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Old 6th December 2006, 02:14 AM   #1
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Default Harmony H303A

Hello all, I'm building my 3rd tube project.

I decided to see what I could do with my 50c5/35w4/12au6 tubes I have, so I found a schem for some old amps people seem to like.

I have decided on the harmony h-303A

Obviously I know you're going to see 50c5 and the like and say AC/DC. Rest assured I'm using an iso tranny. Heres where my question comes in:

http://www.schematicheaven.com/barga...mony_h303a.pdf
Here is the schematic for the H-303A. As you can see it already has a power transformer from the factory, but instead of being a direct isolation transformer, it has a 48v tap to run the heaters.

Here is a schematic of the Kay 703 :
http://www.schematicheaven.com/bargainbin/kay703.pdf

As you can see in it, its basically the same layout, except the heaters are taken off the main AC line instead of after the transformer. This causes an added advantage: Since the heaters are not in the circuit, this reduces the current needed from the total circuit allowing a smaller (cheaper) isolation transformer.

My question is: Is this an ok practice, are the heaters pretty much isolated from the rest of the tube ? If not, does it hurt to just buy a larger 1:1 transformer with the added current capability?

It would be really nice if I didn't have to run the heaters after the transformer, as long as I dont reference them to ground I should be ok right?

Tsd88~
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Old 6th December 2006, 04:16 PM   #2
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I would say it's OK to run the heaters from the line but...

1) You MUST use a 3-wire power cord and fuse the HOT side of the line (otherwise a tube shorting could make the chassis hot)

2) The low-level preamp tube must be at the neutral end of the heater string - you may still have hum.

3) The half-wave current (with a DC component...) drawn from the transformer will require a bigger transformer than you might think... it won't be much smaller than if you ran the heaters from it. One way around this is to use two 35W4s (and a 35C5 output). Use them as half of a full-wave bridge (solid state for the other half) and there won't be a DC current in the secondary.
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Old 7th December 2006, 05:28 PM   #3
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I can't say this is a good idea, and if you go that route you must go with the 3 wire line cord as Tom suggests. Note that the original Harmony design is fully isolated from the line, that's not an auto transformer.

Even with the 50C5 on the isolated side you are not talking about much current. I can't see that you would saving much if any money on that isolation transformer.

Were it me I would go with a conventional power transformer, full wave rectification (6X4?) and use 6AQ5 & 6AU6 instead of the 50C5 & 12AU6. Little if any additional cost and a little more deluxe in keeping with the amount of time, effort, and cost required to build the thing.
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Old 7th December 2006, 08:04 PM   #4
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If I were building a simple amp from scratch, I would do as Kevin said and use a 6AQ5, and a 6AU6. In fact I have built such an amp and when I get back to it I will refine it further and make a PC board for it, but it won't be any time soon.

Sevaral years ago I bought and old "Electrolab" guitar amp at a swap meet. It should have been called an "Electrocution Lab" amp because it has NO power transformer at all, and it will shock the ********** out of the user.

I added an isolation transformer (Triad N68X, $12 from Mouser) that powers the whole amp, tubes and all (50C5, 35W4, and 12AU6). I replaced the fried electrolytic and the amp worked, for a while. My extreme tendencies made a short unhappy life for the 50+ year old speaker cone. The 50 VA isolation transformer gets mildly warm. The amp sounded pretty decent until I got carried away with the effects pedal and ventillated the speaker cone.

I often use cheap 6X9 or 6 inch round car speakers for small low cost guitar amps(10 watts and under). They sound decent (somewhat clean) and even I can't blow them up.

For the ultimate low buck guitar amp, I use cheap car speakers from Wal Mart, the Triad isolation transformer feeding a SS voltage doubler, a Mouser $7 filament transformer, the $16 Edcor P-P output transformer, 2 X 6AQ5 in P-P with a 6AV6 phase splitter, driven by a 12AX7. It is possible to build a guitar amp for under $100 with some parts scrounging.
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Old 8th December 2006, 03:13 AM   #5
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Hey tubelab , are you talking about durabands? I have some of those 6x9's in my car... with a 20 watt amp driving each- they really are something special for the money! As far as speakers I have plenty of vintage small woofers.

My last amp I built was a SE EL84/ high gain pre ( 2 12ax7s) EZ81 rectifier running into a Jensen c10q. I also added a switch to run it triode strapped- which by the way sounds superior to the pentode and you don't loose too much volume. Compression is way better also, estimated power output 1- 2watts in triode? I just played it in an large auditorium tonight with as much clean headroom as I could get and it filled the whole room- amazing what low wattage amps can do!

Anyway, Ive heard good things about these little 50c5 amps as far as sound.. and having some junked aa5's around Id like to see if I can do something for the heck of it. My cost is around $40 for the transformers/sockets/a few caps and resistors I need, and I need to order some other parts online as well so might as well now...

Now I know the 50c5 is a current hog as far as the heaters are concerned, thats why I asked about the iso transformer.

Now to figure out the rectifier. While fullwave would be ideal, with the tubes I have my only choice is to use a 35w4 because of the series heaters, a 35c5 is out of the question as I think it sounds bad for any audio, guitar or not. I built some a while back and liked the 50c5's breakup better.

I don't know how much voltage the 35w4 drops, I'm sure I could find out from the datasheet....but would adding in a solid state diode as the other half ( and possibly a resistor ) hurt anything? Also wouldn't I need a 100-0-100 power supply with half wave rectification?

I'm ordering my parts from tubesandmore because everything is on sale right now untill dec. 30th. They have some really good prices on transformers and stuff.

Hey tubelab: Isnt the 6av6 a ddt? I have a 12av6 cleartop .. Don't you need two triodes for a phase splitter?
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Old 8th December 2006, 05:47 PM   #6
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The last pair of cheap 6X9's that I got were Audiovox. I got several pairs when a local K-mart store closed down. I still have one pair left. It has been a while since I have built any guitar amps. I have an almost finished amp (no time to finish it) that uses 2 5X7 JBL car speakers that I pulled out of my car before trading it in. I have fed this cabinet with a 40 watt amp without blowing anything, and it positively screams. They were not cheap speakers though.

The 35W4, 12AU6, 12AV6, 50C5 all have a 150 mA heater, wire them all in series, and run them off of the isolation transformer with an appropriate dropping resistor. As stated before put the 12AU6 (or whatever input tube that you use) at the cold end of the filament string. The 35W4 can be used for a half wave rectifier in the same manner that it is used in an AA5 radio. This is the simplest circuit but gives the worst regulation and most voltage drop. A full wave center tapped circuit (the most common) would require a 100-0-100 or 125-0-125 volt transformer. These are available but cost considerably more than an isolation transformer. A full wave bridge requires 4 diodes but could be operated directly off of an isolation transformer. You could use 4 silicon diodes, 2 silicon and 2 tubes, or 4 tubes to build this.

You could build a P-P amp with a 12AU6 input tube, a 12AV6 phase splitter, and 2 X 50C5 output tubes. If you wanted more gain the 12AV6 could be replaced with a 12AX7. All are 150 mA tubes which could be wired in series. I may have to try this some day since I have a good supply of all of the tubes.

The Electrolab amp has a 35W4 as a half wave rectifier. Its filament is connected through the power switch to the line cord (no fuse) next in the string is a big resistor (no markings left) then the 50C5 and the 12AU6 the other side of the 12AU6 filament goes directly to the line cord. This connection is circuit ground for the rest of the amp.

If I remember right the B+ was about 130 volts. The amp is wired up like an AA5 radio. The guitar input jack is grounded to the metal chassis, which is connected to an old (leaky) paper capacitor which goes to circuit ground (one side of the power line). DO NOT EVEN THINK ABOUT DOING THIS! IT COULD KILL YOU!

I can't believe that something like this was actually sold. I added the isolation transformer (in the bottom of the cabinet) which is between the power line and the original amp. I also added a 3 wire power cord with the ground wire connected to the chassis. The isolation transformer is on whenever the amp is plugged in since the rest of the amp is original and wired to the secondary of the transformer.

I don't know what Antique Electronics supply has for isolation transformers, but they may be more expensive than Mouser even on sale. I buy parts from both places.

Yes the 6AV6 (or 6AT6, 6AQ6) is a DDT, the 12AV6 is the same thing with a 12 volt heater. I ignore the diodes and use them like 1/2 of a 12AX7. Each has different gain though. A "split load" or "Concertina" phase splitter can be made using only one tube although it has no gain.

Look here for an example. This is the schematic that I started with a long time ago. I have made dozens of amps that started with either the Fender Champ or Fender Harvard schematic. I started building them when I was about 14 years old. You can't get much simpler. Most of my amps were built before I got smart enough to write anything down, so I have very little documentation of the stuff that I actually built.

I know that I put the 6AT6 (or 6AV6) in the phase splitter position because they are more microphonic than the 12AX7 type tubes. I have also used a 6C4 for the phase splitter. This also allows the use of a 7025 or 5751 tube in the input for a quieter amp. This helps when building a one piece (amp and speaker in same cabinet) design.

http://www.schematicheaven.com/fende...5f10_schem.pdf

Start here if you want to build an amp with a pentode (12AU6) input tube. The grid resistors need to be much smaller, and you need a cathode resistor on the 12AU6. Start with the resistor values from the IF amp in the donor AA5.

http://www.schematicheaven.com/fende..._5c1_schem.pdf

I made many of my amps when I was a teenager using radios, TV sets, or old HiFi sets. They were easy to get at the local trash dump. The OPT from an old radio will work for a 1 or 2 watt guitar amp and will give that Fender saturated transformer sound. Vertical output transformers from old TV's work for 2 to 5 watt SE amps. Old HiFi sets gave up their power and OPT's for P-P amps. I liked Magnavoxes with P-P 6V6's.

I found a schematic of one of my "Turbo Champs" (built about 10 years ago) and posted it on my web site. I built several variations of those, along with dozens of P-P amps. All were sold years ago. I see one from time to time, but I haven't had the time to make any new ones in several years.
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Old 8th December 2006, 09:12 PM   #7
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I have some opt's from the AA5's, so I will probably use those or get a $4 3 watt one on tubesandmore. Their prices are pretty good, a hammond 35va iso transformer is $18 right now.

I do have two 50c5s, but their not matched.. and I'm pretty sure one of them is a bit worn out. One came from an RCA radio , the other a GE. I wonder, how does a pentode pre sound compared to a triode? I'd have to say triode is superior in the output stage though.

I may build an amp similar to the champ or similar to mine with a 6L6 instead of a 6v6 or el84. I wonder how much power you could get out of a 6L6 triode strapped

I really think 5 watts of triode distortion would be the best sound ever.. My amp sounds very good when it begins to clip and gets into moderate clipping, but it gets a bit too chunky after that , and in triode it doesnt do that , it compresses the signal more too.

It really makes no sence to use a 35w4 unless you use two,because obviously with the half wave my ripple will be much worse. The only reason I'd use it is for the heater, unless I could substitute a resistor for that? I'd imagine it would get pretty warm wouldn't it..

Now I wonder.. would it be worth it to get the LM317hv and make a 50v regulator, and use a transformer that has a 12v tap for the amp? Does the 50c5 do better on ac or dc?

If nothing else... does anyone see anything wrong with just building it the way it was originally (isolation transformer included though)
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Old 8th December 2006, 09:34 PM   #8
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I'm going to advocate again for the 6AX4, 6AQ5, 6AU6 approach. No oddball tubes, fullwave rectification. Cheap and totally safe. Amenable to lots of tube rolling as well.

I'm pretty sure with a little judicious shopping on eBay and elsewhere most of the parts can be secured dirt cheap. (Not that 50C5 aren't cheap, just is it worth the trouble if you have to buy some anyway??)

Start throwing in things like 50V regulators, etc and it is no longer cheap or simple.

I'll probably design and build something similar for my niece.
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Old 8th December 2006, 10:53 PM   #9
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Quote:
hammond 35va iso transformer is $18 right now.
The Triad from Mouser is 50 VA and costs under $12 all of the time. I have used a few (5 or 6) of these without issue. The Hammond transformers are OK too.

I again tend to agree with Kevin. If you even have to think about using 50 volt regulators and such, the amp has lost the only appeal that I would give to a 50C5 amp, CHEAPNESS, utter cheapness. I was thinking of how to build a $50 guitar amp with 50C5's powered by a $12 isolation transformer. A 50 volt regulator would cost more in money, hassle, and reliability than a $8 Mouser 6 volt filament transformer. 6 volts allows the use of standard tubes that you can get anywhere.

6AQ5's are cheap, sound awesome in a SE guitar amp. One of the Gibson Skylark amps use a 6AQ5, driven by a 6EU7, powered by a 6X4. I have one, but an idiot with a 20 amp fuse smoked the power transformer because of a bad 6X4.

I have a few hundred 6AQ5's (there is a project coming here) and I have tortured several of them to find their limits (they had severely corroded pins, no good tubes were fried). I had one that put out 5 watts in triode for over an hour (glowing bright red) until the glass finally melted and the tube sucked air. How is that for a tough tube. Millions of them were made for radios and military stuff. There are dozens of different manufuacturers. A commercial grade is the 6005. They can usually be used in an amplifier circuit that was designed for a 6V6 by reducing the supply voltage slightly.

If you are buying parts, then consider the 6AQ5, or the slightly bigger 6BQ5 - EL84, or even the 6V6, all are good sounding guitar amp tubes. If you must have the sound of a 50C5, there was a 6 volt version. I don't have the number handy (I am at work) but I think it is 6CU5.

If you want to build the Harmony as is you can probably find a 48 volt transformer to light the filaments. Look through the surplus catalogs, or on Ebay. The 35W4 half wave does have some ripple, but it is 60 Hz not 120 as with a full wave rectifier. Many cheap amps got away with it because the cheap speaker doesn't make much sound at 60 Hz and the cheap OPT is lossy at 60 Hz. Millions of AA5's were made, they don't hum much for the same reason.

Quote:
I wonder how much power you could get out of a 6L6 triode strapped
That is a good question, and the answer depends a lot on the 6L6 and the efficiency of the OPT. I wouldn't push a 6L6GB past 2 or 3 watts. A 6L6GC will make 3 or 4 without breaking a sweat and 5 if you push it. Some of the current production tubes being sold as "6L6 types" can be pushed harder. My favorite "power producer" is the Sovtek 6L6WXT, also sold as the Electro Harmonix 6L6GC. These can be cranked to 7 or 8 watts in triode and 10 to 14 in Ultralinear. I haven't tried pentode mode. The Electro Harmonix 6550 or KT88 will do 10 watts in triode and 18 in Ultralinear but you need about 500 volts of B+ (cathode bias). This is what I used to call the "Turbo Champ".

The $18 Edcor SE OPT's made the most power of any that I tested. The big Hammond 1628SE was the lossiest, and was a watt behind the Edcor with an EL34 tube in triode, it is overkill for a guitar amp.
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Old 9th December 2006, 01:32 AM   #10
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I have a 10 watt hammond 125DSE in mine. Are there any better ones that what I have in it ?

Heh thats amazing what that 6aq5 did. Wonder what an EL84's max limits are? The EL84/6bq5 is a good sounding tube but can be a bit harsh unless you tame it. I think Rock when I see an EL84, I think blusey when I see a 6v6. I'm thinking about throwing a 6v6 in mine to see what the diffs are.

I did not know the skylark used a 6aq5! I know some of the later GA5's used EL84's for power.

The 50v regulator probably isn't worth it.. so nevermind on that. I was planning on using a 6 inch or smaller speaker in my amp anyways. The radios I had were good working radios ( ugly, but good working- and not worth anything) and they had no hum really.

I really just wanted to use these tubes for something... Now if I were building an amp for someone else I would definatly not use obsolete tubes and they would have to be 12/6v tubes.



I was aware of the 12c5 but not the 6cu5. When I search for it at some common tube places only the 12cu5 comes up... and it lists it as a ballast tube?

Btw: since we're on ths subject... I have an ez81 in my amp, not because it really does anything to the sound because its CLass A, but because it drops more voltage than SS and looks kinda cool too. Sometimes people think its got two el84's in it and say it sounds amazing-you should see their jaws drop when I tell them its only one and running 5 watts at best!

The rerason I have a tube recto in it is because the 269ex was designed for 117v lines, and where I live its about 120-122v. B+ voltage is supposed to be 250v, but with the raise in mains it comes out to about 270v with the tube recto. With SS it jumps up to about 287v. I wasn't sure if this would hurt anything when I built it so I didn't mess with it. Now I am thinking about making a plug in SS recto and just turning down the bias a bit ( I added adjustable bias- a REALLY usefull feature!)

Now I was wondering, I know my opt's max current is spec.d at 70ma, and the EL84 is 43ma @ 250v. I know max plate dissipation is 12 watts, but since were on this subject how far can you push it without giving up too much reliability? It seems to sound better when pushed when its biased a bit hotter. As long as my plates don't glow is it ok to bump up the bias a bit and rais the main rails? The pre's seem to like more voltage, because when the pre's are dimed the high-gain distortion is a bit more defined. That being said, I read somewhere that amps sound better on lower plate voltages and higher currents, instead of upping the plate voltage.. Am I missing something here?
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