Univox 1221 Rebuild-Mod

[silverfox]

http://www.freeinfosociety.com/electronics/schematics/audio/univox1221.pdf

I thought I'd wait on the questions until I was ready to "Rebuild-Mod" but can't since this looks so interesting.


PRR- On first glance it appears from a light white powder when I removed the output tubes that there has indeed been and arcing at some pointl Haven't pulled the chassis yet. The plastic tube sockets will go. Fortunately I've bought all sorts of parts I didn't need at the foreseeable future so they're in stock- ceramic base.


Question: I researched this and got conflicting answers-  Will the Sovtek 5881 WXT work as a replacement for the 6L6GC output tubes in this amp. I've got the 5881's (When I first studied the schematic I thought the 620 volt plate voltage was a typo)



What is the benefit of using the 6AN8 or really, what benefit in the use of the pentode and is that a gain stage or coupling it is being used for?

If it were jumpered or rewired the input stages of the two channels look like the plexi dual preamp inputs.

Silverfox.

[eleventeen]

That link doesn't work for me.

[HotBluePlates]

http://www.freeinfosociety.com/electronics/schematics/audio/univox1221.pdf

I can't get that site to load.

But we have the same schematic in possibly higher quality right here at Hoffman's.

On first glance it appears from a light white powder when I removed the output tubes that there has indeed been and arcing at some point

Arcing usually results in dark carbonized tracks. Whitish powder, especially if on the metal chassis, could be oxidation of metal and is pretty common on older electronics.

Will the Sovtek 5881 WXT work as a replacement for the 6L6GC output tubes in this amp. I've got the 5881's (When I first studied the schematic I thought the 620 volt plate voltage was a typo)

Almost certainly not.

Russian tubes with American-like numbers (6L6, 5881, 6550, etc.) are Russian tube types which are relabeled with number the American market is familiar with. But there is no guarantee these tubes meet those ratings. For example, the 6P3S-E, often relabeled as a 6L6, has a maximum dissipation of only 20.5w (3rd line of numbers on the 2nd page). A different data sheet suggests a maximum plate voltage is 400v.

I'll admit I'm unsure if the 5881WXT is indeed a 6P3S-E, though some information from other here suggest it is. That particular data sheet doesn't seem to give a max plate voltage, but I'd be uncomfortable applying more than 450v or so.

I think you need real old American 6L6GC's for this amp, unless you completely rework the power supply to drop the operating voltage.

What is the benefit of using the 6AN8 or really, what benefit in the use of the pentode and is that a gain stage or coupling it is being used for?

The style of phase inverter that is being used in that amp provides no gain on its own, so it needs a stage ahead of the inverter to develop a big enough signal to drive the output tubes. Prior to the pentode portion of the 6AN8, there is only a single triode, followed with volume and tone controls. The controls toss away some amount of signal, so the pentode is needed rather than a triode to develop sufficient signal level in a single gain stage.

If you got rid of the 6AN8, you'd now need 2 triodes to replace it, one to drive the phase inverter and another to make up the signal loss of the volume and tone circuits.

The Univox just took a different design approach in using the 6AN8's in the 70's than the approach used by Fender, which was largely set in stone by the early 60's. The 6AN8 was a later development and was used in products by a number of manufacturers around this time.

[PRR]

The 6AN8 is fine. Leave all that alone *or* do a total re-design.

IMHO, the amp was *designed* for 6550/KT88 (or just-maybe a good grade of EL34). But the final budget wouldn't stretch that far (remember: Univox never commanded the sale-price of a Fender or Marshall), so 6L6GC were tried; and apparently lasted long enough so the amps didn't come-back too soon.

Don't fork-around with "6L6 workalikes". Few amps beat the tubes as hard as this. You'll just melt valuable glass, maybe fry a transformer. Top-grade 6L6GC or EL34, or very preferably 6550/KT88.

Add small cathode resistors and check the bias. At 660V, 32mA is as hot as any of these tubes should run. 25mA is a better target to allow for drift in tubes and line-voltage. To adjust the bias, change the 10K resistor at the "-33.5V BIAS" node.

Cathodyne cathode is 74V, not 7.4V.

12AU7 may have to be "selected". This scheme depends on the very-low-voltage behavior of the tube, which the factory does not worry about. If sure you have wobble on the grids, but trem action is weak or none, try different 12AU7, new-made and golden-age, try AT7 even AX7.

[silverfox]

Since I'm apprenticing the tube amp field, I'm getting the feeling here that it may not be a bad option to complete the EL34 based power amp-power supply I put together and fit that in this chassis (it's on perf board so real easy to remove). Then, mode the two channels to a Plexi style design. Keep the reverb since it sounds great and use the remaining sockets for 12AX7 based effects channels.

As it stands it will need a cap job and replacement of the output tube sockets.

As mentioned, the envelope pushing high voltage on the output tubes and consequential limited life span kinda work opposite to the direction I'm going with my designs. It seems that the margin for the caps is pretty close too. With the EL34's I would also have the option to run 5881's if desired.

I'm suspicious of the Iron. I just doesn't look like 100 watt and barely 50 if that. The wire colors match and everything else looks good. Just undersized.

I checked the link again and it worked for me. Don't know what's up with that but Dougs sch seems to be cleaner.

The tremolo works pretty good as tested with- "House of The Rising Sun". Just not something I use.

Silverfox.

[PRR]

> high voltage on the output tubes and consequential limited life span

There's nothing wrong with 600V *on the right tubes* (and sockets).

I got 750 Watts of 600V tube-amps which were being retired from a film-to-video application. F2V needs a speed-shave (this was before frame-store) so the projectors were fed 59.9Hz of 120V AC from large amplifiers. These amps had been in daily service for a decade.

These amps ran 8417, which was a cheap hotrod of 6550, and today very rare.

I've also had an Ampex 6550 servo-amp from the video-tape machines which covered Eisenhower's inauguration. GOOD sockets and all tubes worked great.

Gennie 6550/KT88 on clean ceramic will handle 600V E-Z.

Note also the 300V on G2, which helps a lot.

> suspicious of the Iron

Yes-- I think this company used more modern iron techniques while Fender/Marshall clung to older formulas. Those 300 Watt Bogens had even smaller iron (per Watt) and measured terrific.

If you rip out the circuit, and rip out the iron, and change the tubes... what's left? Knobs and Naugahyde. I submit that a minimally molested Univox is, or may someday, be worth more than a chop-job.

[silverfox]

Point taken. I powered up my other amp project last night so I may be close to done on that and the deck clear.

Observed while playing the 1221- Buried in the hum of the amps age induced decline I heard a truly vintage voice. Perhaps it was saying "help me Obi Wan Kanobi, you're my only hope".

In my limited research I did find that Unicord, a transformer company, (Univox being the instrument name), was an importer of Marshall amplifiers and a big concern they had regarding Marshall amps- under-sized transformers. Consequently Marshall amps were built with Unicord trannies.



PRR- Gennie 6550/KT88 on clean ceramic will handle 600V E-Z

            Does Gennie mean Genuine?      I searched for Gennie tubes- Not there.


So I'll follow your suggestions and fix the ground issue also- Is that the Death Cap Thing on the Neutral?

If I put 6550's in it will I have to change any components other than the tube sockets or are they circuit compatible? I'll put in an order today if I need any resistors or caps to change the output section or bias circuit.


I've got some ideas for mods but I'll at least get it up and running as is and check it out. After I get it up and running on 6550's, perhaps using the Tremolo circuit for a Dumble Overdrive or Hot tube a channel and adding an effects loop will be sufficient.


To the ready room, Silverfox.

[DummyLoad]

Gennie 6550/KT88

i believe he means Genalex also known as G.E.C.

--pete

[silverfox]

I'm listening to a comparison of EL34, KT77, 6550 and KT88 tubes found here: Orange Rockerverb 50 MkII with EL34, KT77, 6550 and KT88 tubes

One of the better comparisons I've heard, but then the amps no wannabe either.

Apparently any of those tubes will work. I guess it's a question of the filament current as to compatibility with the Univox 1221 rather than the output circuit changes. A bias adjustment too.

Now I'm finding that the 6550 draws quite a bit more current then the 6L6GC; .9 vs 1.6

Is this going to work? The schematic says 4 amps for filament current. Hmmm.

Fox.

[PRR]

> Does Gennie mean Genuine?

Sorry; Yes.



I have no experience with the stuff badged "Genalex" made today. I understand it means as much as the "Murray" badge on my shiny new lawnmower (Murray went broke a decade ago; another outfit bought the rights).

Golden-age (1950-1970) Genalex WAS great stuff; now far too valuable to use.

I do know the Sovtek 6550 that Hoffman sold a few years back was SOLID at 560V and 41 Watts Pdiss. Good as gennie made-in-USA Tung-Sol (another name that has fallen into new hands). We didn't buy enuff so he let them go out of stock.

I would not worry about the heater current.

I would *not* try to judge tones, either from clips, or in other amps not worked at 620V. Modern 6L6GC is much like a small 6550.

[silverfox]

For whatever it's worth I went with the Electro Harmonix... I chose those over the Peavey options at AES.


I listened to a couple of videos comparing the 6550 to various others and could hear a difference. Seemed to have clarity to the highs whereas the KT88's were a little darker and less clear. Same with the EL34's.

I also picked up 3 no brand 6AN8's. They seem to be getting scarce.

btw The chickie looks like she's selling "Tobacco Glassware"...

Fox.

[HotBluePlates]

btw The chickie looks like she's selling "Tobacco Glassware"...

... or blown glass shifter knobs. But I guess we see what we want to see. 

[super&plexi]

my wife said something to the effect of 'what... marital aids on your amp site....now you're onto something... how's that variac thingy coming along for my magic wand'...and with a spin of her heels she was gone. gotta love her. YES, the mind sees what it wants to. and if we're talking about those blue univox 4input bassmanesque jobs w/vvmbtp, we got a few of 'em in our shop, years ago.They were quick sells when hooked up to a good cab. Had a definite Austin Powers vibe going on lookswise, along w/ their bassman/plexi thing kranked,,, do seem to remember they ran a touch hot though. sold for about 450-500, I think in 2004. blue, w/black plexi face, and lg knobs, if that's the one. just needed a couple caps...and good to go.

[silverfox]

I don't think it's a Bass amp. All black. Definitely has a vintage rock tone to it. Lots of hum. It goes up on the bench momentarily.

My first order of business I thought would be to add a grounded cord. Then I saw a post elsewhere stating the amp is double insulated; Like a drill motor?

It appears to me the line could still short to the chassis and energize portions of the guitar.

Should I remove the ground cap on the fuse side of the line and add a three prong cord? I think so.

Silverfox.

[phsyconoodler]

Gennie sells shifter knobs and oldie style car parts.

I had a 1221 Univox in for repairs.It had the original 6L6's in it.They were all toast,so 6550's is definitely the way to go.
  If I had my way I would have dropped in a new PT.

[silverfox]

http://min.us/mLXGCvMQYCk3a   <Pics.

Here's what I've got. It definitely arced.

I couldn't get a reading from nearly any of the caps. The gray ones are Atlas oil.

I'm quite surprised I got any sound out of the amp at the residence when under test. I guess one output tube is all it takes to get an output but it didn't really sound as bad as this would lead one to believe. Single Ended but only half the waveform? I don't understand that...

The high tension lead on the output transformer got hot but it is not burnt all the up or even close. Just changed color a little near the base of the tube.

OT reads: Red common, Yel- 217 ohms, Org- 234 ohms, Yel to Org 455. I know that won't mean much but at least it's not open and above the chassis the wires didn't look hot.

When I touched the orange lead it broke off the tube socket. This guy was very lucky it lasted as long as it did.

Any consensus? Perhaps I'll wire the OT to a Variac and check the relationship. What should I look for?

btw- I have reached the point where you've got to treat these amp purchases as if you're dealing with a used car salesman. From now on if they won't let me take the chassis out of the amp and inspect it I won't even consider a purchase. This guy had to have known something was seriously wrong with the amp having played through it and suddenly it don't even sound close to what it used to.

Perhaps, and I say perhaps since there was no indication anything like an arc had occurred while I was testing it- Perhaps it did this when I plugged my effects box into the input. Since there is no indication on the head regarding impedance, it was likely run at the wrong impedance. I'm feeling like the apprentice that's sent to get a bucket of steam or a left handed monkey wrench.

Silverfox.

[HotBluePlates]

Should I remove the ground cap on the fuse side of the line and add a three prong cord? I think so.

Yes, that cap will eventually fail, and it's too old to be a class Y cap (which is designed and rated to run from line to ground, and fail safely if it ever fails).

... I guess one output tube is all it takes to get an output but it didn't really sound as bad as this would lead one to believe. Single Ended but only half the waveform? I don't understand that...

Nearly all push-pull amps are "class A" until you slam them with a big enough drive signal. Meaning all output tubes conducting all the time. With a large enough input signal, each side may be driven to cutoff during a portion on the input signal cycle.

So if you test at low volume, no output tube cuts off. You could have 1x 6L6 in a Twin Reverb and still get decent sound though power will be much reduced. Crank it loud enough, and things will get nasty.

OT reads: Red common, Yel- 217 ohms, Org- 234 ohms, Yel to Org 455. I know that won't mean much but at least it's not open and above the chassis the wires didn't look hot.

Look again at the schematic I linked (maybe yours is the same; I couldn't get the site to open).

Red is the OT center-tap; Orange and Yellow are either end. You measured largely the same resistance from Red to either end, so I'd call the OT good (until you prove by applying a signal that it is not good).

When I touched the orange lead it broke off the tube socket.

Solid-core wire? Stranded?

Should be stranded for a transformer. But if solid-core, those don't react well to movement and will eventually break.

Perhaps, and I say perhaps since there was no indication anything like an arc had occurred while I was testing it- Perhaps it did this when I plugged my effects box into the input. Since there is no indication on the head regarding impedance, it was likely run at the wrong impedance.

That probably won't matter, unless you ran the amp fully cranked with no load at all or fully cranked with only 1 functioning output tube. Even then, you'd know something happened due to a blown fuse, an almighty spark/flash/bang or smoke.

I'm feeling like the apprentice that's sent to get a bucket of steam or a left handed monkey wrench.

I need you to go get me 50 yards of gig-line. 

[John]

Nearly all push-pull amps are "class A" until you slam them with a big enough drive signal. Meaning all output tubes conducting all the time. With a large enough input signal, each side may be driven to cutoff during a portion on the input signal cycle.

So if you test at low volume, no output tube cuts off.

Something else I'm sure I've read different ways, but never realized  until now.

[silverfox]

HPB- testing at low volume sounded good, it began to fade and never sounded like 100 watts. That was my experience.

The Red wire broke off as a result of both tubes having Arced in the past. The connection was fried. If you saw the pics I posted you can see the Yel wire melted off. The wire was stranded and is an OT lead.

I tested the transformer by connecting the primary to a variac and monitored the output jack with a second meter. 10 volts in, equated to .4 volts out on each separately tested output primary connection. I then did some math based on 660 - 320 = 340 {actual working volts} (340 / (10/.4)) *2 and got about 26 which I took to be the output voltage at full power. The OT seems to be okay.

I don't know how legitimate that previous formula is but I think it's right.

On measuring the primary leads- The Org lead measured 20 ohms higher. Don't know why but I bet it has to be that way for this design.

Start New Findings

I checked the Filter caps and most seem to be open. No variation in resistance. Hence the Hum.

The .1 caps and other coupling caps appear to polarized oil filled caps. I posted a link to the Amp shots and the caps have a black line on one end. I'm wondering if I need to use the same cap type. I was going to use "Xicon coupling caps". Along with all the carbon resistors there seems to be a whole lot of Mojo in this amp, MOD oil filled caps are 5.25 ea for .1.

Overall if you saw the pics, this appears to be very well done. I'm speculating were I to charge for this work it would be in the $400 range.

Silverfox.

[phsyconoodler]

The line on the oil coupling caps is the outer foil.They are not polarized.

[silverfox]

phsyconoodler said: "The line on the oil coupling caps is the outer foil.They are not polarized."

That sounds like a very controversial statement. I know in the traditional sense they are not polarized, why the line then? I assume the foil side would connect to the most positive side of the circuit. The forward point in the signal flow?

Will a high quality metal foil polypropylene cap work about as well?

Fox.

[PRR]

The mark on the cap only matters for Radio Frequency work.

When you put the large cap in a chassis there is a few pFd more capacitance on the marked (outside) end than the other end. In some radio circuits, if you stuff caps random ways, half your radios will tune-up different from the other half. They may also have different sneak-paths from one cap to another via the 0.01pFd more/less capacitance between outside foils.

In audio, this is a VERY minor difference. Few-pFd is "nothing". Especially since there is 10,000pFd from one end to the other. There is a custom to orient the mark toward the lower impedance side, usually the preceding plate; but even this is "nothing".

Replace those arced sockets before any more testing.

Install grounded cord. No it is not double insulated. Between PT primary and PT secondary is only a layer of paper.

Don't fuss with the OT. It played (even sick); it's fine. Replace sockets and wire it just like before.

Filter caps may be sick (tho if they were -dead- I'd expect the amp to be unplayable).

Don't replace the coupling caps until you probe the grids and *prove* you have a leaker. I suspect those caps will live longer than all of us.

[silverfox]

Roger That PRR. I will say I have increasing concerns as a result of several wires in the power supply breaking upon touching. Lot's of corrosion but I know how to deal with that.

I'll check the various wires and change the sockets first. Then power it up and check the supply voltages ect.

6550's are on the way and I posted pics in a post above.

Sounds like you know your way around, Ultra Highly Freqy stuff, TV dinners, perhaps even an array of phases but that would be totally off topic. Maybe a different place and topic; and I know even less about that stuff but always curious about new thoughts.

I'll post after I get the suggested work done.

Thanks all,

Fox.

[HotBluePlates]

Sounds like you know your way around, Ultra Highly Freqy stuff, TV dinners, perhaps even an array of phases but that would be totally off topic. Maybe a different place and topic; and I know even less about that stuff but always curious about new thoughts.

Problem is, 99% of that stuff has no application to guitar amps. The other 1% is the audio amplifier section of the TV.

But it's good to know about generally, and sweep tubes can be used to build amps around if you get all the pieces cheap.

[silverfox]

It lives!

Got the tube sockets replaced and added the ground cord. Put the old tubes back in, replaced several broken or questionable wires and it sounds like is should. Initially I had taken the ground cap out but there seemed to be substantial hum and thunking sounds when the chords  were struck to hard, it also seemed to lack power. Turned out I had forgotten to plug the Reverb unit back in. Once that was done everything sounded right. I'm not sure if I need to leave the cap in that goes from neutral to ground on the fuse, but it's put back with jumpers so I'll take it out again later.

I don't want to leave it in the circuit. Could it be affecting the circuit when removed? Don't see how.

Okay, the amp sounds very plain. It definitely sounds better than a basic solid state amp. Now that I've had a chance to play it and get things adjusted it's starting to sound really cool. It doesn't lose power when turned up.

When tapping on the caps though it creates noise- Haven't tried that since I installed the Reverb I had left out and perhaps after warming up those problems dissipated. One of the caps pops in after warm up and then it quiets down to normal.

As a Jazz or basic Blues amp it would work. With a distortion pedal and a Mid Boost EQ it sounds like Carlos Santana's tone. Haven't jumped the inputs yet to see how well that works. Takes the Amp Emulator okay. Best on moderate gain amp models.

Like you said PRR- It was sick and now it appears well and not suffering a burnt OT.

There is a 1K resistor between pins 5-6 on the 6L6GC tubes. Don't know what that is for.

Are there any changes I need to make to run 6550's or are the "Plug and Play" Found it above!

Thanks for the assistance, this is my first repair.

Silverfox.

[PRR]

What is the combined output-tube cathode current? (Voltage across 1 ohm resistor in cathodes return.) If over 0.1V (100mV), that's hot for 6550, consider a bias mod. (Over 70mV is hot for 6L6GC.)

Note all as-working grid plate cathode voltages on a copy of the schematic for future reference. Anything look real wrong, post details here.

[silverfox]

PRR- Clever?. Thanks for the hint.

Another observation I've made: At this time the amp is running off the Variac as the 120V pushes the supply voltage up a little over 700VDC; Even when things have settled out. At 110 AC everything is just right. This could be due to a bad supply cap. Still hums substantially at first till it POPS!.

Today I'll replace 3 supply caps. The other 2 50uf's are replaced with a 100uf by the factory and that may be okay. Schematic is off there.

Silverfox.