1/2 watt per channel. (about half the volume you have now.)

this means that you will need to come up with a schematic for the power-amp, a voltmeter, and an oscilloscope.



2 - using an attenuator will be much easier than modifying your amp.

it will not cure any possible short-comings of your amp's design, like reaching cutoff before saturation (a less "pretty" distortion)

unless you want to get VERY complicated in the design (i won't be helping with that), the most readily available variable attenuators are call L-pads.

they are designed to keep a fairly constant load on the amp even when they are set for maximum attenuation.

pay a few extra dollars and get one that is rated for a much higher power capacity, for instance 50 watts. at the same power level, it will still radiate as much energy (heat) as the smaller ones, but at a lower temperature.



unk

[spiggy]

Hey, thanks for the great reply unk!

As for the first half of your reply, I'll have to take it to a local amp tech to see if he understands your strange tongue and can translate it into some dialect of heathen English for me. Lol. Some great info there. I'm fairly pleased with the overall sound of my setup, but I feel it could be better. I may commission this guy to do some work for me anyway, so I'll throw your comments at him and see what he says.

The second half of you post... so the cheap hi-fi L-pads will work with a tube amp despite what Weber said? (As long as its the right resistance (8 ohm) and an appropriate wattage?) Again, he said they wouldn't because tube amps required variable resistance. I'll go for the 50 watt L-Pads, but will that affect the range of attenuation, since each channel is only 5 watts? Mounting two of them to the face of the housing I'm going to build, maybe finding some chicken-head knobs to fit, etc, is very appealing to me. I'll hafta post a pic of my custom rack mount job if/when it ever gets done. Thanks again for all the great info. Cheers.

[spiggy]

...for instance, if I just mounted one of these AT50Ds to the face, that would take care of it and shouldn't fry my transformers if properly connected? www.starkelectronic.com/12.htm

Thanks

[UnklMickey]

hi Spiggy,

the L-pads WILL work with your tube amp.

they will sound a bit different from having a speaker connected directly to the output.

the more attenuation you use, the more resistive, rather than reactive, the load will be.

it certainly won't harm the amp.

i wouldn't describe the tube amp as needing a "variable resistance".

because of the voice coil moving within a magnetic field, the speaker will look like a complicated network primarily comprised of resistors and capacitors.

some speaker emulators are built like that.

others use a weighted voice coil and magnet assembly.

that requires an enclosure, so you don't hear the sound produced by the voice coil.

again these lack interactions will affect the tone, but not be harmful to the amp.

long story short: i think for the money, you will be fairly happy with the results from an L-pad.



regardless of the power capacity of the L-pad, the range of attenuation will be the same.




good luck,


unk

[UnklMickey]

the AD50D should work just fine.

make sure if you put it in a rack, it has some place for the hot air to "breathe".

you shouldn't need a fan or anything, as long as the air is free to circulate.

if you enclose it in a box, with no vents, it could get a bit hot.

[spiggy]

Okay, unklmickey...

I wanted to wait until I had some more info before following up on this post. I received my AT-50D L-Pad in the mail a week or so ago. Just temporarily taped it in place until I could test to see if I had it installed correctly and if smoke would start pouring out of my amp. The experiment was most definitely worth the measly $20 investment. I've played with it installed 4 or 5 times now for about 30 mins or so each, and have some mixed feelings in general. I think the main thing that I discovered was how much preamp gain I am actually getting from the Motorola. It was so much more obvious while playing with everything maxed out. It even made some of my cleaner patches sound like death metal, Lol. I did design a new patch on my preamp which was tweaked while the Motorola was maxed out. I used hardly any gain at all from the Voodu Valve, just kept tweaking the pre and post parametric EQs, etc until I shaped the tone of the motorola's distortion to my liking.

I re-read your first post to me, and this time it didn't sound quite so obscure - I feel like I actually understand a wee bit of it.
"if that were the case you would almost certainly also have some gas filled tubes ( perhaps 0A2s or similar) as voltage references." I'll hafta get back to you on that, as I'm at work at the moment. Heheh. There is one metal jacketed (5Y3?) rectifier, and one other metal jacketed tube that I'm unsure about besides the pair of pres and EL84s. I'm not sure what the other one is, but I do have a schematic glued to the top of the amp so I'll let you know out of curiosity. After hearing the amp being pushed as hard as my Voodu Valve could push it, I must say that I think at some point in the near future I may have someone take a stab at your recommendations for redesigning the amp by dropping the supply voltage, re-biasing the tubes, and replacing the appropriate resisters with grid-snubbers. I was planning on having a local amp tech do a cap job on it anyway, so I'll see what kind of quote he'll give me for the whole shebang.

On another note, I talked some tube to the fellow at JJ/Tesla Eurotubes (who was very nice and extremely helpful such as yourself) and mentioned the rectifier. He said that the 5Y3 was way overkill for the 5 watt per channel EL84s and that there was no way to push the amp hard enough to get any recto sag. He said that I could use a diff rectifier (I forget which one at the moment) but that I would have to totally redesign the amp. Would you agree with this? Is this something that would be easy to do in conjunction with the other modifications you recommended to the amp, or would that require a lot of additional work? I wouldn't mind getting a little sag when totally saturated (the amp that is, not me).

As far as the l-pad attenuator goes, I think it probably did the job just as well as one of the more expensive load dumps (maybe not the ones that use the actual speaker drivers). There is definitely a trade off in tone as more attenuation is applied. I was surprised to find that with the dial turned all the way to the left, the level was just barely audible. But with this much attenuation, you can imagine that the tone was extremely dull and flat with no life in it. In order to get a sound that had minimal (acceptable) tone loss I had to turn it up to the level of say a very loud TV, but I could max the amp out without having it "piss your neighbors off/call the cops" loud. It is definitely a BIG help in that regard, as now I can dial in some sounds for recording that I couldn't have gotten before without everyone else knowing about it. You weren't kidding about it throwing off some heat were you? Sheesh, after playing for some time - the L-Pad got VERY hot. So much so that it was uncomfortable to hold in the hand while turning the pot. I'll definitely have it well ventilated when I go to mount it with my amp into a rack.

Sorry for the long rambling post again... I get very excited about sound, and I'm starting to learn some about tube amps in the process. I really, really appreciate the help. Thanks again.

[UnklMickey]

hi Spiggy,

good to hear from you again.

if you have the time, let me know what rectifier the folks at Eurotubes suggested.

as far as tubes with an octal socket, i thought the 5y3 was about as wimpy as you get.

if they have something in a 7-pin or 9-pin that uses a 5volt heater, and is sufficiently wimpy, i'd like to add that to my knowledge base.

i never looked at rectifier characteristic other than 5y3, 5u4, and 5ar4, so there's a ton on the subject that i don't know about.




thanks for all the info.

i'm sure there will be many readers that will find it useful.



unk

unk

[spiggy]

Howdy unk!

Wow, thanks for the quick reply man. I couldn't remember if the Eurotubes email was in my home email or work mailbox. A quick search of my inbox here and I found it. (I work in IT for a state agency and have some slow days now and again, Lol) Here's what Bob at Eurotubes had to say concerning the rectifier...

"The 5Y3 is not directly in the tone circuit. All it does is convert AC to DC voltage for the power tubes and will out last your other tubes 20 to 1 so if it works your fine. Using a 5Y3 for two EL84's is WAY overkill so even the weakest 5Y3 will still not get sag. I know your next question... The rectifier that would sag would be an EZ81 but rewiring for this would be a BIG Job."


So, after re-reading that... I may just hold off on that one, maybe indefinitely. I'd much rather mod the amp to get rid of as much preamp distortion as possible than have tons of work done just to get a little sag. I'll hafta see where my finances stand after my next check before I commission any work from the local amp tech, but I'll be sure to let you know how I make out after it's done. I'll also post a pic or two when I finally get my frankenstein rack mount job completed. Thanks again for all the help.

spiggs

[UnklMickey]

i looked up a data sheet on that tube.

should have lots more series resistance.

it will be a much bigger job.

not such a big deal to change to a 9-pin socket.

but this tube requires a 6 volt heater supply.

and that should be separate from the 6 volt supply that's running the other tubes.

it's doubtful the power transformer in your amp has a 5-volt winding and 2 6-volt windings.

so that would mean adding another filament transformer.

do-able, but it could get a bit klugy.



getting rid of the preamp distortion should be a fairly easy concept to deal with.

have your tech review the architecture of the amp and make sure there is plenty of headroom throughout, until the output section "runs out of steam".

of course as you continue to overdrive the output into rediculous levels of distortion, you will eventually start clipping in the pre-amp too.

it would seem to me, the easiest way to have the output section be the primary source of distortion, would be to decrease the supply voltage to the EL84s.


unk

[sumgai]

So, would this be a good time to ask a question?

How does one make a rectifier sag?


sumgai

[UnklMickey]

gravity and loose straps.



it's not exactly the rectifier that "sags".


the effective series resistance of a valve rectifier allows the power supply to "droop" or "sag" when heavily loaded.

so after you reach a certain point, the more current the amp demands, the more the PS voltage droops.


as it turns out, this was a GREAT time to ask!

it now occurs to me that since this is a single-ended output, the AVERAGE current in the output stage will remain fairly constant, regardless of the size of the signal.

so trying to design in some sag will not be straight-forward at all.



thanks,


unk

[sumgai]

unk,

......
gravity and loose straps.

LS/MFT?

.....
thanks


unk

That's what I'm here for! At least until they let me back out in the play-yard with the other kids. ;D


sumgai

p.s. Spiggy - tubes don't sag, capacitors sag when they aren't large enough to hold the load. Best be getting a second opinion on how to design for intentional sag from the power supply circuit, not the rectifier component of that circuit.

[UnklMickey]

sumgai said:

unk,

......
gravity and loose straps.

LS/MFT?

so you remember!

[sumgai]

unk,

LS/MFT?

so you remember!

Yeah, but I never inhaled, OK? ;D


sumgai