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Old 25th September 2012, 07:41 PM   #21
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Location: was Chicago IL, now Long Beach CA
Keep in mind this amp was a wreck rescued via ebay from a pawnshop, and I was the only one bidding on it. It's a good educational platform for me, and I'm saving it from the dumpster no matter what I do to it.

I think I know what I didn't like about my Marshall; irritating too-treble distortion and too little control of the tone of the distortion. The amp and speakers did only one thing, and the controls didn't do much. Even some pedals are more versatile than some Marshalls. With the two Alembic preamp channels in series, I could apply pre-emphasis early and de-emphasis late, with some distortion added in-between. So for instance I could boost the treble on the guitar in an early stage and cut it in a later stage...but the distortion generated in-between only had its treble cut. I didn't have enough independent control over the tone of the distortion on the Marshall. I'm not all that into the tone of some 'classic' speakers either, but so far I like the Blue Marvel 10's more than I expected, and I think I'll like them even more when the second bottom arrives and I close the backs, especially for practicing bass thru this rig, which works better than I expected. IMHO getting my ideal bass bite/grit distortion is even more difficult than for guitar. I would definitely like some kind of 'deep' switch or control in the power amp section, more than the current 'presence' control, but I don't know whether to put it in the feedback circuit like I assume the presence control is.

So far the EL84s seem to have a nice creamy distortion that comes on much earlier than with 6L6s (and I'm just no lover of EL34s). I like the basic character, and it's not what I'm used to. So I'd like to study that a bit. Not power supply sag or output transformer saturation. These are my first el84s and I want to hear them.

I'm also thinking about adding some kid of tone and/or volume control switches or pots between the two stages that the dirty relay inserts; some control of tone caps byapssing a resistor or pot and/or a switch for the coupling cap. I want more control over that voicing; that's what people love or get frustrated with, and why they frequently call the dirty channel 'thin'. I've got to play it more and decide whether the completely stock values should remain available, as well as new options. LIke if I add a bunch of sub-mini toggles, but all down is back to stock.

And I'm still considering wiring the valverb in direct. Which would give me another place to mount a pot on the classic 50 (instead of the reverb dry/wet mix knob on the c50, since I'd have drive and output controls on the Valverb).

But this is all still conjecture, dreams, hardly plans right now. Baby steps. I'm not home enough, and I have many projects already in progress. But I've got to decide on the layout long before I build a cabinet. For now I might just put a metal bottom on the chassis. I've got to dig the Valverb out of the garage...

If anyone ever placed the back of this amp case against a wall it would suck in its own hot exhaust. I'm probably going to make some exhaust exit downward and intake from upward and/or the side. Maybe install a center divider behind the amp and some vox-ish vents in the case top or bottom. I'll have to think about it.

The new fan arrived, so that will make it a lot more lovable when it stops sounding like an old refrigerator. The stock fan has a lot of axial play; the shaft isn't loose and doesn't wobble radially, but it can be pulled in & out with a clunk at each end of travel. It's a 117 v AC shaded-pole motor. It seems like mounted horizontally with it blowing down into the chassis the blade wants to lift up in opposition to flow but gravity wants to pull it back down, and the magnetic field tries to center it, so it rattles only when mounted horizontally blowing down, in the stock position. These types of fans often work best mouinted vertically blowing horizontally. The fan would actually work better and quieter mounted vertically to the rear panel with a labyrinth behind it, with the air exiting in some direction away from the air slots by the tubes. It is a nice heavy housing, made in America. Sleeve-bearing fans (like the stock one) are usually quieter than ball-bearing. But the new fan will work a lot better in the stock location and last longer. It is very quiet for a ball-bearing fan, and a bit more powerful though the speed and airflow (on full wall voltage) is the same, so I think it's not ever going to stall or have problems restarting from a stall. It draws more current, so when it's in series with the speed-reduction resistor there will be a bit more current thru both but the voltage will divide very differently. I might add a 2-speed switch, or I might wire it to more throws on the half-power switch. What I eventually do depends on whether I need the fan space for transformers, in which case the fan will get relocated.

Some air flow holes in the bottom of the MDF case near the peak of the tube top tits, right near the stock slots, might also be protection against somebody placing the back of the amp against a wall.

I'm still leaning toward an output transformer upgrade and dual power transformers.
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Old 26th September 2012, 05:12 PM   #22
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Anybody ever use a HI-FI output transformer for a guitar amp? What were the results? Mercury Magnetics claims all kinds of horrors, but I conjecture that the guitar amp transformer input impedance loads the tubes differently, and that makes more difference than the "sound" of the transformer itself. I imagine guitar amp OP trannies were originally just economical HI-FI transformers of their day (LO-FI??). Of course, I could be all wrong, the guitar trans can be purposely narrow-band and more resonant in some "pleasing" range. I once jacked my Alembic preamp into dad's class-A/B Citation II with 4 KT88's into a pair of SVT bottoms and it was nice...for bass guitar. Insufficient data.

After some advice to do so, I've started looking at other (cheaper than Mercury Magnetics) output transformers that might still be an upgrade over the stock one. And by "upgrade" I mean suiting my tastes better. But my fear is that my tastes are for 4 6L6s and I need to give 4 EL84s a chance to grow on me, or I may need 2 amps for guitar, to be a happy camper. Anyway, I've gotten interested in the Edcor 60-watt output transformers, which can push more and lower bass. Seems to me the input impedance would be really critical to the general sound and character of the amp, with a lower impedance making the tubes deliver more current. I wish I still had a 'scope, there's a lot of things I would like to look at and measure. Anyway, the only things Mercury Magnetics will say about the specifics of their transformer is that it's 4K input impedance, and the output impedances. Edcor makes a 60-watt with 4K input impedance, and another different one with a 4.2K input impedance. The 4.2K might be easier on the output tubes and a bit cleaner, but might lose the essential character of the C50 and reduce the output and make it more like a heavy C30 with worse tone. Might make it a bit cleaner, a bit more hi-fi and more like what I'm used to but not necessarily great guitar tone anymre. In the opposite direction, there's also a 3.5K. I don't have the data sheets on EL84's and I've never worked with them before, don't really know what they can tolerate. I see Blue Guitar mods bumping up the voltage, but I have no idea what king of current they can deliver at what voltage... I've got more research and learning to do if I want to avoid expensive experiments.

So what's the disadvantage, besides weight, of using a transformer that has too high a power rating, like using a 60-watt or even 100-watt transformer? Too bass-heavy? I came to Edcor because so many mfgrs make 50-watt transformers then jump to 100-watt transformers, like the guitar amp market categories, whereas Edcor makes this sweet 60-watt series. I'm assuming I can just ignore the screen windings, but that does imply this is a hi-fi transformer and not a guitar transformer.

ENZO, you really seem to have the technical knowledge and a lot of practical experience, and also understand guitar amps as musical generators rather than just signal amplifiers. So, what do you think about trying the 4K 60-watt transformer? Does 4K sound right to you? It's only 70 bucks, so I could buy and try 5 different edcor transformers for what the Mercury Magnetics costs. But this is a different animal; the Mercury Magnetics transformers seem to give better treble clarity and like most guitar transformers they still limit the bass somewhat and limit the exreme treble and seem to resonate some in the high mids. This 6.75 pound 60-watt Edcor will carry a lot more bass. Will it give the "shimmery bell-like clear and disctinct treble notes even with heavy distortion" that the Mercury Magnetics claims? I haven't a clue. It might do exactly what I want. I'm inclined to install two big heavy-duty molex type plugs, one for primary and another for secondary, and try the Edcor for $70. If it's not any better, I can plug the stock one back in, sell the Edcor, and buy the Mercury Magnetics to compare. Is there some compelling reason to stick with 50 watt transformers? Is it ridiculous for some reason to consider a 60-watt OP tranny for 4 EL84's? Has my perspective been skewed by looking at too many class-a designs where the tranny has to be enormous to handle the DC offset component of the input?

EDCOR - CXPP60-MS-4K
EDCOR - CXPP60-MS-4K
http://www.edcorusa.com/Content/DWG/...125-200-0X.pdf

Last edited by cyclecamper; 26th September 2012 at 05:21 PM.
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Old 26th September 2012, 05:20 PM   #23
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If you get the input impedance of an output transformer off by a little, can you compensate the loading on the output tubes somewhat by varying what output impedance speaker taps you use? Obviously that under or overloading of the transformer is reflected to the output tubes too...

Edcor also makes a 70-watt output transformer with a 3.5K input impedance. Unfortunately it only has one output, without additional taps.

Last edited by cyclecamper; 26th September 2012 at 05:34 PM.
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Old 26th September 2012, 08:01 PM   #24
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I read an interesting paper about different core materials for output transformers. One interesting part was a rule of thumb for steel-core trannys, estimating how many watts a HI-FI transformer could handle with acceptable distortion and frequency response based on what it weighs. It ends by suggesting doubling that weight if you really want low distortion. Based on that, all guitar amplifier transformers are wildly undersized. But this sure ain't HI-FI is it. And maybe I'm all wrong, and output transformer limitations ARE more important to good tone than I give credit for? I've got lots to learn about guitar amps...
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Old 26th September 2012, 11:01 PM   #25
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I believe Hartley. I've met the man, and saying "he's forgotten more than others have learned...." would be accurate, except he hasn't forgotten anything. He can rattle off the specs of any obscure tube you might mention. He's done extensive research, dating back to the '60s, on what makes guitar amps sound good. He has often tried to target niche markets, so Fender and Marshall fans don't always think of all his amps as successful, but talk to some guys in Nashville about Peavey. You'll find them in every studio.

Anyway, yes, I think the output transformer's non-linearities have much to do with a "toneful" guitar amp. Tubes can clip, but we know what fuzz is. We can do that with transistors and diodes. There's more to a tube amp, and getting just the right distortion from the OT is a big part of it, IMHO.

Sub-hifi-standard power supplies are another big part of the subtle nuances a tube amp can produce. Think of all the jazz guys and steel players that want clean, clean, clean, yet still insist on tube amps.


And yes on the input impedance output tap thing. If you pull two tubes from a four tube amp to cut the power, you use different taps to compensate. (Often done on 100 watt Marshalls and Classic 30s)
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Old 27th September 2012, 01:47 AM   #26
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Yes, on the Classic 50 I can pull two output tubes, on the classic 30 something is in series...good point, with two parallel tubes pulled the transformer input impedance should realy be half...but the mismatch makes it quieter and easier on the tubes, so no harm done, but the tubes are delivering less current and perhaps sound different in half-power mode too. Interesting point you make.

I'm going to play thru it a bit more before I do anything. Wonderful oportunity right now, as nobody's home. But for that cheap I think I'll install plugs and try the Edcor, as a learning experiment if nothing else. Maybe I should be asking these things on a guitar/amp specific forum or the Peavey forums?

SO, anyone: for matching 4 EL84's, should I go for the 4K or the 4.2K primary? Any clue why they offer both with specs so similar?
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Old 27th September 2012, 06:23 AM   #27
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You should probably try asking in the Technical Section on The Gear Page. We have some pretty sharp people here, but there's more there. Definitely guitar oriented.

My impression is, the guys at the Peavey forum, while they know their stuff, aren't as adventurous as you are.


It's the heaters that are in series on the 30, but you can fix that by breaking some pins off old tubes so they only link the heaters. That is covered on the Peavey forum.
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Old 27th September 2012, 04:55 PM   #28
Struth is offline Struth  Canada
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Hi Guys

The advice above about checking out the amp prior to making any mods is something everyone should take to heart. The PT and OT are the most expensive components, so best to make sure they work properly before investing any time in the rest of it.

Assuming they are okay, you can get a fuller tone out the iron than PV does by doing some very simple things. If you are a fan of EL-84s, you owe it to the amp to treat them properly. Add individual screen-stops for each tube, of at least 1k-5W - 2k2-5W is even better as EL-84s have very sensitive screens. This will protect the tube when over-driven.

Grid-stops could also be added/increased in value to help control some of the harshness of the 84s. Skewing the drive slightly will enhance even harmonic content and give a richer sound.

As far as subbing Mercury Magnetics parts: DON"T. They are a complete waste of money. I've heard more stories than I can remember about players asking for MMs to be put in their amps, only to have them removed because they did not sound as good as the original or than a generic replacement.

MM is highly unprofessional in how they deal with potential customers. A fellow I know was going to use their OT with a toroidal PT. The MM dude went ballistic claiming that the sound will be all wrong and there is no way to get the full MM magic from their OT without also buying an MM PT. What a load of crap!

You only need to replace the OT if you want properly extended bass, or wish to have the sound be more consistent from quiet to loud. PV always uses undersized OTs, which is a common design approach in instrument amps. The assumption is that a guitar is a midrange instrument so why have bass response? That was okay in the days of only clean playing. With modern expectations of tone, relying on the OT to bandwidth limit the sound also requires that your hearing suffer with the SPL. The output stage contributes to the tone at ALL loudness levels. So, if you had a master volume preamp you can still hear the differences of all the tubes installed but be able to dial the loudness up/down independently.

Frankly, I don't like EL-84s - at least not how everyone uses them. I would dump them as there are no subs for this tube. With octal-based tubes there are several plug-n-play types with the same pinout, so you can mix and match types for whatever tone you like. Way more versatile.

Since you seem intent on modifying most of the amp anyway, why not replace the PCB with an eyelet board. This will allow easier tinkering and eliminate all the service compromises of PV's folded hard-wired boards. That looked good on the computer screen but you go in to repair one problem and create seven new ones trying to wrestle the board in and out. With a hand-wired assembly you don't have that issue.

As the PV fan suggested, if gutting and changing everything is the plan, it may be easier overall to start with something else. Why not start from scratch? Hammond offers great value in their "classic" PTs and OTs. Heybor is also very good and both of these companies have knowledgable and decent people working there. Hammond also has generic chassis you have to drill yourself - not ones designed to fit a specific vintage cabinet, or with predrilled control compliments to limit what you do.

There are also excellent resources to learn about circuitry and proper wiring. "The Ultimate Tone vol.3" shows proper wiring for low noise and best note articulation. See my site for a detailed description.

Have fun
Kevin O'Connor
londonpower.com
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Old 27th September 2012, 07:01 PM   #29
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Yes, eventually I'll build a bass amp handwired on turrets with 6L6s or KT88s in good sockets. This 'rescue amp' will eventually become a second amp. The 12AX7's are chinese, and the EL84's are Mesa, and all are in decent condition. This is just a low-bucks project, with considerable potential.

The C50 preamp actually does a lot of what I like, I just want more variety and control. The 'dirty' channel automatically does one of my favorite tones, by boosting the treble between the stages it inserts, just before most of the distortion is added. But I want more switches and knobs to achieve more variety. The 'middle' midrange tone knob does almost nothing, and the tone knobs interact too much. The clean mode could use more bass and doesn't have any way to share in the treble that the dirty mode has in excess. The 'bright' input doesn't work well with my guitar pickups that have a LOT of inductance. To me, the amp has two nice 'presets' but could easily be much more versatile. I'm going to keep its basic dual characters, but make it more versatile.

All in all, I like the C50 a lot more now that the new fan is quiet! I tapped the screw holes in the new fan. I didn't use any rubber gasket or silicone, though there are anti-vibration mounts for computer fans the same size. I used some blue Loctite. Besides the rattly bearings in the old fan, the stamping machine that did the big hole for the fan airflow was not indexed to the chassic steel the same as the stamping machine that made the mounting holes, so the fan was partially blocked on one side and made excessive noise with every pass of the blade. Rather than file the big hole larger, I just slotted the mounting screw holes and slid the fan over a bit; much quieter now. Since it's a seperate head and not a combo being vibrated to death, I also cut a bunch of the tidy cable-ties and spread the wires. They're not supported as well, but just that reduced the hum and I can turn up the presence and gain a bit more before getting squealing oscillation now. I tried cleaning the pots, but the only openings in the pots face the wrong way, so I have to remove the board. If I have to remove the board, I'll just replace all the pots.

In very light use, the power transformer gets warm but the output transformer doesn't. If I convert to two power transformers, one will mount where the fan is now, and I'll relocate the fan; since I have to relocate the larger reverb tank there's plenty of new places to put the fan.

Grid-stoppers, eh? I'll look into it. Thanks for the advice.
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Old 28th September 2012, 06:44 PM   #30
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It has some bandwidth-limiting grid-stopper devices. I'll put in some switchable bandwidth-limiting devices for the first stage.

I ordered the Edcor 4K-primary output transformer. It may take as much as 5 weeks, as each is custom-built. Price is excellent, compared to Mercury Magnetics. It's about a half-inch larger in all dimensions than the stock Peavey OT.

Now I need some advice for some changes to the power supply. First I need to find some affordable chokes. And then I need to make some measurements. Instead of an overpriced Mercury Magnetics power transformer upgrade, I'm considering one transformer etc. for just the 4 EL84s and another smaller one for everything else. But I've got to watch the voltage in action; the stock tranny gets hot but it might suffice if I replace some resistors with chokes and add some caps. Mercury Magnetics replaces R58 400 ohm 5 watt resistor with an 8 Henry choke. I'd want something similar, and to also replace R59 22K 1 watt resistor with a small choke, and R60 10K 1 watt resistro with a small choke.

I don't understand why there's a 100 ohm 5 watt resistor (R46 on the schematic) between the two screens of the paralleled push tubes and another similar 100 ohm 5 watt resistor (R55 on the schematic) between the two screens of the paralleled pull tubes. Is this normal?

Last edited by cyclecamper; 28th September 2012 at 06:47 PM.
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